Protection & Guide Network Automation
Network P rotection & Automation Guide First edition July 2002 Previously called Protective Relays Application Guide First printed Reprinted Ju ne 1966 January 1967 August 1968 November 1970 September 1971 February 1973 Janu ary 1974 March 1975 November 1977 December 1979 November 1982 October 1983 Octob er 1985 June 1987 September 1990 March 1995 Second edition First printed Reprinted Third edition First printed Reprinted All rights reserved Copyright AREVA 2005 AREVA T&D - 1, Place de la Coupole - 92084 Paris La Défense - France. www.areva-td .com AREVA T&D Worlwide Contact Centre: http://www.areva-td.com/contactcentre/ T el.: +44 (0) 1785 250 070 ISBN : 2-9518589-0-6 Layout by Flash Espace, Montpellier, France - Printed by Cayfosa, Barcelona, Spa in
Acknowledgements This book is the result of the co-operation and teamwork of the many specialist engineers employed by AREVA T&D Automation & Information Systems. The Company wo uld like to acknowledge their assistance in producing this edition. AREVA T&D wo uld also like to acknowledge gratefully the co-operation of the following compan ies in providing material for this edition. ALSTOM Power AREVA T&D Transformers AREVA T&D Instrument Transformers AREVA T&D Distribution Switchgear AREVA T&D Ne twork Planning ALSTOM Electrical Machines ALSTOM Transport/Virgin Trains The inv aluable contributions of PB Power within the review process are also acknowledge d gratefully. Peter Rush Network Protection & Automation Guide Acknowledgements •
Contents 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Appendix 1 App endix 2 Appendix 3 Index Introduction Fundamentals of Protection Practice Fundam ental Theory Fault Calculations Equivalent Circuits and Parameters of Power Syst em Plant Current and Voltage Transformers Relay Technology Protection: Signallin g and Intertripping Overcurrent Protection for Phase and Earth Faults Unit Prote ction of Feeders Distance Protection Distance Protection Schemes Protection of C omplex Transmission Circuits Auto-Reclosing Busbar Protection Transformer and Tr ansformer-Feeder Protection Generator and Generator-Transformer Protection Indus trial and Commercial Power System Protection A.C. Motor Protection Protection of A.C. Electrified Railways Relay Testing and Commissioning Power System Measurem ents Power Quality Substation Control and Automation Distribution System Automat ion Terminology ANSI/IEC Relay Symbols Application Tables .................... p2 p4 .................... ................ p16 p30 p46 p78 p98 ................. ................. ................. ................. ............... p112 p122 p152 p170 p192 p202 p218 p232 p254 p280 p316 p336 p352 p370 p398 p410 p422 p442 p454 p466 p468 p476 .............. .............. .............. .............. .............. .............. .............. .............. .............. .............. .............. ..............
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• 1 • Introduction
• 1 • Introduction ‘Relay hardware is becoming even more standardised, to the point at which versions of a relay may differ only by the software they contain’. This accurate predictio n in the preface to the Third Edition of the Protective Relay Application Guide (PRAG), 1987, has been followed by the rapid development of integrated protectio n and control devices. The change in technology, together with significant chang es in Utility, Industrial and Commercial organisations, has resulted in new emph asis on Secondary Systems Engineering. In addition to the traditional role of pr otection & control, secondary systems are now required to provide true added val ue to organisations. When utilised to its maximum, not only can the integration of protection & control functionality deliver the required reduction in life-tim e cost of capital, but the advanced features available (Quality of Supply, distu rbance recording and plant monitoring) enable system and plant performance to be improved, increasing system availability. The evolution of all secondary connec ted devices to form digital control systems continues to greatly increase access to all information available within the substation, resulting in new methodolog ies for asset management. In order to provide the modern practising substation e ngineer with reference material, the Network Protection & Automation Guide provi des a substantially revised and expanded edition of PRAG incorporating new chapt ers on all levels of network automation. The first part of the book deals with t he fundamentals, basic technology, fault calculations and the models of power sy stem plant, including the transient response and saturation problems that affect instrument transformers. The typical data provided on power system plant has be en updated and significantly expanded following research that showed its popular ity. The book then provides detailed analysis on the application of protection s ystems. This includes a new Chapter on the protection of a.c. electrified railwa ys. Existing chapters on distance, busbar and generator protection have been com pletely revised to take account of new developments, including improvements due to numerical protection techniques and the application problems of embedded gene ration. The Chapter on relay testing and commissioning has been completely updat ed to reflect modern techniques. Finally, new Chapters covering the fields of po wer system measurements, power quality, and substation and distribution automati on are found, to reflect the importance of these fields for the modern Power Sys tem Engineer. The intention is to make NPAG the standard reference work in its’ su bject area - while still helping the student and young engineer new to the field . We trust that you find this book invaluable and assure you that any comments w ill be carefully noted ready for the next edition. Network Protection & Automation Guide • 3 •
• 2 • Fundamentals of Protection Practice Introduction Protection equipment Zones of protection Reliability Selectivity St ability Speed Sensitivity Primary and back-up protection Relay output devices Re lay tripping circuits 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 Trip circuit supervision 2.12
• 2 • Fundamentals of P rotection P ractice 2.1 INTRODUCTION The purpose of an electrical power system is to generate and su pply electrical energy to consumers. The system should be designed and managed t o deliver this energy to the utilisation points with both reliability and econom y. Severe disruption to the normal routine of modern society is likely if power outages are frequent or prolonged, placing an increasing emphasis on reliability and security of supply. As the requirements of reliability and economy are larg ely opposed, power system design is inevitably a compromise. A power system comp rises many diverse items of equipment. Figure 2.2 shows a hypothetical power sys tem; this and Figure 2.1 illustrates the diversity of equipment that is found. Figure 2.1: Power station Network Protection & Automation Guide • 5 •
Hydro power station R1 G1 G2 R2 T1 T2 380kV L2 L1B A L1A Fundamentals of P rotection P ractice 380kV C L3 380kV L4 B T5 T6 T3 T4 1 10kV Steam power station R3 G3 G4 R4 C 33kV CCGT power station R5 G5 G6 B R6 G7 R7 T9 T10 T11 T7 T8 220kV D L7A T14 380kV E
L6 • 2• L7B T15 T12 T13 Grid substation F 380kV L5 G T16 L8 T17 33kV D Grid 380kV F 1 10kV G e 2. Figur Figure 2.1: Example power system Figure 2.2: Example power system • 6 • Network Protection & Automation Guide
Figure 2.4: Possible consequence of inadequate protection 2 . 2 P R OT E C T I O N E Q U I P M E N T a. Protection System: a complete arrangement of protection equipment and other d evices required to achieve a specified function based on a protection principal (IEC 60255-20) b. Protection Equipment: a collection of protection devices (rela ys, fuses, etc.). Excluded are devices such as CT’s, CB’s, Contactors, etc. Figure 2.3: Onset of an overhead line fault Many items of equipment are very expensive, and so the complete power system rep resents a very large capital investment. To maximise the return on this outlay, the system must be utilised as much as possible within the applicable constraint s of security and reliability of supply. More fundamental, however, is that the power system should operate in a safe manner at all times. No matter how well de signed, faults will always occur on a power system, and these faults may represe nt a risk to life and/or property. Figure 2.3 shows the onset of a fault on an o verhead line. The destructive power of a fault arc carrying a high current is ve ry great; it can burn through copper conductors or weld together core lamination s in a transformer or machine in a very short time – some tens or hundreds of mill iseconds. Even away from the fault arc itself, heavy fault currents can cause da mage to plant if they continue for more than a few seconds. The provision of ade quate protection to detect and disconnect elements of the power system in the ev ent of fault is therefore an integral part of power system design. Only by so do ing can the objectives of the power system be met and the investment protected. Figure 2.4 provides an illustration of the consequences of failure to provide ap propriate protection. This is the measure of the importance of protection system s as applied in power system practice and of the responsibility vested in the Pr otection Engineer. c. Protection Scheme: a collection of protection equipment providing a defined f unction and including all equipment required to make the scheme work (i.e. relay s, CT’s, CB’s, batteries, etc.) In order to fulfil the requirements of protection wi th the optimum speed for the many different configurations, operating conditions and construction features of power systems, it has been necessary to develop ma ny types of relay that respond to various functions of the power system quantiti es. For example, observation simply of the magnitude of the fault current suffic es in some cases but measurement of power or impedance may be necessary in other s. Relays frequently measure complex functions of the system quantities, which a re only readily expressible by mathematical or graphical means. Relays may be cl assified according to the technology used: a. electromechanical b. static c. dig ital d. numerical The different types have somewhat different capabilities, due to the limitations of the technology used. They are described in more detail in Chapter 7. Network Protection & Automation Guide • 7 • Fundamentals of P rotection P ractice • The definitions that follow are generally used in relation to power system prote ction: 2•
In many cases, it is not feasible to protect against all hazards with a relay th at responds to a single power system quantity. An arrangement using several quan tities may be required. In this case, either several relays, each responding to a single quantity, or, more commonly, a single relay containing several elements , each responding independently to a different quantity may be used. The termino logy used in describing protection systems and relays is given in Appendix 1. Di fferent symbols for describing relay functions in diagrams of protection schemes are used, the two most common methods (IEC and IEEE/ANSI) are provided in Appen dix 2. 2 . 3 Z O N E S O F P R OT E C T I O N A Busbar protection rotection Feed Feeder protection (a) CT s on both sides of circuit breaker Busbar protecti on rotection F Fundamentals of P rotection P ractice To limit the extent of the power system that is disconnected when a fault occurs , protection is arranged in zones. The principle is shown in Figure 2.5. Ideally , the zones of protection should overlap, so that no part of the power system is left unprotected. This is shown in Figure 2.6(a), the circuit breaker being inc luded in both zones. Feed Feeder protection (b) CT s on circuit side of circuit breaker Figure 2.6: C T Locations Figure 2.6: CT Locations Zone 1 the circuit breaker A that is not completely protected against faults. In Figure 2.6(b) a fault at F would cause the busbar protection to operate and open the c ircuit breaker but the fault may continue to be fed through the feeder. The feed er protection, if of the unit type (see section 2.5.2), would not operate, since the fault is outside its zone. This problem is dealt with by intertripping or s ome form of zone extension, to ensure that the remote end of the feeder is tripp ed also. The point of connection of the protection with the power system usually defines the zone and corresponds to the location of the current transformers. U nit type protection will result in the boundary being a clearly defined closed l oop. Figure 2.7 illustrates a typical arrangement of overlapping zones. Zone 2 Zone 3 • 2• Zone 5 Zone 4 ~ ~ Figure 2.7 Figure 2.7: Overlapping zones of protection systems Zone 7
Feeder 1 Feeder 2 Zone 6 Feeder 3 Figure 2.5: Division of power system FigureFigure 2.5: Division of power system into protection zones 2.52.6 into protection zones For practical physical and economic reasons, this ideal is not always achieved, accommodation for current transformers being in some cases available only on one side of the circuit breakers, as in Figure 2.6(b). This leaves a section betwee n the current transformers and Alternatively, the zone may be unrestricted; Figure 2.7: Overlapping zones of pr otection systems the start will be defined but the extent (or ‘reach’) will depend on measurement of the system quantities and will therefore be subject to variation, owing to chang es in system conditions and measurement errors. • 8 • Network Protection & Automation Guide
2.4 RELIABILITY The need for a high degree of reliability is discussed in Sectio n 2.1. Incorrect operation can be attributed to one of the following classificat ions: a. incorrect design/settings b. incorrect installation/testing c. deterior ation in service 2.4.1 Design The design of a protection scheme is of paramount importance. This is to ensure that the system will operate under all required co nditions, and (equally important) refrain from operating when so required (inclu ding, where appropriate, being restrained from operating for faults external to the zone being protected). Due consideration must be given to the nature, freque ncy and duration of faults likely to be experienced, all relevant parameters of the power system (including the characteristics of the supply source, and method s of operation) and the type of protection equipment used. Of course, no amount of effort at this stage can make up for the use of protection equipment that has not itself been subject to proper design. 2.4.2 Settings It is essential to ens ure that settings are chosen for protection relays and systems which take into a ccount the parameters of the primary system, including fault and load levels, an d dynamic performance requirements etc. The characteristics of power systems cha nge with time, due to changes in loads, location, type and amount of generation, etc. Therefore, setting values of relays may need to be checked at suitable int ervals to ensure that they are still appropriate. Otherwise, unwanted operation or failure to operate when required may occur. 2.4.3 Installation The need for c orrect installation of protection systems is obvious, but the complexity of the interconnections of many systems and their relationship to the remainder of the installation may make checking difficult. Site testing is therefore necessary; s ince it will be difficult to reproduce all fault conditions correctly, these tes ts must be directed to proving the installation. The tests should be limited to such simple and direct tests as will prove the correctness of the connections, r elay settings, and freedom from damage of the equipment. No attempt should be ma de to type test the equipment or to establish complex aspects of its technical performance. Network Protection & Automation Guide • 9 • 2.4.4 Testing Comprehensive testing is just as important, and this testing shoul d cover all aspects of the protection scheme, as well as reproducing operational and environmental conditions as closely as possible. Type testing of protection equipment to recognised standards fulfils many of these requirements, but it ma y still be necessary to test the complete protection scheme (relays, current tra nsformers and other ancillary items) and the tests must simulate fault condition s realistically. 2.4.5 Deterioration in Service Subsequent to installation in pe rfect condition, deterioration of equipment will take place and may eventually i nterfere with correct functioning. For example, contacts may become rough or bur nt owing to frequent operation, or tarnished owing to atmospheric contamination; coils and other circuits may become open-circuited, electronic components and a uxiliary devices may fail, and mechanical parts may seize up. The time between o perations of protection relays may be years rather than days. During this period defects may have developed unnoticed until revealed by the failure of the prote ction to respond to a power system fault. For this reason, relays should be regu larly tested in order to check for correct functioning. Testing should preferabl y be carried out without disturbing permanent connections. This can be achieved by the provision of test blocks or switches. The quality of testing personnel is an essential feature when assessing reliability and considering means for impro vement. Staff must be technically competent and adequately trained, as well as s elf-disciplined to proceed in a systematic manner to achieve final acceptance. I mportant circuits that are especially vulnerable can be provided with continuous electrical supervision; such arrangements are commonly applied to circuit break er trip circuits and to pilot circuits. Modern digital and numerical relays usua lly incorporate selftesting/diagnostic facilities to assist in the detection of failures. With these types of relay, it may be possible to arrange for such fail ures to be automatically reported by communications link to a remote operations centre, so that appropriate action may be taken to ensure continued safe operati on of that part of the power system and arrangements put in hand for investigati
on and correction of the fault. 2.4.6 Protection Performance Protection system p erformance is frequently assessed statistically. For this purpose each system fa ult is classed Fundamentals of P rotection P ractice • 2•
as an incident and only those that are cleared by the tripping of the correct ci rcuit breakers are classed as correct . The percentage of correct clearances ca n then be determined. This principle of assessment gives an accurate evaluation of the protection of the system as a whole, but it is severe in its judgement of relay performance. Many relays are called into operation for each system fault, and all must behave correctly for a correct clearance to be recorded. Complete reliability is unlikely ever to be achieved by further improvements in construct ion. If the level of reliability achieved by a single device is not acceptable, improvement can be achieved through redundancy, e.g. duplication of equipment. T wo complete, independent, main protection systems are provided, and arranged so that either by itself can carry out the required function. If the probability of each equipment failing is x/unit, the resultant probability of both equipments failing simultaneously, allowing for redundancy, is x2. Where x is small the res ultant risk (x2) may be negligible. Where multiple protection systems are used, the tripping signal can be provided in a number of different ways. The two most common methods are: a. all protection systems must operate for a tripping operat ion to occur (e.g. ‘two-out-of-two’ arrangement) b. only one protection system need operate to cause a trip (e.g. ‘one-out-of two’ arrangement) The former method guards against maloperation while the latter guards against failure to operate due to an unrevealed fault in a protection system. Rarely, three main protection system s are provided, configured in a ‘two-out-of three’ tripping arrangement, to provide both reliability of tripping, and security against unwanted tripping. It has lon g been the practice to apply duplicate protection systems to busbars, both being required to operate to complete a tripping operation. Loss of a busbar may caus e widespread loss of supply, which is clearly undesirable. In other cases, impor tant circuits are provided with duplicate main protection systems, either being able to trip independently. On critical circuits, use may also be made of a digi tal fault simulator to model the relevant section of the power system and check the performance of the relays used. 2.5 SELECTIVITY When a fault occurs, the pro tection scheme is required to trip only those circuit breakers whose operation i s required to isolate the fault. This property of selective tripping is also cal led discrimination and is achieved by two general methods. • 10 • 2.5.1 Time Grading Protection systems in successive zones are arranged to operat e in times that are graded through the sequence of equipments so that upon the o ccurrence of a fault, although a number of protection equipments respond, only t hose relevant to the faulty zone complete the tripping function. The others make incomplete operations and then reset. The speed of response will often depend o n the severity of the fault, and will generally be slower than for a unit system . 2.5.2 Unit Systems It is possible to design protection systems that respond on ly to fault conditions occurring within a clearly defined zone. This type of pro tection system is known as unit protection . Certain types of unit protection a re known by specific names, e.g. restricted earth fault and differential protect ion. Unit protection can be applied throughout a power system and, since it does not involve time grading, is relatively fast in operation. The speed of respons e is substantially independent of fault severity. Unit protection usually involv es comparison of quantities at the boundaries of the protected zone as defined b y the locations of the current transformers. This comparison may be achieved by direct hard-wired connections or may be achieved via a communications link. Howe ver certain protection systems derive their restricted property from the confi guration of the power system and may be classed as unit protection, e.g. earth f ault protection applied to the high voltage delta winding of a power transformer . Whichever method is used, it must be kept in mind that selectivity is not mere ly a matter of relay design. It also depends on the correct coordination of curr ent transformers and relays with a suitable choice of relay settings, taking int o account the possible range of such variables as fault currents, maximum load c urrent, system impedances and other related factors, where appropriate. 2 . 6 S TA B I L I T Y The term ‘stability’ is usually associated with unit protection schem es and refers to the ability of the protection system to remain unaffected by co
nditions external to the protected zone, for example through load current and ex ternal fault conditions. 2.7 SPEED The function of protection systems is to isol ate faults on the power system as rapidly as possible. The main objective is to safeguard continuity of supply by removing each disturbance before it leads to w idespread loss of synchronism and consequent collapse of the power system. Network Protection & Automation Guide Fundamentals of P rotection P ractice • 2•
As the loading on a power system increases, the phase shift between voltages at different busbars on the system also increases, and therefore so does the probab ility that synchronism will be lost when the system is disturbed by a fault. The shorter the time a fault is allowed to remain in the system, the greater can be the loading of the system. Figure 2.8 shows typical relations between system lo ading and fault clearance times for various types of fault. It will be noted tha t phase faults have a more marked effect on the stability of the system than a s imple earth fault and therefore require faster clearance. Figure 2.8 Phase-earth Load power 2 . 9 P R I M A R Y A N D B A C K - U P P R OT E C T I O N The reliability of a power system has been discussed earlier, including the use of more than one prim ary (or ‘main’) protection system operating in parallel. In the event of failure or non-availability of the primary protection some other means of ensuring that the fault is isolated must be provided. These secondary systems are referred to as ‘b ack-up protection’. Back-up protection may be considered as either being ‘local’ or ‘rem ote’. Local back-up protection is achieved by protection which detects an un-clear ed primary system fault at its own location and which then trips its own circuit breakers, e.g. time graded overcurrent relays. Remote back-up protection is pro vided by protection that detects an un-cleared primary system fault at a remote location and then issues a local trip command, e.g. the second or third zones of a distance relay. In both cases the main and back-up protection systems detect a fault simultaneously, operation of the back-up protection being delayed to ens ure that the primary protection clears the fault if possible. Normally being uni t protection, operation of the primary protection will be fast and will result i n the minimum amount of the power system being disconnected. Operation of the ba ck-up protection will be, of necessity, slower and will result in a greater prop ortion of the primary system being lost. The extent and type of back-up protecti on applied will naturally be related to the failure risks and relative economic importance of the system. For distribution systems where fault clearance times a re not critical, time delayed remote back-up protection may be adequate. For EHV systems, where system stability is at risk unless a fault is cleared quickly, m ultiple primary protection systems, operating in parallel and possibly of differ ent types (e.g. distance and unit protection), will be used to ensure fast and r eliable tripping. Back-up overcurrent protection may then optionally be applied to ensure that two separate protection systems are available during maintenance of one of the primary protection systems. Back-up protection systems should, ide ally, be completely separate from the primary systems. For example a circuit pro tected by a current differential relay may also have time graded overcurrent and earth fault relays added to provide circuit breaker tripping in the event of fa ilure of the main primary unit protection. To maintain complete separation and t hus integrity, current transformers, voltage transformers, relays, circuit break er trip coils and d.c. supplies would be duplicated. This ideal is rarely attain ed in practice. The following compromises are typical: a. separate current trans formers (cores and secondary windings only) are provided. This involves little e xtra cost or accommodation compared with the use of • 11 • Phase-phase-earth Three-phase Time Figure 2.8: Typical power/time relationship for various fault types System stability is not, however, the only consideration. Rapid operation of pro tection ensures that fault damage is minimised, as energy liberated during a fau lt is proportional to the square of the fault current times the duration of the fault. Protection must thus operate as quickly as possible but speed of operatio n must be weighed against economy. Distribution circuits, which do not normally
require a fast fault clearance, are usually protected by time-graded systems. Ge nerating plant and EHV systems require protection gear of the highest attainable speed; the only limiting factor will be the necessity for correct operation, an d therefore unit systems are normal practice. 2.8 SENSITIVITY Sensitivity is a t erm frequently used when referring to the minimum operating level (current, volt age, power etc.) of relays or complete protection schemes. The relay or scheme i s said to be sensitive if the primary operating parameter(s) is low. With older electromechanical relays, sensitivity was considered in terms of the sensitivity of the measuring movement and was measured in terms of its volt-ampere consumpt ion to cause operation. With modern digital and numerical relays the achievable sensitivity is seldom limited by the device design but by its application and CT /VT parameters. Network Protection & Automation Guide Fundamentals of P rotection P ractice • Phase-phase 2•
common current transformers that would have to be larger because of the combined burden. This practice is becoming less common when digital or numerical relays are used, because of the extremely low input burden of these relay types b. volt age transformers are not duplicated because of cost and space considerations. Ea ch protection relay supply is separately protected (fuse or MCB) and continuousl y supervised to ensure security of the VT output. An alarm is given on failure o f the supply and, where appropriate, prevent an unwanted operation of the protec tion c. trip supplies to the two protections should be separately protected (fus e or MCB). Duplication of tripping batteries and of circuit breaker tripping coi ls may be provided. Trip circuits should be continuously supervised d. it is des irable that the main and back-up protections (or duplicate main protections) sho uld operate on different principles, so that unusual events that may cause failu re of the one will be less likely to affect the other Digital and numerical rela ys may incorporate suitable back-up protection functions (e.g. a distance relay may also incorporate time-delayed overcurrent protection elements as well). A re duction in the hardware required to provide back-up protection is obtained, but at the risk that a common relay element failure (e.g. the power supply) will res ult in simultaneous loss of both main and back-up protection. The acceptability of this situation must be evaluated on a case-by-case basis. 2 . 10 R E L AY O U T P U T D E V I C E S In order to perform their intended function, relays must be fitted with some means of providing the various output signals required. Cont acts of various types usually fulfil this function. 2.10.1 Contact Systems Relay s may be fitted with a variety of contact systems for providing electrical outpu ts for tripping and remote indication purposes. The most common types encountere d are as follows: a. Self-reset The contacts remain in the operated condition on ly while the controlling quantity is applied, returning to their original condit ion when it is removed b. Hand or electrical reset These contacts remain in the operated condition after the controlling quantity is removed. They can be reset either by hand or by an auxiliary electromagnetic element • 12 • The majority of protection relay elements have self-reset contact systems, which , if so desired, can be modified to provide hand reset output contacts by the us e of auxiliary elements. Hand or electrically reset relays are used when it is n ecessary to maintain a signal or lockout condition. Contacts are shown on diagra ms in the position corresponding to the un-operated or deenergised condition, re gardless of the continuous service condition of the equipment. For example, an u ndervoltage relay, which is continually energised in normal circumstances, would still be shown in the deenergised condition. A make contact is one that close s when the relay picks up, whereas a break contact is one that is closed when the relay is de-energised and opens when the relay picks up. Examples of these c onventions and variations are shown in Figure 2.9. Self reset Fundamentals of P rotection P ractice Hand reset make contacts (normally open) break contacts (normally open) Time delay on pick up Time delay on drop-off Figure 2.9: Contact types Figure 2.9: Contact types • 2• A protection relay is usually required to trip a circuit breaker, the tripping m
echanism of which may be a solenoid with a plunger acting directly on the mechan ism latch or an electrically operated valve. The power required by the trip coil of the circuit breaker may range from up to 50 watts for a small distribution circuit breaker, to 3000 watts for a large, extra-highvoltage circuit breaker. The relay may therefore energise the tripping coil directly, or, according to th e coil rating and the number of circuits to be energised, may do so through the agency of another multi-contact auxiliary relay. The basic trip circuit is simpl e, being made up of a handtrip control switch and the contacts of the protection relays in parallel to energise the trip coil from a battery, through a normally open auxiliary switch operated by the circuit breaker. This auxiliary switch is needed to open the trip circuit when the circuit breaker opens since the protec tion relay contacts will usually be quite incapable of performing the interrupti ng duty. The auxiliary switch will be adjusted to close as early as possible in the closing stroke, to make the protection effective in case the breaker is bein g closed on to a fault. Network Protection & Automation Guide
Where multiple output contacts, or contacts with appreciable current-carrying ca pacity are required, interposing, contactor type elements will normally be used. In general, static and microprocessor relays have discrete measuring and trippi ng circuits, or modules. The functioning of the measuring modules is independent of operation of the tripping modules. Such a relay is equivalent to a sensitive electromechanical relay with a tripping contactor, so that the number or rating of outputs has no more significance than the fact that they have been provided. For larger switchgear installations the tripping power requirement of each circ uit breaker is considerable, and further, two or more breakers may have to be tr ipped by one protection system. There may also be remote signalling requirements , interlocking with other functions (for example auto-reclosing arrangements), a nd other control functions to be performed. These various operations may then be carried out by multicontact tripping relays, which are energised by the protect ion relays and provide the necessary number of adequately rated output contacts. 2.10.2 Operation Indicators Protection systems are invariably provided with ind icating devices, called flags , or targets , as a guide for operations personn el. Not every relay will have one, as indicators are arranged to operate only if a trip operation is initiated. Indicators, with very few exceptions, are bi-sta ble devices, and may be either mechanical or electrical. A mechanical indicator consists of a small shutter that is released by the protection relay movement to expose the indicator pattern. Electrical indicators may be simple attracted arm ature elements, where operation of the armature releases a shutter to expose an indicator as above, or indicator lights (usually light emitting diodes). For the latter, some kind of memory circuit is provided to ensure that the indicator re mains lit after the initiating event has passed. With the advent of digital and numerical relays, the operation indicator has almost become redundant. Relays wi ll be provided with one or two simple indicators that indicate that the relay is powered up and whether an operation has occurred. The remainder of the informat ion previously presented via indicators is available by interrogating the relay locally via a ‘manmachine interface’ (e.g. a keypad and liquid crystal display scree n), or remotely via a communication system. 2 . 11 T R I P P I N G C I R C U I T S There are three main circuits in use for circuit breaker tripping: a. series sealing b. shunt reinforcing c. shunt reinfo rcement with sealing These are illustrated in Figure 2.10. PR 52a TC (a) Series sealing PR 52a TC (b) Shunt reinforcing PR 52a TC (c) Shunt reinforcing with series sealing Figure 2.10: Typical relay tripping circuits Figure 2.10: Typical relay tripping circuits For electromechanical relays, elect rically operated indicators, actuated after the main contacts have closed, avoid imposing an additional friction load on the measuring element, which would be a serious handicap for certain types. Care must be taken with directly operated i
ndicators to line up their operation with the closure of the main contacts. The indicator must have operated by the time the contacts make, but must not have do ne so more than marginally earlier. This is to stop indication occurring when th e tripping operation has not been completed. With modern digital and numerical relays, the use of various alternative methods of providing trip circuit functions is largely obsolete. Auxiliary miniature co ntactors are provided within the relay to provide output contact functions and t he operation of these contactors is independent of the measuring system, as ment ioned previously. The making current of the relay output contacts and the need t o avoid these contacts breaking the trip coil current largely dictates circuit b reaker trip coil arrangements. Comments on the various means of providing trippi ng arrangements are, however, included below as a historical reference applicabl e to earlier electromechanical relay designs. Network Protection & Automation Guide • 13 • Fundamentals of P rotection P ractice • 2•
2.11.1 Series sealing The coil of the series contactor carries the trip current initiated by the protection relay, and the contactor closes a contact in paralle l with the protection relay contact. This closure relieves the protection relay contact of further duty and keeps the tripping circuit securely closed, even if chatter occurs at the main contact. The total tripping time is not affected, and the indicator does not operate until current is actually flowing through the tr ip coil. The main disadvantage of this method is that such series elements must have their coils matched with the trip circuit with which they are associated. T he coil of these contacts must be of low impedance, with about 5% of the trip su pply voltage being dropped across them. is countered by means of a further contact on the auxiliary unit connected as a retaining contact. This means that provision must be made for releasing the seal ing circuit when tripping is complete; this is a disadvantage, because it is som etimes inconvenient to find a suitable contact to use for this purpose. 2.12 TRI P CIRCUIT SUPERVISION The trip circuit includes the protection relay and other c omponents, such as fuses, links, relay contacts, auxiliary switch contacts, etc. , and in some cases through a considerable amount of circuit wiring with interme diate terminal boards. These interconnections, coupled with the importance of th e circuit, result in a requirement in many cases to monitor the integrity of the circuit. This is known as trip circuit supervision. The simplest arrangement co ntains a healthy trip lamp, as shown in Figure 2.11(a). The resistance in series with the lamp prevents the breaker being tripped by an internal short circuit c aused by failure of the lamp. This provides supervision while the circuit breake r is closed; a simple extension gives pre-closing supervision. Figure 2.11(b) sh ows how, the addition of a normally closed auxiliary switch and a resistance uni t can provide supervision while the breaker is both open and closed. PR 52a TC Fundamentals of P rotection P ractice When used in association with high-speed trip relays, which usually interrupt th eir own coil current, the auxiliary elements must be fast enough to operate and release the flag before their coil current is cut off. This may pose a problem i n design if a variable number of auxiliary elements (for different phases and so on) may be required to operate in parallel to energise a common tripping relay. 2.11.2 Shunt reinforcing Here the sensitive contacts are arranged to trip the c ircuit breaker and simultaneously to energise the auxiliary unit, which then rei nforces the contact that is energising the trip coil. Two contacts are required on the protection relay, since it is not permissible to energise the trip coil a nd the reinforcing contactor in parallel. If this were done, and more than one p rotection relay were connected to trip the same circuit breaker, all the auxilia ry relays would be energised in parallel for each relay operation and the indica tion would be confused. The duplicate main contacts are frequently provided as a three-point arrangement to reduce the number of contact fingers. 2.11.3 Shunt r einforcement with sealing This is a development of the shunt reinforcing circuit to make it applicable to situations where there is a possibility of contact bou nce for any reason. Using the shunt reinforcing system under these circumstances would result in chattering on the auxiliary unit, and the possible burning out of the contacts, not only of the sensitive element but also of the auxiliary uni t. The chattering would end only when the circuit breaker had finally tripped. T he effect of contact bounce • 14 • (a) Supervision while circuit breaker is closed (scheme H4) PR 52a 52b (b) Super vision while circuit breaker is open or closed (scheme H5) PR A TC
• 2• 52a B TC C Alarm (c) Supervision with circuit breaker open or closed with remote alarm (sch eme H7) Trip Trip Circuit breaker 52a TC 52b (d) Implementation of H5 scheme in numerical relay Figure 2.11: Trip circuit supervision circuits. Network Protection & Automation Guide
In either case, the addition of a normally open pushbutton contact in series wit h the lamp will make the supervision indication available only when required. Sc hemes using a lamp to indicate continuity are suitable for locally controlled in stallations, but when control is exercised from a distance it is necessary to us e a relay system. Figure 2.11(c) illustrates such a scheme, which is applicable wherever a remote signal is required. With the circuit healthy, either or both o f relays A and B are operated and energise relay C. Both A and B must reset to a llow C to drop-off. Relays A, B and C are time delayed to prevent spurious alarm s during tripping or closing operations. The resistors are mounted separately fr om the relays and their values are chosen such that if any one component is inad vertently short-circuited, tripping will not take place. The above schemes are commonly known as the H4, H5 and H7 schemes, arising from the diagram references of the Utility specification in which they originally app eared. Figure 2.11(d) shows implementation of scheme H5 using the facilities of a modern numerical relay. Remote indication is achieved through use of programma ble logic and additional auxiliary outputs available in the protection relay. Network Protection & Automation Guide • 15 • Fundamentals of P rotection P ractice • The alarm supply should be independent of the tripping supply so that indication will be obtained in case of failure of the tripping supply. 2•
• 3 • Fundamental Theory Introduction Vector algebra 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Manipulation of complex quantities Circuit quantities and conventions Impedance notation Basic circuit laws, theorems and network reduction References
• 3 • Fundamental Theor y 3.1 INTRODUCTION The Protection Engineer is concerned with limiting the effects of disturbances in a power system. These disturbances, if allowed to persist, ma y damage plant and interrupt the supply of electric energy. They are described a s faults (short and open circuits) or power swings, and result from natural haza rds (for instance lightning), plant failure or human error. To facilitate rapid removal of a disturbance from a power system, the system is divided into protec tion zones . Relays monitor the system quantities (current, voltage) appearing i n these zones; if a fault occurs inside a zone, the relays operate to isolate th e zone from the remainder of the power system. The operating characteristic of a relay depends on the energizing quantities fed to it such as current or voltage , or various combinations of these two quantities, and on the manner in which th e relay is designed to respond to this information. For example, a directional r elay characteristic would be obtained by designing the relay to compare the phas e angle between voltage and current at the relaying point. An impedance-measurin g characteristic, on the other hand, would be obtained by designing the relay to divide voltage by current. Many other more complex relay characteristics may be obtained by supplying various combinations of current and voltage to the relay. Relays may also be designed to respond to other system quantities such as frequ ency, power, etc. In order to apply protection relays, it is usually necessary t o know the limiting values of current and voltage, and their relative phase disp lacement at the relay location, for various types of short circuit and their pos ition in the system. This normally requires some system analysis for faults occu rring at various points in the system. The main components that make up a power system are generating sources, transmission and distribution networks, and loads . Many transmission and distribution circuits radiate from key points in the sys tem and these circuits are controlled by circuit breakers. For the purpose of an alysis, the power system is treated as a network of circuit elements contained i n branches radiating from nodes to form closed loops or meshes. The system varia bles are current and voltage, and in Network Protection & Automation Guide • 17 •
steady state analysis, they are regarded as time varying quantities at a single and constant frequency. The network parameters are impedance and admittance; the se are assumed to be linear, bilateral (independent of current direction) and co nstant for a constant frequency. 3 . 2 V E C TO R A L G E B R A A vector represe nts a quantity in both magnitude and direction. In Figure 3.1 the vector OP has a magnitude |Z| at an angle θ with the reference axis OX. Y Figure 3.1 The representation of a vector uantity algebraically in terms of its rectangula r co-ordinates is called a 'complex uantity'. Therefore, x + jy is a complex u antity and is the rectangular form of the vector |Z| θ where: + y2 y θ = tan −1 x x = Z cos θ — y = Z sin θ Z= 2 (x ) …Euation 3.2 From Euations 3.1 and 3.2: — Z = |Z| (cos θ + jsin θ) P …Euation 3.3 and since cos θ and sin θ may be expressed in exponential form by the identities: sin θ = X |Z| y q e jθ − e − jθ 2j 0 Figure 3.1: Vector OP Figure 3.1: Vector OP x Fundamental Theor y It may be resolved into two components at right angles to each other, in this ca se x and y. The magnitude or scalar value of vector Z is known as the modulus |Z |, and — the angle θ is the argument, and is written as arg. Z. — The conventional met hod of expressing a vector Z is to write simply |Z| θ. This form completely specifie s a vector for graphical representation or conversion into other forms. For vect ors to be useful, they must be expressed — algebraically. In Figure 3.1, the vecto r Z is the resultant of vectorially adding its components x and y; algebraically this vector may be written as: — Z = x + jy …Euation 3.1 e jθ − e − jθ 2 — it follows that Z may also be written as: — Z = |Z|e jθ cosθ = …Euation 3.4 Therefore, a vector uantity may also be represented trigonometrically and expon entially. 3 . 3 M A N I P U L AT I O N OF COMPLEX QUANTITIES Complex uantities may be represented in any of the four co ordinate systems given below: a. Polar b. Rectangular c. Trigonometric d. Exponential Z θ x + jy |Z| (cos θ + jsin θ) |Z|e jθ
• 3• where the operator j indicates that the component y is perpendicular to componen t x. In electrical nomenclature, the axis OC is the 'real' or 'in phase' axis, a nd the vertical axis OY is called the 'imaginary' or 'uadrature' axis. The oper ator j rotates a vector anticlockwise through 90°. If a vector is made to rotate a nticlockwise through 180°, then the operator j has performed its function twice, a nd since the vector has reversed its sense, then: j x j or j2 = 1 whence j = √ 1 The modulus |Z| and the argument θ are together known as 'polar co ordinates', and x and y are described as 'cartesian co ordinates'. Conversion between coordinat e systems is easily achieved. As the operator j obeys the ordinary laws of algeb ra, complex uantities in rectangular form can be manipulated algebraically, as can be seen by the following: — — Z1 + Z2 = (x1+x2) + j(y1+y2) …Euation 3.5 — — Z1 Z2 = (x1 x2) + j(y1 y2) …Euation 3.6 (see Figure 3.2) • 18 • Network Protection & Automation Guide
Z1Z2 = Z1 Z2
θ1 + θ2
Z1 Z1 = θ1 − θ2
Z2 Z2
3.3.2 Complex Numbers A complex number may be defined as a constant that represe nts the real and imaginary components of a physical uantity. The impedance para meter of an electric circuit is a complex number having real and imaginary compo nents, which are described as resistance and reactance respectively. Confusion o ften arises between vectors and complex numbers. A vector, as previously defined , may be a complex number. In this context, it is simply a physical uantity of constant magnitude acting in a constant direction. A complex number, which, bein g a physical uantity relating stimulus and response in a given operation, is kn own as a 'complex operator'. In this context, it is distinguished from a vector by the fact that it has no direction of its own. Because complex numbers assume a passive role in any calculation, the form taken by the variables in the proble m determines the method of representing them. 3.3.3 Mathematical Operators Mathe matical operators are complex numbers that are used to move a vector through a g iven angle without changing the magnitude or character of the vector. An operato r is not a physical uantity; it is dimensionless. The symbol j, which has been compounded with uadrature components of complex uantities, is an operator that rotates a uantity anti clockwise through 90°. Another useful operator is one whi ch moves a vector anti clockwise through 120°, commonly represented by the symbol a. Operators are distinguished by one further feature; they are the roots of uni ty. Using De Moivre's theorem, the nth root of unity is given by solving the exp ression: 11/n = (cos2πm + jsin2πm)1/n where m is any integer. Hence: • …Equation 3.7 Y |Z2| y2 |Z1| y1 0 x1 x2 X Figure 3.2: Addition of vectors Figure 3.2: Addition of vectors 3.3.1 Complex variables Some complex quantities are variable with, for example, time; when manipulating such variables in differential equations it is expedient to write the complex quantity in exponential form. When dealing with such funct ions it is important to appreciate that the quantity contains real and imaginary components. If it is required to investigate only one component of the complex variable, separation into components must be carried out after the mathematical operation has taken place. Example: Determine the rate of change of the real com ponent of a vector |Z| wt with time. |Z| wt = |Z| (coswt + jsinwt) = |Z|e jwt The re al component of the vector is |Z|coswt. Differentiating |Z|e jwt with respect to time: d Z e jwt = jw Z e jwt dt = jw|Z| (coswt + jsinwt) Separating into real and imaginary components: 2 πm 2 πm + j sin n n where m has values 1, 2, 3, ... (n-1) 11/ n = cos From the above exression j is found to be the 4th root and a the 3rd root of un ity, as they have four and three distinct values resectively. Table 3.1 gives s ome useful functions of the a oerator. d Z e jwt = Z ( − w sin wt + jw cos wt ) dt Thus, the rate of change of the real c omponent of a vector |Z| wt is: -|Z| w sinwt
( ) Network Protection & Automation Guide • 19 • Fundamental Theor y 3•
1 3 a=− + j =e 2 2 4π j 1 3 =e 3 a2 = − − j 2 2 j0 1=1+ j0 = e 2π j 3 For examle, the instantaneous value, e, of a voltage varying sinusoidally with time is: e=Emsin(wt+δ) where: Em is the maximum amplitu e of the waveform; ω=2πf, the angular velocity, δ is the argument efining the amlitu e of the voltage at a tim e t=0 At t=0, the actual value of the voltage is Emsin δ. So if Em is regar e as the mo ulus of a vector, hose argument is δ, then Emsin δ is the imaginarycomonen t of the vector |Em| δ. Figure 3.3 illustrates this quantity as a vector an as a si nusoi al function of time. Figure 3.3 Y |Em| e Em X t …Equation 3.8 1+ a + a2 = 0 1−a = j 3a2 1−a2 = − j 3a a −a2 = j 3 j= a −a2 3 Table 3.1: Proerties of the a oerator
3.4CIRCUIT QUANTITIES AND CONVENTIONS Circuit analysis may be escribe as the stu y of the resonse of a circuit to an imose con ition, for examle a short circuit. The circuit variables are current an voltage. Conventionally, current flo results from the alication of a riving voltage, but there is comlete u ality beteen the variables an either may be regar e as the cause of the other . When of energy; a circuit may be es a circuit exists, there is an interchange cribe as The arts of a circ being ma e u of 'sources' an 'sinks' for energy. uitare escribe as elements; a 'source' may be regar e as an 'active' element an a 'sink' as a 'assive' element. Some circuit elements are issiative, tha t is, they are continuous sinks for energy, for examle resistance. Other circui t elements may be alternately sources an sinks, an in for exam le ca acitance uctance. The elements of a circuit are form a netork havi connecte together to ng no es (terminals or junctions) an branches (series grou s of elements) that form close loos (meshes). In stea y state a.c. circuit theory, the ability of a circuit toaccet a current flo resulting from a given riving voltage iscal le the ime ance of the circuit. Since current an voltage are uals the im e a nce arametermust also have a ual, calle a mittance. 3.4.1 Circuit Variables As current an voltage are sinusoi alfunctions of time, varying at a single an constant frequency, they are regar e as rotatingvectors an can be ran as lan vectors (that is, vectors efine by to co or inates) on a vector iagram. X' 0 Y' t=0 Fun amental Theor y
Figure 3.3:Reresentation of a sinusoi al function Figure 3.3: Reresentation o f a sinusoi al function The current resulting from alying a voltage to a circuit een s uon the circ uit imeance. If the voltage is a sinusoi al function at a given frequency an the ime ance is constant the current ill alsovary harmonically at the same fr equency, so it can be shon on the same vector iagram as the voltage vector, an
is given by the equation i= Em Z sin (t + δ − φ ) • 3• …Equation 3.9 where: Z = R2 + X 2
1
X =
ωL −
ωC
φ = tan −1 X R
…Equation 3.10 From Equations 3.9 and 3.10 it can be seen that the angular displacement φ between the current and voltage vectors and the current magnitude |Im|=|Em|/|Z| is — depe ndent upon the impedance Z . In complex form the — impedance may be written Z=R+jX . The real component , R, is the circuit resistance, and the • 20 • Network Protection & Automation Guide
imaginary component , X, is the circuit reactance. When the circuit reactance i s inductive (that is, wL>1/wC), the current lags the voltage by an angle φ, and when it is capacitive (that is, 1/wC>wL) it 'leads' the voltage by an angle φ. Whe n drawing vector diagrams, one vector is chosen as the 're erence vector' and al l other vectors are drawn relative to the re erence vector in terms o magnitude and angle. The circuit impedance |Z| is a complex operator and is distinguished from a vector only by the fact that it has no direction of its own. A further c onvention is that sinusoidally varying quantities are described by their effect ive or root mean square (r.m.s.) values; these are usually written using the relevant symbol without a suffix. Thus: 2 E = Em 2 …Equation 3.11 The root mean square value is that value which has the same heating effect as a direct current quantity of that value in the same circu it, and this definition applies to non-sinusoidal as well as sinusoidal quantiti es. I = Im steady state terms Equation 3.12 may be written: ∑E = ∑I Z …Equation 3.13 and this is known as the equated-voltage equation [3.1]. It is the equation most usually adopted in electrical network calculations, since it equates the drivin g voltages, which are known, to the passive voltages, which are functions of the currents to be calculated. In describing circuits and drawing vector diagrams, for formal analysis or calculations, it is necessary to adopt a notation which d efines the positive direction of assumed current flow, and establishes the direc tion in which positive voltage drops and voltage rises act. Two methods are avai lable; one, the double suffix method, is used for symbolic analysis, the other, the single suffix or diagrammatic method, is used for numerical calculations. In the double suffix method the positive direction of current flow is assumed to b e from node a to node b and the current is designated Iab . With the diagrammati c method, an arrow indicates the direction of current flow. The voltage rises ar e positive when acting in the direction of current flow. It can be seen from Fig ure 3.4 — — — that E1 and Ean are positive voltage rises and E2 and — Ebn are negative v oltage rises. In the diagrammatic method their direction of action is simply ind icated by an — — arrow, whereas in the double suffix method, Ean and Ebn indicate th at there is a potential rise in directions na and nb. Figure 3.4 Methods or representing a circuit 3.4.2 Sign Conventions In describing the electrical state of a circuit, it is of ten necessary to refer to the potential difference existing between two points in the circuit. Since wherever such a potential difference exists, current will flow and energy will either be transferred or absorbed, it is obviously necessa ry to define a potential difference in more exact terms. For this reason, the te rms voltage rise and voltage drop are used to define more accurately the nature of the potential difference. Voltage rise is a rise in potential measured in the direction of current flow between two points in a circuit. Voltage drop is the converse. A circuit element with a voltage rise across it acts as a source of en ergy. A circuit element with a voltage drop across it acts as a sink of energy. Voltage sources are usually active circuit elements, while sinks are usually pas sive circuit elements. The positive direction of energy flow is from sources to sinks. Kirchhoff s first law states that the sum of the driving voltages must eq ual the sum of the passive voltages in a closed loop. This is illustrated by the fundamental equation of an electric circuit: Ldi 1 iR + + idt = e …Equation 3.12 dt C ∫ where the terms on the left hand side of the equation are voltage drops across the circuit elements. Expressed in Z3 Z1 E1 I Z2 E2
E1-E2=(Z1+Z2+Z3)I (a) Diagrammatic a Zan Ean Iab Zab b Zbn Ebn n Ean-Ebn=(Zan+Zab+Zbn)Iab (b) Double suffix Figure 3.4 Methods of representing a circuit Figure 3.4: Circuit representation methods Network Protection & Automation Guide • 21 • Fundamental Theor y • 3•
Voltage drops are also positive when acting in the direction of current flow. Fr om Figure 3.4(a) it can be — — — — seen that ( Z1+ Z2+ Z3) I is the total voltage drop i n the loop in the direction of current flow, and must equate to — — the total voltag e rise E1- E2. In Figure 3.4(b), the voltage — drop between nodes a and b designat ed Vab indicates that point b is at a lower potential than a, and is positive — wh en current flows from a to b. Conversely Vba is a negative voltage drop. Symboli cally: — — — Vabab =V an −V bn V = Van Vbn — — — Vbaba =V bn −V an V = Vbn Van common re erence point. 3.4.3 Power The product o the potential di erence acro ss and the current through a branch o a circuit is a measure o the rate at whi ch energy is exchanged between that branch and the remainder o the circuit. I the potential di erence is a positive voltage drop, the branch is passive and a bsorbs energy. Conversely, i the potential di erence is a positive voltage ris e, the branch is active and supplies energy. The rate at which energy is exchang ed is known as power, and by convention, the power is positive when energy is be ing absorbed and negative when being supplied. With a.c. circuits the power alte rnates, so, to obtain a rate at which energy is supplied or absorbed, it is nece ssary to take the average power over one whole cycle. I e=Emsin(wt+δ) an i=Imsin (t+δ φ), then the power equation is: p=ei=P[1 cos2(wt+δ)]+Qsin2(t+δ) here: P=|E||I|co s φ Q=|E||I|sin φ From Equation 3.15 it can be seen that the quantity P varies rom 0 to 2P and quantity Q varies rom Q to +Q in one cycle, and that the wave orm is o twice the periodic requency o the current voltage wave orm. The average value o the power exchanged in one cycle is a constant, equal to quantity P, an d as this quantity is the product o the voltage and the component o current wh ich is 'in phase' with the voltage it is known as the 'real' or 'active' power. The average value o quantity Q is zero when taken over a cycle, suggesting that energy is stored in one hal cycle and returned to the circuit in the remaining hal cycle. Q is the product o voltage and the quadrature and …Equation 3.15 component o current, and is known as 'reactive power'. As P and Q are constants which speci y the power exchange in a given circuit, and are products o the — cu rrent and voltage vectors, then i S is the vector — —— product E I it ollows that wi th E as the re erence vector — — and φ as the angle between E and I : — S = P + jQ …Equati on 3.16 — The quantity S is described as the 'apparent power', and is the term use d in establishing the rating o a circuit. — S has units o VA. 3.4.4 Single Phase and Polyphase Systems A system is single or polyphase depending upon whether th e sources eeding it are single or polyphase. A source is single or polyphase ac cording to whether there are one or several driving voltages associated with it. For example, a three phase source is a source containing three alternating driv ing voltages that are assumed to reach a maximum in phase order, A, B, C. Each p hase driving voltage is associated with a phase branch o the system network as shown in Figure 3.5(a). I a polyphase system has balanced voltages, that is, eq ual in magnitude and reaching a maximum at equally displaced time intervals, and the phase branch impedances are identical, it is called a 'balanced' system. It will become 'unbalanced' i any o the above conditions are not satis ied. Calc ulations using a balanced polyphase system are simpli ied, as it is only necessa ry to solve or a single phase, the solution or the remaining phases being obta ined by symmetry. The power system is normally operated as a three phase, balanc ed, system. For this reason the phase voltages are equal in magnitude and can be represented by three vectors spaced 120° or 2π/3 ra ians aart, as shon in Figure 3.5(b). A Ecn C Ean N Ebn B A' N' C' Phase branches B' …Equation 3.14 Fun amental Theor y • 3•
(a) Three hase system Ea Direction 120° of rotation 120° Ec=aEa 120° Eb=a2Ea
(b) Balance system of vectors Figure 3.5: Three hase systems Figure 3.5: Three hase systems • 22 •
Netork Protection & Automation Gui e
Since the voltages are symmetrical, they may be exresse in terms of one, that is: — =— E E Eaa= Eaa — = 2— E aE Ebb= a 2 Eaa — — Ec c= aEa E = Ea system ime ances may be converte to those base quantities by using the equatio ns given belo: MVAb 2 MVAb1 2 kVb1 Zb 2 = Zb1 × kVb 2 Zb 2 = Zb1 × …Equation 3.17
here a is the vector oerator e j2π/3. Further, if the hase branch imeances ar e i entical currents are als in a balance system, it follo s that the resulting ection of any oer sys o balance . 3.5 IMPEDANCE NOTATION It can be seen by ins tem iagram that: a. several voltage levels exist in a system b. it is common r actice to refer to lant MVA in terms of er unit or ercentage values c. transm ission line an cable constants are given in ohms/km Before any system calculati lace, the system arameters must be referre to 'base quantities' ons can take an reresente as a unifie system of ime ances in either ohmic, ercentage, o r er unit values. The base quantities are o er an voltage. Normally, they are given interms of the three hase oer in MVA an the line voltage in kV. The baseime ance resulting from the above base quantities is: ohms …Equation 3.18 MV A an , rovi e the system is balance , the base ime ance may be calculate usi ng either single hase or three hase quantities. Zb …Equation 3.20
here suffix b1 enotes the value to the original base an b2 enotes the value to ne base The choice of ime ancenotation een s uon the comlexity of the lant ime ance notation an the nature of the system calculations envis system, age . If the system is relatively simlean contains transmission line mainly ata, given in ohms, then the ohmic metho can be a o te ith a vantage. Hoever , theer unit metho ofime ance notation is the most common for general syste m stu ies since: 1. ime ances are the same referre to either si e of a transfo rmer if the ratio of base voltages on the to si es of a transformer is equal to oers o the transformer turns ratio 2. confusion cause by the intro uction of f 100 in ercentage calculations is avoi e 3. by a suitable choice of bases, th t ithin a reictable range, an he e magnitu es of the ata an results are ke nce errors in ata an comutations are easier to sot Most oer system stu ies Irres ective ofthe meth are carrie out using soft are in er unit quantities. o of calculation, the choice of base voltage, an unifying system ime ances to this base, shoul be a roache ith caution, as sho n in the folloing examle . (kV )2 =
The er unit or ercentage value of any ime ance in the system is the ratio of actual to base im e ance values. Hence: 11.8kV 11.8/141kV 132kV Overhea line Wrong selection of base voltage …Equation 3.19 132/11kV 11kV Distribution MVAb (kVb )2 Z (% ) = Z ( .u .) ×100 here MVAb = base MVA kVb = base kV 11.8kV Right selection 11.8kV 132kV 11kV
Z ( .u .) = Z (ohms ) ×
141kV 141 x 11=11.7kV 132
Simle transosition of the above formulaeill refer the ohmic value of ime an ce to the er unit or ercentage values an base quantities. Having chosen base quantities of suitable magnitu e all Figure 3.6: Selection of base voltages Figure 3.6: Selection of base voltages Netork Protection & Automation Gui e • 23 • Fun amental Theor y • 3•
From Figure 3.6 it can be seen that the base voltages in the three circuits are relate by the turns ratios of the intervening transformers.Care is require as the nominal transformation ratios of the transformers quote may be ifferent f rom the turns ratios e.g. a 110/33kV (nominal) transformer may have a turns rat io of 110/34.5kV. Therefore, the rule for han calculations is: 'to refer an im e ance in ohms from one circuit to another multi ly the given im e ance by the s quare of the turns ratio (oen circuit voltage ratio) of the intervening transfo are is use, the softare normally has rmer'. Where oer system simulation soft calculation routines built in to a just transformer account arameters to take of ifferences bet een the nominal rimary an secon ary voltages an turns rati os. In this case, the choice of base voltages may be more conveniently ma eas t he nominal voltages of each section of the oer system. This a roach avoi s co nfusion hen er unit or ercent values are use in calculations in translating the final results into volts, ams, etc. For examle, in Figure 3.7, generators G1 an G2 have a sub transient reactance of 26% on 66.6MVA rating at 11kV, an t ransformers T1 an T2 a voltage ratio of 11/145kV an an im e ance of 12.5% on 7 5MVA.Choosing 100MVA as base MVA an 132kV as base voltage, fin the ercentage ime ances to ne base quantities. a. Generator reactances to ne bases are: 3 . 6 B A S I C C I R C U I T L AW S, THEOREMS ANDNETWORK REDUCTION Most ract ical oer system roblems are solve by using stea y state analytical metho s. tions mae are that the circuit arameters are linear an bilateral an The assum constant for constant frequency circuit variables. In some roblems, escribe as initial value roblems, it is necessary to stuy the behaviour of a circuit in the transient state. Such roblems can be solve using oerational metho s. A gain, in other roblems, hich fortunately are fe in number, the assumtion of linear, bilateral are solv circuit arameters is no longer vali .These roblems e using a vance mathematical techniques that are beyon the scoe of this book . 3.6.1 Circuit Las In linear, bilateral circuits, three basic netork las a ly, regar less of the state of the circuit, at anyarticular instant of time. T s, ue to Ohm an Kirchhoff, an hese la sare the branch, junction an mesh la are state belo, using stea y state 3.6.1.1 Branch la — — The c a.c. nomenclature. urrent I in a given branch of im e ance Z is — ro ortional to the otential iffe rence V aearing — —— across the branch, that is, V = I Z. 3.6.1.2 Junction la The algebraic sum of all currents entering any junction (or no e) in a netork is ze ro, that is: Fun amental Theor y (11) =0.27% 100 26 × × 66.6 (132 )2 2 b. Transformer reactances to ne bases are: 100 (145 ) 12.5 × × = 20.1% 75 (132 )2 2 ∑ I =0 3.6.1.3 Mesh la The algebraic sum of all the riving voltages in any close at h (or mesh) in a net orkis equalto the algebraic sum of all the assive voltag es (ro ucts of the ime ances an the currents) in the comonents branches, tha t is: • 3•
NOTE: The base voltages of the generator an circuits are 11kV an 145kV resect ively, that is, the The correson ing turns ratio of the transformer. er unit v alues can be foun by ivi ing by 100, an the ohmic value can be foun by using Equation 3.19. Figure 3.7
T1 G1 132kV overhea lines T2 ∑ E = ∑Z I Alternatively, the total change in otential aroun a close loo is zero. 3.6.2 Circuit Theorems From the above netork las, many theorems have been erive f or the rationalisation of net orks, either to reach a quick, sim le, solution to resent a comlicate circuit by an equivalent. These theorem a roblem or to re s are ivie into to classes: those concerne ith the general roerties of n et orks an those G2 Figure 3.7: Section of a oer system Figure 3.7: Section of a oer system • 24 • Netork Protection & Automation Gui e
concerne ith netork re uction. Of the many theorems that exist, the three mos Theorem, Thévenin's Theorem an t im ortant are given. These are: the Su erosition Kennelly's Star/Delta Theorem. 3.6.2.1 Su erosition Theorem (general netork theorem) 3.6.3 Netork Re uction The aim of netork re uction is to re uce a system to a simleequivalent hile retaining the i entity of that art of the system to be stu ie . For exam le, consi er the system shon in Figure 3.9. The netork has t o sources E ’ an E ’’, a line AOB shunte by an impeance, which may be regare as the between A an B, an a loa connect re uction of a further network connecte e between O an N. The object of the re uction is to stu y the effect of openin g a breaker at A or B system operations, or of a fault at A or B. uring normal Thus the i entity of no es A an B must be retaine together withthe sources, b ut the branch ON can be eliminate , simplifying the stu y. Procee ing, A, B, N, forms a star branch an can therefore be converte to an equivalent elta. Figur e 3.9 2.55Ω 1.6Ω A 0.75Ω E 18.85Ω B 0.45Ω E 0.4Ω The resultant current that flows in any branch of a network ue to the simultane ous action of several riving voltages is equal to the algebraic sum ofthe comp onent currents ue to each riving voltage acting alone with the remain er short -circuite . 3.6.2.2 Thévenins Theorem (active network re uction theorem) Anyacti ve network that may be viewe from two terminalscan be replace by a single ri ving voltage acting in series with a single impe ance. The riving voltage is th e open-circuit voltage between the two terminals an the impe ance is the impe a nce of the network viewe from the terminals with all sources short-circuite . 3 .6.2.3 Kennelly s Star/Delta Theorem (passive network re uction theorem) Any thr ee-terminal network can be replace by a elta or star impe ance equivalent with out isturbing the external network. The formulae relating the replacement of a elta network by the equivalent star network is as follows (Figure 3.8): — — — — — — Zco = 13 Z23 / (Z12 + Z13 + Z23) an so on. a Zao O Zco Zbo b 1 Z13 Z12 2 Z23 0 N Figure 3.9: Typical power system network Figure 3.9: Typical power system netw ork Z AN = Z AO + Z NO + Z AO Z NO Z BO = 0.75 +18.85 + = 51 ohms 0.75 ×18.85 0.45 c (a) Star network 3 (b) Delta network Z BN = Z BO + Z NO +
Figure3.8: Star-Delta network transformation Figure 3.8: Star/Delta network re u ction Figure 3.8: Star-Delta network transformation Z BO Z NO Z AO The impe ance of a elta network correspon ing to an replacing any star network
is: — — Zao Zbo — — — Z12 = Zao + Zbo + ———————— — Zco an so on. = 0.45 +18.85 + =30.6 ohms 0.45 ×18.85 0.75 Z AN = Z AO + Z BO + Z AO Z BO Z NO = 1.2 ohms (since ZNO>>> ZAOZBO) Figure 3.10
Network Protection & Automation Gui e • 25 • Fun amental Theor y • 3•
2.5Ω 1.6Ω A E 51Ω 1.2Ω B 30.6Ω E 0.4Ω Most re uction problems follow the same pattern as the example above. The rules to apply inpractical network re uction are: a. eci e on the nature of the ist urbance or isturbances to be stu ie b. eci e on the information require , for example at a particular location the branch currents in the network for a fault c. re uce all passive sections of the network not irectly involve with the se ction un er examination . re uce all active meshes to a simple equivalent, that is, to a simple source in series with a single impe ance With the wi esprea av ailability of computer-base power system simulation software, it is now usual t o use such software on a routine basis for network calculations without signific ant network re uction taking place. However, the network re uction techniques gi ven above are still vali , as there will be occasions where such software is not imme iately available an a han calculation must be carrie out. In certain ci rcuits, for example parallel lines on the same towers, there is mutual coupling between branches. Correct circuit re uction must take account of this coupling. Figure 3.13 P I Ib Zbb (a) Actual circuit I Z Z -Z2 Z= aa bb ab Zaa+Zbb-2Zab (b) Equivalent when Zaa≠Zbb P I Z= 1 (Zaa+Zbb) 2 (c) Equivalent when Zaa=Zbb Figure 3.13: Re uction of two branches Figure 3.13: Re uction of two branches wi th mutual coupling coupling with mutual N
Figure 3.10: Re uction using star/ elta transform The network is now re uce as shown in Figure 3.10. By applying Thévenin s theorem to the activeloops, these can be replace by a single riving voltage in serie s with an impe ance as shownin Figure 3.11. Figure 3.11 1.6Ω A E 51Ω 51 E 52.6 N (a) Re uction of left active mesh N 0.4 x 30.6 Ω 31 1.6 x 51 Ω 5 2.6 A
Fun amental Theor y B 30.6Ω 0.4Ω B E 30.6 E
31 N
Ia Zaa Zab Q N
(b) Re uction of right active mesh Figure 3.11: Re uction of active meshes: Théven in s Theorem Figure 3.11: Re uction of active meshes: Thévenin s Theorem • 3•
The network shown in Figure 3.9 is now re uce to that shown in Figure 3.12 with
the no es A an B retaining their i entity. Further, the loa impe ancehas bee n completely eliminate . The network shown in Figure 3.12 may now be use to stu y system isturbances, for example power swings with an without faults. Figure 3.12 2.5Ω 1.55Ω A 1.2Ω 0.97E 0.99E B 0.39Ω P Q Q
Three cases are of interest. Theseare: a. two branches connecte together at th eir no es b. two branches connecte together at one no e only c. two branches th at remain unconnecte N Figure 3.12: Re uction of typical Figure 3.12: Re uction of typical power system network power system network • 26 • Network Protection & Automation Gui e
Consi ering each case in turn: a. consi er the circuit shown in Figure 3.13(a). The application of a voltage V between the terminals P an Q gives: V = IaZaa + IbZab V = IaZab + IbZbb where Ia an Ib are the currents in branches a an b, re spectively an I = Ia + Ib , the total current entering at terminal P an leavin g at terminal Q. Solving for Ia an Ib : The assumption is ma e that an equivalent star network can replace the network s hown. From inspection with one terminal isolate in turn an a voltage V impress e across the remaining terminals it can be seen that: Za+Zc=Zaa Zb+Zc=Zbb Za+Zb =Zaa+Zbb-2Zab Solving these equations gives: Ia = from which (Zbb − Zab )V 2 Zaa Zbb − Zab Za = Zaa − Zab Zb = Zbb − Zab see Figure 3.14(b).
Zc = Zab
…Equation 3.23 Ib = and (Zaa − Zab )V 2 Zaa Zbb − Zab I = Ia +Ib = V (Zaa + Zbb −2 Zab ) 2 Zaa Zbb − Zab c. consider the our terminal network given in Figure 3.15(a), in which the bran ches 11' and 22' are electrically separate except or a mutual link. The equatio ns de ining the network are: V1=Z11I1+Z12I2 I1=Y11V1+Y12V2 V2=Z21I1+Z22I2 I2=Y21 V1+Y22V2 where Z12=Z21 and Y12=Y21 , i the network is assumed to be reciprocal. Further, by solving the above equations it can be shown that: 2 Zaa Zbb − Zab V Z= = I Zaa + Zbb − 2 Zab …Equation 3.21 (Figure 3.13(b)), and, i the branch impedances are equal, the usual case, then: Z= (Figure 3.13(c)). 1 (Zaa + Zab ) 2 …Equation 3.22 Y11 = Z22 ∆ b. consider the circuit in Figure 3.14(a). Zaa A Zab B C Y22 = Z11 ∆ …Equation 3.24
Y12 = Z12 ∆
2
∆ = Z11Z22 − Z12
Zbb (a) Actual circuit Za=Zaa Zab There are three independent coe icients, namely Z12, Z11, Z22, so the original circuit may be replaced by an equivalent mesh containing our external terminals , each terminal being connected to the other three by branch impedances as shown in Figure 3.15(b). A C Zc=Zab B Zb=Zbb Zab (b) Equivalent circuit Figure 3.14: Reduction o mutually coupled branches with a common terminal 1 Z11 Z12 1' 1 Z12 Z12 Z11 Z21 Z22 Z12 1' 2 Z22 2' 2 2' (a) Actual circuit 1 Z11 Z12 Z12 • 27 • (b) Equivalent circuit Z11 Figure 3.15 : Equivalent circuits or 1 1' Z12 our t erminal network with mutual coupling Z12 2 Z12 Z12 (d) Equivalent circuit Z12 2' Figure 3.14: Reduction o mutually coupled branches with a common terminal Network Protection & Automation Guide C (c) Equivalent with all nodes commoned Fundamental Theor y • so that the equivalent impedance o the original circuit is: 3•
In order to evaluate the branches o the equivalent mesh let all points o entry o the actual circuit be commoned except node 1 o circuit 1, as shown in Figur e 3.15(c). Then all impressed voltages except V1 will be zero and: I1 = Y11V1 I2 = Y12V1 I the same conditions are applied to the equivalent mesh, then: I1 = V 1Z11 I2 = V1/Z12 = V1/Z12 These relations ollow rom the act that the branch connecting nodes 1 and 1' carries current I1 and the branches connecting nodes 1 and 2' and 1 and 2 carry current I2. This must be true since branches between pairs o commoned nodes can carry no current. By considering each node in turn w ith the remainder commoned, the ollowing relationships are ound: Z11’ = 1/Y11 Z2 2’ = 1/Y22 efining the equivalent mesh inFigure 3.15(b), an inserting ra ial branches ha ving impe ances equal to Z11 an Z22 in terminals 1 an 2, results in Figure 3.1 5( ). 3.7 REFERENCES 3.1 Power System Analysis. J. R. Mortlock an M. W. Humphre y Davies. Chapman & Hall. 3.2 Equivalent Circuits I. Frank M. Starr, Proc. A.I.E .E. Vol. 51. 1932, pp. 287-298. Fun amental Theor y Z12’ = -1/Y12 Z12 = Z1’ 2’ = -Z21’ = -Z12’ Z Z −Z 2 Z ' = 11 22 2 12 Z11’ 11 Z11Z22-Z 12 = _______________ Z22 Z22 2 Z11Z22 − Z= = Z22’ 22 Z11Z22-Z212 _______________ Z11 Z11 2 Z11Z -Z2Z12 Z12 12 Z11Z2222 − 2 Z= = _______________ ZZ12 …Equation 3.25 12 A similar but equally rigorous equiv alent circuit is shown in Figure 3.15(d). This circuit [3.2] ollows rom the a ct that the sel impedance o any circuit Z11 Z11 1 1' 1' is independent o all other circuits. There ore, it need not appear in any o the mutual branches i i t Z12 Z12 Z Z Z is lumped as a radial branch at12 terminals. So the 21 12 puttin g Z11 and Z22 equal to zero in Equation 2' 3.25, 2 2' Z22 Z22 (b) Equivalent circuit 1 Z11 Z12 Z12 C (c) Equivalent with all nodes com moned except 1 2 Z11 Z12 Z12 Z12 Z12 (d) Equivalent circuit Z12 2' 1' Hence: • 3• 1 2 (a) Actual circuit 1 Figure 3.15: Equivalent circuits or our terminal network with mutual coupling Figure 3.15: Equivalent circuits or our terminal network with mutual coupling • 28 • Network Protection & Automation Guide
• 4 • Fault Calculations Introduction Three phase ault calculations 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Symmetrical component analysis o a three phase network Equations and network co nnections or various types o aults Current and voltage distribution in a syst em due to a ault E ect o system earthing on zero sequence quantities Re erenc es
• 4 • Fault Calculations 4.1 INTRODUCTION A power system is normally treated as a balanced symmetrical th ree phase network. When a ault occurs, the symmetry is normally upset, resultin g in unbalanced currents and voltages appearing in the network. The only excepti on is the three phase ault, which, because it involves all three phases equally at the same location, is described as a symmetrical ault. By using symmetrical component analysis and replacing the normal system sources by a source at the ault location, it is possible to analyse these ault conditions. For the correct application o protection equipment, it is essential to know the ault current distribution throughout the system and the voltages in di erent parts o the sy stem due to the ault. Further, boundary values o current at any relaying point must be known i the ault is to be cleared with discrimination. The in ormatio n normally required or each kind o ault at each relaying point is: i. maximum ault current ii. minimum ault current iii. maximum through ault current To o btain the above in ormation, the limits o stable generation and possible operat ing conditions, including the method o system earthing, must be known. Faults a re always assumed to be through zero ault impedance. 4 . 2 T H R E E P H A S E F A U LT C A L C U L AT I O N S Three phase aults are unique in that they are balanced, that is, symmetrical in the three phases, and can be calculated rom the single phase impedance diagram and the operating conditions existing prior t o the ault. A ault condition is a sudden abnormal alteration to the normal cir cuit arrangement. The circuit quantities, current and voltage, will alter, and t he circuit will pass through a transient state to a steady state. In the transie nt state, the initial magnitude o the ault current will depend upon the point on the voltage wave at which the ault occurs. The decay o the transient condit ion, until it merges into steady state, is a unction o the parameters o the c ircuit elements. The transient current may be regarded as a d.c. exponential cur rent Network Protection & Automation Guide • 31 •
superimposed on the symmetrical steady state ault current. In a.c. machines, ow ing to armature reaction, the machine reactances pass through 'sub transient' an d 'transient' stages be ore reaching their steady state synchronous values. For this reason, the resultant ault current during the transient period, rom ault inception to steady state also depends on the location o the ault in the netw ork relative to that o the rotating plant. In a system containing many voltage sources, or having a complex network arrangement, it is tedious to use the norma l system voltage sources to evaluate the ault current in the aulty branch or t o calculate the ault current distribution in the system. A more practical metho d [4.1] is to replace the system voltages by a single driving voltage at the au lt point. This driving voltage is the voltage existing at the ault point be ore the ault occurs. Consider the circuit given in Figure 4.1 where the driving — — vo ltages are E and E’ , the impe ances on either si e of — — fault point F are Z1’ an Z1’’ , an the current through — point F before the fault occurs is I . Figure 4.1: Z 1 I F Z 1 be a e to the currents circulating in the system ue to the fault, to give the total current in any branch of the system at the time of fault inception. Howev er, in most problems, the loa current is small in comparison to the fault curre nt an is usually ignore . In a practical power system, the system regulation is such that the loa voltage at any point in the system is within 10% of the ecl are open-circuit voltage at that point. For this reason, it is usual toregar the pre-fault voltage voltage, an this a at the fault as being the open-circuit ssumption is also ma e in a number of the stan ar s ealing with fault levelcal culations. For an example of practical three-phase fault calculations, consi er a fault at A in Figure 3.9. With the network re uce as shown in Figure 4.2, the loa voltage at A before the fault occurs is: Figure 4.2: 1.55 Ω A 1.2 Ω 0.97E 0.99E 2.5 Ω B 0.39 Ω Fa u l t C a l c u l a t i o n s N E E V
Figure 4.2: Re uction of typical power system network — — — V = 0.97 E’ - 1.55 I N Figure 4.1: Network with fault at F • 4•
— The voltage V at F before fault inception is: — — —— — —— V = E - I Z‘ = E’’ + I Z’’ — Af voltage V is zero. Hence, the change — in voltage is - V . Because of the fault, the change in the current flowing into the network from F is: Z1 + Z1 V = −V ' '' Z1 Z1 Z1 and, since no current was lowing into the netwo rk rom F prior to the ault, the ault current lowing rom the network into th e ault is: ⎛ 1.2 × 2.5 ⎞ + E’’ 〉〉〉1.207 I A an N mo ifies The no e A is the
0.39 ⎟ I V . Hence E’≅ the circuit junction of
= 0.99 E '' + ⎜ ⎝ 2.5 + 1.2 ⎠ — — For practical working co E’’≅ V. — — Replacing the riving voltages E’ an E’’ by the as shown in Figure 4.3(a). three branches. In practice, the no e woul be a b
usbar, an the branches are fee ers ra iating from the bus via circuit breakers, as shown inFigure 4.3(b). There are two possible locations for a fault at A; t he busbar si eof the breakers or the linesi e of the breakers. In this example , it is assume that the fault is at X, an it is require to calculate the curr ent flowing from the bus to X. The network viewe from AN has a riving point im pe ance |Z1| = 0.68 ohms. The current in the fault is ΔI = − ( ) ' '' Z1 Z1 By applying the principle o superposition, the load currents circula ting in the system prior to the ault may I ( Z1' + Z1'' ) = −ΔI = V V Z1 . • 32 • Network Protection & Automation Guide
Let this current be 1.0 per unit. It is now necessary to find the fault current distribution in the various branches of the network and in particular the curren t flowing from A to X on the assumption that a relay at X is to detect the fault condition. The equivalent impedances viewed from either side of the fault are s hown in Figure 4.4(a). Figure 4.3 Figure 4.4 2.5Ω 1.55Ω A V N (a) Three - phase fault iagram for a fault at no e A Busbar Circui t breaker 1.2Ω B 0.39Ω Therefore, current in 2.5 ohm branch 1.2 × 0.563 = 0.183 p.u. 3.7 an the current in 1.2 ohm branch = 2.5 × 0.563 = 0.38 p.u. 3.7 Total current entering X from the left, that is, from A to X, is 0.437+ 0.183 = 0.62 p.u. an from B to X is 0.38p.u. The equivalent network asviewe from the relay is as shown in Figure 4.4(b). The impe ances on either si e are: = an 0.68/0.62 = 1.1 ohms 0.68/0.38 = 1.79 ohms A X
The circuit of Figure 4.4 (b) has been inclu e because the Protection Engineer is intereste in these equivalent parameters when applying certain types of prot ection relay. 4 . 3 S Y M M E T R I C A L C O M P O N E N T A N A LY S I S OF A THREE-PHASE NETWORK 1.21Ω (b) Typical physical arrangement of no e A with a fault shown at X Figure 4.3: Network with fault at no e A 1.55Ω A V
N (a) Impe ance viewe from no e A 1.1Ω X 1.79Ω V
N (b) Equivalentimpe ances viewe from no e X Figure 4.4: Impe ances viewe from fault The Protection Engineer is intereste in a wi er variety of faults than just a t hree-phase fault. The most common fault is a single-phase to earth fault, which, in LV systems, can pro uce a higherfault current than a threephase fault. Simi larly, because protection is expecte to operate correctly for alltypes of faul t, it may be necessary to consi er the fault currents ue to many ifferent type s offault. Since the three-phase fault is uniquein being a balance fault, a m etho of analysis that is applicable to unbalance faults is require . It can be shown [4.2] that, by applying the Principle of Superposition , any general thr ee-phase system of vectors may be replace by three sets of balance (symmetrica l) vectors; two sets are three-phase but having opposite phase rotation an one
set is co-phasal. These vector sets are escribe as the positive, negative an zero sequence sets respectively. The equations between phase an sequence voltag es are given below: E a = E1 + E 2 + E 0 ⎫ ⎪ ⎪ E b = a 2 E1 + aE 2 + E 0 ⎬ ⎪ E c = aE1 + a 2 E 2 + E 0 ⎪ The currents from Figure 4.4(a) are as follows: From the right: 1.55 = 0.563 p.u . 2.76 From the left: 1.21 = 0.437 p.u. 2.76 There is a parallel branch to the r ight of A …Equation 4.1
Network Protection & Automation Gui e • 33 • Fa u l t C a l c u l a t i o n s • 4•
1 ⎫
E1 = E a + aE b + a 2 E c ⎪
3 ⎪ ⎪
1 2 E2 = E a + a E b + aE c ⎬ 3 ⎪
⎪ 1 E0 = Ea
( ( ( ) ) )
— fault branch changes from 0 to I an the positive — — sequence voltage across the br anch changes from V to V1 ; replacing the fault branch by a source equal to the change in voltage an short-circuiting all normal riving voltages — in the system results in a current Δ I flowing into the system, and: …Equation 4.2 where all quantities are referred to the reference phase A. A similar set of equ ations can be written for phase and sequence currents. Figure 4.5 illustrates th e resolution of a system of unbalanced vectors. Figure 4.5 Eo E2 Ec aE1 E1 Ea ΔI = − (V − V ) 1 Z1 …Equation 4.3 a2E2 Eo
— where Z1 is the positive sequence impedance o the system viewed rom the ault. As be ore the ault no current was lowing rom the ault into the system, it — ollows that I1 , the ault current lowing rom the — system into the ault must e qual Δ I . Therefore: — — —— V1 = V - I1 Z1 …Equation 4.4 is the relationship between pos tive sequence currents and voltages in the fault branch during a fault. In Figur e 4.6, which represents a simple system, the — — — — — — voltage drops I1’ Z1’ and I1’ Z1’’ to ( V - V1 ) — — where the currents I1’ and I1’’ enter the fault from the — — left and rig respectively and impedances Z1’ and Z1’’ are the total system impedances viewed from e ither side — of the fault branch. The voltage V is equal to the opencircuit voltag e in the system, and it has been shown that — — — V ≅ E ≅ E ’’ (see Section 3.7). So the po ive sequence voltages in the system due to the fault are greatest at the source, as shown in the gradient diagram, Figure 4.6(b). Figure 4.6 X ZS1 ΔZ 1 I 1 Z 1 E N (a) System diagram N I 1 X V V 1+I 1ΔZ 1 N (b) Gradi ent diagram I 1 Z 1 F V1 F I 1 I1 V1 E Z 1 a2E1 Eb aE2 Eo Fa u l t C a l c u l a t i o n s • Figure 4.5: Resolution of a system of unbalanced vectors 4•
When a fault occurs in a power system, the phase impedances are no longer identi cal (except in the case of three-phase faults) and the resulting currents and vo ltages are unbalanced, the point of greatest unbalance being at the fault point. It has been shown in Chapter 3 that the fault may be studied by short-circuitin g all normal driving voltages in the system and replacing the fault connection b y a source whose driving voltage is equal to the pre-fault voltage at the fault point. Hence, the system impedances remain symmetrical, viewed from the fault, a nd the fault point may now be regarded as the point of injection of unbalanced v oltages and currents into the system. This is a most important approach in defin ing the fault conditions since it allows the system to be represented by sequenc e networks [4.3] using the method of symmetrical components. 4.3.1 Positive Sequ ence Network During normal balanced system conditions, only positive sequence cu rrents and voltages can exist in the system, and therefore the normal system imp edance network is a positive sequence network. When a fault occurs in a power sy stem, the current in the Figure 4.6: Fault at F: Positive sequence diagrams 4.3.2 Negative Sequence Network If only positive sequence quantities appear in a power system under normal conditions, then negative sequence quantities can onl y exist during an unbalanced fault. If no negative sequence quantities are prese nt in the • 34 • Network Protection & Automation Guide
fault branch prior to the fault, then, when a fault occurs, — — the change in voltag e is V2 , and the resulting current I2 flowing from the network into the fault i s: 4.4 EQUATIONS AND NETWORK CONNECTIONS FOR VARIOUS TYPES OF FAULTS The most impor tant types of faults are as follows: a. single-phase to earth b. phase to phase c. phase-phase-earth d. three-phase (with or without earth) The above faults are described as single shunt faults because they occur at one location and involve a connection between one phase and another or to earth. In addition, the Protec tion Engineer often studies two other types of fault: e. single-phase open circu it f. cross-country fault By determining the currents and voltages at the fault point, it is possible to define the fault and connect the sequence networks to r epresent the fault condition. From the initial equations and the network diagram , the nature of the fault currents and voltages in different branches of the sys tem can be determined. For shunt faults of zero impedance, and neglecting load c urrent, the equations defining each fault (using phaseneutral values) can be wri tten down as follows: a. Single-phase-earth (A-E) I b = 0 ⎫ ⎪ ⎪ I c = 0 ⎬ ⎪ V a = 0 ⎪ ⎭ I2 = −V2 Z2 …Equation 4.5 The impedances in the negative sequence network are generally the same as those in the positive sequence — — network. In machines Z1 ≠ Z2 , but the di erence is gene rally ignored, particularly in large networks. The negative sequence diagrams, s hown in Figure 4.7, are similar to the positive sequence diagrams, with two impo rtant di erences; no driving voltages exist be ore — the ault and the negative s equence voltage V2 is greatest at the ault point. Figure 4.7 ZS1 X ΔZ 1 I 2 F I 2 Z 1 I2 V2 N (a) Negative sequence network F X V2 V2 + I 2ΔZ 1 N (b) Gradient diagram Figure 4.7: Fault at F: Negative sequence diagram Z 1 …Equation 4.7 b. Phase-phase (B-C) ⎫ ⎪ ⎪ I b = − I c ⎬ ⎪
V b = V c ⎪ ⎭
c. Phase-phase-earth (B-C-E) Ia = 0 4.3.3 Zero Sequence Network The zero sequence current an voltage relationships uring a fault con ition are the same as those in the negative sequence network. Hence: — —— V0 = - I0 Z0 …Equation 4.6 Also, the zero sequence iagram is that of Figur e 4.7, — — substituting I0 for I2 , an so on. The currents an voltages in the zero sequence network are co-phasal, that is, all the same phase. For zero sequence currents toflow in a system there must be a return connection through either a neutral con uctor or the general mass of earth. Note must be taken of this fact when etermining zero sequenceequivalent circuits. — — — Further, in general Z1 ≠ Z0 an the value of Z0 varies accor ing to the type of plant, the win ing arrangement an the metho of earthing. …Equation 4.8 I a = 0 ⎫ ⎪
⎪ V b = 0 ⎬ ⎪
V c = 0 ⎪
⎭
. Three-phase (A-B-C or A-B-C-E)
…Equation 4.9 I a + I b + I c = 0 ⎫ ⎪
⎪ Va = Vb ⎬ ⎪
Vb = Vc ⎪
…Equation 4.10 ⎭ It shoul be note
for any type of fault there are three equations that efine the fault con ition s. Network Protection & Automation Gui e • 35 • Fa u l t C a l c u l a t i o n s • 4•
When there is a fault impe ance, this must be taken into account when writing o wn the equations. For example, with a single-phase-earth fault through fault imp e ance — Zf , Equations 4.7 are re-written: ⎫ ⎪ ⎪ Ic = 0 ⎬ ⎪ V a = I a Z f ⎭ ⎪ Ib = 0 — —— —— V - I1 Z1 = I2 Z2 — an substituting for I2 fromEquation4.15: — — — — V = I1 ( Z1 uation 4.17 The constraints impose by Equations 4.15 an 4.17 in icate that the re is no zero sequence network connection in the equivalent circuit an that the positivean negative sequence networks are connecte in parallel. Figure 4.9 s hows the efining an equivalent circuits satisfying the above equations. F Figure 4.9 A B C Ic Ia =0 Ib =-Ic Vb=-Vc (a) Definition of fault (b) Equivalen t circuit Figure 4.9: Phase-Phase fault at F …Equation 4.11 Figure 4.8 A B C F Ia Ib Ic Vc Vb Va F 1 Z1 N1 V F2 Z2 N2 F 0 Z0 N0 Va Ia Vc Ib Vb F1 Z1 N1 V F2 Z2 N2 Z0 F0 N0 Ib =0 Ic =0 Va=0 (a) Definition of fault (b) Equivalent circuit Figure 4.8: Single-phase-earth fault at F
4.4.1 Single-phase-earth Fault (A-E) Consi er a fault efine by Equations 4.7 a n by Figure 4.8(a). Converting Equations 4.7 into sequence quantities by using Equations 4.1 an 4.2, then: 4.4.3 4.9 an Equations 4.1 Phase-phase-earth Fault (B-C-E) Again, from Equation an 4.2: — —— I1 = -( I2 + Io ) …Equation 4.18an — — — V1 = V2 = V0 …Equation 4.19 — — ing for V2 an V0 using network Equations 4.5 an 4.6: —— —— I2 Z2 = I0 Z0 thus, using E quation 4.18: Fa u l t C a l c u l a t i o n s • 4•
1 I1 = I 2 = I o = I a …Equation 4.12 3 — — — V1 = - ( V2 + V0 ) …Equation 4.13 — — — Subst ng for V1 , V2 an V0 in Equation 4.13 from Equations 4.4, 4.5 an 4.6: — —— —— —— V - I1 Z I2 Z2 + I0 Z0 — — — but, from Equation 4.12, I1 = I2 = I0 , therefore: — — — — — V = I1 ( Z2 + Z3 ) …Equation 4.14 The constraints impose by Equations 4.12 an 4.14 in icate that the equivalent circuit for the fault is obtaine by connecting the sequence networks in series, as shown in Figure 4.8(b). 4.4.2 Phase-phase Fault (B-C) From Equation 4.8 an using Equations 4.1 an 4.2: — — I1 = - I2 …Equation 4.15 — I0 = 0 — — V1 = V2 …Equation 4. From network Equations 4.4 an 4.5, Equation 4.16 can be re-written: — —— —— —— V - I1 Z1 = Z2 + I0 Z0 I0 = − I2 = −
Z 2 I1 Z0 + Z 2 …Equation 4.20 Z 0 I1 …Equation 4.21 Z0 + Z 2 — — Now equating V1 and V2 and using Equation 4.4 gives : — —— —— V I1 Z1 = I2 Z2 or — —— —— V = I1 Z1 I2 Z2 — Substituting or I2 rom Equation 4.21: or ⎡
Z 0 Z 2 ⎤
V = ⎢ Z1 + I1 Z 0 + Z 2 ⎥
I1 = V (Z 0 + Z2 ) …Equation 4.22 Z1 Z 0 + Z1 Z 2 + Z 0 Z 2 • 36 • Network Protection & Automation Guide
⎦ ⎣
From the above equations it ollows that connecting the three sequence networks in parallel as shown in Figure 4.10(b) may represent a phase phase earth ault. A F1 Figure 4.10a Phase phase earth ault I B C Ib Ic Ia=0 Vb=0 Vc=0 (a) De init ion o ault (b) Equivalent circuit Vb Vc N1 Z1 V F Va F2 Z2 N2 F0 Z0 N0 Hence, rom Equations 4.2, V0 = 1/3 Va V1 = 1/3 Va V2 = 1/3 Va and there ore:
V1 = V 2 = V0 = 1 3 V a ⎫ ⎪ ⎬ I a = I1 + I 2 + I 0 = 0⎪ …Equation 4.28 ⎭ From Equatio can be conclu e that the sequence networks are connecte in parallel, as shown in Figure 4.12(b). Va P νa Q Va' Figure 4.10: Phase-phase-earth fault at F 4.4.4 Three-phase Fault (A-B-C or A-B-C-E) Assumig that the fault icludes eart h, the, from Equatios 4.10 ad 4.1, 4.2, it follows that: V0 = V a ⎫ ⎪ ⎬ = 0 ⎪ ⎭ I1 Vc Ib Vb νb I Vc' c νc (a) Circuit diagram 4.4.6 Cross-coutry Faults A cross-coutry fault is oe where there are two faul ts affectig the same circuit, but i differet locatios ad possibly ivolvig differet phases. Figure 4.13(a) illustrates this. The costraits expressed i terms of sequece quatities are as follows: a) At poit F A B C F Vb Vc c Va F1 Z1 N1 V F2 Z2 N2 F0 Z0 N0 I b + I c = 0 ⎫ Therefore:
⎪ ⎬
Va = 0 ⎪
⎭
…Equation 4.29 Figure 4.11 I Ib Ia Ia+Ib+Ic=0 Va+Vb+Vc=0 (a) Definition of fault (b) Equivalent circuit I a1 = I a 2 = I a 0 V a1 + V a 2 + V a 0 b) At point F’ I a = I c = 0 ⎫ ⎪ ⎬ V b = 0 ⎪ ⎭ ⎫ ⎪
⎬
= 0 ⎭ ⎪
…Equation 4.30 Figure 4.11: Three-phase-earth fault at F 4.4.5 Single-phase Open Circuit Fault The single-phase open circuitfault is sho wn iagrammatically in Figure 4.12(a). At the fault point, the boun ary con itio ns are: …Equation 4.31 an therefore: I ’b1 = I ’b2 = I ’b0 …Equation 4.32 ⎫ ⎪
⎬
V b = V c = 0 ⎭
⎪
Ia = 0 …Equation 4.27
To solve, it is necessary to convert the currents an voltages at point F ’ to the sequence currents in the same phase as those at point F. From Equation 4.32, Network Protection & Automation Gui e • 37 • Fa u l t C a l c u l a t i o n s •
— I0 = 0 …Equation 4.24 — Substituting V2 = 0 in Equation 4.5 gives: — I2 = 0 …Equation 4. 25 — an substituting V1 = 0 in Equation 4.4: — —— 0 = V1 - I1 Z1 or — —— V = I1 Z1 …Equati 26 — Further, since from Equation 4.24 Io = 0 , it follows from — — Equation 4.6 that Vo is zero when Zo is finite. The equivalent sequence connections for a three-ph ase fault are shown in Figure 4.11. an V1 = V 2 …Equation 4.23 N1 +ve Sequence Network P1 ν1 Q1 N2 -ve Sequece Network I2 P2 ν2 Q2 N0 Zero Sequece Network I0
P0 ν0 Q0 (b) Equivalet circuit Figure 4.12: Ope circuit o phase A 4•
F F' a-e b-e (a) `A' phase to groud at F ad `B' phase to groud at F' Ia1 F1 I'a1 F'1 Va1 V'a1 N1 N'1 a I'a2 2 F2 Ia2 I'a2 F'2 1 2 a 2 Va2 N2 V'a2 N'2 a V'a2 Fa u l t C a l c u l a t i o s aI'a0 F0 Ia0 I'a0 F'0 1 a Va0 N0 (b) Equivalet circuit Figure 4.13: Cross - coutry fault - phase A to phase B V'a0 N'0 aV'a0 Figure 4.13: Cross - coutry fault - phase A to phase B • 4•
’ ’ a2 I a1 = aI a2 = I ’a0 or ’ ’ I a1 = a2I a2 = aI ’a0 an , for the voltages V ’b1 + V ’ ’b0 = 0 Converting: ’ ’ a2V a1 + aV a2 +V ’a0 = 0 or ’ ’ V a1 + a2V a2 + aV ’a0 = 0 …Equation 4.34 …Equation 4.33
4.5 CURRENT AND VOLTAGE DISTRIBUTION IN A SYSTEM DUE TO A FAULT Practical fault calculations involve the examination of the effect of a fault in branches of net work other than the faulte branch, so that protection can be applie correctly to isolate the section of the system irectly involve in the fault. It is there fore not in the fault itself; the fault cu enough to calculate the fault current rrent istribution must also be establishe . Further, abnormal voltage stresses may appear in a system because of a fault, an these may affect the operation of the protection. Knowle ge of current an voltage istribution in a network ue to a fault is essential for the application of protection. The approach to netwo rk fault stu ies for assessing the application of protection equipment may be su mmarise as follows: • 38 • Network Protection & Automation Gui e The fault constraints involve phase shifte sequence quantities. To construct th e appropriate sequence networks, it is necessary to intro uce phase-shifting tra nsformers to couple the sequence networks. This is shown in Figure 4.13(b).
a. from the network iagram an accompanying ata, assess the limits of stable g eneration an possible operating con itions for the system NOTE: When full infor mation is not available assumptions may have to bema e b. with faults assume t o occur at each relaying point in turn, maximum an minimum fault currents are c alculate for each type of fault NOTE: The fault is assume to be through zero i mpe ance c. by calculating the current istribution in the network for faults ap plie at ifferent points in the network (from (b)above) the maximum through fa ult currents at each relaying point are establishe for each type of fault . at this stage more or less efinite i eas on the type of protection to be applie are forme . Further calculations for establishing voltage variation at the relay ing limit of the system with a fault on it, are now carr point, orthe stability ie out in or er to etermine the class of protection necessary, such as high or low spee , unit or nonunit, etc. 4.5.1 current i Current Distribution The phase n any branch of a network is etermine from the sequence current istribution i n the equivalent circuit of the fault. The sequence currents are expresse in pe r unit terms of the sequence current in the fault branch. In power system calcul ations, the positive an negative sequence impe ances are normally equal. Thus, the ivision of sequence currents in the two networks will also be i entical. Th e impe ance values an configuration of the zero sequence network are usually i fferent from those sequence networks, so the of the positive an negative zero s equence current istribution is calculate separately. If Co an C1 are escribe as the zero an positive sequence istribution factors then the actual current in a sequence branch is given by multiplying the actual current in the sequence fault branch by the appropriate — — — istribution factor. For this reason, if I1 , I 2 an I0 are sequence currents in an arbitrary branch of a network ue to a faul t at some point in the network, then the phase currents in that branch may be ex presse in terms of the istribution constants an the sequence currents in the fault. These are given below for the various common shunt faults, using Equation 4.1 an the appropriate fault equations: a. single-phase-earth (A-E) I a = ( 2 C1 + C 0 ) I 0 ⎫ ⎪ b. phase-phase (B-C)
⎪ I
b = − (C1 − C 0 ) I 0 ⎬ ⎪
I
c = − (C1 − C 0 ) I
…Equation 4.35 ⎫
⎪
⎪
2 I
b = a − a C1 I1 ⎬ ⎪
I
c = a − a 2 C1 I1 ⎪
⎭ c. phase-phase-earth (B-C-
( ( ) ) …Equation 4.36 I
a = − (C1 − C 0 ) I 0 ⎡
Z I ' b = ⎢ a − a 2 C1 0 Z1 ⎣
( ( ) ) ⎡ Z I ' c = ⎢ a 2 − a C1 0 Z1 ⎣ ⎫ ⎪ ⎪ ⎪ ⎤ ⎪ − a 2 C1 − C 0 ⎥ I 0 ⎬ …Equation 4.37 . three-phase (A-B-C or A-B-C-E)
⎦
⎪ ⎪ ⎤
− aC1 + C 0 ⎥
I 0 ⎪ ⎪ ⎦
⎭
⎫ ⎪ ⎪ I b = a 2 C1 I1 ⎬ ⎪ I c = aC1 I1 ⎪ …Equation 4.38 ⎭ As an example of curre hnique, consi er the system in Figure 4.14(a). The equivalent sequence networks
are given in Figures with 4.14(b)an (c), together typical values of impe ances . A fault is assume at A an it is esire to fin the currents in branch OB u e to the fault. In each network, the istribution factors are given for each bra nch, with the current in the fault branch taken as 1.0p.u. From the iagram, the zero sequence istribution factor Co in branch OB is 0.112 an the positive seq uence factor C1 is 0.373. For an earth fault at A the phase currents in branch O B from Equation 4.35 are: — — Ia = (0.746 + 0.112) I0 — = 0.858 I0 an — — — I ’b = I ’c = 73 + 0.112) I0 — = -0.261 I0 By using network re uction metho s an assuming that all impe ances are reactive , it can be shown that — — Z1 = Z0 = j0.68 ohms. Therefore, from Equation 4.14, the current in fault branch I a = V 0.68
Network Protection & Automation Gui e • 39 • Fa u l t C a l c u l a t i o n s • I 4•
a = C1 I1
Power system A O Fault Loa (a) Single line iagram j7.5Ω j0.9Ω 1.0 A 0.08 B 4.5.2 Voltage Distribution The voltage istribution in any branch of a network i s etermine from the sequence voltage istribution. As shown by Equations 4.4, 4.5 an 4.6 an the gra ient iagrams, Figures 4.6(b) an 4.7(b), the positive s equence voltage is a minimum at the fault, whereas the zero an negative sequenc e voltages are a maximum. Thus, the sequence voltages in any part of the system may be given generally as: j0.4Ω 0.755 B j2.6Ω 0 j1.6Ω 0.112 0.165 0.192 j4.8Ω 0.053 (b) Zero sequence network j2.5Ω 0.183 ⎡
V1' = V − I1 ⎢
Z1 − ⎢
⎣
j1.6Ω 1.0 0.422 j0.4Ω B j0.75Ω 0 j0.45Ω 0.395 0.373 0.556 j18.85Ω A 0.022 ⎥ ⎪ 1
⎦
⎪ n ⎡
⎤ ⎪
⎪
V 2
= − I 2 ⎢
Z1 − ∑ C1 n ΔZ1 n ⎥ ⎬
⎢ ⎥
1 ⎣ ⎦
⎪ ⎪
n ⎡
∑ C1 n ΔZ1 n ⎥ ⎪ n ⎤ ⎫ Using theabove equation, the fault voltages at bus B in the previous example ca n be foun . From the positive sequence istribution iagram Figure 4.8(c): V 1 = V − I1 Z1 − j (0.395 × 0.75 ) + (0.373 × 0.45 ) V ' 2 = V − I1 Z1 − j 0.464 (c) Positive and negative sequence networks Figure 4.14: Typical power system Fa u l t C a l c u l a t i o n s — Assuming that |V | = 63.5 volts, then: 63.5 = 31.2 A I0 = 1 Ia = 3 3 x 0.68 — If V is taken as the reference vector, then : — I ’a = 26.8 -90° A — I ’b = I ’c =8.15 -90° A The vector diagram for the above fault on is shown in Figure 4.15. [ { }] [ ] }] From the zero sequence distribution diagram Figure 4.8(b): V 165 × 2.6 ) + (0.112 × 1.6 )
0 = I 0 Z 0 − j (0.
[ { • 4• Figure 4.15 = I 0 Z 0 − j 0.608 — — — For earth aults, at the ault I1 = I2 = I0 = j31.2A, when — |V | = 63.5 volts and is taken as the reference vector. — — Further, Z1 = Z0 = j0.68 oh ms. [ ] V c =61.5-116.4° Hence: — V’1 = 63.5 - (0.216 x 31.2) = 56.76 0° volts — V’2 = 6.74
180° volts — V’0 = 2.25 180°
I b =I c =8.15-90° V=63.5-0° V a =47.8-0° and, using Equations 4.1: — — — — Va = V1 + V2 + V0 = 56.76 -(6.74 + 2.25) — V’a = 47.8 a2 V’1 + aV’2 + V’0 = 56.76a2 -(6.74a + 2.25) — V’b = 61.5 -116.4° volts • 40 • Network Protection & Automation Guide I a =26.8-90° V b =61.5-116.4° Figure 4.15: Vector diagram: Fault currents and voltages in branch OB due to P-E fault at bus A
0°
— — — — V’c = aV’1 + a2V’2 + V’0 — V’c = 61.5 116.4° volts = 56.75a -(6.74a2 + 2.25)
— — — — and since Vbn = a2 Van , Vcn =aVan then: — — VR = 3Vne …Equation 4.43 — where Vcn ral displacement voltage. Measurements of residual quantities are made using cur rent and voltage transformer connections as shown in Figure 4.16. If relays are connected into the circuits in place of the ammeter and voltmeter, it follows th at earth faults in the system can be detected. These voltages are shown on the vector diagram, Figure 4.15. 4.6 EFFECT OF SYSTE M EARTHING ON ZERO SEQUENCE QUANTITIES It has been shown previously that zero se quence currents flow in the earth path during earth faults, and it follows that the nature of these currents will be influenced by the method of earthing. Becau se these quantities are unique in their association with earth faults they can b e utilised in protection, provided their measurement and character are understoo d for all practical system conditions. 4.6.1 Residual Current and Voltage Residu al currents and voltages depend for their existence on two factors: a. a system connection to earth at two or more points b. a potential difference between the earthed points resulting in a current flow in the earth paths Under normal syste m operation there is a capacitance between the phases and between phase and eart h; these capacitances may be regarded as being symmetrical and distributed unifo rmly through the system. So even when (a) above is satisfied, if the driving vol tages are symmetrical the vector sum of the currents will equate to zero and no current will flow between any two earth points in the system. When a fault to ea rth occurs in a system an unbalance results in condition (b) being satisfied. Fr om the definitions given above it follows that residual currents and voltages ar e the vector sum of phase currents and phase voltages respectively. Hence: Ia Ib Ic V ae A Vbe A B C V ce V (a) Residual current (b) Residual voltage Figure 4.16: Measurement of residual quantities I R = Ia + Ib + Ic and V R = V ae + V be Also, from Equations 4.2: ⎫
⎪
⎪
⎬ ⎪
+ V ce ⎪
⎭
…Equation 4.40
When assessing the istribution of resi ual quantities through a system, it is c onvenient to use the fault point as the reference asit is the point of injectio n of unbalance quantities into the system. The resi ual voltage is measure in relation to the normal phaseneutral system voltage an the resi ual current is c ompare with the three-phase fault current at the fault point. It can be shown [ 4.4/4.5] that the character of these quantities can be expresse in terms of the system — — Z0 / Z1 ratio. The positive sequence impe ance of a systemis mainly rea ctive, whereas the zero sequence impe ance being affecte by the metho of earth ing may contain both resistive an reactive components of comparable — — magnitu e. Thus the express of the Z0 / Z1 ratio approximates to: I R = 3 I 0 ⎫ ⎪ ⎬ V R = 3 V0 ⎪ ⎭ It shoul be further note that: V ae = V an + V ne ⎫ ⎪ ⎪ V be = V bn + V ne ⎬ ⎪ V ce = V cn + V ne ⎭ ⎪ …Equation 4.41 Z0 X R = 0 − j 0 Z1 X1 X1 …Equation 4.42
…Equation 4.44 Expressing the residual current in terms o the three— — phase current and Z0 / Z1 r atio: • 41 • Network Protection & Automation Guide Fa u l t C a l c u l a t i o n s • — — 4.6.2 System Z0 / Z1 Ratio — — The system Z0 / Z1 ratio is de ined as the ratio o z ero sequence and positive sequence impedances viewed rom the ault; it is a var iable ratio, dependent upon the method o earthing, ault position and system op erating arrangement. 4•
a. Single phase earth (A E) 3.0 IR 3V = = 2 Z1 + Z 0 3 (2 + K ) V Z1 VR and IR as multiples o V and I3 Residual current or Double Phase Earth ault 2.5 Residual voltage or Single Ph ase Earth ault 2.0 — — — where K = Z0 / Z1 I3φ = Thus: IR = I3φ V Z1 1.5 1.0 3 Residual voltage or Double Phase Earth ault (2 + K ) 3 Z1 I1 = − Z1 + Z 0 …Equation 4.45 0.5 Residual current or Double Phase Earth ault 0 1 2 K = Z0 Z1 3 4 5 • b. Phase phase earth (B C E) I R = 3I0 I1 = Hence: V Z1 + Z 0 2 Z1 Z 0 + ( ) Z12 Figure 4.17: Variation o residual quantities at ault point 4.6.3 Variation o Residual Quantities
IR = − 3 V Z1 2 Z1 Z 0 + Z12 V = − 2 K + 1 Z1 3 ( ) Fa u l t C a l c u l a t i o n s There ore: IR 3 = − I3ϕ 2K +1 ( ) …Equation 4.46 The variation o residual quantities in a system due to di erent earth arrangem ents can be most readily understood by using vector diagrams. Three examples hav e been chosen, namely solid ault isolated neutral, solid ault resistance neutr al, and resistance ault solid neutral. These are illustrated in Figures 4.18, 4 .19 and 4.20 respectively. X N Iab+Iac Iab Iac Similarly, the residual voltages are ound by multiplying —— Equations 4.45 and 4.46 by K V . a. Single phase each (A E) F A B C Iab Iac VR = − 3K (2 + K ) 3K V …Equation 4.47 Iab+Iac • 4• b. Phase phase earth (B C E) VR = (2 K + 1 )
V …Equation 4.48 (a) Circuit diagram Iac c n Iab b VbF=Eab VcF=Eac a(F) VbF VR (c) Residual vol tage diagram VcF The curves in Figure 4.17 illustrate the variation o the — — above residual quantit ies with the Z0 / Z1 ratio. The residual current in any part o the system can b e obtained by multiplying the current rom the curve by the appropriate zero seq uence distribution actor. Similarly, the residual voltage is calculated by subt racting rom the voltage curve three times the zero sequence voltage drop betwee n the measuring point in the system and the ault. (b) Vector diagram Figure 4.18: Solid ault isolated neutral • 42 • Network Protection & Automation Guide
4.6.3.1 Solid ault isolated neutral From Figure 4.18 it can be seen that the ca pacitance to earth o the aulted phase is short circuited by the ault and the resulting unbalance causes capacitance currents to low into the ault, returnin g via sound phases through sound phase capacitances to earth. At the ault point : and VaF = 0 VR — — = VbF + VcF — = 3 Ean
4.6.3.2 Solid ault resistance neutral Figure 4.19 shows that the capacitance o the aulted phase is short circuited by the ault and the neutral current combi nes with the sound phase capacitive — currents to give Ia in the aulted phase. Wi th a relay at X, residually connected as shown in — Figure 4.16, the residual curr ent will be Ian , that is, the neutral earth loop current.Figure 4.19 At the au lt point: — — — — VR = VbF + VcF since VFe = 0 At source: — — — — VR = VaX + VbX + VcX From residual voltage diagram it is clear that there is little variation in the resid ual voltages at source and ault, as most residual voltage is dropped across the neutral resistor. The degree o variation in residual quantities is there ore d ependent on the neutral resistor value. 4.6.3.3 Resistance ault solid neutral C apacitance can be neglected because, since the capacitance o the aulted phase is not short circuited, the circulating capacitance currents will be negligible. At the ault point: — — — — VR = VFn + Vbn + Vcn At relaying point X: — — — — VR = VXn + V Vcn X At source: — — — VR = 3Vne = 3Ean since — — — Ean + Ebn + Ecn = 0 Thus, with an isolated neutral system, the residual voltage is three times the n ormal phase neutral voltage o the aulted phase and there is no variation betwe en — — VR at source and VR at ault. In practice, there is some leakage impedance be tween neutral and earth and a small residual current would be detected at X i a very sensitive relay were employed. Ia X Iab Iac Ian Ia Iab Iab ZL F A B C ZS IF ZL F (a) Circuit diagram Iac c A B C IF IF
Vc VcX n VXn Vb Iab VbX b (b) Vector diagram X a(F) IaZL Ian Ia Iac Iab (a) Circuit diagram c VcF n Vbn VFn VbF (b) Vector diagram VFn VR VXn Van VXn a IFZS X IF Vcn F IFZL b VR VcX Vb VR (at ault) VcF VR (at source) VbX VaX Vcn (c) Residual voltage at ault Vbn Vcn Vbn
(c) Residual voltage diagram Figure 4.19: Solid ault resistance neutral (d) Residual voltage at relaying point Figure 4.20: Resistance ault solid neutral Network Protection & Automation Guide • 43 • Fa u l t C a l c u l a t i o n s • 4•
From the residual voltage diagrams shown in Figure 4.20, it is apparent that the residual voltage is greatest at the ault and reduces towards the source. I th e ault resistance approaches zero, that is, the ault becomes — solid, then VFn a pproaches zero and the voltage drops in — — — ZS and ZL become greater. The ultimate v alue o VFn will depend on the e ectiveness o the earthing, and this — — is a unc tion o the system Z0 / Z1 ratio. 4.7 REFERENCES 4.1 Circuit Analysis o A.C. Po wer Systems, Volume I. Edith Clarke. John Wiley & Sons. 4.2 Method o Symmetrica l Co ordinates Applied to the Solution o Polyphase Networks. C.L. Fortescue. Tr ans. A.I.E.E.,Vol. 37, Part II, 1918, pp 1027 40. 4.3 Power System Analysis. J.R . Mortlock and M.W. Humphrey Davies. Chapman and Hall. 4.4 Neutral Groundings. R Willheim and M. Waters, Elsevier. 4.5 Fault Calculations. F.H.W. Lackey, Oliver & Boyd. Fa u l t C a l c u l a t i o n s • 4• • 44 • Network Protection & Automation Guide
• 5 • Equivalent Circuits and Parameters o Power System Plant Introduction Synchronous machines Armature reaction Steady state theory Salient pole rotor Transient analysis Asymmetry Machine reactances Negative sequence rea ctance Direct and quadrature axis values 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.1 1 Zero sequence reactance 5.10 E ect o saturation on machine reactances 5.12 Tra ns ormers 5.13 Trans ormer positive sequence equivalent circuits 5.14 Trans orme r zero sequence equivalent circuits 5.15 Auto trans ormers 5.16 Trans ormer impe dances 5.17 Overhead lines and cables 5.18 Calculation o series impedance 5.19 Calculation o shunt impedance 5.20 Overhead line circuits with or without earth wires 5.21 OHL equivalent circuits 5.22 Cable circuits 5.23 Overhead line and c able data 5.24 Re erences 5.25
• 5 • Equivalent Circuits and Parameters o Power System Plant 5.1 INTRODUCTION Knowledge o the behaviour o the principal electrical system p lant items under normal and ault conditions is a prerequisite or the proper ap plication o protection. This chapter summarises basic synchronous machine, tran s ormer and transmission line theory and gives equivalent circuits and parameter s so that a ault study can be success ully completed be ore the selection and a pplication o the protection systems described in later chapters. Only what migh t be re erred to as 'traditional' synchronous machine theory is covered, as that is all that calculations or ault level studies generally require. Readers int erested in more advanced models o synchronous machines are re erred to the nume rous papers on the subject, o which re erence [5.1] is a good starting point. P ower system plant may be divided into two broad groups static and rotating. Th e modelling o static plant or ault level calculations provides ew di iculti es, as plant parameters generally do not change during the period o interest o llowing ault inception. The problem in modelling rotating plant is that the par ameters change depending on the response to a change in power system conditions. 5.2 SYNCHRONOUS MACHINES There are two main types o synchronous machine: cylin drical rotor and salient pole. In general, the ormer is con ined to 2 and 4 pol e turbine generators, while salient pole types are built with 4 poles upwards an d include most classes o duty. Both classes o machine are similar in so ar th at each has a stator carrying a three phase winding distributed over its inner p eriphery. Within the stator bore is carried the rotor which is magnetised by a w inding carrying d.c. current. The essential di erence between the two classes o machine lies in the rotor construction. The cylindrical rotor type has a uni o rmly cylindrical rotor that carries its excitation winding distributed over a nu mber o slots Network Protection & Automation Guide • 47 •
around its periphery. This construction is unsuited to multi polar machines but it is very sound mechanically. Hence it is particularly well adapted or the hig hest speed electrical machines and is universally employed or 2 pole units, plu s some 4 pole units. The salient pole type has poles that are physically separat e, each carrying a concentrated excitation winding. This type o construction is in many ways complementary to that o the cylindrical rotor and is employed in machines having 4 poles or more. Except in special cases its use is exclusive in machines having more than 6 poles. Figure 5.1 illustrates a typical large cylin drical rotor generator installed in a power plant. most common. Two stroke diesel engines are o ten derivatives o marine designs w ith relatively large outputs (circa 30MW is possible) and may have running speed s o the order o 125rpm. This requires a generator with a large number o poles (48 or a 125rpm, 50Hz generator) and consequently is o large diameter and sho rt axial length. This is a contrast to turbine driven machines that are o small diameter and long axial length. Equivalent Circuits and Parameters o Power System Plant Two and our pole generators are most o ten used in applications where steam or gas turbines are used as the driver. This is because the steam turbine tends to be suited to high rotational speeds. Four pole steam turbine generators are most o ten ound in nuclear power stations as the relative wetness o the steam make s the high rotational speed o a two pole design unsuitable. Most generators wit h gas turbine drivers are our pole machines to obtain enhanced mechanical stren gth in the rotor since a gearbox is o ten used to couple the power turbine to t he generator, the choice o synchronous speed o the generator is not subject to the same constraints as with steam turbines. Generators with diesel engine driv ers are invariably o our or more pole design, to match the running speed o th e driver without using a gearbox. Four stroke diesel engines usually have a high er running speed than twostroke engines, so generators having our or six poles are Weak N Strong Weak S Strong Direction o rotation (a) N S N (b) Figure 5.2: Distortion o lux due to armature reaction • 5• Figure 5.1: Large synchronous generator • 48 •
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5 . 3 A R M AT U R E R E A C T I O N Armature reaction has the greatest e ect o n the operation o a synchronous machine with respect both to the load angle at which it operates and to the amount o excitation that it needs. The phenomenon is most easily explained by considering a simpli ied ideal generator with ull p itch winding operating at unity p. ., zero lag p. . and zero lead p. . When oper ating at unity p. ., the voltage and current in the stator are in phase, the sta tor current producing a cross magnetising magneto motive orce (m.m. .) which in teracts with that o the rotor, resulting in a distortion o lux across the pol e ace. As can be seen rom Figure 5.2(a) the tendency is to weaken the lux at the leading edge or e ectively to distort the ield in a manner equivalent to a shi t against the direction o rotation. I the power actor were reduced to ze ro lagging, the current in the stator would reach its maximum 90° a ter the voltag e and the rotor would there ore be in the position shown in Figure 5.2(b). The s tator m.m. . is now acting in direct opposition to the ield. Similarly, or ope ration at zero leading power actor, the stator m.m. . would directly assist the rotor m.m. . This m.m. . arising rom current lowing in the stator is known as 'armature reaction'. 5 . 4 . S T E A DY S TAT E T H E O R Y The vector diagram o a single cylindrical rotor synchronous machine is shown in Figure 5.3, assumi ng that the magnetic circuit is unsaturated, the air gap is uni orm and all vari able quantities are sinusoidal. Further, since the reactance o machines is norm ally very much larger than the resistance, the latter has been neglected. The ex citation ampere turns, ATe, produces a lux Φ across the air-gap thereby inducing a voltage, Et, in the stator. This voltage drives a current I at a power factor cos-1φ and gives rise to an armature reaction m.m. . ATar in phase with it. The m. m. . AT resulting rom the combination o these two m.m. . vectors (see Figure 5.3(a)) is the excitation which must be provided on the rotor to maintain lux Φ a cross the air-gap. Rotating the rotor m.m.f. diagram, igure 5.3(a), clockwise u ntil coincides with Et and changing the scale of the diagram so that ATe becomes the basic unit, where ATe = Et =1, results in igure 5.3(b). The m.m.f. vectors have thus become, in effect, voltage vectors. or example ATar /ATe is a unit o f voltage that is directly proportional to the stator load current. This vector can be fully represented by a reactance and in practice this is called 'armature reaction reactance' and is denoted by Xad. Similarly, the remaining si de of the triangle becomes ATf /ATe , which is the per unit voltage produced on open circuit by ampere-turns ATf . It can be considered as the internal generate d voltage of the machine and is designated Eo . Et(=V) I ATar ATe ATf (a) ATf ATe Et=1=V I ATe ATar ATf (b) IXad IX d EL IXL V I (c) Eo igure 5.3: Vector diagram of synchronous machine The true leakage reactance of the stator winding which gives rise to a voltage d rop or regulation has been neglected. This reactance is designated XL (or Xa in some texts) and the voltage drop occurring in it, IXL, is the difference between the terminal voltage V and the voltage behind the stator leakage reactance, EL. IZL is exactly in phase with the voltage drop due to Xad, as shown on the vecto r diagram igure 5.3(c). It should be noted that Xad and XL can be combined to g ive a simple equivalent reactance; this is known as the 'synchronous reactance', denoted by Xd. The powergenerated by the machine is given by the equation: P = VI cos φ = VE sin δ X …Equation 5.1
here δ is the angle beteen the internal voltage an the terminal voltage an is knon as the loa angle of the machine. Netork Protection & Automation Gui e • 49 •
Equivalent Circuits an Parameters of Poer System Plant • ATar ATe 5•
It follos from the above analysis that, for stea y state erformance, the machi ne may be reresente by the equivalent circuit sho n in Figure 5.4, here XL is a true reactance associate ith flux leakage aroun the stator in an Xa ing is a fictitious reactance, being the ratio of armature reaction an oen circui t excitation magneto motive forces. When a ole ith the assume sine ave m.m.f. set u by the stator, a is aligne corres on ing sine ave flux ill be set u, but hen an inter olar ga is ali gne very severe istortion is cause . The ifference is treate by consi ering on ing to the ole an the inter olar ga, these to axes, that is those corres searately. They are esignate the ' irect' an 'qua rature' axes res ectively , an the general theory is kno n as the 't o axis' theory. The vector iagram f or the salient ole machine is similar tothat for the cylin rical rotor excet ith them are slit into to comonen that the reactance an currents associate ts. The synchronous reactance for the irect axis is X = Xa + XL, hile that i n the qua rature axis is Xq = Xaq + XL. The vector iagram is constructe as bef ore but the aroriate quantities in this case are resolve along to axes. The resultant internal voltage is Eo, as shon in Figure 5.6. ’ In passing it shoul be note that E 0 is the internal voltage which woul begiven, in cylin rical r otor theory, by vectorially a ing the simple vectors IX an V. There ’ is very l ittle ifference in magnitu e between E0 an E0 but a substantial ifference in internal angle; the simple theory is perfectly a equate for calculation of excit ation currents but not for stability consi erations where loa angle is signific ant. IqXq I X EO IX E O Xa XL Eo Et V
Equivalent Circuits an Parameters of Power System Plant Figure 5.4: Equivalent circuit of elementary machine In practice, ue tonecessary constructional features of a cylin rical rotor to accommo ate the Xa is not constant irrespective of rotor win ings, the reactance position, an mo elling procee s as for a generator with a salient pole rotor. However, the numerical ifference between the values of Xa an Xaq is small, mu ch less than for the salient pole machine. 5 . 5 S A L I E N T P O L E R OTO R T he prece ing theory is limite to the cylin rical rotor generator. The basic ass umption that the air-gapis uniform is very obviously not vali when asalient p ole rotor is being consiere . The effect of this is that the flux pro uce by a rmature reaction m.m.f. epen s on the position of the rotor at any instant, as shown in Figure 5.5. • 5• Lag Armature reaction M.M.F. Lea V Flux
Flux I Iq
Qua r Qu rature axis Dire axis po rect ole I Pole axis Figure 5.5: Variation of armature reaction m.m.f. with pole position Figure 5.6: Vector iagram for salient pole machine • 50 •
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5 . 6 T R A N S I E N T A N A LYS I S For normal changes in loa con itions, st ea y state theory is perfectly a equate. However, there are occasions when almos t instantaneous changes are involve ,such as faults or switching operations. Wh en this happens new factors are intro uce within the machine an to represent t hese a equately a correspon ing new set of machine characteristics is require . The generally accepte an most simple way to appreciate the meaning an erivat ion of these characteristics is to consi er a su en three-phase short circuit a pplie to a machine initially runningon open circuit an excite to normal volt age E0. This voltage will be generate by a flux crossing the airgap. It is not possible to confine the flux to one path exclusively in any machine, an as a re sult there will be a leakage flux ΦL that will leak from pole to pole and across t he inter-polar gaps without crossing the main air-gap as shown in igure 5.7. Th e flux in the pole will be Φ + ΦL. XL XL Xad (a) Synchronous reactance XL Xad Xf (b) Transient reactance Xad Xf Xkd
(c) Subtransient reactance igure 5.8: Synchronous machine reactances 2 L 2 L It might be expected that the fault current would be given by E0 /(XL+Xad) equal to E0/Xd , but this is very much reduced, and the machine is operating with no saturation. or this reason, the value of voltage used is the value read from th e air-gap line corresponding to normal excitation and is rather higher than the normal voltage. The steady state current is given by:
Id = igure 5.7: lux paths of salient pole machine Eg Xd …Equation 5.2 where Eg = voltage on air gap line An important point to note now is that betwee n the initial and final conditions there has been a severe reduction of flux. Th e rotor carries a highly inductive winding which links the flux so that the roto
r flux linkages before the short circuit are produced by (Φ + ΦL). In practice the l eakage flux is distributed over the whole pole and all of it does not link all t he winding. ΦL is an equivalent concentrated flux imagined to link all the winding and of such a magnitude that the total linkages are equal to those actually occ urring. It is a fundamental principle that any attempt to change the flux linked with such a circuit will cause current to flow in a direction that will oppose the change. In the present case the flux is being reduced and so the induced cur rents will tend to sustain it. • If the stator winding is then short-circuited, the power factor in it will be ze ro. A heavy current will tend to flow, as the resulting armature reaction m.m.f. is demagnetising. This will reduce the flux and conditions will settle until th e armature reaction nearly balances the excitation m.m.f., the remainder maintai ning a very much reduced flux across the air-gap which is just sufficient to gen erate the voltage necessary to overcome the stator leakage reactance (resistance neglected). This is the simple steady state case of a machine operating on shor t circuit and is fully represented by the equivalent of igure 5.8(a); see also igure 5.4. Network Protection & Automation Guide • 51 • Equivalent Circuits and Parameters of Power System Plant 5•
or the position immediately following the application of the short circuit, it is valid to assume that the flux linked with the rotor remains constant, this be ing brought about by an induced current in the rotor which balances the heavy de magnetising effect set up by the shortcircuited armature. So (Φ + ΦL) remains consta nt, but owing to the increased m.m.f. involved, the flux leakage will increase c onsiderably. With a constant total rotor flux, this can only increase at the exp ense of that flux crossing the air-gap. Consequently, this generates a reduced v oltage, which, acting on the leakage reactance, gives the short circuit current. It is more convenient for machine analysis to use the rated voltage E0 and to i nvent a fictitious reactance that will giverise to the same current. This react ance is called the 'transient reactance' X’ an is efine by the equation: Trans ient current I = Eo X The amperwin ing(s) is subjecte to the full effect of flux transfer to leakag e paths an will carry an in uce current ten ing to oppose it. As long as this current can flow, the air-gap flux will be hel at a value slightly higher than woul be the case if only the excitation win ing were present, but still less th an the original open circuit flux Φ. As before, it is convenient to use rated volt age and to create another fictitious reactance that is considered to be effectiv X ’’ an is efine e over this period.This is known as the 'subtransient reactance' by the equation: Sub-transient current I ’’ = Eo X where X = X L + o r …Equation 5.4 Equivalent Circuits an Parameters of Power System Plant X a X f X k X a X f + X k X f + X a X k X’’ = XL + X’k …Equation 5.3 an Xk = leakage reactance of amper win ing(s) X’k = effective leakage reactanc e of amper win ing(s) It is greater than XL but less than X’ an the correspon i ng equivalent circuit is shown in Figure 5.8(c). Again, the uration of this pha se epen s upon the time constant of the amper win ing. In practice this is app roximately 0.05 secon s - very much less than the transient - hence the term su b-transient . Figure 5.9 shows the envelope of the symmetrical component of an a rmature short circuit current in icating the values escribe in the prece ing a nalysis. The analysis of the stator current waveform resulting from a su en sho rt circuit test is tra itionally the It is greater than XL, an the equivalent circuit is represente by Figure 5.8(b ) where: X = X a X f +XL X a + X f an X f is the leakage reactance of the fiel win ing The above equation may als o be written as: X’ = XL + X’f where X’f = effective leakage reactance of fiel win i value while ng The flux will only be sustaine at its relatively high the in uce current flows inthe fiel win ing. As this current ecays, so con itions will approach the stea y state. Consequently, the uration of this phase will be et ermine by the time constant of the excitation win ing. Thisis usually of the o r er of a secon or less - hence the term transient applie to characteristics associate with it.A further point now arises. All synchronous machines have w hat is usually calle a ‘ amper win ing’ or win ings. In some cases,this may be a p hysical win ing (like a fiel win ing, but of fewer turns an locate separately
), or an ‘effective’ one (for instance, the soli iron rotor of a cylin rical rotor machine). Sometimes, both physical an effective amper win ings may exist (as i nsome esignsof cylin rical rotorgenerators, having both a soli iron rotor a n a physical amper win ing locate in slots in the pole faces). Un er short ci rcuit con itions, there is a transfer of flux fromthe main air-gap to leakage p aths. This iversion is, to a small extent, oppose by the excitation win ing an the main transfer will be experience towar s the pole tips. • I
=
Eo X I =
Current Eo X I = Eair gap X 5• Time Figure 5.9: Transient ecay envelope of short-circuit current • 52 •
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metho by which these reactances are measure. However, the major limitation is that only irect axis parameters are measure .Detaile test metho s for synchro nous machines are given in references [5.2] an [5.3], an inclu e other tests t hat are capable of provi ing more etaile parameter information. 5.7 ASYMMETRY The exact instant at which the short circuit is applie to the stator win ing is of significance. If resistance is negligible compare with reactance, the curre nt in a coil will lag the voltage by 90°, that is, at theinstant when the voltage wave attains a maximum, any current flowing through woul be passing through ze ro. If a short circuit were applie at this instant, the resulting current woul rise smoothly an woul be a simple a.c. component. However, at the moment when the in uce voltage is zero, any current flowing must pass through a maximum (o wing to the 90° lag).If a fault occurs at this moment, the resulting current will assume the correspon ing relationship; it will be at its peak an in the ensuin g 180° will go through zero to maximum in thereverse irection an so on. In fact the current must actually start from zero an so will follow a sinewave that i s completely asymmetrical. Interme iate positions will give varying egrees of a symmetry. This asymmetry can be consi ere to be ue toa .c. component of curr ent which ies away becauseresistance is present. The .c. component of stator current sets up a .c. fiel in the stator which causes a supply frequency rippl e on the fiel current, an this alternating rotor flux has a further effect on the stator.This is best shown by consi ering the supply frequency flux as being represente by two half magnitu e waves each rotating in opposite irections at supply frequency relative to the rotor. So, as viewe from the stator, one is stationary an the other rotating at twice supply freque ncy. The latter sets up secon harmonic currents in the stator. Further evelopm ent along but the resulting harmonics are usually neglig these lines is possible ible an normally neglecte . 5 . 8 M A C H I N E R EA C TAN C E S Table 5.1 gi ves values of machine reactances for salient pole an cylin rical rotor machines typical of latest esign practice. Also inclu e are parameters for synchronous compensators – such machines are now rarely built, but significantnumberscan st ill be foun in operation. 5.8.1 Synchronous Reactance X = XL + Xa The or er o f magnitu e of XL is normally 0.1-0.25p.u., while that of Xa is 1.0-2.5p.u. The leakage reactance XL can be re uce by increasing the machine size ( erating), or increase by artificially increasing the slotleakage, but it will be note t hat XL is only about 10% of the total value of X an exercise much influ cannot ence. The armature reaction reactance can be re uce by ecreasing the armature reaction of the machine, which in esign terms means re ucing the ampere con uct or or electrical (as istinct from magnetic) loa ing - this will often mean a ph ysically larger machine. Alternatively the excitation nee e to generate open-ci rcuit voltage may be increase ; this is simply achieve by increasing the machin e air-gap, but is only possible if the excitation system is mo ifie to meet the increase requirements. In general, control of X is obtaine almost entirely b y varying Xa , an in most cases a re uction in X will mean a larger an more c ostlymachine. It is also worth Cylin rical rotor turbine generators Salient pole generators 4 Pole 0.4-0.6 1.75 -3.0 0.9-1.5 0.26-0.35 0.19-0.25 0.19-0.35 0.16-0.27 0.01-0.1 0.4-1.1 3.0-9.0 0. 02-0.04 0.035-0.06 0.025-0.04 0.13-0.2 Multi-pole 0.6-0.8 1.4-1.9 0.8-1.0 0.24-0 .4 0.16-0.25 0.18-0.24 0.16-0.23 0.045-0.23 0.25-1 1.7-4.0 0.02-0.06 0.03-0.1 0. 025-0.08 0.1-0.35 Type of machine Short circuit ratio Direct axis synchronous reactance X (p.u.) Qua axis transient reactance rature axis synchronous reactance Xq (p.u.) Direct X’ (p.u.) Direct axis sub-transient reactance X’’ (p.u.) Qua rature axis sub-transie nt reactance X’’q (p.u.) Negative sequence reactance X2 (p.u.) Zero sequence reactan ce X0 (p.u.) Direct axis short circuit transient time constant T’ (s) Direct axis open circuit transienttime constant T’ o (s) Direct axis short circuit sub-trans ient- time constant T’’ (s) Direct axis open circuit sub-transient time constant T’’ o( s) Qua rature axis short circuit sub-transient time constant T’’q (s) Qua rature axi s open circuit sub-transient time constant T’’qo (s) Table 5.1: Typical synchronous
generator parameters 4 Pole I Multi-Pole 0.5-0.7 1.6-2.0 1.0-1.23 0.3-0.5 0.2-0.4 0.25-0.6 0.25-0.5 0 .12-0.16 1.5-2.5 5-10 0.04-0.9 0.07-0.11 0.04-0.6 0.1-0.2 1.0-1.2 0.8-1.0 0.5-0. 65 0.2-0.35 0.12-0.25 0.15-0.25 0.14-0.35 0.06-0.10 1.0-2.0 3-7 0.05-0.10 0.08-0 .25 0.05-0.6 0.2-0.9 Air Coole 0.4-0.6 2.0-2.8 1.8-2.7 0.2-0.3 0.15-0.23 0.16-0.25 0.16-0.23 0.06-0. 1 0.6-1.3 6-12 0.013-0.022 0.018-0.03 0.013-0.022 0.026-0.045 Hy rogen Coole 0.4-0.6 2.1-2.4 1.9-2.4 0.27-0.33 0.19-0.23 0.19-0.23 0.19-0.24 0.1-0.15 0.7-1.0 6-10 0.017-0.025 0.023-0.032 0.018-0.027 0.03-0.05 Hy rogen/ Water Coole 0.4-0.6 2.1-2.6 2.0-2.5 0.3-0.36 0.21-0.27 0.21-0.28 0.21 -0.27 0.1-0.15 0.75-1.0 6-9.5 0.022-0.03 0.025-0.035 0.02-0.03 0.04-0.065 NB all reactance values are unsaturate . Network Protection & Automation Gui e • 53 •
Equivalent Circuits an Parameters of Power System Plant • 5•
noting thatXL normally changes in sympathy with Xa , but that it is completely oversha owe by it. The value 1/X has a special significance as it approximates to the short circuit ratio (S.C.R.), the only ifference being that the S.C.R. takes saturation into account whereas X is erive from the air-gap line. 5.8.2 Transient Reactance X’ = XL + X’f The transient reactance covers the behaviour of a machine in the perio 0.1-3.0 secon s after a isturbance. This generally corr espon s to the spee of changes in a system an therefore X’ has a major influenc e in transient stability stu ies. rise to parasitic currents an heating; most machines are quite limite in the a mount of such current which they are able to carry, both in the stea y – state an transiently. An accurate calculation of the negative sequence current capabilit y of a generator involves consi eration of the currentpaths in the rotor bo y. In a turbine generator rotor, forinstance, they inclu e the soli rotor bo y,s lot we ges, excitation win ing an en -win ing retaining rings. There is a ten e ncy for local over-heating to occur an , although possible for the stator, conti nuous local temperature measurement is not practicalin the rotor. Calculation r equires complex mathematical techniquesto be applie , an involves specialist s oftware. In practice an empirical metho is use , base on the fact that a given type of for short perio machine is capable of carrying, s, an amount of heat e termine byits thermal capacity, an for a long perio , a rate of heat input wh ich it can issipate continuously. Synchronous machines are esigne to be capab le of operating continuously on an unbalance system such that, with none of the phase currents excee ing the rate current, the ratio of the negative sequence current I2 to therate current IN oes not excee the values given in Table 5.2 . Un erfault con itions, the machine shall also be capable 2 of operation with the pro uct of ⎛ I 2 ⎞ and time in ⎜ ⎟ ⎝ I N ⎠ seconds (t) not exceeding the values g Equivalent Circuits and Parameters o Power System Plant Generally, the leakage reactance XL is equal to thee ective ield leakage reac tance X’f, about 0.1-0.25p.u. The principal factor etermining the value of X’f is t he fiel leakage. This is largely beyon the control of the esigner, inthat ot her consi erations are at present more significant than fiel leakage an hence takeprece ence in etermining the fiel esign. XL can be varie as alrea y out line , an , in practice, control of transient reactance is usually achieve by v arying XL 5.8.3 Sub-transient Reactance X’’ = XL + X’k The sub-transient reactance etermines the initial current peaks following a isturbance an in the case of a su en fault is of importance for selecting the breaking capacity of associate circuit reach maximum values t breakers. The mechanical stresses on the machine hat epen on this constant. The effective amper wining leakage reactance X’k i s largely etermine by theleakage of the amper win ings an control of thisi s only possible to a limite extent. X’k normally has a value between 0.05an 0. 15 p.u. The major factor is XL which, as in icate previously, is of the or er o f 0.1-0.25 p.u., an control of the sub-transient reactance is normally achieve by varying XL. It shoul be note that goo transient stability is obtaine by keeping the value of X’ low, which therefore also implies a low value of X’’ . The fa ult rating of switchgear, etc. will therefore be relatively high. It is not norm ally possible to improve transient stability performance in a generator without a verse effects on fault levels, an vice versa. 5 . 9 N E G AT I V E S E Q U E N C E R E A C TA N C E Negative sequence currents can arise whenever there is an y unbalance present in the system. Their effect is to set up a fiel rotating in the opposite irection to the main fiel generate by the rotor win ing, so sub jecting the rotor to ouble frequency flux pulsations. This gives • 54 • Rotor construction Rotor Cooling
Machine Type (SN) /Rating (MVA)
Maximum Maximum (I2/IN)2t for I2/IN for continuous operation uring faults opera tion 0.1 0.08 0.1 0.08 0.05 0.08 0.1 0.1 0.08 Note 1 Note 1 0.05 20 20 20 15 15 15 15 10 8 Note 2 5 5 • 5• Salient Cylin rical
motors generators inirect synchronous con ensers motors generators irect synch ronous con ensers in irectly coole (air) all in irectly coole (hy rogen) all < =350 351-900 irectly coole 901-1250 1251-1600 I2 S -350 = 0.08- N IN 3 x 104 Note 1: Calculate as Note 2: Calculate as () I2 2 t = 8-0.00545(SN-350) IN Table 5.2: Unbalance operating con itions for synchronous machines (from IEC 60 034-1) Network Protection & Automation Gui e
5 . 10 ZE R O S E Q U E N C E R E A C TA N C E If a machine is operating with a n earthe neutral, a system earth fault will give rise to zero sequence currents in themachine. This reactance represents the machine s contribution to the tot al impe ance offere to these currents. In practice it is generally low an ofte n outweighe by other impe ances present in the circuit. 5.11 DIRECT AND QUADRAT URE AXIS VALUES The transient reactance is associate with the fiel win ing an since on salient polemachines this is concentrate on the irect axis, there i s no correspon ing qua rature axis value. The value of reactance applicable in t he qua rature axis is the synchronous reactance, that is, X’q = Xq. The amper win ing (or its equivalent) is more wi ely sprea an hence the sub-transient react ance associate with this has a efinitequa rature axis value X”q, which iffers significantly in many generators from X” . 5.12EFFECTOF SATURATION ON MACHINE RE ACTANCES In general, any electrical machine is esigne to avoi severe saturati on of its magnetic circuit. However, it is not economically possible to operate at such low flux ensities as to re uce saturation to negligible proportions, an in practice a mo erate egree of saturation is accepte . Since the armature re action reactance Xa is a ratio ATar /ATe it is evi ent that ATe will notvary i n a linear manner for ifferent voltages, while ATar will remain unchange . The value of Xa will vary withthe egree ofsaturation present in the machine, an for extreme accuracy shoul be etermine for the particular con itions involve in any calculation. All the other reactances, namely XL , X’ an X’’ are true reac tances an actually arise from flux leakage. Much ofthis leakage occurs in the iron parts of the machines an hence must be affecte by saturation. For a given set of con itions, the leakage flux exists as a result of the net m.m.f. which causes it. If the iron circuit is unsaturate its reactance is low an leakage f lux iseasily establishe . If the circuits are highly saturate the reverse is t rue an the leakage flux is relatively lower, so the reactance un er saturate c on itions is lower thanwhen unsaturate . Most calculation metho s assume infini te iron permeability an for this reason lea tosomewhat i ealise unsaturate reactance values. The recognition of a finite an varying permeability makes a s olution extremely laborious an in practice a simple factor of approximately 0.9 is taken as representing the re uction in reactance arising from saturation. Network Protection & Automation Gui e It is necessary to istinguish which value of reactance isbeing measure when o n test. The normal instantaneous short circuit test carrie out from rate open circuit voltage gives a currentthat is usually several times full loa value, s o that saturation is present an the reactance measure will be the saturate va lue. This value is also known as the rate voltage value since it is measure by a short circuit applie with the machine excite to rate voltage. In some ca ses, if it is wishe to avoi the severe mechanical strain towhich a machine is subjecte by such a irect short circuit, the test may be ma e from a suitably re uce voltage so that the initial current is approximately full loa value. Sa turationis very much re uce an the reactance values measure are virtually un saturate values. They are also known as rate current values, for obvious rea sons. 5.13 TRANSFORMERS A transformer may be replace in a power system by an eq uivalent circuit representing the self-impe ance of, an the mutual coupling bet ween, the win ings. A twowin ing transformer can be simply represente as a T network in which thecross member is the short-circuit impe ance, an the column the excitation impe ance. It is rarely necessary in fault stu ies to consi er e xcitationimpe ance as this is usually many times the magnitu e of the short-cir cuit impe ance. With these simplifying assumptions a three-win ing transformer b ecomes a star of three impe ances an a four-win ing transformer a mesh of six i mpe ances. The impe ancesof a transformer, in common with other plant, can be g iven or in per unit or percentage terms in ohms an qualifie by a base voltage, an qualifie by abase MVA. Care shoul be taken with multiwin ing transformer s to refer all impe ances to a common base MVA or to statethe base on which eac h is given. The impe ances of static apparatus are in epen ent of the phase sequ ence of the applie voltage; in consequence, transformer negative sequence an p ositive sequence impe ances are i entical. In etermining the impe ance to zero
phase sequence currents, account must be taken of the win ing connections, earth ing, an , in some cases, the construction type. The existence of a path for zero sequence currents implies a fault to earth an a flow of balancing currents in the win ings of the transformer. Practical three-phase transformers may have a p hase shift between primary an secon ary win ings epen ing on the connections o f the win ings – elta or star. The phase shift that occurs isgenerally of no sig nificance in fault level calculationsas all phases are shifte equally. It is t herefore ignore . It is normal to fin elta-star transformers at the transmitti ng en of a • 55 • Equivalent Circuits an Parameters of Power System Plant • 5•
transmission system an in istribution systems for the following reasons: a. at the transmitting en , a higher step-up voltage ratiois possible than with othe r win ing arrangements, while the insulation to groun of the star secon ary win ing oes not increase by the same ratio b. in istribution systems, the star wi n ing allows a neutral connection to be ma e, which may be important in consi er ing system earthing arrangements c. the elta win ing allows circulation of zero sequencecurrents within the elta, thus preventing transmission of these from the secon ary (star) win ing into the primary circuit. This simplifies protectio n consi erations 5.14 TRANSFORMER POSITIVE SEQUENCE EQUIVALENT CIRCUITS The tran sformer is a relatively simple evice. However, the equivalent circuits for faul t calculations nee not necessarily be quite so simple, especially where earth f aults are concerne . The following two sections iscuss the equivalent circuits of various types of transformers. 5.14.1 Two-win ing Transformers The two-win in g transformer has four terminals, but in most system problems, two-terminal or t hree-terminal equivalent circuits as shown inFigure 5.10 can represent it. In F igure 5.10(a), terminals A an B are assume to be at thesame potential. Henc e if the per unit selfimpe ances of the win ings are Z11 an Z22 respectively an the mutual impe ance between them Z12, the A EB C Loa transformer may be represente by Figure 5.10(b). The circuit in Figure 5.10(b) is similar to that shown in Figure 3.14(a), an can therefore be replace by an equivalent T as shown in Figure 5.10(c) where: Z1 = Z11 − Z12 ⎫ ⎪ ⎪ Z2 = Z22 − Z12 ⎬ ⎪ Z3 = Z12 ⎪ ⎭ …Equation 5.5 Z1 is escribe as the leakage impe ance of win ing AA an Z2 the leakage impe ance of win ing BB. Impe ance Z3 is the mutual impe ance between the win ings, usually represente by XM, the magnetizing reactance parallele with the hystere sis an e y current loops asshown in Figure 5.10( ). If the secon ary of the t ransformers is short-circuite , an Z3 is assume to be large with respect to Z1 an Z2,then the short-circuit impe ance viewe from the terminals AA’ is ZT = Z1 + Z2 an the transformer can be replace by a two-terminalequivalent circuit a s shown inFigure 5.10(e). The relative magnitu es of ZT an XM are of the or er of 10% an 2000% respectively. ZT an XM rarely have to be consi ere together, so that the transformer may be represente either as a series impe ance or as a n excitation impe ance, accor ing to the problem being stu ie . A typical power transformer is illustrate in Figure 5.11. 5.14.2 Three-win ing Transformers If excitation impe ance is neglecte the equivalent circuit of a three-win ing tran sformer may in Figure 5.12, whe be represente by a star of impe ances, as shown re P,T an S are the primary, tertiary an secon ary win ings respectively. The impe ance of any of these branches can be etermine by consi ering the short-c ircuit impe ance between pairs of win ings with the thir open. S Zs Zp Equivalent Circuits an Parameters of Power System Plant • 5• A Z11 A B ~ A
Z12 Z22
B C
(a) Mo el of transformer Z1 =Z11-Z12 Z2=Z22-Z12 A B Z3=Z12 A Zero bus (c) T e quivalent circuit A A B B Zero bus (b) Equivalent circuit of mo el r2+jx2 r1+jx1 A B R jXM B Secon ary P Tertiary Primary A
Zero bus ( ) π' equivalent circuit B ZT =Z1+Z2 Zt
B' Zero bus (e) Equivalent circuit: secon ary in ing s/c Figure 5.10: Equivalent circuits for a t o in ing transformer T Zero bus Figure 5.12: Equivalent circuit for a three in ing transformer • 56 •
Netork Protection & Automation Gui e
Figure 5.11: Large transformer 5.15 TRANSFORMER ZERO SEQUENCE EQUIVALENT CIRCUITS The flo of zero sequence cur rents in a transformer is only ossible henthe transformer forms art ofa clo se loo for uni irectional currents an am ere turn balance is maintaine bet een in ings. The ositive sequence equivalent circuit is still maintaine to re resent the transformer, but no there are certain conitions attache to its co nnection into the external circuit. The or er of excitation ime ance is very mu er than for the ositive sequence circuit; it ch lo ill be roughly bet een 1 an stu ies. The m 4 er unit, but still high enough to be neglecte in most fault o e of connection of atransformer to theexternal circuit is etermine by taki ng account of each in ing arrangement an its connection or other ise to groun . If zero sequence currents can flo into an out of a in ing, the in ing term inal is connecte to the external circuit (that is, linka is close in Figure 5 .13). If zero sequence currents can circulate in the ining ithout flo ing in the external circuit, the in ing terminal is connecte irectly to the zero bus (that is, link b is close in Figure 5.13). Table 5.3 gives the zero sequence c onnections of some common to an threein ing transformer arrangements alyin g the above rules. Netork Protection & Automation Gui e The excetions to the general rule of neglecting magnetising im e ance occur he n the transformer is star/star an either or both neutrals are earthe . In these circumstances the transformer is connecte to the zero bus through the magnetis ing ime ance. Where a three hase transformer bank is arrange ithout interlin king magnetic flux (that isa three hase shell ty e, or three single hase unit s) an rovi e there is a ath for zero sequence currents, the zero sequence im eance is equal to the ositive sequence imeance. In the case of three hase coretye units, the zero sequence fluxes ro uce by zero sequence currents can fin a high reluctance ath, the effect being to re uce the zero sequence im e ance to about 90% of the ositive sequence im in han calculati e ance. Ho ever, ons, it isusual to ignore this variation an consi er the ositive an zero seq uence ime ances to be equal. It is common hen using fault soft are to erform ith the calculations to enter a value of zero sequence im e ance in accor ance above gui elines, if the manufacturer is unable to rovi e a value. • 57 •
Equivalent Circuits an Parameters of Poer System Plant • 5•
Connections an zero hase sequence currents a Zero hase sequence netork ZT b Zero bus b a a b ZT b Zero bus a a b ZT b Zero bus a
Equivalent Circuits an Parameters of Poer System Plant a b ZT b Zero bus a a b ZT b Zero bus a a b ZT b Zero bus a Zero bus ZT Zs a b Z Zero bus Zt b a a b Zs • 5• a b Z Zero bus Zt b a a b
Zs a b Z Zero bus Zt b Zs a b Z Zero bus Zt b a a b a a b Zs a b Z Zero bus Zt b a a b Table 5.3: Zero sequence equivalent circuit connections • 58 • Netork Protection & Automation Gui e
5 . 1 6 A U TO T R A N S F O R M E R S The auto transformer is characterise b y a single continuous in ing, art of hich is share by boththe high an lo in ing is the inin voltage circuits, as shon in Figure 5.14(a). The 'common' g beteen the lo voltage terminals hereas the remain erof the in ing, belong ingexclusively to the high voltage circuit, is esignate the 'series' in ing, an , combine ith the 'common' in ing, forms the 'series common' in ing bet een the high voltage terminals. The a vantage of using an auto transformer as o ose to a to ining transformer is that the auto transformer is smaller an l ighter fora given rating. The isa vantage is thatgalvanic isolation bet een t he t o in ings oes not exist, giving rise to the ossibility of largeovervolt ages on the loer voltage system in the event of major insulation break o n. Thr ee hase auto transformer banks generally have main in ings, the star connecte neutral of hich is normally connecte soli ly to earth. In a ition, it is com ractice to inclu e a thir ining connecte in elta calle the tertiary mon in ing, as shon in Figure 5.14(b). b a ZT 2 ZT 2 a b Ze b
Zero otential bus (a) To in ings a Z a Zt a b
Ze b Zero otential bus (b) Three in ings 5.16.1 Positive Sequence Equivalent Circuit The ositive sequence equivalent cir cuit of a three hase auto transformer bank is the same as that of a t o or thr ee in ing transformer. The star equivalent for a three in ing transformer, for ith the ifference that the imeance examle, is obtaine in the same manner, s beteen in ings are esignate as follos: Figure 5.13: Zero sequence equivalent circuits 1 (Z sc−c + Zc−t − Z sc−t ) ⎫ ⎪ H IH IL L L IL IL-IH N IL-IH VL IN ZN IT IH T
2 ⎪ ⎪
1 Z H = (Z sc−c + Z sc−t − Zc−t )⎬
2 ⎪ ⎪
1 ZT
H VH …Equation 5.8
where: Zsc-t = impe ance between series common an tertiary win ings Zsc-c = i mpe ance between series common an common win ings Zsc-t = impe ance between common an tertiary win ings When no loa is connecte to the elta tertiary, the point T will be open-circuite an the short-circuit impe ance of the trans former becomes ZL + ZH = Zsc-c’ , that is, similar to the equivalent circuit of a two-win ing transformer, with magnetising impe ance neglecte ; see Figure 5.14(c ). IH N IL (a) Circuit iagram L ZH IL1 IT1 ZL ZT (b)Circuit iagram with tertiary win ing H L ZX IL0 ZY ZZ IT0 Zero potential bu s ( ) Zero sequence equivalent circuit IH0 ZHT T IT0 H T H IH0 IH1
T (c) Positive sequence impe ance L IL0 ZLT ZLH
Zero potential bus (e) Equivalent circuit with isolate neutral Figure 5.14: Equivalent circuit of auto-transformer Network Protection & Automation Gui e • 59 •
Equivalent Circuits an Parameters of Power System Plant • Zs 5•
5.16.2Zero Sequence Equivalent Circuit The zero sequence equivalent circuit is erive in a similar manner to the positive sequence circuit, except that, as th ere is no i entity for the neutral point, the current in the neutral an the neu tral voltage cannot be given irectly. Furthermore, in eriving the branch impe ances, account must be taken of an impe ance in the neutral Zn, as shown in the following equations, where Zx, Zy an Zz are the impe ances of the low, high an tertiary win ings respectively an N is the ratio between the series an common win ings. ⎫ N ⎪ ( N +1) ⎪ ⎪ N ⎪ Z y = Z H −3 Zn ⎬ ( N +1)2 ⎪ ⎪ 1 ⎪ Z z = ZT +3 Zn ( N +1) ⎪ With the equivalent elta replacing the star impe ances in the auto-transformer zero sequence equivalent circuit the transformer can be combine with the system impe ances in the usual manner to obtain the system zero sequence iagram. 5.17 TRANSFORMER IMPEDANCES In the the Protecti vast majority of fault calculations, on Engineer is only concerne with the transformer leakage impe ance; the magnet ising impe ance is neglecte , as it is very much higher. Impe ances for transfor mers rate 200MVA or less are given in IEC 60076 an repeate in Table 5.4, toge ther with an in ication of X/R values (not part ofIEC 60076). These impe ances are commonly use for transformers installe in in ustrial plants. Some variatio n is possible to assist in controlling fault levels or motor starting, an typic ally up to 10% variation on the impe ance values given in the table is possible w ithout incurring a significant cost penalty. For these transformers, the tapping rangeis small, an the variation of impe ance with tap position is normally ne glecte in fault level calculations. For transformers use in electricity istri bution networks, the situation is more complex, ue to an increasing tren to as sign importance to the staning (or no-loa) losses represente by the magnetisi ng impe ance. This can be a juste at the esign stage but there is often an imp act on the leakage reactance in consequence. In a ition, it may be more importa nt to control fault levels on the LV si e than to improve motor starting voltage rops. Therefore, epartures from the IEC 60076 values are commonplace. IEC 600 76 oes not make recommen ations of nominal impe ance in respect of transformers rate over 200MVA, while generator transformers an a.c. traction supply transf ormers have impe ances that are usually specifie as a result of Power Systems S tu ies to ensure satisfactory performance. Typical values of transformer impe an ces covering a variety of transformer esigns are given in Tables 5.5 – 5.9. Where appropriate, they inclu e an in ication ofthe impe ance variation at the extre mes of the taps given. Transformers esigne to work at 60Hz will have substanti ally the same impe ance as their 50Hz counterparts. MVA <0.630 0.631-1.25 1.251 - 3.15 3.151 - 6.3 6.301-12.5 12.501- 25.0 25.001 200 >200 Z% HV/LV 4.00 5.00 6.25 7.15 8.35 10.00 12.50 X/R 1.5 3.5 6.0 8.5 13.0 20.0 45.0 by agreement Tolerance on Z% 10 10 10 10 10 7.5 7.5 Equivalent Circuits an Parameters of Power System Plant …Equation 5.9 Figure 5.14( ) shows the equivalent circuit of the transformer bank. Currents IL O an IHO are those circulating in the low an high voltage circuits respectivel y. The ifference between these currents, expresse in amperes, is the current i n the common win ing. The current in the neutral impe ance is three times the cu rrent in the common win ing. 5.16.3 Special Con itions of Neutral Earthing With a soli ly groun e neutral, Zn = O, the branch impe ances Zx, Zy , Zz, become ZL , ZH, ZT, that is, i entical to the correspon ing positive sequence equivalent c ircuit, except that the equivalent impe ance ZT of the elta tertiary is connect e to the zero potential bus in the zero sequence network. When the neutral is u ngroun e Zn = ∞ an the impe ances of the equivalent star also become infinite be cause there are apparently no paths for zero sequence currents between the win i ngs, although a physical circuit exists an ampere-turn balance can be obtaine . A solution is to use an equivalent elta circuit (see Figure 5.14(e)), an eval uate the elements of the elta irectly from the actual circuit. The metho requ
ires three equations correspon ingto three assume operating con itions. Solvin g these equations will relate the elta impe ances to the impe ance between the series an tertiary win ings, as follows: Z LH = Z s−t N 2 ⎫ (1 + N ) ⎪ ⎪ ⎪ Z LT = Z s−t N ⎬ ⎪ N ⎪ Z HT = Z s−t (1 + N ) ⎪ • 5•
Table 5.4: Transformer impe ances - IEC 60076 …Equation 5.10 • 60 • Network Protection & Automation Gui e
⎭
MVA 7.5 7.5 8 11.5 11.5 11.5 11.5 11.5 12 12 12 15 15 16 16 16 19 30 Primary kV 33 33 33 33 33 33 33 33 33 33 33 66 66 33 33 33 33 33 Primary Taps +5.72% -17.16% +5.72% -17.16% +5.72% -17.16% +5.72% -17.16% +5.72% -17.16% +5.72% -17.16% +5.72% -17.16% +4.5% -18% +5% -15% 10% 10% +9% -15% +9% -15 % 10% +5.72% -17.16% +5.72% -17.16% +5.72% -17.16% +5.72% -17.16% Secon ary kV 11 11 11 6.6 6.6 11 11 6.6 11.5 11.5 11.5 11.5 11.5 11.5 11 6.6 11 11 Z% HV/LV 7.5 7.5 8 11.5 11.5 11.5 11.5 11.5 12 12 12 15 15 16 16 16 19 30 X/R ratio 15 17 9 24 24 24 26 24 27 27 25 14 16 16 30 31 37 40 MVA 24 30 30 30 30 40 45 60 60 60 60 60 60 60 65 90 90 Primary kV 33 33 132 132 132 132 132 132 132 132 132 132 132 132 140 132 132 Primary Taps 10% 10% +10% -20% +10% -20% +10% -20% +10% -20% +10% -20% +10% -20% + 10% -20% +10% -20% +10% -20% +10% -20% +9.3% -24% +9.3% -24% +7.5% -15% +10% -20 % +10% -20% Secon ary kV 6.9 6.9 11 11 11 11 33 33 33 33 66 11/11 11/11 11/11 11 33 66 Z% HV/LV 24 24 21.3 25 23.5 27.9 11.8 16.7 17.7 14.5 11 35.5 36 35.9 12.3 24.4 1 5.1 X/R ratio 25 25 43 30 46 37 18 28 26 25 25 52 75 78 28 60 41 Table 5.5: Impe ances of two win ing istribution transformers – Primary voltage < 200kV MVA 20 20 57 74 79.2 120 125 125 180 255 Primary kV 220 230 275 345 220 275 230 230 275 230 Primary Taps +12.5% -7.5% +12.5% -7.5% 10% +14.4% -10% +10% -15% +10% -15% 16.8% n ot known 15% +10% Secon ary kV 6.9 6.9 11.8 96 11.6 34.5 66 150 66 16.5 Tertiary kV 7.2 12 11 13 Z% HV/LV 9.9 10-14 18.2 8.9 18.9 22.5 13.1 10-14 22.2 14.8 X/R ratio 18 13 34 25 35 63 52 22 38 43 MVA 95 140 141 151 167 180 180 247 250 290 307 346 420 437.8 450 600 716 721 736 900 Primary kV 132 157.5 400 236 145 289 Primary Taps 10% 10% 5% 5% +7.5% -16.5% 5% Secon ary kV 11 11.5 15 15 15 16 15 15.5 15 15 15.5 17.5 22 21 19 21 19 Z% HV/LV 13.5 12.7 14.7 13.6 25.7 13.4 13.8 15.2 28.6 15.7 15.3 16.4 16 14.6 14 16.2 15.7 15.2 15.5 15.7
X/R ratio 46 41 57 47 71 34 40 61 70 43 67 81 87 50 49 74 61 83 73 67 Table 5.6: Impe ances of two win ing istribution transformers – Primary voltage > 200kV 132 10% 432 +3.75% -16.25% 300 +11.2% -17.6% 420 10% 432 +3.75% -16.25% 435 +5% -1 5% 432 +5.55% -14.45% 144.1 +10.8% -21.6% 132 420 525 362 245 525 10% 11.25% 10% MVA 100 180 240 240 240 250 500 750 1000 1000 333.3 Primary Primary Secon ary Secon ary Tertiary Z% kV Taps kV HV/LV Taps kV 66 33 1 0.7 275 132 15% 13 15.5 13 20.2 +15% -5% 132 400 400 132 +15% -5% 13 20.0 400 132 +15% -5% 13 20.0 +15% -5% 13 10-13 132 400 14.3 22 +0% -15% 132 400 400 275 13 12.1 275 13 15.8 400 33 17.0 275 400 − 500√3 X/R ratio 28 55 83 51 61 50 51 90 89 91 101 +6.25% 13.75% 22 +7% 13% 22 +7% 13% 23 (a) Three phase units MVA/ phase 266.7 266.7 277 375 375 ±10% − 230√3
22 18.2 Table 5.8: Autotrans ormer data Primary Primary Secondary kV Taps kV 432/√3 +6.67% 13.33% 23.5 23.5 432/√3 +6.6% 1 3.4% 22 515/√3 ±5% 525/√3 +6.66% 13.32% 26 26 420/√3 +6.66% 13.32% (b) Single phase un its Z% HV/LV 15.8 15.7 16.9 15 15.1 X/R ratio 92 79 105 118 112 Table 5.7: Impedances o generator trans ormers Network Protection & Automation Guide • 61 • Equivalent Circuits and Parameters o Power System Plant • 5•
5.18 OVERHEAD LINES AND CABLES In this section a description o common overhead lines and cable systems is given, together with tables o their important charac teristics. The ormulae or calculating the characteristics are developed to giv e a basic idea o the actors involved, and to enable calculations to be made o r systems other than those tabulated. A transmission circuit may be represented by an equivalent π or T netork using lume constants as shon inFigure 5.15. Z is the total series ime ance (R + jX)L an Y is thetotal shunt a mittance (G + jB)L, here L is the circuit length. The terms insi e the brackets in Figure 5. 15 are correction factors that allo for the fact that in the actual circuit the arameters are istribute over the hole length of the circuit an not lume, as in the equivalent circuits. With short lines it is usually ossible to ignor hich greatly simlifies calculations, but on longer lin e the shunt a mittance, es it must be inclu e . Another simlification that can be ma e is that of assum ing to be symmetrical. The self ime ance of each co the con uctorconfiguration n uctor becomes Z , an the mutual im e ance R G B X G B R X beteen con uctors becomes Zm. Hoever, for rigorous calculations a etaile tre atment is necessary, ith account being taken of the s acing of a con uctor in r elation to its neighbour an earth. 5 . 1 9 C A L C U L AT I O N O F S E R IE S I M P E D A N C E The self ime ance of a con uctor ith an earth return an th e mutual ime ance beteen to arallel con uctors ith a common earth return ar e given by the Carson equations: De ⎫ c ⎪ ⎪ ⎬ …Equation 5.11 De ⎪ Zm = 0.000988 f + j0.0029 f log10 ⎪ D ⎭ Z p = R +0 f log10 Equivalent Circuits an Parameters of Power System Plant where: R = con uctor a.c. resistance (ohms/km) c = geometric mean ra ius of a s ingle con uctor D = spacing between the parallel con uctors f = system frequency De = equivalent spacing of the earth return path The above formulae give the im pe ances in ohms/km. It shoul benote that the last terms in Equation 5.11 are very similar to the classical in uctance formulae for long straightcon uctors. The geometric means ra ius (GMR) ofa con uctor is an equivalent ra ius that al lowsthe in uctance formula to be re uce to a single term. It arises because th e in uctance of a soli con uctor is a function of theinternal flux linkages in a ition to those external to it. If the original con uctor can be replace by an equivalent that is a hollow cylin er with infinitesimally thin walls, the cur rent is confine to the surface of the con uctor, an there can be no internal f lux. The geometric mean ra ius is the ra ius of the equivalent con uctor. If the original con uctor is a soli cylin er having a ra iusr its equivalent has a r a ius of 0.779r. It can be shown that the sequence impe ances for a symmetrical three-phase circuit are: = 216√p/f where p is earth resistivity (ohms/cm3) Series impe ance Z = R + jX per unit length Shunt a mittance Y = G + jB per unit length (a) Actual transmission circuit ⎛ sinh ZY Z ⎜ Y 2 ⎛ ⎜ Y 2
⎝
ZY
⎝
⎛ tanh ZY 2⎛ ⎜ ⎜ ⎝ (b) π Equivalent
ZY 2 ⎝
⎛ tanh ZY 2⎛
ZY 2 ⎝
•
⎜ ⎜ ⎝
5• Z 2 ⎛ tanh ZY 2⎛
⎜ ⎜ ⎝
ZY 2 ⎝
⎜ ⎜ ⎝
ZY 2 ⎝
Z 2 ⎛ tanh ZY 2⎛
⎛
⎜ ⎝
⎛ sinh ZY Y ⎜ ⎝ ZY (c) T Equivalent Z1 = Z2 = Z p − Zm ⎫
⎪ ⎬ Zo = Z p + 2 Zm ⎪
…Equation 5.12 Note: Z an Y in (b) an (c) respectively. Z=(R+jX)L an sinh ZY ZY tanh ZY =1+ = 1ZY where Zp an Zm are given by on 5.12 gives:
⎭
are the total series impe ance an shunt a mittance Y=(G+jB)L where L is the circuit length. 6 ZY 12 + Z2Y2 120 Z2Y2 120 + Z3Y3 5040 + ... Equation 5.11. Substituting Equation 5.11 in Equati
Z1 = Z2 = R + j0.0029 f log10 D c 3 + + 17Z3Y3 20160 + ... ZY Zo = R +0.00296 f + j0.00869 f log10 ⎫ ⎪ ⎪ ⎬ De ⎪ cD 2 ⎪ ⎭ …Equation 5.13 Figure 5.15: Transmission circuit equivalents • 62 •
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In the formula for Z0 the expression √ cD2 is the geometric mean ra ius of the con uctor group.Where the circuit is not symmetrical, the usualcase, symmetry can be maintaine by transposing the con uctors so that each conuctor is in each p hase positionfor one thir ofthe circuit length. If A, B an C are the spacing s between con uctors bc, ca an abthen D in the above equations becomesthe geo metric mean3 istance between con uctors, equal to √ABC. Writing Dc = √ cD2, the se quence impe ances in ohms/km at 50Hz become: 3 3 a D b
Con uctor Ra ius r h D Earth h 5 . 2 0 C A L C U L AT IO N O F S H U N T IMP E D A N C E It can be shown tha t the potential of a con uctor a above groun ue to its own charge qa an a cha rge -qa on its image is: a Figure 5.16 Geometry of two parallel con uctors a an b an the image of a (a ) Va =2 qaloge 2h r …Equation 5.15
where h is the height above groun of the con uctor an r is the ra ius of the c on uctor,as shownin Figure 5.16. Similarly, it can be shown that the potential of a con uctor a ue to a charge qb on a neighbouring con uctor b an the charg e -qb on its image is: Va =2 qbloge to the con uctor spacing, which is the case with overhea lines, 2h=D’. From Equat ion 5.12, the sequence impe ances of a symmetrical three-phase circuit are: Z1 = Z2 = − j0.132 log10 Zo = − j0.396 log10 3 D' D …Equation 5.16 D ⎫ r ⎪
⎪ ⎬ D
⎪ rD 2 ⎪
⎭
…Equation 5.18
where D is the spacing betweencon uctors a an b an D’ is the spacing between co n uctor b an the image of con uctor a as shown in Figure 5.14. Since the capaci tance C=q/V an the capacitive reactance Xc =1/ωC, it follos that the self an mu
tual caacitive reactance of thecon uctor system in Figure 5.16 can be obtaine irectly from Equations 5.15 an 5.16. Further, as leakage can usually be negle cte , the self an mutual shunt ime ances Z’p an Z’m in megohm-km at a system freq uency of 50Hz are: Z p = − j0.132 log10 2 h ⎫ r ⎪ ⎪ ⎬ m = − j0.132 log10 D ⎪ Z D ⎪ ⎭ It shoul be note that the logarithmic terms above aresimilar to those in Equa tion 5.13 except that r is the actual ra ius of the con uctors an D’ is the spaci ng between the con uctors an their images. Again, where the con uctors are not symmetrically space but transpose , Equation 5.18 can be re-written making use of the geometric mean istance between 3 con uctors, √ABC, an giving the istance of each con uctor above groun , that is, ha , h2 , hc , as follows: ABC ⎫ ⎪ r ⎪ ⎬ 8 ha hbhb ⎪ Z0 = − j0.132 log10 r 3 A 2 B 2 C 2 ⎪ …Equation 5.19 ⎭ Z1 3 …Equation 5.17 Where the distances above ground are great in relation Network Protection & Automation Guide • 63 • Equivalent Circuits and Parameters o Power System Plant • ABC ⎫ Z1 = Z 2 = R + j0.145 log10 ⎪ c ⎪ ⎬ D Zo = ( R +0.148 ) + j0.434 log10 e ⎪ 4 ⎭ 3 5•
Dc ⎪
3.80 A a B a C 0.50 A 6.0 U n (kV) 3.3 6.6 11 22 33 a (m) 0.55 0.67 0.8 1 1.25 A A=3.5m R2 R1 W X Y Single circuit Un= 63kV/66kV/90kV Single circuit Un= 90kV Equivalent Circuits an Parameters of Power System Plant Single circuit 1.75 - K 2.00 - N c a b 3.30 6.6 2 2 a 2.70 Un(kV) a (m) 4.00 2.50 3.30
2.50 a a 63 kV(K) 3.0 b 3.7 3.8 c 3.0 1.4 3.50 2.8 3.5 2.8 3.5 a U n (kV) a (m) 3.50 R1 W Y Single circuit Un= 63kV/90kV 63 90 1.4 1.85 R1 W Y 90 kV (N) 3.1
3.8 1.85 63 66 1.40 1.40 1.85 3.0 3.0 90 Double circuit Un= 63kV/90kV Double circuit Un= 63kV/66kV/90kV • 5• 6.60 2 2 2.75 2.75 3.10 3.4 4.1 3.7 a 3.9 b a a=3.7m b=4.6m 6.20 3.9 5.80 4.2 8.0 8.0 4.2 R1 W Y Single circuit Un= 110kV R1 W Y Double circuit Un= 138kV Double circuit Un= 170kV Figure 5.17: Typical OHL configurations (not to scale) • 64 • Network Protection & Automation Gui e
8.45 12.2 1.75 5.0 5.0 16.4 2.5 a b c
7.5 a b c
5.20 p 7.50 n1 n2 n n 9.5 9.8 n1 n2 p 6.3 6.3 6.0 6.0 R1 W X Y R2 R1 W X Single circuit Un= 245kV A 3.5 3.8 4.1 2.8 B 4.2 4.5 4.8 2.8 C 4.2 4.5 4.8 2.8 5.0 4.5 5.0 4.8 Double circuit Un= 245kV Double circuit Un= 245kV 9.74 25.1 7.0 8.5 6.7 11.3 8.5 7.7 R1 W X 8.5 7.4 8.5 7.8 7.8 7.4 6.7 2.40 9.2 32.4 R1 W X Single circuit Un= 245kV Double circuit Un= 420kV Double circuit Un= 420kV 7.5 0 20.0 0 10.0 8.0 9.5 9.5 9.5 16.0 23.0 8.0 5.0 0 0 0 12.0 12.0 37.0 Single circuit Un= 550kV Double circuit Un= 550kV Single circuit Un= 800kV Figure 5.17(cont): Typical OHL configurations (not to scale)
Network Protection & Automation Gui e • 65 • Equivalent Circuits an Parameters of Power System Plant • 5•
5 . 21 O V E R H E A D L I NE C I R C U I T S WITH OR WITHOUT EARTH WIRES Typic al configurations of overhea line circuits are given in Figure 5.17. Towerheig hts are not given as they vary consi erably accor ing to the esign span an nat ure ofthe groun . As in icate in some of the tower outlines, some tower esign s are esigne with a number of base extensions for this purpose. Figure 5.18 sh ows a typical tower. In some cases, the phase con uctors are not symmetrically ispose to each other an therefore, as previously in icate , electrostatic an electromagnetic unbal ance will result, which can be largely eliminate by transposition. Mo ern pract ice is to buil overhea lines without transposition towers to re uce costs; thi s must be taken into account in rigorous calculations of theunbalances. In othe r cases, lines are forme of bun le con uctors, that is con uctors forme of tw o, three or four separate con uctors. minimises losses when vol This arrangement tages of220kV an above are involve . It shoul be note that the line configur ation an con uctor spacings are influence , not only by voltage, but also by ma ny other factors inclu ing type of insulators, type of support, span length, con uctor sag an the nature of terrain an external climatic loa ings. Therefore, there can be large variations in spacings between ifferent line esigns for the same voltage level, so those epicte in Figure5.17 are only typical examples. When calculating the phase self an mutual impe ances, Equations 5.11 an 5.17 may be use in this case Zp is calculate for , but it shoul be remembere that each conuctor an Zm for each pair of con uctors. This section is not, therefor e, inten e to give a etaile analysis, but rather to show the general metho o f formulating the equations, taking the calculation of series impe ance as an ex amplean assuming a single circuit line with a single earth wire. Thephase vol tage rops Va,Vb,Vb of a single circuit line with a single earth wire ue to cur rents Ia, Ib, Ib flowing in the phases an Ie in the earth wire are: Equivalent Circuits an Parameters of Power System Plant • 5• where: Va = Zaa I a + Zab I b + Zac I c + Zae I e ⎫ ⎪ Vb = Zba I a + Zbb I b + Zbc I c + Zbe I e ⎪ ⎬ Vc = Zca I a + Zcb I b + Zcc I c + Z ce I e ⎪ ⎪ 0 = Zea I a + Zeb I b + Z ec I ee I e ⎭ …Equation 5.20 Zaa = R +0.000988 f + j0.0029 f log10 Zab = 0.000988 f + j0.0029 f log10 De D De c an so on. The equation require for the calculation of shunt voltage rops is i entical to Equation 5.20 in form, except that primes must be inclu e , the impe ances being erive from Equation 5.17. Figure 5.18: Typical overhea line tower • 66 •
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Sequence impe ance Z00 = (Z0’0’) Z11 = Z22 = (Z1’1’) (Z0’0 =Z00’) Z01 = Z20 = (Z0’1’ = Z2’0 Z10 = (Z0’2’ = Z1’0’) Z12 = (Z1’2’) Z21 = (Z2’1’) (Z11’=Z1’1 = Z22’ = Z2’2) (Z02’ = Z0’2 = = Z1’0 = Z10’ (Z1’2 = Z12’) (Z21’ = Z2’1) 132kV Single circuit line (400 mm2) 1.0782 .0256 -139°1’ -
73°54’ 0.3947
78°54’ 0.0116
-166°52’
5°8
380kV Single circuit line (400 mm2) 0.8227 275 147°26’ 0.0275 27°29’ -
70°36’ 0.3712
75°57’ 0.0094
-39°28’ 0.0153
132kV Double circuit line (200 mm2) 1.1838 71°6’ 66°19’ 0.6334 71°2’ 0.0257 -63°25’ .0276 161°17’ 0.0277 37°13’ 0.0114 88°6’ 0.0140 -93°44’ 0.0150 -44°11’ 0.0103 1
275kV Double circuit line (400 mm2) 0.9520 76°46’ 0.3354 74°35’ 0.5219 75°43’ 0.0241 217 -100°20’ 0.0281 149°46’ 0.0282 29°6’ 0.0129 88°44’ 0.0185 -91°16’ 0.0173 -7 1’ From Equation 5.20 it can be seen that: −Ie = Z Z Zea I a + eb I b + ec I c Zee Zee Zee Making use o this relation, the sel and mutual impedances o the phase conduct ors can be modi ied using the ollowing ormula: J nm = Znm − For example: J aa = Zaa − Zne Zme Zee …Equation 5.21 2 Zae Zee J ab = Zab − and so on. Zae Zbe Zee So Equation 5.20 can be simpli ied while still taking account o the e ect o t he earth wire by deleting the ourth row and ourth column and substituting Jaa or Zaa, Jab or Zab , and so on, calculated using Equation 5.21. The single cir cuit line with a single earth wire can there ore be replaced by an equivalent si ngle circuit line having phase sel and mutual impedances Jaa , Jab and so on. I t can be shown rom the symmetrical component theory given in Chapter 4 that the sequence voltage drops o a general three phase circuit are:
⎫ ⎪ ⎪ 1 1 ⎪ Z11 = ( J aa + J bb + J cc ) − ( J ab + J bc + J ac ) ⎪ 3 3 ⎪ 1 2 2 J + a ⎪ Z12 = ( J aa + a bb (aJ ab + a ac bc ⎪ cc 3 3 ⎪ 1 2 2 2 J )+ Z21 = ( J aa + aJ bb + a cc (a J ab + aJ ac + J bc ) ⎪ ⎪ 3 3 ⎪ ⎪ 1 1 2 J + aJ ) − ( aJ + a 2 J + J ) Z20 = ( J a b cc ab ac bc ⎬ 3 3 ⎪ ⎪ 1 1 2 Z10 = ( J aa + aJ bb + a 2 J cc ) − (a J ab + aJ ac + Jbc ⎪ ⎪ Z22 = Z11 ⎪ ⎪ Z01 = Z20 ⎪ Z02 = Z10 ⎪ ⎪ ⎪ …Equation 5.23 ⎪ ⎭ The evelopment circuit lines with two earth wires is similar except that more terms are involve . Z00 = 1 ( J aa + J bb + J cc ) + 2 ( J ab + J bc + J ac ) 3 3 V0 = Z00 I 0 + Z01 I1 + Z02 I 2 ⎫
⎪
⎪ V1 = Z10 I 0 + Z11 I1 + Z12 I 2 ⎬ ⎪
V2 = Z20 I 0
+ Z22 I 2 ⎭
⎪
The sequence mutual impe ances are very small an can usually be neglecte ; this also applies for ouble circuit lines except for the mutual impe ance between t he zero sequence circuits, namely(ZOO’ = ZO’O). Table5.10 gives typical values of all sequence self an mutual impe ances some single an ouble circuit lines wit h earth wires. All con uctors are 400mm2 ACSR, except for the 132kV ouble circu it example where they are 200mm2. 5.22 OHL EQUIVALENT CIRCUITS Consi er an earth e , infinite busbar source behin a length of transmission line as shown in Figu re 5.19(a). An earth fault involving phase A is assume to occur at F. If the r iving voltage is E an the fault current is Ia • 67 • …Equation 5.22 An , from Equation 5.20 mo ifie as in icate above an Equation 5.22, the seque nce impe ances are: Network Protection & Automation Gui e Equivalent Circuits an Parameters of Power System Plant • Table 5.10: Sequence self an mutual impe ances for various lines 5•
Source Line F ~ ~ ~ C istance relay the phase an earth fault relays are set to applications because measure Z2 an are compensate for the earth return impe ance (Z0-Z1)/3. It is c ustomary to quote the impe ances of a transmission circuit in terms of Z1 an th e ratio Z0/Z1 , since in this form they are most irectly useful. By efinition, the positive sequence impe ance Z1 is a function of the con uctor spacing an r a ius, whereas the Z0/Z1 ratio is epen ent primarily on the level of earth resi stivity ρ. Fu the details may be found in Chapte 12. 5.23 CABLE CIRCUITS B A E (a) Actual ci cuit S Ic Ib Z1 Z1 F C Equivalent Ci cuits and Pa amete s of Powe System Plant 3E B Ia Z1 (Z0-Z )/3 (b) Equivalent ci cuit A E Figu e 5.19: Th ee-phase equivalent of a t ansmission ci cuit then the ea th fault impedance is Ze . F om symmet ical component theo y (see Ch apte 4): 3E Ia = Z1 + Z2 + Z0 thus
The basic fo mulae fo calculating the se ies and shunt impedances of a t ansmis sion ci cuit, Equations 5.11 and 5.17 may be applied fo evaluating cable pa ame te s; since the conducto configu ation is no mally symmet ical GMD and GMR valu es can be used without isk of app eciable e o s. Howeve , the fo mulae must be modified by the inclusion of empi ical facto s to take account of sheath and sc een effects. A useful gene al efe ence on cable fo mulae is given in efe ence [5.4]; mo e detailed info mation on pa ticula types of cables should be obtain ed di ect f om the manufactu e s. The equivalent ci cuit fo dete mining the pos itive and negative sequence se ies impedances of a cable is shown in Figu e 5.20 . F om this ci cuit it can be shown that: ⎧ X 2 ⎫ Z1 = Z2 = ⎨ Rc + Rs 2 cs 2 ⎬ Rs + X s ⎭ ⎩ ⎧ X 2 ⎫ + j ⎨ X c − X s 2 cs 2 ⎬ 2 Z +Z Ze = 1 0 3 …Equation 5.24
• 5• since, as shown, Z1 = Z2 for a transmission circuit. From Equations 5.12, Z1=ZpZm an ZO=Zp+2Zm. Thus, substituting these values in the above equation gives Ze =Zp. This relation is physically vali because Zp is the self-impe ance of a sin gle con uctor with an earth return. Similarly, for a phase fault between phases B an C at F: 3E Ib = −Ic = 2 Z1 _ where √3E is the voltage between phases and 2Z is the impedance o the ault loop.
where Rc, Rs are the core and sheath (screen) resistances per unit length, Xc an d Xs core and sheath (screen) reactances per unit length and Xcs the mutual reac tance between core and sheath (screen) per unit length. Xcs is in general equal to Xs. The zero sequence series impedances are obtained directly using Equation 5.11 and account can be taken o the sheath in the same way as an earth wire in the case o an overhead line. The shunt capacitances o a sheathedcable can be calculated rom the simple ormula: ⎧ ⎪ 1 C = 0.0241ε ⎨ ⎪ log + 2 T ⎩ ⎫ ⎪ ⎬ µF / Making use of the above relations a transmission circuit may be represente , wit hout any loss in generality, by the equivalent of Figure5.19(b), where Z1 is th e phase impe ance to the fault an (Z0-Z1)/3 is the impe ance of the earth path, there being no mutualimpe ance between the phases or between phase an earth. The equivalent is vali for single an ouble circuit lines except that for oub le circuit lines there is zero sequence mutual impe ance, hence Z0=(Z00-Z0’0). The equivalent circuit of Figure 5.19(b) is valuable in • 68 • …Equation 5.25 where is the overall iameter for a roun con uctor, T core insulation thickne ss an ε p rmittivity of di l ctric. Wh n th conductors ar oval or shap d, an q uival nt N twork Prot ction & Automation Guid
diam t r d’ may b us d wh r d’=(1/π) x eri hery of con uctor. No sim le formula exi sts for belte or unscreene cables, but an em irical formula that gives reasona ble results is: C= 0.0555 ε µF / km G …Equation 5.26 wh r G is a g om tric factor which is a function of cor and b lt insulation th ickn ss and ov rall conductor diam t r. 5 . 2 4 O V E R H E A D L I N E A N D C A B L E D ATA Th following tabl s contain typical data on ov rh ad lin s and ca bl s that can b us d in conjunction with th various quations quot d in this t xt. It is not int nd d that this data should r plac that suppli d by manufactu r rs. Wh r th r sults of calculations ar important, r lianc should not b pl ac d on th data in th s Tabl s and data should b sourc d dir ctly from a manu factur r/suppli r. At th conc ptual d sign stag , initial s l ction of ov rh ad lin conductor siz will b d t rmin d by four factors: a. maximum load to b c arri d in MVA b. l ngth of lin c. conductor mat rial and h nc maximum t mp rat ur d. cost of loss s Tabl 5.21 giv s indicativ d tails of th capability of v arious siz s of ov rh ad lin s using th abov factors, for AAAC and ACSR conduc tor mat rials. It is bas d on commonly us d standards for voltag drop and ambi nt t mp ratur . Sinc th s factors may not b appropriat for any particular pr oj ct, th Tabl should only b us d as a guid for initial sizing, with appropr iat ly d tail d calculations carri d out to arriv at a final proposal. Numb r of Strands 7 19 37 61 91 127 169 Solid GMR 0.726r 0.758r 0.768r 0.772r 0.774r 0.776r 0.776r 0.779r Tabl 5.11: GMR for strand d copp r, aluminium and aluminium alloy conductors (r = conductor radius) * - Indicativ valu s only, sinc GMR for singl lay r conductors is aff ct d by cyclic magn tic flux, which d p nds on various factors. Tabl 5.12: GMR for aluminium conductor st l r inforc d (ACSR) (r = conductor r adius) Sh ath circuit (s) Xcs P r unit l ngth Ic Cor circuit (c) Is V Rs Xs P r unit l ngth Rc Xc P r unit l ngth V is voltag p r unit l ngth Figur 5.20: Equival nt circuit for d t rmining positiv or n gativ imp danc o f cabl s N twork Prot ction & Automation Guid • 69 • Equival nt Circuits and Param t rs of Pow r Syst m Plant • Numb r of Lay rs 1 1 2 2 2 2 2 3 3 3 3 3 4 4 4
Numb r of Al Strands 6 12 18 24 26 30 32 36 45 48 54 66 72 76 84 GMR 0.5r* 0.75r* 0.776r 0.803r 0.812r 0.826r 0.833r 0.778r 0.794r 0.799r 0.81r 0 .827r 0.789r 0.793r 0.801r 5•
Stranding ar a (mm2) 10.6 21.2 26.7 33.6 42.4 53.5 67.4 85.0 107.2 126.6 152.0 1 77.3 202.7 228.0 253.3 278.7 304.3 329.3 354.7 380.0 405.3 456.0 506.7 Wir 7 7 7 7 7 7 7 7 7 19 19 19 19 37 37 37 37 61 61 61 61 61 61 Ov rall Diam t r (mm) 1.38 1.96 2.20 7.00 2.77 3.12 3.50 3.93 4.42 2.91 3.19 3.4 5 3.69 2.80 2.95 3.10 3.23 2.62 2.72 2.82 2.91 3.09 3.25 (a) ASTM Standards RDC Diam t r (mm) 4.17 5.89 6.60 7.42 8.33 9.35 10.52 11.79 13.26 14.58 15.98 17 .25 18.44 19.61 20.65 21.67 22.63 23.60 24.49 25.35 26.19 27.79 29.26 (20°C) (Ohm/km) 1.734 0.865 0.686 0.544 0.431 0.342 0.271 0.215 0.171 0.144 0.120 0.103 0.090 0.080 0.072 0.066 0.060 0.056 0.052 0.048 0.045 0.040 0.036 D signation Stranding and wir diam t r (mm) Aluminium St l 1 1 1 1 1 1 7 7 1 7 7 7 19 1 7 7 7 19 7 19 7 2.67 3 3.37 3.78 4.25 4.77 2 2.21 3.47 2.92 2.67 2.89 2.16 3.38 2. 25 3.28 3.52 2.29 2.85 2.62 2.94 Total Approx. RDC ar a ov rall at 20 °C 2 (mm ) diam t r (Ohm/km) Aluminium St l (mm) 33.6 42.4 53.5 67.4 85.0 107.2 135.2 152.0 170.5 201.4 241.7 282.0 306.6 32 2.3 402.8 456.1 523.7 565.0 644.5 805.7 1100.0 5.6 7.1 8.9 11.2 14.2 17.9 22.0 2 6.9 9.5 46.9 39.2 45.9 69.6 9.0 27.8 59.1 68.1 78.3 44.7 102.4 47.5 39.2 49.5 62 .4 78.6 99.2 125.1 157.2 178.9 179.9 248.3 280.9 327.9 376.2 331.2 430.7 515.2 5 91.8 643.3 689.2 908.1 1147.5 8.01 9 10.11 11.34 12.75 14.31 16.28 17.28 17.35 2 0.44 21.79 23.55 25.24 23.62 27.03 29.52 31.68 33.35 34.17 39.26 44.07 0.854 0.6 77 0.536 0.426 0.337 0.268 0.214 0.191 0.169 0.144 0.120 0.103 0.095 0.089 0.072 0.064 0.055 0.051 0.045 0.036 0.027 S ctional ar a (mm2) Equival nt Circuits and Param t rs of Pow r Syst m Plant Stranding ar a (mm2) 11.0 13.0 14.0 14.5 16.1 18.9 23.4 32.2 38.4 47.7 65.6 70.1 97.7 129.5 132.1 164.0 165.2 Wir 1 1 1 7 1 1 1 1 7 7 7 1 7 19 7 7 19 Ov rall Diam t r (mm) 3.73 4.06 4.22 1.63 4.52 4.90 5.46 6.40 2.64 2.95 3.45 9.4 5 4.22 2.95 4.90 5.46 3.33 (b) BS Standards RDC Diam t r (mm) 3.25 4.06 4.22 4.88 4.52 4.90 5.46 6.40 7.92 8.84 10.36 9.45 1 2.65 14.73 14.71 16.38 16.64 (20°C) (Ohm/km) 1.617 1.365 1.269 1.231 1.103 0.938 0.756 0.549 0.466 0.375 0.273 0.252 0.183 0.139 0.135 0.109 0.109 Sparrow Robin Rav n Quail Pig on P nguin Partridg Ostrich M rlin Lark Hawk Dov T al Swift T rn Canary Curl w Finch Bitt rn Falcon Kiwi 6 6 6 6 6 6 26 26 18 30 26 26 30 36 45 54 54 54 45 54 72 2.67 3 3.37 3.78 4.25 4.77 2.57 2.73 3.47 2.92 3.44 3.72 3.61 3.38 3.38 3.28 3.5 2 3.65 4.27 4.36 4.41 (a) to ASTM B232 D signation Goph r W as l F rr t Rabbit Hors Dog Tig r Wolf Dingo Lynx Caracal Jaguar Panth r Z bra
Stranding and wir diam t r (mm) Aluminium 6 2.36 6 2.59 6 3 6 3.35 12 2.79 6 4. 72 30 2.36 30 2.59 18 3.35 30 2.79 18 3.61 18 3.86 30 3 54 3.18 1 1 1 1 7 7 7 7 1 7 1 1 7 7 • 5• Tabl 5.13: Ov rh ad lin conductor - hard drawn copp r Total Approx. RDC ar a ov rall at 20 °C 2 (mm ) diam t r (Ohm/km) St l Aluminium St l (mm) 2.36 26.2 4.4 30.6 7.08 1.093 2.59 31.6 5.3 36.9 7.77 0.908 3 42.4 7. 1 49.5 9 0.676 3.35 52.9 8.8 61.7 10.05 0.542 2.79 73.4 42.8 116.2 13.95 0.393 1 .57 105.0 13.6 118.5 14.15 0.273 2.36 131.2 30.6 161.9 16.52 0.220 2.59 158.1 36 .9 194.9 18.13 0.182 3.35 158.7 8.8 167.5 16.75 0.181 2.79 183.4 42.8 226.2 19.5 3 0.157 3.61 184.2 10.2 194.5 18.05 0.156 3.86 210.6 11.7 222.3 19.3 0.137 3 212 .1 49.5 261.5 21 0.136 3.18 428.9 55.6 484.5 28.62 0.067 (b) to BS 215.2 S ctional ar a (mm2) Tabl 5.14: Ov rh ad lin conductor data - aluminium conductors st l r inforc d (ACSR). • 70 • N twork Prot ction & Automation Guid
D signation 35/6 44/32 50/8 70/12 95/15 95/55 120/70 150/25 170/40 185/30 210/50 265/35 305/40 380/50 550/70 560/50 650/45 1045/45 Stranding and wir diam t r (mm) Aluminium 6 2.7 14 2 6 3.2 26 1.85 26 2.15 12 3 .2 12 3.6 26 2.7 30 2.7 26 3 30 3 24 3.74 54 2.68 54 3 54 3.6 48 3.86 45 72 4.3 4.3 1 7 1 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 19 19 19 37 37 37 37 37 37 37 37 3.686 3.909 4.12 3.096 3.233 3.366 3.493 3.617 3.734 3.962 4.176 202.7 228.0 253.3 278.5 303.7 329.2 354.6 380.2 405.2 456.2 506.8 18.4 19.6 20.6 21.7 22.6 23.6 24.5 25.3 26.1 27.7 29.2 0.165 0.147 0.132 0.120 0.110 0.102 0.094 0.088 0.083 0.073 0.066 2.87 2.87 653.5 1045.6 45.3 698.8 45.3 1090.9 34.4 43 0.044 0.028 (c) to DIN 48204 Total Approxi. RDC ar a ov rall at 20 °C 2 (mm ) diam t r (Ohm/km ) Aluminium St l Aluminium St l (mm) 12 2 7 2 37.7 22.0 59.7 10 0.765 12 2.25 7 2.25 47.7 27.8 75.5 11.25 0.604 12 30 30 30 30 30 36 30 30 32 66 2.5 2 2 2.25 2.5 2.8 2.8 3.15 3.15 3.6 3.13 7 7 7 7 7 7 19 7 7 19 19 19 2.5 2 2 2.25 2.5 2.8 2.25 3.15 3.15 2.4 2.65 3.15 58.9 94.2 94.2 119.3 147.3 184.7 221.7 233.8 233.8 325.7 507.8 717.3 34.4 22.0 22.0 27.8 34.4 43.1 75.5 54.6 54.6 86.0 104.8 93.3 1 16.2 116.2 147.1 181.6 227.8 297.2 288.3 288.3 411.7 612.6 12.5 14 14 15.75 17.5 19.6 22.45 22.05 22.05 26.4 32.03 38.01 0.489 0.306 0.306 0.243 0.197 0.157 0.1 31 0.124 0.124 0.089 0.057 0.040 Stranding and wir diam t r (mm) S ctional ar a (mm2) D signation CANNA 59.7 CANNA 75.5 CANNA 93.3 CANNA 116.2 CROCUS 116.2 CANNA 147. 1 CROCUS 181.6 CROCUS 228 CROCUS 297 CANNA 288 CROCUS 288 CROCUS 412 CROCUS 612 CROCUS 865 ASTM B-399 ASTM B-399 (a) ASTM Standard BS 3242 BS 3242 BS 3242 BS 3242 BS 3242 BS 3242 BS 3242 BS 324 2 BS 3242 BS 3242 BS 3242 BS 3242 BS 3242 BS 3242 BS 3242 BS 3242 BS 3242 BS 324 2 BS 3242 BS 3242 BS 3242 D signation Box Acacia Almond C dar Fir Haz l Pin Wil low Oak Mullb rry Ash Elm Poplar Sycamor Upas Y w Totara Rubus Araucaria No. Wi r S ctional Ov rall RDC of Al diam t r ar a diam t r at 20°C Strands (mm) (mm2) ( mm) (Ohm/km) 7 7 7 7 7 7 7 7 7 7 7 19 19 19 37 37 37 37 37 61 61 1.85 2.08 2.34 2.54 2.95 3.3 3.61 4.04 4.19 4.45 4.65 3.18 3.48 3.76 2.87 3.23 3.53 4.06 4.14 3 .5 4.14 (b) BS Tabl 5.15: Ov rh ad lin conductor data - aluminium alloy. 18.8 23.8 30.1 35.5 47.8 59.9 71.6 89.7 96.5 108.9 118.9 150.9 180.7 211.0 239.4 303. 2 362.1 479.0 498.1 586.9 821.1 5.6 6.2 7.0 7.6 8.9 9.9 10.8 12.1 12.6 13.4 14.0 15.9 17.4 18.8 20.1 22.6 24.7 28.4 29.0 31.5 28.4 1.750 1.384 1.094 0.928 0.688 0.550 0.460 0.367 0.341 0.302 0.277 0.219 0.183 0.157 0.139 0.109 0.092 0.069 0 .067 0.057 0.040
66 3.72 148.1 865.4 (d) to NF C34-120 Tabl 5.14: Ov rh ad lin conductor data - aluminium conductor s st l r inforc d (ACSR). N twork Prot ction & Automation Guid • 71 • Equival nt Circuits and Param t rs of Pow r Syst m Plant • Total Approx. RDC ar a ov rall at 20 °C (mm2) diam t r (Ohm/km) St l Aluminium St l (mm) 2.7 34.4 5.7 40.1 8.1 0.834 2.4 44.0 31.7 75.6 11.2 0.652 3.2 48.3 8.0 56.3 9.6 0.594 1.44 69.9 11.4 81.3 11.7 0.413 1.67 94.4 15.3 109.7 13.6 0.305 3. 2 96.5 56.3 152.8 16 0.299 3.6 122.1 71.3 193.4 18 0.236 2.1 148.9 24.2 173.1 17 .1 0.194 2.7 171.8 40.1 211.8 18.9 0.168 2.33 183.8 29.8 213.6 19 0.157 3 212.1 49.5 261.5 21 0.136 2.49 263.7 34.1 297.7 22.4 0.109 2.68 304.6 39.5 344.1 24.1 0.095 3 381.7 49.5 431.2 27 0.076 3.6 549.7 71.3 620.9 32.4 0.052 3 561.7 49.5 6 11.2 32.2 0.051 S ctional ar a (mm2) Standard ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-397 ASTM B-3 97 ASTM B-399 ASTM B-399 ASTM B-399 ASTM B-399 ASTM B-399 ASTM B-399 ASTM B-399 ASTM B-399 ASTM B-399 D signation K nch Kib Kayak Kop ck Kittl Radian R d Ragout R x R m x Rubl Ru n Spar Solar No. Wir S ctional Ov rall RDC of Al diam t r ar a diam t r at 20°C Strands (mm) ( mm2) (mm) (Ohm/km) 7 7 7 7 7 19 19 19 19 19 19 19 37 37 2.67 3.37 3.78 4.25 4.77 3.66 3.78 3.98 4.14 4.36 4.46 4.7 3.6 4.02 39.2 62.4 78.6 99.3 125.1 199.9 212. 6 236.4 255.8 283.7 296.8 330.6 376.6 469.6 8.0 10.1 11.4 12.8 14.3 18.3 18.9 19 .9 19.9 21.8 22.4 23.6 25.2 28.2 0.838 0.526 0.418 0.331 0.262 0.164 0.155 0.140 0.129 0.116 0.111 0.100 0.087 0.070 5•
Standard CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 C SA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M 87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49 .1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 CSA C49.1-M87 D sign. 10 16 25 40 63 100 125 160 200 250 315 400 450 500 560 630 710 800 900 1 000 1120 1250 1400 1500 No. Wir S ctional Ov rall RDC of Al diam t r ar a diam t r at 20°C Strands (mm) ( mm2) (mm) (Ohm/km) 7 7 7 7 7 19 19 19 19 19 37 37 37 37 37 61 61 61 61 91 91 91 91 91 1.45 1.83 2.29 2.89 3.63 2.78 3.1 3.51 3.93 4.39 3.53 3.98 4.22 4.45 4.71 3.89 4.13 4.38 4.65 4.01 4.25 4.49 4.75 4.91 (c) CSA No. of Wir S ctional Ov ra ll RDC Al diam t r ar a diam t r at 20°C 2 Strands (mm) (mm ) (mm) (Ohm/km) 7 7 7 19 7 19 19 19 37 37 61 61 61 61 1.7 2.1 2.5 1.8 3 2.1 2.5 2.8 2.25 2.5 2.25 2.5 2.89 3.23 (d) DIN 15.9 24.3 34.4 48.4 49.5 65.8 93.3 117.0 147.1 181.6 242.5 299 .4 400.1 499.8 5.1 6.3 7.5 9.0 9.0 10.5 12.5 14.0 15.7 17.5 20.2 22.5 26.0 29.1 2.091 1.370 0.967 0.690 0.672 0.507 0.358 0.285 0.228 0.184 0.138 0.112 0.084 0. 067 11.5 18.4 28.8 46.0 72.5 115.1 143.9 184.2 230.2 287.7 362.1 460.4 517.9 575 .5 644.5 725.0 817.2 920.8 1035.8 1150.9 1289.1 1438.7 1611.3 1726.4 4.3 5.5 6.9 8.7 10.9 13.9 15.5 17.6 19.6 22.0 24.7 27.9 29.6 31.2 33.0 35.0 37.2 39.5 41.9 44.1 46.7 49.4 52.2 54.1 2.863 1.788 1.142 0.716 0.454 0.287 0.230 0.180 0.144 0 .115 0.092 0.072 0.064 0.058 0.051 0.046 0.041 0.036 0.032 0.029 0.026 0.023 0.0 21 0.019 Standard NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C3 4-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-1 25 NF C34-125 D signation ASTER 22 ASTER 34-4 ASTER 54-6 ASTER 75-5 ASTER 93,3 ASTER 117 ASTER 148 ASTER 181-6 ASTER 228 ASTER 288 ASTER 366 ASTER 570 ASTER 851 ASTER 1144 AS TER 1600 No. of Wir S ctional Ov rall RDC Al diam t r ar a diam t r at 20°C Strands (mm) ( mm2) (mm) (Ohm/km) 7 7 7 19 19 19 19 37 37 37 37 61 91 91 127 ( ) NF 2 2.5 3.15 2.25 2.5 2.8 3.15 2.5 2.8 3.15 3.55 3.45 3.45 4 4 22.0 34.4 54.6 75.5 93.3 117.0 148.1 181.6 227.8 288.3 366.2 570.2 850.7 1143.5 1595.9 6.0 7.5 9.5 11.3 12.5 1 4.0 15.8 17.5 19.6 22.1 24.9 31.1 38.0 44.0 52.0 1.497 0.958 0.604 0.438 0.355 0 .283 0.223 0.183 0.146 0.115 0.091 0.058 0.039 0.029 0.021 Equival nt Circuits and Param t rs of Pow r Syst m Plant Tabl 5.15 (cont): Ov rh ad lin conductor data - aluminium alloy. Standard DIN 48201 DIN 48201 DIN 48201 DIN 48201 DIN 48201 DIN 48201 DIN 48201 D IN 48201 DIN 48201 DIN 48201 DIN 48201 DIN 48201 DIN 48201 DIN 48201 D signation 16 25 35 50 50 70 95 120 150 185 240 300 400 500 • 5• • 72 • N twork Prot ction & Automation Guid
Standard D signation Alloy Stranding and wir diam t r (mm) St l 2.62 2.97 2.76 3.13 3.08 3.5 3.7 3.66 3.8 8 4.12 3.26 3.63 3.85 4.34 4.12 4.35 7 7 7 7 7 7 7 19 19 19 19 19 19 19 19 19 2. 04 2.31 2.76 2.43 3.08 2.72 2.88 2.2 2.33 2.47 1.98 2.18 2.31 2.6 2.47 2.61 (a) ASTM Stranding and wir diam t r (mm) Alloy St l 1.85 2.15 2.33 2.7 2.7 3 3 3.5 3.74 2.68 3 3.45 3.86 4 7 7 7 7 7 7 7 7 7 7 7 7 7 19 (b) DIN Stranding and wir diam t r (mm) Alloy St l 2 2.25 2.25 2.5 2.5 2.8 2.8 3.15 3.15 2.5 2.8 19 19 7 19 7 19 7 19 7 19 37 2 2.25 2.25 2.5 2.5 2.8 2.8 3.15 3.15 2.5 2.8 (c) NF 1.44 1.67 2.33 2.1 2.7 2.33 3 2.33 2.49 2.68 3 2.68 3 2.4 S ctional ar a (mm2) Alloy 140.2 180.1 179.5 200.1 223.5 250.1 279.6 315.6 354.7 399.9 450.7 558.9 628.6 798.8 1119.9 1248.4 St l 22.9 29.3 41.9 32.5 52.2 40.7 45.6 72.2 81.0 91.0 58.5 70.9 79.6 100.9 91.0 101.7 Total ar a (mm2) 163.1 209.5 221.4 232.5 275.7 290.8 325.2 387.9 435.7 491.0 509 .2 629.8 708.3 899.7 1210.9 1350.0 Approximat ov rall diam t r (mm) 7.08 11.08 12.08 13.08 16.08 17.08 19.08 22.08 24.08 26.08 27.08 29.08 30.08 32.08 35.08 36.08 RDC at 20 °C (ohm/km) 0.240 0.187 0.188 0.168 0.151 0.135 0.120 0.107 0.095 0.084 0.075 0.060 0.054 0.042 0.030 0.027 Standard D signation S ctional ar a (mm2) Alloy 69.9 94.4 127.9 148.9 171.8 183.8 212.1 230.9 263.7 3 04.6 381.7 448.7 561.7 678.6 St l 11.4 15.3 29.8 24.2 40.1 29.8 49.5 29.8 34.1 39.5 49.5 39.5 49.5 86.0 Total ar a (mm2) 81.3 109.7 157.8 173.1 211.8 213.6 261.5 260.8 297.7 344.1 431. 2 488.2 611.2 764.5 Approximat ov rall diam t r (mm) 11.7 13.6 16.3 17.1 18.9 19 21 21 22.4 24.1 27 28.7 32.2 36 RDC at 20 °C (ohm/km) 0.479 0.355 0.262 0.225 0.195 0.182 0.158 0.145 0.127 0.110 0.088 0.075 0.060 0.049 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 DIN 48206 70/12 95/15 125/30 150/25 170/40 185/30 210/50 230/30 265/35 305/40 380/50 450/4 0 560/50 680/85 26 26 30 26 30 26 30 24 24 54 54 48 48 54 Standard D signation S ctional ar a (mm2) Alloy 56.5 71.6 119.3 88.4 147.3 110.8 184.7 140.3 233.8 20 6.2 147.8 St l 59.7 75.5 27.8 93.3 34.4 117.0 43.1 148.1 54.6 93.3 227.8 Total ar a (mm2) 116.2 147.1 147.1 181.6 181.6 227.8 227.8 288.3 288.3 299.4 375 .6
Approximat ov rall diam t r (mm) 14 15.75 15.75 17.5 17.5 19.6 19.6 22.05 22.05 22.45 26.4 RDC at 20 °C (ohm/km) 0.591 0.467 0.279 0.378 0.226 0.300 0.180 0.238 0.142 0.162 0.226 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 NF C34-125 PHLOX 116.2 PHLOX 147.1 PASTEL 147.1 PHLOX 181.6 PASTEL 181.6 PHLOX 228 PASTEL 2 28 PHLOX 288 PASTEL 288 PASTEL 299 PHLOX 376 18 18 30 18 30 18 30 18 30 42 24 Tabl 5.16: Ov rh ad lin conductor data – aluminium alloy conductors, st l r -in forc d (AACSR) N twork Prot ction & Automation Guid • 73 • Equival nt Circuits and Param t rs of Pow r Syst m Plant • ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 ASTM B711 26 26 30 26 30 26 26 30 30 30 54 54 54 54 84 84 5•
XAC at 50 Hz 66kV S ctional ar a of aluminium mm2 13.3 15.3 21.2 23.9 26.2 28.3 33.6 37.7 42.4 44.0 47.7 51.2 58.9 63.1 67.4 73.4 79.2 85.0 94.4 105.0 121.6 127 .9 131.2 135.2 RDC (20°C) RAC at 50Hz @ 20°C Ω/km 2.159 1.877 1.356 1.201 1.093 1.025 0.854 0.765 0.677 0.652 0.604 0.564 0.490 0.455 0.426 0.393 0.362 0.338 0.306 0. 274 0.237 0.226 0.220 0.214 0.194 0.182 0.170 0.157 0.144 0.137 0.131 0.126 0.12 0 0.110 0.103 0.095 0.090 0.086 0.081 0.076 0.073 0.068 0.066 0.065 0.061 0.060 0.058 0.057 3.3kV 6.6kV 11kV 22kV 33kV Flat circuit C nF/km 8.7 8.8 9.0 9.1 9.2 9.4 9.4 9.7 9.6 9.9 9.9 10.0 10.1 9.9 10.0 10.3 10.1 10.2 10.3 10.4 10.6 10.7 10 .7 10.5 10.8 10.7 10.8 10.9 11.0 11.0 11.3 11.2 11.2 11.3 11.3 11.5 11.5 11.6 11 .7 11.8 11.8 11.5 11.9 12.0 12.0 12.1 12.1 12.1 X Ω/km 0.503 0.499 0.488 0.484 0.4 82 0.460 0.474 0.435 0.467 0.427 0.427 0.425 0.421 0.454 0.452 0.414 0.447 0.445 0.410 0.438 0.402 0.398 0.397 0.405 0.396 0.400 0.398 0.395 0.388 0.391 0.381 0 .384 0.387 0.380 0.382 0.375 0.378 0.373 0.369 0.369 0.368 0.374 0.364 0.365 0.3 63 0.362 0.360 0.360 C nF/km 7.6 7.7 7.8 7.9 8.0 8.2 8.1 8.4 8.3 8.5 8.5 8.6 8.7 8.5 8.5 8.8 8.7 8.7 8.8 8.8 9.0 9.0 9.1 9.0 9.1 9.1 9.1 9.2 9.3 9.3 9.5 9.4 9.4 9.5 9.5 9.7 9.6 9.7 9.8 9.8 9.9 9.7 10.0 10.0 10.0 10.0 10.1 10.1 Double vertic al X Ω/km 0.513 0.508 0.498 0.494 0.491 0.470 0.484 0.444 0.476 0.437 0.437 0.434 0.430 0.464 0.462 0.423 0.457 0.454 0.420 0.448 0.412 0.407 0.407 0.415 0.406 0. 410 0.407 0.405 0.398 0.401 0.391 0.393 0.396 0.389 0.392 0.384 0.387 0.383 0.37 9 0.379 0.378 0.384 0.374 0.374 0.372 0.372 0.370 0.370 XAC at 50 Hz an shunt capacitance Triangle Double vertical X Ω/km 0.541 0.537 0.5 27 0.522 0.520 0.498 0.512 0.473 0.505 0.465 0.465 0.463 0.459 0.492 0.490 0.452 0.485 0.483 0.448 0.476 0.440 0.436 0.435 0.443 0.434 0.438 0.436 0.433 0.426 0 .429 0.419 0.422 0.425 0.418 0.420 0.413 0.416 0.411 0.408 0.407 0.407 0.413 0.4 02 0.403 0.401 0.400 0.398 0.398 C nF/km 7.0 7.1 7.2 7.3 7.3 7.5 7.5 7.7 7.6 7.8 7.8 7.9 7.9 7.8 7.8 8.1 7.9 7.9 8.0 8.1 8.2 8.2 8.3 8.2 8.3 8.3 8.3 8.4 8.5 8.4 8.6 8.6 8.5 8.6 8.6 8.7 8.7 8.8 8.9 8.9 8.9 8.7 9.0 9.0 9.0 9.0 9.1 9.1 132kV D ouble triangle X Ω/km 0.528 0.523 0.513 0.509 0.506 0.485 0.499 0.459 0.491 0.452 0.452 0.449 0.445 0.479 0.477 0.438 0.472 0.469 0.435 0.463 0.427 0.422 0.421 0. 430 0.420 0.425 0.422 0.420 0.412 0.416 0.406 0.408 0.411 0.404 0.406 0.399 0.40 2 0.398 0.394 0.393 0.393 0.399 0.389 0.389 0.387 0.387 0.385 0.385 C nF/km 7.2 7.3 7.4 7.5 7.5 7.7 7.7 7.9 7.8 8.0 8.1 8.1 8.2 8.0 8.1 8.3 8.2 8.2 8.3 8.3 8.4 8.5 8.5 8.4 8.6 8.5 8.6 8.6 8.8 8.7 8.9 8.9 8.8 8.9 8.9 9.1 9.0 9.1 9.2 9.2 9.2 9.0 9.3 9.3 9.4 9.4 9.4 9.4 Flat circuit X Ω/km 0.556 0.552 0.542 0.537 0.535 0.51 3 0.527 0.488 0.520 0.481 0.480 0.478 0.474 0.507 0.505 0.467 0.500 0.498 0.463 0.491 0.455 0.451 0.450 0.458 0.449 0.453 0.451 0.449 0.441 0.444 0.435 0.437 0. 440 0.433 0.435 0.428 0.431 0.426 0.423 0.422 0.422 0.428 0.418 0.418 0.416 0.41 5 0.413 0.413 C nF/km 6.8 6.9 7.0 7.1 7.1 7.3 7.2 7.4 7.3 7.5 7.6 7.6 7.7 7.5 7. 6 7.8 7.6 7.7 7.8 7.8 7.9 8.0 8.0 7.9 8.0 8.0 8.0 8.1 8.2 8.1 8.3 8.3 8.2 8.3 8. 3 8.4 8.4 8.5 8.5 8.6 8.6 8.4 8.7 8.7 8.7 8.7 8.7 8.7 Ω/km 2.1586 1.8771 1.3557 1.2013 1.0930 1.0246 0.8535 0.7647 0.6768 0.6516 0.6042 0.5634 0.4894 0.4545 0.4255 0.3930 0.3622 0.3374 0.3054 0.2733 0.2371 0.2254 0.2 197 0.2133 Ω/km 0.395 0.391 0.381 0.376 0.374 0.352 0.366 0.327 0.359 0.320 0.319 0.317 0.313 0.346 0.344 0.306 0.339 0.337 0.302 0.330 0.294 0.290 0.289 0.297 0.288 0.292 0 .290 0.287 0.280 0.283 0.274 0.276 0.279 0.272 0.274 0.267 0.270 0.265 0.262 0.2 61 0.261 0.267 0.257 0.257 0.255 0.254 0.252 0.252 Ω/km 0.409 0.405 0.395 0.390 0.388 0.366 0.380 0.341 0.373 0.334 0.333 0.331 0.327 0.360 0.358 0.320 0.353 0.351 0.316 0.344 0.308 0.304 0.303 0.311 0.302 0.306 0 .304 0.302 0.294 0.297 0.288 0.290 0.293 0.286 0.288 0.281 0.284 0.279 0.276 0.2 75 0.275 0.281 0.271 0.271 0.269 0.268 0.266 0.266 Ω/km 0.420 0.415 0.405 0.401 0.398 0.377 0.390 0.351 0.383 0.344 0.344 0.341 0.337 0.371 0.369 0.330 0.363 0.361 0.327 0.355 0.318 0.314 0.313 0.322 0.312 0.316 0 .314 0.312 0.304 0.308 0.298 0.300 0.303 0.296 0.298 0.291 0.294 0.289 0.286 0.2 85 0.285 0.291 0.281 0.281 0.279 0.279 0.277 0.277
Ω/km 0.434 0.429 0.419 0.415 0.412 0.391 0.404 0.365 0.397 0.358 0.358 0.355 0.351 0.385 0.383 0.344 0.377 0.375 0.341 0.369 0.332 0.328 0.327 0.336 0.326 0.330 0 .328 0.326 0.318 0.322 0.312 0.314 0.317 0.310 0.312 0.305 0.308 0.303 0.300 0.2 99 0.299 0.305 0.295 0.295 0.293 0.293 0.291 0.291 X Ω/km 0.445 0.441 0.430 0.426 0.424 0.402 0.416 0.376 0.409 0.369 0.369 0.367 0.3 62 0.396 0.394 0.356 0.389 0.387 0.352 0.380 0.344 0.340 0.339 0.347 0.338 0.342 0.340 0.337 0.330 0.333 0.323 0.326 0.329 0.321 0.324 0.317 0.320 0.315 0.311 0 .311 0.310 0.316 0.306 0.307 0.305 0.304 0.302 0.302 C nF/km 7.4 7.5 7.7 7.8 7.8 8.0 7.9 8.2 8.1 8.3 8.3 8.4 8.5 8.3 8.3 8.6 8.4 8.5 8.6 8.6 8.8 8.8 8.8 8.8 8.9 8.9 8.9 9.0 9.1 9.1 9.3 9.2 9.2 9.3 9.3 9.4 9.4 9.5 9.6 9.6 9.6 9.4 9.7 9.7 9.8 9.8 9.8 9.8 X Ω/km 0.520 0.515 0.505 0.501 0.498 0.477 0.491 0.451 0.483 0.444 0.444 0.441 0.4 37 0.471 0.469 0.430 0.464 0.461 0.427 0.455 0.419 0.414 0.414 0.422 0.413 0.417 0.414 0.412 0.405 0.408 0.398 0.400 0.403 0.396 0.399 0.391 0.394 0.390 0.386 0 .386 0.385 0.391 0.381 0.381 0.379 0.379 0.377 0.377 C nF/km 7.3 7.4 7.6 7.6 7.7 7.8 7.8 8.1 7.9 8.2 8.2 8.2 8.3 8.2 8.2 8.5 8.3 8.4 8.4 8.5 8.6 8.7 8.7 8.6 8.7 8.7 8.8 8.8 8.9 8.9 9.1 9.0 9.0 9.1 9.1 9.2 9.2 9.3 9.4 9.4 9.4 9.2 9.5 9.5 9.6 9.6 9.6 9.6 Equivalent Circuits an Parameters of Power System Plant 148.9 0.1937 158.7 0.1814 170.5 0.1691 184.2 0.1565 201.4 0.1438 210.6 0.1366 22 1.7 230.9 241.7 263.7 282.0 306.6 322.3 339.3 362.6 386.0 402.8 428.9 448.7 456. 1 483.4 494.4 510.5 523.7 0.1307 0.1249 0.1193 0.1093 0.1022 0.0945 0.0895 0.085 0.0799 0.0747 0.0719 0.0671 0.0642 0.0635 0.0599 0.0583 0.0565 0.0553 • 5•
Table 5.17: Fee er circuits ata - overhea lines • 74 •
Network Protection & Automation Gui e
XAC at 60 Hz 66kV Sectional RDC RAC at area of (20°C) 60Hz aluminium 20°C mm2 13.3 15.3 21.2 23.9 26.2 28.3 33.6 37.7 42.4 44.0 47.7 51.2 58.9 63.1 67.4 73.4 79.2 85.0 94.4 105.0 121.6 127.9 131.2 135.2 Ω/km 2.1586 1.8771 1.3557 1.2013 1.0930 1 .0246 0.8535 0.7647 0.6768 0.6516 0.6042 0.5634 0.4894 0.4545 0.4255 0.3930 0.36 22 0.3374 0.3054 0.2733 0.2371 0.2254 0.2197 0.2133 Ω/km 2.159 1.877 1.356 1.201 1 .093 1.025 0.854 0.765 0.677 0.652 0.604 0.564 0.490 0.455 0.426 0.393 0.362 0.3 38 0.306 0.274 0.238 0.226 0.220 0.214 0.194 0.182 0.170 0.157 0.145 0.137 0.132 0.126 0.120 0.110 0.103 0.096 0.091 0.086 0.081 0.076 0.074 0.069 0.066 0.065 0 .062 0.060 0.059 0.057 3.3kV 6.6kV 11kV 22kV 33kV Flat circuit XAC at 60 Hz an shunt capacitance Double vertical Triangle Double vertical X C Ω/ km nF/km 0.649 0.644 0.632 0.627 0.624 0.598 0.614 0.567 0.606 0.559 0.558 0.555 0.550 0.591 0.588 0.542 0.582 0.579 0.538 0.572 0.528 0.523 0.522 0.532 0.521 0 .526 0.523 0.520 0.511 0.515 0.503 0.506 0.510 0.501 0.504 0.495 0.499 0.493 0.4 89 0.488 0.488 0.495 0.483 0.480 0.481 0.480 0.478 0.478 7.0 7.1 7.2 7.3 7.3 7.5 7.5 7.7 7.6 7.8 7.8 7.9 7.9 7.8 7.8 8.1 7.9 7.9 8.0 8.1 8.2 8.2 8.3 8.2 8.3 8.3 8.3 8.4 8.5 8.4 8.6 8.6 8.5 8.6 8.6 8.7 8.7 8.8 8.9 8.9 8.9 8.7 9.0 9.0 9.0 9.0 9.1 9.1 132kV Double triangle Flat circuit Ω/km 0.474 0.469 0.457 0.452 0.449 0.423 0.439 0.392 0.431 0.384 0.383 0.380 0.375 0.416 0.413 0.367 0.407 0.404 0.363 0.396 0.353 0.348 0.347 0.357 0.346 0.351 0 .348 0.345 0.336 0.340 0.328 0.331 0.335 0.326 0.329 0.320 0.324 0.318 0.314 0.3 13 0.313 0.320 0.308 0.305 0.306 0.305 0.303 0.303 Ω/km 0.491 0.486 0.474 0.469 0.466 0.440 0.456 0.409 0.447 0.400 0.400 0.397 0.392 0.432 0.430 0.384 0.424 0.421 0.380 0.413 0.370 0.365 0.364 0.374 0.362 0.367 0 .365 0.362 0.353 0.357 0.345 0.348 0.351 0.343 0.346 0.337 0.341 0.335 0.331 0.3 30 0.330 0.337 0.325 0.322 0.323 0.322 0.320 0.320 Ω/km 0.503 0.498 0.486 0.481 0.478 0.452 0.468 0.421 0.460 0.413 0.412 0.409 0.404 0.445 0.442 0.396 0.436 0.433 0.392 0.426 0.382 0.377 0.376 0.386 0.375 0.380 0 .377 0.374 0.365 0.369 0.357 0.360 0.364 0.355 0.358 0.349 0.353 0.347 0.343 0.3 42 0.342 0.349 0.337 0.334 0.335 0.334 0.332 0.332 Ω/km 0.520 0.515 0.503 0.498 0.495 0.469 0.485 0.438 0.477 0.429 0.429 0.426 0.421 0.462 0.459 0.413 0.453 0.450 0.409 0.442 0.399 0.394 0.393 0.403 0.392 0.397 0 .394 0.391 0.382 0.386 0.374 0.377 0.381 0.372 0.375 0.366 0.370 0.364 0.360 0.3 59 0.359 0.366 0.354 0.351 0.352 0.351 0.349 0.349 X Ω/km 0.534 0.529 0.516 0.511 0.508 0.483 0.499 0.452 0.490 0.443 0.443 0.440 0.4 35 0.475 0.473 0.427 0.467 0.464 0.423 0.456 0.413 0.408 0.407 0.416 0.405 0.410 0.408 0.405 0.396 0.400 0.388 0.391 0.394 0.386 0.389 0.380 0.384 0.378 0.374 0 .373 0.372 0.380 0.367 0.364 0.366 0.365 0.362 0.363 C nF/km 8.7 8.8 9.0 9.1 9.2 9.4 9.4 9.7 9.6 9.9 9.9 10.0 10.1 9.9 10.0 10.3 10.1 10.2 10.3 10.4 10.6 10.7 10.7 10.5 10.8 10.7 10.8 10.9 11.0 11.0 11.3 11.2 11.2 11.3 11.3 11.5 11.5 11.6 11.7 11.8 11.8 11.5 11.9 12.0 12.0 12.1 12.1 12.1 X C X Ω/km nF/km Ω/km 0.604 0.598 0.586 0.581 0.578 0.552 0.569 0.521 0.560 0.513 0. 513 0.510 0.505 0.545 0.543 0.496 0.536 0.534 0.492 0.526 0.482 0.477 0.476 0.48 6 0.475 0.480 0.477 0.474 0.466 0.469 0.458 0.460 0.464 0.455 0.458 0.450 0.453 0.448 0.443 0.443 0.442 0.449 0.437 0.434 0.435 0.435 0.432 0.432 7.6 7.7 7.8 7. 9 8.0 8.2 8.1 8.4 8.3 8.5 8.5 8.6 8.7 8.5 8.5 8.8 8.7 8.7 8.8 8.8 9.0 9.0 9.1 9. 0 9.1 9.1 9.1 9.2 9.3 9.3 9.5 9.4 9.4 9.5 9.5 9.7 9.6 9.7 9.8 9.8 9.9 9.7 10.0 1 0.0 10.0 10.0 10.1 10.1 0.615 0.610 0.598 0.593 0.590 0.564 0.580 0.533 0.572 0. 525 0.524 0.521 0.516 0.557 0.554 0.508 0.548 0.545 0.504 0.537 0.494 0.489 0.48 8 0.498 0.487 0.492 0.489 0.486 0.477 0.481 0.469 0.472 0.476 0.467 0.470 0.461 0.465 0.459 0.455 0.454 0.454 0.461 0.449 0.446 0.447 0.446 0.444 0.444 C X nF/km Ω/km 7.4 7.5 7.7 7.8 7.8 8.0 7.9 8.2 8.1 8.3 8.3 8.4 8.5 8.3 8.3 8.6 8.4
8.5 8.6 8.6 8.8 8.8 8.8 8.8 8.9 8.9 8.9 9.0 9.1 9.1 9.3 9.2 9.2 9.3 9.3 9.4 9.4 9.5 9.6 9.6 9.6 9.4 9.7 9.7 9.8 9.8 9.8 9.8 0.624 0.619 0.606 0.601 0.598 0.572 0.589 0.541 0.580 0.533 0.533 0.530 0.525 0.565 0.563 0.516 0.556 0.554 0.512 0 .546 0.502 0.497 0.496 0.506 0.495 0.500 0.497 0.494 0.486 0.489 0.478 0.480 0.4 84 0.476 0.478 0.470 0.473 0.468 0.463 0.463 0.462 0.469 0.457 0.454 0.455 0.455 0.452 0.452 C nF/km 7.3 7.4 7.6 7.6 7.7 7.8 7.8 8.1 7.9 8.2 8.2 8.2 8.3 8.2 8.2 8.5 8.3 8.4 8.4 8.5 8.6 8.7 8.7 8.6 8.7 8.7 8.8 8.8 8.9 8.9 9.1 9.0 9.0 9.1 9.1 9.2 9.2 9.3 9.4 9.4 9.4 9.2 9.5 9.6 9.6 9.6 9.6 9.6 X C X C Ω/km nF/km Ω/km nF/km 0.633 0.628 0.616 0.611 0.608 0.582 0.598 0.551 0.589 0.542 0.542 0.539 0.534 0.574 0.572 0.526 0.566 0.563 0.522 0.555 0.512 0.507 0. 506 0.516 0.504 0.509 0.507 0.504 0.495 0.499 0.487 0.490 0.493 0.485 0.488 0.47 9 0.483 0.477 0.473 0.472 0.472 0.479 0.467 0.463 0.465 0.464 0.462 0.462 7.2 7. 3 7.4 7.5 7.5 7.7 7.7 7.9 7.8 8.0 8.1 8.1 8.2 8.0 8.1 8.3 8.2 8.2 8.3 8.3 8.4 8. 5 8.5 8.4 8.6 8.5 8.6 8.6 8.8 8.7 8.9 8.9 8.8 8.9 8.9 9.1 9.0 9.1 9.2 9.2 9.2 9. 0 9.3 9.4 9.4 9.4 9.4 9.4 0.668 0.662 0.650 0.645 0.642 0.616 0.633 0.585 0.624 0.577 0.576 0.573 0.568 0.609 0.606 0.560 0.600 0.598 0.556 0.590 0.546 0.541 0. 540 0.550 6.8 6.9 7.0 7.1 7.1 7.3 7.2 7.4 7.3 7.5 7.6 7.6 7.7 7.5 7.6 7.8 7.6 7. 7 7.8 7.8 7.9 8.0 8.0 7.9 148.9 0.1937 158.7 0.1814 170.5 0.1691 184.2 0.1565 201.4 0.1438 210.6 0.1366 22 1.7 230.9 241.7 263.7 282.0 306.6 322.3 339.3 362.6 386.0 402.8 428.9 448.7 456. 1 483.4 494.4 510.5 523.7 0.1307 0.1249 0.1193 0.1093 0.1022 0.0945 0.0895 0.085 0 0.0799 0.0747 0.0719 0.0671 0.0642 0.0635 0.0599 0.0583 0.0565 0.0553 0.539 8.0 0.544 8.0 0.541 8.0 0.538 8.1 519 0.522 0.514 0.517 0.511 0.507 0.506 6 0.496 8.3 8.3 8.2 8.3 8.3 8.4 8.4 8.5 Table 5.17 (cont): Fee er circuits ata Network Protection & Automation Gui e
0.529 8.2 0.533 8.1 0.522 0.524 0.528 0. 0.506 0.513 0.501 0.498 0.499 0.498 0.49 8.5 8.6 8.6 8.4 8.7 8.7 8.7 8.7 8.7 8.7 - overhea lines
• 75 •
Equivalent Circuits an Parameters of Power System Plant • 5•
Con uctor size mm2 Series Resistance Series Reactance Susceptance Series Resista nce Series Reactance Susceptance Series Resistance Series Reactance Susceptance Series Resistance Series Reactance Susceptance Series Resistance Series Reactanc e Susceptance Series Resistance Series Reactance Susceptance Series Resistance S eries Reactance Susceptance R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/ X (Ω/km) ωC (mS/km) R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/km) X ( S/km) R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/km) X (Ω/km) ωC (mS/km 927 0.097 0.059 0.927 0.121 0.085 0.927 0.128 0.068 35 0.669 0.092 0.067 0.669 0 .113 0.095 0.669 0.119 0.074 0.669 0.136 0.053 0.669 0.15 0.042 50 0.494 0.089 0 .079 0.494 0.108 0.104 0.494 0.114 0.082 0.494 0.129 0.057 0.494 0.143 0.045 70 0.342 0.083 0.09 0.342 0.102 0.12 0.342 0.107 0.094 0.348 0.121 0.065 0.348 0.13 4 0.05 95 0.247 0.08 0.104 0.247 0.096 0.136 0.247 0.101 0.105 0.247 0.114 0.072 0.247 0.127 0.055 120 0.196 0.078 0.111 0.196 0.093 0.149 0.196 0.098 0.115 0.1 96 0.11 0.078 0.196 0.122 0.059 150 0.158 0.076 0.122 0.158 0.091 0.16 0.158 0.0 95 0.123 0.158 0.107 0.084 0.158 0.118 0.063 185 0.127 0.075 0.133 0.127 0.088 0 .177 0.127 0.092 0.135 0.127 0.103 0.091 0.127 0.114 0.068 240 0.098 0.073 0.146 0.098 0.086 0.189 0.098 0.089 0.15 0.098 0.1 0.1 0.098 0.109 0.075 300 0.08 0.0 72 0.16 0.08 0.085 0.195 0.08 0.087 0.165 0.08 0.094 0.109 0.08 0.105 0.081 400 0.064 0.071 0.179 0.064 0.083 0.204 0.064 0.084 0.182 0.064 0.091 0.12 0.064 0.1 02 0.089 *500 0.051 0.088 0.19 0.057 0.088 0.205 0.051 0.089 0.194 0.051 0.096 0 .128 0.051 0.103 0.094 0.0387 0.117 0.079 0.0387 0.13 0.053 *630 0.042 0.086 0.2 02 0.042 0.086 0.228 0.042 0.086 0.216 0.042 0.093 0.141 0.042 0.1 0.103 0.031 0 .113 0.082 0.031 0.125 0.06 *800 *1000 *1200 *1600 3.3kV 6.6kV 11kV 22kV
Equivalent Circuits an Parameters of Poer System Plant 33kV 66kV* 145kV* 0.0254 0.109 0.088 0.0254 0.12 0.063 0.0215 0.102 0.11 0.0215 0.115 0.072 0.0215 0.123 0.057 0.0215 0.156 0.05 0.0161 0.119 0.063 0.0161 0.151 0.057 0.0126 0.113 0.072 0.0126 0.144 0.063 245kV* Series Resistance Series Reactance Suscetance Series Resistance 420kV* Series R eactance Suscetance 0.0487 0.0387 0.0310 0.0254 0.145 0.137 0.134 0.128 0.044 0.047 0.05 0.057 0.031 0 0.0254 0.172 0.162 0.04 0.047 For aluminium con uctors of the same cross section, the resistance increases by 60 65 ercent, the series reactance an shunt caacitance is virtually unaltere .* single core cables in trefoil. Different values aly if lai in sace fla t formation. Series Resistance a.c. resistance @ 90°C. Series reactance equiva lent star reactance. cables may vary significantly from Data for 245kV an 420kV that given, een ent on manufacturer an construction. Table 5.18: Characteris tics of olyethylene insulate cables (XLPE)
Con uctor Size (mm2) Series Resistance 3.3kV Series Reactance Suscetance Series Resistance 6.6kV Series Reactance Suscetance Series Resistance 11kV Series Rea ctance Suscetance Series Resistance 22kV Series Reactance Suscetance Series Re sistance 33kV Series Reactance Suscetance R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/km) X (Ω/km) ω (mS/km) R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/km) X (Ω/km) ωC (mS/km) R (Ω/km) X (Ω/km) ωC (mS 16 25 35 50 70 95 120 150 185 240 2063 1289 825.5 595 439.9 304.9 220.4 174.5 1 42.3 113.9 87.6 87.7 83.6 76.7 74.8 72.5 70.2 67.5 66.6 65.7 64.7 63.8 514.2 326 26.2 24.3 111 9.26 206.4 148.8 110 22 21.2 20.4 76.2 19.6 55.1 18.7 43.6 18.3 3 5.6 17.9 28.5 17.6 21.9 17.1 300 70.8 62.9 17.6 16.9 400 56.7 62.4 14.1 16.5 *50 0 45.5 73.5 11.3 18.8 *630 37.1 72.1 9.3 18.4 *800 *1000 31.2 27.2 71.2 69.8 7.8 18 6.7 17.8 • 5• 0.87 0.63 0.46 0.32 0.23 0.184 0.15 0.12 0.092 0.074 0.059 0.048 0.039 0.033 0.0 28 0.107 0.1 0.096 0.091 0.087 0.085 0.083 0.081 0.079 0.077 0.076 0.085 0.083 0 .081 0.08 17.69 12.75 9.42 2.89 2.71 2.6 4.19 1.16 6.53 2.46 2.9 1.09 4.71 2.36 2.09 1.03 3.74 2.25 3.04 2.19 2.44 2.11 1.87 2.04 1.51 1.97 1.21 1.92 0.96 1.9 0 .79 1.84 0.66 1.8 0.57 1.76 0.181 0.147 0.118 0.09 0.073 0.058 0.046 0.038 0.031 0.027 0.107 0.103 0.101 0.0 97 0.094 0.09 0.098 0.097 0.092 0.089 0.104 0.116 0.124 0.194 0.151 0.281 0.179 0.198 0.22 0.245 Cables are of the soli tye, 3 core excet for those marke *. Ime ances at 50 Hz frequency Table 5.19: Characteristics of aer insulate cables • 76 •
Netork Protection & Automation Gui e
Con uctor size (mm2) 16 25 35 50 70 95 120 150 185 240 300 400 *500 *630 *800 *1 000 3 core Coer con uctors, 50Hz values. * single core cables in trefoil Tab le 5.20: 3.3 kV PVC insulate cables 3.3kV R Ω/km 1.380 0.870 0.627 0.463 0.321 0.232 0.184 0.150 0.121 0.093 0.075 0.0 60 0.049 0.041 0.035 0.030 X Ω/km 0.106 0.100 0.094 0.091 0.086 0.084 0.081 0.079 0.077 0.076 0.075 0.075 0.089 0.086 0.086 0.084 5.25 REFERENCES 5.1 Physical significance of sub-subtransient quantities in yna mic behaviour of synchronous machines. I.M. Canay. Proc. IEE, Vol. 135, Pt. B, N ovember 1988. Metho s for etermining 5.2 IEC 60034-4. synchronous machine quant ities from tests. 5.3 IEEE Stan ar s 115/115A. IEEE TestProce ures for Synchron ous Machines. 5.4 Power System Analysis. J.R.Mortlock an M.W.Humphrey Davies (C hapman & Hall, Lon on). Voltage Level Un kV Um kV Cross Sectional Area mm2 30 50 90 120 150 1 50 90 120 150 50 90 120 150 90 150 2 50 250 150 250 250 400 400 400 400 400 400 Con uctors per phase 1 1 1 1 1 1.2 1 1 1 1 1 1 1 1 1 1 1 2 1 1 2 1 2 1 2 4 2 4 2 3 Surge Impe ance Loa ing MVA 0.3 0.3 0.4 0.5 0.5 44 1.2 1.2 1.4 1.5 2.7 2.7 3.1 3 .5 11 11 11 15 44 44 58 56 73 130 184 260 410 582 482 540 Voltage Drop Loa ing MWkm 11 17 23 27 30 5.8 66 92 106 119 149 207 239 267 827 1 068 1240 1790 4070 4960 7160 6274 9057 15600 22062 31200 58100 82200 68200 81200 In icative Thermal Loa ing MVA A 2.9 3.9 5.1 6.2 7.3 151 7.8 10.2 12.5 14.6 11.7 15.3 18.7 21.9 41 59 77 153 85 115 230 160 320 247 494 988 850 1700 1085 1630 1 51 204 268 328 383 204 268 328 383 204 268 328 383 268 383 502 1004 370 502 1004 698 1395 648 1296 2592 1296 2590 1650 2475 11 12 24 30 33 36 66 72.5 132 145 220 245 380
420 400 550 550 Table 5.21: OHL capabilities
Network Protection & Automation Gui e • 77 •
Equivalent Circuits an Parameters of Power System Plant • 5•
• 6 •
C u rr e n t a n Vo l t a g e Transformers Intro uction Electromagnetic voltage transformers Capacitor voltage transformers Current transformers Novel instrument transformers 6.1 6.2 6.3 6.4 6.5
•
6 • Current an Voltage Transformers 6.1 INTRODUCTION Whenever the values of voltage or current in a power circuit ar e too high to permitconvenient irect connection of measuring instruments or re lays, couplingis ma e through transformers. Such measuring transformers are r equire to pro uce a scale own replica of the input quantity to the accuracy e xpecte for the particular measurement; this is ma e possible by the high effici ency of the transformer. The performance of measuring transformers uring an fo llowing large instantaneous changes in the input quantity is important, in that this quantity may epart from the sinusoi al waveform. The eviation may consist of a step change in magnitu e, or a transient component that persists for an ap preciable perio , or both. The resulting effect on instrument performance is usu ally negligible, although for precision metering a persistent change in the accu racy of the transformer However,many protection systems are may be significant. require to operate uring the perio of transient isturbance in the output of the measuring transformers that follows a system fault. The errors in transform er output may abnormally elay the operation of the protection, or cause unneces sary operations. The functioning of such transformers must, therefore, be examin e analytically. It can be shown that the transformer can be represente by the equivalent circuit of Figure 6.1, where all quantities are referre to the secon ary si e. 1/1 Rp Lp Rs Ls Ze
Bur en Figure 6.1: Equivalent circuit of transformer Network Protection & Automation Gui e • 79 •
When the transformer is not 1/1 ratio, this con ition can be represente by ener gising the equivalent circuit with an i eal transformer of the given ratio but h aving no losses. 6.1.1 Measuring Transformers Voltagean current transformers f or low primary voltage or current ratings are not rea ily istinguishable; for h igher ratings, issimilarities of construction are usual. Nevertheless the iffe rences between these evices lie principally in the way they are connecte into are much like small power transformers, the power circuit.Voltage transformers iffering only in etails of esign that control ratio accuracyover the specifi e range of output. Current transformers have their primary win ings connecte i n series with the power circuit, an soalso inseries with the system impe ance . The response of the transformer is ra ically ifferent in these two mo es of o peration. voltage rops are ma e small, an the normal flux ensity in the core is esigne to be well ensity, in or er that the exciting current ma below the saturation y be low an the exciting impe ance substantially constant witha variation of a pplie voltage over the esire operating range inclu ing some egree of overvol tage. These limitations in esign result in a VT for a given bur en being much l arger than a typical power transformer of similar rating. The exciting current, in consequence, will not be as small, relative to the rate bur en, as it woul be for a typical power transformer. 6.2.1 Errors The ratio an phase errors of t he transformer can be calculate using the vector iagram of Figure 6.2. The rat io error is efine as: C u r r e n t a n Vo l t a g e T r a n s f o r m e r s ( K nV s ) × 100% Vp where: Kn is the nominal ratio Vp is theprimary voltage Vs is the secon ary vol tage If the error is positive, the secon ary voltage excee s the nominal value. The turns ratio of the transformer nee not be equal to the nominal ratio; a sma ll turns compensation will usually be employe , so that the error will be positi ve for low bur ens an negative for high bur ens. 6.2 ELECTROMAGNETIC VOLTAGE TRANSFORMERS In the shunt mo e, the system voltage i s applie across the input terminals of the equivalent circuit of Figure 6.1. Th e vector iagram for this circuit is shown in Figure 6.2. Vp IpXp IpRp Ep θ
Vs V Ip L Ie Im Ic E Ie p • 6• Is Vs IsXs Φ The phase error is the phase difference between the reversed secondary and the p rimary voltage vectors. It is positive when the reversed secondary voltage leads the primary vector. Requirements in this respect are set out in IEC 60044-2. Al l voltage transformers are required to comply with one of the classes in Table 6 .1. or protection purposes, accuracy of voltage measurement may be important du ring fault conditions, as the system voltage might be reduced by the fault to a
low value. Voltage transformers for such types of service must comply with the e xtended range of requirements set out in Table 6.2. Accuracy class 0.1 0.2 0.5 1.0 3.0 0.8 - 1.2 x rated voltage 0.25 - 1.0 x rated burden at 0.8pf voltage ratio error phase displacement (%) (minutes) +/- 0.1 +/5 +/- 0.2 +/- 10 +/- 0.5 +/- 20 +/- 1.0 +/- 40 +/- 3.0 not specified Φ Ie Im I θ I Ip Es Is R s I I I s = exciting current = phase angle error s s = secondary current p = primary current Ipp Figure 6.2: Vector diagram for voltage transformer The secondary output voltage Vs is reuired to be an accurate scaled replica of the input voltage Vp over a specified range of output. To this end, the winding • 80 • Table 6.1: Measuring voltage transformer error limits Network Protection & Automation Guide
Accuracy class 3P 6P 0.25 1.0 x rated burden at 0.8pf 0.05 Vf x rated primary voltage voltage rat io error phase displacement (%) (%) +/ 3.0 +/ 120 +/ 6.0 +/ 240 Table 6.2: Additional limits for protection voltage transformers. as possible. A short circuit on the secondary circuit wiring will produce a curr ent of many times the rated output and cause excessive heating. Even where prima ry fuses can be fitted, these will usually not clear a secondary side short circ uit because of the low value of primary current and the minimum practicable fuse rating. 6.2.5 Construction The construction of a voltage transformer takes into account the following factors: a. output – seldom more than 200 300VA. Cooling is rarely a problem b. insulation – designed for the system impulse voltage level. I nsulation volume is often larger than the winding volume c. mechanical design – no t usually necessary to withstand short circuit currents. Must be small to fit th e space available within switchgear Three phase units are common up to 36kV but for higher voltages single phase units are usual. Voltage transformers for mediu m voltage circuits will have dry type insulation, but for high and extra high vo ltage systems, oil immersed units are general. Resin encapsulated designs are in use on systems up to 33kV. Figure 6.3 shows a typical voltage transformer. 6.2.2 Voltage Factors The uantity Vf in Table 6.2 is an upper limit of operatin g voltage, expressed in per unit of rated voltage. This is important for correct relay operation and operation under unbalanced fault conditions on unearthed or impedance earthed systems, resulting in a rise in the voltage on the healthy ph ases. Voltage factors, with the permissible duration of the maximum voltage, are given in Table 6.3. Voltage factor Vf 1.2 1.2 1.5 1.2 1.9 1.2 1.9 Time rating continuous continuous 30 s continuous 30 s continuous 8 hours Primary winding connection/system earthi ng conditions Between lines in any network. Between transformer star point and e arth in any network Between line and earth in an effectively earthed network Bet ween line and earth in a non effectively earthed neutral system with automatic e arth fault tripping Between line and earth in an isolated neutral system without automatic earth fault tripping, or in a resonant earthed system without automat ic earth fault tripping Table 6.3: Voltage transformers: Permissible duration of maximum voltage 6.2.3 Secondary Leads Voltage transformers are designed to maintain the specifie d accuracy in voltage output at their secondary terminals. To maintain this if l ong secondary leads are reuired, a distribution box can be fitted close to the VT to supply relay and metering burdens over separate leads. If necessary, allow ance can be made for the resistance of the leads to individual burdens when the particular euipment is calibrated. 6.2.4 Protection of Voltage Transformers Vol tage Transformers can be protected by H.R.C. fuses on the primary side for volta ges up to 66kV. Fuses do not usually have a sufficient interrupting capacity for use with higher voltages. Practice varies, and in some cases protection on the primary is omitted. The secondary of a Voltage Transformer should always be prot ected by fuses or a miniature circuit breaker (MCB). The device should be locate d as near to the transformer Network Protection & Automation Guide Figure 6.3: Typical voltage transformer 6.2.6 Residually Connected Voltage Transformers The three voltages of a balanced system summate to zero, but this is not so when the system is subject to a sing le phase earth fault. The residual voltage of a system is measured by connecting the secondary windings of a VT in 'broken delta' as shown in Figure 6.4. The ou tput of the secondary windings connected in broken delta is zero when balanced s
inusoidal voltages are applied, but under conditions of unbalance a residual • 81 • C u r r e n t a n d Vo l t a g e T r a n s f o r m e r s • 6•
voltage eual to three times the zero seuence voltage of the system will be dev eloped. A B C If a voltage is suddenly applied, an inrush transient will occur, as with power transformers. The effect will, however, be less severe than for power transforme rs because of the lower flux density for which the VT is designed. If the VT is rated to have a fairly high voltage factor, little inrush effect will occur. An error will appear in the first few cycles of the output current in proportion to the inrush transient that occurs. When the supply to a voltage transformer is i nterrupted, the core flux will not readily collapse; the secondary winding will tend to maintain the magnetising force to sustain this flux, and will circulate a current through the burden which will decay more or less exponentially, possib le with a superimposed audio freuency oscillation due to the capacitance of the winding. Bearing in mind that the exciting uantity, expressed in ampere turns, may exceed the burden, the transient current may be significant. 6.2.8 Cascade Voltage Transformers The capacitor VT (section 6.3) was developed because of the high cost of conventional electromagnetic voltage transformers but, as shown in Section 6.3.2, the freuency and transient responses are less satisfactory than those of the orthodox voltage transformers. Another solution to the problem is the cascade VT (Figure 6.5). A Residual voltage Figure 6.4: Residual voltage connection C u r r e n t a n d Vo l t a g e T r a n s f o r m e r s In order to measure this component, it is necessary for a zero seuence flux to be set up in the VT, and for this to be possible there must be a return path for the resultant summated flux. The VT core must have one or more unwound limbs li nking the yokes in addition to the limbs carrying windings. Usually the core is made symmetrically, with five limbs, the two outermost ones being unwound. Alter natively, three single phase units can be used. It is eually necessary for the primary winding neutral to be earthed, for without an earth, zero seuence excit ing current cannot flow. A VT should be rated to have an appropriate voltage fac tor as described in Section 6.2.2 and Table 6.3, to cater for the voltage rise o n healthy phases during earth faults. Voltage transformers are often provided wi th a normal star connected secondary winding and a broken delta connected ‘tertiar y’ winding. Alt rnativ ly th r sidual voltag can b xtract d by using a star/br ok n-d lta conn ct d group of auxiliary voltag transform rs n rgis d from th s condary winding of th main unit, providing th main voltag transform r fulfi ls all th r quir m nts for handling a z ro s qu nc voltag as pr viously d scr ib d. Th auxiliary VT must also b suitabl for th appropriat voltag factor. It should b not d that third harmonics in th primary voltag wav , which ar of z ro s qu nc , summat in th brok nd lta winding. 6.2.7 Transi nt P rformanc Transi nt rrors caus f w difficulti s in th us of conv ntional voltag tra nsform rs although som do occur. Errors ar g n rally limit d to short tim p r iods following th sudd n application or r moval of voltag from th VT primary. C P C • 6• C C
P - primary winding C - coupling windings S - s condary winding C n S N a Figur 6.5: Sch matic diagram of typical cascad voltag transform r • 82 • N twork Prot ction & Automation Guid
Th conv ntional typ of VT has a singl primary winding, th insulation of whic h pr s nts a gr at probl m for voltag s abov about 132kV. Th cascad VT avoids th s difficulti s by br aking down th primary voltag in s v ral distinct and s parat stag s. Th compl t VT is mad up of s v ral individual transform rs, th primary windings of which ar conn ct d in s ri s, as shown in Figur 6.5. Each magn tic cor has primary windings (P) on two opposit sid s. Th s condary winding (S) consists of a singl winding on th last stag only. Coupling windi ngs (C) conn ct d in pairs b tw n stag s, provid low imp danc circuits for th transf r of load amp r -turns b tw n stag s and nsur that th pow r fr qu n cy voltag is qually distribut d ov r th s v ral primary windings. Th pot nti als of th cor s and coupling windings ar fix d at d finit valu s by conn ctin g th m to s l ct d points on th primary windings. Th insulation of ach windin g is suffici nt for th voltag d v lop d in that winding, which is a fraction o f th total according to th numb r of stag s. Th individual transform rs ar m ount d on a structur built of insulating mat rial, which provid s th int rstag insulation, accumulating to a valu abl to withstand th full syst m voltag across th compl t h ight of th stack. Th ntir ass mbly is contain d in a h ollow cylindrical porc lain housing with xt rnal w ath r-sh ds; th housing is fill d with oil and s al d, an xpansion b llows b ing includ d to maintain h rm tic s aling and to p rmit xpansion with t mp ratur chang . 6.3 CAPACITOR VOLT AGE TRANSFORMERS Th siz of l ctromagn tic voltag transform rs for th high r voltag s is larg ly proportional to th rat d voltag ; th cost t nds to incr a s at a disproportionat rat . Th capacitor voltag transform r (CVT) is oft n mor conomic. This d vic is basically a capacitanc pot ntial divid r. As with r sistanc -typ pot ntial divid rs, th output voltag is s riously aff ct d by load at th tapping point. Th capacitanc divid r diff rs in that its quival nt sourc imp danc is capacitiv and can th r for b comp nsat d by a r actor conn ct d in s ri s with th tapping point. With an id al r actor, such an arran g m nt would hav no r gulation and could supply any valu of output. A r actor poss ss s som r sistanc , which limits th output that can b obtain d. For a s condary output voltag of 110V, th capacitors would hav to b v ry larg to p rovid a us ful output whil k ping rrors within th usual limits. Th solutio n is to us a high s condary voltag and furth r transform th output to N twork Prot ction & Automation Guid th normal valu using a r lativ ly in xp nsiv l ctromagn tic transform r. Th succ ssiv stag s of this r asoning ar indicat d in Figur 6.6. C1 C1 L C2 Zb C2 Zb (a) Basic capacitiv voltag divid r C1 L C2 (b) Capacitiv divid r with inductiv comp nsation T Zb (c) Divid r with E/M VT output stag Figur 6.6: D v lopm nt of capacitor voltag transform r
Th r ar num rous variations of this basic circuit. Th inductanc L may b a s parat unit or it may b incorporat d in th form of l akag r actanc in th t ransform r T. Capacitors C1 and C2 cannot conv ni ntly b mad to clos tol ranc s, so tappings ar provid d for ratio adjustm nt, ith r on th transform r T, or on a s parat auto-transform r in th s condary circuit. Adjustm nt of th tu ning inductanc L is also n d d; this can b don with tappings, a s parat tap p d inductor in th s condary circuit, by adjustm nt of gaps in th iron cor s, or by shunting with variabl capacitanc . A simplifi d quival nt circuit is sho wn in Figur 6.7. C L Rp Rs Vi Z Zb L - tuning inductanc Rp - primary winding r sistanc (plus loss s) Z - xcitin g imp danc of transform r T Rs - s condary circuit r sistanc Zb - burd n imp d anc C - C1 + C2 (in Figur 6.6) Figur 6.7: Simplifi d quival nt circuit of capacitor voltag transform r It will b s n that th basic diff r nc b tw n Figur 6.7 and Figur 6.1 is t h pr s nc of C and L. At normal fr qu ncy wh n C and L ar in r sonanc and th r for • 83 • C u r r n t a n d Vo l t a g T r a n s f o r m r s • 6•
canc l, th circuit b hav s in a similar mann r to a conv ntional VT. At oth r f r qu nci s, how v r, a r activ compon nt xists which modifi s th rrors. Stan dards g n rally r quir a CVT us d for prot ction to conform to accuracy r quir m nts of Tabl 6.2 within a fr qu ncy rang of 97-103% of nominal. Th corr spon ding fr qu ncy rang of m asur m nt CVT’s is much l ss, 99%-101%, as r ductions in accuracy for fr qu ncy d viations outsid this rang ar l ss important than fo r prot ction applications. 6.3.1 Voltag Prot ction of Auxiliary Capacitor If th burd n imp danc of a CVT w r to b shortcircuit d, th ris in th r actor v oltag would b limit d only by th r actor loss s and possibl saturation, that is, to Q x E2 wh r E2 is th no-load tapping point voltag and Q is th amplif ication factor of th r sonant circuit. This valu would b xc ssiv and is th r for limit d by a spark gap conn ct d across th auxiliary capacitor. Th volt ag on th auxiliary capacitor is high r at full rat d output than at no load, a nd th capacitor is rat d for continuous s rvic at this rais d valu . Th spark gap will b s t to flash ov r at about twic th full load voltag . Th ff ct of th spark gap is to limit th short-circuit curr nt which th VT will d liv r and fus prot ction of th s condary circuit has to b car fully d sign d with this point in mind. Faciliti s ar usually provid d to arth th tapping point, ith r manually or automatically, b for making any adjustm nts to tappings or c onn ctions. 6.3.2 Transi nt B haviour of Capacitor Voltag Transform rs A CVT is a s ri s r sonant circuit. Th introduction of th l ctromagn tic transform r b tw n th int rm diat voltag and th output mak s possibl furth r r sonanc involving th xciting imp danc of this unit and th capacitanc of th divid r stack. Wh n a sudd n voltag st p is appli d, oscillations in lin with th s diff r nt mod s tak plac , and will p rsist for a p riod gov rn d by th total r sistiv damping that is pr s nt. Any incr as in r sistiv burd n r duc s th tim constant of a transi nt oscillation, although th chanc of a larg initial amplitud is incr as d. For v ry high-sp d prot ction, transi nt oscillations should b minimis d. Mod rn capacitor voltag transform rs ar much b tt r in th is r sp ct than th ir arli r count rparts, but high p rformanc prot ction sch m s may still b adv rs ly aff ct d. 6.3.3 F rro-R sonanc Th xciting imp danc Z of th auxiliary transform r T • 84 • and th capacitanc of th pot ntial divid r tog th r form a r sonant circuit th at will usually oscillat at a sub-normal fr qu ncy. If this circuit is subj ct d to a voltag impuls , th r sulting oscillation may pass through a rang of fr qu nci s. If th basic fr qu ncy of this circuit is slightly l ss than on -thir d of th syst m fr qu ncy, it is possibl for n rgy to b absorb d from th sys t m and caus th oscillation to build up. Th incr asing flux d nsity in th tr ansform r cor r duc s th inductanc , bringing th r sonant fr qu ncy n ar r to th on -third valu of th syst m fr qu ncy. Th r sult is a progr ssiv buildup until th oscillation stabiliz s as a third sub-harmonic of th syst m, which can b maintain d ind finit ly. D p nding on th valu s of compon nts, oscillat ions at fundam ntal fr qu ncy or at oth r sub-harmonics or multipl s of th supp ly fr qu ncy ar possibl but th third sub-harmonic is th on most lik ly to b ncount r d. Th principal manif station of such an oscillation is a ris in o utput voltag , th r.m.s. valu b ing p rhaps 25%50% abov th normal valu ; th output wav form would g n rally b of th form shown in Figur 6.8. C u r r n t a n d Vo l t a g T r a n s f o r m r s Amplitud Tim Figur 6.8: Typical s condary voltag wav form with third sub-harmonic oscillati on. •
6• Such oscillations ar l ss lik ly to occur wh n th circuit loss s ar high, as is th cas with a r sistiv burd n, and can b pr v nt d by incr asing th r si stiv burd n. Sp cial anti-f rro-r sonanc d vic s that us a parall ltun d circ uit ar som tim s built into th VT. Although such arrang m nts h lp to suppr ss f rro-r sonanc , th y t nd to impair th transi nt r spons , so that th d sign is a matt r of compromis . Corr ct d sign will pr v nt a CVT that suppli s a r sistiv burd n from xhibiting this ff ct, but it is possibl for non-lin ar in ductiv burd ns, such as auxiliary voltag transform rs, to induc f rro-r sonan c . Auxiliary voltag transform rs for us with capacitor voltag transform rs s hould b d sign d with a low valu of flux d nsity that pr v nts transi nt volta g s from causing cor saturation, which in turn would bring high xciting curr n ts. N twork Prot ction & Automation Guid
6.4 CURRENT TRANSFORMERS Th primary winding of a curr nt transform r is conn ct d in s ri s with th pow r circuit and th imp danc is n gligibl compar d wit h that of th pow r circuit. Th pow r syst m imp danc gov rns th curr nt pass ing through th primary winding of th curr nt transform r. This condition can b r pr s nt d by ins rting th load imp danc , r f rr d through th turns ratio, in th input conn ction of Figur 6.1. This approach is d v lop d in Figur 6.9 , taking th num rical xampl of a 300/5A CT appli d to an 11kV pow r syst m. T h syst m is consid r d to b carrying rat d curr nt (300A) and th CT is f din g a burd n of 10VA. 6.4.1 Errors Th g n ral v ctor diagram (Figur 6.2) can b simplifi d by th om ission of d tails that ar not of int r st in curr nt m asur m nt; s Figur 6. 10. Errors aris b caus of th shunting of th burd n by th xciting imp danc . This us s a small portion of th input curr nt for xciting th cor , r ducing th amount pass d to th burd n. So Is = Ip - I , wh r I is d p nd nt on Z , th xciting imp danc and th s condary .m.f. Es, giv n by th quation Es = I s (Zs + Zb), wh r : Zs = th s lf-imp danc of th s condary winding, which can g n rally b tak n as th r sistiv compon nt Rs only Zb = th imp danc of th burd n Z=21.2Ω Is R s E=6350V 300/5A Bur en 10VA Is X s (a) Physical arrangement Ir Iq Ip
Z=21.2Ω I eal CT E=6350V r=300/5 0.2Ω Vs j50Ω 150Ω 0.4Ω Is θ (b) Euivalent circuit of (a) E2 =21.2Ω x 602 r =76.2kΩ 0.2Ω Ie Φ E = Secondary induced e.m.f. Vs Secondary output voltage Er =6350V x 60 =381kV j50Ω 150Ω 0.4Ω
I θ p Phase angle error (c) Euivalent circuit, all uantities referred to secondary side Figure 6.9: Derivation of euivalent circuit of a current transformer Ie I r Is Is A study of the final euivalent circuit of Figure 6.9(c), taking note of the typ ical component values, will reveal all the properties of a current transformer. It will be seen that: a. the secondary current will not be affected by change of the burden impedance over a considerable range b. the secondary circuit must no t be interrupted while the primary winding is energised. The induced secondary e .m.f. under these circumstances will be high enough to present a danger to life and insulation c. the ratio and phase angle errors can be calculated easily if t he magnetising characteristics and the burden impedance are known Network Protection & Automation Guide Figure 6.10: Vector diagram for current transformer (referred to secondary) 6.4.1.1 Current or Ratio Error This is the difference in magnitude between Ip an d Is and is eual to Ir, the component of Ie which is in phase with Is. 6.4.1.2 Phase Error This is represented by I, the component of Ie in uadrature with Is and results in the phase error . The values of the current error and phase erro r depend on the phase displacement between Is and Ie, but neither current nor ph ase error can exceed the vectorial error Ie. It will be seen that with a moderat ely inductive burden, resulting in Is and Ie approximately in phase, there will • 85 • C u r r e n t a n d Vo l t a g e T r a n s f o r m e r s • Es 6•
be little phase error and the exciting component will result almost entirely in ratio error. A reduction of the secondary winding by one or two turns is often u sed to compensate for this. For example, in the CT corresponding to Figure 6.9, the worst error due to the use of an inductive burden of rated value would be ab out 1.2%. If the nominal turns ratio is 2:120, removal of one secondary turn wou ld raise the output by 0.83% leaving the overall current error as 0.37%. For lo wer value burden or a different burden power factor, the error would change in t he positive direction to a maximum of +0.7% at zero burden; the leakage reactanc e of the secondary winding is assumed to be negligible. No corresponding correct ion can be made for phase error, but it should be noted that the phase error is small for moderately reactive burdens. 6.4.2 Composite Error Class 5P 10P Current error at Phase displacement Composite error at rated primary at rated cu rrent rated accuracy limit current (%) (minutes) primary current (%) +/ 1 +/ 60 5 +/ 3 10 Standard accuracy limit factors are 5, 10, 15, 20, and 30 Table 6.5: Protection CT error limits for classes 5P and 10P Even though the burden of a protection CT is only a few VA at rated current, the output reuired from the CT may be considerable if the accuracy limit factor is high. For example, with an accuracy limit factor of 30 and a burden of 10VA, th e CT may have to supply 9000VA to the secondary circuit. Alternatively, the same CT may be subjected to a high burden. For overcurrent and earth fault protectio n, with elements of similar VA consumption at setting, the earth fault element o f an electromechanical relay set at 10% would have 100 times the impedance of th e overcurrent elements set at 100%. Although saturation of the relay elements so mewhat modifies this aspect of the matter, it will be seen that the earth fault element is a severe burden, and the CT is likely to have a considerable ratio er ror in this case. So it is not much use applying turns compensation to such curr ent transformers; it is generally simpler to wind the CT with turns correspondin g to the nominal ratio. Current transformers are often used for the dual duty of measurement and protection. They will then need to be rated according to a clas s selected from both Tables 6.4 and 6.5. The applied burden is the total of inst rument and relay burdens. Turns compensation may well be needed to achieve the m easurement performance. Measurement ratings are expressed in terms of rated burd en and class, for example 15VA Class 0.5. Protection ratings are expressed in te rms of rated burden, class, and accuracy limit factor, for example 10VA Class 10 P10. 6.4.4 Class PX Current Transformers The classification of Table 6.5 is only used for overcurrent protection. Class PX is the definition in IEC 60044 1 for the uasi transient current transformers formerly covered by Class X of BS 3938, commonly used with unit protection schemes. Guidance was given in the specifica tions to the application of current transformers to earth fault protection, but for this and for the majority of other protection applications it is better to r efer directly to the maximum useful e.m.f. that can be obtained from the CT. In this context, the 'knee point' of the excitation curve is defined as 'that point at which a further increase of 10% of secondary e.m.f. would reuire an increme nt of exciting current of 50%’; s Figur 6.11. • 86 • N twork Prot ction & Automation Guid C u r r n t a n d Vo l t a g T r a n s f o r m r s This is d fin d in IEC 60044-1 as th r.m.s. valu of th diff r nc b tw n th id al s condary curr nt and th actual s condary curr nt. It includ s curr nt a nd phas rrors and th ff cts of harmonics in th xciting curr nt. Th accura cy class of m asuring curr nt transform rs is shown in Tabl 6.4. Accuracy class 0.1 0.2 0.5 1 Accuracy class 3 +/- P rc ntag curr nt (ratio) rr
or % curr nt 5 20 100 120 0.4 0.2 0.1 0.1 0.75 0.35 0.2 0.2 1.5 3 0.75 1.5 0.5 1 .0 0.5 1.0 +/- Phas displac m nt (minut s) 5 20 100 120 15 8 5 5 30 15 10 10 90 180 45 90 30 60 30 60 (a) Limits of rror accuracy for rror class s 0.1 - 1.0 +/- curr nt (ratio) rr or, % % curr nt 50 3 5 120 3 5 • 6• 5 Tabl 6.4: CT rror class s (b) Limits of rror for rror class s 3 and 5 6.4.3 Accuracy Limit Curr nt of Prot ction Curr nt Transform rs Prot ction quip m nt is int nd d to r spond to fault conditions, and is for this r ason r quir d to function at curr nt valu s abov th normal rating. Prot ction class curr nt transform rs must r tain a r asonabl accuracy up to th larg st r l vant curr nt. This valu is known as th ‘accuracy limit curr nt’ and may b xpr ss d in prim ary or quival nt s condary t rms. Th ratio of th accuracy limit curr nt to th rat d curr nt is known as th accuracy limit factor . Th accuracy class of p rot ction curr nt transform rs is shown in Tabl 6.5.
VK + 10%V V + 50%I K I Exciting voltag (Vs) V n c ssary primary insulation. In oth r cas s, th bushing of a circuit br ak r o r pow r transform r is us d for this purpos . At low primary curr nt ratings it may b difficult to obtain suffici nt output at th d sir d accuracy. This is b caus a larg cor s ction is n d d to provid nough flux to induc th s cond ary .m.f. in th small numb r of turns, and b caus th xciting amp r -turns f orm a larg proportion of th primary amp r -turns availabl . Th ff ct is part icularly pronounc d wh n th cor diam t r has b n mad larg so as to fit ov r larg EHV bushings. 6.4.5.3 Cor -balanc curr nt transform rs Th cor -balanc CT (or CBCT) is normally of th ring typ , through th c ntr of which is pass d cabl that forms th primary winding. An arth fault r lay, conn ct d to th s condary winding, is n rgis d only wh n th r is r sidual curr nt in th primary syst m. Exciting voltag (I ) I Figur 6.11: D finition of kn -point of xcitation curv D sign r quir m nts for curr nt transform rs for g n ral prot ction purpos s ar fr qu ntly laid out in t rms of kn -point .m.f., xciting curr nt at th kn -point (or som oth r sp cifi d point) and s condary winding r sistanc . Such cu rr nt transform rs ar d signat d Class PX. 6.4.5 CT Winding Arrang m nts A numb r of CT winding arrang m nts ar us d. Th s ar d scrib d in th following s c tions. 6.4.5.1 Wound primary typ This typ of CT has conv ntional windings form d of copp r wir wound round a cor . It is us d for auxiliary curr nt transform rs and for many low or mod rat ratio curr nt transform rs us d in switchg ar o f up to 11kV rating. 6.4.5.2 Bushing or bar primary typ Many curr nt transform rs hav a ring-shap d cor , som tim s built up from annular stampings, but oft n consisting of a singl l ngth of strip tightly wound to form a clos -turn d spi ral. Th distribut d s condary winding forms a toroid which should occupy th wh ol p rim t r of th cor , a small gap b ing l ft b tw n start and finish l ads for insulation. Such curr nt transform rs normally hav a singl conc ntrically plac d primary conductor, som tim s p rman ntly built into th CT and provid d with th N twork Prot ction & Automation Guid Cor -balanc transform rs ar normally mount d ov r a cabl at a point clos up to th cabl gland of switchg ar or oth r apparatus. Physically split cor s ( sl ip-ov r typ s) ar normally availabl for applications in which th cabl s ar alr ady mad up, as on xisting switchg ar. 6.4.5.4 Summation curr nt transform rs Th summation arrang m nt is a winding arrang m nt us d in a m asuring r lay or on an auxiliary curr nt transform r to giv a singl -phas output signal havi ng a sp cific r lationship to th thr -phas curr nt input. 6.4.5.5 Air-gapp d curr nt transform rs Th s ar auxiliary curr nt transform rs in which a small a ir gap is includ d in th cor to produc a s condary voltag output proportiona l in magnitud to curr nt in th primary winding. Som tim s t rm d transactors and quadratur curr nt transform rs , this form of curr nt transform r has b n us d as an auxiliary compon nt of unit prot ction sch m s in which th outputs into multipl s condary circuits must r main lin ar for and proportion d to th wid st practical rang of input curr nts. • 87 • C u r r n t a n d Vo l t a g T r a n s f o r m r s
• I K Th advantag in using this m thod of arth fault prot ction li s in th fact th at only on CT cor is us d in plac of thr phas CT s whos s condary winding s ar r sidually conn ct d. In this way th CT magn tising curr nt at r lay op r ation is r duc d by approximat ly thr -to-on , an important consid ration in s nsitiv arth fault r lays wh r a low ff ctiv s tting is r quir d. Th numb r of s condary turns do s not n d to b r lat d to th cabl rat d curr nt b cau s no s condary curr nt would flow und r normal balanc d conditions. This allows th numb r of s condary turns to b chos n such as to optimis th ff ctiv pr imary pickup curr nt. 6•
6.4.6 Lin Curr nt CT’s CT’s for m asuring lin curr nts fall into on of thr typ s. 6.4.6.1 Ov rdim nsion d CT’s Ov rdim nsion d CT’s ar capabl of transforming ful ly offs t fault curr nts without distortion. In cons qu nc , th y ar v ry larg , as can b d duc d from S ction 6.4.10. Th y ar pron to rrors du to r man n t flux arising, for instanc , from th int rruption of h avy fault curr nts. 6.4 .6.2 Anti-r man nc CT’s This is a variation of th ov rdim nsion d curr nt transf orm r and has small gap(s) in th cor magn tic circuit, thus r ducing th possi bl r man nt flux from approximat ly 90% of saturation valu to approximat ly 10 %. Th s gap(s) ar quit small, for xampl 0.12mm total, and so th xcitation charact ristic is not significantly chang d by th ir pr s nc . How v r, th r s ulting d cr as in possibl r man nt cor flux confin s any subs qu nt d.c. flux xcursion, r sulting from primary curr nt asymm try, to within th cor saturat ion limits. Errors in curr nt transformation ar th r for significantly r duc d wh n compar d with thos with th gapl ss typ of cor . Transi nt prot ction cu rr nt transform rs ar includ d in IEC 60044-6 as typ s TPX, TPY and TPZ and thi s sp cification giv s good guidanc to th ir application and us . 6.4.6.3 Lin ar curr nt transform rs Th lin ar curr nt transform r constitut s an v n mor radical d partur from th normal solid cor CT in that it incorporat s an appr ciabl air gap, for xampl 7.510mm. As its nam impli s th magn tic b haviour t nds to lin arisation by th i nclusion of this gap in th magn tic circuit. How v r, th purpos of introducin g mor r luctanc into th magn tic circuit is to r duc th valu of magn tisin g r actanc . This in turn r duc s th s condary tim -constant of th CT, th r by r ducing th ov rdim nsioning factor n c ssary for faithful transformation. Fig ur 6.12 shows a typical mod rn CT for us on MV syst ms. 6.4.7 S condary Windin g Imp danc As a prot ction CT may b r quir d to d liv r high valu s of s conda ry curr nt, th s condary winding r sistanc must b mad as low as practicabl . S condary l akag r actanc also occurs, particularly in wound primary curr nt transform rs, although its pr cis m asur m nt is difficult. Th non-lin ar natu r of th CT magn tic circuit mak s it difficult to ass ss th d finit ohmic va lu r pr s nting s condary l akag r actanc . It is, how v r, normally acc pt d that a curr nt transform r is of th low r actanc typ provid d that th follow ing conditions pr vail: a. th cor is of th jointl ss ring typ (including spi rally wound cor s) b. th s condary turns ar substantially v nly distribut d a long th whol l ngth of th magn tic circuit c. th primary conductor(s) pass s through th approximat c ntr of th cor ap rtur or, if wound, is approximat ly v nly distribut d along th whol l ngth of th magn tic circuit d. flux q ualising windings, wh r fitt d to th r quir m nts of th d sign, consist of at l ast four parall l-conn ct d coils, v nly distribut d along th whol l ngth of th magn tic circuit, ach coil occupying on quadrant Alt rnativ ly, wh n a curr nt transform r do s not obviously comply with all of th abov r quir m nts , it may b prov d to b of low-r actanc wh r : . th composit rror, as m as ur d in th acc pt d way, do s not xc d by a factor of 1.3 that rror obtain d dir ctly from th V-I xcitation charact ristic of th s condary winding 6.4.8 S condary Curr nt Rating Th choic of s condary curr nt rating is d t rmin d la rg ly by th s condary winding burd n and th standard practic of th us r. Sta ndard CT s condary curr nt ratings ar 5A and 1A. Th burd n at rat d curr nt im pos d by digital or num rical r lays or • 88 • N twork Prot ction & Automation Guid C u r r n t a n d Vo l t a g T r a n s f o r m r s • 6• Figur 6.12: Typical mod rn CT for us on MV syst ms
instrum nts is larg ly ind p nd nt of th rat d valu of curr nt. This is b caus th winding of th d vic has to d v lop a giv n numb r of amp r -turns at rat d curr nt, so that th actual numb r of turns is inv rs ly proportional to th curr nt, and th imp danc of th winding vari s inv rs ly with th squar of th curr nt rating. How v r, l ctrom chanical or static arth-fault r lays may ha v a burd n that vari s with th curr nt tapping us d. Int rconn ction l ads do not shar this prop rty, how v r, b ing commonly of standard cross-s ction r gar dl ss of rating. Wh r th l ads ar long, th ir r sistanc may b appr ciabl , and th r sultant burd n will vary with th squar of th curr nt rating. For x ampl a CT l ad run of th ord r of 200 m tr s, a typical distanc for outdoor E HV switchg ar, could hav a loop r sistanc of approximat ly 3 ohms. Th CT l ad VA burd n if a 5A CT is us d would b 75VA, to which must b add d th r lay bu rd n (up to of p rhaps 10VA for an l ctrom chanical r lay, but l ss than 1VA fo r a num rical r lay), making a total of 85VA. Such a burd n would r quir th CT to b v ry larg and xp nsiv , particularly if a high accuracy limit factor w r also applicabl . With a 1A CT s condary rating, th l ad burd n is r duc d to 3VA, so that with th sam r lay burd n th total b com s a maximum of 13VA. Th is can b provid d by a CT of normal dim nsions, r sulting in a saving in siz , w ight and cost. H nc mod rn CT’s t nd to hav s condary windings of 1A rating. H ow v r, wh r th primary rating is high, say abov 2000A, a CT of high r s cond ary rating may b us d, to limit th numb r of s condary turns. In such a situat ion s condary ratings of 2A, 5A or, in xtr m cas s, 20A, might b us d. 6.4.9 Rat d Short-Tim Curr nt A curr nt transform r is ov rload d whil syst m shortc ircuit curr nts ar flowing and will b short-tim rat d. Standard tim s for whi ch th CT must b abl to carry rat d short-tim curr nt (STC) ar 0.25, 0.5, 1. 0, 2.0 or 3.0 s conds. A CT with a particular short-tim curr nt/ tim rating wi ll carry a low r curr nt for a long r tim in inv rs proportion to th squar o f th ratio of curr nt valu s. Th conv rs , how v r, cannot b assum d, and lar g r curr nt valu s than th S.T.C. rating ar not p rmissibl for any duration u nl ss justifi d by a n w rating t st to prov th dynamic capability. 6.4.10 Tra nsi nt R spons of a Curr nt Transform r Wh n accuracy of r spons during v ry s hort int rvals is b ing studi d, it is n c ssary to xamin what happ ns N twork Prot ction & Automation Guid wh n th primary curr nt is sudd nly chang d. Th ff cts ar most important, an d w r first obs rv d in conn ction with balanc d forms of prot ction, which w r liabl to op rat unn c ssarily wh n short-circuit curr nts w r sudd nly sta blish d. 6.4.10.1 Primary curr nt transi nt Th pow r syst m, n gl cting load ci rcuits, is mostly inductiv , so that wh n a short circuit occurs, th fault curr nt that flows is giv n by: ip = Ep R 2 + ω 2 L2 − ( R L) t [ sin ( ωt + β − α ) + sin ( α − β ) e where: Ep R L β α = pe k system e.m.f. = system resist nce = system induct nce ] …Equ tion 6.1
= initi l ph se ngle governed y inst nt of f ult occurrence = system power f c tor ngle = t n 1 ωL/R The first term of Equation 6.1 reresents the stea y state alternating current, hile the secon is a transient quantity resonsible for islacing the aveform asymmetrically. E R + ω 2 L 2 is the stea y state eak current I. The maximum transient occurs hen sin = (α β) = 1; no other condition need e ex mined. So: 2
π
− R i = I
sin
ωt −
+ e ( 2
L) t
...Equation 6.2
When the current is asse through the rimary in ing of a current transformer, the res onse can be examine by re lacing the CT ith an equivalent circuit as shon in Figure 6.9(b). As the 'i eal' CT has no losses, it ill transfer the en tire function, an all further analysis can be carrie out in terms of equivalen t secon ary quantities (is an Is). A simlifie solution is obtainable by negle e in an inuctance is obt cting the exciting current of the CT. The flux evelo aine by integrating the alie e.m.f. through a time interval: φ = K ∫t vdt 1 t2 For the CT equivalent circuit, the voltage is the drop on • 89 • …Equation 6.3 C u r r e n t a n d Vo l t a g e T r a n s o r m e r s • 6•
the burden resistance Rb. Flux (multiples o steady value) Integrating or each component in turn, the steady state peak lux is given by: 3π 2ω 20 16 12 8 T = 0.06s 4 φ A = KR b I s π ω ∫ π
sin
ωt −
t
2
KR b I s ω The transient flux is given by: = φ B = KR b I s ∫ e − ( R 0 α L) t ...Equ tion 6.4 dt = KR I s L R 0 0.05 0.1 Time (seconds) 0.15 0.2 ...Equ tion 6.5 Hence, the r tio of the tr nsient flux to the ste dy st te v lue is: B A T time const nt of prim ry circuit = wL X = R R Figure 6.13: Response of CT of infinite shunt imped nce to tr nsient symmetri c prim ry current C u r r e n t n d Vo l t g e T r n s f o r m e r s where X nd R re the prim ry system re ct nce nd resist nce v lues. The CT cor e h s to c rry oth fluxes, so th t: C = A
+ B = A Xˆ Ê Á1 + ˜ Ë R‾ ...Equ tion 6.6
The term (1+X/R) h s een c lled the 'tr nsient f ctor' (TF), the core flux ein g incre sed y this f ctor during the tr nsient symmetric current period. From this it c n e seen th t the r tio of re ct nce to resist nce of the power syste m is n import nt fe ture in the study of the eh viour of protection rel ys. Al tern tively, L/R is the prim ry system time const nt T, so th t the tr nsient f ctor c n e written: Since CT requires finite exciting current to m int in flux, it will not re m in m gnetised (neglecting hysteresis), nd for this re son complete represen t tion of the effects c n only e o t ined y including the finite induct nce of the CT in the c lcul tion. The response of current tr nsformer to tr nsient symmetric current is shown in Figure 6.14. 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.1 Ie = Tr nsient exciting current i's e 1 T1 1 e T =1 + ωL = 1 + ωT R Ie • 6•
Again, fT is the time constant exresse in cycles of the a.c. quantity T’, so tha t: TF = 1 + 2πfT = 1 + 2πT’ This latt r xpr ssion is particularly us ful wh n ass ssi ng a r cording of a fault curr nt, b caus th tim constant in cycl s can b a sily stimat d and l ads dir ctly to th transi nt factor. For xampl , a syst m cycl s r sults in a transi nt factor of (1+6π), or 19.85; tim constant of thr thatis,theCT oul be require to han le almost tenty times the maximum flux ro uce un er stea y state con itions. The above theory is sufficient to give a generalvieof the roblem. In this simlifie treatment, no reverse voltage is alie to emagnetise the CT, so that the flux oul buil u as shon in Fi gure 6.13. • 90 • Time Figure 6.14: Resonse of a current transformer to a transient asymmetric current
Netork Protection & Automation Gui e
Let: is = the nominal secon ary current i’s = th actual s condary output curr nt i = th xciting curr nt th n: is = i + i’s also, ...Equation 6.7 cor to r tain a r man nt flux m ans that th valu of B d v lop d in Equation 6.5 has to b r gard d as an incr m nt of flux from any possibl r man nt valu positiv or n gativ . Th formula would th n b r asonabl provid d th appli d curr nt transi nt did not produc saturation It will b s n that a pr cis cal culation of th flux and xcitation curr nt is not f asibl ; th valu of th st udy is to xplain th obs rv d ph nom na. Th asymm tric (or d.c.) compon nt can b r gard d as building up th m an flux ov r a p riod corr sponding to s v ral cycl s of th sinusoidal compon nt, during which p riod th latt r compon nt pr oduc s a flux swing about th varying m an l v l stablish d by th form r. Th asymm tric flux c as s to incr as wh n th xciting curr nt is qual to th t otal asymm tric input curr nt, sinc b yond this point th output curr nt, and h nc th voltag drop across th burd n r sistanc , is n gativ . Saturation mak s th point of quality b tw n th xcitation curr nt and th input occur at a flux l v l low r than would b xp ct d from lin ar th ory. Wh n th xpon ntial compon nt driv s th CT into saturation, th magn tising inductanc d cr as s, causing a larg incr as in th alt rnating compon nt i . Th total xciting cur r nt during th transi nt p riod is of th form shown in Figur 6.15 and th cor r sponding r sultant distortion in th s condary curr nt output, du to saturati on, is shown in Figur 6.16. L wh nc : di = R b i s′ dt ...Equation 6.8 R i di R i + b = b s L L dt which giv s for th transi nt t rm ...Equation 6.9 i = I1 T −t T − e −t T T1 − T 1 ( )
T1 = CT second ry circuit time const nt Le/R I1 = prospective pe k second ry cu rrent 6.4.10.2 Pr ctic l conditions Pr ctic l conditions differ from theory for the following re sons: . no ccount h s een t ken of second ry le k ge or urd en induct nce. This is usu lly smll comp red with Le so th t it h s little effe ct on the m ximum tr nsient flux . iron loss h s not een considered. This h s the effect of reducing the second ry time const nt, ut the v lue of the equiv l ent resist nce is v ri le, depending upon oth the sine nd exponenti l terms. Consequently, it c nnot e included in ny line r theory ndis too complic ted for s tisf ctory tre tment to e evolved c. the theory is sed upon line r excit tion ch r cteristic. This is only pproxim tely true up to the knee point llowing for non line rity is not pr of theexcit tion curve. Aprecise solution ctic le. Solutions h ve een sought y repl cing the excit tion curve with n um er of chords; line r n lysis c n then e m de for the extent of e ch chord The ove theory is sufficient, however, to give good insight into the pro le m nd to llow most pr ctic l issues to e decided. d. the effect of hysteresis, p rt from loss s discussed under ( ) ove, is not included. Hysteresis m kes
the induct nce different for flux uild up nd dec y, so th t the second ry tim e const nt is v ri le. Moreover, the ility of the Network Protection & Autom tion Guide Exciting current Time Figure 6.15: Typic l exciting current of CT during tr nsient symmetric input cu rrent Current Prim ry current referred to second ry
0 Second ry current Residu l flux = 0 Resistive urden Power system T.C. = 0.05s Figure 6.16: Distortion in second ry current due to s tur tion Time • 91 • C u r r e n t n d Vo l t g e T r n s f o r m e r s • where: T = prim ry system time const nt L/R 6•
The presence of residu l flux v ries the st rting point of the tr nsient flux ex cursion on the excit tion ch r cteristic. Rem nence of like pol rity to the tr n sient will reduce the v lue of symmetric current of given time const nt which th e CT c n tr nsform without severe s tur tion; conversely, reverse rem nence will gre tly incre se the ility of CT to tr nsform tr nsient current. If the CT were theline r non s tur le device considered in the n lysis, the sine curren t would e tr nsformed without loss of ccur cy. In pr ctice the v ri tion in ex cit tion induct nce c used y tr nsferring the centre of the flux swing to other points on the excit tion curve c uses n error th t m y e very l rge. The effe ct on me surement is of little consequence, ut for protection equipment th t is required to function during f ult conditions, the effect is more serious. The o utput current is reduced during tr nsient s tur tion, which m y prevent the rel ys from oper ting if the conditions re ne r to the rel y setting. This must not e confused with the incre sed r.m.s. v lue of the prim ry current due to the fe ture which sometimes offsets the incre se r tio error. symmetric tr nsient, In the c se of l nced protection, during through f ultsthe errors of the sev er l current tr nsformers m y differ nd produce n out of l nce qu ntity, c us ing unw nted oper tion. 6.4.11 H rmonics during the Tr nsient Period When CT i s required to develop high second ry e.m.f. under ste dy st te conditions, the non line rity of the excit tion imped nce c uses some distortion of the output w veform; such w veform will cont in, in ddition to the fund ment l current, odd h rmonics only. When, however, the CT is s tur ted uni direction lly while eing simult neously su jected to sm ll .c. qu ntity, s in the tr nsient cond ition discussed ove, the output will cont in oth odd nd even h rmonics. Usu lly the lower num ered h rmonics re of gretest mplitude nd the second nd th ird h rmonic components m y e of consider le v lue. This m y ffect rel ys th t re sensitive to h rmonics. 6.4.12 Test Windings On site conjunctive testing o f current tr nsformers nd the pp r tus th t they energise is often required. I t m y e difficult, however, to p ss suit le v lue of current through the pri m ry windings, ec use of the sc le of such current nd in m ny c ses ec use c cess to the prim ry conductors is difficult. Addition l windings m y e provided to m ke such tests e sier, these windings usu lly eing r ted t 10A. The test winding will inevit ly occupy ppreci le sp ce nd the • 92 • chieved; CT will cost more. This f ct should e weighed g inst the convenience very often it will e found th t the tests in question c n e repl ced y lter n tive procedures. 6.5 NOVEL INSTRUMENT TRANSFORMERS The preceding types of inst rument tr nsformers h ve ll een sed on electrom gnetic principles using m gnetic core. There re now v il le sever l new methods of tr nsforming the me sured qu ntity using optic l nd m ss st te methods. 6.5.1 Optic l Instrument Tr nsducers The key fe tures of freest nding optic l instrument tr nsducer c n e illustr ted with the function l di gr m of Figure 6.17. C u r r e n t n d Vo l t g e T r n s f o r m e r s HV Bus Insul ting function Sensor E/O converter + Communic tion Sensing function Instrument Tr nsformer Optic l link (fi re optics) Electronic interf ce Communic tion + O/E converter S econd ry output • 6•
Figure 6.17: Function l di gr m of optic l instrument tr nsducer Optic l converters nd optic l gl ss fi re ch nnels implement the link etween t he sensor nd the lowvolt ge output. The fund ment l difference etween n instr ument tr nsducer nd convention l instrument tr nsformer is the electronic int erf ce needed for its oper tion. This interf ce is required oth for the sensing function nd for d pting the new sensor technology to th t of the second ry ou tput currents nd volt ges. Non convention l optic l tr nsducers lend themselves to sm ller, lighter devices where the over ll size nd power r ting of the unit does not h ve ny signific nt e ring on the size nd the complexity of the sen sor. Sm ll, lightweight insul tor structures m y e t ilor m de to Network Protection & Autom tion Guide
fit optic l sensing devices s n integr l p rt of the insul tor. Addition lly, the non line r effects nd electrom gnetic interference pro lems in the second r y wiring of convention l VT’s and CT’s ar minimis d. Optical transduc rs can b s p arat d in two famili s: firstly th hybrid transduc rs, making us of conv ntion al l ctrical circuit t chniqu s to which ar coupl d various optical conv rt r syst ms, and s condly th ‘all-optical’ transduc rs that ar bas d on fundam ntal, o ptical s nsing principl s. 6.5.1.1 Optical s nsor conc pts C rtain optical s nsi ng m dia (glass, crystals, plastics) show a s nsitivity to l ctric and magn tic fi lds and that som prop rti s of a probing light b am can b alt r d wh n pas sing through th m. On simpl optical transduc r d scription is giv n h r in Fi gur . 6.18. Consid r th cas of a b am of light passing through a pair of polar ising filt rs. If th input and output polarising filt rs hav th ir ax s rotat d 45° from ach oth r, only half th light will com through. Th r f r nc light input int nsity is maintain d constant ov r tim . Now if th s two polarising fi lt rs r main fix d and a third polarising filt r is plac d in b tw n th m, a ra ndom rotation of this middl polaris r ith r clockwis or count r-clockwis wil l b monitor d as a varying or modulat d light output int nsity at th light d t ctor. Wh n a block of optical s nsing mat rial (glass or crystal) is imm rs d i n a varying magn tic or l ctric fi ld, it plays th rol of th ‘odd’ polaris r. Chang s in th magn tic or l ctric fi ld in which th optical s nsor is imm rs d ar monitor d as a varying int ns ity of th probing light b am at th light d t ctor. Th light output int nsity fluctuat s around th z ro-fi ld l v l qual to 50% of th r f r nc light input . This modulation of th light int nsity du to th pr s nc of varying fi lds i s conv rt d back to tim -varying curr nts or voltag s. A transduc r us s a magn to-optic ff ct s nsor for optical curr nt m asuring applications. This r fl cts th fact that th s nsor is not basically s nsitiv to a curr nt but to th mag n tic fi ld g n rat d by this curr nt. Although ‘all-fibr ’ approach s ar f asibl , most comm rcially availabl optical curr nt transduc rs r ly on a bulk-glass s nsor. Most optical voltag transduc rs, on th oth r hand, r ly on an l ctro-op tic ff ct s nsor. This r fl cts th fact that th s nsor us d is s nsitiv to t h impos d l ctric fi ld. 6.5.1.2 Hybrid transduc rs Odd polaris r input polaris r optical fibr in light sourc 45° optical s nsing m dium 90° output polaris r optical fibr out s nsing light d t ctor 1.0 1.0 + z ro fi ld l v l 0.5 0 r f r nc light input int nsity 0.5 0 t t modulat d light input int nsity Figur . 6.18: Sch matic r pr s ntation of th conc pts b hind th optical s nsin g of varying l ctric and magn tic fi lds N twork Prot ction & Automation Guid • 93 •
C u r r n t a n d Vo l t a g T r a n s f o r m r s • Th hybrid family of non-conv ntional instrum nt transduc rs can b divid d in t wo typ s: thos with activ s nsors and thos with passiv s nsors. Th id a b h ind a transduc r with an activ s nsor is to chang th xisting output of th c onv ntional instrum nt transform r into an optically isolat d output by adding a n optical conv rsion syst m (Figur 6.18). This conv rsion syst m may r quir a pow r supply of its own: this is th activ s nsor typ . Th us of an optical i solating syst m s rv s to d -coupl th instrum nt transform r output s condary voltag s and curr nts 6•
from arth d or galvanic links. Thus th only link that r mains b tw n th cont rol-room and th switchyard is a fibr optic cabl . Anoth r typ of hybrid non-c onv ntional instrum nt transform r is achi v d by r trofitting a passiv optical s nsing m dium into a conv ntional ‘hard-wir s condary’ instrum nt transform r. Th is can b t rm d as a passiv hybrid typ sinc no pow r supply of any kind is n d d at th s condary l v l. 6.5.1.3 ‘All-optical’ transduc rs Th s instrum nt tra nsform rs ar bas d ntir ly on optical mat rials and ar fully passiv . Th s n sing function is achi v d dir ctly by th s nsing mat rial and a simpl fibr op tic cabl running b tw n th bas of th unit and th s nsor location provid s th communication link. Th s nsing l m nt is mad of an optical mat rial that is position d in th l ctric or magn tic fi ld to b s ns d. In th cas of a c urr nt m asuring d vic th s nsitiv l m nt is ith r locat d fr in th magn tic fi ld (Figur 6.19(a)) or it can b imm rs d in a fi ld-shaping magn tic ‘gap’ (Figur 6.19(b)). In th cas of a voltag -s nsing d vic (Figur 6.20) th sam alt rnativ s xist, this tim for l m nts that ar s nsitiv to l ctric fi ld s. Th possibility xists of combining both s nsors within a singl housing, thu s providing both a CT and VT within a singl compact housing that giv s ris to spac savings within a substation. I AC lin curr nt analysing circuitry. In sharp contrast with a conv ntional fr -standing instrum nt transform r, th optical instrum nt transform r n ds an l ctronic int rfac modul in ord r to function. Th r for its s nsing principl (th optical mat rial) is passiv but its op rational int grity r li s on th int rfac that is p ow r d in th control room (Figur 6.21). Floating l ctrod AC lin voltag El ctro-optic s nsor Optical fibr s (a) Fr -fi ld typ R f r nc l ctrod R f r nc l ctrod C u r r n t a n d Vo l t a g T r a n s f o r m r s AC lin voltag Light path El ctro-optic s nsor (b) Fi ld shaping typ R f r nc l ctrod Optical fibr s Figur 6.20: Optical voltag s nsor bas d on th l ctrical prop rti s of optica l mat rials Optical fibr Magn to-optic s nsor Magn tic fi ld (a) Fr -fi ld typ AC lin curr nt I Optical fibr • 6• High voltag s nsor ass mbly Magn tic fi ld Gapp d Magn to-optic s nsor magn tic cor (b) Fi ld-shaping typ
Optical fibr s Fibr optic cabl Junction box Optical int rfac unit Figur 6.19: Optical curr nt s nsor bas d on th magn tic prop rti s of optical mat rials In all cas s th r is an optical fibr that chann ls th probing r f r nc light from a sourc into th m dium and anoth r fibr that chann ls th light back to • 94 • AC/DC sourc Figur 6.21: Nov l instrum nt transduc r conc pt r quiring an l ctronic int rfa c in th control room N twork Prot ction & Automation Guid
Dom El ctro-optic s nsor (bulk-glass transduc r) El ctro-optic s nsor ( all-fibr ransduc r)
t
AC lin curr nt I Bulk-glass s nsing l m nt Light in Optical fibr s Light out AC lin I curr nt H1 H2 I Fibr optic cabl conduit Liquid /solid/ gas ous int rnal insulation (a) Glass s nsor approach Insulator column Fibr junction box S nsor #1 S nsor #2 Fibr optic cabl s AC lin curr nt I Light in Optical fibr s Fibr s nsing l m nt (b) All-fibr s nsor conc pt Figur 6.22: Conc ptual d sign of a doubl -s nsor optical CT Similar to conv ntional instrum nt transform rs th r ar ‘liv tank’ and ‘d ad tank’ op tical transduc rs. Typically, curr nt transduc rs tak th shap of a clos d loo p of lighttranspar nt mat rial, fitt d around a straight conductor carrying th lin curr nt (Figur 6.22). In this cas a bulkglass s nsor unit is d pict d (Fi gur 6.22(a)), along with an ‘all-optical’ s nsor xampl , as shown in Figur 6.22(b ). Light d t ctors ar basically v ry s nsitiv d vic s and th s nsing mat rial can thus b s l ct d in such a way as to scal -up r adily for larg r curr nts. ‘A ll-optical’ voltag transduc rs how v r do not l nd th ms lv s asily for xtr m l y high lin voltag s. Two conc pts using a fullvoltag s nsor ar shown in Fig ur 6.23. Although ‘all-optical’ instrum nt transform rs w r first introduc d 10-15 y ars ago , th r ar still only a f w in s rvic nowadays. Figur 6.24 shows a fi ld inst allation of a combin d optical CT/VT. Conductor (a)
Liv tank
(b) D ad tank Figur 6.23: Optical voltag transduc r conc pts, using a ‘full-voltag ’ s nsor Figur 6.24: Fi ld installation of a combin d optical CT/VT N twork Prot ction & Automation Guid • 95 • C u r r n t a n d Vo l t a g T r a n s f o r m r s
• Light out 6•
6.5.2 Oth r S nsing Syst ms Th r ar a numb r of oth r s nsing syst ms that can b us d, as d scrib d b low. 6.5.2.1 Z ro-flux (Hall Eff ct) curr nt transform r In this cas th s nsing l m nt is a s mi-conducting waf r that is plac d in th gap of a magn tic conc ntrating ring. This typ of transform r is also s nsi tiv to d.c. curr nts. Th transform r r quir s a pow r supply that is f d from th lin or from a s parat pow r supply. Th s nsing curr nt is typically 0.1% of th curr nt to b m asur d. In its simpl st shap , th Hall ff ct voltag is dir ctly proportional to th magn tising curr nt to b m asur d. For mor accur at and mor s nsitiv applications, th s nsing curr nt is f d through a s cond ary, multipl -turn winding, plac d around th magn tic ring in ord r to balanc out th gap magn tic fi ld. This z ro-flux or null-flux v rsion allows v ry accu rat curr nt m asur m nts in both d.c. and highfr qu ncy applications. A sch mat ic r pr s ntation of th s nsing part is shown in Figur 6.25. Magn tic conc ntrator (gapp d magn tic cor ) of insulation mat rial. In most cas s th Rogowski coil will b conn ct d to an amplifi r, in ord r to d liv r suffici nt pow r to th conn ct d m asuring or pr ot ction quipm nt and to match th input imp danc of this quipm nt. Th Rogow ski coil r quir s int gration of th magn tic fi ld and th r for has a tim and phas d lay whilst th int gration is compl t d. This can b corr ct d for with in a digital prot ction r lay. Th sch matic r pr s ntation of th Rogowski coil s nsor is shown in Figur 6.27. El ctrical to optical conv rt r/transmitt r Burd n I Optical fibr s C u r r n t a n d Vo l t a g T r a n s f o r m r s Curr nt transform r I Figur 6.26: D sign principl of a hybrid magn tic curr nt transform r fitt d wi th an optical transmitt r Air cor toroidal coil i V i S nsing curr nt S nsing l m nt El ctrical to optical conv rt r Optical fibr s Figur 6.25: Conc ptual d sign of a Hall- ff ct curr nt s nsing l m nt fitt d i n a fi ld-shaping gap Curr nt carrying conductor Figur 6.27: Sch matic r pr s ntation of a Rogowski coil, us d for curr nt s nsi ng • 6• 6.5.2.2 Hybrid magn tic-optical s nsor This typ of transform r is mostly us d i n applications such as s ri s capacitiv comp nsation of long transmission lin s , wh r a non-ground d m asur m nt of curr nt is r quir d. In this cas , s v ral curr nt s nsors ar r quir d on ach phas in ord r to achi v capacitor surg prot ction and balanc . Th pr f rr d solution is to us small toroidally wound
magn tic cor transform rs conn ct d to fibr optic isolating syst ms. Th s s n sors ar usually activ s nsors in th s ns that th isolat d syst ms r quir a pow r supply. This is illustrat d in Figur 6.26. 6.5.2.3 Rogowski coils Th Ro gowski coil is bas d on th principl of an aircor d curr nt transform r with a v ry high load imp danc . Th s condary winding is wound on a toroid • 96 • N twork Prot ction & Automation Guid
• 7 • R lay T chnolog y Introduction El ctrom chanical r lays Static r lays Digital r lays Num rical r l ays 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 Additional f atur s of num rical r lays Num rical r lay issu s R f r nc s
• 7 • R lay T ch n o l o g y 7. 1 I N T R OD U C T I O N Th last thirty y ars hav s n normous chang s in r lay t chnology. Th l ctrom chanical r lay in all of its diff r nt forms has b n r plac d succ ssiv ly by static, digital and num rical r lays, ach chang bringing with it r ductions and siz and improv m nts in functionality. At th s am tim , r liability l v ls hav b n maintain d or v n improv d and availabil ity significantly incr as d du to t chniqu s not availabl with old r r lay typ s. This r pr s nts a tr m ndous achi v m nt for all thos involv d in r lay d s ign and manufactur . This chapt r charts th cours of r lay t chnology through th y ars. As th purpos of th book is to d scrib mod rn prot ction r lay pra ctic , it is natural th r for to conc ntrat on digital and num rical r lay t c hnology. Th vast numb r of l ctrom chanical and static r lays ar still giving d p ndabl s rvic , but d scriptions on th t chnology us d must n c ssarily b som what bri f. For thos int r st d in th t chnology of l ctrom chanical and static t chnology, mor d tail d d scriptions can b found in r f r nc [7.1]. 7. 2 E L E C T R OM EC H A NI C AL RE L AY S Th s r lays w r th arli st form s of r lay us d for th prot ction of pow r syst ms, and th y dat back n arly 1 00 y ars. Th y work on th principl of a m chanical forc causing op ration of a r lay contact in r spons to a stimulus. Th m chanical forc is g n rat d thr ough curr nt flow in on or mor windings on a magn tic cor or cor s, h nc th t rm l ctrom chanical r lay. Th principl advantag of such r lays is that th y provid galvanic isolation b tw n th inputs and outputs in a simpl , ch ap and r liabl form – th r for for simpl on/off switching functions wh r th outp ut contacts hav to carry substantial curr nts, th y ar still us d. El ctrom ch anical r lays can b classifi d into s v ral diff r nt typ s as follows: a. attr act d armatur b. moving coil c. induction d. th rmal . motor op rat d f. m cha nical How v r, only attract d armatur typ s hav significant N twork Prot ction & Automation Guid • 99 •
application at this tim , all oth r typ s having b n sup rs d d by mor mod rn quival nts. 7.2.1 Attract d Armatur R lays Th s g n rally consist of an ironcor d l ctromagn t that attracts a hing d armatur wh n n rgis d. A r storing forc is provid d by m ans of a spring or gravity so that th armatur will r tu rn to its original position wh n th l ctromagn t is d - n rgis d. Typical form s of an attract d armatur r lay ar shown in Figur 7.1. Mov m nt of th armatu r caus s contact closur or op ning, th armatur ith r carrying a moving cont act that ngag s with a fix d on , or caus s a rod to mov that brings two conta cts tog th r. It is v ry asy to mount multipl contacts in rows or stacks, and h nc caus a singl input to actuat a numb r of outputs. Th contacts can b m ad quit robust and h nc abl to mak , carry and br ak r lativ ly larg curr n ts und r quit on rous conditions (highly inductiv circuits). This is still a s ignificant advantag of this typ of r lay that nsur s its continu d us . Figur 7.2: Typical polaris d r lay • 7• Figur 7.1: Typical attract d armatur r lays Th n rgising quantity can b ith r an a.c. or a d.c. curr nt. If an a.c. curr nt is us d, m ans must b provid d to pr v nt th chatt r that would occur from th flux passing through z ro v ry half cycl . A common solution to th probl m is to split th magn tic pol and provid a copp r loop round on half. Th fl ux chang is now phas -shift d in this pol , so that at no tim is th total flu x qual to z ro. Conv rs ly, for r lays n rgis d using a d.c. curr nt, r man nt flux may pr v nt th r lay from r l asing wh n th actuating curr nt is r mov d . This can b avoid d by pr v nting th armatur from contacting th l ctromagn t by a non-magn tic stop, or constructing th l ctromagn t using a mat rial wi th v ry low r man nt flux prop rti s. Figur 7.3: Typical attract d armatur r lay mount d in cas Op rating sp d, pow r consumption and th numb r and typ of contacts r quir d ar a function of th d sign. Th typical attract d armatur r lay has an op rat ing sp d of b tw n 100ms and 400ms, but r d r lays (whos us spann d a r lat iv ly short p riod in th history of prot ction r lays) with light curr nt conta cts can b d sign d to hav an op rating tim of as littl as 1ms c. Op rating p ow r is typically 0.05-0.2 watts, but could b as larg as 80 watts for a r lay with s v ral h avy-duty contacts and a high d gr of r sistanc to m chanical s hock. Som applications r quir th us of a polaris d r lay. This • 100 • N twork Prot ction & Automation Guid
can b simply achi v d by adding a p rman nt magn t to th basic l ctromagn t. Both s lf-r s t and bi-stabl forms can b achi v d. Figur 7.2 shows th basic construction. On possibl xampl of us is to provid v ry fast op rating tim s for a singl contact, sp ds of l ss than 1ms b ing possibl . Figur 7.3 illus trat s a typical xampl of an attract d armatur r lay. 7 . 3 S TAT IC RE L AY S Figur 7.4: Circuit board of static r lay Th t rm ‘static’ impli s that th r lay has no moving parts. This is not strictly t h cas for a static r lay, as th output contacts ar still g n rally attract d armatur r lays. In a prot ction r lay, th t rm ‘static’ r f rs to th abs nc of moving parts to cr at th r lay charact ristic. Introduction of static r lays b gan in th arly 1960’s. Th ir d sign is bas d on th us of analogu l ctronic d vic s inst ad of coils and magn ts to cr at th r lay charact ristic. Early v rsions us d discr t d vic s such as transistors and diod s in conjunction with r sistors, capacitors, inductors, tc., but advanc s in l ctronics nabl d th us of lin ar and digital int grat d circuits in lat r v rsions for signal proc ssing and impl m ntation of logic functions. Whil basic circuits may b common to a numb r of r lays, th packaging was still ss ntially r strict d to a sing l prot ction function p r cas , whil compl x functions r quir d s v ral cas s of hardwar suitably int rconn ct d. Us r programming was r strict d to th basi c functions of adjustm nt of r lay charact ristic curv s. Th y th r for can b vi w d in simpl t rms as an analogu l ctronic r plac m nt for l ctrom chanic al r lays, with som additional fl xibility in s ttings and som saving in spac r quir m nts. In som cas s, r lay burd n is r duc d, making for r duc d CT/VT output r quir m nts. A numb r of d sign probl ms had to b solv d with static r lays. In particular, th r lays g n rally r quir a r liabl sourc of d.c. pow r and m asur s to pr v nt damag to vuln rabl l ctronic circuits had to b d vis d. Substation nvi ronm nts ar particularly hostil to l ctronic circuits du to l ctrical int r f r nc of various forms that ar commonly found ( .g. switching op rations and th ff ct of faults). Whil it is possibl to arrang for th d.c. supply to b g n rat d from th m asur d quantiti s of th r lay, this has th disadvantag of incr asing th burd n on th CT’s or VT’s, and th r will b a minimum primary cu rr nt or voltag b low which th r lay will not op rat . This dir ctly aff cts t h possibl s nsitivity of th r lay. So provision of an ind p nd nt, highly r l iabl and s cur sourc of r lay pow r supply was an important consid ration. To pr v nt malop ration or d struction of l ctronic d vic s during faults or swit ching op rations, s nsitiv circuitry is hous d in a shi ld d cas to xclud co mmon mod and radiat d int rf r nc . Th d vic s may also b s nsitiv to static charg , r quiring sp cial pr cautions during handling, as damag from this caus may not b imm diat ly appar nt, but b com appar nt lat r in th form of pr m atur failur of th r lay. Th r for , radically diff r nt r lay manufacturing f aciliti s ar r quir d compar d to l ctrom chanical r lays. Calibration and r p air is no long r a task p rform d in th fi ld without sp cialis d quipm nt. Fi gur 7.4 shows th circuit board for a simpl static r lay and Figur 7.5 shows xampl s of simpl and compl x static r lays. • 7• Figur 7.5: S l ction of static r lays N twork Prot ction & Automation Guid • 101 •
7 . 4 D I G I TA L R E L AY S Digital prot ction r lays introduc d a st p chang in t chnology. Microproc ssors and microcontroll rs r plac d analogu circuits us d in static r lays to impl m nt r lay functions. Early xampl s b gan to b i ntroduc d into s rvic around 1980, and, with improv m nts in proc ssing capacit y, can still b r gard d as curr nt t chnology for many r lay applications. How v r, such t chnology will b compl t ly sup rs d d within th n xt fiv y ars by num rical r lays. Compar d to static r lays, digital r lays introduc A/D conv rsion of all m asur d analogu quantiti s and us a microproc ssor to impl m nt th prot ction algorithm. Th microproc ssor may us som kind of counting t chn iqu , or us th Discr t Fouri r Transform (DFT) to impl m nt th algorithm. Ho w v r, th typical microproc ssors us d hav limit d proc ssing capacity and m m ory compar d to that provid d in num rical r lays. Th functionality t nds th r for to b limit d and r strict d larg ly to th prot ction function its lf. Add itional functionality compar d to that provid d by an l ctrom chanical or stati c r lay is usually availabl , typically taking th form of a wid r rang of s tt ings, and gr at r accuracy. A communications link to a r mot comput r may also b provid d. Th limit d pow r of th microproc ssors us d in digital r lays r s tricts th numb r of sampl s of th wav form that can b m asur d p r cycl . Thi s, in turn, limits th sp d of op ration of th r lay in c rtain applications. Th r for , a digital r lay for a particular prot ction function may hav a long r op ration tim than th static r lay quival nt. How v r, th xtra tim is no t significant in t rms of ov rall tripping tim and possibl ff cts of pow r sy st m stability. Exampl s of digital r lays ar shown in Figur 7.6. 7. 5 N U M E RI C A L RE L AY S Th distinction b tw n digital and num rical r lay r sts on points of fin t chnical d tail, and is rar ly found in ar as oth r than Prot ction. Th y can b vi w d as natural d v lopm nts of digital r lays a s a r sult of advanc s in t chnology. Typically, th y us a sp cialis d digital signal proc ssor (DSP) as th computational hardwar , tog th r with th associat d softwar tools. Th input analogu signals ar conv rt d into a digital r pr s ntation and proc ss d according to th appropriat math matical algorithm. Pro c ssing is carri d out using a sp cialis d microproc ssor that is optimis d for signal proc ssing applications, known as a digital signal proc ssor or DSP for s hort. Digital proc ssing of signals in r al tim r quir s a v ry high pow r micr oproc ssor. In addition, th continuing r duction in th cost of microproc ssors and r lat d digital d vic s (m mory, I/O, tc.) naturally l ads to an approach wh r a singl it m of hardwar is us d to provid a rang of functions (‘on -box solution’ approach). By using multipl microproc ssors to provid th n c ssary co mputational p rformanc , a larg numb r of functions pr viously impl m nt d in s parat it ms of hardwar can now b includ d within a singl it m. Tabl 7.1 pr ovid s a list of typical functions availabl , whil Tabl 7.2 summaris s th adv antag s of a mod rn num rical r lay ov r th static quival nt of only 10-15 y a rs ago. Figur 7.7 shows typical num rical r lays, and a circuit board is shown in Figur 7.8. Figur 7.9 provid s an illustration of th savings in spac possi bl on a HV f d r showing th spac r quir m nt for r lays with l ctrom chanic al and num rical r lay t chnology to provid th sam functionality. Distanc Prot ction- s v ral sch m s including us r d finabl ) Ov rcurr nt Prot ction (dir ctional/non-dir ctional) S v ral S tting Groups for prot ction valu s Switch-on-to-Fault Prot ction • 7• Pow r Swing Blocking Voltag Transform r Sup rvision N gativ S qu nc Curr nt P rot ction Und rvoltag Prot ction Ov rvoltag Prot ction CB Fail Prot ction Faul t Location CT Sup rvision VT Sup rvision Ch ck Synchronisation Autor clos CB Co ndition Monitoring CB Stat Monitoring Us r-D finabl Logic Brok n Conductor D t ction M asur m nt of Pow r Syst m Quantiti s (Curr nt, Voltag , tc.) Fault/Ev nt/Disturbanc r cord r Tabl 7.1: Num rical distanc r lay f atur s Figur 7.6:
S l ction of digital r lays • 102 • N twork Prot ction & Automation Guid
Figur 7.7: Typical num rical r lays S v ral s tting groups Wid r rang of param t r adjustm nt R mot communications built in Int rnal Fault diagnosis Pow r syst m m asur m nts availabl Distanc to fault locator Disturbanc r cord r Auxiliary prot ction functions ( brok n co nductor, n gativ s qu nc , tc.) CB monitoring (stat , condition) Us r-d finabl logic Backup prot ction functions in-built Consist ncy of op ration tim s - r duc d grading margin Tabl 7.2: Advantag s of num rical prot ction r lays ov r s tatic B caus a num rical r lay may impl m nt th functionality that us d to r quir s v ral discr t r lays, th r lay functions (ov rcurr nt, arth fault, tc.) ar now r f rr d to as b ing ‘r lay l m nts’, so that a singl r lay (i. . an it m of hardwar hous d in a singl cas ) may impl m nt s v ral functions using s v ral r lay l m nts. Each r lay l m nt will typically b a softwar routin or routi n s. Th argum nt against putting many f atur s into on pi c of hardwar c ntr s on th issu s of r liability and availability. A failur of a num rical r lay may caus many mor functions to b lost, compar d to applications wh r diff r nt functions ar impl m nt d by s parat hardwar it ms. Comparison of r liabil ity and availability b tw n th two m thods is compl x as int rd p nd ncy of l m nts of an application provid d by s parat r lay l m nts n ds to b tak n i nto account. With th xp ri nc gain d with static and digital r lays, most har dwar failur m chanisms ar now w ll • 7• N twork Prot ction & Automation Guid • 103 •
und rstood and suitabl pr cautions tak n at th d sign stag . Softwar probl ms ar minimis d by rigorous us of softwar d sign t chniqu s, xt nsiv prototyp t sting (s Chapt r 21) and th ability to download am nd d softwar into m m ory (possibly using a r mot t l phon link for download). Practical xp ri nc indicat s that num rical r lays ar at l ast as r liabl and hav at l ast as go od a r cord of availability as r lays of arli r t chnologi s. As th t chnology of num rical r lays has only b com availabl in r c nt y ars, a pr s ntation o f th conc pts b hind a num rical r lay is pr s nt d in th following s ctions. 7.5.1 Hardwar Archit ctur Th typical archit ctur of a num rical r lay is sho wn in Figur 7.10. It consists of on or mor DSP microproc ssors, som m mory, digital and analogu input/output (I/O), and a pow r supply. Wh r multipl proc ssors ar provid d, it is usual for on of th m to b d dicat d to x cuting th prot ction r lay algorithms, whil th r maind r impl m nts any associat d log ic and handl s th Human Machin Int rfac (HMI) int rfac s. By organising th I /O on a s t of plug-in print d circuit boards (PCB’s), additional I/O up to th li mits of th hardwar /softwar can b asily add d. Th int rnal communications b us links th hardwar and th r for is critical compon nt in Figur 7.8: Circuit board for num rical r lay th d sign. It must work at high sp d, us low voltag l v ls and y t b immun to conduct d and radiat d int rf r nc from th l ctrically noisy substation nvironm nt. Exc ll nt shi lding of th r l vant ar as is th r for r quir d. Dig ital inputs ar optically isolat d to pr v nt transi nts b ing transmitt d to th int rnal circuitry. Analogu inputs ar isolat d using pr cision transform rs to maintain m asur m nt accuracy whil r moving harmful • 7• Figur 7.9: Spac r quir m nts of diff r nt r lay t chnologi s for sam function ality • 104 • N twork Prot ction & Automation Guid
Figur 7.10: R lay modul s and information flow transi nts. Additionally, th input signals must b amplitud limit d to avoid t h m xc ding th pow r supply voltag s, as oth rwis th wav form will app ar d istort d, as shown in Figur 7.11. Analogu signals ar conv rt d to digital for m using an A/D conv rt r. Th ch ap st m thod is to us a singl A/D conv rt r, pr c d d by a multipl x r to conn ct ach of th input signals in turn to th co nv rt r. Th signals may b initially input to a numb r of simultan ous sampl -a nd–hold circuits prior to multipl xing, or th tim r lationship b tw n succ ssiv sampl s must b known if th phas r lationship b tw n signals is important. Th alt rnativ is to provid ach input with a d dicat d A/D conv rt r, and log ic to nsur that all conv rt rs p rform th m asur m nt simultan ously. Th fr qu ncy of sampling must b car fully consid r d, as th Nyquist crit rion appli s: fs 2 x fh wh r : fs = sampling fr qu ncy fh = high st fr qu ncy of int r st If too low a s ampling fr qu ncy is chos n, aliasing of th input signal can occur (Figur 7.12 ), r sulting in high fr qu nci s app aring as part of signal in th fr qu ncy ra ng of int r st. Incorr ct r sults will th n b obtain d. Th solution is to app ly an anti-aliasing filt r, coupl d with an appropriat choic of sampling fr qu ncy, to th analogu signal, so thos fr qu ncy compon nts that could caus ali asing ar filt r d out. Digital sin and cosin filt rs ar us d (Figur 7.13), with a fr qu ncy r spons shown in Figur 7.14, to xtract th r al and imaginar y compon nts of th signal. Fr qu ncy tracking of th input signals is appli d t o adjust th sampling fr qu ncy so that th d sir d numb r of sampl s/cycl is a lways obtain d. A mod rn num rical r lay may sampl ach analogu input quantity at b tw n 16 and 24 sampl s p r cycl . • 7• N twork Prot ction & Automation Guid • 105 •
Figur 7.11: Signal distortion du to xc ssiv amplitud All subs qu nt signal proc ssing is carri d out digitally in softwar , final dig ital outputs us r lays to provid isolation or ar s nt via an xt rnal communi cations bus to oth r d vic s. 7.5.2 R lay Softwar Th softwar provid d is comm only organis d into a s ri s of tasks, op rating in r al tim . An ss ntial comp on nt is th R al Tim Op rating Syst m (RTOS), whos function is to nsur that th oth r tasks ar x cut d as and wh n r quir d, on a priority basis. Oth r t ask softwar provid d will naturally vary according to th function of th sp ci fic r lay, but can b g n ralis d as follows: a. syst m s rvic s softwar – this i s akin to th BIOS of an ordinary PC, and controls th low-l v l I/O for th r l ay (i. . driv rs for th r lay hardwar , boot-up s qu nc , tc.) • b. HMI int rfac softwar – th high l v l softwar for communicating with a us r, via th front pan l controls or through a data link to anoth r comput r running suitabl softwar , storag of s tting data, tc. c. application softwar – this i s th softwar that d fin s th prot ction function of th r lay d. auxiliary fu nctions – softwar to impl m nt oth r f atur s off r d in th r lay – oft n structur d as a s ri s of modul s to r fl ct th options off r d to a us r by th manufa ctur r 7.5.3 Application Softwar Th r l vant softwar algorithm is th n appli d. Firstly, th valu s of th quantiti s of int r st hav to b d t rmin d from th availabl information contain d in th data sampl s. This is conv ni ntly do n by th application of th Discr t Fouri r Transform (DFT), and 7• Figur 7.12: Signal aliasing probl m • 106 • N twork Prot ction & Automation Guid
b. tim r xpir d – action alarm/trip c. valu r turn d b low s tting – r s t tim rs, tc. d. valu b low s tting – do nothing . valu still abov s tting – incr m nt t im r, tc. Sinc th ov rall cycl tim for th softwar is known, tim rs ar g n rally impl m nt d as count rs. 7. 6 A D D I T IO N A L F E AT U R E S OF N U M E RI C A L RE L AY S Th DSP chip in a num rical r lay is normally of suffici n t proc ssing capacity that calculation of th r lay prot ction function only occ upi s part of th proc ssing capacity. Th xc ss capacity is th r for availabl to p rform oth r functions. Of cours , car must b tak n n v r to load th pr oc ssor b yond capacity, for if this happ ns, th prot ction algorithm will not compl t its calculation in th r quir d tim and th prot ction function will b compromis d. Typical functions that may b found in a num rical r lay b sid s prot ction functions ar d scrib d in this s ction. Not that not all functions may b found in a particular r lay. In common with arli r g n rations of r lays , manufactur rs, in accordanc with th ir p rc iv d mark t s gm ntation, will of f r diff r nt v rsions off ring a diff r nt s t of functions. Function param t r s will g n rally b availabl for display on th front pan l of th r lay and al so via an xt rnal Figur 7.13: Digital filt rs th r sult is magnitud and phas information for th s l ct d quantity. This ca lculation is r p at d for all of th quantiti s of int r st. Th quantiti s can th n b compar d with th r lay charact ristic, and a d cision mad in t rms of th following: a. valu abov s tting – start tim rs, tc. • 7• Figur 7.14: Filt r fr qu ncy r spons N twork Prot ction & Automation Guid • 107 •
communications port, but som by th ir natur may only b availabl at on outpu t int rfac . 7.6.1 M asur d Valu s Display This is p rhaps th most obvious and simpl function to impl m nt, as it involv s th l ast additional proc ssor tim . Th valu s that th r lay must m asur to p rform its prot ction function hav alr ady b n acquir d and proc ss d. It is th r for a simpl task to display t h m on th front pan l, and/or transmit as r quir d to a r mot comput r/HMI sta tion. L ss obvious is that a numb r of xtra quantiti s may b abl to b d riv d from th m asur d quantiti s, d p nding on th input signals availabl . Th s might includ : a. s qu nc quantiti s (positiv , n gativ , z ro) b. pow r, r act iv pow r and pow r factor c. n rgy (kWh, kvarh) d. max. d mand in a p riod (kW , kvar; av rag and p ak valu s) . harmonic quantiti s f. fr qu ncy g. t mp rat ur s/RTD status h. motor start information (start tim , total no. of starts/r ac c l rations, total running tim i. distanc to fault Th accuracy of th m asur d valu s can only b as good as th accuracy of th transduc rs us d (VT’s CT’s, A/D conv rt r, tc.). As CT’s and VT’s for prot ction functions may hav a diff r nt ac curacy sp cification to thos for m t ring functions, such data may not b suffi ci ntly accurat for tariff purpos s. How v r, it will b suffici ntly accurat for an op rator to ass ss syst m conditions and mak appropriat d cisions. position-switch outputs can b conn ct d to th r lay digital inputs and h nc p rovid th indication of stat via th communications bus to a r mot control c ntr . Circuit br ak rs also r quir p riodic maint nanc of th ir op rating m ch anisms and contacts to nsur th y will op rat wh n r quir d and that th fault capacity is not aff ct d adv rs ly. Th r quir m nt for maint nanc is a functi on of th numb r of trip op rations, th cumulativ curr nt brok n and th typ of br ak r. A num rical r lay can r cord all of th s param t rs and h nc b co nfigur d to s nd an alarm wh n maint nanc is du . If maint nanc is not carri d out within d fin d crit ria (such as a pr -d fin d tim or numb r of trips) aft r maint nanc is r quir d, th CB can b arrang d to trip and lockout, or inhib it c rtain functions such as auto-r clos . Finally, as w ll as tripping th CB a s r quir d und r fault conditions, it can also b arrang d for a digital output to b us d for CB closur , so that s parat CB clos control circuits can b li minat d. 7.6.4 Disturbanc R cord r Th r lay m mory r quir s a c rtain minimum numb r of cycl s of m asur d data to b stor d for corr ct signal proc ssing and d t ction of v nts. Th m mory can asily b xpand d to allow storag of a gr at r tim p riod of input data, both analogu and digital, plus th stat of th r lay outputs. It th n has th capability to act as a disturbanc r cord r for th circuit b ing monitor d, so that by fr zing th m mory at th instant of f ault d t ction or trip, a r cord of th disturbanc is availabl for lat r downl oad and analysis. It may b inconv ni nt to download th r cord imm diat ly, so faciliti s may b provid d to captur and stor a numb r of disturbanc s. In ind ustrial and small distribution n tworks, this may b all that is r quir d. In tr ansmission n tworks, it may b n c ssary to provid a singl r cord r to monitor a numb r of circuits simultan ously, and in this cas , a s parat disturbanc r cord r will still b r quir d. 7.6.5 Tim Synchronisation Disturbanc r cords a nd data r lating to n rgy consumption r quir s tim tagging to s rv any us ful purpos . Although an int rnal clock will normally b pr s nt, this is of limit d accuracy and us of this clock to provid tim information may caus probl ms if th disturbanc r cord has to b corr lat d with similar r cords from oth r s ourc s to obtain a compl t pictur of an v nt. Many num rical r lays hav th facility for tim synchronisation from an xt rnal clock. Th standard normally us d is an IRIG-B signal, which may b d riv d from a numb r of sourc s, th lat st b ing from a GPS sat llit syst m. • 7• 7.6.2 VT/CT Sup rvision If suitabl VT’s ar us d, sup rvision of th VT/CT suppli s can b mad availabl . VT sup rvision is mad mor complicat d by th diff r
nt conditions und r which th r may b no VT signal – som of which indicat VT fa ilur and som occur b caus of a pow r syst m fault having occurr d. CT sup rvi sion is carri d out mor asily, th g n ral principl b ing th calculation of a l v l of n gativ s qu nc curr nt that is inconsist nt with th calculat d va lu of n gativ s qu nc voltag . 7.6.3 CB Control/Stat Indication /Condition M onitoring Syst m op rators will normally r quir knowl dg of th stat of all c ircuit br ak rs und r th ir control. Th CB • 108 • N twork Prot ction & Automation Guid
7.6.6 Programmabl Logic Logic functions ar w ll suit d to impl m ntation using microproc ssors. Th impl m ntation of logic in a r lay is not n w, as function s such as int rtripping and autor clos r quir a c rtain amount of logic. How v r, by providing a substantial numb r of digital I/O and making th logic capabl of b ing programm d using suitabl off-lin softwar , th functionality of suc h sch m s can b nhanc d and/or additional f atur s provid d. For instanc , an ov rcurr nt r lay at th r c iving nd of a transform r f d r could us th t m p ratur inputs provid d to monitor transform r winding t mp ratur and provid alarm/trip faciliti s to th op rator/upstr am r lay, liminating th n d for a s parat winding t mp ratur r lay. This is an l m ntary xampl , but oth r ad vantag s ar vid nt to th r lay manufactur r – diff r nt logic sch m s r quir d by diff r nt Utiliti s, tc., no long r n d s parat r lay v rsions or som har d-wir d logic to impl m nt, r ducing th cost of manufactur . It is also asi r to customis a r lay for a sp cific application, and liminat oth r d vic s tha t would oth rwis b r quir d. 7.6.7 Provision of S tting Groups Historically, l ctrom chanical and static r lays hav b n provid d with only on group of s t tings to b appli d to th r lay. Unfortunat ly, pow r syst ms chang th ir topo logy du to op rational r asons on a r gular basis. ( .g. supply from normal/ m rg ncy g n ration). Th diff r nt configurations may r quir diff r nt r lay s t tings to maintain th d sir d l v l of n twork prot ction (sinc , for th abov xampl , th fault l v ls will b significantly diff r nt on parts of th n twor k that r main n rgis d und r both conditions). This probl m can b ov rcom by th provision within th r lay of a numb r of s tting groups, only on of which is in us at any on tim . Chang ov r b tw n groups can b achi v d from a r mo t command from th op rator, or possibly through th programmabl logic syst m. This may obviat th n d for duplicat r lays to b fitt d with som form of s witching arrang m nt of th inputs and outputs d p nding on n twork configuratio n. Th op rator will also hav th ability to r mot ly program th r lay with a group of s ttings if r quir d. 7.6.8 Conclusions Th provision of xtra faciliti s in num rical r lays may avoid th n d for oth r m asur m nt/control d vic s to b fitt d in a substation. A tr nd can th r for b disc rn d in which prot c tion r lays ar provid d with functionality that in th past has b n provid d u sing s parat quipm nt. Th prot ction r lay no long r p rforms a basic prot ction function; but is b coming an int gral and major part of a substation automation sch m . Th choic of a prot ction r lay rath r than som oth r d vic is logical, as th prot ction r lay is probably th only d vi c that is virtually mandatory on circuits of any significant rating. Thus, th functions pr viously carri d out by s parat d vic s such as bay controll rs, di scr t m t ring transduc rs and similar d vic s ar now found in a prot ction r lay. It is now possibl to impl m nt a substation automation sch m using num ri cal r lays as th principal or ind d only hardwar provid d at bay l v l. As th pow r of microproc ssors continu s to grow and pr ssur on op rators to r duc costs continu s, this tr nd will probably continu , on obvious d v lopm nt b i ng th provision of RTU faciliti s in d signat d r lays that act as local conc n trators of information within th ov rall n twork automation sch m . 7. 7 N U M E RI C A L RE L AY I S S U E S Th introduction of num rical r lays r plac s som of th issu s of pr vious g n rations of r lays with n w on s. Som of th n w issu s that must b addr ss d ar as follows: a. softwar v rsion control b. r lay data manag m nt c. t sting and commissioning 7.7.1 Softwar V rsion Control Num rical r lays p rform th ir functions by m ans of softwar . Th proc ss us d for softwar g n ration is no diff r nt in principl to that for any oth r d vic using r al-tim softwar , and includ s th difficulti s of d v loping cod tha t is rror-fr . Manufactur rs must th r for pay particular att ntion to th m thodology us d for softwar g n ration and t sting to nsur that as far as poss ibl , th cod contains no rrors. How v r, it is virtually impossibl to p rfor m int rnal t sts that cov r all possibl combinations of xt rnal ff cts, tc., and th r for it must b acc pt d that rrors may xist. In this r sp ct, softw ar us d in r lays is no diff r nt to any oth r softwar , wh r us rs acc pt tha t fi ld us may uncov r rrors that may r quir chang s to th softwar . Obvious
ly, typ t sting can b xp ct d to prov that th prot ction functions impl m n t d by th r lay ar carri d out prop rly, but it has b n known for failur s of rar ly us d auxiliary functions to occur und r som conditions. Wh r probl ms ar discov r d in softwar subs qu nt to th r l as of a num rical r lay for sa l , a n w v rsion of th softwar may b consid r d n c ssary. This proc ss th n r quir s som form of softwar v rsion control to b impl m nt d to k p track of: • 7• N twork Prot ction & Automation Guid • 109 •
a. th diff r nt softwar v rsions in xist nc b. th diff r nc s b tw n ach v rsion c. th r asons for th chang d. r lays fitt d with ach of th v rsions With an ff ctiv v rsion control syst m, manufactur rs ar abl to advis us r s in th v nt of r port d probl ms if th probl m is a known softwar r lat d p robl m and what r m dial action is r quir d. With th aid of suitabl softwar h ld by a us r, it may b possibl to download th n w softwar v rsion inst ad o f r quiring a visit from a s rvic ngin r. 7.7.2 R lay Data Manag m nt A num r ical r lay usually provid s many mor f atur s than a r lay using static or l c trom chanical t chnology. To us th s f atur s, th appropriat data must b n t r d into th m mory of th r lay. Us rs must also k p a r cord of all of th data, in cas of data loss within th r lay, or for us in syst m studi s, tc. Th amount of data p r num rical r lay may b 10-50 tim s that of an quival nt l ctrom chanical r lay, to which must b add d th possibility of us r-d fin d logic functions. Th task of nt ring th data corr ctly into a num rical r lay b com s a much mor compl x task than pr viously, which adds to th possibility of a mistak b ing mad . Similarly, th amount of data that must b r cord d is much larg r, giving ris pot ntially to probl ms of storag . Th probl ms hav b n addr ss d by th provision of softwar to automat th pr paration and downl oad of r lay s tting data from a portabl comput r conn ct d to a communications port of th r lay. As part of th proc ss, th s tting data can b r ad back fr om th r lay and compar d with th d sir d s ttings to nsur that th download has b n rror-fr . A copy of th s tting data (including us r d fin d logic sc h m s wh r us d) can also b stor d on th comput r, for lat r printout and/or upload to th us rs databas faciliti s. Mor advanc d softwar is availabl to p rform th abov functions from an Engin ring Comput r in a substation automat ion sch m – s Chapt r 24 for d tails of such sch m s). 7.7.3 R lay T sting and Commissioning Th t sting of r lays bas d on softwar is of n c ssity radically diff r nt from arli r g n rations of r lays. Th topic is d alt with in d tail in Chapt r 21, but it can b m ntion d h r that sit commissioning is usually r strict d to th in-built softwar s lf-ch ck and v rification that curr nts and voltag s m asur d by th r lay ar corr ct. Probl ms r v al d by such t sts r q uir sp cialist quipm nt to r solv , and h nc fi ld policy is usually on a r pair-by-r plac m nt basis. 7. 8 R E F E RE N CE S 7.1 Prot ctiv R lays Application Guid , 3rd dition. AREVA T&D Prot ction and Control, 1987. • 7• • 110 • N twork Prot ction & Automation Guid
• 8 • Prot ction: Signalling and Int rtripping Introduction Unit prot ction sch m s T l prot ction commands Int rtripping P rfo rmanc r quir m nts Transmission m dia, int rf r nc and nois M thods of signal ling 8.1 8.2 8.3 8.4 8.5 8.6 8.7
• 8 • P rot ction: Signalling and Int rtripping 8.1 INTRODUCTION Unit prot ction sch m s, form d by a numb r of r lays locat d r mot ly from ach oth r, and som distanc prot ction sch m s, r quir som form of communication b tw n ach location in ord r to achi v a unit prot ction fu nction. This form of communication is known as prot ction signalling. Additional ly communications faciliti s ar also r quir d wh n r mot op ration of a circui t br ak r is r quir d as a r sult of a local v nt. This form of communications is known as int rtripping. Th communication m ssag s involv d may b quit simp l , involving instructions for th r c iving d vic to tak som d fin d action (trip, block, tc.), or it may b th passing of m asur d data in som form from on d vic to anoth r (as in a unit prot ction sch m ). Various typ s of commun ication links ar availabl for prot ction signalling, for xampl : i. privat p ilot wir s install d by th pow r authority ii. pilot wir s or chann ls communic ations company r nt d from a iii. carri r chann ls at high fr qu nci s ov r th pow r lin s iv. radio chann l s at v ry high or ultra high fr qu nci s v. optical fibr s Wh th r or not a part icular link is us d d p nds on factors such as th availability of an appropriat communication n twork, th distanc b tw n prot ction r laying points, th t rrain ov r which th pow r n twork is construct d, as w ll as cost. Prot ction s ignalling is us d to impl m nt Unit Prot ction sch m s, provid t l prot ction c ommands, or impl m nt int rtripping b tw n circuit br ak rs. 8 . 2 U N I T P R OT E C T I O N S C H E M E S Phas comparison and curr nt diff r ntial sch m s u s signalling to conv y information conc rning th r laying quantity - phas ang l of curr nt and phas and • 113 • N twork Prot ction & Automation Guid
Pow r transmission lin Trip I V Int rtrip Int rtrip V I Trip P rmissiv trip P rmissiv trip Blocking Prot ction r lay sch m T l prot ction command (s nd) Communication link Blocking T l prot ction command (r c iv ) Prot ction r lay sch m T l m try T l m try P rot ction: Signalling and Int rtripping T l control T l control T l phon T l phon Data Communication syst ms Data Communication syst ms Figur 8.1: Application of prot ction signalling and its r lationship to oth r s yst ms using communication (shown as a unidir ctional syst m for simplicity) magnitud of curr nt r sp ctiv ly - b tw n local and r mot r laying points. Co mparison of local and r mot signals provid s th basis for both fault d t ction and discrimination of th sch m s. Figur of Application of prot ction signalling and its r lationship to oth r D t ails8.1: Unit Prot ction sch m s ar giv n in Chapt r 10. syst ms using communic ation a. a f d r with a w ak inf d at on nd, insuffici nt Communications m t hods (Shown as a unidir ctionalthis Chapt r. ar cov r d lat r in syst m for sim plicity) to op rat th prot ction for all faults pi c of apparatus in sympathy with th tripping of oth r circuit br ak rs. Th main us of such sch m s is to nsur that prot ction at both nds of a fault d circuit will op rat to isolat th quipm nt conc rn d. Possibl circumstanc s wh n it may b us d ar : • 8• 8 . 3 T E L E P R OT E C T I O N C O M M A N D S Som Distanc Prot ction sch m s d scrib d in Chapt r 12 us signalling to conv y a command b tw n local and r mot r laying points. R c ipt of th information is us d to aid or sp d up cl aranc of faults within a prot ct d zon or to pr v nt tripping from faults outs id a prot ct d zon . T l prot ction syst ms ar oft n r f rr d to by th ir mod of op ration, or th rol of th t l prot ction command in th syst m. 8.4 INTE RTRIPPING Int rtripping is th controll d tripping of a circuit br ak r so as to compl t th isolation of a circuit or
b. f d r prot ction appli d to transform r –f d r circuits. Faults on th transf orm r windings may op rat th transform r prot ction but not th f d r prot ct ion. Similarly, som arth faults may not b d t ct d du to transform r conn ct ions c. faults b tw n th CB and f d r prot ction CT’s, wh n th s ar locat d o n th f d r sid of th CB. Bus-zon prot ction do s not r sult in fault cl ara nc – th fault is still f d from th r mot nd of th f d r, whil f d r unit prot ction may not op rat as th fault is outsid th prot ct d zon d. som di stanc prot ction sch m s us int rtripping to improv fault cl aranc tim s for som kinds of fault – s Chapt rs 12/13 Int rtripping sch m s us signalling to conv y a trip • 114 • N twork Prot ction & Automation Guid
command to r mot circuit br ak rs to isolat circuits. For high r liability EHV prot ction sch m s, int rtripping may b us d to giv back-up to main prot ctio ns, or back-tripping in th cas of br ak r failur . Thr typ s of int rtrippin g ar commonly ncount r d, and ar d scrib d b low. 8.4.1 Dir ct Tripping In di r ct tripping applications, int rtrip signals ar s nt dir ctly to th mast r tr ip r lay. R c ipt of th command caus s circuit br ak r op ration. Th m thod of communication must b r liabl and s cur b caus any signal d t ct d at th r c iving nd will caus a trip of th circuit at that nd. Th communications sys t m d sign must b such that int rf r nc on th communication circuit do s not caus spurious trips. Should a spurious trip occur, consid rabl unn c ssary iso lation of th primary syst m might r sult, which is at b st und sirabl and at w orst quit unacc ptabl . 8.4.2 P rmissiv Tripping P rmissiv trip commands ar always monitor d by a prot ction r lay. Th circuit br ak r is tripp d wh n r c ipt of th command coincid s with op ration of th prot ction r lay at th r c i ving nd r sponding to a syst m fault. R quir m nts for th communications chann l ar l ss on rous than for dir ct tripping sch m s, sinc r c ipt of an incorr ct signal must coincid with op ration of th r c iving nd prot ction for a tr ip op ration to tak plac . Th int ntion of th s sch m s is to sp d up trippi ng for faults occurring within th prot ct d zon . 8.4.3 Blocking Sch m Blockin g commands ar initiat d by a prot ction l m nt that d t cts faults xt rnal to th prot ct d zon . D t ction of an xt rnal fault at th local nd of a prot c t d circuit r sults in a blocking signal b ing transmitt d to th r mot nd. At th r mot nd, r c ipt of th blocking signal pr v nts th r mot nd prot cti on op rating if it had d t ct d th xt rnal fault. Loss of th communications c hann l is l ss s rious for this sch m than in oth rs as loss of th chann l do s not r sult in a failur to trip wh n r quir d. How v r, th risk of a spurious trip is high r. Figur 8.1 shows th typical applications of prot ction signall ing and th ir r lationship to oth r signalling syst ms commonly r quir d for con trol and manag m nt of a pow r syst m. Of cours , not all of th prot ction sign als shown will b r quir d in any particular sch m . 8.5 PERFORMANCE REQUIREMENTS Ov rall fault cl aranc tim is th sum of: a. sign alling tim b. prot ction r lay op rating tim c. trip r lay op rating tim d. c ircuit br ak r op rating tim Th ov rall tim must b l ss than th maximum tim for which a fault can r main on th syst m for minimum plant damag , loss of s tability, tc. Fast op ration is th r for a pr -r quisit of most signalling sy st ms. Typically th tim allow d for th transf r of a command is of th sam o rd r as th op rating tim of th associat d prot ction r lays. Nominal op ratin g tim s rang from 5 to 40ms d p nd nt on th mod of op ration of th t l prot ction syst m. Prot ction signals ar subj ct d to th nois and int rf r nc ass ociat d with ach communication m dium. If nois r produc s th signal us d to c onv y th command, unwant d commands may b produc d, whilst if nois occurs wh n a command signal is b ing transmitt d, th command may b r tard d or miss d c ompl t ly. P rformanc is xpr ss d in t rms of s curity and d p ndability. S cu rity is ass ss d by th probability of an unwant d command occurring, and d p nd ability is ass ss d by th probability of missing a command. Th r quir d d gr of s curity and d p ndability is r lat d to th mod of op ration, th charact ristics of th communication m dium and th op rating standards of th particula r pow r authority. Typical d sign obj ctiv s for t l prot ction syst ms ar not mor than on incorr ct trip p r 500 quipm nt y ars and l ss than on failur t o trip in v ry 1000 att mpts, or a d lay of mor than 50ms c should not occur m or than onc p r 10 quipm nt y ars. To achi v th s obj ctiv s, sp cial mpha sis may b attach d to th s curity and d p ndability of th t l prot ction comm and for ach mod of op ration in th syst m, as follows. 8.5.1 P rformanc R qu ir m nts – Int rtripping Sinc any unwant d command caus s incorr ct tripping, v r y high s curity is r quir d at all nois l v ls up to th maximum that might v r b ncount r d. 8.5.2 P rformanc R quir m nts – P rmissiv Tripping S curity so m what low r than that r quir d for int rtripping is usually satisfactory, sinc incorr ct tripping can occur only if an unwant d command happ ns to coincid wi th op ration of th prot ction r lay for an out-of-zon fault.
N twork Prot ction & Automation Guid • 115 • P rot ction: Signalling and Int rtripping • 8•
For p rmissiv ov r-r ach sch m s, r s tting aft r a command should b highly d p ndabl to avoid any chanc of malop rations during curr nt r v rsals. 8.5.3 P rformanc R quir m nts – Blocking Sch m s Low s curity is usually ad quat sinc a n unwant d command can n v r caus an incorr ct trip. High d p ndability is r qu ir d sinc abs nc of th command could caus incorr ct tripping if th prot cti on r lay op rat s for an out-of-zon fault. Typical p rformanc r quir m nts ar shown in Figur 8.2. C physical fibr conn ction and thus nabl s mor compr h nsiv monitoring of th pow r syst m to b achi v d by th provision of a larg numb r of communication chann ls. 8.6.1 Privat Pilot Wir s and Chann ls Pilot wir s ar continuous copp r conn ctions b tw n signalling stations, whil pilot chann ls ar discontinuo us pilot wir s with isolation transform rs or r p at rs along th rout b tw n signalling stations. Th y may b laid in a tr nch with high voltag cabl s, laid by a s parat rout or strung as an op n wir on a s parat wood pol rout . Di stanc s ov r which signalling is r quir d vary consid rably. At on nd of th s cal , th distanc may b only a f w t ns of m tr s, wh r th d vic s conc rn d ar locat d in th sam substation. For applications on EHV lin s, th distanc b tw n d vic s may b b tw n 10100km or mor . For short distanc s, no sp cial m asur s ar r quir d against int rf r nc , but ov r long r distanc s, sp cial s nd and r c iv r lays may b r quir d to boost signal l v ls and provid immun ity against induc d voltag s from pow r circuits, lightning strik s to ground ad jac nt to th rout , tc. Isolation transform rs may also hav to b provid d to guard against ris s in substation ground pot ntial du to arth faults. Th cap acity of a link can b incr as d if fr qu ncy division multipl xing t chniqu s a r us d to run parall l signalling syst ms, but som Utiliti s pr f r th link t o b us d only for prot ction signalling. Privat pilot wir s or chann ls can b attractiv to an Utility running a v ry d ns pow r syst m with short distanc s b tw n stations. 8.6.2 R nt d Pilot Wir s and Chann ls P rot ction: Signalling and Int rtripping S c 0.06 0.05 0.04 0.03 0.02 0.01 0 10-2 10-3 10-4 10-5 10 10 TOP Analogu Digital Analogu Int rtrip nt rtrip T - 0.04s c PUC -1.00E-03 P -1.00E01 TOP - 0.015s c P -1.00E-01 PMC -1.00E-01 T - 0.015s c -2.00E-02 -1.00E-01 Int rtrip Digital Int rtrip nt rtrip T - 0.04s c P P T P P T P T PMC TOP Blocking -1.00E-01 - 0.015s c - Maximum op rating tim imum D p ndability UC )% 100(1-PMC )% P Figur 8.2: Typical p rformanc r quir m nts for prot ction signalling wh n th communication link is subj ct d to nois
8.6 TRANSMISSION MEDIA INTERFERENCE AND NOISE • 8• Th transmission m dia that provid th communication links involv d in prot cti on signalling ar : Th s ar r nt d from national communication authoriti s and, apart from th con n ction from th r laying point to th n ar st t l phon xchang , th routing F igur privat p rformanc r quir m nts for prot ction signalling wh n th communi cation link is subj ct d to nois will b through cabl s forming part of th a. 8.2: Typical pilots national communication n twork. b. r nt d pilots or chann ls c. pow r lin carri r d. radio . optical fibr s An conomic d cision has to b mad b tw n th us of privat or r nt d pilots. If privat pilots ar us d, t h own r has compl t control, but b ars th cost of installation and maint nanc . If r nt d pilots ar us d, most of th s costs ar liminat d, but f s must b paid to th own r of th pilots and th signal path may b chang d without wa rning. This may b a probl m in prot ction applications wh r signal transmissio n tim s ar critical. Th chanc of voltag s b ing induc d in r nt d pilots is s mall r than for privat pilots, as th pilot rout is normally not r lat d to th rout of th pow r lin with which it is associat d. How v r, som d gr of s curity N twork Prot ction & Automation Guid Historically, pilot wir s and chann ls (discontinuous pilot wir s with isolation transform rs or r p at rs along th rout b tw n signalling points) hav b n th most wid ly us d du to th ir availability, follow d by Pow r Lin Carri r C ommunications (PLCC) t chniqu s and radio. In r c nt y ars, fibr -optic syst ms hav b com th usual choic for n w installations, primarily du to th ir compl t immunity from l ctrical int rf r nc . Th us of fibr -optic cabl s also gr atly incr as s th numb r of communication chann ls availabl for ach • 116 •
and prot ction against induc d voltag s must b built into signalling syst ms. E l ctrical int rf r nc from oth r signalling syst ms, particularly 17, 25 and 50 Hz ringing ton s up to 150V p ak, and from nois g n rat d within th quipm nt us d in th communication n twork, is a common hazard. Similarly, th signalling syst m must also b proof against int rmitt nt short and op n circuits on th p ilot link, incorr ct conn ction of 50 volts d.c. across th pilot link and oth r similar faults. Station arth pot ntial ris is a significant factor to b tak n into account and isolation must b provid d to prot ct both th p rsonn l and quipm nt of th communication authority. Th most significant hazard to b with stood by a prot ction signalling syst m using this m dium aris s wh n a lin sman inadv rt ntly conn cts a low imp danc t st oscillator across th pilot link th at can g n rat signalling ton s. Transmissions by such an oscillator may simula t th op rating cod or ton s qu nc that, in th cas of dir ct int rtripping sch m s, would r sult in incorr ct op ration of th circuit br ak r. Communicat ion b tw n r laying points may b ov r a two-wir or four-wir link. Cons qu nt ly th ff ct of a particular human action, for xampl an incorr ct disconn cti on, may disrupt communication in on dir ction or both. Th signals transmitt d must b limit d in both l v l and bandwidth to avoid int rf r nc with oth r sig nalling syst ms. Th own r of th pilots will impos standards in this r sp ct that may limit transmission capacity and/or transmission distanc . With a pow r syst m op rating at, say, 132kV, wh r r lativ ly long prot ctio n signalling tim s ar acc ptabl , signalling has b n achi v d abov sp ch tog th r with m t ring and control signalling on an stablish d control n twork. Co ns qu ntly th prot ction signalling was achi v d at v ry low cost. High voltag syst ms (220kV and abov ) hav d mand d short r op rating tim s and improv d s curity, which has l d to th r nting of pilot links xclusiv ly for prot ction s ignalling purpos s. 8.6.3 Pow r Lin Carri r Communications T chniqu s Wh r lon g lin s ctions ar involv d, or if th rout involv s installation difficulti s , th xp ns of providing physical pilot conn ctions or op rational r striction s associat d with th rout l ngth r quir that oth r m ans of providing signall ing faciliti s ar r quir d. Pow r Lin Carri r Communications (PLCC) is a t chn iqu that involv s high fr qu ncy signal transmission along th ov rh ad pow r l in . It is robust and th r for r liabl , constituting a low loss transmission p ath that is fully controll d by th Utility. High voltag capacitors ar us d, a long with drainag coils, for th purpos of inj cting th signal to and xtract ing it from th lin . Inj ction can b carri d out by impr ssing th carri r sig nal voltag b tw n on conductor and arth or b tw n any two phas conductors. Th basic units can b built up into a high pass or band pass filt r as shown i n Figur 8.3. Lin trap To lin To station S ri s tuning unit Capacitor VT To E/M VT To E/M VT Shunt filt r unit 75 ohms Coaxial cabl To HF quipm nt Figur 8.3: Typical phas -to-phas coupling quipm nt Figur 8.3: Typical phas to-phas coupling quipm nt N twork Prot ction & Automation Guid • 117 • P rot ction: Signalling and Int rtripping • 8•
Th att nuation of a chann l is of prim importanc in th application of carri r signalling, b caus it d t rmin s th amount of transmitt d n rgy availabl a t th r c iving nd to ov rcom nois and int rf ring voltag s. Th loss of ach lin t rminal will b 1 to 2dB through th coupling filt r, a maximum of 3dB th rough its broad-band trap and not mor than 0.5dB p r 100 m tr s through th hig h fr qu ncy cabl . An installation of PLCC quipm nt including capacitor voltag transform rs and lin traps, in a lin -to-lin inj ction arrang m nt, is shown in Figur 8.4. Th high fr qu ncy transmission charact ristics of pow r circuits ar good th loss amounting to 0.02 to 0.2dB p r kilom tr d p nding upon lin voltag and fr qu ncy. Lin att nuation is not aff ct d appr ciably by rain, but s rious incr as in loss may occur wh n th phas conductors ar thickly coat d with hoar-frost or ic . Att nuations of up to thr tim s th fair w ath r valu hav b n xp ri nc d. R c iving quipm nt commonly incorporat s automatic gai n control (AGC) to comp nsat for variations in att nuation of signals. High noi s l v ls aris from lightning strik s and syst m fault inc ption or cl aranc . Although th s ar of short duration, lasting only a f w millis conds at th mos t, th y may caus ov rloading of pow r lin carri r r c iving quipm nt. Signall ing syst ms us d for int rtripping in particular must incorporat appropriat s curity f atur s to avoid malop ration. Th most s v r nois l v ls ar ncount r d with op ration of th lin isolators, and th s may last for som s conds. A lthough malop ration of th associat d t l prot ction sch m may hav littl op rational significanc , sinc th circuit br ak r at on nd at l ast is normally alr ady op n, high s curity is g n rally r quir d to cat r for nois coupl d b tw n parall l lin s or pass d through lin traps from adjac nt lin s. Signallin g for p rmissiv int rtrip applications n ds sp cial consid ration, as this inv olv s signalling through a pow r syst m fault. Th incr as in chann l att nuati on du to th fault vari s according to th typ of fault, but most pow r author iti s s l ct a nominal valu , usually b tw n 20 and 30dB, as an application gui d . A prot ction signal boost facility can b mploy d to cat r for an incr as in att nuation of this ord r of magnitud , to maintain an acc ptabl signal-to-n ois ratio at th r c iving nd, so that th p rformanc of th s rvic is not i mpair d. Most dir ct int rtrip applications r quir signalling ov r a h althy po w r syst m, so boosting is not normally n d d. In fact, if a voic fr qu ncy in t rtrip syst m is op rating ov r a carri r b ar r chann l, th dynamic op rating rang of th r c iv r must b incr as d to accommodat a boost d signal. This m ak s it l ss inh r ntly s cur in th pr s nc of nois during a qui sc nt signa lling condition. N twork Prot ction & Automation Guid P rot ction: Signalling and Int rtripping Figur 8.4: Carri r coupling quipm nt Th high voltag capacitor is tun d by a tuning coil to pr s nt a low imp danc at th signal fr qu ncy; th parall l circuit pr s nts a high imp danc at th s ignal fr qu ncy whil providing a path for th pow r fr qu ncy curr nts pass d b y th capacitor. Th compl t arrang m nt is d sign d as a balanc d or unbalanc d half-s ction band pass filt r, according to wh th r th transmission is phas phas or phas - arth; th pow r lin charact ristic imp danc , b tw n 400 and 6 00 ohms, d t rmin s th d sign imp danc of th filt r. It is n c ssary to minim iz th loss of signal into oth r parts of th pow r syst m, to allow th sam f r qu ncy to b us d on anoth r lin . This is don with a lin trap or wav tr ap , which in its simpl st form is a parall l circuit tun d to pr s nt a v ry hi gh imp danc to th signal fr qu ncy. It is conn ct d in th phas conductor on th station sid of th inj ction quipm nt. Th compl t carri r coupling quip m nt is shown in Figur 8.4. Th singl fr qu ncy lin trap may b tr at d as an int gral part of th compl t inj ction quipm nt to accommodat two or mor ca rri r syst ms. How v r, difficulti s may aris in an ov rall d sign, as, at c rt ain fr qu nci s, th actual station r actanc , which is normally capacitiv , wil l tun with th trap, which is inductiv b low its r sonant fr qu ncy; th r sul
t will b a low imp danc across th transmission path, pr v nting op ration at th s fr qu nci s. This situation can b avoid d by th us of an ind p nd nt d oubl fr qu ncy or broad-band trap. Th coupling filt r and th carri r quip m nt ar conn ct d by high fr qu ncy cabl of pr f rr d charact ristic imp danc 75 ohms. A matching transform r is incorporat d in th lin coupling filt r to match it to th hf cabl . Surg div rt rs ar fitt d to prot ct th compon nts a gainst transi nt ov r voltag s. • 8• • 118 •
8.6.4 Radio Chann ls At first consid ration, th wid bandwidth associat d with radio fr qu ncy transmissions could allow th us of mod ms op rating at v ry hi gh data rat s. Prot ction signalling commands could b s nt by s rial cod d m ss ag s of suffici nt l ngth and compl xity to giv high s curity, but still achi v fast op rating tim s. In practic , it is s ldom conomic to provid radio qui pm nt xclusiv ly for prot ction signalling, so standard g n ral-purpos t l com munications chann l quipm nt is normally adopt d. Typical radio b ar r quipm n t op rat s at th microwav fr qu nci s of 0.2 to 10GHz. B caus of th r lativ ly short rang and dir ctional natur of th transmitt r and r c iv r a rial sys t ms at th s fr qu nci s, larg bandwidths can b allocat d without much chanc of mutual int rf r nc with oth r syst ms. Multipl xing t chniqu s allow a numb r of chann ls to shar th common b ar r m dium and xploit th larg bandwidth . In addition to voic fr qu ncy chann ls, wid r bandwidth chann ls or data chan n ls may b availabl , d p nd nt on th particular syst m. For instanc , in anal ogu syst ms using fr qu ncy division multipl xing, normally up to 12 voic fr q u ncy chann ls ar group d tog th r in bas bands at 12-60kHz or 60-108kHz, but a lt rnativ ly th bas band may b us d as a 48kHz signal chann l. Mod rn digital syst ms mploying puls cod modulation and tim division multipl xing usually p rovid th voic fr qu ncy chann ls by sampling at 8kHz and quantising to 8 bits ; alt rnativ ly, acc ss may b availabl for data at 64kbits/s ( quival nt to on voic fr qu ncy chann l) or high r data rat s. Radio syst ms ar w ll suit d t o th bulk transmission of information b tw n control c ntr s and ar wid ly us d for this. Wh n th rout of th trunk data n twork coincid s with that of tra nsmission lin s, chann ls can oft n b allocat d for prot ction signalling. Mor g n rally, radio communication is b tw n major stations rath r than th nds o f individual lin s, b caus of th n d for lin -of-sight op ration b tw n a ri als and oth r r quir m nts of th n twork. Roundabout rout s involving r p at r stations and th addition of pilot chann ls to int rconn ct th radio installati on and th r lay station may b possibl , but ov rall d p ndability will normall y b much low r than for PLCC syst ms in which th communication is dir ct from on nd of th lin to th oth r. Radio chann ls ar not aff ct d by incr as d a tt nuation du to pow r syst m faults, but fading has to b tak n into account w h n th signal-to-nois ratio of a particular installation is b ing consid r d. Most of th nois in such a prot ction signalling syst m will b g n rat d withi n th radio quipm nt its lf. A pollut d atmosph r can caus radio b am r fraction that will int rf r with ffici nt signalling. Th h ight of a rial tow r should b limit d, so that winds and t mp ratur chang s hav th minimum ff ct on th ir position. 8.6.5 Optica l Fibr Chann ls Optical fibr s ar fin strands of glass, which b hav as wav guid s for light. This ability to transmit light ov r consid rabl distanc s can b us d to provid optical communication links with normous information carryi ng capacity and an inh r nt immunity to l ctromagn tic int rf r nc . A practica l optical cabl consists of a c ntral optical fibr which compris s cor , claddi ng and prot ctiv buff r coating surround d by a prot ctiv plastic ov rsh ath c ontaining str ngth m mb rs which, in som cas s, ar nclos d by a lay r of armo uring. To communicat information a b am of light is modulat d in accordanc wit h th signal to b transmitt d. This modulat d b am trav ls along th optical fi br and is subs qu ntly d cod d at th r mot t rminal into th r c iv d signal. On/off modulation of th light sourc is normally pr f rr d to lin ar modulatio n sinc th distortion caus d by non-lin ariti s in th light sourc and d t cto rs, as w ll as variations in r c iv d light pow r, ar larg ly avoid d. Th ligh t transmitt r and r c iv r ar usually las r or LED d vic s capabl of mitting and d t cting narrow b ams of light at s l ct d fr qu nci s in th low att nuati on 850, 1300 and 1550 nanom tr sp ctral windows. Th distanc ov r which ff ct iv communications can b stablish d d p nds on th att nuation and disp rsion of th communication link and this d p nds on th typ and quality of th fibr and th wav l ngth of th optical sourc . Within th fibr th r ar many mod s of propagation with diff r nt optical paths that caus disp rsion of th light s ignal and r sult in puls broad ning. Th d grading of th signal in this way ca
n b r duc d by th us of grad d ind x fibr s that caus th various mod s to follow n arly qual paths. Th distanc ov r which signals can b transmitt d i s significantly incr as d by th us of monomod fibr s that support only on mod of propagation. With optical fibr chann ls, communication at data rat s of hundr ds of m gah rtz is achi vabl ov r a f w t ns of kilom tr s, whilst gr at r distanc s r quir th us of r p at rs. An optical fibr can b us d as a d d icat d link b tw n two t rminal quipm nts, or as a multipl x d link that carri s all communication traffic such as voic , t l control and prot ction signallin g. In th latt r cas th availabl bandwidth of a link is divid d by m ans of t im division multipl xing (T.D.M.) t chniqu s into a numb r of chann ls, ach of 64kbits/s ( quival nt to on N twork Prot ction & Automation Guid • 119 • P rot ction: Signalling and Int rtripping • 8•
voic fr qu ncy chann l which typically us s an 8-bit analogu -to-digital conv r sion at a sampling rat of 8kHz). A numb r of Utiliti s s ll surplus capacity on th ir links to t l communications op rators. Th tr nd of using r nt d pilot ci rcuits is th r for b ing r v rs d, with th Utiliti s moving back towards own r ship of th communication circuits that carry prot ction signalling. Th quipm nts that carry out this multipl xing at ach nd of a lin ar known as Puls C od Modulation (P.C.M.) t rminal quipm nts. This approach is th on adopt d b y t l communications authoriti s and som Utiliti s favour its adoption on th ir privat syst ms, for conomic consid rations. Optical fibr communications ar w ll stablish d in th l ctrical supply industry. Th y ar th pr f rr d m ans for th communications link b tw n a substation and a t l phon xchang wh n r nt d circuits ar us d, as trials hav shown that this link is particularly su sc ptibl to int rf r nc from pow r syst m faults if copp r conductors ar us d . Whilst such fibr s can b laid in cabl tr nch s, th r is a strong tr nd to a ssociat th m with th conductors th ms lv s by producing composit cabl s compr ising optical fibr s mb dd d within th conductors, ith r arth or phas . For ov rh ad lin s us of OPGW (Optical Ground Wir ) arth conductors is v ry common , whil an alt rnativ is to wrap th optical cabl h lically around a phas or arth conductor. This latt r t chniqu can b us d without r stringing of th li n . 8.7 SIGNALLING METHODS Various m thods ar us d in prot ction signalling; no t all n d b suit d to v ry transmission m dium. Th m thods to b consid r d bri fly ar : a. D.C. voltag st p or d.c. voltag r v rsals b. plain ton k y d signals at high and voic fr qu nci s c. fr qu ncy shift k y d signals involving two or mor ton s at high and voic fr qu nci s G n ral purpos t l communicati ons quipm nt op rating ov r pow r lin carri r, radio or optical fibr m dia in corporat fr qu ncy translating or multipl xing t chniqu s to provid th us r w ith standardis d communication chann ls. Th y hav a nominal bandwidth/chann l o f 4kHz and ar oft n r f rr d to as voic fr qu ncy (vf) chann ls. Prot ction si gnalling quipm nts op rating at voic fr qu nci s xploit th standardisation o f th communication int rfac . Wh r voic fr qu ncy chann ls ar not availabl or suitabl , prot ction signalling may mak us of a m dium or sp cialis d quip m nt d dicat d ntir ly to th signalling r quir m nts. Figur 8.5 illustrat s th communication arrang m nts commonly ncount r d in pr ot ction signalling. 8.7.1 D.C. Voltag Signalling A d.c. voltag st p or d.c. v oltag r v rsals may b us d to conv y a signalling instruction b tw n prot cti on r laying points in a pow r syst m, but th s ar suit d only to privat pilot wir s, wh r low sp d signalling is acc ptabl , with its inh r nt s curity. 8. 7.2 Plain Ton Signals Plain high fr qu ncy signals can b us d succ ssfully for th signalling of blocking information ov r a pow r lin . A normally qui sc nt pow r lin carri r quipm nt can b d dicat d ntir ly to th transf r to t l pr ot ction blocking commands. Phas comparison pow r lin carri r unit prot ction sch m s oft n us such quipm nt and tak advantag of th v ry high sp d and d p ndability of th signalling syst m. Th sp cial charact ristics of d dicat d on/off k y d carri r syst ms ar discuss d lat r. A r lativ ly ins nsitiv r c iv r is us d to discriminat against nois on an amplitud basis, and for som applications th s curity may b satisfactory for p rmissiv tripping, particula rly if th normal high-sp d op ration of about 6ms is sacrific d by th additio n of d lays. Th n d for r gular r fl x t sting of a normally qui sc nt chann l usually pr clud s any us for int rtripping. Plain ton pow r lin carri r sign alling syst ms ar particularly suit d to providing th blocking commands oft n associat d with th prot ction of multi- nd d f d rs, as d scrib d in Chapt r 1 3. A blocking command s nt from on nd can b r c iv d simultan ously at all th oth r nds using a singl pow r lin carri r chann l. Oth r signalling syst ms usually r quir discr t communication chann ls b tw n ach of th nds or inv olv r p at rs, l ading to d cr as d d p ndability of th blocking command. Plai n voic fr qu ncy signals can b us d for blocking, p rmissiv int rtrip and dir ct int rtrip applications for all transmission m dia but op ration is at such a low signal l v l that s curity from malop ration is not v ry good. Op ration in th ton on to ton off mod giv s th b st chann l monitoring, but off rs
littl s curity; to obtain a satisfactory p rformanc th output must b d lay d . This r sults in r lativ ly slow op ration: 70 millis conds for p rmissiv int rtripping, and 180 millis conds for dir ct int rtripping. P rot ction: Signalling and Int rtripping • 8• • 120 • N twork Prot ction & Automation Guid
Pilot wir s Pilot chann l Voic fr qu ncy Prot ction r lay sch m Carri r fr qu ncy shift On/off k y d carri r Fr qu ncy division multipl x Radio transmitt r PC M primary multipl x Optical transmitt r Pow r lin carri r communication chann l Pow r lin carri r Digital Optical fibr g n ral purpos Optical fibr d dicat d Optical Prot ction signalling quipm nt Communication q uipm nt Transmission m dia Figur 8.5: Communication arrang m nts commonly ncount r d in prot ction signal ling giv th r quir d d gr of s curity in dir ct int rtrip sch m s: th short op r ating tim s n d d may r sult in Fr qu ncy shift k y d high fr qu ncy signals ca n b un conomical us of th availabl voic fr qu ncy us d ov r a pow r lin ca rri r link to giv short sp ctrum, particularly if th chann l is not xclusiv l y op rating tim s (15 millis conds for blocking and mploy d for prot ction sign alling. As nois pow r is p rmissiv int rtripping, 20 millis conds for dir ct i n prot ction signalling Figur 8.5: Communication arrang m nts commonly ncount r d dir ctly proportional to bandwidth, a larg bandwidth int rtripping) for all applications of prot ction caus s an incr as in th nois l v l admitt d to th signalling. Th r quir d amount of s curity can b d t ctor, making op ration in th pr s nc of nois mor achi v d by using a broadband nois d t ctor to di fficult. So, again, it is difficult to obtain both high monitor th actual op ra tional signalling quipm nt. d p ndability and high s curity. Fr qu ncy shift k y d voic fr qu ncy signals can b Th signal fr qu ncy shift t chniqu has adva ntag s us d for all prot ction signalling applications ov r all wh r fast signa lling is n d d for block d distanc and transmission m dia. Fr qu ncy modulatio n t chniqu s p rmissiv int rtrip applications. It has littl inh r nt mak poss ibl an improv m nt in p rformanc , b caus s curity, but additional circuits r sponsiv to v ry typ amplitud limiting r j cts th amplitud modulation of in t rf r nc can giv acc ptabl s curity. This syst m compon nt of nois , l aving only th phas modulation do s not r quir a chann l capabl of high transmissi on compon nts to b d t ct d. rat s, as th fr qu ncy chang s onc only; th Th op rational prot ction signal may consist of ton bandwidth can th r for b na rrow r than in cod d s qu nc cod s with, say, thr ton s, or a multi-bit syst ms, giving b tt r nois r j ction as w ll as b ing cod using two discr t ton s for succ ssiv bits, or of a advantag ous if th chann l is shar d with t l m t ry singl fr qu ncy shift. and control signalling, which will in vitably b th cas if a pow r lin carri r b ar r is mploy d. Mod rn high-sp d syst ms us m ulti-bit cod or singl 8.7.3 Fr qu ncy Shift K y d Signals fr qu ncy shift t ch niqu s. Compl x cod s ar us d to N twork Prot ction & Automation Guid • 121 • P rot ction: Signalling and Int rtripping • Radio 8•
• 9 • Ov rcurr nt Prot ction for Phas and Earth Faults Introduction Co-ordination proc dur Principl s of tim /curr nt grading Standard I.D.M.T. ov rcurr nt r lays Combin d I.D.M.T. and high s t instantan ous ov rcu rr nt r lays V ry Inv rs ov rcurr nt r lays Extr m ly Inv rs ov rcurr nt r lay s Oth r r lay charact ristics Ind p nd nt (d finit ) tim ov rcurr nt r lays R l ay curr nt s tting R lay tim grading margin R comm nd d grading margins 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17 9.18 9.19 9 .20 9.21 Calculation of phas fault ov rcurr nt r lay s ttings Dir ctional phas fault ov rcurr nt r lays Ring mains Earth fault prot ction Dir ctional arth fault ov rc urr nt prot ction Earth fault prot ction on insulat d n tworks Earth fault prot ction on P t rs n Coil arth d n tworks Exampl s of tim and curr nt grading R f r nc s
• 9 • Ov rcurr nt P rot ction for Phas and Earth Faults 9.1 INTRODUCTION Prot ction against xc ss curr nt was naturally th arli st pr ot ction syst m to volv . From this basic principl , th grad d ov rcurr nt sys t m, a discriminativ fault prot ction, has b n d v lop d. This should not b c onfus d with ‘ov rload’ prot ction, which normally mak s us of r lays that op rat in a tim r lat d in som d gr to th th rmal capability of th plant to b pr ot ct d. Ov rcurr nt prot ction, on th oth r hand, is dir ct d ntir ly to th cl aranc of faults, although with th s ttings usually adopt d som m asur of ov rload prot ction may b obtain d. 9.2 CO-ORDINATION PROCEDURE Corr ct ov rcur r nt r lay application r quir s knowl dg of th fault curr nt that can flow in ach part of th n twork. Sinc larg -scal t sts ar normally impracticabl , sy st m analysis must b us d – s Chapt r 4 for d tails. Th data r quir d for a r lay s tting study ar : i. a on -lin diagram of th pow r syst m involv d, showi ng th typ and rating of th prot ction d vic s and th ir associat d curr nt tr ansform rs ii. th imp danc s in ohms, p r c nt or p r unit, of all pow r transf orm rs, rotating machin and f d r circuits iii. th maximum and minimum valu s of short circuit curr nts that ar xp ct d to flow through ach prot ction d v ic iv. th maximum load curr nt through prot ction d vic s v. th starting curr nt r quir m nts of motors and th starting and lock d rotor/stalling tim s of i nduction motors vi. th transform r inrush, th rmal withstand and damag charact ristics vii. d cr m nt curv s showing th rat of d cay of th fault curr nt su ppli d by th g n rators viii. p rformanc curv s of th curr nt transform rs Th r lay s ttings ar first d t rmin d to giv th short st op rating tim s at ma ximum fault l v ls and N twork Prot ction & Automation Guid • 123 •
th n ch ck d to s if op ration will also b satisfactory at th minimum fault curr nt xp ct d. It is always advisabl to plot th curv s of r lays and oth r prot ction d vic s, such as fus s, that ar to op rat in s ri s, on a common sc al . It is usually mor conv ni nt to us a scal corr sponding to th curr nt xp ct d at th low st voltag bas , or to us th pr dominant voltag bas . Th alt rnativ s ar a common MVA bas or a s parat curr nt scal for ach syst m v oltag . Th basic rul s for corr ct r lay co-ordination can g n rally b stat d as follows: a. wh n v r possibl , us r lays with th sam op rating charact ris tic in s ri s with ach oth r is som tim s d scrib d as an ‘ind p nd nt d finit -tim d lay r lay’, sinc its op r ating tim is for practical purpos s ind p nd nt of th l v l of ov rcurr nt. It is th tim d lay l m nt, th r for , which provid s th m ans of discriminatio n. Th r lay at B is s t at th short st tim d lay possibl to allow th fus t o blow for a fault at A on th s condary sid of th transform r. Aft r th tim d lay has xpir d, th r lay output contact clos s to trip th circuit br ak r. Th r lay at C has a tim d lay s tting qual to t1 s conds, and similarly for th r lays at D and E. If a fault occurs at F, th r lay at B will op rat in t s conds and th subs qu nt op ration of th circuit br ak r at B will cl ar th fault b for th r lays at C, D and E hav tim to op rat . Th tim int rval t1 b tw n ach r lay tim s tting must b long nough to nsur that th upstr am r lays do not op rat b for th circuit br ak r at th fault location has trip p d and cl ar d th fault. Th main disadvantag of this m thod of discriminatio n is that th long st fault cl aranc tim occurs for faults in th s ction clos st to th pow r sourc , wh r th fault l v l (MVA) is high st. 9.3.2 Discrimin ation by Curr nt Discrimination by curr nt r li s on th fact that th fault cur r nt vari s with th position of th fault b caus of th diff r nc in imp danc valu s b tw n th sourc and th fault. H nc , typically, th r lays controll ing th various circuit br ak rs ar s t to op rat at suitably tap r d valu s o f curr nt such that only th r lay n ar st to th fault trips its br ak r. Figur 9.2 illustrat s th m thod. For a fault at F1, th syst m short-circuit curr n t is giv n by: I = 6350 A Z S + Z L1 Ov rcurr nt Prot ction for Phas and Earth Faults b. mak sur that th r lay farth st from th sourc has curr nt s ttings qual to or l ss than th r lays b hind it, that is, that th primary curr nt r quir d to op rat th r lay in front is always qual to or l ss than th primary curr nt r quir d to op rat th r lay b hind it. 9.3 PRINCIPLES OF TIME/CURRENT GRADI NG Among th various possibl m thods us d to achi v corr ct r lay co-ordinatio n ar thos using ith r tim or ov rcurr nt, or a combination of both. Th comm on aim of all thr m thods is to giv corr ct discrimination. That is to say, ach on must isolat only th faulty s ction of th pow r syst m n twork, l avin g th r st of th syst m undisturb d. 9.3.1 Discrimination by Tim In this m tho d, an appropriat tim s tting is giv n to ach of th r lays controlling th ci rcuit br ak rs in a pow r syst m to nsur that th br ak r n ar st to th fault op ns first. A simpl radial distribution syst m is shown in Figur 9.1, to ill ustrat th principl . • 9• E D C B
A wh r Zs = sourc imp danc = 112 250 = 0.485Ω t1 t1 t1 F
Figure 9.1: Ra ial system with time iscrimination ZL1 = cable impe ance between C an B = 0.24Ω Hence I= 11 3 ×0.725 = 8800 A Overcurrent protection is provi e at B, C, D an E, that is, at the infee en of each section of the power system. Each protection unit comprises a efinite-t ime elay overcurrent relay inwhich the operationof the current sensitive elem ent simply initiates the time elay element. Provi e the setting of the current element is below the fault current value, this element plays no part in the ach ievement of iscrimination. For this reason, the relay So, a relay controlling the circuit breaker at C an set to operate at a fault c urrent of 8800A woul in theory protect the whole of the cable section between C an B. However, there are two important practical points that affect this metho of co-or ination: Network Protection & Automation Gui e • 124 •
a. it is not practical to istinguish between a fault at F1 an a fault atF2, s ince the istance between these points may be only a few metres, correspon ing t o a change in fault current of approximately 0.1% b. in practice, there woul be variations in the source fault level, typically from 250MVA to 130MVA. At this lower fault level the fault current woul not excee 6800A, even for a cable fau lt close to C. A relay set at 8800A woul not protect any part of the cable sect ion concerne Discrimination by current is therefore not a practical proposition for correct gra ing between the circuit breakers at C an B. However, the probl em changes appreciably impe when there is significant ance between the two circu it breakers concerne . Consi er the gra ing require between the circuit breaker s at C an A in Figure 9.2. Assuming a fault at F4, the shortcircuit current is given by: I = 6350 A Z S + Z L1 margin of 20% to allow for relay errors an a further 10% for variations in the system impe ance values, it is reasonable to choose a relay setting of 1.3 x 220 0A, that is 2860A, for the relay at B. Now, assuming a fault at F3, at the en o f the 11kV cable fee ing the 4MVA transformer, the short-circuit current is give n by: I= 11 3 (ZS + Z L1 + Z L 2 ) 11 3 (0.485 + 0.24 + 0.04 ) Thus, assuming a 250MVA source fault level: I= = 8300 A 11 3 (0.93 + 0.214 + 0.04 ) I=
= 5250 A In other wor s, for either value of source level, the relay at B woul operate correctly for faults anywhereon the 11kV cable fee ing the transformer. 9.3.3 Discrimination by both Time an Current Each of the two metho s escribe so far has afun amental isa vantage. In the case of iscrimination by time al one, the isa vantage is ue to the fact that the severe faults are cleare more in the longest operating time. On the other han, iscrimination by current can be applie only the two br where there is appreciable impe ance between circuit eakers concerne . It is because of the limitations impose by the in epen ent us e of either time or currentco-or ination that the inverse time overcurrent rela y characteristic has evolve . With this characteristic, the time of operation is inversely proportional to the fault current level an the actual characteristic is a function of both ‘time’ an current settings. Figure 9.3 illustrates the cha racteristics of two relays given ifferent current/time settings. For a large va riation in fault current between the two en s of the fee er, faster operating ti mes can be achieve by the relays nearest to the source, where the fault level i s the highest. The isa vantages of gra ing by time or current alone are overcom e. where ZS = source ZL1 = cable impe ance between C an B = 0.24Ω Z impe ance = 0.485Ω L2 = cable impe ance between B an 4 MVA transformer = 0.04Ω ZT = transformer impe ance 112 = 0.07 4 = 2.12Ω Hence I= 11 3 ×2.885 = 2200 A 11kV 250MVA Source 200 metres 240mm2 P.I.L.C. Cable 200 metres 240mm2 P.I.L.C. C able 4MVA 11/3.3kV 7% C F1 B F2 F3
A F4
Figure 9.2: Ra ial system with current iscrimination For this reason, a relay controlling the circuitbreaker at B an set to operate at of a current 2200A plus a safety margin woul not operate for a fault at F4 an woul thus iscriminate with the relay at A. Assuming a safety Network Protection & Automation Gui e The selection of overcurrent relay characteristics generally startswith selecti on of the correct characteristic to be use for each relay, followe by choice o f the relay current settings.Finally the gra ing margins an hence time setting s of the relays are etermine . An iterative proce ure is often require to reso lve conflicts, an may involve use of non-optimal characteristics, current or ti me gra ing settings. • 125 •
Overcurrent Protection for Phase an Earth Faults • Alternatively, assuming a source fault level of 130MVA: 9•
1000. Relay Characteristic IEEE Mo erately Inverse Equation (IEC 60255) t= t= t= t= t = TD 7 TD
0.0515 I r2 −1 0.02394
+0.114 0.02
7
I r0.02 −1
+ 0.01694
TD −1
19.61
+0.491
I r
IEEE Very Inverse 100. Extremely Inverse (EI) US CO8 Inverse US CO2 Short Time Inverse 10.00 Overcurrent Protection for Ph se nd E rth F ults Ir = (I/Is), where Is = rel y setting current TMS = Time multiplier Setting TD = Time Di l setting ( ): North Americ n IDMT rel y ch r cteristics Time (s) time T le 9.1: Definitions of st nd rd rel y ch r cteristics Rel y A o per ting time 1000.00 1.00 0.10 100 1000 Current (A) Rel y A: Current Setting = 100A, TMS = 1.0 Rel y B: Current Set ting = 125A, TMS = 1.3 10,000 100.00 9.4 STANDARD I.D.M.T. OVERCURRENT RELAYS The current/time tripping ch r cteristi cs of IDMT rel ys m y need to e v ried ccording to the tripping time required nd the ch r cteristics of other protection devices used in the network. For the se purposes, IEC 60255 defines num er of st nd rd ch r cteristics s follows: St nd rd Inverse (SI) Very Inverse (VI) Extremely Inverse (EI) Definite Time (DT ) Rel y Ch r cteristic St nd rd Inverse (SI) Very Inverse (VI) Extremely Inverse (EI) Long time st nd r d e rth f ult Oper ting Time (seconds) Figure 9.3: Rel y ch r cteristics for different settings 10.00 •
7
I r2
9• 1.00 Equ tion (IEC 60255) t = TMS × 0.14 −1 13.5 t = TMS × I r −1 0 I r .02 0.10 1 t = TMS × t = TMS × 80 2 I r −1 120 I r −1 Current (multiples of IS) ( ) IEC 60255 ch r cteristics ; TMS=1.0 10 100 Figure 9.4 ( ): IDMT rel y ch r cteristics ( ): Rel y ch r cteristics to IEC 60255 • 126 • Network Protection & Autom tion Guide
1000.00 10 8 6 4 3 TMS 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 100.00 2 Time (seconds) 10.00 1 0.8 0.6 0.4 0.3 0.2 Oper ting Time (seconds) 0.1 1.00 Moder tely Inverse 0.1 1 2 3 4 6 8 10 Current (multiples of plug settings) 20 30 Time Inverse CO 8 Inverse Extremely Inverse 0.10 1 Figure 9.5: Typic l time/current ch r cteristics of st nd rd IDMT rel y Current (multiples of IS) ( ) North Americ n ch r cteristics; TD=7 10 100 Rel ys for power systems designed to North Americ n pr ctice utilise ANSI/IEEE c urves. T le 9.1( ) gives the m them tic l description of these ch r cteristics nd Figure 9.4( ) shows the curves st nd rdised to time di l setting of 1.0. 9 .5 COMBINED I.D.M.T. ANDHIGH SET INSTANTANEOUS OVERCURRENT RELAYS A high set in st nt neous element c n e used where the source imped nce is sm ll in comp riso n with the protected circuit imped nce. This m kes reduction in the tripping t ime t high f ult levels possi le. It lso improves the over ll system gr ding y llowing the 'discrimin ting curves' ehind the high set inst nt neous element s to e lowered. As shown in Figure 9.6, one of the dv nt ges of the high set i nst nt neous elements is to reduce the oper ting time of the circuit protection ting curves'. If the source imped nce re y the sh ded re elow the 'discrimin m ins const nt, it is then possi le to chieve highspeed protection over l rge section of the protected circuit. The r pid f ult cle r nce time chieved helps to minimise d m ge t the fult loc tion. Figure 9.6 lso illustr tes further import nt dv nt ge g ined ythe use of high set inst nt neous elements. Gr di ng with the rel y immedi tely ehind the rel y th t h s the inst nt neous elemen
ts en led is c rried out t the current setting of the inst nt neous elements nd not t Figure 9.4 ( ): IDMT rel y ch r cteristics The m them tic l descriptions of the curves re given in T le 9.1( ), nd the c urves sed on common setting current nd time multiplier setting of 1 second re shown in Figure 9.4( ). The tripping ch r cteristics for different TMS setti ngs using the SI curve re illustr ted in Figure 9.5. Although the curves re on ly shown for discrete v lues of TMS, continuous djustment m y e possile in n electromech nic l rel y. For other rel y types, the setting steps m y e so sm ll s to effectively provide continuous djustment. In ddition, lmost ll over current rel ys re lso fitted with high set inst nt neous element. In most c ses, use of the st nd rd SI curve proves s tisf ctory, ut if s tisf ctory grdi ng c nnot e chieved, use of the VI or EI curves m y help to resolve the pro le m. When digit l or numeric rel ys re used,other ch r cteristics m y e provide d, including the possi ility of user defin le curves. More det ils re provided in the following sections. Network Protection & Autom tion Guide • 127 • Overcurrent Protection for Ph se nd E rth F ults • 0.2 9•
the m ximum f ult level. For ex mple, in Figure 9.6, rel y R2 is gr ded with rel y R3 t 500A nd not 1100A, llowing rel y R2to e set with TMS of 0.15 inst e d of 0.2 while m int ining gr ding m rgin etween rel ys of 0.4s. Simil rly, rel y R1 is gr ded with R2 t 1400A nd not t 2300A. 3 2 R3 1 R2 9.6 VERY INVERSE Very inverse overcurrent rel ys re p r (VI) OVERCURRENT RELAYS ticul rly suit le if there is su st nti l reduction of fult current s the d ist nce from the power source incre ses, i.e. there is su st nti l incre se in f ult imped nce. The VI oper ting ch r cteristic is such th t the oper ting tim e is pproxim tely dou led for reduction in current from 7 to 4 times the rel y current setting. This permits the use of the s me time multiplier setting for se ver l rel ys in series. Figure 9.7 provides comp rison of the SI nd VI curves for rel y. The VI curve is much steeper nd therefore the oper tion incre ses comp red to the SI curve. This en much f ster for the s me reduction incurrent les the requisite gr ding m rgin to e o t ined with lower TMS for the s me setting current, nd hence the tripping time t source c n e minimised. R1 Overcurrent Protection for Ph se nd E rth F ults Time (seconds) 0.1 100 Source 250 MVA 11kV R1 R2 1000 0 R tio R3 10,000 0 100.00 400/1A 100/1A F ult level 13.000A F ult level 2300A 500A 0.125 TMS 62.5A 0.10 TM S 300A 500A 50/1A F ult level 1100A 9.5.1 Tr nsient Overre ch The re ch of rel y is th t p rt of the system protec ted y the rel y if f ult occurs. A rel y th t oper tes for f ult th t lies eyond the intended zone of protectionis s id to overre ch. When using inst nt neous overcurrent elements, c re must e exercised in choosing the settings to p revent them oper ting for f ults eyond the protected section. The initi l curre nt due to d.c. offset in the current w ve m y e gre ter th n the rel y pick u p v lue nd c use it to oper te. This m y occur even though the ste dy st te r.m .s. v lue of the f ult current for f ult t point eyond the required re ch point m y e less th n the rel y setting. This phenomenon is c lled tr nsient ov erre ch, nd is defined s: I −I % tr nsient overre ch = 1 2 ×100% I2 …Equ tion 9.1 wh ere: I1 = r.m.s ste dy st te rel y pick up current I2 = ste dy st te r.m.s. curr ent which when fully offset just c uses rel y pick up When pplied to power tr n sformers, the high set inst nt neous overcurrent elements must e set ove the m ximum through f ult current th n the power tr nsformer c n supply for f ult cross its LV termin ls, in order to m int in discrimin tion with the rel ys on the LV side of the tr nsformer. Oper ting time (seconds)
Figure 9.6: Ch r cteristics of com ined IDMT nd high set inst nt neous overcurr ent rel ys 10.00
St nd rd Inverse (SI) 1.00 Very Inverse (VI) • 9• 0.10 1 10 Current ( multiples of Is ) 100 Figure 9.7: Comp rison of SI nd VI rel y ch r cteristics 9.7 EXTREMELY INVERSE (EI) OVERCURRENT RELAYS With this ch r cteristic, the oper tion time is pproxim tely inversely proportion l to the squ re of the pplied current. This m kes it suit le for the protection of distri ution feeder circui ts in which the feeder is su jected to pe k currents on switching in, s would e the c se on power circuit supplying refriger tors, pumps, w ter he ters nd so on, which rem in connected even fter prolonged interruption of supply. The long time oper ting ch r cteristic of the extremely Network Protection & Autom tion Guide • 128 •
200.0 100.0 10.0 Time (secs) St nd rd inverse (SI) Overcurrent rel ys re norm lly lso provided with elements h ving independent o r definite time ch r cteristics. These ch r cteristics provide re dy me ns of co ordin ting sever l rel ys in series in situ tions in which the system f ult c urrent v ries very widely due to ch nges in source imped nce, s there is no ch nge in time with the v ri tion of f ult current. The time/current ch r cteristic s of this curve re shown in Figure 9.9, together with those of the st nd rd I.D .M.T. ch r cteristic, to indic te th t lower oper ting times re chieved y the inverse rel y t the higher v lues of f ult current, where s the definite time rel y h s lower oper ting times t the lower current v lues. Vertic l lines T1, T2, T3, nd T4 indic te the reduction in oper ting times chieved y the inverse rel y t high f ult levels. 9.10 RELAY CURRENT SETTING An overcurrent rel y h s minimum oper ting current, known s the current setting of the rel y. The cur rent setting must e chosenso th t the rel y does not oper te for the m ximum l o d current in the circuit eing protected, ut does oper tefor current equ l or gre ter to the minimumexpected f ult current. Although y using current s etting th t is only just ove the m ximum lo d current in thecircuit cert in degree of protection g inst overlo ds s well s f ults m y e provided, the m in function of overcurrent protection is to isol te prim ry system f ults nd n ot to provide overlo d protection. In gener l, the current setting will e selec ted to e ove the m ximum short time r ted current of the circuit involved. Si nce ll rel ys h ve hysteresis in their current settings, the setting must e su fficiently high to llow the rel y to reset when the r ted current of the circui t is eing c rried. The mount of hysteresis in the current setting is denoted y the pick up/drop off r tio of rel y – the v lue for modern rel y is typic ll y 0.95. Thus, rel y minimum current 1.0 inverse (EI) v s E 200A Fuse A us 0.1 100 1000 Current ( mps) 10,000 Figure 9.8: Comp rison of rel y nd fuse ch r cteristics 9.8 OTHER RELAY CHARACTERISTICS User defin le curves m y e provided on some ty pes of digit l or numeric l rel ys. The gener l principle is th t the user enter s series of current/time co ordin tes th t re stored in the memory of the rel is used to provide smooth trip ch r cteristic y. Interpol tion etween points . Such fe ture, if v il le, m y eused in speci l c ses if none of the st n d rd tripping ch r cteristics is suit le. However, gr ding of upstre m protecti on m y ecome more difficult, nd it is necess ry to ensure th t the curve Network Protection & Autom tion Guide • 129 • Overcurrent Protection for Ph se nd E rth F ults • inverse rel y t norm l pe k lo d v lues of current lso m kes this rel y p rtic ul rly suit le forgr ding with fuses. Figure 9.8 shows typic l curves to illus tr te this. It c n e seen th t use of the EI ch r cteristic gives s tisf ctor y gr ding m rgin, ut use of the VI or SI ch r cteristics t the s me settings d oes not. Another pplic tion of this rel y is in conjunction with utoreclosers in low volt ge distriution circuits. The m jority of f ults re tr nsient in n nd repl cing of the fuses present in fin l circuit ture nd unnecess ry lowing s of such system c n e voided if the uto reclosers re set to oper te efor
e the fuse lows. If the f ult persists, the uto recloser locks itself in the c losed position fter one opening nd the fuse lows to isol te the f ult. is properly documented, long with the re sons for use. Since the st nd rd curve s provided cover most c ses with dequ te tripping times, nd most equipment is designed with st nd rd protection curves in mind, the need to utilise this form of protection is rel tively r re. Digit l nd numeric l rel ys m y lso include predefined logic schemes utilising digit l (rel y) I/O provided in the rel y to implement st nd rd schemes such s CB f ilure nd trip circuit supervision. This s ves the provision of sep r te rel y or PLC (Progr mm le Logic Controller) h rdw re to perform these functions. 9.9 INDEPENDENT (DEFINITE) TIME OVERCURRENT R ELAYS 9•
10 Gr ding m rgin
etween rel ys: 0.4s R4 R3 R2 R1 R1A
R R 3A 2A R4A Time (seconds) T4 1 T 2 T3 Overcurrent Protection for Ph se nd E rth F ults T 1 0.1 10 R1 R1A 100 R2 R2A F ult current ( mps) R3 R3A 1 1000 R4 R4A 10.000 F ult level 6000A 3500A 2000A 1200A Settings of independent (definite) time rel y R 300A 1.8s 1A R 175A 1.4s R 100A 1.0s R set t 57.5A 0.6s 4A Settings of I.D.M.T. rel y with st nd rd inverse ch r cteristic R 1A R R R set t 4A 300A 175A 100A 57.5A 0.2TMS 0.3TMS 0.37TMS 0.42TMS Figure 9.9: Comp rison of definite time nd st nd rd I.D.M.T. rel y •
9• setting of t le st 1.05 times the short time r ted current of the circuit is li kely to e required. 9.11 RELAY TIME GRADING MARGIN The time interv l th t must e llowed etween the oper tion of two dj cent rel ys in order to chieve corr ect discrimin tion etween them is c lled the gr ding m rgin. If gr ding m rgi n is not provided, or is insufficient, more th n one rel y will oper te for f ult, le ding to difficulties in determining the loc tion of the f ult nd unnece ss ry loss of supply to some consumers. The gr ding m rgin depends on num er o f f ctors: i. the f ult current interrupting time of the circuit re ker ii. rel y timing errors iii. the overshoot time of the rel y iv. CT errors v. fi n l m rgin on completion of oper tion F ctors (ii) nd (iii) ove depend to c ert in extent on the rel y technology used – n electromech nic l rel y, for inst nce, will h ve l rger overshoot time th n numeric l rel y. Gr ding is initi lly c rried out for the m ximum f ult level t the rel ying point under consider tion, ut check is lso m de th t the required gr ding m rgin exists for ll current levels etween rel y pick up current nd m ximum f ult level. Network Protection & Autom tion Guide • 130 •
9.11.1 Circuit Bre ker Interrupting Time The circuit re ker interrupting the f ult must h ve completely interrupted the current efore the discrimin ting rel y ce ses to e energised. The time t ken is dependent on the type of circuit re ker used nd the f ult current to e interrupted. M nuf cturers norm lly provide the f ult interrupting time t r ted interrupting c p city nd this v lue is in v ri ly used in the c lcul tion of gr ding m rgin. 9.11.2 Rel y Timing Error Al l rel ys h ve errors in their timing comp red to the ide l ch r cteristic s def ined in IEC 60255. For rel y specified to IEC 60255, rel y error index is qu oted th t determines the m ximum timing error of the rel y. The timing error mus t e t ken into ccount when determining the gr ding m rgin. 9.11.3 Overshoot Wh en the rel y is de energised, oper tion m y continue for little longer until ny stored energy h s een dissip ted. For ex mple, n induction disc rel y will h ve stored kinetic energy in the motion of the disc; st tic rel y circuits m y h ve energy stored in cp citors. Rel y design is directed to minimising nd s or ing these energies, ut some llow nce is usu lly necess ry. The overshoot ti me is defined s the difference etween the oper ting time of rel y t speci fied v lue of inputcurrent nd the m ximum dur tion of input current, which whe n suddenly reduced elow the rel y oper ting level, is insufficient to c use rel y oper tion. 9.11.4 CT Errors Current tr nsformers h ve ph se nd r tio errors due to the exciting current required to m gnetise their cores. The result is th t the CT second ry current is not n identic l sc led replic of the prim ry cur rent. This le ds to errors in the oper tion of rel ys, especi lly in the time of oper tion. CT errors re not relev nt when independent definite time del y over current rel ys re eing considered. 9.11.5 Fin l M rgin After the ove llow n ces h ve een m de, the discrimin ting rel y must just f il to complete its oper tion. Some extr llow nce, or s fety m rgin, is required to ensure th t rel y oper tion does not occur. 9.11.6 Over ll Accur cy The over ll limits of ccur cy ccording to IEC 60255 4 for n IDMT rel y with st nd rd inverse ch r cteristic re shown in Figure 9.10. Network Protection & Autom tion Guide 50 40 3 20 Time (seconds) 10 8 6 4 3 1 1 2 3 4 5 6 8 10 20 30
Time/Current ch r cteristic llow le limit At 2 times setting 2.5 x Decl red er ror At 5 times setting 1.5 x Decl red error At 10 times setting 1.0 x Decl red e rror At 20 times setting 1.0 x Decl red error Figure 9.10: Typic l limits of cc ur cy from IEC 60255 4 for n inverse definite minimum time overcurrent rel y
9.12 RECOMMENDED GRADING INTERVALS The following sections give the recommended o ver ll gr ding m rgins for etween different protection devices. 9.12.1 Gr ding: Rel y to Rel y The tot l interv l required to cover the ove items depends on the oper ting speed of the circuit re kers nd the rel y perform nce. At one ti me 0.5s w s norm l gr ding m rgin. With f ster modern circuit re kers nd l ower rel y overshoot time, 0.4s is re son le, while under the est conditions e ven lower interv ls m y e pr ctic l. The use of fixed gr ding m rgin is popul r, ut it m y e etter to c lcul te the required v lue for e ch rel y loc tion . This more precise m rgin comprises fixed time, covering circuit re kerf ul t interrupting time, rel y overshoot time nd s fety m rgin, plus v ri le t ime th t llows for rel y nd CT errors. T le 9.2 gives typic l rel y errors c cording to the technology used. It should e noted th t use of fixed gr ding m rgin is only ppropri te t high f ult levels th t le d to short rel y oper tin g times. At lower f ult current levels, with longer oper ting times, the permitt ed error specified in IEC 60255 (7.5% of oper ting time) m y exceed the fixed gr ding m rgin, resulting in the possi ility th t the rel y f ils to gr de correct ly while rem ining within • 131 • Overcurrent Protection for Ph se nd E rth F ults • 2 9•
specific tion. This requires consider tion when considering the gr ding m rgin t low f ult current levels. A pr ctic l solution for determining the optimum gr ding m rgin is to ssume th t the rel yne rer to the f ult h s m ximum possi le timing error of +2E, where E is the sic timing error. To this tot l effecti ve error for the rel y, further 10% should e dded for the over ll current tr nsformer error. Electromech nic l 7.5 0.05 0.1 0.4 Rel y Technology St tic 5 0.03 0.05 0.35 Digi t l 5 0.02 0.03 0.3 Numeric l 5 0.02 0.03 0.3 follows n I2t l w. So, to chieve proper co ordin tion etween two fuses in ser ies, it is necess ry to ensure th t the tot l I2t t ken y the sm ller fuse is n ot gre ter th n the pre rcing I2t v lue of the l rger fuse. It h s een est li shed y tests th t s tisf ctory gr ding etween the two fuses will gener lly e 9.12.3 Gr chieved if the current r ting r tio etween them is gre ter th n two. ding: Fuse to Rel y For gr ding inverse time rel ys with fuses, the sic ppro ch is to ensure whenever possi le th t the rel y cks up the fuse nd not vice vers . If the fuse is upstre m of the rel y, it is very difficult to m int in c orrect discrimin tion t high v lues of f ult current ec use of the f st oper t ion of the fuse. The rel y ch r cteristic est suited for this co ordin tion wit h fuses is norm lly the extremely inverse (EI) ch r cteristic s it follows si mil r I2t ch r cteristic. To ensure s tisf ctory co ordin tion etween rel y nd fuse, the prim ry current setting of the rel y should e pproxim tely three ti mes the current r ting of the fuse. The gr ding m rgin for proper co ordin tion, when expressed s fixed qu ntity, should not e less th n 0.4s or, when expre ssed s v ri le qu ntity, should h ve minimum v lue of: t’ = 0.4t+0.15 s cond s …Equation 9.4 Ov rcurr nt Prot ction for Phas and Earth Faults Typical basic timing all grading margin -
rror (%) Ov rshoot tim (s) Saf ty margin (s) Typical ov r r lay to r lay(s)
Tabl 9.2: Typical r lay timing rrors - standard IDMT r lays
A suitabl minimum grading tim int rval, t’, may b calculat d as follows: 2E +E t ′ = R CT t +t CB +t o +t s s conds 100 …Equation 9.2 wh r : Er = r lay timing rr r (IEC 60255-4) Ect = allowanc for CT ratio rror (%) t = op rating tim of r l ay n ar r fault (s) tCB = CB int rrupting tim (s) to = r lay ov rshoot tim (s) ts = saf ty margin (s) If, for xampl t=0.5s, th tim int rval for an l ctro m chanical r lay tripping a conv ntional circuit br ak r would b 0.375s, wh r a s, at th low r xtr m , for a static r lay tripping a vacuum circuit br ak r, t h int rval could b as low as 0.24s. Wh n th ov rcurr nt r lays hav ind p nd nt d finit tim d lay charact ristics, it is not n c ssary to includ th allow anc for CT rror. H nc : 2E t ′ = R t +t CB +t o +t s s conds 100 …Equation 9. culation of sp cific grading tim s for ach r lay can oft n b t dious wh n p rf orming a prot ction grading calculation on a pow r syst m. Tabl 9.2 also giv s practical grading tim s at high fault curr nt l v ls b tw n ov rcurr nt r lays for diff r nt t chnologi s. Wh r r lays of diff r nt t chnologi s ar us d, th tim appropriat to th t chnology of th downstr am r lay should b us d. 9.12 .2 Grading: Fus to Fus Th op rating tim of a fus is a function of both th pr -arcing and arcing tim of th fusing l m nt, which wh r t is th nominal op rating tim of fus . S ction 9.20.1 giv s an xampl o f fus to r lay grading. 9.13 CALCULATION OF PHASE FAULT OVERCURRENT RELAY SETTI NGS Th corr ct co-ordination of ov rcurr nt r lays in a pow r syst m r quir s t h calculation of th stimat d r lay s ttings in t rms of both curr nt and tim . Th r sultant s ttings ar th n traditionally plott d in suitabl log/log form at to show pictorially that a suitabl grading margin xists b tw n th r lays at adjac nt substations. Plotting may b don by hand, but nowadays is mor comm
only achi v d using suitabl softwar . Th information r quir d at ach r laying point to allow a r lay s tting calculation to proc d is giv n in S ction 9.2. Th principal r lay data may b tabulat d in a tabl similar to that shown in Ta bl 9.3, if only to assist in r cord k ping. • 9• Location Maximun Load Curr nt (A) Maximun Minimun Fault Curr nt (A) CT Ratio R lay Curr nt S tting P r C nt R lay Tim Primary Multipli r S tting Curr nt (A) Tabl 9.3: Typical r lay data tabl • 132 • N twork Prot ction & Automation Guid
It is usual to plot all tim /curr nt charact ristics to a common voltag /MVA bas on log/log scal s. Th plot includ s all r lays in a singl path, starting wit h th r lay n ar st th load and finishing with th r lay n ar st th sourc of supply. A s parat plot is r quir d for ach ind p nd nt path, and th s ttings of any r lays that li on multipl paths must b car fully consid r d to nsur that th final s tting is appropriat for all conditions. Earth faults ar consi d r d s parat ly from phas faults and r quir s parat plots. Aft r r lay s tti ngs hav b n finalis d, th y ar nt r d in a tabl . On such tabl is shown in Tabl 9.3. This also assists in r cord k ping and during commissioning of th r lays at sit . 9.13.1 Ind p nd nt (d finit ) Tim R lays Th s l ction of s tti ngs for ind p nd nt (d finit ) tim r lays pr s nts littl difficulty. Th ov rc urr nt l m nts must b giv n s ttings that ar low r, by a r asonabl margin, t han th fault curr nt that is lik ly to flow to a fault at th r mot nd of th syst m up to which backup prot ction is r quir d, with th minimum plant in s r vic . Th s ttings must b high nough to avoid r lay op ration with th maximum probabl load, a suitabl margin b ing allow d for larg motor starting curr nt s or transform r inrush transi nts. Tim s ttings will b chos n to allow suitab l grading margins, as discuss d in S ction 9.12. 9.13.2 Inv rs Tim R lays Wh n th pow r syst m consists of a s ri s of short s ctions of cabl , so that th total lin imp danc is low, th valu of fault curr nt will b controll d princ ipally by th imp danc of transform rs or oth r fix d plant and will not vary g r atly with th location of th fault. In such cas s, it may b possibl to grad th inv rs tim r lays in v ry much th sam way as d finit tim r lays. How v r, wh n th prosp ctiv fault curr nt vari s substantially with th location of th fault, it is possibl to mak us of this fact by mploying both curr nt and tim grading to improv th ov rall p rformanc of th r lay. Th proc dur b gins by s l ction of th appropriat r lay charact ristics. Curr nt s ttings a r th n chos n, with finally th tim multipli r s ttings to giv appropriat gr ading margins b tw n r lays. Oth rwis , th proc dur is similar to that for d finit tim d lay r lays. An xampl of a r lay s tting study is giv n in S ctio n 9.20.1. 9.14 DIRECTIONAL PHASE FAULT OVERCURRENT RELAYS Wh n fault curr nt can flow in b oth dir ctions through th r lay location, it may b n c ssary to mak th r spo ns of th r lay dir ctional by th introduction of a dir ctional control facili ty. Th facility is provid d by us of additional voltag inputs to th r lay. 9 .14.1 R lay Conn ctions Th r ar many possibiliti s for a suitabl conn ction o f voltag and curr nt inputs. Th various conn ctions ar d p nd nt on th phas angl , at unity syst m pow r factor, by which th curr nt and voltag appli d t o th r lay ar displac d. R f r nc [9.1] d tails all of th conn ctions that h av b n us d. How v r, only v ry f w ar us d in curr nt practic and th s ar d scrib d b low. In a digital or num rical r lay, th phas displac m nts ar r alis d by th us of softwar , whil l ctrom chanical and static r lays g n ra lly obtain th r quir d phas displac m nts by suitabl conn ction of th input quantiti s to th r lay. Th history of th topic r sults in th r lay conn ctio ns b ing d fin d as if th y w r obtain d by suitabl conn ction of th input qu antiti s, irr sp ctiv of th actual m thod us d. 9.14.2 90° R lay Quadratur Conn ction This is th standard conn ction for static, digital or num rical r lays. D p nding on th angl by which th appli d voltag is shift d to produc maximu m r lay s nsitivity (th R lay Charact ristic Angl , or RCA) two typ s ar avail abl . Ia Z ro torqu lin Va A MT 30° V bc 150° 30° Vbc
Vc Vb A phas l m nt conn ct d Ia Vbc B phas l m nt conn ct d Ib Vca C phas l m n t conn ct d Ic Vab Figur 9.11: V ctor diagram for th 90°-30° conn ction (phas A l m nt) N twork Prot ction & Automation Guid • 133 • Ov rcurr nt Prot ction for Phas and Earth Faults • 9•
9.14.2.1 90°-30° charact ristic (30° RCA) Th A phas r lay l m nt is suppli d with I a curr nt and Vbc voltag displac d by 30° in an anti-clockwis dir ction. In this cas , th r lay maximum s nsitivity is produc d wh n th curr nt lags th syst m phas to n utral voltag by 60°. This conn ction giv s a corr ct dir ctional tri pping zon ov r th curr nt rang of 30° l ading to 150° lagging; s Figur 9.11. T h r lay s nsitivity at unity pow r factor is 50% of th r lay maximum s nsitivi ty and 86.6% at z ro pow r factor lagging. This charact ristic is r comm nd d wh n th r lay is us d for th prot ction of plain f d rs with th z ro s qu nc sourc b hind th r laying point. 9.14.2.2 90°-45° charact ristic (45° RCA) For a digital or num rical r lay, it is common to allow us r-s l ction of th RC A angl within a wid rang . Th or tically, thr fault conditions can caus mal op ration of th dir ctional l m nt: i. a phas -phas -ground fault on a plain f d r ii. a phas -ground fault on a transform r f d r with th z ro s qu nc so urc in front of th r lay iii. a phas -phas fault on a pow r transform r with th r lay looking into th d lta winding of th transform r It should b r m mb r d, how v r, that th conditions assum d abov to stablish th maximum angular displac m nt b tw n th curr nt and voltag quantiti s at th r lay ar such t hat, in practic , th magnitud of th curr nt input to th r lay would b insuf fici nt to caus th ov rcurr nt l m nt to op rat . It can b shown analyticall y that th possibility of malop ration with th 90°-45° conn ction is, for all pract ical purpos s, non- xist nt. 9.14.3 Application of Dir ctional R lays If non-uni t, non-dir ctional r lays ar appli d to parall l f d rs having a singl g n ra ting sourc , any faults that might occur on any on lin will, r gardl ss of th r lay s ttings us d, isolat both lin s and compl t ly disconn ct th pow r sup ply. With this typ of syst m configuration, it is n c ssary to apply dir ctiona l r lays at th r c iving nd and to grad th m with th nondir ctional r lays a t th s nding nd, to nsur corr ct discriminativ op ration of th r lays duri ng lin faults. This is don by s tting th dir ctional r lays R1’ and R2’ in Figur 9.13 with th ir dir ctional l m nts looking into th prot ct d lin , and givin g th m low r tim and curr nt s ttings than r lays R1 and R2. Th usual practic is to s t r lays R1’ and R2’ to 50% of th normal full load of th prot ct d circui t and 0.1TMS, but car must b tak n to nsur that th continuous th rmal ratin g of th r lays of twic rat d curr nt is not xc d d. An xampl calculation i s giv n in S ction 9.20.3 R1 R 1 Ov rcurr nt Prot ction for Phas and Earth Faults Th A phas r lay l m nt is suppli d with curr nt Ia and voltag Vbc displac d by 45° in an anti-clockwis dir ction. Th r lay maximum s nsitivity is produc d w h n th curr nt lags th syst m phas to n utral voltag by 45°. This conn ction g iv s a corr ct dir ctional tripping zon ov r th curr nt rang of 45° l ading to 135° lagging. Th r lay s nsitivity at unity pow r factor is 70.7% of th maximum torqu and th sam at z ro pow r factor lagging; s Figur 9.12. This conn cti on is r comm nd d for th prot ction of transform r f d rs or f d rs that hav a z ro s qu nc sourc in front of th r lay. It is ss ntial in th cas of pa rall l transform rs or transform r f d rs, in ord r to nsur corr ct r lay op ration for faults b yond th star/d lta transform r. This conn ction should also b us d wh n v r singl -phas dir ctional r lays ar appli d to a circuit wh r a curr nt distribution of th form 2-1-1 may aris . Ia Va • 9• Z ro torqu lin 45° 45° 135°
M V bc TA Vbc Vc Vb Sourc I> Fault R 2 I> Load R2 A phas l m nt conn ct d Ia Vbc B phas l m nt conn ct d Ib Vca C phas l m n t conn ct d Ic Vab Figur 9.12: V ctor diagram for th 90°-45° conn ction (phas A l m nt) I> I> Figur 9.13: Dir ctional r lays appli d to parall l f d rs • 134 • N twork Prot ction & Automation Guid
9.15 RING MAINS A particularly common arrang m nt within distribution n tworks i s th Ring Main. Th primary r ason for its us is to maintain suppli s to consu m rs in cas of fault conditions occurring on th int rconn cting f d rs. A typ ical ring main with associat d ov rcurr nt prot ction is shown in Figur 9.14. C urr nt may flow in ith r dir ction through th various r lay locations, and th r for dir ctional ov rcurr nt r lays ar appli d. In th cas of a ring main f d at on point only, th s ttings of th r lays at th supply nd and at th mid point substation ar id ntical. Th y can th r for b mad non-dir ctional, if, in th latt r cas , th r lays ar locat d on th sam f d r, that is, on at ach nd of th f d r. It is int r sting to not that wh n th numb r of f d rs round th ring is an v n numb r, th two r lays with th sam op rating tim a r at th sam substation. Th y will th r for hav to b dir ctional. Wh n th numb r of f d rs is an odd numb r, th two r lays with th sam op rating tim ar at diff r nt substations and th r for do not n d to b dir ctional. It may also b not d that, at int rm diat substations, wh n v r th op rating tim of th r lays at ach substation ar diff r nt, th diff r nc b tw n th ir op ra ting tim s is n v r l ss than th grading margin, so th r lay with th long r o p rating tim can b non-dir ctional. With mod rn num rical r lays, a dir ctiona l facility is oft n availabl for littl or no xtra cost, so that it may b sim pl r in practic to apply dir ctional r lays at all locations. Also, in th v n t of an additional f d r b ing add d subs qu ntly, th r lays that can b nondi r ctional n d to b r -d t rmin d and will not n c ssarily b th sam – giving r is to probl ms of changing a non-dir ctional r lay for a dir ctional on . If a VT was not provid d originally, this may b v ry difficult to install at a lat r dat . 9.15.1 Grading of Ring Mains Th usual grading proc dur for r lays in a ring main circuit is to op n th ring at th supply point and to grad th r lay s first clockwis and th n anti-clockwis . That is, th r lays looking in a cloc kwis dir ction around th ring ar arrang d to op rat in th s qu nc 1-2-34-5 -6 and th r lays looking in th anti-clockwis dir ction ar arrang d to op rat in th s qu nc 1’-2’3’-4’-5’-6’, as shown in Figur 9.14. Th arrows associat d with th r laying points indicat th dir ction of curr nt flow that will caus th r lay to op rat . A doubl -h ad d arrow is us d to indicat a non-dir ctional r lay, such as thos at th supply point wh r th pow r can flow only in on dir ction . A singl -h ad d arrow is us d to indicat a dir ctional N twork Prot ction & Automation Guid 6 1 0.1 5
1.7
6 2.1 2.1 0.1 1.7 5 1 Ix Fault 0.5 1.3 4 2 Iy 1.3 0.5 2 0.9 0.9 3 3
1.3
4 1.3 4 0.9 3 1.7 5 0.5 2
2.1 6 0.1 1 0.5 2 1.7 5 0.9 3 0.1 1 2.1 6 Figur 9.14: Grading of ring mains r lay, such as thos at int rm diat substations around th ring wh r th pow r can flow in ith r dir ction. Th dir ctional r lays ar s t in accordanc with th invariabl rul , applicabl to all forms of dir ctional prot ction, that th curr nt in th syst m must flow from th substation busbars into th prot ct d lin in ord r that th r lays may op rat . Disconn ction of th fault d lin is carri d out according to tim and fault curr nt dir ction. As in any parall l s yst m, th fault curr nt has two parall l paths and divid s its lf in th inv rs ratio of th ir imp danc s. Thus, at ach substation in th ring, on s t of r lays will b mad inop rativ b caus of th dir ction of curr nt flow, and th oth r s t op rativ . It will also b found that th op rating tim s of th r lay s that ar inop rativ ar fast r than thos of th op rativ r lays, with th xc ption of th mid-point substation, wh r th op rating tim s of r lays 3 and 3’ happ n to b th sam . Th r lays that ar op rativ ar grad d downwards towar ds th fault and th last to b aff ct d by th fault op rat s first. This appli s to both paths to th fault. Cons qu ntly, th fault d lin is th only on to b disconn ct d from th ring and th pow r supply is maintain d to all th sub stations. Wh n two or mor pow r sourc s f d into a ring main, tim grad d ov r curr nt prot ction is difficult to apply • 135 • Ov rcurr nt Prot ction for Phas and Earth Faults • 4 9•
and full discrimination may not b possibl . With two sourc s of supply, two sol utions ar possibl . Th first is to op n th ring at on of th supply points, which v r is mor conv ni nt, by m ans of a suitabl high s t instantan ous ov r curr nt r lay. Th ring is th n grad d as in th cas of a singl inf d. Th s cond m thod is to tr at th s ction of th ring b tw n th two supply points as a continuous bus s parat from th ring and to prot ct it with a unit prot ctio n syst m, and th n proc d to grad th ring as in th cas of a singl inf d. S ction 9.20.4 provid s a work d xampl of ring main grading. 9.16 EARTH FAULT PROTECTION A B C I (a) A B C > Ov rcurr nt Prot ction for Phas and Earth Faults In th for going d scription, att ntion has b n principally dir ct d towards ph as fault ov rcurr nt prot ction. Mor s nsitiv prot ction against arth faults can b obtain d by using a r lay that r sponds only to th r sidual curr nt of th syst m, sinc a r sidual compon nt xists only wh n fault curr nt flows to arth. Th arth-fault r lay is th r for compl t ly unaff ct d by load curr nts, wh th r balanc d or not, and can b giv n a s tting which is limit d only by th d sign of th quipm nt and th pr s nc of unbalanc d l akag or capacitanc curr nts to arth. This is an important consid ration if s ttings of only a f w p rc nt of syst m rating ar consid r d, sinc l akag curr nts may produc a r sidual quantity of this ord r. On th whol , th low s ttings p rmissibl for a rthfault r lays ar v ry us ful, as arth faults ar not only by far th most fr qu nt of all faults, but may b limit d in magnitud by th n utral arthing im p danc , or by arth contact r sistanc . Th r sidual compon nt is xtract d by conn cting th lin curr nt transform rs in parall l as shown in Figur 9.15. Th simpl conn ction shown in Figur 9.15(a) can b xt nd d by conn cting ov rcu rr nt l m nts in th individual phas l ads, as illustrat d in Figur 9.15(b), and ins rting th arth-fault r lay b tw n th star points of th r lay group a nd th curr nt transform rs. Phas fault ov rcurr nt r lays ar oft n provid d o n only two phas s sinc th s will d t ct any int rphas fault; th conn ctions to th arth-fault r lay ar unaff ct d by this consid ration. Th arrang m nt i s illustrat d in Figur 9.15(c). Th typical s ttings for arth-fault r lays ar 30%-40% of th full-load curr nt or minimum arth-fault curr nt on th part of th syst m b ing prot ct d. How v r, account may hav to b tak n of th variati on of s tting with r lay burd n as d scrib d in S ction 9.16.1 b low. If gr at r s nsitivity than this is r quir d, on of th m thods d scrib d in S ction 9.16 .3 for obtaining s nsitiv arth-fault prot ction must b us d. I> I> (b) I> I > A B C I> I> I
> (c) Figur 9.15: R sidual conn ction of curr nt transform rs to arth-fault r la ys • 9• 9.16.1 Eff ctiv S tting of Earth-Fault R lays Th primary s tting of an ov rcur r nt r lay can usually b tak n as th r lay s tting multipli d by th CT ratio. Th CT can b assum d to maintain a suffici ntly accurat ratio so that, xpr s s d as a p rc ntag of rat d curr nt, th primary s tting will b dir ctly propo rtional to th r lay s tting. How v r, this may not b tru for an arth-fault r lay. Th p rformanc vari s according to th r lay t chnology us d. 9.16.1.1 St atic, digital and num rical r lays Wh n static, digital or num rical r lays ar us d th r lativ ly low valu and limit d variation of th r lay burd n ov r th r lay s tting rang r sults in th abov stat m nt holding tru . Th variation of input burd n with curr nt should b ch ck d to nsur that th N twork Prot ction & Automation Guid • 136 •
variation is suffici ntly small. If not, substantial rrors may occur, and th s tting proc dur will hav to follow that for l ctrom chanical r lays. 9.16.1.2 El ctrom chanical r lays Wh n using an l ctrom chanical r lay, th arth-fault l m nt g n rally will b similar to th phas l m nts. It will hav a similar VA consumption at s tting, but will impos a far high r burd n at nominal or ra t d curr nt, b caus of its low r s tting. For xampl , a r lay with a s tting o f 20% will hav an imp danc of 25 tim s that of a similar l m nt with a s ttin g of 100%. V ry fr qu ntly, this burd n will xc d th rat d burd n of th curr nt transform rs. It might b thought that corr spondingly larg r curr nt transf orm rs should b us d, but this is consid r d to b unn c ssary. Th curr nt tra nsform rs that handl th phas burd ns can op rat th arth fault r lay and th incr as d rrors can asily b allow d for. Not only is th xciting curr nt o f th n rgising curr nt transform r proportionat ly high du to th larg burd n of th arth-fault r lay, but th voltag drop on this r lay is impr ss d on t h oth r curr nt transform rs of th parall l d group, wh th r th y ar carrying primary curr nt or not. Th total xciting curr nt is th r for th product of th magn tising loss in on CT and th numb r of curr nt transform rs in parall l. Th summat d magn tising loss can b appr ciabl in comparison with th op ra ting curr nt of th r lay, and in xtr m cas s wh r th s tting curr nt is low or th curr nt transform rs ar of low p rformanc , may v n xc d th output to th r lay. Th ‘ ff ctiv s tting curr nt’ in s condary t rms is th sum of th r lay s tting curr nt and th total xcitation loss. Strictly sp aking, th ff c tiv s tting is th v ctor sum of th r lay s tting curr nt and th total xciti ng curr nt, but th arithm tic sum is n ar nough, b caus of th similarity of pow r factors. It is instructiv to calculat th ff ctiv s tting for a rang of s tting valu s of a r lay, a proc ss that is s t out in Tabl 9.4, with th r sults illustrat d in Figur 9.16. Th ff ct of th r lativ ly high r lay imp d anc and th summation of CT xcitation loss s in th r sidual circuit is augm n t d still furth r by th fact that, at s tting, th flux d nsity in th curr nt transform rs corr sponds to th bottom b nd of th xcitation charact ristic. Th xciting imp danc und r this condition is r lativ ly low, causing th ratio rror to b high. Th curr nt transform r actually improv s in p rformanc with i ncr as d primary curr nt, whil th r lay imp danc d cr as s until, with an inp ut curr nt s v ral tim s gr at r than th primary s tting, th multipl of s tti ng curr nt in th r lay is appr ciably high r than th multipl of primary s tti ng curr nt which is appli d to th primary circuit. This caus s th r lay op rat ing tim N twork Prot ction & Automation Guid to b short r than might b xp ct d. At still high r input curr nts, th CT p r formanc falls off until finally th output curr nt c as s to incr as substanti ally. B yond this valu of input curr nt, op ration is furth r complicat d by di stortion of th output curr nt wav form. 30 S condary voltag 20 10 0 0.5 1.0 Exciting curr nt (amp r s) 1.5 100 Eff ctiv s tting (p r c nt) 80 60 40 20 0 20
40 60 R lay s tting (p r c nt) 80 100 Figur 9.16: Eff ctiv s tting of arth-fault r lay R lay Plug Coil voltag S tting at S tting (V) % Curr nt (A) 5 0.25 12 10 0.5 6 15 0.75 4 20 1 3 40 2 1.5 60 3 1 80 4 0.75 100 5 0.6 Exciting Curr nt I 0.583 0.405 0.3 0.27 0.17 0.12 0.1 0.08 Eff ctiv S tting Curr nt % (A) 2 40 1.715 34.3 1.65 33 1.81 36 2.51 50 3.36 67 4.3 86 5.24 105 Tabl 9.4: Calculation of ff ctiv s ttings 9.16.2 Tim Grading of Earth-Fault R lays Th tim grading of arth-fault r lays can b arrang d in th sam mann r as for phas fault r lays. Th tim /primary curr nt charact ristic for l ctrom chanical r lays cannot b k pt proportionat to th r lay charact ristic with anything lik th accuracy that is possibl fo r phas fault r lays. As shown abov , th ratio rror of th curr nt transform r s at r lay s tting curr nt may b v ry high. It is cl ar that tim grading of l ctrom chanical arth-fault r lays is not such a simpl • 137 • Ov rcurr nt Prot ction for Phas and Earth Faults • Curr nt transform r xcitation charact ristic 9•
matt r as th proc dur adopt d for phas r lays in Tabl 9.3. Eith r th abov factors must b tak n into account with th rrors calculat d for ach curr nt l v l, making th proc ss much mor t dious, or long r grading margins must b al low d. How v r, for oth r typ s of r lay, th proc dur adopt d for phas fault r lays can b us d. 9.16.3 S nsitiv Earth-Fault Prot ction LV syst ms ar not n ormally arth d through an imp danc , du to th r sulting ov rvoltag s that may occur and cons qu ntial saf ty implications. HV syst ms may b d sign d to acco mmodat such ov rvoltag s, but not th majority of LV syst ms. How v r, it is qu it common to arth HV syst ms through an imp danc that limits th arth-fault curr nt. Furth r, in som countri s, th r sistivity of th arth path may b v ry high du to th natur of th ground its lf ( .g. d s rt or rock). A fault to arth not involving arth conductors may r sult in th flow of only a small cur r nt, insuffici nt to op rat a normal prot ction syst m. A similar difficulty a lso aris s in th cas of brok n lin conductors, which, aft r falling on to h d g s or dry m tall d roads, r main n rgis d b caus of th low l akag curr nt, and th r for pr s nt a dang r to lif . To ov rcom th probl m, it is n c ssary to provid an arth-fault prot ction syst m with a s tting that is consid rably low r than th normal lin prot ction. This pr s nts no difficulty to a mod rn digital or num rical r lay. How v r, old r l ctrom chanical or static r lays ma y pr s nt difficulti s du to th high ff ctiv burd n th y may pr s nt to th CT. Th r quir d s nsitivity cannot normally b provid d by m ans of conv ntiona l CT’s. A cor balanc curr nt transform r (CBCT) will normally b us d. Th CBCT is a curr nt transform r mount d around all thr phas (and n utral if pr s nt) conductors so that th CT s condary curr nt is proportional to th r sidual (i. . arth) curr nt. Such a CT can b mad to hav any conv ni nt ratio suitabl f or op rating a s nsitiv arth-fault r lay l m nt. By us of such t chniqu s, arth fault s ttings down to 10% of th curr nt rating of th circuit to b prot ct d can b obtain d. Car must b tak n to position a CBCT corr ctly in a cabl circuit. If th cabl sh ath is arth d, th arth conn ction from th cabl gl and/sh ath junction must b tak n through th CBCT primary to nsur that phas s h ath faults ar d t ct d. Figur 9.17 shows th corr ct and incorr ct m thods. With th incorr ct m thod, th fault curr nt in th sh ath is not s n as an unb alanc curr nt and h nc r lay op ration do s not occur. Th normal r sidual cur r nt that may flow during h althy • 138 • Cabl gland Cabl box Cabl gland /sh ath ground conn ction I > (a) Physical conn ctions Ov rcurr nt Prot ction for Phas and Earth Faults No op ration I > (b) Incorr ct positioning Op ration I > Figur 9.17: Positioning of cor balanc curr nt transform rs • 9•
conditions limits th application of non-dir ctional s nsitiv arth-fault prot ction. Such r sidual ff cts can occur du to unbalanc d l akag or capacitanc in th syst m. 9.17 DIRECTIONAL EARTH-FAULT OVERCURRENT PROTECTION Dir ctional arth-fault ov rcurr nt may n d to b appli d in th following situations: i. fo r arth-fault prot ction wh r th ov rcurr nt prot ction is by dir ctional r la ys ii. in insulat d- arth n tworks iii. in P t rs n coil arth d n tworks iv. wh r th s nsitivity of s nsitiv arth-fault prot ction is insuffici nt – us of a dir ctional arth-fault r lay may provid gr at r s nsitivity N twork Prot ction & Automation Guid
Th r lay l m nts pr viously d scrib d as phas fault l m nts r spond to th f low of arth fault curr nt, and it is important that th ir dir ctional r spons b corr ct for this condition. If a sp cial arth fault l m nt is provid d as d scrib d in S ction 9.16 (which will normally b th cas ), a r lat d dir ctiona l l m nt is n d d. 9.17.1 R lay Conn ctions Th r sidual curr nt is xtract d as shown in Figur 9.15. Sinc this curr nt may b d riv d from any phas , in or d r to obtain a dir ctional r spons it is n c ssary to obtain an appropriat qu antity to polaris th r lay. In digital or num rical r lays th r ar usually t wo choic s provid d. 9.17.1.1 R sidual voltag A suitabl quantity is th r sidu al voltag of th syst m. This is th v ctor sum of th individual phas voltag s. If th s condary windings of a thr -phas , fiv limb voltag transform r or thr singl -phas units ar conn ct d in brok n d lta, th voltag d v lop d ac ross its t rminals will b th v ctor sum of th phas to ground voltag s and h nc th r sidual voltag of th syst m, as illustrat d in Figur 9.18. Th prima ry star point of th VT must b arth d. How v r, a thr -phas , thr limb VT i s not suitabl , as th r is no path for th r sidual magn tic flux. Wh n th main voltag transform r associat d with th high voltag syst m is not provid d with a brok n d lta s condary winding to polaris th dir ctional art h fault r lay, it is p rmissibl to us thr singl -phas int rposing voltag t ransform rs. Th ir primary windings ar conn ct d in star and th ir s condary wi ndings ar conn ct d in brok n d lta. For satisfactory op ration, how v r, it is n c ssary to nsur that th main voltag transform rs ar of a suitabl constr uction to r produc th r sidual voltag and that th star point of th primary winding is solidly arth d. In addition, th star point of th primary windings of th int rposing voltag transform rs must b conn ct d to th star point of t h s condary windings of th main voltag transform rs. Th r sidual curr nt is phas offs t from th r sidual voltag and h nc angl a djustm nt is r quir d. Typically, th curr nt will lag th polarising voltag . T h m thod of syst m arthing also aff cts th R lay Charact ristic Angl (RCA), and th following s ttings ar usual: i. r sistanc - arth d syst m: 0° RCA ii. dis tribution syst m, solidly- arth d: -45° RCA iii. transmission syst m, solidly- art h d: -60° RCA Th diff r nt s ttings for distribution and transmission syst ms ari s from th diff r nt X/R ratios found in th s syst ms. 9.17.1.2 N gativ s qu nc curr nt Th r sidual voltag at any point in th syst m may b insuffici nt to polaris a dir ctional r lay, or th voltag transform rs availabl may not s atisfy th conditions for providing r sidual voltag . In th s circumstanc s, n gativ s qu nc curr nt can b us d as th polarising quantity. Th fault dir ct ion is d t rmin d by comparison of th n gativ s qu nc voltag with th n gati v s qu nc curr nt. Th RCA must b s t bas d on th angl of th n gativ phas s qu nc sourc voltag . 9.18 EARTH-FAULT PROTECTION ON INSULATED NETWORKS Occ asionally, a pow r syst m is run compl t ly insulat d from arth. Th advantag of this is that a singl phas arth fault on th syst m do s not caus any arth fault A B C I > (a) R lay conn ctions V a V a Va2 3IO I 3V 3VO V c V b V c V b
(b) Balanc d syst m (z ro r sidual volts) (c) Unbalanc d syst m fault (3Vo r sidual volts) Figur 9.18: Voltag polaris d dir ctional arth fault r lay N twork Prot ction & Automation Guid • 139 • Ov rcurr nt Prot ction for Phas and Earth Faults • Th r sidual voltag will b z ro for balanc d phas voltag s. For simpl arthfault conditions, it will b qual to th d pr ssion of th fault d phas voltag . In all cas s th r sidual voltag is qual to thr tim s th z ro s qu nc v oltag drop on th sourc imp danc and is th r for displac d from th r sidual curr nt by th charact ristic angl of th sourc imp danc . Th r sidual quant iti s ar appli d to th dir ctional l m nt of th arth-fault r lay. 9•
curr nt to flow, and so th whol syst m r mains op rational. Th syst m must b d sign d to withstand high transi nt and st ady-stat ov rvoltag s how v r, so its us is g n rally r strict d to low and m dium voltag syst ms. It is vital t hat d t ction of a singl phas - arth fault is achi v d, so that th fault can b trac d and r ctifi d. Whil syst m op ration is unaff ct d for this condition, th occurr nc of a s cond arth fault allows substantial curr nts to flow. Th abs nc of arth-fault curr nt for a singl phas - arth fault cl arly pr s nts som difficulti s in fault d t ction. Two m thods ar availabl using mod rn r l ays. 9.18.1 R sidual Voltag Wh n a singl phas - arth fault occurs, th h althy phas voltag s ris by a factor of √3 and th thr phas voltag s no long r hav a phasor sum of z ro. H nc , a r sidual voltag l m nt can b us d to d t ct t h fault. How v r, th m thod do s not provid any discrimination, as th unbala nc d voltag occurs on th whol of th aff ct d s ction of th syst m. On adva ntag of this m thod is that no CT’s ar r quir d, as voltag is b ing m asur d. H ow v r, th r quir m nts for th VT’s as giv n in S ction 9.17.1.1 apply. Grading is a probl m with this m thod, sinc all r lays in th aff ct d s ction will s th fault. It may b possibl to us d finit -tim grading, but in g n ral, it is not possibl to provid fully discriminativ prot ction using this t chniqu . 9.18.2 S nsitiv Earth Fault This m thod is principally appli d to MV syst ms, a s it r li s on d t ction of th imbalanc in th p r-phas charging curr nts tha t occurs. Figur 9.19 illustrat s th situation that occurs wh n a singl phas arth fault is pr s nt. Th r lays on th h althy f d rs s th unbalanc in c harging curr nts for th ir own f d rs. Th r lay in th fault d f d r s s th charging curr nts in th r st of th syst m, with th curr nt of its’ own f d rs canc ll d out. Figur 9.20 shows th phasor diagram. Vaf Ov rcurr nt Prot ction for Phas and Earth Faults IR1 Vapf Ib1 R strain Ia1 Op rat Vbf Vcpf Vr s (= -3Vo) An RCA s tting of +90° shifts th "c nt r of th charact ristic " to h r Figur 9.20: Phasor diagram for insulat d syst m with C phas - arth fa ult Vbpf IR3= -(IH1 IH2) I Ia1 Ib1 IR1 jXc1 X IH1 • 9• IR2 Ia2 Ib2 Us of Cor Balanc CT’s is ss ntial. With r f r nc to Figur 9.20, th unbalanc
curr nt on th h althy f d rs lags th r sidual voltag by 90°. Th charging cu rr nts on th s f d rs will b √3 tim s th normal valu , as th phas - arth volt ag s hav ris n by this amount. Th magnitud of th r sidual curr nt is th r fo r thr tim s th st ady-stat charging curr nt p r phas . As th r sidual curr nts on th h althy and fault d f d rs ar in antiphas , us of a dir ctional arth fault r lay can provid th discrimination r quir d. Th polarising quantit y us d is th r sidual voltag . By shifting this by 90°, th r sidual curr nt s n by th r lay on th fault d f d r li s within th ‘op rat ’ r gion of th dir ctio nal charact ristic, whil th r sidual curr nts on th h althy f d rs li withi n th ‘r strain’ r gion. Thus, th RCA r quir d is 90°. Th r lay s tting has to li b tw n on and thr tim s th p r-phas charging curr nt. This may b calculat d at th d sign stag , but confirmation by m ans of t sts on-sit is usual. A si ngl phas - arth fault is d lib rat ly appli d and th r sulting curr nts not d, a proc ss mad asi r in a mod rn digital or num ric r lay by th m asur m nt f aciliti s provid d. As not d arli r, application of such a fault for a short p riod do s not involv any disruption jXc2 X IH2 Ia3 IH1+ IR3 H3 jXc3 X IR3 =I +IH2+IH3-IH3 I I I =IH1 IH2 I I I +IH2 I Figur 9.19: Curr nt distribution in an insulat d syst m with a C phas – arth fau lt • 140 • N twork Prot ction & Automation Guid
to th n twork, or fault curr nts, but th duration should b as short as possib l to guard against a s cond such fault occurring. It is also possibl to disp n s with th dir ctional l m nt if th r lay can b s t at a curr nt valu that li s b tw n th charging curr nt on th f d r to b prot ct d and th charging curr nt of th r st of th syst m. 9.19 EARTH FAULT PROTECTION ON PETERSEN COIL EARTHED NETWORKS P t rs n Coil arthing is a sp cial cas of high imp danc ar thing. Th n twork is arth d via a r actor, whos r actanc is mad nominally qual to th total syst m capacitanc to arth. Und r this condition, a singl ph as - arth fault do s not r sult in any arth fault curr nt in st ady-stat condi tions. Th ff ct is th r for similar to having an insulat d syst m. Th ff ct iv n ss of th m thod is d p nd nt on th accuracy of tuning of th r actanc va lu – chang s in syst m capacitanc (du to syst m configuration chang s for insta nc ) r quir chang s to th coil r actanc . In practic , p rf ct matching of th coil r actanc to th syst m capacitanc is difficult to achi v , so that a sma ll arth fault curr nt will flow. P t rs n Coil arth d syst ms ar commonly fou nd in ar as wh r th syst m consists mainly of rural ov rh ad lin s, and ar pa rticularly b n ficial in locations subj ct to a high incid nc of transi nt faul ts. To und rstand how to corr ctly apply arth fault prot ction to such syst ms, syst m b haviour und r arth fault conditions must first b und rstood. IR1 Ia1 Ib1 -X -jXC1 IH1 IL Ia2 Ib2 IR2 jXL X -X -jXC2 IH2 IR3 IF -X -jXC3 IL=IF IH1 IH2-IH3 I I IL I H1+IH2 Figur 9.22: Distribution of curr nts during a C phas - arth fault – radial distri bution syst m Figur 9.21 illustrat s a simpl n twork arth d through a P t rs n Coil. Th q uations cl arly show that, if th r actor is corr ctly tun d, no arth fault cur r nt will flow. Figur 9.22 shows a radial distribution syst m arth d using a P t rs n Coil. On f d r has a phas - arth fault on phas C. Figur 9.23 shows t h r sulting phasor diagrams, assuming that no r sistanc is pr s nt. IH3 IH2 I Ia1 N C B a) Capacitiv t inductiv curr nts IL H1 b1 IL Sourc -IB I -IC I Van L A 3VO V If IB- C+ P t rs n coil If =O if
Van jXL X =IB+IC I I -X -jX (=IL) I jXL X Vab jXC X (=-Ib I -X -jXC Vac C an jXL X Ic) -X -jXC -IC I A -IB I Vac N C B Vab IL Ib1 Ia1 IR1=IH1 -IH1 -I IR3 IR3 =-I +I =-IH2 Vr s=-3VO V b) Unfault d lin Vr s=-3VO V c) Fault d lin Curr nt v ctors for A phas fault Figur 9.21: Earth fault in P t rs n Coil art h d syst m Figur 9.23: C phas - arth fault in P t rs n Coil arth d n twork: th or tical c as –no r sistanc pr s nt in XL or XC N twork Prot ction & Automation Guid • 141 • Ov rcurr nt Prot ction for Phas and Earth Faults • Ia3 Ib3 I =IF I 9•
In Figur 9.23(a), it can b s n that th fault caus s th h althy phas voltag s to ris by a factor of √3 and th charging curr nts l ad th voltag s by 90°. Usi ng a CBCT, th unbalanc curr nts s n on th h althy f d rs can b s n to b a simpl v ctor addition of Ia1 and Ib1, and this li s at xactly 90° lagging to t h r sidual voltag (Figur 9.23(b)). Th magnitud of th r sidual curr nt IR1 is qual to thr tim s th st ady-stat charging curr nt p r phas . On th faul t d f d r, th r sidual curr nt is qual to IL-IH1-IH2, as shown in Figur 9.23 (c) and mor cl arly by th z ro s qu nc n twork of Figur 9.24. IROF IOF Fault d f d r H althy f d rs How v r, in practical cas s, r sistanc is pr s nt and Figur 9.25 shows th r s ulting phasor diagrams. If th r sidual voltag Vr s is us d as th polarising v oltag , th r sidual curr nt is phas shift d by an angl l ss than 90° on th fau lt d f d r and gr at r than 90° on th h althy f d rs. H nc a dir ctional r lay can b us d, and with an RCA of 0°, th h althy f d r r sidual curr nt will fall in th ‘r strain’ ar a of th r lay charact ristic whil th fault d f d r r sidua l curr nt falls in th ‘op rat ’ ar a. Oft n, a r sistanc is d lib rat ly ins rt d in parall l with th P t rs n Coil to nsur a m asurabl arth fault curr nt an d incr as th angular diff r nc b tw n th r sidual signals to aid r lay appl ication. Having stablish d that a dir ctional r lay can b us d, two possibilit i s xist for th typ of prot ction l m nt that can b appli d – s nsitiv arth fault and z ro s qu nc wattm tric. 9.19.1 S nsitiv Earth Fault Prot ction To apply this form of prot ction, th r lay must m t two r quir m nts: a. curr nt m asur m nt s tting capabl of b ing s t to v ry low valu s b. an RCA of 0°, and c apabl of fin adjustm nt around this valu Th s nsitiv curr nt l m nt is r q uir d b caus of th v ry low curr nt that may flow – so s ttings of l ss than 0.5 % of rat d curr nt may b r quir d. How v r, as comp nsation by th P t rs n Coi l may not b p rf ct, low l v ls of st ady-stat arth-fault curr nt will flow a nd incr as th r sidual curr nt s n by th r lay. An oft n us d s tting valu is th p r phas charging curr nt of th circuit b ing prot ct d. Fin tuning of th RCA is also r quir d about th 0° s tting, to comp nsat for coil and f d r r sistanc s and th p rformanc of th CT us d. In practic , th s adjustm nts a r b st carri d out on sit through d lib rat application of faults and r cordi ng of th r sulting curr nts. 9.19.2 S nsitiv Wattm tric Prot ction It can b s n in Figur 9.25 that a small angular diff r nc xists b tw n th spill curr nt on th h althy and fault d f d rs. Figur 9.26 illustrat s how this angular diff r nc giv s ris to activ compon nts of curr nt which ar in antiphas to ach oth r. Cons qu ntly, th activ compon nts of z ro s qu nc pow r will als o li in similar plan s and a r lay capabl of d t cting activ pow r can mak a discriminatory Ov rcurr nt Prot ction for Phas and Earth Faults IROH IL 3XL IROH -VO IH3 Xco IH2 IH1 K y: IROF=r sidual curr nt on fault d f d r IROH=r sidual curr nt on h althy f d r It can th r for b s n that: -IOF=IL-IH1-IH2-IH3 IROF=IH3+IOF So: -IROF=I L=IH1-IH2 Figur 9.24: Z ro s qu nc n twork showing r sidual curr nts R sistiv compon nt in grounding coil I L (I
1+IH2+IH3) R sistiv compon nt in f d r 3VO A • 9• C N B a) Capacitiv and inductiv curr nts with r sistiv compon nts IL IR1=IH1 -IH1 -IH2 IR3 =I +I IR3 F H3 =IL-IH1-IH2 Vr s=-3VO Op rat R strain Op rat Z ro torqu lin for O° RCA Vr s=-3VO Z ro torqu lin for 0° RCA b) Unfault d lin R strain c) Fault d lin Figur 9.25: C phas - arth fault in P t rs n Coil arth d n twork: practical cas with r sistanc pr s nt in XL or XC • 142 • N twork Prot ction & Automation Guid
Vr s=-3VO V Activ compon nt of r sidual curr nt: fault d f d r IR3 IH1-IH2 I I L IR1 of r sidual curr nt: h althy f d r Op rat Z ro torqu lin for O° RCA R strain Figur 9.26: R sistiv compon nts of spill curr nt Figur 9.27: MiCOM P140 Consid r th syst m shown in Figur 9.28. 500 MVA 11kV Utility sourc 11kV 3000/5 Max load 2800A Utility cli nt Wattm tric pow r is calculat d in practic using r sidual quantiti s inst ad of z ro s qu nc on s. Th r sulting valu s ar th r for nin tim s th z ro s qu nc quantiti s as th r sidual valu s of curr nt and voltag ar ach thr tim s th corr sponding z ro s qu nc valu s. Th quation us d is: V r s ×I r s ×cos ( ϕ−ϕ c ) = 9 ×V O ×I O ×cos ( ϕ−ϕ c ) where: Vres = residual voltage Ires = residual current Vo = ero sequence voltage Io φ φc = zero sequence current = angle between Vres and Ires = relay characteristic angle setting …Equation 9.5 5 I>> I> I > 4 I> I > Bus A 11kV I>> I> Cable C1 : 5 x3 x1c x 630mm2 XLPE Z = 0.042 + j 0.086Ω/km/cable L = 2km 3000/1 3 1000/1 Max loa 1000A Bus B 11kV I> I > 1 500/1 I Reactor R1 : Z=4% on 20MVA I> 2 > 500/1 Cables C2,C3: 1 x 3c x 185mm2XLPE Z = 0.128 + j 0.093Ω/km L = Ikm Max loa 400A/fee er C2 C3 The current an RCA settings are as for a sensitiveearth fault relay. 9.20 EXAM PLES OF TIME AND CURRENT GRADING This section provi es etails of the time/curre nt gra ing of some example networks, to illustrate the process ofrelay setting calculations an relay gra ing. They are base on the use of a mo ern numerical overcurrent relay illustrate in Figure 9.27, with setting ata taken from this relay. Network Protection & Automation Gui e Bus C 11kV FS2 160A F2 I> 200/5 150/5 F1 I> FS1 125A
IS = 120% IS = 110% TMS = 0.25 TMS = 0.1 Max loa 190A Max loa 130A Figure 9.28 : IDMT relay gra ing example
Max loa 90A The problem is to calculate appropriate relay settings for relays 1-5 inclusive. Because the example is concerne with gra ing, consi erations such as bus-zone • 143 •
Overcurrent Protection for Phase an Earth Faults • ecision. component of zero sequence power is etecte in the If the wattmetric forwar irection, it in icates a fault on that fee er, while a power in the rev erse irection in icates a fault elsewhere on the system. This metho ofprotect ion is more popular than the sensitive earth fault metho , an can provi e great er security against false operation ue to spurious CBCT output un er non-earth fault con itions. 9.20.1 Relay Phase Fault Setting Example – IDMT Relays/Fuses 9•
protection, an CT knee-point voltage requirements, etc., are not ealt with. Al l curves are plotte to an 11kV base. The contactors in series with fuses FS1/FS 2 have a maximum breaking capacity of 3kA, an relay F2 has been set to ensure t hat the fuse operates prior to the contactor for currents in excess of this valu e. CT’s for relays F1, F2 an 5 are existing CT’s with 5A secon aries, while the rem aining CT’s are new with 1A secon aries. Relay 5 is the property of the supply uti lity, an is require to be set using an SI characteristic in or er to ensure gr a ing with upstream relays. 9.20.1.1 Impe ance Calculations All impe ances must first be referre to a common base, taken as 500MVA, as follows: (ii) At bus B Fault Level = 500 ×100 MVA ZS + ZC1 + Z R1 = 232MVA =12.2 kA (iii) At bus A Fault Level = 500 ×100 MVA ZS + ZC1 = 432MVA = 22.7kA (iv) Source CT ratio se Fault Level = 500MVA = 26.3kA 9.20.1.3 lection This requires consi eration not only of the maximum loa current, but al so of the maximum secon ary current un er fault con itions. CT secon aries are g enerally rate to carry a short-term current equal to 100 x rate secon ary curr ent. Therefore, a check is require that none of the new CT secon aries has a cu rrent of more than 100A when maximum fault current is flowingin the primary. Us ing the calculate fault currents, this con ition is satisfie , so mo ifications to the CT ratios are not require . 9.20.1.4 Relay overcurrent settings – Relays 1 of the cable fee ers, Busbar C an /2 These relays perform overcurrent protection backup-protection to relays F1, F2 an their associate fuses FS1 an FS2. The settings for Relays 1 an 2 will be i entical, so calculations will only be per forme for Relay 1. Consi er first the current setting of the relay. Relay 1 mus t be able to reset at a current of 400A – the rating of the fee er. The relay has a rop-off/pick-up ratio of 0.95, so the relay current setting must not be less than 400/0.95, or 421A. A suitable setting that is greater than this valueis 45 0A. However, Section 9.12.3 also recommen s that the current setting shoul be t hree times the largest fuse rating (i.e. 3 x 160A, the rating of the largest fus e on the outgoing circuits from Busbar C), lea ing to a current setting of 480A, or 96% of relay rate primary current. Note that in this application of relays to a istribution system, the question of maximum an minimum fault levels are p robably not relevant as the ifference between maximum an minimum fault levels will be very small. However in other applications where significant ifferences between maximum an minimum fault levels exist, it is essential to ensure that Overcurrent Protection for Phase an Earth Faults Reactor R1 4 ×500 Z R1 = = 100% 20 Cable C1 0.096 ZC1 = ×2 = 0.038 Ω 5 On 500MVA base, 0.038 ×100 ×500 ZC1 = (11)2 = 15.7% Cables C2,C3 ZC2, ZC3 = 0.158 Ω On 500MVA base, Z C2, ZC 3 = 0.158 ×100 ×500 (11) 2
= 65.3% Source Impe ance (500MVA base) 500 ZS = × 100% 500 = 100% 9.20.1.2 Fault L evels The fault levels are calculate as follows: (i) At bus C For 2 fee ers, Fa ult Level= 500 ×100 MVA Z R1 + ZS + ZC1 + ZC 2 2 • 9•
= 10.6 kA on 11kV base For a single fee er, fault level = 178MVA = 9.33kA • 144 •
Network Protection & Automation Gui e
the selection of a current setting that is greater than full loa current oes n ot result in the relay failing to operate un er minimum fault current con itions . Such a situation may arise for example in a self-containe power system with i ts own generation. Minimum generation may be represente by the presence of asi ngle generator an the ifference between minimum fault level an maximum loa l evel may make the choice of relay current settings ifficult. The gra ing margin now has to be consi ere . For simplicity, a fixe gra ing margin of 0.3s betwee n relays is use in the calculations, in accor ance with Table 9.2. Between fuse an relay, Equation 9.4 isapplie , an with fuse FS2 pre-arcing time of 0.01s (from Figure 9.29), the gra ing margin is 0.154s. Consi er first the IDMT overcu rrent protection. Select the EI characteristic, as fuses exist ownstream, to en sure gra ing. The relay must iscriminate with the longest operating time betwee n relays F1, F2 an fuse FS2 (being the largest fuse) at the maximum fault level seen by relays 1 an 2. The maximum fault current seen by relay 1 for a fault a t Busbar C occurs when only one of cables C2, C3 is in service. This is because the whole of the fault current then flows through the fee er that is in service. With two fee ers in service, although the fault level at Busbar C is higher, ea ch relay only sees half ofthe total fault current, which is less than the fault current with a single fee er in service. With EI characteristics use for relay s F1 an F2, the operating time for relay F1 is 0.02s at TMS=0.1 because the fau lt current is greater than 20 times relay setting, at which point the EI charact eristic becomes efinite time (Figure 9.4) an 0.05s for relay F2 (TMS=0.25). He nce select relay 1 operating time =0.3+0.05=0.35s, to ensure gra ing with relay F2 at a fault current of 9.33kA. With a primary setting of 480A, a fault current of 9.33kA represents 9330/480 = 19.44 times setting Thus relay 1 operating time at TMS=1.0 is 0.21s. The require TMS setting is given by the formula: TMS = op eration time require Actual op. time require at TMS =1.0 ∴ TMS = characteristic: I sr1 f = where t is the require operation time (s) Isr1f = set ting of relay at fault current Hence, with t = 0.35, Isr1f = 15.16 or, I sr1 = I sr1 = 9330 =615.4 A 15.16 616 =1.232 500 80 +1 t Use 1.24 = 620A nearest available value At a TMS of 1.0, operation time at 9330A = 80 9330 −1 620 0.35 = 0.99 0.355 2 = 0.355 Hence, required TMS = for convenience, use TMS of 1.0, slightly gre ter th n the required v lue. Fro mthe gr ding curves of Figure 9.29, it c n e seen th t there re no gr ding pr o lems with fuse FS1 or rel ys F1 nd F2. 9.20.1.5 Rel y overcurrent settings Rel y 3 This rel y providesovercurrent protection for re ctor R1, nd ckup ov ercurrent protection for c les C2 nd C3. The overcurrent protection lso provi des us r protection for Bus r B. Ag in, the EI ch r cteristic is used to ensu re gr ding with rel ys 1 nd 2. The m ximum lo d current is 1000A. Rel y 3 curre nt setting is therefore feeder flc I sr 3 > CT prim ry current ×0.95 Su stituting v lues, Isr3 >1052A Use setting of 106% or 1060A, ne rest v il le setting ove 1052A.Rel y 3 h s to gr de with rel ys 1/2 under two conditions: 1. for f ult just eyond rel ys 1 nd 2 where the f ult current will e the us r f ult current of 12.2kA 2. for f ult t Bus C where the f ult current seen y 0.35 =1.66 0.21 This v lue of TMS is outsidethe sett le r nge of the rel y (m ximum setting1.2). Therefore, ch nges must e m de to the rel y current setting in order to ring the v lue ofTMS required into the r nge v il le, provided this does not result in the in ility of the rel y to oper te t the minimum f u lt level. By re rr ngement of the formul for the EI Network Protection & Autom tion Guide
• 145 • Overcurrent Protection for Ph se nd E rth F ults • 9•
either rel y 1 or 2 will e h lf the tot l Bus C f ult current of 10.6kA, i.e. 5 .3kA Ex mining first condition 1. With current setting of 620A, TMS of 1.0 nd f ult current of 12.2kA, rel y 1 will oper te in 0.21s. Using gr ding int erv l of 0.3s, rel y 3 must therefore oper te in 0.3 + 0.21 = 0.51s t f ult c urrent of 12.2kA. 12.2kA represents 12200/1060 = 11.51 times setting for rel y 3 nd thus the time multiplier setting of rel y 3 should e 0.84 to give n oper ting time of 0.51s t 11.51 times setting. Rel y
setting must e t le st 2800 = 98% 3000 ×0.95 . For convenience, use v lue of 1 00% (=3000A). Thus rel y 4 must oper te in 0.605s t 15860/3000 = 5.29 times set ting. Thus select time multiplier setting of 0.15, giving rel y oper ting ti me of 0.62s for norm l inverse type ch r cteristic. 4 At this st ge, it is ins tructive in Figure 9.29( ). While to review the gr ding curves, which re shown it c n e seen th t there re no gr ding pro lems etween the fuses nd rel ys 1 /2, nd etween rel ys F1/2 nd rel ys 1/2, it is cle r th t rel y 3 nd rel y 4 do not gr de over the whole r nge of f ult current. This is consequence of th e ch nge in ch r cteristic for rel y 4 to SI from the EI ch r cteristic of rel y 3 to ensure gr ding of rel y 4 with rel y 5. The solution is to incre se the TM S setting of rel y 4 until correct gr ding is chieved. The ltern tive is to in cre se the current setting, ut this is undesir le unless thelimit of the TMS setting is re ched, ec use the current setting should lw ys e s low s possi le to help ensure positive oper tion of the rel y nd provide overlo d protecti on. Tri l nd error is often used, ut suit le softw re c n speed the t sk – for inst nce it is not difficult to construct spre dsheet with the fuse/rel y oper tion times nd gr ding m rgins c lcul ted. S tisf ctory gr ding c n e found fo r rel y 4 setting v lues of: Ist4 = 1.0 or 3000A TMS = 0.275 At 22.7kA, the oper tion time of rel y 4 is 0.93s. The revised gr ding curves re shown in Figure 9 .29( ). 9.20.1.7 Rel y 5 Rel y 5 must gr de with rel y 4 t f ult current of 2 2.7kA. At this f ult current, rel y 4 oper tes in 0.93s nd thus rel y 5 must op er te in 0.3 + 0.93 = 1.23s t 22.7kA. A current setting of 110% of rel y 4 curr ent setting (i.e. 110% or 3300A) is chosen to ensure rel y 4 picks up prior to r el y 5. Thus 22.7kA represents 6.88 times the setting of rel y 5. Rel y 5 must g r de with rel y 4 t f ult current of 22.7kA, where the required oper tion tim e is 1.23s. At TMS of 1.0, rel y 5 oper tion time is current Overcurrent Protection for Ph se nd E rth F ults Consider now condition 2. With settings of 620A nd TMS of 1.0 nd f ult curre nt of 5.3kA, rel y 1 will oper te in 1.11s. Using gr ding interv l of 0.3s, re l y 3 must therefore oper te in 0.3 + 1.11 = 1.41s t f ult current of 5.3kA. 5.3kA represents 5300/1060 = 5 times setting for rel y 3, nd thus the time mult iplier setting of rel y 3 should e 0.33 to give n oper ting time of 1.11s t 5 times setting. Thus condition 1 represents the worst c se nd the time multipli er setting of rel y 3 should e set t 0.84. In pr ctice, v lue of 0.85 is use d s the ne rest v il le setting on the rel y. Rel y 3 lso h s n inst nt neo us element. Thisis set such th t it will not oper te for the m ximum throughf u lt current seen y the rel y, setting of 130% of this v lue eing s tisf ctory . The setting is therefore: 1.3x12.2kA =15.86kA This is equ l to current setti ng of 14.96 times the setting of rel y 3. 9.20.1.6 Rel y 4 This must gr de with rel y 3 nd rel y 5. The supply uthority requires th t rel y 5 use n SI ch r c teristic to ensure gr ding with rel ys further upstre m, so the SI ch r cteristi c will e used for rel y 4 lso. Rel y 4 must gr de with rel y 3 t Bus A m ximu m f ult level of 22.7kA. However with theuse of n inst nt neous high set eleme nt for rel y 3, the ctu l gr ding point ecomes the point t which the high set setting of rel y 3 oper tes, i.e. 15.86kA. At this current, the oper tion time of rel y 3 is 80 × 0.85s = 0.305s 2 (14.96 ) − 1 Thus, rel y 4 required oper ting ti
me is 0.305 + 0.3 = 0.605s t f ult level of 15.86kA. • 9• (6.88 ) 0.14 0.02 −1 = 3.56 s
Therefore, the required TMS is 1.23/3.56 = 0.345, use 0.35 ne rest v il le v l ue. The protection gr ding curves th t result re shown in Figure 9.30 nd the s etting v lues in T le 9.5. Gr ding is now s tisf ctory. • 146 • Network Protection & Autom tion Guide
100.00 10.00 Rel y F1 Rel y F2 Time (sec) Fuse FS1 1.00 Fuse FS2 Rel ys 1/2 Rel y 4 0.10 0.01 100 1000 Current (A) ( ) Initi l gr ding curves 10000 100000 100.00 10.00 Rel y F1 Rel y F2 Fuse FS1 1.00 Fuse FS2 Rel ys 1/2 Rel y 3 Rel y 4 0.10
0.01 100 1000 Current (A) ( ) Revised initi l gr ding curves Figure 9.29: Initi l rel y gr ding curves – overcurrent rel y ex mple 10000 100000 Network Protection & Autom tion Guide • 147 • Overcurrent Protection for Ph se nd E rth F ults • Rel y 3 Time (sec) 9•
In situ tions where one of the rel ys to e gr ded is provided y third p rty, it is common for the settings of the rel y to e specified nd this m y le d to l ck of co ordin tion etween this rel y nd others (usu m lly those downstre ). Negoti tion is then required to try nd chieve ccept le settings, ut it i s often the c se th t no ch nge to the settings of the rel y provided y the thi rd p rty is llowed. A l ck of co ordin tion etween rel ys then h s to e ccep ted over t le st p rt of the r nge of f ult currents. Rel y Settings Lo d M x Rel y/ current F ult CT Fuse Ch r c Current Setting TMS Fuse Current R tio R ting teristic Prim ry Per Amps Cent (A) kA F1 190 10.6 200 /5 EI 100 100 0.1 F2 130 10.6 150/5 EI 150 120 0.25 FS1 90 10.6 125A FS2 130 10. 6 160A 1 400 12.2 500/1 EI 620 124 1 2 400 12.2 500/1 EI 620 124 1 EI 1060 106 0 .85 3 1000 22.7 1000/1 Inst nt. 15860 14.96 4 3000 22.7 3000/1 SI 3000 100 0.275 5 3000 26.25 3000/5 SI 3300 110 0.35 T le 9.5: Rel y settings for overcurrent rel y ex mple order to c lcul te f ult levels. However, such imped nces re frequently not v il le, or known only pproxim tely nd the ph se f ult current levels h ve to e used. Note th t e rth f ult levels c n e higher th n ph se f ult levels if th e system cont ins multiple e rth points, or if e rth f ult levels re considered on the st r side of delt /st r tr nsformer when the st r winding is solidly e rthed. On the circuit with fuse F2, low level e rth f ults m y not e of suffic ient m gnitude to low the fuse. Attempting to grde the e rth f ult element of the upstre m rel y with fuse F2 will not e possi le. Simil rly, rel ys F1 nd F 2 h ve ph se f ult settings th t do not provide effective protection g inst e r th f ults. The remedy would e to modify the downstre m protection, ut such con sider tions lie outside the scope of this ex mple. In gener l therefore, the e r th f ult elements of rel ys upstre m of circuits with only ph se f ult protectio n (i.e. rel ys with only ph se f ult elements or fuses) willh ve to e set with compromise th t they will detect downstre m e rth f ults ut will not provide discrimin tive trip. This illustr tes the pr ctic l point th t it is r re in nything other th n very simple network to chieve s tisf ctory gr ding for l l f ults throughout the network. In the ex mple of Figure 9.27, it is likely th t the difference in f ult levels etween ph se to ph se nd ph se to e rth f ult s will e very sm ll nd thus the only function of e rth f ult elements is to de tect nd isol te low level e rth f ults not seen y the ph se f ult Overcurrent Protection for Ph se nd E rth F ults 9.20.2 Rel y E rth F ult Settings The procedure for setting the e rth f ult elem ents is identic l to th t for the overcurrent elements, except th t zero sequenc e imped nces must e used if v il le nd different from positive sequence impe d nces in 100.00 Rel y F1 Rel y F2 10.00 • 9• Time (sec) 1.00 Fuse FS1 Fuse FS2 Rel ys 1/2 Rel y 3 Rel y 4 Rel y 5 0.10 0.01 100
1000 Current (A) 10000 100000 Figure 9.30: Fin l rel y gr ding curves for overcurrent rel y ex mple • 148 • Network Protection & Autom tion Guide
elements. Following the guidelines of Section 9.16, rel ys 1/2 c n use current setting of 30% (150A) nd TMS of 0.2, using theEI ch r cteristic. Gr ding of rel ys 3/4/5 follows the s me procedure s descri ed for the ph se f ult elemen ts of these rel ys. 9.20.3 Protection of P r llel Feeders Figure 9.31( ) shows t wo p r llel tr nsformer feeders forming p rt of supply circuit. Imped nces re s given in the di gr m. T1 4 IF4 Bus P 220k Ie 5 I I> I Source 0.01pu I f 6 > Bus P 4 I Z=0.25puI 50MVA Z=12.5% If rel ys 2 nd 3 re non direction l, then, using SI rel y ch r cteristics for ll rel ys, gr ding of the rel ys is dict ted y the following: ) f ult t loc tion F1, with 2 feeders in service ) f ult t loc tion F4, with one feeder in s ervice Current setting 1 1.1 1.1 0.61 0.61 0.7 Rel y 1 2 3 4 5 6 100.00 CT Prim ry 300 300 300 300 300 300 TMS 0.2 0.3 0.3 0.425 0.425 0.7 Ch r cteristic SI SI SI SI SI SI Time (sec) Source 10000MVA If 6 > I
Rel y 1 F3 Bus Q 110kV 3 I I> 2 > 1 I I> 10.00 1.00 0.10 100 Rel ys 2/3 Rel ys 4 /5 Rel y 6 1000 10000 100000 Current (A) ( ) Rel y gr ding curves non direction l rel ys Figure 9.32: Rel y gr ding for p r llel feeder ex mple – non direction l rel ys Bus Q I I> 3 Ie I 3 I I> I I> All imped nces to p 100MVA, 110kV
se ( ) Imped nce di gr m
Figure 9.31: System di gr m: P r llel feeder ex mple The settings shown in Figure9.32( ) c n e rrived t, with the rel y oper tion times shown in Figure 9.32( ). It is cle r th t for f ult t F3 with oth tr nsformer feeders in service, rel y 3 oper tes t the s me time s rel y 2 nd re sults in tot l disconnection of Bus Q nd ll consumers supplied solely from it. This is undesir le – the dv nt ges of duplic ted 100% r ted tr nsformers h ve een lost. By m king rel ys 2 nd 3 direction l s shown in Figure 9.33( ), lower settingsfor these rel ys c n e dopted – they c n e set s low s re son ly p r ctic l ut norm lly current setting of out 50% of feeder full lo d current is used, with TMS of 0.1. Gr ding rules c n e est lished s follows: . rel y 4 is gr ded with rel y 1 for f ults t loc tion F1 with one tr nsformer feede
r in service . rel y 4 is gr ded with rel y 3 for f ults t loc tion F3 with tw o tr nsformer feeders in service c. rel y 6 gr des with rel y 4 for f ults t F4 d. rel y 6 lso h s to gr de with rel y 4 for f ults t F1 with oth tr nsforme r feeders in service – rel y 6sees the tot l f ult current ut rel y 4 only 50% o f this current. Norm l rules out c lcul ting current setting v lues of rel ys in series pply. The settings nd resulting The ex mple shows th t unless rel ys 2 nd 3 re m de direction l, they will m l oper te for f ult t F3. Also shown is how to c lcul te ppropri te rel y sett ings for ll six rel ys to ensure s tisf ctory protection for f ults t loc tion s F1 F4. Figure 9.31( ) shows the imped nce di gr m, to 100MVA, 110kV se. The f ult currents for f ults with v rious system configur tions re shown in T le 9.6. F ult System Position Config. F1 F1/F2 F2 F3 F4 2 fdrs 1 fdr 2 fdrs 2 fdrs 1 fdr Currents (A) Ic 0 0 0 1944 0 F ult 3888 2019 3888 3888 26243 I 1944 2019 1944 1944 0 I 1944 0 1944 1944 0 Id 972 1009 972 972 26243 Ie 972 0 972 972 0 If 1944 1009 1944 1944 26243 T le 9.6: F ult currents for p r llel feeder ex mple Network Protection & Autom tion Guide • 149 • Overcurrent Protection for Ph se nd E rth F ults • 50MVA Z=12.5% ( ) Rel y settings non direction l rel ys 2 9•
oper tion times re given in Figure 9.33( ) nd(c) respectively. T1 4 I> IF4 Bus P 220kV I> I e 11kV f ult level =200MVA 11kV Id If 6 I> 50MVA Z=12,5% I 2 Ic F3 Bus Q 110kV 3 I> IF2 F2 Ch r cteristic SI SI SI SI SI SI CB7 1 I>
5MVA Z=7.15% 1000/1 5MVA Z=7.15% 1000/1 A 3.3kV I> R1 CB2 Source 10000MVA
IF3 F4 T2 220/110kV 50MVA Z I di gr m F1 F1 R8 I> CB8 1000/1 C1 =1km 1000/1 C4 =1.5km CB1 5 I> Rel y 1 2 3 4 5 6 100.00 Overcurrent Protection for Ph se nd E rth F ults CT Prim ry 300 300 300 300 300 300
Current setting TMS 1 0.2 0.42 0.1 0.42 0.1 0.6 0.275 0.275 0.6 0.7 0.475 ( ) Re l y settings 1000/1 1000/1 D CB6 3.3kV R7 I> 1000/1 C3 =2km 1000/1 CB4 I> R2 1000/1 C2 =1.3km 1000/1 CB5 CB3 B Time (sec) 10.00 1.00 0.10 100 1000 10000 Rel y 1 Rel ys 2/3 Rel ys 4/5 Rel y 6 (i) F ult current 3888A f ults F1, F2 1 feeder (iii) F ult current 26243A f ult F4 1 feeder R6 R5
I> I> I> I> R3 R4 3.3kV (ii) (i) (iii) Current (A) referred to 110kV (c) Rel y ch r cteristics Figure 9.33: Rel y gr ding for p r llel feeder ex mple – direction l rel ys C 3.3kV All c les re 3 x 1c x 1200mm2, AI conductor, Z = 0.09 Ω/km VT s omitte for clar ity Figure 9.34: Ring main gra ing example – circuit iagram In practice, a complete protection stu ywoul inclu e instantaneous elements on the primary si e of the transformers an analysis of the situation with only on e transformer in service. These have been omitte from this example, as the purp ose is to illustrate the principles of parallel fee er protection in a simple fa shion. 9.20.4 Gra ing of a Ring Main Figure 9.34 shows a simple ring main, with a single infee at Bus A an three loa busbars. Settings for the irectional re lays R2-R7 an non- irectional relays R1/R8 are require . Maximum loa current i n the ring is 785A (maximum continuous current with one transformer out of servi ce), so 1000/1A CT’s are chosen. The relay consi ere is a MiCOM P140 series. The first step is to establish the maximum fault current at each relay location. Ass uming a fault at Bus B (the actual location conf is not important), two possible igurations of the ring have to be consi ere , firstly a close ring an secon ly an open ring. For convenience, the ring will be consi ere to be open at CB1 (C B8 is the other possibility to be consi ere , but the conclusion will be the sam e). Figure 9.35 shows the impe ance iagram for these two cases. V ZS+ZT 6.08% ZAB 6.2% B ZBC 8.26% V V ZS+ZT 6.08% A ZAD 4.13% D ZCD 5.37% ZAB 6.2% B ZBC 8.26% • 9• A ZAD 4.13% D ZCD 5.37% I 1= I1 C I1 ZS
1+
ZBC+ZCD+ZAD ZAB +ZBC+ZCD+ZAD
C V I 1= +ZS+ZBC+ZCD+ZAD (b) Ring open at CB1 (a) Ring close Figure 9.35: Impe ance iagrams with ring open Three-phase fault currents I1 an I’1 can be calculate as 2.13kA an 3.67kA respe ctively, so that the worst case is with the ring open (this can also be seen fro m consi eration of the impe ance relationships, without the necessity of perform ing the calculation). Network Protection & Automation Gui e • 150 •
Clockwise Open Point CB8 Fault Bus Current kA D 7.124 C 4.259 B 3.376 Bus B C D Anticlockwise Open Point CB1 Fault Current kA 3.665 5.615 8.568 Table 9.8 summarises the relay settings, while Figure 9.36 illustrates the relay gra ing curves. 100.00 Table 9.7: Fault current tabulation with ring open
Table 9.7 shows the fault currents at each bus for open points atCB1 an CB8. F or gra ing of the relays, consi er relays looking in a clockwise irection roun the ring, i.e. relays R1/R3/R5/R7. 9.20.4.1 Relay R7 Loa current cannot flow f rom Bus D to Bus A since Bus A is the onlysource. Hence low relay current an T MS settings can be chosen to ensure a rapi fault clearance time. These can be c hosen arbitrarily, so long as they are above the cable charging current an with in the relay setting characteristics. Select a relay current setting of 0.8 (i.e . 800A CT primary current) an TMS of 0.05. This ensures that the other relays w ill not pick up un er con itions of normal loa current. At a fault current of 3 376A, relay operating time on the SI characteristic is 0.14 s 0.05 × 0.02 ( 4.22 −1 = 0.24s 9.20.4.2 Rel y R5 This rel y must gr de with rel y R7 t 3376A nd h v e minimum oper tion time of 0.54s. Rel y R5 current setting must e t le st 1 10% of rel y R7 to prevent unw nted pickup, so select rel y R5 current setting o f 0.88 (i.e. 880A CT prim ry current). Rel y R5 oper ting time t TMS = 1.0 0.14 s = 5.14s = 0.02 ( 3.84 ) −1 0.54 = 5.14s 5.14 Use ne rest sett le v lue of TM f 0.125. Hence, rel y R5 TMS = Bus Rel y Rel y Ch r cteristic SI SI SI SI SI SI SI SI CT R tio M x M x Lo d F u lt Current Current (A) (A) (3.3kV se) 874 3376 874 4259 874 7124 874 14387 874 14387 874 8568 874 5615 874 3665 Current Setting p.u. 0.8 0.88 0.97 1.07 1.07 0 .97 0.88 0.8 TMS 10.00 Time (sec) 1.00 Rel y R5 y 0.10 Rel y R7 y 10000 100,000 Current (A) ( ) Clockwise gr ding of rel ys (ring open t CB8) 1000 100.00 10.00 Time (sec) 1.00 0.10 Rel y R2 y 1000
100,000 10000 Current (A) ( ) Anticlockwise gr ding of rel ys (ring open t CB1) Figure 9.36: Ring m in ex mple – rel y gr ding curves 9.21 REFERENCES 0.05 0.125 0.2 0.275 0.3 0.2 0.125 0.05 D C B A A D C B
R7 R5 R3 R1 R8 R6 R4 R2 1000/1 1000/1 1000/1 1000/1 1000/1 1000/1 1000/1 1000/1 9.1. Direction l Element Connections for Ph se Rel ys. W.K Sonnem nn, Tr ns ctio ns A.I.E.E. 1950. T le 9.8: Ring m in ex mple rel y settings Network Protection & Autom tion Guide • 151 • Overcurrent Protection for Ph se nd E rth F ults • 9•
• 10 • Unit Protection of Feeders Introduction Convention of direction Conditions for direction comp rison Circul ting current system B l nced volt ge system Summ tion rr ngements Ex mples of e lectromech nic l nd st tic unit protection systems Digit l/Numeric l current di fferenti l protection systems C rrier unit protection schemes Current differenti l scheme – n logue techniques Ph se comp rison protection scheme consider tions Ex mples References 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10. 12 10.13
• 10 • Unit P rotection of Feeders 10 . 1 I N T R O D U C T I O N The gr ded overcurrent systems descri ed in Ch pt er 9, though ttr ctively simple in principle, do not meet ll the protection re quirements of power system. Applic tion difficulties re encountered for two r e sons: firstly, s tisf ctory gr ding c nnot lw ys e rr nged for complex ne twork, nd secondly, the settings m y le d to m ximum tripping times t points i n thesystem th t re too long to prevent excessive distur nces occurring. Thes e pro lems led to the concept of 'Unit Protection', where y sections of the powe r system re protected individu lly s complete unit without reference to othe r sections. One form of ‘Unit Protection’ is also known as ‘Diff r ntial Prot ction’, as th principl is to s ns th diff r nc in curr nts b tw n th incoming and o utgoing t rminals of th unit b ing prot ct d. Oth r forms can b bas d on dir c tional comparison, or distanc t l prot ction sch m s, which ar cov r d in Chap t r 12, or phas comparison prot ction, which is discuss d lat r in this chapt r . Th configuration of th pow r syst m may l nd its lf to unit prot ction; for instanc , a simpl arth fault r lay appli d at th sourc nd of a transform rf d r can b r gard d as unit prot ction provid d that th transform r winding associat d with th f d r is not arth d. In this cas th prot ction cov rag is r strict d to th f d r and transform r winding b caus th transform r cann ot transmit z ro s qu nc curr nt to an out-of-zon fault. In most cas s, how v r, a unit prot ction syst m involv s th m asur m nt of fault curr nts (and poss ibly voltag s) at ach nd of th zon , and th transmission of information b tw n th quipm nt at zon boundari s. It should b not d that a stand-alon dist anc r lay, although nominally r sponding only to faults within th ir s tting zo n , do s not satisfy th conditions for a unit syst m b caus th zon is not cl arly d fin d; it is d fin d only within th accuracy limits of th m asur m nt. Also, to cat r for som conditions, th s tting of a stand-alon distanc r lay may also xt nd outsid of th prot ct d zon to cat r for som conditions. M r z and Pric [10.1] first stablish d th principl of curr nt diff r ntial unit syst ms; th ir fundam ntal diff r ntial syst ms hav form d th basis of many N twork Prot ction & Automation Guid • 153 •
highly d v lop d prot ction arrang m nts for f d rs and num rous oth r it ms of plant. In on arrang m nt, an auxiliary ‘pilot’ circuit int rconn cts similar curr nt transform rs at ach nd of th prot ct d zon , as shown in Figur 10.1. Curr nt transmitt d through th zon caus s s condary curr nt to circulat round th pilot circuit without producing any curr nt in th r lay. For a fault within th prot ct d zon th CT s condary curr nts will not balanc , compar d with th t hroughfault condition, and th diff r nc b tw n th curr nts will flow in th r lay. An alt rnativ arrang m nt is shown in Figur 10.2, in which th CT s con dary windings ar oppos d for through-fault conditions so that no curr nt flows in th s ri s conn ct d r lays. Th form r syst m is known as a ‘Circulating Curr nt’ syst m, whil th latt r is known as a ‘Balanc d Voltag ’ syst m. End G End H 10 . 2 C O N V E N T I O N O F D I R E C T I O N It is us ful to stablish a con v ntion of dir ction of curr nt flow; for this purpos , th dir ction m asur d f rom a busbar outwards along a f d r is tak n as positiv . H nc th notation of curr nt flow shown in Figur 10.3; th s ction GH carri s a through curr nt whi ch is count d positiv at G but n gativ at H, whil th inf ds to th fault d s ction HJ ar both positiv . Sourc + _ + + Sourc G H Fault J Figur 10.3: Conv ntion of curr nt dir ction Id> R lay U n i t P ro t c t i o n F d r s Figur 10.1: Circulating curr nt syst m N gl ct of this rul has oft n l d to anomalous arrang m nts of quipm nt or dif ficulty in d scribing th action of a compl x syst m. Wh n appli d, th rul wil l normally l ad to th us of id ntical quipm nts at th zon boundari s, and i s qually suitabl for xt nsion to multi- nd d syst ms. It also conforms to th standard m thods of n twork analysis. End H End G 10 . 3 C O N D I T I O N S F O R D I R E C T I O N C O M PA R I S O N Th circul ating curr nt and balanc d voltag syst ms of Figur s 10.1 and 10.2 p rform full v ctorial comparison of th zon boundary curr nts. Such syst ms can b tr at d as analogu s of th prot ct d zon of th pow r syst m, in which CT s condary q uantiti s r pr s nt primary curr nts and th r lay op rating curr nt corr sponds to an in-zon fault curr nt. Th s syst ms ar simpl in conc pt; th y ar n v rth l ss applicabl to zon s having any numb r of boundary conn ctions and for a ny patt rn of t rminal curr nts. To d fin a curr nt r quir s that both magnitud and phas b stat d. Comparison in t rms of both of th s quantiti s is p rfor
m d in th M rz-Pric syst ms, but it is not always asy to transmit all this in formation ov r som pilot chann ls. Chapt r 8 provid s a d tail d d scription of mod rn m thods that may b us d. 10 . 4 C I R C U L AT I N G C U R R E N T S Y S T E M Th principl of this syst m is shown in outlin in Figur 10.1. If th curr nt transform rs ar id al, th functioning of th syst m is straightforward . Th Id> R lay G Id> R lay H Figur 10.2: Balanc d voltag syst m • 10 • Most syst ms of unit prot ction function through th d t rmination of th r lati v dir ction of th fault curr nt. This dir ction can only b xpr ss d on a com parativ basis, and such a comparativ m asur m nt is th common factor of many syst ms, including dir ctional comparison prot ction and distanc t l prot ction sch m s with dir ctional imp danc m asur m nt. A major factor in consid ration of unit prot ction is th m thod of communication b tw n th r lays. This is c ov r d in d tail in Chapt r 8 in r sp ct of th lat st fibr -optic bas d digital t chniqu s. For old r ‘pilot wir ’ syst ms, only bri f m ntion is mad . For mor d tail d d scriptions of ‘pilot wir ’ t chniqu s, s r f r nc [10.2] in S ction 10.1 3. • 154 • N twork Prot ction & Automation Guid
transform rs will, how v r, hav rrors arising from both Wattm tric and magn ti sing curr nt loss s that caus d viation from th id al, and th int rconn ction s b tw n th m may hav un qual imp danc s. This can giv ris to a ‘spill’ curr nt through th r lay v n without a fault b ing pr s nt, thus limiting th s nsitiv ity that can b obtain d. Figur 10.4 illustrat s th quival nt circuit of th circulating curr nt sch m . If a high imp danc r lay is us d, th n unl ss th r lay is locat d at point J in th circuit, a curr nt will flow through th r lay v n with curr nts IPg and IPh b ing id ntical. If a low imp danc r lay is us d, voltag FF ’ will b v ry small, but th CT xciting curr nts will b un qual d u to th un qual burd ns and r lay curr nt IR will still b non-z ro. End G End H unacc ptabl . On solution is to includ a stabilising r sistanc in s ri s with th r lay. D tails of how to calculat th valu of th stabilising r sistor ar usually includ d in th instruction manuals of all r lays that r quir on . Wh n a stabilising r sistor is us d, th r lay curr nt s tting can b r duc d to a ny practical valu , th r lay now b ing a voltag -m asuring d vic . Th r is obv iously a low r limit, b low which th r lay l m nt do s not hav th s nsitivit y to pick up. R lay calibration can in fact b in t rms of voltag . For mor d t ails, s r f r nc [10.2]. 10.4.2 Bias Th spill curr nt in th r lay arising from th s various sourc s of rror is d p nd nt on th magnitud of th throug h curr nt, b ing n gligibl at low valu s of through-fault curr nt but som tim s r aching a disproportionat ly larg valu for mor s v r faults. S tting th o p rating thr shold of th prot ction abov th maximum l v l of spill curr nt pr oduc s poor s nsitivity. By making th diff r ntial s tting approximat ly propor tional to th fault curr nt, th lowl v l fault s nsitivity is gr atly improv d. Figur 10.5 illustrat s a typical bias charact ristic for a mod rn r lay that o v rcom s th probl m. At low curr nts, th bias is small, thus nabling th r la y to b mad s nsitiv . At high r curr nts, such as would b obtain d from inrus h or through fault conditions, th bias us d is high r, and thus th spill curr nt r quir d to caus op ration is high r. Th r lay is th r for mor tol rant o f spill curr nt at high r fault curr nts and th r for l ss lik ly to malop rat , whil still b ing s nsitiv at low r curr nt l v ls. IPg IPh RSh i g Z g iSg RLg Id R RLh Sh RSh i h Z h R lay (a) G G
G F J F
Subscripts: scripts S - CT S condary L G h - nd H H H GG GG H H El ctro-motiv forc s with low imp danc r lay (b) Figur 10.4: Equival nt circuit of circulating curr nt sch m I1 I3
I2 • 10.4.1 Transi nt Instability It is shown in S ction 6.4.10 that an asymm trical curr nt appli d to a curr nt transform r will induc a flux that is gr at r than th p ak flux corr sponding to th st ady stat alt rnating compon nt of th cu rr nt. It may tak th CT into saturation, with th r sult that th dynamic xci ting imp danc is r duc d and th xciting curr nt gr atly incr as d. Wh n th b alancing curr nt transform rs of a unit prot ction syst m diff r in xcitation c haract ristics, or hav un qual burd ns, th transi nt flux build-ups will diff r and an incr as d spill curr nt will r sult. Th r is a cons qu nt risk of r lay op ration on a h althy circuit und r transi nt conditions, which is cl arly • 155 • Idiff = I1+I2+I3 Op rat P rc ntag bias k2 P rc ntag bias k1 Is1 R strain Is2 Ibias= I1 + I2 + I3 2 Figur 10.5: Typical bias charact ristic of r lay N twork Prot ction & Automation Guid U n i t P ro t c t i o n F d r s 10 • RR
10 . 5 B A L A N C E D V O LTA G E S Y S T E M This s ction is includ d for hist orical r asons, mainly b caus of th numb r of such sch m s still to b found i n s rvic – for n w installations it has b n almost compl t ly sup rs d d by circ ulating curr nt sch m s. It is th dual of th circulating curr nt prot ction, a nd is summaris d in Figur 10.2 as us d in th ‘Translay H04’ sch m . With primary t hrough curr nt, th s condary .m.f.’s of th curr nt transform rs ar oppos d, an d provid no curr nt in th int rconn cting pilot l ads or th s ri s conn ct d r lays. An in-zon fault l ads to a circulating curr nt condition in th CT s co ndari s and h nc to r lay op ration. An imm diat cons qu nc of th arrang m n t is that th curr nt transform rs ar in ff ct op n-circuit d, as no s condary curr nt flows for any primary throughcurr nt conditions. To avoid xc ssiv sat uration of th cor and s condary wav form distortion, th cor is provid d with non-magn tic gaps suffici nt to absorb th whol primary m.m.f. at th maximum curr nt l v l, th flux d nsity r maining within th lin ar rang . Th s condary winding th r for d v lops an .m.f. and can b r gard d as a voltag sourc . T h shunt r actanc of th transform r is r lativ ly low, so th d vic acts as a transform r load d with a r activ shunt; h nc th Am rican nam of transactor . Th quival nt circuit of th syst m is as shown in Figur 10.6. Th s ri s co nn ct d r lays ar of r lativ ly high imp danc ; b caus of this th CT s condar y winding r sistanc s ar not of gr at significanc and th pilot r sistanc can b mod rat ly larg without significantly aff cting th op ration of th syst m . This is why th sch m was d v lop d for f d r prot ction. End G End H
xciting curr nt, b caus th whol of th primary curr nt is xp nd d as xciti ng curr nt. In cons qu nc , th s condary .m.f. is an accurat m asur of th p rimary curr nt within th lin ar rang of th transform r. Provid d th transfor m rs ar d sign d to b lin ar up to th maximum valu of fault curr nt, balanc is limit d only by th inh r nt limit of accuracy of th transform rs, and as a r sult of capacitanc b tw n th pilot cor s. A brok n lin in th quival nt circuit shown in Figur 10.6 indicat s such capacitanc . Und r through-fault con ditions th pilots ar n rgis d to a proportionat voltag , th charging curr n t flowing through th r lays. Th stability ratio that can b achi v d with this syst m is only mod rat and a bias t chniqu is us d to ov rcom th probl m. 1 0 . 6 S U M M AT I O N A R R A N G E M E N T S Sch m s hav so far b n discuss d as though th y w r appli d to singl -phas syst ms. A polyphas syst m could b provid d with ind p nd nt prot ction for ach phas . Mod rn digital or num ri cal r lays communicating via fibr -optic links op rat on this basis, sinc th amount of data to b communicat d is not a major constraint. For old r r lays, u s of this t chniqu ov r pilot wir s may b possibl for r lativ ly short dista nc s, such as would b found with industrial and urban pow r distribution syst m s. Cl arly, ach phas would r quir a s parat s t of pilot wir s if th prot c tion was appli d on a p r phas basis. Th cost of providing s parat pilot-pair s and also s parat r lay l m nts p r phas is g n rally prohibitiv . Summation t chniqu s can b us d to combin th s parat phas curr nts into a singl r l aying quantity for comparison ov r a singl pair of pilot wir s. For d tails of such t chniqu s, s r f r nc [10.2]. 10 . 7 E X A M P L E S O F E L E C T R O M E C H A N I C A L A N D S TAT I C U N I T P R OT E C T I O N S Y S T E M S As m ntion d abov , th basic balanc d voltag principl of prot ction volv d to b ias d prot ction syst ms. S v ral of th s hav b n d sign d, som of which app ar to b quit diff r nt from oth rs. Th s dissimilariti s ar , how v r, sup r ficial. A numb r of th s syst ms that ar still in common us ar d scrib d b l ow. 10.7.1 ‘Translay’ Balanc d Voltag El ctrom chanical Syst m A typical bias d, l ctrom chanical balanc d voltag syst m, trad nam ‘Translay’, still giving us ful s rvic on distribution syst ms is shown in Figur 10.7. • U n i t P ro t c t i o n F d r s 10 •
RSg RLg RLh RSh Z g Pilot Param t rs Z h Id> R lay G Id> R lay H Figur 10.6: Equival nt circuit for balanc d voltag syst m 10.5.1 Stability Limit of th Voltag Balanc Syst m Unlik normal curr nt trans form rs, transactors ar not subj ct to rrors caus d by th progr ssiv build-u p of • 156 • N twork Prot ction & Automation Guid
A B C End G End H Bias is produc d by a copp r shading loop fitt d to th pol of th upp r magn t , th r by stablishing a F rraris motor action that giv s a r v rs or r straini ng torqu proportional to th squar of th upp r magn t flux valu . Typical s t tings achi vabl with such a r lay ar : L ast s nsitiv arth fault - 40% of rat ing L ast s nsitiv phas -phas fault - 90% of rating Thr -phas fault - 52% of rating 10.7.2 Static Circulating Curr nt Unit Prot ction Syst m – ‘Translay ‘S’ ’ A typic al static modular pilot wir unit prot ction syst m op rating on th circulating curr nt principl is shown in Figur 10.8. This us s summation transform rs wit h a n utral s ction that is tapp d, to provid alt rnativ arth fault s nsitivi ti s. Phas comparators tun d to th pow r fr qu ncy ar us d for m asur m nt an d a r straint circuit giv s a high l v l of stability for through faults and tra nsi nt charging curr nts. High-sp d op ration is obtain d with mod rat ly siz d curr nt transform rs and wh r spac for curr nt transform rs is limit d and wh r th low st possibl op rating tim is not ss ntial, small r curr nt transfo rm rs may b us d. This is mad possibl by a sp cial adjustm nt (Kt) by which t h op rating tim of th diff r ntial prot ction can b s l ctiv ly incr as d if n c ssary, th r by nabling th us of curr nt transform rs having a corr spond ingly d cr as d kn -point voltag , whilst nsuring that through-fault stability is maintain d to gr at r than 50 tim s th rat d curr nt. Int rnal faults giv simultan ous tripping of r lays at both nds of th lin , providing rapid fault cl aranc irr sp ctiv of wh th r th fault curr nt is f d from both lin nds o r from only on lin nd. Summation winding S condary ry winding Bias loop Pilot Figur 10.7: Typical bias d l ctrom chanical diff r ntial prot ction syst m. Th l ctrom chanical d sign d riv s its balancing voltag s from th transactor incorporat d in th m asuring r lay at ach lin nd. Th latt r ar bas d on th induction-typ m t r l ctromagn t as shown in Figur 10.7. Th upp r magn t c arri s a summation winding to r c iv th output of th curr nt transform rs, an d a s condary winding which d liv rs th r f r nc .m.f. Th s condary windings of th conjugat r lays ar int rconn ct d as a balanc d voltag syst m ov r th pilot chann l, th low r l ctromagn ts of both r lays b ing includ d in this circuit. Through curr nt in th pow r circuit produc s a stat of balanc in th pilot circuit and z ro curr nt in th low r l ctromagn t coils. In this condit ion, no op rating torqu is produc d. An in-zon fault causing an inflow of curr nt from ach nd of th lin produc s circulating curr nt in th pilot circuit and th n rgisation of th low r l ctromagn ts. Th s co-op rat with th flux of th upp r l ctromagn ts to produc an op rating torqu in th discs of both r lays. An inf d from on nd only will r sult in r lay op ration at th f di ng nd, but no op ration at th oth r, b caus of th abs nc of upp r magn t fl ux. • A B C T1 - Summation transform r T2 - Auxiliary transform r RVO - Non lin ar r s istor Trip Rs T1 c O T2 Tr Pr
Pr T2 Tr Trip T1 c To - Op rating winding Rs Tr - R straining winding Ro - Lin ar r sistor Pr - Pil ots padding r sistor Pilot wir s Ro Ro TO RVO V RVO V c - Phas comparator Figur 10.8: Typical static circulating curr nt f d r unit prot ction circuit d iagram N twork Prot ction & Automation Guid • 157 • U n i t P ro t c t i o n F d r s 10 •
10 . 8 D I G I TA L / N U M E R I C A L C U R R E N T D I F F E R E N T I A L P R OT E C T I O N S Y S T E M S A digital or num rical unit prot ction r lay may typically provid phas -s gr gat d curr nt diff r ntial prot ction. This m ans t hat th comparison of th curr nts at ach r lay is don on a p r phas basis. F or digital data communication b tw n r lays, it is usual that a dir ct optical conn ction is us d (for short distanc s) or a multipl x d link. Link sp ds of u p to 64kbit/s (56kbit/s in N. Am rica) ar normal. Through curr nt bias is typic ally appli d to provid through fault stability in th v nt of CT saturation. A dual slop bias t chniqu (Figur 10.5) is us d to nhanc stability for throug h faults. A typical trip crit rion is as follows: For |Ibias| < Is2 |Idiff | < k 1 |Ibias| + Is1 For |Ibias| < Is2 |Idiff | < k2 |Ibias| - (k2 - k1) Is2 + Is1 On ce the relay at one end of the protected section has determined that a trip cond ition exists, an intertrip signal is transmitted to the relay at the other end. Relays that are supplied with information on line currents at all ends of the li ne may not need to implement intertripping facilities. However, it is usual to p rovide intertripping in any case to ensure the protection operates in the event of any of the relays detecting a fault. A facility for vector/ratio compensation of the measured currents, so that transformer feeders can be included in the un it protection scheme without the use of interposing CT’s or defining the transform er as a separate zone increases versatility. Any interposing CT’s required are imp lemented in software. Maloperation on transformer inrush is prevented by second harmonic detection. Care must be taken if the transformer has a wide-ratio on-lo ad tap changer, as this results in the current ratio departing from nominal and may cause maloperation, depending on the sensitivity of the relays. The initial bias slope should be set taking this into consideration. Tuned measurement of po wer frequency currents provides a high level of stability with capacitance inrus h currents during line energisation. The normal steadystate capacitive charging current can be allowed for if a voltage signal can be made available and the sus ceptance of the protected zone is known. Where an earthed transformer winding or earthing transformer is included within the zone of protection, some form of ze ro sequence current filtering is required. This is because there will be an in-z one source of zero sequence current for an external earth fault. The differentia l protection will see zero sequence differential current for an external fault a nd it could incorrectly operate as a result. In older protection schemes, the problem was eliminated by delta connection of the CT secondary windings. For a digital or numerical relay, a selectable software zero sequence filter is typically employed. The problem r emains of compensating for the time difference between the current measurements made at the ends of the feeder, since small differences can upset the stability of the scheme, even when using fast direct fibre-optic links. The problem is ove rcome by either time synchronisation of the measurements taken by the relays, or calculation of the propagation delay of the link continuously. 10.8.1 Time Sync hronisation of Relays Fibre-optic media allow direct transmission of the signals between relays for distances of up to several km without the need for repeaters . For longer distances repeaters will be required. Where a dedicated fibre pair is not available, multiplexing techniques can be used. As phase comparison techn iques are used on a per phase basis, time synchronisation of the measurements is vitally important. This requires knowledge of the transmission delay between th e relays. Four techniques are possible for this: a. assume a value b. measuremen t during commissioning only c. continuous online measurement d. GPS time signal Method (a) is not used, as the error between the assumed and actual value will b e too great. Method (b) provides reliable data if direct communication between r elays is used. As signal propagation delays may change over a period of years, r epeat measurements may be required at intervals and relays re-programmed accordi ngly. There is some risk of maloperation due to changes in signal propagation ti me causing incorrect time synchronisation between measurement intervals. The tec hnique is less suitable if rented fibre-optic pilots are used, since the owner m ay perform circuit re-routing for operational reasons without warning, resulting in the propagation delay being outside of limits and leading to scheme malopera
tion. Where re-routing is limited to a few routes, it may be possible to measure the delay on all routes and pre-program the relays accordingly, with the relay digital inputs and ladder logic being used to detect changes in route and select the appropriate delay accordingly. Method (c), continuous sensing of the signal propagation delay, is a robust technique. One method of achieving this is shown in Figure 10.9. • U n i t P ro te c t i o n Fe e d e r s 10 • • 158 • Network Protection & Automation Guide
A End A Measured sampling time TB3*=(TA*-Tp2) TA1 TA2 TA3 TB3* TA4 TA* TA5 Tp2 T p1 Digital communications link B End B Propagation delay time Tp1=Tp2=1/2(TA*-TA1-T d) TA1 TA2 - sampling instants of relay A TB1 TB2 - sampling instants of relay B Tp1 - propagation delay time from relay A to B Tp2 - propagation delay time f rom relay B to A Td - time between the arrival of message TA1 at relay B and des patch of message TB3 TA1* - arrival time of message TB3 and relay A TB* - arriva l time of message TA1 and relay B TB3* - the measured sampling time of TB3 by re lay A Current v ectors TA1 TB1 TB2 Td TB3 TB4 TB5 TB* TB3 TA1 Td vectors Current Figure 10.9: Signal propagation delay measurement Relays A and B sample signals at time TA1,TA2 …and TB1,TB2 …respectively. The times will not be coincident, even if they start coincidentally, due to slight differe nces in sampling frequencies. At time TA1 relay A transmits its data to relay B, containing a time tag and other data. Relay B receives it at time TA1 +Tp1 wher e Tp1 is the propagation time from relay A to relay B. Relay B records this time as time TB*. Relay B also sends messages of identical format to relay A. It tra nsmits such a message at time TB3, received by relay A at time TB3 +Tp2 (say tim e TA*), where Tp2 is the propagation time from relay B to relay A. The message f rom relay B to relay A includes the time TB3, the last received time tag from re lay A (TA1) and the delay time between the arrival time of the message from A (T B*) and TB3 – call this the delay time Td. The total elapsed time is therefore: (T A* - TA1) = (Td + Tp1 + Tp2) If it is assumed that Tp1 = Tp2, then the value of Tp1 and Tp2 can be calculated, and hence also TB3. The relay B measured data as received at relay A can then be adjusted to enable data comparison to be perform ed. Relay B performs similar computations in respect of the data received from r elay A (which also contains similar time information). Therefore, continuous mea surement of the propagation delay is made, thus reducing the possibility of malo peration due to this cause to a minimum. Comparison is carried out on a per-phas e basis, so signal transmission and the calculations are required for each phase . A variation of this technique is available that can cope with unequal propagat ion delays in the two communication channels under well-defined conditions. The technique can also be used with all types of pilots, subject to provision of appropriate interfacing d evices. Method (d) is also a robust technique. It involves both relays being cap able of receiving a time signal from a GPS satellite. The propagation delay on e ach communication channel is no longer required to be known or calculated as bot h relays are synchronised to a common time signal. For the protection scheme to meet the required performance in respect of availability and maloperation, the G PS signal must be capable of reliable receipt under all atmospheric conditions. There is extra satellite signal receiving equipment required at both ends of the line, which implies extra cost. The minimum setting that can be achieved with s
uch techniques while ensuring good stability is 20% of CT primary current. 10.8. 2 Application to Mesh Corner and 1 1/2 Breaker Switched Substations These substa tion arrangements are quite common, and the arrangement for the latter is shown in Figure 10.10. Problems exist in protecting the feeders due to the location of the line CT’s, as either Bus 1 or Bus 2 or both can supply the feeder. Two altern atives are used to overcome the problem, and they are illustrated in the Figure. The first is to common the line CT inputs (as shown for Feeder A) and the alter native is to use a second set of CT inputs to the relay (as shown for Feeder B). • Network Protection & Automation Guide • 159 • U n i t P ro te c t i o n Fe e d e r s 10 •
Bus 1 B2 IF B1 Bus 2 F Id > Stub bus inputs A B Figure 10.10: Breaker and a half switched substation I d> power system currents since the systems are designed to operate at much higher f requencies, but each medium may be subjected to noise at the carrier frequencies that may interfere with its correct operation. Variations of signal level, rest rictions of the bandwidth available for relaying and other characteristics uniqu e to each medium influence the choice of the most appropriate type of scheme. Me thods and media for communication are discussed in Chapter 8. 10 . 10 C U R R E N T D I F F E R E N T I A L S C H E M E – ANALOGUE TECHNIQUES The carrier channel is used in this type of scheme to convey both the phase and magnitude of the cur rent at one relaying point to another for comparison with the phase and magnitud e of the current at that point. Transmission techniques may use either voice fre quency channels using FM modulation or A/D converters and digital transmission. Signal propagation delays still need to be taken into consideration by introduci ng a deliberate delay in the locally derived signal before a comparison with the remote signal is made. A further problem that may occur concerns the dynamic ra nge of the scheme. As the fault current may be up to 30 times the rated current, a scheme with linear characteristics requires a wide dynamic range, which impli es a wide signal transmission bandwidth. In practice, bandwidth is limited, so e ither a non-linear modulation characteristic must be used or detection of fault currents close to the setpoint will be difficult. 10.10.1 Phase Comparison Schem e The carrier channel is used to convey the phase angle of the current at one re laying point to another for comparison with the phase angle of the current at th at point. The principles of phase comparison are illustrated in Figure 10.11. Th e carrier channel transfers a logic or on/off signal that switches at the zero crossing points of the power frequency waveform. Comparison of a local logic si gnal with the corresponding signal from the remote end provides the basis for th e measurement of phase shift between power system currents at the two ends and h ence discrimination between internal and through faults. Current flowing above t he set threshold results in turnoff of the carrier signal. The protection operat es if gaps in the carrier signal are greater than a set duration – the phase angle setting of the protection. Load or through fault currents at the two ends of a protected feeder are in antiphase (using the normal relay convention for directi on), whilst during an internal fault the (conventional) currents tend towards th e in-phase Network Protection & Automation Guide In the case of a through fault as shown, the relay connected to Feeder A theoret ically sees no unbalance current, and hence will be stable. However, with the li ne disconnect switch open, no bias is produced in the relay, so CT’s need to be we ll matched and equally loaded if maloperation is to be avoided. For Feeder B, th e relay also theoretically sees no differential current, but it will see a large bias current even with the line disconnect switch open. This provides a high de gree of stability, in the event of transient asymmetric CT saturation. Therefore , this technique is preferred. Sensing of the state of the line isolator through auxiliary contacts enables the current values transmitted to and received from remote relays to be set to zero when the isolator is open. Hence, stub-bus prote ction for the energised part of the bus is then possible, with any fault resulti ng in tripping of the relevant CB. 10 . 9 C A R R I E R U N I T P R OT E C T I O
N S C H E M E S In earlier sections, the pilot links between relays have been t reated as an auxiliary wire circuit that interconnects relays at the boundaries of the protected zone. In many circumstances, such as the protection of longer l ine sections or where the route involves installation difficulties, it is too ex pensive to provide an auxiliary cable circuit for this purpose, and other means are sought. In all cases (apart from private pilots and some short rented pilots ) power system frequencies cannot be transmitted directly on the communication m edium. Instead a relaying quantity may be used to vary the higher frequency asso ciated with each medium (or the light intensity for fibre-optic systems), and th is process is normally referred to as modulation of a carrier wave. Demodulation or detection of the variation at a remote receiver permits the relaying quantit y to be reconstituted for use in conjunction with the relaying quantities derive d locally, and forms the basis for all carrier systems of unit protection. Carri er systems are generally insensitive to induced • U n i t P ro te c t i o n Fe e d e r s 10 • • 160 •
condition. Hence, if the phase relationship of through fault currents is taken a s a reference condition, internal faults cause a phase shift of approximately 18 0° with respect to the reference condition. Phase comparison schemes respond to an y phase shift from the reference conditions, but tripping is usually permitted o nly when the phase shift exceeds an angle of typically 30 to 90 degrees, determi ned by the time delay setting of the measurement circuit, and this angle is usua lly referred to as the Stability Angle. Figure 10.12 is a polar diagram that ill ustrates the discrimination characteristics that result from the measurement tec hniques used in phase comparison schemes. Since the carrier channel is required to transfer only End G binary information, the techniques associated with sending teleprotection comman ds. Blocking or permissive trip modes of operation are possible, however Figure 10.11 illustrates the more usual blocking mode, since the comparator provides an output when neither squarer is at logic 1 . A permissive trip scheme can be re alised if the comparator is arranged to give an output when both squarers are at logic 1 . Performance of the scheme during failure or disturbance of the carri er channel and its ability to clear single-end-fed faults depends on the mode of operation, the type and function of fault detectors or starting units, and the use of any additional signals or codes for channel monitoring and transfer tripp ing. End H Summation network A Squarer B D Signalling equipment and communication channel Transmitter Receiver D C E Phase comparator Pulse length discrimination F Load or through fault G IG IH H G IG Internal fault IH H A. Summation voltage at end G B. Squarer output at end G 1 0 1 0 • C. Summation voltage at end H D. Squarer output at end H (Received at end G via ideal carrier system as D 1 0 1 0 E. Comparator output at end G E=B+D
F. Discriminator output at end G
1 0 1 0 1 0 Stability setting 1 0 Figure 10.11: Principles of phase comparison protection. Network Protection & Automation Guide • 161 • U n i t P ro te c t i o n Fe e d e r s 10 •
θ=90° θ=180° Tripping O Stability θ=0° R θ=270° θ System differential phase shift referred to through fault reference condition IG IH OR Through fault IG= IH reference condition H G (IG' IH conventional rela y currents at ends of protected feeder) Discriminator stability angle setting. Figure 10.12: Polar diagram for phase comparison scheme both ends are nominally eual, so the receiver responds eually to blocks of car rier from either end. Throughfault current results in transmission of blocks of carrier from both ends, each lasting for half a cycle, but with a phase displace ment of half a cycle, so that the composite signal is continuously above the thr eshold level and the detector output logic is continuously '1'. Any phase shift relative to the through fault condition produces a gap in the composite carrier signal and hence a corresponding '0' logic level from the detector. The duration of the logic '0' provides the basis for discrimination between internal and ext ernal faults, tripping being permitted only when a time delay setting is exceede d. This delay is usually expressed in terms of the corresponding phase shift in degrees at system freuency ϕs in Figure 10.12. The advantages generally associate d with the use o the power line as the communication medium apply namely, that a power line provides a robust, reliable, and low loss interconnection between t he relaying points. In addition dedicated 'on/o ' signalling is particularly su ited or use in phase comparison blocking mode schemes, as signal attenuation is not a problem. This is in contrast to permissive or direct tripping schemes, wh ere high power output or boosting is required to overcome the extra attenuation due to the ault. The noise immunity is also very good, making the scheme very r eliable. Signal propagation delay is easily allowed or in the stability angle s etting, making the scheme very sensitive as well. 10 . 11 P H A S E C O M PA R I S I O N P R OT E C T I O N S C H E M E C O N S I D E R AT I O N S One type o u nit protection that uses carrier techniques or communication between relays is phase comparison protection. Communication between relays commonly uses PLCC or requency modulated carrier modem techniques. There are a number o consideratio ns that apply only to phase comparison protection systems, which are discussed i n this section. 10.11.1 Lines with Shunt Capacitance A problem can occur with th e shunt capacitance current that lows rom an energising source. Since this cur rent is in addition to the load current that lows out o the line, and typicall y leads it by more than 90°, signi icant di erential phase shi ts between the cur rents at the ends o the line can occur, particularly when load current is low. The system di erential phase shi t may encroach into the tripping region o the simple discriminator characteristic, regardless o how large the stability angl e setting may be. Figure 10.14 illustrates the e ect and indicates techniques t hat are commonly used to ensure stability. U n i t P ro te c t i o n Fe e d e r s • Signal transmission is usually per ormed by voice requency channels using requ ency shi t keying (FSK) or PLC techniques. Voice requency channels involving FS K use two discrete requencies either side o the middle o the voice band. This arrangement is less sensitive to variations in delay or requency response than i the ull bandwidth was used. Blocking or permissive trip modes o operation
may be implemented. In addition to the two requencies used or conveying the sq uarer in ormation, a third tone is o ten used, either or channel monitoring or trans er tripping dependent on the scheme. For a sensitive phase comparison sche me, accurate compensation or channel delay is required. However, since both the local and remote signals are logic pulses, simple time delay circuits can be us ed, in contrast to the analogue delay circuitry usually required or current di erential schemes. The principles o the Power Line Carrier channel technique ar e illustrated in Figure 10.13. The scheme operates in the blocking mode. The 'sq uarer' logic is used directly to turn a transmitter 'on' or 'o ' at one end, an d the resultant burst (or block) o carrier is coupled to and propagates along t he power line which is being protected to a receiver at the other end. Carrier s ignals above a threshold are detected by the receiver, and hence produce a logic signal corresponding to the block o carrier. In contrast to Figure 10.11, the signalling system is a 2 wire rather than 4 wire arrangement, in which the local transmission is ed directly to the local receiver along with any received sign al. The transmitter requencies at 10 • • 162 • Network Protection & Automation Guide
End G Line trap Line trap End H Summation network Squarer C Pulse length discriminator D Trip A Transmitter Rece iver Coupling ilter B Identical relay to end G 1 0 Load or through ault 1 0 Internal ault Trip A. Squarer output at end G Blocks o carrier transmitted rom end G Squarer output at end H 1 0 1 0 Blocks o carrier transmitted rom end H B. Composite carrier signal at end G C. Carrier detector output 1 0 1 1 0 1 0 Stability setting D. Discriminator output 0 Figure 10.13: Principles o power line carrier phase comparison A θc O ϕs IC IL Through Fault Re erence Squarer Threshold Starter Threshold Limits o di erential phase shi t due to ca pacitive current IC Encroachment into tripping region or discriminator with sta bility angle setting ϕs `Keyhole' characteristic capacitive current Minimum starte r threshold = sin ϕs 1 IC where ϕs = tan IL Characteristic o system with amplitude dependent compensation ϕs = angular compensation or current o magnitude OA IC or squarer threshold IC 2sin 1 OA IL = load current
Operation o the discriminator can be permitted only when current is above some threshold, so that measurement o the large di erential phase shi ts which occu r near the origin o the polar diagram is avoided. By choice o a suitable thres hold and stability angle, a 'keyhole' characteristic can be provided such that t he capacitive current characteristic alls within the resultant stability region . Fast resetting o the ault detector is required to ensure stability ollowing the clearance o a through ault when the currents tend to all towards the ori gin o the polar diagram. The mark space ratio o the squarer (or modulating) wa ve orm can be made dependent on the current amplitude. Any decrease in the mark space ratio will permit a corresponding di erential phase shi t to occur betwee n the currents be ore any output is given rom the comparator or measurement in the discriminator. A squarer circuit with an o set or bias can provide a • Figure 10.14: Capacitive current in phase comparison schemes and techniques used to avoid instability Network Protection & Automation Guide • 163 • U n i t P ro te c t i o n Fe e d e r s 10 •
decreasing mark space ratio at low currents, and with a suitable threshold level the extra phase shi t θc which is permitted can be arranged to eual or exceed th e phase shift due to capacitive current. At high current levels the capacitive c urrent compensation falls towards zero and the resultant stability region on the polar diagram is usually smaller than on the keyhole characteristic, giving imp rovements in sensitivity and/or dependability of the scheme. Since the stability region encompasses all through fault currents, the resetting speed of any fault detectors or starter (which may still be reuired for other purposes, such as t he control of a normally uiescent scheme) is much less critical than with the k eyhole characteristic. 10.11.2 System Tripping Angles For the protection scheme to trip correctly on internal faults the change in differential phase shift, θ0, f rom the through fault condition taken as reference, must exceed the effective st ability angle of the scheme. Hence: θ0 = ϕs + θc where …Euation 10.1 In the absence of pre fault load current, the voltages at the two ends of a line are in phase. Internal faults are fed from both ends with fault contributions w hose magnitudes and angles are determined by the position of the fault and the s ystem source impedances. Although the magnitudes may be markedly different, the angles (line plus source) are similar and seldom differ by more than about 20°. He nce |θG θH| ≤ 20° and the requirements of Equation 10.3 are very easily satisfied. The addition of arc or fault resistance makes no difference to the reasoning above, so the scheme is inherently capable of clearing such faults. 10.11.3 Effect of Load Current When a line is heavily loaded prior to a fault the e.m.f. s of the sources which cause the fault current to flow may be displaced by up to about 50°, that is, the power system stability limit. To this the differential line and so urce angles of up to 20° mentioned above need to be added. So |θG θH| ≤ 70° and the requ irements of Equation 10.3 are still easily satisfied. For three phase faults, or solid earth faults on phase-byphase comparison schemes, through load current fa lls to zero during the fault and so need not be considered. For all other faults , load current continues to flow in the healthy phases and may therefore tend to increase |θG θH| towards the through fault reference value. For low resistance fa ults the fault current usually far exceeds the load current and so has little ef fect. High resistance faults or the presence of a weak source at one end can pro ve more difficult, but high performance is still possible if the modulating quan tity is chosen with care and/or fault detectors are added. 10.11.4 Modulating Qu antity Phase-by-phase comparison schemes usually use phase current for modulatio n of the carrier. Load and fault currents are almost in antiphase at an end with a weak source. Correct performance is possible only when fault current exceeds load current, or for IF < IL’ |θG θH| ≈ 180° for IF > IL’ |θG θH| ≈ 180° …Equation 10.4 U n i t P ro te c t i o n Fe e d e r s
ϕs = stability angle setting θc = capacitive current compensation (when applicable) The currents at the ends of a transmission line IG and IH may be expressed in te rms of magnitude and phase shift θ with respect a common system voltage. IG = |IG| θG IH = |IH| θH Using the relay convention described in Section 10.2, the reference through fault condition is IG = IH ∴ IG θG = IH θH = IH θH 180° ∴ |θG θH| =180 al faults, the system tripping angle θ0 is the differential phase shift relative t o the reference condition. ∴ θ0 =180° |θG θH| Substituting θ0 in Euation 10.1, the con itions for tripping are: 180 |θG θH| ≥ ϕS + θc ∴ |θG θH| ≤ 180 - (ϕS + θc) …Euation 10.2 • 10 • where IF = fault current contribution from weak source IL = load current flowing towards weak source To avoid any risk of failure to operate, fault detectors wi
th a setting greater than the maximum load current may be applied, but they may limit the sensitivity of scheme. When the fault detector is not operated at one end, fault clearance invariably involves seuential tripping of the circuit brea kers. The term (ϕs + θc) is the effective stability angle setting of the scheme. Substitut ing a typical value of 60° in Euation 10.2. gives the tripping condition as |θG θH| ≤ 120° …Equation 10.3 • 164 • Network Protection & Automation Guide
Most phase comparison schemes use summation techniques to produce a single modul ating quantity, responsive to faults on any of the three phases. Phase sequence components are often used and a typical modulating quantity is: IM = MI2 + NI1 w here I1 = Positive phase sequence component I2 = Negative phase sequence compone nt M,N = constants With the exception of three phase faults all internal faults give rise to negative phase sequence (NPS) currents, I2, which are approximately in phase at the ends of the line and therefore could form an ideal modulating q uantity. In order to provide a modulating signal during three phase faults, whic h give rise to positive phase sequence (PPS) currents, I1, only, a practical mod ulating quantity must include some response to I1 in addition to I2. Typical val ues of the ratio M: N exceed 5:1, so that the modulating quantity is weighted he avily in favour of NPS, and any PPS associated with load current tends to be swa mped out on all but the highest resistance faults. For a high resistance phase-e arth fault, the system remains well balanced so that load current IL is entirely positive sequence. The fault contribution IF provides equal parts of positive, negative and zero sequence components IF /3. Assuming the fault is on A phase and the load is resistive, all sequence components are in phase at the infeed en d G: ∴ I mG = NI L + and θG ≈ 0 MI FG NI FG + 3 3 …Euation 10.5 The fault current in Euation 10.6 is the effective earth fault sensitivity IE o f the scheme. For the typical values of M = 6 and N = 1 M = −6 N 3 ∴ IE =− IL 5 Comparing this with Euation 10.4, a scheme using summation is potentially 1.667 times more sensitive than one using phase current for modulation. Even though t he use of a negative value of M gives a lower value of IE than if it were positi ve, it is usually preferred since the limiting condition of Im = 0 then applies at the load infeed end. Load and fault components are additive at the outfeed en d so that a correct modulating uantity occurs there, even with the lowest fault levels. For operation of the scheme it is sufficient therefore that the fault c urrent contribution from the load infeed end exceeds the effective setting. For faults on B or C phases, the NPS components are displaced by 120° or 240° with respe ct to the PPS components. No simple cancellation can occur, but instead a phase displacement is introduced. For tripping to occur, Euation 10.2 must be satisfi ed, and to achieve high dependability under these marginal conditions, a small e ffective stability angle is essential. Figure 10.15 illustrates operation near t o the limits of earth fault sensitivity. Very sensitive schemes may be implement ed by using _ high values of M but the scheme then becomes more N sensitive to d ifferential errors in NPS currents such as the unbalanced components of capaciti ve current or spill from partially saturated CT's. Techniues such as capacitive current compensation and _ reduction of M at high fault levels may be reuired to N ensure stability of the scheme. 10.11.5 Fault Detection and Starting For a scheme using a carrier system that continuously transmits the modulating uantit y, protecting an ideal line (capacitive current=0) in an interconnected transmis sion system, measurement of current magnitude might be unnecessary. In practice, fault detector or starting elements are invariably provided and the scheme then becomes a permissive tripping scheme in which both the fault detector and the d iscriminator must operate to provide a trip output, and the fault detector may l imit the sensitivity of the scheme. Reuirements for the fault detectors vary ac cording to the type of carrier channel used, mode of operation used in the At the outfeed end load current is negative, • ∴ I mH = − NI L +
MI FH NI FH + 3 3
Now, for ImH > 0,θH = 0, and |θG θH| = 0° and for ImH < 0,θH = 180°, and |θG θH| = 180° Hence for correct operation ImH ≥ 0 Let ImH = 0 Then I FH = 3I L = IE M +1 N …Equation 10.6 Network Protection & Automation Guide • 165 • U n i t P ro te c t i o n Fe e d e r s 10 •
System voltage reference 1.1 NILH MIE 3 1.1 ImG θG=180° NIE 3 NILH MIE 0.9 3 NI 0.9 E 3 MIE 3 ImH θH=0 ImG θG=0 ensure that during through faults, a High Set is never operated when a Low Set h as reset and potential race conditions are often avoided by the transmitting of an unmodulated (and therefore blocking) carrier for a short time following the r eset of low set; this feature is often referred to as 'Marginal Guard.' 10.11.7 Scheme without Capacitive Current Compensation The 'keyhole' discrimination char acteristic of depends on the inclusion of a fault detector to ensure that no mea surements of phase angle can occur at low current levels, when the capacitive cu rrent might cause large phase shifts. Resetting must be very fast to ensure stab ility following the shedding of through load. 10.11.8 Scheme with Capacitive Cur rent Compensation (Blocking Mode) When the magnitude of the modulating uantity is less than the threshold of the suarer, transmission if it occurred, would be a continuous blocking signal. This might occur at an end with a weak source, re mote from a fault close to a strong source. A fault detector is reuired to perm it transmission only when the current exceeds the modulator threshold by some mu ltiple (typically about 2 times) so that the effective stability angle is not ex cessive. For PLCC schemes, the low set element referred to in Section 10.11.6 is usually used for this purpose. If the fault current is insufficient to operate the fault detector, circuit breaker tripping will normally occur seuentially. 1 0.11.9 Fault Detector Operating Quantities Most faults cause an increase in the corresponding phase current(s) so measurement of current increase could form the basis for fault detection. However, when a line is heavily loaded and has a low fault level at the outfeed end, some faults can be accompanied by a fall in cur rent, which would lead to failure of such fault detection, resulting in seuenti al tripping (for blocking mode schemes) or no tripping (for permissive schemes). Although fault detectors can be designed to respond to any disturbance (increas e or decrease of current), it is more usual to use phase seuence components. Al l unbalanced faults produce a rise in the NPS components from the zero level ass ociated with balanced load current, whilst balanced faults produce an increase i n the PPS components from the load level (except at ends with very low fault lev el) so that the use of NPS and PPS fault detectors make the scheme sensitive to all faults. For schemes using summation of NPS and PPS components for the modula ting uantity, the use of NPS and PPS fault NIE 3 (a) A phase to earth fault IF = 0.9IE |θG θH |=180° 0.9 NIE 3 MIE 3 ImH 0.9 NILG 1.1 NIE 1.1 3 (b) A phase to earth fault IF = 1.1 IE |θG θH |=0° NIE 3 MIE 3 NILG NILH NILH θH MIE 3 ImH
θG θH NILG θG NILG 120° NI E 3 ImG MIE 3 (c) B phase to earth fault IF = IE |θG θH |=70° NIE 120° ImG 3 MIE 3 (d) C phase to earth fault IF = IE U n i t P ro te c t i o n Fe e d e r s Assumptions for examples: Infeed of load IL at end G Outfeed of load IL at end G M =-6 therefore I = 6I - I and from Equation 10.6 m 2 2 N 3 effective earth fau lt sensitivity IE =- IL 5 IF also IF1 = 3 Figure 10.15: Effect of load current on differential phase shift |θg θH| for resis tive earth faults at the effective earth fault sensitivity IE phase angle measurement, that is, blocking or permissive, and the features used to provide tolerance to capacitive current. 10.11.6 Normally Quiescent Power Lin e Carrier (Blocking Mode) To ensure stability of through faults, it is essential that carrier transmission starts before any measurement of the width of the gap is permitted. To allow for equipment tolerances and the difference in magnitude of the two currents due to capacitive current, two starting elements are used, usually referred to as Low Set and High Set respectively. Low Set controls t he start-up of transmission whilst High Set, having a setting typically 1.5 to 2 times that of the Low Set element, permits the phase angle measurement to proce ed. The use of impulse starters that respond to the change in current level enab les sensitivities of less than rated current to be achieved. Resetting of the st arters occurs naturally after a swell time or at the clearance of the fault. Dwe ll times and resetting characteristics must • 10 • • 166 • Network Protection & Automation Guide
detectors is particularly appropriate since, in addition to any reductions in ha rdware, the scheme may be characterized entirely in terms of sequence components . Fault sensitivities IF for PPS and NPS impulse starter settings I1S and I2S re spectively are as follows: Three phase fault Phase-phase fault Phase-earth fault 10 . 1 2 E X A M P L E S This section gives examples of setting calculations fo r simple unit protection schemes. It cannot and is not intended to replace a pro per setting calculation for a particular application. It is intended to illustra te the principles of the calculations required. The examples use the AREVA MiCOM P541 Current Differential relay, which has the setting ranges given in Table 10 .1 for differential protection. The relay also has backup distance, high-set ins tantaneous, and earth-fault protection included in the basic model to provide a complete ‘one-box’ solution of main and backup protection. 10.12.1 Unit Protection o f a Plain Feeder The circuit to be protected is shown in Figure 10.16. It consis ts of a plain feeder circuit formed of an overhead line 25km long. The relevant properties of the line are: Line voltage: 33kV Z = 0.157 + j0.337Ω/km Shunt chargi ng current = 0.065A/km To settings, the arrive at thecorrect characteristics of the relays to be applie must be consi ere . The recommen e settings for three of the a justable values (taken from the relay manual) are: Is2 = 2.0pu IF = I1 S IF = √3I2S IF = 3I2S Parameter Differential Current Setting, Is1 Bias Current Threshol Setting, Is2 Lower Percentage Bias Setting, k1 Higher Precentage Bias Setting, k2 In - CT rat e secon ary current Table 10.1: Relay Setting Ranges Setting Range 0.2 -2.0 In 1-30 In 0.3-1.5 0.3-1.5 k1 = 30% k2 = 150% To provi e immunity from the effects of line charging current , the setting of IS1 must be at least 2.5 times the stea y-state charging curren t, i.e. 4.1A or 0.01p.u., after taking into consi eration the CT ratio of 400/1. The nearest available setting above this is 0.20p.u. This gives the points on t he as shown in Figure 10.17. Theminimum operating current relay characteristic I minis relate to the value of Is1 by the formula I min = (k1IL + Is1)/(1-0.5k 1) an for Ibias
Digital communications link I > I > Stea y state charging current = 0.065A/km Figure 10.16: Typical plain fee er circuit Network Protection & Automation Gui e • 167 •
U n i t P ro te c t i o n Fe e e r s 10 • I min = (k2IL -(k2-k1)Is2 + Is1)/(1-0.5k2) for Ibias >Is2 where IL = loa curren t an hence the minimum operating current at no loa is 0.235p.u. or 94A.
8 7
The elta/star transformer connection requires phase shift correction of CT seco n ary currents across the transformer, an in this case software equivalents of interposing CT’s are use . Since the LV si e quantities lag the HV si e quantities by 30°, it is necessary to correct this phase shift by using software CT settings a. HV si e: that prouce a 30° phaseshift. Thereare two obvious possibilities: Y 1 LV si e: Yy0 b. HV si e: Yy0 LV si e: Y 11 Only the secon combination is sa tisfactory, since only this one provi es the necessary zero-sequence current tra p to avoi maloperation of the protection scheme for earth faults on the LV si e of thetransformer outsi e of the protecte zone. Ratio correction must alsobe applie , in or er to ensure that therelays see currents fromtheprimary an s econ ary si es of the transformer fee er that are well balance un er full loa con itions. This is not always inherently the case, ue to selection of the main CT ratios. For the example of Figure 10.18, Transformer turns ratio at nominal tap 6 5 I iff 4 3 2 1 0 0 1 2 3 Ibias 4 5 6 U n i t P ro te c t i o n Fe e
e r s
Figure 10.17: Relay characteristic; plain fee er example 10.12.2 Unit Protection of a Transformer Fee shows unit protecti er Figure 10.18 on applie to a transformer fee er. The fee er is assume to be a 100m length of cable, such as might befoun in some in ustrial plants or where a short istan ce separates the 33kV an 11kV substations. While 11kV cable capacitance will ex ist, it can be regar e as negligible for the purposes of this example. =
11 = 0.3333 33 Require turns ratio accor ing to the CT ratios use = 1 = 0.32 1250 1 400 33kV 400/1 350A 0° 20 MVA 33/11kV Dyn1 1050A -30° • 10 • Cable 100m 1250/1 11kV 0.875A 0.84A
Digital communication channel I > I > Ratio correction: 1.19 software CT: Y 11 Figure 10.18: Unit protection of a transformer fee er Ratio correction: 1.14 software CT: Yy0 • 168 •
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Spill current that will arise ue to the incompatibility of the CT ratios use w ith the power transformer turns ratio may cause relay maloperation. This has to be eliminate by using the facility in the relay for CT ratio correction factors . Forthis particular relay, the correction factors are chosen such that the ful l loa current seen by the relay software is equal to 1A. The appropriate correc tion factors are: HV: 400/350 = 1.14 LV: 1250/1050 = 1.19 where: transformer rat e primary current = 350A transformer rate secon ary current = 1050A With the l ine charging current being negligible, the following relay settings are then sui table, an allow for transformer efficiency an mismatch ue to tapchanging: IS1 = 20% (minimum possible) IS1 = 20% k2 = 150%10 . 1 3 R E F E R E NC E S 10.1 Merz-Price Protective Gear. K. Faye-Hansen an G.Harlow. IEE Procee ings, 1911. 10.2 Protective Relays Application Gui e – 3r E ition. AREVA Transmission an Di stribution Protection an Control, 1987. k1 = 30% •
Network Protection & Automation Gui e • 169 • U n i t P ro te c t i o n Fe e
e r s 10 •
• 11 • Distance Protection Intro uction Principles of istance relays Relay performance Relationship betwee n relay voltage an ZS/ZL ratio Voltage limit for accurate reach point measureme nt Zones of protection Distance relay characteristics Distance relay implementat ion Effect of source impe ance an earthing metho s Distance relay application p roblems Other istance relay features Distance relay application example Referen ces 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.13
• 11 • Distance P rotection 11.1 INTRODUCTION The problem of combining fast fault clearance with selective t ripping of plant is a key To meet aim for the protection of power systems. these requirements, highspee protection systems for transmission an primary istrib ution circuits that are suitable for use withthe automatic reclosureof circuit breakers are un er continuous evelopment an are very wi ely applie . Distance offering cons protection, in its basic form, is a non-unit system of protection i erable economic an technical a vantages. Unlike phase an neutral overcurrent protection, thekey a vantage of istance is that itsfault coverage protection of the protecte circuit is virtually in epen ent of source impe ance variation s. Zs=10Ω Z1=4Ω Zs=10Ω 115kV IF1= x I >> R1 3 F1 =7380A √ √3 + Relay R1 (a) Zs=10Ω Z1=4Ω F2 115kV IF2= > I >> 115x103 =6640A √ √3x10(b) Therefore, for relay operation for line faults, Relay cur rent setting <6640A an >7380A This is impractical, overcurrent relay not suitab le Must use Distance or Unit Protection Figure 11.1: A vantages of istance over overcurrent protection This is illustrate in Figure 11.1, where it can be seen that overcurrent protec tion cannot be applie satisfactorily. Network Protection & Automation Gui e • 171 •
Distance protection iscomparatively simple to apply an it can be fast in opera tion for faults locate along most of a protecte circuit. It can also provi e b oth primary an remote back-up functions in a singlescheme. It can easily be a apte to create a unit protection scheme when applie with a signalling channel. In this form it is eminently suitable for application with high-spee autoreclo sing, for the protection of critical transmission lines. 11.2 PRINCIPLES OF DIST ANCE RELAYS Since the impe ance of a transmission line is proportional to its le ngth, for istance measurement it is appropriate to use a relay capable of measu ring the impe ance of a line up to a pre etermine point(the reach point). Such a relay is escribe as a istance relay an is esigne to operate only for fa ultsoccurring between the relay location an the selecte reach point, thus giv ing iscrimination for faults that may occur in ifferent line sections. The bas ic principle of istance protection involves the ivisionof the voltage atthe relaying point by the measure current. The apparent impe ance so calculate is compare with the reach point impe ance. If the measure impe ance is less than the reach point impe ance, it is assume that a fault exists on the line between the relay an the reach point. The reachpoint of a relay is the point along th e line impe ance locusthatis intersecte by the boun arycharacteristic of the relay. Since this is epen ent on theratio of voltage an current an the phas e angle between them, it may be plotte on an R/X iagram. The loci of power sys tem impe ances asseen by the relay uring faults, power swings an loa variati ons may be plotte on the same iagram an in of the this manner the performance relay in the presence of system faults an isturbances may be stu ie . 11.3 RE LAYPERFORMANCE Distance relay performance is efine in terms of reach accuracy an operating is a comparison of the actual ohmic reach of time. Reach accuracy the relay un er practical con itions with the relay setting value in ohms. Reac haccuracy particularly epen s on the level of voltage presente to the relay u n er fault con itions. The impe ance measuring techniques employe in particular relay esigns also have an impact. Operating times can vary with fault current, with fault position relative to the relay setting, an with the point on the vo ltage wave at which the fault occurs. Depen ing on the measuring techniques empl oye ina particular relay esign, measuring signal transient errors, such as th ose pro uce by Capacitor Voltage Transformers or saturating CT’s, can also a versely elay relay operation for faul ts close to the reach point. It isusual for electromechanical an static istan ce relays to claim both maximum an minimum operating times. However, for mo ern igital or numerical istance relays, between these is small over the variation a wi e range of system operating con itions an fault positions. 11.3.1 Electro mechanical/Static Distance Relays With electromechanical an earlier static rela y esigns, the magnitu e of input quantities particularly influence both reach accuracy an operating time. It was customary to present information on relay pe rformance by voltage/reach curves, as shown in Figure 11.2, an operating time/f ault position curves for various values of system impe ance ratios (S.I.R.’s) as s hown in Figure 11.3, where: S.I .R. = ZS an ZS = ZL
system source impe ance behin the relay location ZL = line impe ance equivalent to relay reach setting Distance P rotection Impe ance reach (% Zone 1 setting) 105 100 95 0
10 20 30 40 50 60 65
% relay rate voltage (a) Phase-earth faults Impe ance reach (% Zone 1 setting) 105 100 95 0 20 40 80 100 60 % relay rate voltage (b) Phase-phase faults • 11 • Impe ance reach (% Zone 1 setting)
105 100 95 0 20 40 60 80 100 % relay rate voltage (c) Three-phase an three-pha se-earth faults Figure 11.2: Typical impe ance reach accuracy characteristics fo r Zone 1 • 172 •
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11.3.2 Digital/Numerical Distance Relays Operation time (ms) 50 40 30 20 10 0 Max Min 10 20 30 40 50 60 70 80 90 100 Faul t position (% relay setting) Digital/Numerical istance relays ten to have more consistentoperating times. They are usually slightly slower than some of the ol er relay esigns when opera ting un er the best con itions, but their maximum operatingtimes are also less un er a verse waveform con itions or for boun ary fault con itions. 11.4 RELATIO NSHIP BETWEEN RELAY VOLTAGE AND ZS/ZL RATIO A single, generic, equivalent circui t, as shown in Figure 11.5(a), may represent anyfault con ition in a threephase power system. The voltage V applie to the impe ance loop is the open circuit v oltage of the power system. Point R represents the relay location; IR an VR are the measure by the relay, respectively. The impe ances ZS current an voltage an ZL are escribe as source an line impeances because of their position wit h respect to the relay location. Source impe ance ZS is a measure of the fault l evel at the relaying point. For faults involving earth it is epen ent on the me tho of system earthing behin the relaying point. Line impe ance ZL is a measur e of the impe ance of the protecte section. The voltage VR applie to the relay is, therefore, IRZL. For a fault at the reach point, this may be alternatively expresse in terms of source to line impe ance ratio ZS/ZL by means of the follo wing expressions: where: VR=IRZL IR = Therefore : VR = ZL V ZS + Z L 1 V ZL ) +1 • (a) With system impe ance ratio of 1/1 Operation time (ms) 50 40 30 20 10 0 Max Min 10 20 30 40 50 60 70 80 90 100 Fault position (% relay setting) (b) With system impe ance ratio of 30/1 Figure 11.3: Typical operation time char acteristics for Zone 1 phase-phase faults Fault position (p.u. relay setting ZL) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.01 Boun ary 13ms 9ms V ZS + Z L 0.1 1 Z S/ZL 10 1 100 1000 or VR = Fault position (p.u. relay setting ZL) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.01 Boun ary y
(ZS ...Equation 11.1 15ms
The above generic relationship between VR an ZS/ZL, illustrate in Figure 11.5( b), is vali for all types of short circuits provi e a few simple rules are obs erve . These are: 1 Z S/ZL 10 100 1000 0.1 (b) Zone 1 phase-phase fault: maximum operation times Figure 11.4: Typical operation-time contours
i. for phase faults, V is the phase-phase source voltage an ZS/ZL is the positi vesequence source to line impe ance ratio. VR is the phase-phase relay voltage an IR is the phase-phase relay current, for the faulte phases • 173 • Network Protection & Automation Gui e Distance P rotection 11 •
Alternatively, the above information was combine in a family of contour curves, where the fault position expresse as a percentage of the relay setting is plot te against the source to line impe ance ratio, as illustrate in Figure 11.4.
VR = (ZS 1 V p−p ZL ) +1 11.5 VOLTAGE LIMIT FOR ACCURATE REACH POINT MEASUREMENT …Equ tion 11.2 ii. for e rth f ults, V is the ph se neutr l source volt ge nd ZS/ZL is compo site r tio involving the positive nd zero sequence imped nces. VR is the ph se neutr l rel y volt ge nd IR is the rel y current for the f ulted ph se VR = The ility of dist nce rel y to me sure ccur tely for re ch point f ult de pends on the minimum volt ge t the rel y loc tion under this condition eing ove decl red v lue. This volt ge, which depends on the rel y design, c n lso e quoted in terms of n equiv lent m ximum ZS/ZL or S.I.R. Dist nce rel ys re designed so th t, provided the re ch point volt ge criterion is met, ny incre s ed me suring errors for f ults closer to the rel y will not prevent rel y oper t ion. Most modern rel ys re provided with he lthy ph se volt ge pol ris tion nd /or memory volt ge pol ris tion. The prime purpose of the rel y pol rising volt ge is to ensure correct rel y direction l response for close up f ults, in the f orw rd or reverse direction, where the f ult loop volt ge me sured y the rel y m y e very sm ll. 11.6 ZONES OF PROTECTION (ZS ZL ) 1 V l −n
2 + p
+1
2 +q
...Equ tion 11.3 where ZS = 2ZS1 + ZS0 = ZS1(2+p) ZL = 2ZL1 + ZL0 = ZL1(2+q) nd p= q= ZS0 Z S1 Z L0 Z L1 Source R Line VS ZS IR VR VL=VR ZL V ( ) Power system configur tion 10 7.5 100 VR (%) 5.0 2.5 0 10 20 30 40 50 ZS ZL C reful selection of times for the v rious zones the re ch settings nd tripping of me surement en les correct coordin tion etween dist nce rel ys on power system. B sic dist nce protection will comprise inst nt neous direction l Zone 1 protection nd one or more timedel yed zones. Typic l re ch nd time settings f or 3zone dist nce protection re shown in Figure 11.6. Digit l nd numeric l d ist nce rel ys m y h ve up to five zones, some set to me sure in the reverse dir ection. Typic l settings for three forw rd looking zones of sic dist nce prote ction re given in the following su sections. To determine the settings for p rticul r rel y design or for p rticul r dist nce teleprotection scheme, invol
ving end to end sign lling, the rel y m nuf cturer’s instructions should b r f rr d to. 11.6.1 Zon 1 S tting El ctrom chanical/static r lays usually hav a r ac h s tting of up to 80% of th prot ct d lin imp danc for instantan ous Zon 1 prot ction. For digital/num rical distanc r lays, s ttings of up to 85% may b saf . Th r sulting 15-20% saf ty margin nsur s that th r is no risk of th Zo n 1 prot ction ov r-r aching th prot ct d lin du to rrors in th curr nt an d voltag transform rs, inaccuraci s in lin imp danc data provid d for s tting purpos s and rrors of r lay s tting and m asur m nt. Oth rwis , th r would b a loss of discrimination with fast op rating prot ction on th following lin s ction. Zon 2 of th distanc prot ction must cov r th r maining 15-20% of th lin . Distanc P rot ction 11 • Voltag VR (% rat d voltag ) • 90 80 70 60 50 40 30 20 10 0 0.1 0.2 0.3 VR (%) 2 3 4 5 ZS Syst m imp danc ratio ZL 0.5 1 10 (b) Variation of r lay voltag with syst m sourc to lin imp danc ratio Figur 11.5: R lationship b tw n sourc to lin ratio and r lay voltag • 174 • N twork Prot ction & Automation Guid
11.6.2 Zon 2 S tting To nsur full cov r of th lin with allowanc for th so urc s of rror alr ady list d in th pr vious s ction, th r ach s tting of th Zon 2 prot ction should b at l ast 120% of th prot ct d lin imp danc . In ma ny applications it is common practic to s t th Zon 2 r ach to b qual to th prot ct d lin s ction +50% of th short st adjac nt lin . Wh r possibl , this nsur s that th r sulting maximum ff ctiv Zon 2 r ach do s not xt nd b yon d th minimum ff ctiv Zon 1 r ach of th adjac nt lin prot ction. This avoid s th n d to grad th Zon 2 tim s ttings b tw n upstr am and downstr am r l ays. In l ctrom chanical and static r lays, Zon 2 prot ction is provid d ith r by s parat l m nts or by xt nding th r ach of th Zon 1 l m nts aft r a tim d lay that is initiat d by a fault d t ctor. In most digital and num rical r lays, th Zon 2 l m nts ar impl m nt d in softwar . Zon 2 tripping must b tim -d lay d to nsur grading with th primary r laying appli d to adjac nt ci rcuits that fall within th Zon 2 r ach. Thus compl t cov rag of a lin s cti on is obtain d, with fast cl aranc of faults in th first 80-85% of th lin an d som what slow r cl aranc of faults in th r maining s ction of th lin . Z3JR Z3JF Z2J Z1J K b tak n into account wh n s tting Zon 3. In som syst ms, variations in th r mot busbar inf d can pr v nt th application of r mot back-up Zon 3 prot cti on but on radial distribution syst ms with singl nd inf d, no difficulti s sh ould aris . 11.6.4 S ttings for R v rs R ach and Oth r Zon s Mod rn digital or num rical r lays may hav additional imp danc zon s that can b utilis d to pro vid additional prot ction functions. For xampl , wh r th first thr zon s a r s t as abov , Zon 4 might b us d to provid back-up prot ction for th loca l busbar, by applying a r v rs r ach s tting of th ord r of 25% of th Zon 1 r ach. Alt rnativ ly, on of th forwardlooking zon s (typically Zon 3) could b s t with a small r v rs offs t r ach from th origin of th R/X diagram, in a ddition to its forward r ach s tting. An offs t imp danc m asur m nt charact ri stic is nondir ctional. On advantag of a non-dir ctional zon of imp danc m a sur m nt is that it is abl to op rat for a clos -up, z ro-imp danc fault, in situations wh r th r may b no h althy phas voltag signal or m mory voltag signal availabl to allow op ration of a dir ctional imp danc zon . With th of fs t-zon tim d lay bypass d, th r can b provision of ‘Switch-on-toFault’ (SOTF) prot ction. This is r quir d wh r th r ar lin voltag transform rs, to provi d fast tripping in th v nt of accid ntal lin n rgisation with maint nanc arthing clamps l ft in position. Additional imp danc zon s may b d ploy d as p art of a distanc prot ction sch m us d in conjunction with a t l prot ction si gnalling chann l. 11.7 DISTANCE RELAY CHARACTERISTICS Som num rical r lays m as ur th absolut fault imp danc and th n d t rmin wh th r op ration is r quir d according to imp danc boundari s d fin d on th R/X diagram. Traditional dist anc r lays and num rical r lays that mulat th imp danc l m nts of traditio nal r lays do not m asur absolut imp danc . Th y compar th m asur d fault vo ltag with a r plica voltag d riv d from th fault curr nt and th zon imp dan c s tting to d t rmin wh th r th fault is within zon or out-of-zon . Distanc r lay imp danc comparators or algorithms which mulat traditional comparator s ar classifi d according to th ir polar charact ristics, th numb r of signal inputs th y hav , and th m thod by which signal comparisons ar mad . Th commo n typ s compar ith r th r lativ amplitud or phas of two input quantiti s t o obtain op rating charact ristics that ar ith r straight lin s or circl s wh n plott d on an R/X diagram. At ach stag of distanc r lay d sign volution, t h d v lopm nt of Tim Sourc Y Z1L Sourc
0 H J Z1H H Tim Z1K Z2K Z3KF Z3KR Zon 1 = 80-85% of prot ct d lin imp danc Zon 2 (minimum) = 120% of prot ct d lin Zon 2 (maximum) < Prot ct d lin + 50% of short st s cond lin Zon 3F = 1.2 (prot ct d lin + long st s cond lin ) Zon 3R = 20% of prot ct d lin X = C ircuit br ak r tripping tim Y = Discriminating tim Figur 11.6: Typical tim /distanc charact ristics for thr zon distanc prot ction • 11.6.3 Zon 3 S tting R mot back-up prot ction for all faults on adjac nt lin s can b provid d by a third zon of prot ction that is tim d lay d to discrimin at with Zon 2 prot ction plus circuit br ak r trip tim for th adjac nt lin . Zon 3 r ach should b s t to at l ast 1.2 tim s th imp danc pr s nt d to th r lay for a fault at th r mot nd of th s cond lin s ction. On int rconn ct d pow r syst ms, th ff ct of fault curr nt inf d at th r mot busbars will caus th imp danc pr s nt d to th r lay to b much gr at r than th actual im p danc to th fault and this n ds to N twork Prot ction & Automation Guid • 175 • Distanc P rot ction 11 •
imp danc op rating charact ristic shap s and sophistication has b n gov rn d b y th t chnology availabl and th acc ptabl cost. Sinc many traditional r lay s ar still in s rvic and sinc som num rical r lays mulat th t chniqu s of th traditional r lays, a bri f r vi w of imp danc comparators is justifi d. 1 1.7.1 Amplitud and Phas Comparison R lay m asuring l m nts whos functionalit y is bas d on th comparison of two ind p nd nt quantiti s ar ss ntially ith r amplitud or phas comparators. For th imp danc l m nts of a distanc r lay , th quantiti s b ing compar d ar th voltag and curr nt m asur d by th r la y. Th r ar num rous t chniqu s availabl for p rforming th comparison, d p nd ing on th t chnology us d. Th y vary from balanc d-b am (amplitud comparison) and induction cup (phas comparison) l ctromagn tic r lays, through diod and o p rational amplifi r comparators in static-typ distanc r lays, to digital s qu nc comparators in digital r lays and to algorithms us d in num rical r lays. A ny typ of imp danc charact ristic obtainabl with on comparator is also obtai nabl with th oth r. Th addition and subtraction of th signals for on typ o f comparator produc s th r quir d signals to obtain a similar charact ristic us ing th oth r typ . For xampl , comparing V and I in an amplitud comparator r sults in a circular imp danc charact ristic c ntr d at th origin of th R/X di agram. If th sum and diff r nc of V and I ar appli d to th phas comparator th r sult is a similar charact ristic. 11.7.2 Plain Imp danc Charact ristic Th is charact ristic tak s no account of th phas angl b tw n th curr nt and th voltag appli d to it; for this r ason its imp danc charact ristic wh n plott d on an R/X diagram is a circl with its c ntr at th origin of th co-ordinat s and of radius qual to its s tting in ohms. Op ration occurs for all imp danc valu s l ss than th s tting, that is, for all points within th circl . Th r lay charact ristic, shown in Figur 11.7, is th r for nondir ctional, and in t his form would op rat for all faults along th v ctor AL and also for all fault s b hind th busbars up to an imp danc AM. It is to b not d that A is th r la ying point and RAB is th angl by which th fault curr nt lags th r lay voltag for a fault on th lin AB and RAC is th quival nt l ading angl for a fault on lin AC. V ctor AB r pr s nts th imp danc in front of th r lay b tw n th r laying point A and th nd of lin AB. V ctor AC r pr s nts th imp danc of lin AC b hind th r laying point. AL r pr s nts th r ach of instantan ous Zon 1 prot ction, s t to cov r 80% to 85% of th prot ct d lin . Lin AC C A Z Z< X B L Op rat s A AC M C Imp danc p r lay Figur 11.7: Plain im p danc r lay charact ristic Lin AB B R strains Lin AB R B L Imp danc l m nt RZ< R strains Dir ctional l m nt RD A Q R Distanc P rot ction (a) Charact ristic of combin d dir ctional/imp danc r lay A Sourc Z< C D IF1 I F2 B Sourc •
11 • F (b) Illustration of us of dir ctional/imp danc r lay: circuit diagram RAZ< R AD RAD AZ< & & Trip r lay RAD: dir ctional l m nt at A (c) Logic for dir ctional and imp danc l m nts a t A Figur 11.8: Combin d dir ctional and imp danc r lays A r lay using this charact ristic has thr important disadvantag s: i. it is no n-dir ctional; it will s faults both in front of and b hind th r laying point , and th r for • 176 • N twork Prot ction & Automation Guid
r quir s a dir ctional l m nt to giv it corr ct discrimination ii. it has nonuniform fault r sistanc cov rag iii. it is susc ptibl to pow r swings and h a vy loading of a long lin , b caus of th larg ar a cov r d by th imp danc ci rcl Dir ctional control is an ss ntial discrimination quality for a distanc r lay, to mak th r lay non-r sponsiv to faults outsid th prot ct d lin . Thi s can b obtain d by th addition of a s parat dir ctional control l m nt. Th imp danc charact ristic of a dir ctional control l m nt is a straight lin on th R/X diagram, so th combin d charact ristic of th dir ctional and imp danc r lays is th s mi-circl APLQ shown in Figur 11.8. If a fault occurs at F cl os to C on th parall l lin CD, th dir ctional unit RD at A will r strain du to curr nt IF1. At th sam tim , th imp danc unit is pr v nt d from op ratin g by th inhibiting output of unit RD. If this control is not provid d, th und r imp danc l m nt could op rat prior to circuit br ak r C op ning. R v rsal o f curr nt through th r lay from IF1 to IF2 wh n C op ns could th n r sult in in corr ct tripping of th h althy lin if th dir ctional unit RD op rat s b for th imp danc unit r s ts. This is an xampl of th n d to consid r th prop r co-ordination of multipl r lay l m nts to attain r liabl r lay p rformanc d uring volving fault conditions. In old r r lay d signs, th typ of probl m to b addr ss d was commonly r f rr d to as on of ‘contact rac ’. 11.7.3 S lf-Polaris d Mho R lay Th mho imp danc l m nt is g n rally known as such b caus its cha ract ristic is a straight lin on an admittanc diagram. It cl v rly combin s th discriminating qualiti s of both r ach control and dir ctional control, th r b y liminating th ‘contact rac ’ probl ms that may b ncount r d with s parat r ac h and dir ctional control l m nts. This is achi v d by th addition of a polari sing signal. Mho imp danc l m nts w r particularly attractiv for conomic r asons wh r l ctrom chanical r lay l m nts w r mploy d. As a r sult, th y ha v b n wid ly d ploy d worldwid for many y ars and th ir advantag s and limita tions ar now w ll und rstood. For this r ason th y ar still mulat d in th al gorithms of som mod rn num rical r lays. Th charact ristic of a mho imp danc l m nt, wh n plott d on an R/X diagram, is a circl whos circumf r nc pass s through th origin, as illustrat d in Figur 11.9(b). This d monstrat s that th imp danc l m nt is inh r ntly dir ctional and such that it will op rat only for faults in th forward dir ction along lin AB. IX IZn V-IZn V R strain j Op rat p IR (a) Phas comparator inputs IX B R strain Zn ZF j A Op rat IR R strain K (b) Mho imp danc charact ristic IX B P Q
q j A IR AP R lay imp danc s tting j AB Prot ct d lin Arc r sistanc q Lin angl (c) Incr as d arc r sistanc cov rag Figur 11.9: Mho r lay charact ristic • K Th imp danc charact ristic is adjust d by s tting Zn, th imp danc r ach, alo ng th diam t r and ϕ, the angle o displacement o the diameter rom the R axis. Angle ϕ is known as the Relay Characteristic Angle (RCA). The relay operates or v alues o ault impedance ZF within its characteristic. Network Protection & Automation Guide • 177 • Distance P rotection 11 •
It will be noted that the impedance reach varies with ault angle. As the line t o be protected is made up o resistance and inductance, its ault angle will be dependent upon the relative values o R and X at the system operating requency. Under an arcing ault condition, or an earth ault involving additional resista nce, such as tower ooting resistance or ault through vegetation, the value o the resistive component o ault impedance will increase to change the impedance angle. Thus a relay having a characteristic angle equivalent to the line angle will under reach under resistive ault conditions. It is usual, there ore, to se t the RCA less than the line angle, so that it is possible to accept a small amo unt o ault resistance without causing under reach. However, when setting the r elay, the di erence between the line angle θ and the relay characteristic angle ϕ m ust be known. The resulting characteristic is shown in Figure 11.9(c) where AB c orresponds to the length o the line to be protected. With ϕ set less than θ, the ac tual amount of line protected, AB, would be eual to the relay setting value AQ multiplied by cosine (θ ϕ). There ore the required relay setting AQ is given by: AQ = AB cos ( θ − ϕ ) with regard to the relay settings other than the e ect that reduced ault curre nt may have on the value o arc resistance seen. The earthing resistance is in t he source behind the relay and only modi ies the source angle and source to line impedance ratio or earth aults. It would there ore be taken into account only when assessing relay per ormance in terms o system impedance ratio. 11.7.4 O set Mho/Lenticular Characteristics Under close up ault conditions, when the rel ay voltage alls to zero or near zero, a relay using a sel polarised mho charac teristic or any other orm o sel polarised directional impedance characteristi c may ail to operate when it is required to do so. Methods o covering this con dition include the use o non directional impedance characteristics, such as o set mho, o set lenticular, or cross polarised and memory polarised directional impedance characteristics. I current bias is employed, the mho characteristic i s shi ted to embrace the origin, so that the measuring element can operate or c lose up aults in both the orward and the reverse directions. The o set mho re lay has two main applications: Distance P rotection Due to the physical nature o an arc, there is a non linear relationship between arc voltage and arc current, which results in a non linear resistance. Using th e empirical ormula derived by A.R. van C. Warrington, [11.1] the approximate va lue o arc resistance can be assessed as: Ra = where: Ra = arc resistance (ohms) L = length o arc (metres) I = arc current (A) On long overhead lines carried o n steel towers with overhead earth wires the e ect o arc resistance can usuall y be neglected. The e ect is most signi icant on short overhead lines and with ault currents below 2000A (i.e. minimum plant condition), or i the protected l ine is o wood pole construction without earth wires. In the latter case, the ea rth ault resistance reduces the e ective earth ault reach o a mho Zone 1 ele ment to such an extent that the majority o aults are detected in Zone 2 time. This problem can usually be overcome by using a relay with a cross polarised mho or a polygonal characteristic. Where a power system is resistance earthed, it s hould be appreciated that this does not need to be considered • 178 • X 28710 I 1.4 L ...Equation 11.4 Zone 1 Busbar zone Zone 3 Zone 2 R
• 11 • (a) Busbar zone back up using an o set mho relay X J H Zone 3 Zone 2 Zone 1 G C arrier start Carrier stop R K (b) Carrier starting in distance blocking schemes Figure 11.10: Typical applicat ions or the o set mho relay Network Protection & Automation Guide
11.7.4.1 Third zone and busbar back up zone In this application it is used in co njunction with mho measuring units as a ault detector and/or Zone 3 measuring u nit. So, with the reverse reach arranged to extend into the busbar zone, as show n in Figure 11.10(a), it will provide back up protection or busbar aults. This acility can also be provided with quadrilateral characteristics. A urther ben e it o the Zone 3 application is or Switch on to Fault (SOTF) protection, wher e the Zone 3 time delay would be bypassed or a short period immediately ollowi ng line energisation to allow rapid clearance o a ault anywhere along the prot ected line. 11.7.4.2 Carrier starting unit in distance schemes with carrier bloc king I the o set mho unit is used or starting carrier signalling, it is arran ged as shown in Figure 11.10(b). Carrier is transmitted i the ault is external to the protected line but inside the reach o the o set mho relay, in order to prevent accelerated tripping o the second or third zone relay at the remote st ation. Transmission is prevented or internal aults by operation o the local m ho measuring units, which allows highspeed ault clearance by the local and remo te end circuit breakers. 11.7.4.3 Application o lenticular characteristic There is a danger that the o set mho relay shown in Figure 11.10(a) may operate unde r maximum load trans er conditions i Zone 3 o the relay has a large reach sett ing. A large Zone 3 reach may be required to provide remote back up protection or aults on the adjacent eeder.
To avoid this, a shaped type o characteristic may be used, where the resistive coverage is restricted. With a ‘lenticul r’ charact ristic, th asp ct ratio of th l ns a is adjustabl , nabling b it to b s t to provid th maximum fault r sist nc cov rag consist nt with non-op ration und r maximum load transf r condition s. Figur 11.11 shows how th l nticular charact ristic can tol rat much high r d gr s of lin loading than offs t mho and plain imp danc charact ristics. R duction of load imp danc from ZD3 to ZD1 will corr spond to an quival nt incr as in load curr nt. 11.7.5 Fully Cross-Polaris d Mho Charact ristic Th pr viou s s ction show d how th non-dir ctional offs t mho charact ristic is inh r ntly abl to op rat for clos -up z ro voltag faults, wh r th r would b no polar ising voltag to allow op ration of a plain mho dir ctional l m nt. On way of nsuring corr ct mho l m nt r spons for z ro-voltag faults is to add a p rc n tag of voltag from th h althy phas (s) to th main polarising voltag as a su bstitut phas r f r nc . This t chniqu is call d cross-polarising, and it has th advantag of pr s rving and ind d nhancing th dir ctional prop rti s of t h mho charact ristic. By th us of a phas voltag m mory syst m, that provid s s v ral cycl s of pr -fault voltag r f r nc during a fault, th cross-polari sation t chniqu is also ff ctiv for clos -up thr -phas faults. For this typ of fault, no h althy phas voltag r f r nc is availabl . Early m mory syst m s w r bas d on tun d, r sonant, analogu circuits, but probl ms occurr d wh n a ppli d to n tworks wh r th pow r syst m op rating fr qu ncy could vary. Mor m od rn digital or num rical syst ms can off r a synchronous phas r f r nc for v ariations in pow r syst m fr qu ncy b for or v n during a fault. As d scrib d in S ction 11.7.3, a disadvantag of th s lf-polaris d, plain mho imp danc cha ract ristic, wh n appli d to ov rh ad lin circuits with high imp danc angl s, is that it has limit d cov rag of arc or fault r sistanc . Th probl m is aggra vat d in th cas of short lin s, sinc th r quir d Zon 1 ohmic s tting is low . Th amount of th r sistiv cov rag off r d by th mho circl is dir ctly r l at d to th forward r ach s tting. H nc , th r sulting r sistiv cov rag may b too small in r lation to th xp ct d valu s of fault r sistanc . On addition al b n fit of applying cross-polarisation to a mho imp danc l m nt is that its r sistiv cov rag will b nhanc d. This ff ct is illustrat d in Figur 11.12 , for th cas wh r a mho l m nt has 100% X Offs t L nticular charact ristic b a •
Offs t Mho charact ristic Z D1 Z D2 Z D3 0 Load ar a R Imp danc charact ristic Figur 11.11: Minimum load imp danc p rmitt d with l nticular, offs t mho and i mp danc r lays N twork Prot ction & Automation Guid • 179 • Distanc P rot ction 11 •
cross-polarisation. With cross-polarisation from th h althy phas (s) or from a m mory syst m, th mho r sistiv xpansion will occur during a balanc d thr pha s fault as w ll as for unbalanc d faults. Th xpansion will not occur und r lo ad conditions, wh n th r is no phas shift b tw n th m asur d voltag and th polarising voltag . Th d gr of r sistiv r ach nhanc m nt d p nds on th ra tio of sourc imp danc to r lay r ach (imp danc ) s tting as can b d duc d by r f r nc to Figur 11.13. It must b mphasis d that th appar nt xt nsion of a fully cross-polaris d imp danc charact ristic into th n gativ r actanc quadrants of Figur 11.13 do s not imply that th r would b op ration for r v rs faults. With cross-polarisa tion, th r lay charact ristic xpands to ncompass th origin of th imp danc diagram for forward faults only. For r v rs faults, th ff ct is to xclud th origin of th imp danc diagram, th r by nsuring prop r dir ctional r spons s for clos -up forward or r v rs faults. Fully cross-polaris d charact ristics h av now larg ly b n sup rs d d, du to th t nd ncy of comparators conn ct d to h althy phas s to op rat und r h avy fault conditions on anoth r phas . This i s of no cons qu nc in a switch d distanc r lay, wh r a singl comparator is c onn ct d to th corr ct fault loop imp danc by starting units b for m asur m n t b gins. How v r, mod rn r lays off r ind p nd nt imp danc m asur m nt for ac h of th thr arth-fault and thr phas -fault loops. For th s typ s of r lay , malop ration of h althy phas s is und sirabl , sp cially wh n singl pol trip ping is r quir d for singl -phas faults. 11.7.6 Partially Cross-Polaris d Mho C haract ristic Wh r a r liabl , ind p nd nt m thod of fault d phas s l ction is not provid d, a mod rn non-switch d distanc r lay may only mploy a r lativ ly small p rc ntag of cross polarisation. X ZS =25 ZL ZS 0 ZL R Figur 11.12: Fully cross-polaris d mho r lay charact ristic with variations of ZS/ZL ratio Distanc P rot ction Sourc ZS Positiv curr nt dir ction for r lay ZL R lay location Va1 ZS1 Ia1 ZS2 Va2 Mho unit charact ristic (not cross-polariz d) Ia2 ZL2 IF ZL1 F1 N1 E1 Shi ld-shap d charact ristic with 16% squar -wav cr cross-polarisation S lf-polaris d Mho circl X Fully cross-polaris d Mho circl c N2 F2 -R
Zn R Extra r sistiv cov rag of shi ld Conv ntional 16% partially cross-polaris d Mho circl • 11 • X S 2=ZL1+Zn1 Z Zn1 ZL1 R -X (a) Comparison of polaris d charact ristics drawn for S.I.R. = 6 X Zn2 30° ZS1 0 S 1=ZL1+Zn2 Z 1 6 12 24 60 R Mho unit charact ristic (fully cross-polariz d) -X (b) R sistiv xpansion of shap d partially cross-polaris d Mho with incr asing valu s of S.I.R. Figur 11.14: Partially cross-polaris d charact ristic with sh i ld shap Figur 11.13: Illustration of improv m nt in r lay r sistiv cov rag for fully crosspolaris d charact ristic • 180 • N twork Prot ction & Automation Guid
Th l v l s l ct d must b suffici nt to provid r liabl dir ctional control in th pr s nc of CVT transi nts for clos -up faults, and also attain r liabl fa ult d phas s l ction. By mploying only partial cross-polarisation, th disadva ntag s of th fully cross-polaris d charact ristic ar avoid d, whil still r ta ining th advantag s. Figur 11.14 shows a typical charact ristic that can b ob tain d using this t chniqu . 11.7.7 Quadrilat ral Charact ristic This form of po lygonal imp danc charact ristic is shown in Figur 11.15. Th charact ristic is provid d with forward r ach and r sistiv r ach s ttings that ar ind p nd ntly adjustabl . It th r for provid s b tt r r sistiv cov rag than any mho-typ c haract ristic for short lin s. This is sp cially tru for arth fault imp danc m asur m nt, wh r th arc r sistanc s and fault r sistanc to arth contribut to th high st valu s of fault r sistanc . To avoid xc ssiv rrors in th zon r ach accuracy, it is common to impos a maximum r sistiv r ach in t rms of t h zon imp danc r ach. R comm ndations in this r sp ct can usually b found in th appropriat r lay manuals. X impl m nting this charact ristic using discr t compon nt l ctrom chanical or arly static r lay t chnology do not aris . 11.7.8 Prot ction against Pow r Swing s – Us of th Ohm Charact ristic During s v r pow r swing conditions from which a syst m is unlik ly to r cov r, stability might only b r gain d if th swingin g sourc s ar s parat d. Wh r such sc narios ar id ntifi d, pow r swing, or ou t-ofst p, tripping prot ction can b d ploy d, to strat gically split a pow r sy st m at a pr f rr d location. Id ally, th split should b mad so that th plan t capacity and conn ct d loads on ith r sid of th split ar match d. This typ of disturbanc cannot normally b corr ctly id ntifi d by an ordinary distanc prot ction. As pr viously m ntion d, it is oft n n c ssary to pr v nt distanc prot ction sch m s from op rating during stabl or unstabl pow r swings, in ord r to avoid cascad tripping. To initiat syst m s paration for a prosp ctiv un stabl pow r swing, an out-of-st p tripping sch m mploying ohm imp danc m asu ring l m nts can b d ploy d. Ohm imp danc charact ristics ar appli d along t h forward and r v rs r sistanc ax s of th R/X diagram and th ir op rating bo undari s ar s t to b parall l to th prot ct d lin imp danc v ctor, as shown in Figur 11.16. Th ohm imp danc l m nts divid th R/X imp danc diagram in to thr zon s, A, B and C. As th imp danc chang s during a pow r swing, th p oint r pr s nting th imp danc mov s along th swing locus, nt ring th thr zon s in turn and causing th ohm units to op rat in s qu nc . Wh n th imp dan c nt rs th third zon th trip s qu nc is compl t d and th circuit br ak r trip coil can b n rgis d at a favourabl angl b tw n syst m sourc s for arc int rruption with littl risk of r striking. Locus of Zon 3 C Zon 2 B Zon 1 Zon s 1&2 A R Zon 3 RZ1 RZ2 RZ3 Figur 11.15: Quadrilat ral charact ristic • X H Zon C Lin imp danc Quadrilat ral l m nts with plain r actanc r ach lin s can introduc r ach rro r probl ms for r sistiv arth faults wh r th angl of total fault curr nt dif f rs from th angl of th curr nt m asur d by th r lay. This will b th cas wh r th local and r mot sourc voltag v ctors ar phas shift d with r sp ct to ach oth r du to pr -fault pow r flow. This can b ov rcom by s l cting an alt rnativ to us of a phas curr nt for polarisation of th r actanc r ach l
in . Polygonal imp danc charact ristics ar highly fl xibl in t rms of fault i mp danc cov rag for both phas and arth faults. For this r ason, most digital and num rical distanc r lays now off r this form of charact ristic. A furth r factor is that th additional cost implications of Zon B Zon A G R Out-of-st p tripping r lay charact ristic Figur 11.16: Application of out-of-st p tripping r lay charact ristic N twork Prot ction & Automation Guid • 181 • Distanc P rot ction 11 •
Only an unstabl pow r swing condition can caus th imp danc v ctor to mov su cc ssiv ly through th thr zon s. Th r for , oth r typ s of syst m disturbanc , such as pow r syst m fault conditions, will not r sult in r lay l m nt op rat ion. 11.7.9 Oth r Charact ristics Th x cution tim for th algorithm for tradi tional distanc prot ction using quadrilat ral or similar charact ristics may r sult in a r lativ ly long op ration tim , possibly up to 40ms in som r lay d si gns. To ov rcom this, som num rical distanc r lays also us alt rnativ algor ithms that can b x cut d significantly fast r. Th s algorithms ar bas d g n rally on d t cting chang s in curr nt and voltag that ar in xc ss of what is xp ct d, oft n known as th ‘D lta’ algorithm. This algorithm d t cts a fault by co mparing th m asur d valu s of curr nt and voltag with th valu s sampl d pr vi ously. If th chang b tw n th s sampl s xc ds a pr d fin d amount (th ‘d lta’) , it is assum d a fault has occurr d. In parall l, th distanc to fault is also comput d. Provid d th comput d distanc to fault li s within th Zon r ach of th r lay, a trip command is issu d. This algorithm can b x cut d significant ly fast r than th conv ntional distanc algorithm, r sulting in fast r ov rall tripping tim s. Fault d phas s l ction can b carri d out by comparing th sign s of th chang s in voltag and curr nt. R lays that us th ‘D lta’ algorithm g n r ally run both this and conv ntional distanc prot ction algorithms in parall l, as som typ s of fault ( .g. high-r sistanc faults) may not fall within th fau lt d t ction crit ria of th D lta algorithm. 11.8 DISTANCE RELAY IMPLEMENTATION Discriminating zon s of prot ction can b achi v d using distanc r lays, provi d d that fault distanc is a simpl function of imp danc . Whil this is tru in principl for transmission circuits, th imp danc s actually m asur d by a dist anc r lay also d p nd on th following factors: 1. th magnitud s of curr nt an d voltag (th r lay may not s all th curr nt that produc s th fault voltag ) 2. th fault imp danc loop b ing m asur d 3. th typ of fault 4. th fault r sistanc 5. th symm try of lin imp danc 6. th circuit configuration (singl , doubl or multit rminal circuit) It is impossibl to liminat all of th abov factors for all possibl op ratin g conditions. How v r, consid rabl succ ss can b achi v d with a suitabl dist anc r lay. This may compris r lay l m nts or algorithms for starting, distanc m asuring and for sch m logic. Th distanc m asur m nt l m nts may produc imp danc charact ristics s l ct d from thos d scrib d in S ction 11.7. Various distanc r lay formats xist, d p nding on th op rating sp d r quir d and cos t consid rations r lat d to th r laying hardwar , softwar or num rical r lay p roc ssing capacity r quir d. Th most common formats ar : a. a singl m asuring l m nt for ach phas is provid d, that cov rs all phas faults b. a mor cono mical arrang m nt is for ‘start r’ l m nts to d t ct which phas or phas s hav suf f r d a fault. Th start r l m nts switch a singl m asuring l m nt or algorit hm to m asur th most appropriat fault imp danc loop. This is commonly r f rr d to as a switch d distanc r lay c. a singl s t of imp danc m asuring l m n ts for ach imp danc loop may hav th ir r ach s ttings progr ssiv ly incr as d from on zon r ach s tting to anoth r. Th incr as occurs aft r zon tim d l ays that ar initiat d by op ration of start r l m nts. This typ of r lay is c ommonly r f rr d to as a r ach-st pp d distanc r lay d. ach zon may b provid d with ind p nd nt s ts of imp danc m asuring l m nts for ach imp danc loop . This is known as a full distanc sch m , capabl of off ring th high st p rfo rmanc in t rms of sp d and application fl xibility Furth rmor , prot ction aga inst arth faults may r quir diff r nt charact ristics and/or s ttings to thos r quir d for phas faults, r sulting in additional units b ing r quir d. A tota l of 18 imp danc -m asuring l m nts or algorithms would b r quir d in a full d istanc r lay for thr -zon prot ction for all typ s of fault. With l ctrom ch anical t chnology, ach of th m asuring l m nts would hav b n a s parat r l ay hous d in its own cas , so that th distanc r lay compris d a pan l-mount d ass mbly of th r quir d r lays with suitabl int r-unit wiring. Figur 11.17(a) shows an xampl of such a r lay sch m . Digital/num rical distanc r lays (Fig ur 11.17(b)) ar lik ly to hav all of th abov functions impl m nt d in softw ar . Start r units may not b n c ssary. Th compl t distanc r lay is hous d i
n a singl unit, making for significant conomi s in spac , wiring and incr as d d p ndability, through th incr as d availability that st ms from th provision of continuous s lf-sup rvision. Wh n N twork Prot ction & Automation Guid • Distanc P rot ction 11 • • 182 •
th additional f atur s d tail d in S ction 11.11 ar tak n into consid ration, such quipm nt off rs substantial us r b n fits.
conomy for oth r applications, only on m asuring l m nt was provid d, tog th r with ‘start r’ units that d t ct d which phas s w r fault d, in ord r to switch t h appropriat signals to th singl m asuring function. A distanc r lay using this t chniqu is known as a switch d distanc r lay. A numb r of diff r nt typ s of start rs hav b n us d, th most common b ing bas d on ov rcurr nt, und rv oltag or und r-imp danc m asur m nt. Num rical distanc r lays p rmit dir ct d t ction of th phas s involv d in a fault. This is call d fault d phas s l cti on, oft n abbr viat d to phas s l ction. S v ral t chniqu s ar availabl for f ault d phas s l ction, which th n p rmits th appropriat distanc -m asuring zo n to trip. Without phas s l ction, th r lay risks having ov r or und rr ach p robl ms, or tripping thr phas wh n singl -pol fault cl aranc is r quir d. S v ral t chniqu s ar availabl for fault d phas s l ction, such as: a. sup rimp os d curr nt comparisons, comparing th st p chang of l v l b tw n pr -fault l oad, and fault curr nt (th ‘D lta’ algorithm). This nabl s v ry fast d t ction of th fault d phas s, within only a f w sampl s of th analogu curr nt inputs b. chang in voltag magnitud c. chang in curr nt magnitud Num rical phas s l c tion is much fast r than traditional start r t chniqu s us d in l ctrom chanica l or static distanc r lays. It do s not impos a tim p nalty as th phas s l ction and m asuring zon algorithms run in parall l. It is possibl to build a f ullsch m r lay with th s num rical t chniqu s. Th phas s l ction algorithm p rovid s fault d phas s l ction, tog th r with a s gr gat d m asuring algorithm for ach phas -ground and phas to phas fault loop (AN, BN, CN, AB, BC, CA), th us nsuring fullsch m op ration. How v r, th r may b occasions wh r a num ri cal r lay that mimics arli r switch d distanc prot ction t chniqu s is d sir d . Th r asons may b conomic (l ss softwar r quir d – thus ch ap r than a r lay that contains a full-sch m impl m ntation) and/or t chnical. Figur 11.17 (a): First g n ration of static distanc r lay • Figur 11.17 (b): MiCOM P440 s ri s num rical distanc r lay 11.8.1 Start rs for switch d distanc prot ction El ctrom chanical and static di stanc r lays do not normally us an individual imp danc -m asuring l m nt p r phas . Th cost and th r sulting physical sch m siz mad this arrang m nt imp ractical, xc pt for th most d manding EHV transmission applications. To achi v Som applications may r quir th num rical r lay charact ristics to match thos of arli r g n rations alr ady install d on a n twork, to aid s l ctivity. Such r lays ar availabl , oft n with r fin m nts such as multi-sid d polygonal imp danc charact ristics that assist in avoiding tripping du to h avy load conditi ons. With l ctrom chanical or static switch d distanc r lays, a s l ction of a vailabl start rs oft n had to b mad . Th choic of start r was d p nd nt on p ow r syst m param t rs such as maximum load transf r in N twork Prot ction & Automation Guid • 183 • Distanc P rot ction 11 •
r lation to maximum r ach r quir d and pow r syst m arthing arrang m nts. Wh r ov rcurr nt start rs ar us d, car must b tak n to nsur that, with minimum g n rating plant in s rvic , th s tting of th ov rcurr nt start rs is s nsitiv nough to d t ct faults b yond th third zon . Furth rmor , th s start rs r q uir a high drop-off to pick-up ratio, to nsur that th y will drop off und r m aximum load conditions aft r a s cond or third zon fault has b n cl ar d by th first zon r lay in th faulty s ction. Without this f atur , indiscriminat t ripping may r sult for subs qu nt faults in th s cond or third zon . For satisf actory op ration of th ov rcurr nt start rs in a switch d distanc sch m , th following conditions must b fulfill d: a. th curr nt s tting of th ov rcurr n t start rs must b not l ss than 1.2 tim s th maximum full load curr nt of th prot ct d lin b. th pow r syst m minimum fault curr nt for a fault at th Zon 3 r ach of th distanc r lay must not b l ss than 1.5 tim s th s tting of th ov rcurr nt start rs On multipl - arth d syst ms wh r th n utrals of all th pow r transform rs ar solidly arth d, or in pow r syst ms wh r th fault cur r nt is l ss than th full load curr nt of th prot ct d lin , it is not possibl to us ov rcurr nt start rs. In th s circumstanc s und rimp danc start rs ar typically us d. Th typ of und r-imp danc start r us d is mainly d p nd nt o n th maximum xp ct d load curr nt and quival nt minimum load imp danc in r l ation to th r quir d r lay s tting to cov r faults in Zon 3. This is illustrat d in Figur 11.11 wh r ZD1, ZD2, and ZD3 ar r sp ctiv ly th minimum load imp danc s p rmitt d wh n l nticular, offs t mho and imp danc r lays ar us d. 11. 9 EFFECT OF SOURCE IMPEDANCE AND EARTHING METHODS For corr ct op ration, distanc r lays must b capabl of m asuring th distanc to th fault accurat ly. To nsur this, it is n c ssary to provid th corr ct m asur d quantiti s to th m asur m nt l m nts. It is not always th cas that us of th voltag and curr n t for a particular phas will giv th corr ct r sult, or that additional comp n sation is r quir d. 11.9.1 Phas Fault Imp danc M asur m nt Figur 11.18 shows th curr nt and voltag r lations for th diff r nt typ s of fault. If ZS1 and Z L1 ar th sourc and lin positiv s qu nc imp danc s, vi w d from th r layin g point, th curr nts and voltag s at this point for doubl phas faults ar d p nd nt on th sourc imp danc as w ll as th lin im p danc . Th r lationships ar giv n in Figur 11.19. Applying th diff r nc of th phas voltag s to th r lay liminat s th d p nd nc on ZS1. For xampl : V bc = a 2 − Z L1 I '1 V ' c = 2 2 − Z L1 I '1 ( ( )
( for 3 ph se f ults ) ( for dou le ph se f ults ) ) F
A B Ic I I C Vc V V V =0 Ic=0 I =0 ( ) Single ph se to e rth (A E) F A B Ic I C I V Vc V Dist nce P rotection V =V =Vc=0 I +I +Ic=0 ( ) Three ph se (A B C or A B C E) F A B Ic I C I V Vc V •
11 •
Vc=0 V =0 I =0 (c) Dou le ph se to e rth (B C E) F A B C Ic I I Vc V V I =0 V =Vc I = Ic (d) Dou le ph se (B C) Figure 11.18: Current nd volt ge rel t ionships for some shunt f ults • 184 • Network Protection & Autom tion Guide
F ult qu ntity I' I' I'c V' V' V'c Three ph se (A B C) I'1 2I'1 I'1 ZL1I'1 2ZL1I'1 ZL1I'1 Dou le ph se (B C) 0 ( 2 )I'1 ( 2)I'1 2(ZS1+ZL1)I'1 (2 2ZL1 ZS1)I'1 (2 ZL1 ZS 1)I'1 where I’a, I’b, I’c ar th phas curr nts at th r laying point. From th abov xpr ssions, th voltag at th r laying point can b xpr ss d in t rms of: 1. th p has curr nts at th r laying point, 2. th ratio of th transmission lin z ro s qu nc to positiv s qu nc imp danc , K, (=ZL0/ZL1), 3. th transmission lin positiv s qu nc imp danc ZL1: K −1 V ' = Z L1 I ' + ( I ' + I ' + I ' c ) 3 …Equ tion 11.5 Rel ying point F 1 0 0 Note: I'1 = 1 (I' + I' + 2I'c) 3 I' nd V' re t rel y loc tionFigure 11.19: P h se currents nd volt ges t rel ying point for 3 ph se nd dou le ph se f ults A B C Supply A B C
Dist nce me suring elements re usu lly c li r ted in terms of the positive sequ ence imped nce. Correct me surement for oth ph se ph se nd three ph se f ults is chieved y supplying e ch ph se ph se me suring element with its correspondi ng ph se ph se volt ge nd difference of ph se currents. Thus, for the B C eleme nt, the current me sured will e: I ' − I ' c = − I '1 I ' − I ' c = 2 − I ' 1 (K 1) Z where K= L0 Z= 1+ Z 3 L1 ZL1 ( ) System e rthed t one point only ehind he rel ying point Rel ying point A B C 1 1 1 F 2 1 1 Supply A B C 2 ( 2 )
( dou le ph se f ults ) nd the rel y will me sure ZL1 in e ch c se. 11.9.2 E rth F ult Imped nce Me sur ement When ph se e rth f ult occurs, the ph se e rth volt ge t the f ult loc tion is zero. It would ppe r th t the volt ge drop to the f ult is simply the p roduct of the ph se current nd line imped nce. However, the current in the f ul t loop depends on the num er of e rthing points, the method of e rthing nd sequ ence imped nces of the f ult loop. Unless these f ctors re t ken into ccount, the imped nce me surement will e incorrect. The volt ge drop to the f ult is th e sum of the sequence volt ge drops etween the rel ying point nd the f ult. Th e volt ge drop to the f ult nd current in the f ult loop re: V ' = I '1 Z L1 + I ' 2 Z L1 + I ' 0 Z L 0 I ' = I '1 + I ' 2 + I ' 0 nd the residu l curren t I’N at th r laying point is giv n by: I n = I a + I b + I c = 3 I 0 N twork Prot ction & Automation Guid Z= ZL1 (b) Syst m arth d at on point only in front of th r laying point R lay ing point 1 1 1 A B C
F 2 1 1 Supply A B C • Z=KZL1 (c) As for (b) but with r laying point at r c iving nd Figur 11.20: Eff ct of inf d and arthing arrang m nts on arth fault distanc m asur m nt Th voltag app aring at th r laying point, as pr viously m ntion d, vari s wit h th numb r of inf ds, th m thod of syst m arthing and th position of th r lay r lativ to th inf d and arthing points in th syst m. Figur 11.20 illu strat s th thr possibl arrang m nts that can occur in practic with a singl inf d. In Figur 11.20(a), th h althy phas curr nts ar z ro, so that th • 185 • Distanc P rot ction 11 • ( ) ( 3 - phas faults )
phas curr nts Ia, Ib and Ic hav a 1-0-0 patt rn. Th imp danc s n by a r lay comparing Ia and Va is: ( K − 1 ) Z Z = 1 + L1 3 3 I1 ( Z1 + Z N ) = I1 ( 2 Z1 + ZN ) ZN = …Equ tion 11.6 = ( Z0 Z 0 − Z1 3 3 Z1 − Z1 ) Z1 …Equ tion 11.7
In Figure 11.20( ), the currents entering the f ult from the rel y r nch h ve 2 1 1 distri ution, so: Z=ZL1 In Figure 11.20(c), the ph securrents h ve 1 1 1 distri ution, nd hence: Z=KZL1 If there were infeeds t oth ends of the lin e, the imped nce me sured would e superposition of ny two of the ove ex mp les, with the rel tive m gnitudes of the infeeds t ken into ccount. This n lys is shows th t the rel y c n only me sure n imped nce which is independent of in feed nd e rthing ( K − 1) rr ngements if proportion K N = of the 3 residu l cu rrent In=I +I +Ic is dded to the ph se current I . This technique is known s ‘re sidu l compens tion’. Most distanc r lays comp nsat for th arth fault conditio ns by using an additional r plica imp danc ZN within th m asuring circuits. Wh r as th phas r plica imp danc Z1 is f d with th phas curr nt at th r layi ng point, ZN is f d with th full r sidual curr nt. Th valu of ZN is adjust d so that for a fault at th r ach point, th sum of th voltag s d v lop d across Z1 and ZN quals th m asur d phas to n utral voltag in th fault d phas . Th r quir d s tting for ZN can b d t rmin d by consid ring an arth fault at th r ach point of th r lay. This is illustrat d with r f r nc to th A-N fault w ith singl arthing point b hind th r lay as in Figur 11.20(a). Voltag suppli d from th VT’s: = I1(Z1+Z2+Z0) = I1(2Z1+Z0) Voltag across th r plica imp danc s: = IaZ1+INZN = Ia(Z1+ZN) = 3I1(Z1+ZN) H nc , th r quir d s tting of ZN for ba lanc at th r ach point is giv n by quating th abov two xpr ssions: Z 0 − Z1 , e rth 3 f ult me suring elements will me sure the f ult imped nce corre ctly, irrespective of the num er of infeeds nd e rthing points on the system. W ith the replic imped nce set to11.10 DISTANCE RELAY APPLICATION PROBLEMS Dist nce rel ys my suffer from num er of difficulties in their pplic tion. M ny o f them h ve een overcome in the l test numeric l rel ys. Nevertheless, n w re ness of the pro lems is useful where protection engineer h s to de l with olde r rel ys th t re lre dy inst lled nd not due for repl cement. 11.10.1 Minimum Volt ge t Rel y Termin ls To tt in their cl imed ccur cy, dist nce rel ys th t do not employ volt ge memory techniques require minimum volt ge t the rel y termin ls under f ult conditions. This volt ge should e decl red in the d t sheet for the rel y. With knowledge of the sequence imped nces involved in the f the f ult MVA, the system volt ge nd the e rthing rr ng ult, or ltern tively ements, it is possi le to c lcul te the minimum volt ge t the rel y termin ls f or f ult t the re ch point of the rel y. It is then only necess ry to check t
h t the minimum volt ge for ccur te re chme surement c n e tt ined for giv en pplic tion. C re should e t ken th t oth ph se nd e rth f ults re consid ered. 11.10.2 Minimum Length of Line To determine the minimum length of line th t c n e protected y dist nce rel y, it is necess ry to check first th t ny minimum volt ge requirement of the rel y for f ult t the Zone 1 re ch is with in the decl red sensitivity for the rel y. Secondly, the ohmic imped nce of the line (referred if necess ry to VT/CT second ry side qu ntities) must f ll within the ohmic settingr nge for Zone 1 re ch of the rel y. For very short lines nd especi lly for c le circuits, it m y e found th t the circuit imped nce is le ss th n the minimum setting r nge of the rely. In such c ses, n ltern tive me thod of protection will e required. A suit le ltern tive might e current dif ferenti l Network Protection & Autom tion Guide • Dist nce P rotection 11 • • 186 •
protection, s the line length will pro ly e shortenough for the cost effect ive provision of high ndwidth communic tion link etween the rel ys fitted t the ends of the protected circuit. However, the l test numeric l dist nce rel ys h ve very wide r nge of imped nce setting r nges nd good sensitivity with low levels of rel ying volt ge, so such pro lems re now r rely encountered. App lic tion checks re still essenti l, though. When considering e rth f ults, p rt icul r c re must e t ken to ensure th t the ppropri te e rth f ult loop imped nce is used in the c lcul tion. 11.10.3 Under Re ch Effect of Remote Infeed A dist nce rel y is s id to under re ch when the imped nce presented to it is pp rently gre ter th n the imped nce to the f ult. Percent ge under re ch is define d s: ZR − ZF ×100% ZR where: ZR = intended rel y re ch (rel y re ch setting) ZF = e ffective re ch Them in c use ofunderre ching isthe effect of f ult current in feed t remote us rs. This is est illustr ted y n ex mple. ZA + So, for rel y l nce: IA + IB × x × ZC IA Z A + ZC = Z A + (I A + IB ) IA × x × ZC Therefore the effective re ch is
IA ZA +
ZC
IA + IB
...Equ tion 11.8 It is cle r from Equ tion 11.8 th t the rel y will underre ch. It is rel tively e sy to compens te for this y incre sing the re ch setting of the rel y, ut c re h s to e t ken. Should there e possi ility of the remote infeed eing red uced or zero, the rel y will then re ch further th n intended. For ex mple, sett ing Zone 2 to re ch specific dist nce into n dj cent line section under p r llel circuit conditions m y me n th t Zone 2 re ches eyond the Zone 1 re ch of the dj cent line protection under single circuit oper tion. If IB=9IA nd the r el y re ch is set to see f ults t F, then in the sence of the remote infeed, the rel y effective setting ecomes ZA+10ZC. 11.10.4 Over Re ch A dist nce rel y is s id to over re ch when the pp rent impe d nce presented to it is less th n the imped nce to the f ult. IB ZC IA+IB A IA ZA Z< Rel ying point Rel y setting: ZA+ZC Rel y ctu l re ch du e to p r llel lineinfeed: ZA+xZC Figure 11.21: Effect on dist nce rel ys of inf eed t the remote us r xZC F Percent ge over re ch is defined y the equ tion: ZF − ZR ×100% ZR where: ZR = rel y re ch setting ZF = effective re ch An ex mple of the over re ching effect is wh en dist nce rel ys re pplied on p r llel lines nd one line is t ken out of se rvice nd e rthed t e ch end. This is covered in Section 13.2.3. 11.10.5 Forw r d Re ch Limit tions There re limit tions on the m ximum forw rd re ch setting t h t c n e pplied to dist nce rel y. For ex mple, with reference to Figure 11 .6, Zone 2 of one line section should not re ch eyond the Zone 1 cover ge of Source ...Equ tion 11.9 •
In Figure 11.21, the rel y t A will not me sure the correct imped nce for f u lt on line section ZC due to current infeed IB. Consider rel y setting of ZA+Z C. For f ult t point F, the rel y is presented with n imped nce: Network Protection & Autom tion Guide • 187 • Dist nce P rotection 11 • C re should lso e t ken th t l rge forw rd re ch settings will not result in o per tion of he lthy ph se rel ys for reverse e rth f ults, see Section 11.10.5.
the next line section rel y. Where there is link etween the forw rd re ch set ting nd the rel y resistive cover ge (e.g. Mho Zone 3 element), rel y must not oper te under m ximum lo d conditions. Also, if the rel y rech is excessive , the he lthy ph se e rth f ult units ofsome rel y designs m y e prone to oper tion for he vy reverse f ults. This pro lem only ffected older rel ys pplied tothree termin l lines th t h ve signific nt line section length symmetry. A n um er of the fe tures offered with modern rel ys c n elimin te this pro lem. 11. 10.6 Power Swing Blocking Power swings re v ri tions in power flow th t occur w hen the intern l volt ges of gener tors t different points of the power system slip rel tive to e ch other. The ch nges in lo d flows th t occur s result of f ults nd their su sequent cle r nce re one c use of power swings. A power sw ing m y c use the imped nce presented to dist nce rel y to move w y from the norm l lo d re nd into the rel y ch r cteristic. In the c se of st le powe r swing it is especi lly import nt th t the distnce rel y should not trip in or der to llow the power system to return to st le conditions. For this re son, pplied to tr nsmission systems h ve power s mostdist nce protection schemes wing locking f cility v il le. Different rel ys m y use different principles for detection of power swing, ut ll involve recognising th t the movement of the me sured imped nce in rel tion to the rel y me surement ch r cteristics is t r te th t is signific ntly less th n the r te of ch nge th t occurs during f ult conditions. oper tion of the rel When the rel y detects such condition, y elements c n e locked. Power swing locking m y e pplied individu lly to e ch of the rel y zones, or on n ll zones pplied/inhi ited sis, depending on the p rticul r rel y used. V rious techniques re used in different rel y desig ns to inhi it power swing locking in the event of f ult occurring while pow er swing is in progress. This is p rticul rly import nt, for ex mple, to llow t he rel y to respond to f ult th t develops on line during the de d time of te cert in points on t single pole utoreclose cycle. Some Utilities m y design he network s split points, where the network should e split in the event of n unst le power swing or poleslipping occurring. A dedic ted power swing trippin tively, it g rely m y e employed for this purpose (see Section 11.7.8). Altern m y e possi le to chieve splitting y str tegiclly limiting the dur tion for which the oper tion specific dist nce rel y is locked during power swing con ditions. 11.10.7 Volt ge Tr nsformer Supervision Fuses or sensitive mini ture circuit re kers norm lly protect the second ry wiring etween the volt ge tr nsformer seco nd ry windings nd the rel y termin ls. Dist nce rel ys h ving: . self pol rise d offset ch r cteristics encomp ssing the zero imped nce point of the R/X di gr m . sound ph se pol ris tion c. volt ge memory pol ris tion m y m loper te if o ne or more volt ge inputs re removed due to oper tion of these devices. For the se types of dist nce of the volt ge inputs is recommended. Th rel y, supervision e supervision m y e provided y extern l me ns, e.g. sep r te volt ge supervisi on circuits, or it m y e incorpor ted into the dist nce rel y itself. On detect ion of VT f ilure, tripping of the dist nce rel y c n e inhi ited nd/or n l rm is given. Modern dist nce protection rel ys employ volt ge supervision th t o per tes from sequence volt ges nd currents. Zero or neg tive sequence volt ges nd corresponding zero or neg tive sequence currents re derived. Discrimin tion etween prim ry power system f ults nd wiring f ults or loss of supply due to individu l fuses lowing or MCB’s b ing op n d is obtain d by blocking th distanc prot ction only wh n z ro or n gativ s qu nc voltag is d t ct d without th pr s nc of z ro or n gativ s qu nc curr nt. This arrang m nt will not d t ct th simultan ous loss of all thr voltag s and additional d t ction is r quir d that op rat s for loss of voltag with no chang in curr nt, or a curr nt l ss than that corr sponding to th thr phas fault curr nt und r minimum fault in f d conditions. If fast-acting miniatur circuit br ak rs ar us d to prot ct t h VT s condary circuits, contacts from th s may b us d to inhibit op ration o f th distanc prot ction l m nts and pr v nt tripping. 11.11 OTHER DISTANCE RE LAY FEATURES A mod rn digital or num rical distanc r lay will oft n incorporat additional f atur s that assist th prot ction ngin r in providing a compr h
nsiv solution to th prot ction r quir m nts of a particular part of a n twork. Tabl 11.1 provid s an indication of th additional f atur s that may b provid d in such a r lay. Th combination of f atur s that ar actually provid d is ma nufactur r and r lay mod l d p nd nt, but it can b s n from th Tabl that st ady progr ssion is b ing mad towards a ‘on -box’ solution that incorporat s all th prot ction and control r quir m nts for a lin or cabl . How v r, at th high s t transmission voltag s, th l v l of d p ndability r quir d for rapid cl aranc of any prot ct d circuit fault will still d mand th us of two ind p nd nt pro t ction syst ms. N twork Prot ction & Automation Guid • Distanc P rot ction 11 • • 188 •
Fault Location (Distanc to fault) Instantan ous Ov rcurr nt Prot ction T ’d f d r prot ction Alt rnativ s tting groups CT sup rvision Ch ck synchronis r Autor clos CB stat monitoring CB condition monitoring CB control M asur m nt of vo ltag s, curr nts, tc. Ev nt R cord r Disturbanc R cord r CB failur d t ction/ logic Dir ctional/Non-dir ctional phas fault ov rcurr nt prot ction (backup to distanc prot ction) Dir ctional/Non-dir ctional arth fault ov rcurr nt prot ct ion (backup to distanc prot ction) N gativ s qu nc prot ction Und r/Ov rvolta g prot ction Stub-bus prot ction Brok n conductor d t ction Us r-programmabl s ch m logic Tabl 11.1: Additional f atur s in a distanc r lay R lay param t r ZL1 (mag) ZL1 (ang) ZLO (mag) ZLO (ang) KZO (mag) KZO (ang) Z1 ( mag) Z1 (ang) Z2 (mag) Z2 (ang) Z3 (mag) Z3 (ang) R1ph R2ph R3ph TZ1 TZ2 TZ3 R1G R2G R3G Param t r d scription Lin positiv s qu nc imp danc (magnitud ) Lin positiv s qu nc imp danc (phas angl ) Lin z ro s qu nc imp danc (magnitud ) Lin z ro s qu nc imp danc (phas angl ) D fault r sidual comp nsation factor (magn itud ) D fault r sidual comp nsation factor (phas angl ) Zon 1 r ach imp danc s tting (magnitud ) Zon 1 r ach imp danc s tting (phas angl ) Zon 2 r ach i mp danc s tting (magnitud ) Zon 2 r ach imp danc s tting (phas angl ) Zon 3 r ach imp danc s tting (magnitud ) Zon 3 r ach imp danc s tting (phas angl ) Phas fault r sistiv r ach valu - Zon 1 Phas fault r sistiv r ach valu Zon 2 Phas fault r sistiv r ach valu - Zon 3 Tim d lay - Zon 1 Tim d la y - Zon 2 Tim d lay - Zon 3 Ground fault r sistiv r ach valu - Zon 1 Groun d fault r sistiv r ach valu - Zon 2 Ground fault r sistiv r ach valu - Zon 3 Param t r valu 48.42 79.41 163.26 74.87 0.79 -6.5 38.74 80 62.95 80 83.27 80 78 78 78 0 0.35 0.8 104 104 104 Units Ω eg Ω eg eg Ω eg Ω eg Ω eg Ω Ω Ω s s s Ω Ω Ω Table 11.2: Distance relay parameters for example
11.12 DISTANCE RELAY APPLICATION EXAMPLE The system iagram shown in Figure 11.2 2 shows a simple 230kV network. The following example shows the calculations nec essary to apply three-zone istance protection to the line interconnecting subst ations ABC an XYZ. All relevant ata for this exercise are given in the iagram . The MiCOM P441 relay with qua rilateral characteristics is consi ere in this example. Relay parameters use in the example are liste in Table 11.2. Calculat ions are carrie out in terms of primary system impe ances in ohms, rather than the tra itional practice of using seconary impe ances. With numerical relays, w here the CT an VT ratios may be entere as parameters, the scaling between prim ary an secon ary ohms can be performe by the relay. This simplifies the exampl e by allowing calculations to be carrie out in ABC 1000/1A XYZ PQR primary quantities an eliminates consi erations of VT/CT ratios. For simplicity , it is assume that only a conventional 3zone istance is to be set protection an that there is no teleprotection scheme to be consi ere . In practice, a tele protection scheme woul normally be applie to a line at this voltage level. 11. 12.1 Line Impe ance The line impe ance is: ZL = (0.089 + j0.476) x 100 = 8.9 + j 47.6Ω = 48.42 79.41 Ω 0 •
Use values of 48.42Ω (magnitu e) an 800 settable values. 11.12 (angle) as nearest .2 Resi ual Compensation The relays use are calibrate in terms of thepositive sequence impe ance of the protecte line. Since the zero sequence impe ance of
the line between substations ABC an XYZ is ifferent from the positive sequence impe ance, the impe ance seen by the relay in the case of an earth fault, invol ving the passage of zero sequence current, will be ifferent to that seen for a phase fault. 60km 230kV 230kV 230kV/110V 230kV
Z< Source Impe ance: 5000MVA max = + Ω/km Ω/km Figure 11.22: Example network for istance relay setting calculation • 189 • Network Protection & Automation Gui e Distance P rotection 11 •
Hence, the earth fault reach of the relay requires zero sequencecompensation (s ee Section 11.9.2). For the relay use , this a justment is provi e by the resi ual (or neutral) compensation factor KZ0, set equal to: K Z0 = 11.12.5 Zone 3 Phase Reach 120% of the sum of the lines b Zone 3 is set to cover etween substations ABC an PQR,provi e this oes not result in any transformer s at substation XYZ being inclu e . It is assume that this constraint is met. H ence, Zone 3 reach: 48.42 79.41 o + = Ω 1.2 × 60 × 0.484 79.41 o = 83.27 tting of 83.27 80 0Ω, nearest available setting. 11.12.6 Zone Time Delay Settings Pr oper co-or ination of the istance relay settings with those of other relays is require . In epen ent timers are available for the three zones to ensure this. F or Zone 1, instantaneous tripping is normal. A time elay is use only in cases where large .c. offsets occur an ol circuit breakers, incapable of breaking t he instantaneous .c. component, are involve . The Zone 2 element has to gra e w ith the relays protecting the line between substations XYZ an PQR since the Zon e 2 element covers part of these lines. Assuming that thisline has istance, un it or instantaneous high-set overcurrent protection applie , the time elay requ ire is that to cover the total clearance time of the ownstream relays. To this must be a e the reset time for the Zone 2 element following clearance of a fa ult on the a jacent line, an a suitable safety margin. A typical time elay is 350ms, an the normal range is 200-500ms. The consi erationsfor the Zone 3 elem ent are the same as for the Zone 2 element, except that the ownstream fault cle arance time is that for the Zone 2 element of a istance relay or IDMT overcurre nt protection. Assuming istance relays are use , a typical time is 800ms. In su mmary: TZ1 = 0ms (instantaneous) TZ2 = 250ms TZ3 = 800ms 11.12.7 Phase Fault Res istive Reach Settings With the use of a qua rilateral characteristic,the resist ive reach settings for each zone can be set in epen ently of the impe ance reach settings. The resistive reach setting represents the maximum amount of a ition al fault resistance (in excess of the line impe ance) for which a zone will trip , regar less of the fault within the zone. ( Z0 − Z1 ) 3 Z1 K Z0 = ( Z0 − Z1 ) 3 Z1 For e ch of the tr nsmission lines: Z L1 = 0.089 + j 0.476 Ω 0.484 o ( ) = 0.426 + j1.576 Ω (1.632 K Z 0 = 0.792 K Z 0 = −6.5 o
79.41o Ω Z L0
74.87 Ω )
Hence, 11.12.3 Zone 1 Ph se Re ch Dist nce P rotection The required Zone 1 re ch is 80% of the line imped nce. Therefore, 0.8 × 48.42 79. 41 o = 38.74 79.41 o Ω Use 38.74 80° Ω nearest settable value. 11.12.4 Zone 2 Phase Reac h I eally, the requirements for setting Zone 2 reach are: 1. at least 120% of th
e protecte line 2. less than the protecte line + 50% of the next line Sometime s, the two requirements are in conflict. In this case, both requirements can be met. A setting of the whole of the line between substations ABC an XYZ, plus 50 % of the a jacent line section to substation PQR is use . Hence, Zone 2 reach: 4 8.42 79.41 o + = Ω 0.5 × 60 × 0.089 + j 0.476 = 62.95 79.41 Ω Use 62.95 80 0 Ω ne le setting. o ( ) • 11 • • 190 •
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Two constraints are impose upon the settings, as follows: i. it must be greater than the maximum expecte phase-phase fault resistance (principallythat of the fault arc) ii. it must be less than the apparent resistance measure ue to the heaviest loa on the line The minimum fault current at Substation ABC is of the or er of 1.8kA, lea ing to a typical arc resistance Rarc using the van Warringt on formula (Equation 11.4)of 8Ω. Using the current transformer ratio as a gui e t o the maximum expecte loa current, the minimum loa impe ance Zlmin will be 13 0Ω. Typically, the resistive reaches will be set to avoi the minimum loa impe an ce by a 40% margin for the phase elements, lea ing to a maximum resistivereach setting of 78Ω. Therefore, the resistive reach setting lies between 8Ω an 78Ω. Allowa nce shoul be ma e for the effects of any remote fault infee , by using the maxi mum resistive reach possible. While each zone can have its own resistive reach s etting, for thissimple example they can all be set equal. This nee not always be the case, it epen s on the particular istance protection scheme use an th e nee to inclu e Power Swing Blocking. 11.13 REFERENCES 11.1Protective Relays – their Theory an Practice. A.R. van C. W arrington. Chapman an Hall, 1962. R3ph = 78Ω R2ph = 78Ω R1ph = 78Ω 11.12.8 Earth Fault Impe ance Reach Settings By efau lt, the resi ual compensation factor as calculate in Section 11.12.2is use to a just the phase fault reach setting in the case of earth faults, an is applie Earth FaultResistive Reach Settings The margin for avoi to all zones. 11.12.9 ing the minimum loa impe ance nee only be 20%. Hence the settings are: R3G = 104Ω R2G = 104Ω R1G = 104ΩThis completes the setting of the relay. Table 11.2 also sh ows the settings calculate . •
Network Protection & Automation Gui e • 191 • Distance P rotection 11 •
Suitable settings are chosen to be 80% of the loa resistance:
• 12 • Distance Protection Schemes Intro uction Zone 1 extension scheme Transfer trip schemes Blocking scheme Direc tional comparison unblocking scheme Comparison of transfer trip an blocking rel aying schemes 12.1 12.2 12.3 12.4 12.5 12.6
• 12 • Distance P rotection Schemes 12.1 INTRODUCTION Conventional time-steppe istance protection is illustrate i n Figure 12.1. One of the main isa vantages of this scheme is that the instanta neous Zone 1 protection at each en of the protecte line cannot be set to cover thewhole of the fee er length an is usually setto about 80%. This leaves two en zones , each being about 20% of the protecte fee er length. Faults in the se zones are cleare in Zone 1 time by the protection at one en of the fee er a n in Zone 2 time (typically 0.25 to 0.4 secon s) by the protection at the other en of the fee er. Relay A en zone Z2A Time Z3G 0 A Z1A F B C Z1B Z3B B
Relay B en zone (a) Steppe time/ istance characteristics Z1 Z2 Z3 Z2T 0 Z3 0 ( b) Trip circuit (soli state logic) Figure 12.1: Conventional istance scheme ≥1 Trip
This situation cannot be tolerate in some applications, for two main reasons: a . faults remaining on the fee er for Zone 2 time may cause the system to become unstable b. where high-spee auto-reclosing is use , the nonsimultaneous opening of thecircuit breakers at both en s of the faulte section resultsin no ea time uring the auto-reclose cycle for the fault to be extinguishe an for io nise gases to clear. This results in the possibility that atransient fault wil l cause permanent lockout of the circuit breakers at each en of the line sectio n Network Protection & Automation Gui e • 193 •
Even where instability oes not occur, the increase uration of the isturbance may give rise to power quality problems, an may result in increase plant ama ge. Unit schemes of protection that compare the con itions at the two en s of th e fee er simultaneously positively i entify whether the fault is internal or ext ernal to the protecte f section an provi e high-spee protection for the whole ee er length. This a vantage is balance by the fact that the unit scheme oes n ot provi e the back up protection for a jacent fee ers given by a istance schem e. The most esirable scheme is obviously a combination of the best features of both arrangements, that is, instantaneous tripping over the whole fee er length plus back-up protection to a jacent fee ers. This can be achieve byinterconnec ting the istance protection relays at each en ofthe protecte fee er by a com munications channel. Communication techniques are escribe in etail in Chapter 8. The purpose channel is to transmit information about t of the communications he system con itions from one en of the protecte line to the other, inclu ing requests to initiate or prevent tripping of the remote circuit breaker. The form er arrangement is generally known as a transfer tripping scheme while the latt er is generally known as a blocking scheme . However, the terminology of the va rious schemes varies wi ely, accor ing to local custom an practice. 12.2 ZONE 1 EXTENSION SCHEME (Z1X SCHEME) This scheme is inten e for use with an auto-recl ose facility, or where no communications or the channel ha channel is available, sfaile . Thus it may be use on ra ial istribution fee ers, or on interconnect e lines as a fallback when no communications channel is available, e.g. ue to maintenance or temporary fault. The scheme is shown in Figure 12.2. The Zone 1 e lements of the istance relay have twosettings. One is set to cover 80% of the protecte line length as in the basic istance scheme.The other, known as Exte n e Zone 1or ‘Z1X’, is set to overreach the protecte line, a setting of 120% of t he protecte line being common. The Zone 1 reach is normally controlle by the Z 1X setting an is reset to the basic Zone 1 setting when a comman from the auto -reclose relay is receive . Z1A A Z1extB Z2B Z3B Z1B Z2A Z1extA B Z3A C (a) Distance/time characteristics Auto-reclose Reset Zone 1ext Zone 1ext ≥1 Zone 1 Zone 2 Zone 3 Z2T O Z3T O & ≥1 Trip
(b) Simplifie logic Figure 12.2: Zone 1 extension scheme • 12 • On occurrence of a fault at any point within the Z1X reach, the relay operates i n Zone 1 time, trips the circuit breaker an initiates auto-reclosure. The Zone 1 reach of the istance relay is also resetto the basic value of 80%, prior to the auto-reclose closing pulse being applie to the breaker. This shoul also oc
cur when the autoreclose facility is out of service. Reversion to theZ1X reach setting occurs only at theen of the reclaim time. For interconnecte lines, th e Z1X scheme is establishe (automatically or manually) upon loss of the communi cations channel by selection of the appropriate relay setting (setting group in a numerical relay). If the fault is transient, the trippe circuit breakers will reclose successfully, but otherwise further tripping uring the reclaim time is subject to the iscrimination obtaine with normal Zone 1 an Zone 2 settings. The isa vantage of the Zone 1 extension scheme is that external faults within t he Z1X reach of the relay result in tripping of circuit breakers external to the faulte section, increasing the amount of breaker maintenance nee e an nee le ss transient loss of supply to some consumers. This is illustrate in Figure 12. 3(a) for a single circuit line where three circuit breakers operate an in Figur e 12.3(b) for a ouble circuit line, where five circuit breakers operate. Distance P rotection Schemes • 194 •
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Z1A A Z1extA B C
Z1extB1 Breakers marke thus auto-reclose Z1B1 Z1extC Z1B2 Z1extB2 Z1C
A contact operate by the Zone 1 relay element is arrange to sen a signal to t he remote relay requesting a trip. The scheme may be calle a irect un er-reac h transfer tripping scheme’, ‘transfer trip un er-reaching scheme , or‘intertripping un er-reach istance protection scheme’, asthe Zone 1 relay elements o not cover the whole of the line. A fault F in theen zone at en B in Figure 12.1(a) res ults in operation of the Zone 1 relay an tripping ofthe circuit breaker at en B. A request to trip is also sent to the relay at en A. The receipt of a signa l at the receive relay contact is conne A initiates tripping imme iatelybecause cte irectly to the trip relay. The isa vantage of this scheme is the possibil ity of un esire tripping by acci ental operation or maloperation of signalling equipment, orinterference on the communications channel. As a result, it is not commonly use . 12.3.2 Permissive Un er-reach Transfer Tripping (PUP) Scheme (a) Fault within Zone 1 extension reach of istance relays (single circuit lines ) Z1A A Z1extA B Z1extB Z1B C Z1extD Z1C Z1D Z1extC D Z1P P Z1extN Z1N N Z1extP L M Z1M Z1extM Z1extL Z1L
(b) Fault within Zone 1 extension reach of istance relays ( ouble circuit lines ) Figure 12.3: Performance of Zone 1 extension scheme in conjunction with auto-rec lose relays 12.3 are available, as escr TRANSFER TRIPPING SCHEMES A number of these schemes ibe below. Selection of epen s on the requirements of th an appropriate scheme e system being protecte . 12.3.1 Direct Un er-reach Transfer Tripping Scheme The simplest way of re ucing the fault clearance time at the terminal that clears a n en zone fault in Zone 2 time is to a opt a irect transfer trip or intertrip technique,the logic of which is shown in Figure 12.4. Signal sen Z1 Z2 Z2T O Signal sen Z1 Z2 Z3 Signal receive 0 T Z2T 0 Z3T 0 ≥1 Trip •
& (a) Signal logic Distance relay Signal sen Signal receive Signal sen ≥1 Z3 Signal receive Z3T O Trip
Receive circuit (f1) Signalling equipment -En A Receive Signal circuit receive (f1) Signalling equipment -En B (b) Signalling arrangement Figure 12.5: Permissive un er-reach transfer tripping scheme Figure 12.4: Logic for irect un er-reach transfer tripping scheme Network Protection & Automation Gui e • 195 • Distance relay Sen circuit (f1) Sen circuit (f1) Distance P rotection Schemes 12 • The irect un er-reach transfer tripping scheme escribe above is ma e more sec ure by supervising the receive signal with the operation of the Zone 2 relay el ement before allowing an instantaneous trip, as shown in Figure 12.5. The scheme is then known as a permissive un er-reach transfer tripping scheme (sometimes abbreviate as PUP Z2 scheme) or ‘permissive un er-reach istance protection’, as b oth relays must etect a fault before the remote en relay is permitte to trip in Zone 1 time.
A variant of this scheme, foun on some 3 elemen relays, allows tripping byZone t operation as well asZone 2, provi e the fault is in the forwar irection. T his is sometimes calle the PUP-Fw scheme. Time elaye resetting of the signa l receive element is require to ensurethat the relays at both en s of a sing le-en fe faulte line of a parallel fee er circuit have time to trip when the fault is close to one en . Consi er a fault F in a ouble circuit line, asshown inFigure 12.6. The fault is close to en A, so there is negligible infee from en B when the fault at Foccurs. The protection at B etects a Zone 2 fault on ly after the breaker at en A has trippe . It is possible for the Zone 1 element at A toreset, thus removing the permissive signal to B an causing the signal receive element at B to reset before the Zone 2 unit at en B operates. It is therefore necessary to elay the resetting of the signal receive element to ensure high spee tripping at en B. A B relays that share the same measuring elements for both Zone 1 an Zone 2. In the se relays, the reach of the measuring elements is exten e from Zone 1 to Zone 2 by means of arange change signal imme iately, instea of after Zone 2 time. It is also calle an ‘accelerate un erreach istance protection scheme’. The un er-re aching Zone 1 unit is arrange to sen a signal to the remote en of the fee er in a ition to tripping the local circuit breaker. The receive relay contact is arrange to exten the reach of the measuring element from Zone 1 to Zone 2. Thi s accelerates the fault clearance at the remote en for faults that lie in the r egion the Zone 1an Zone 2 reaches. The scheme is shown in Figure 12.7. between Mo ern istance relays o not employ switche measuring elements, so the scheme is likely to fall into isuse. Z3A Z1A Z2A B Z1B (a) Distance/time characteristics C F A Distance P rotection Schemes Z2B Z3B Z1 & Z2
(a) Fault occurs-bus bar voltage low so negligible fault current via en B A Ope n F B Z3 Z3T O ≥1 Trip Z2T O ≥1 Range change signal
Signal receive & (b) Signal logic Signal sen (b) En A relay clears fault an current starts fee ing from en B • 12 •
Figure 12.7: Permissive un er-reaching acceleration scheme Figure 12.6: PUP scheme: Single-en fe close-up fault on ouble circuit line 12.3.4 Permissive Over-Reach Transfer Tripping (POP) Scheme In this scheme, a i stance relayelement set to reach beyon the remote en of the protecte line is use to sen an intertripping signal to theremoteen . However, it is essentia l that the receive relay contact is monitore by a irectional relay contact to ensure that tripping oes not take place unless the fault is within the protecte section; see Figure 12.8. The contacts of theZone 2 unit are ar instantaneous range to sen the signal, an the receive signal, supervise by Zone 2 operati on, is use to energise the trip circuit. The scheme is then known as a permiss ive over-reach transfer tripping scheme (sometimes abbreviate to ‘POP’), irectio nal comparison scheme , or ‘permissive overreach istance protection scheme’. Network Protection & Automation Gui e The PUP schemes requireonly a single communications channelfor two-way signall ing between the line en s, as the channel iskeye by the un er-reaching Zone 1 elements. When the circuit breaker at one en is open, or there is a weak infee such that the relevant relay element oes not operate, instantaneous clearance cannot be achieve for en-zone faults near the breaker open terminal unless s pecial features are inclu e , as etaile in section 12.3.5. 12.3.3 Permissive U n er-reaching Acceleration Scheme This scheme is applicable only to zone switche istance • 196 •
Signal sen Z1 Z1 Z2 Z2T O Z3T O ≥1 Z2 Z3 Z2T O Trip Z3 Z3T O ≥1 Trip Signal receive &
tp t & Signal receive & (a) Signal logic & Distance relay Signal sen Distance relay Signal sen Signal receive Sen f1 circuit (f1) Receive circuit (f2) f2 f2 Sen circuit (f2) f1 Receive circuit (f1) Signal sen Signal receive Figure 12.9: Current reversal guar logic – permissive over-reach scheme Signalling equipment -En A Signalling equipment -En B (b) Signalling arrangement Figure 12.8: Permissive over-reach transfer tripping scheme The above scheme using Zone heme. An alternative exists s is referre to as the POP If istance relays with mho
2 relay elements is often referre to as a POP Z2 sc that uses Zone 1 elements instea of Zone 2, an thi Z1 scheme. 12.3.5 Weak Infee Con itions characteristics are use , the scheme may be more a v
antageous than the permissive un er-reaching scheme for protecting short lines, because the resistive coverage of the Zone 2 unit maybe greater than that of Zo ne 1. To prevent operation un er current reversal con itions in a parallel fee e r circuit, it is necessary to use a current reversal guar timer to inhibit the tripping of the forwar Zone 2 elements. Otherwise maloperation of the scheme ma y occur un er current reversal con itions, see Section 11.9.9 for more etails. It is necessary only when the Zone 2 reach is set greater than 150% ofthe prote cte line impe ance. The timer is use to block the permissive trip an signal s en circuits as shown in Figure 12.9. The timer is energise if a signal is rece ive an there is no operation of Zone 2 elements. An a justable time elay on p ick-up (tp) is usually set to allow instantaneous tripping to take place for any internal faults, takinginto of Zone 2. account a possible slower operation The timer will have operate an blocke the ‘permissive trip’ an ‘signal sen ’ circuits b y the time the current reversal takes place.The timer is e-energise if the Zo ne 2 elements operate or the signal receive element resets. The reset time e lay (t ) of the timer is set to cover any overlap in time cause by Zone 2 eleme nts operatingan the signal resetting at the remote en , when the current in th ehealthy fee er reverses. Using a timer in this manner means that no extra time elay is a e in the permissive trip circuit for an internal fault. The Weak Infee Echo feature available in some protection relays allows the remo te relay to echo the trip signal back to the sen ing relay even if the appropria te remote relay element has not operate . This caters for con itions of the remo te en having a weak infee or circuit breaker open con ition, so that the relev ant remote relay element oes not operate. Fast clearance for these faults is no w obtaine at both en s of the line. The logic is shown in Figure 12.10. A time elay (T1) is require in the echo circuit to prevent tripping of the remote en breaker when the local breaker is trippe by the busbar protection or breaker f ail protection associate with other fee ers connecte to the busbar. The time elay ensures that the remote en Zone 2 element will reset by the time the echoe signal is receive at that en . From POP signal sen logic (Figure 12.8) To POP trip logic (Figure 12.8) Bre aker open Signal receive Figure 12.10: Weak Infee Echo logic circuit T1 0 • & T2 0 & ≥1
Signal sen Network Protection & Automation Gui e • 197 • Distance P rotection Schemes 12 •
Since the signalling channel is keye by over-reaching Zone 2 elements, the sche me requires uplex communication channels - one frequency for each irection of signalling.
In the stan ar permissive over-reach scheme, as with the permissive un er-reach scheme,instantaneous clearance cannot be achieve foren -zone faults un er we ak infee or breaker open con itions. To overcome this isa vantage, two possibi lities exist.
Signal transmission can take place even after the remote en breaker has trippe . This gives rise to the possibility of continuous signal transmission ue to lo ck-up of both signals. Timer T2 is use to prevent this. After this time elay, signal sen is blocke . A variation on the Weak Infee Echo feature is to allo w tripping of the remote relay un er the circumstances escribe above, provi in g that an un ervoltage con ition exists, ue to the fault. This is known as the Weak Infee Trip feature an ensures that both en s are trippe if thecon ition s are satisfie . 12.4 BLOCKING OVER-REACHING SCHEMES The arrangements escribe so far have use the signalling channel(s) to transmit atripping instruction. I f the signalling channel fails or there is no Weak Infee feature provi e , en zone faults maytake longer to becleare . Blocking over-reaching schemes use an over-reaching istance scheme an inverse logic. Signalling is initiate only f or external faults an signalling transmission takes place over healthy line sec tions. Fastfault clearance occurs when no signal is receive an the over-reach ing Zone 2 istance measuring elements looking into the line operate.The signal ling channel is keye byreverselooking istance elements (Z3 in the iagram, th ough which zone is use epen s on the particular relay use ). An i eal blocking scheme is shown in Figure 12.11. The single frequency signalling channel receive r operatesboth local an remote elays when a block signal is initiate at any en of the protecte section. 12.4 .1 Practical Blocking Schemes A blocking instruction has to be sent by the rever selooking relay elements to prevent instantaneous tripping of the remote relay f or Zone 2 faults external to the protecte section. To achieve this, the reverse -looking elements an the signalling channel must operate faster than the forwar -looking elements. In practice, this is selom the case an to ensure iscrimin ation, a short time elay is generally intro uce into the blocking mo e trip ci rcuit. Either the Zone 2 or Zone 1 element can be use as the forwar -looking el ement, giving rise to two variants of the scheme. 12.4.1.1 Blocking over-reachin g protection scheme using Zone 2 element This scheme (sometimes abbreviate to B OP Z2) is base on the i eal blocking scheme of Figure 12.11, but has the signal logic illustrate in Figure 12.12. It is also known as a ‘ irectional comparison blocking scheme’ or a ‘blocking over-reach istance protection scheme’. Z1 Z2 Z2T O Distance P rotection Schemes Signal sen Z3A Z1A A F1
Z2A F2 Z1B Z3B Signal sen B C Z3 Z3T O ≥1 Trip STL O &
Z2B Signal receive O t (a) Distance/time characteristics • 12 • Z1 Z2 Z3 Z2T O Channel in service Figure 12.12: Signal logic for BOP Z2 scheme Z3T O ≥1 Trip Signal receive
& (b) Simplifie logic Operation of the scheme can be un erstoo by consi ering the faults shown at F1, F2 an F3 in Figure 12.11 along with the signal logic 12.12. A fault of Figure at F1 is seen by the Zone 1 relay elements atboth en s A an B; as a result, th e fault is cleare instantaneously at both en s of the protecte line. Signallin g is controlle by the Z3 elements looking away from the protecte section, so n o transmission takes place, thus giving fast tripping via the forwar -looking Zo ne 1 elements. A fault at F2 is seen by the forwar -looking Zone 2 elements at e n s A an B an by the Zone 1 elements at Distance relay Signal sen Signal receive Signal sen Signal receive Receive circuit (f1) Receive circuit (f1)
Signalling equipment -En A Signalling equipment -En B (c) Signalling arrangement Figure 12.11: I eal istance protection blocking scheme Distance relay Sen circuit (f1)
Sen circuit (f1) • 198 •
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en B. No signal transmission takes place,since the fault is internal an the f ault is cleare in Zone 1 time at en B an after the short time lag (STL) at en A. A fault at F3 is seen by the reverse-looking Z3 elements at en B an the f orwar looking Zone 2 elements aten A. The Zone 1 relay elements at en B asso ciate with line section B-C woul normally clear the fault at F3. To prevent th e Z2 elements at en A from tripping, the reverse-looking Zone3 elements at en B sen a blocking signal to en A. If the fault is not cleare instantaneously by the protection on line section B-C, the trip signal will be given at en B fo r section A-B after the Z3 time elay. The setting of the reverse-looking Zone 3 elements must be greater than that of the Zone 2 elements at the remote en of the fee er, otherwise there is the possibility of Zone 2 elements initiating tri pping an the reverse looking Zone 3 elements failing to see an external fault. This woul result in instantaneous tripping for an external fault. When the sign alling channel is use for a stabilising signal, as in the above case, transmiss ion takes place over a healthy line section if power line carrier is use . The s ignalling channelshoul then be more reliable when use in the blocking mo e th an in tripping mo e. It that the operating times of the various rel is essential ays be skilfully co-or inate for all system con itions, so that sufficient time is always allowe for the receipt of a blocking signal from the remote en of t he fee er. If this is not one accurately, the scheme may trip for an external f ault or alternatively, the en zone tripping times may be elaye longer than is necessary. If the signalling channel fails, the scheme must be arrange to reve rt to conventional basic istance protection. Normally, the blocking mo e tripc ircuit is supervise by a channel-in-service contact so that the blocking mo e trip circuit is isolate when the channel is out of service, as shown in Figure 12.12. Z3G Z2G Z1G G Z1H Z2H Z3H (a) Distance/time characteristics Z3 Z2 & H In a practical application, the reverse-looking relay elements may be set with a forwar offset characteristic to provi e back-up protection for busbar faults a fter the zone time elay. It is then necessary to stop the blocking signal being sent for internal faults. This is achieve by making the ‘signal sen ’ circuit con itional upon nonoperation of the forwar -looking Zone 2 elements, as shown in Fi gure 12.13. Blocking schemes, like the permissiveover-reach scheme, are also af fecte by the current reversal in thehealthy fee er ue to a fault in a ouble circuit line. If current reversal con itions occur, as escribe in section 11.9 .9, it may be possible for the maloperation of a breaker on the healthy line to occur. To avoi this, theresetting of the ‘signal receive ’ element provi e in the blocking scheme is time elaye . The timer with elaye resetting (t ) is set t o cover the time ifference between the maximum resetting time of reverse-lookin g Zone 3 elements an the signalling channel. So, if there isa momentary loss o f the blocking signal uring the timer oes not have time the current reversal, to reset in the blocking mo e trip circuit an no false tripping takes place. 12 .4.1.2 Blocking over-reaching protection scheme using Zone 1 element This is sim ilar to the BOP Z2 scheme escribe above, except that an over-reaching Zone 1 e lement is use in the logic, instea of the Zone 2 element. It may also be known as theBOP Z1 scheme. 12.4.2 Weak Infee Con itions The protection at the stron g infee terminal will operate for all internal faults, since a blocking signal is not receive from the weak infee terminal en . In the case of externalfault s behin the weak infee terminal, the reverse-looking elements at that en will see the fault current fe from the strong infee terminal an operate, initiati ng a block signal to the remote en . The relay at the strong infee en operates correctly without the nee for any a itional circuits. The relay at the weak i nfee en cannot operate for internal faults, an so tripping of that breaker is possible only by means of irect intertripping from the strong source en . 12.5 DIRECTIONAL COMPARISON UNBLOCKING SCHEME •
Signal sen
(b) Soli state logic of sen circuit Figure 12.13: Blocking scheme using reverselooking relays with offset The permissive to over-reach scheme escribe in Section 12.3.4 can be arrange operate on a irectional comparison unblocking principle by provi ing a itional circuitryin the signalling equipment. In this scheme (also calle a ’ eblocking overreach istance protection Network Protection & Automation Gui e • 199 • Distance P rotection Schemes 12 •
scheme’), a continuous block (or guar ) signal is transmitte . When the over-reach ing istance elements operate, the frequency of the signal transmitte is shifte to an unblock (trip) frequency. The receipt of the unblock frequency signal an the operation of overreaching istance elements allow fast tripping to occur for faults within the protecte zone. In principle, the scheme is similar to th e permissive over-reach scheme. The scheme is ma e more epen able than the stan ar permissive over-reach scheme by provi ing a itional circuits in the receiv er equipment. for internal faults even ifthe These allow tripping to take place transmitte unblock signal is short-circuite by the fault. This is achieve by allowing ai e tripping for a short time interval, typically 100 to 150 millise con s, after the loss of both the block an the unblock frequency signals. After this time interval, ai e tripping is permitte only if theunblock frequency s ignal is receive . This arrangement gives the scheme improve security over a bl ocking scheme, since tripping for external faults is possible only if the fault occurs within the above time interval of channel failure. Weak Infee terminal c on itions can be catere for by the techniques etaile in Section 12.3.5. In th is way, the scheme has the epen ability of a blocking scheme an the security o f a permissive overreach scheme. This scheme is generally preferre when power l ine carrier is use , except when continuous transmission of signal is not accept able. 12.6 COMPARISON OF TRANSFER TRIP AND BLOCKING RELAYING SCHEMES On normal t wo-terminal lines the main eci ing factors in the choice of the type of scheme, apart from the of reliability the signalling channel previously iscusse , are operating spee an the metho of operation of the system. Table 12.1 compares t he important characteristics of the various types of scheme. Criterion Spee of operation Spee with in-service testing Suitable for auto-rec lose Securityagainst maloperation ue to: Current reversal Loss of communicatio ns Weak Infee /Open CB Transfer tripping scheme Fast Slower Yes Blocking scheme Not as fast As fast Yes • Distance P rotection Schemes 12 • Special features require Poor Special features require Special features require Goo Special features require Table 12.1: Comparison of ifferent istance protection schemes Mo ern igital or numerical istance relays are provi e with a choice ofsevera l schemes in the same relay. Thus scheme selection is now largely in epen ent of relay selection, an the user is assure that a relay is available with all the require features to cope with changing system con itions. • 200 • Network Protection & Automation Gui e
• 13 • Protection of Complex Transmission Circuits Intro uction Parallel fee ers Multi-en Multi-en e fe e fee ers – unit protection e ers – istance protection Multi-en e fee ers application of istance protection schemes Protection of series compensate lines Examples References 13.1 13.2 13 .3 13.4 13.5 13.6 13.7 13.8
• 13 • P rotection of Complex Transmission Circuits 13.1 INTRODUCTION Chapters 10-12 have covere the basic principles of protection for twoterminal, single circuit lines whose circuit impe ance is ue solely to the con uctors use . However parallel transmission circuits are often installe , either as uplicate circuits on a common structure, or as separate lines conne cting the same two terminal points via ifferent routes. Also, circuits may be m ulti-en e , a three-en e circuit being the most common. For economic reasons, t ransmission an istribution lines can be much more complicate , maybe having th ree or more terminals (multi-en e fee er), or with more than one circuit carrie on a common structure (parallel fee ers), as shown in Figure 13.1. Other possi bilities are the use of series capacitors or irectconnecte shunt reactors. The protection of such lines is more complicate an requires the basic schemes es cribe in the above chapters to be mo ifie . The purpose of this chapter is to e xplain thespecial requirements of some of these situations in respect of protec tion an i entify which protection schemes are particularly appropriate for use in these situations. Bus C Source Source Bus A Bus B
Figure 13.1: Parallel an Multi-en e fee ers 13.2 PARALLEL FEEDERS If two overhea lines are supporte on the same structures or are otherwise in close proximity over part Network Protection & Automation Gui e • 203 •
or whole of their length, there is a mutual coupling between the two circuits. T he positive an negative sequence coupling between the two circuits is small an is usually neglecte . The zero sequence coupling can be strong an its effect c annot be ignore . The other situation that requires mutual effects to be taken i nto account is when there is an earth fault on a fee er when the parallel fee er is out of service an earthe at both en s. An earth fault in the fee er that i s in service can in uce current in the earth loop of the earthe fee er, causing a mislea ing mutual compensation signal. 13.2.1 Unit Protection Systems Types o f protection that use current only, for example unit protection systems, are not affecte by the coupling between the fee ers. Therefore, compensation for the e ffects of mutual coupling is not require for the relay tripping elements. If th e relay has a istance-to-fault feature, mutual compensation is require for an accurate measurement. Refer to Section 13.2.2.3 for how this is achieve . 13.2.2 Distance Protection There are a number of problems applicable to istance relay s, as escribe in the following sections. 13.2.2.1 Current reversal on ouble c ircuit lines When a fault is cleare sequentially on one circuit of a ouble cir cuit line with generation sources at both en s of the circuit, the current in th e healthy line can reverse for a short time. Unwante tripping of CB’s on the heal thy line can then occur if a Permissive Over-reach or Blocking istance scheme ( see Chapter 12) is use . Figure 13.2 shows how the situation can arise. The CB a t D clears the fault at F faster than the CB at C. Before CB D opens, the Zone 2 elements at A may see the fault an operate, sen ing a trip signal to the relay for of the relay at CB B also sees the fault CB B. The reverse looking element an inhibits tripping of CB’s A an B. However, once CB D opens, the relay element at A starts to reset, while the forwar looking elements at B pick up ( ue to c urrent reversal) an initiate tripping. If the reset times of the forwar -lookin g elements of the relay at A are longer than the operating time of the forwar lo oking elements at B, the relays will trip the healthy line. The solution is to i ncorporate a blocking time elay that prevents the tripping of the forwar -looki ng elements of the relays an is initiate by the reverselooking element. The ti me elay must be longer than the reset times of the relay elements at A. C Z< Fault F Z< D A Z< Z< B
(a) Fault current istribution at instant of fault C Z< Fault F Open D Z< P rotection of Complex Transmission Circuits A Z< Z< B
(b) Fault current istribution with circuitbreaker K open Figure 13.2: Fault current istribution in ouble-circuit line
13.2.2.2 Un er-reach on parallel lines If a fault occurs on a line that lies bey on the remote terminal en of a parallel line circuit, the istance relay will un er-reach for those zones set to reach into the affecte line. Analysis shows that un er these con itions, because the relay sees only50% (for two parallel c ircuits) of the totalfault current for afault in the a jacent line section, th e relay sees the impe ance of the affecte section as twice the correct value. T his may have to be allowe for in the settings of Zones 2 an 3 of conventionall y set istance relays. Since the requirement for the minimum reach of Zone 2 is to the en of the protecte line section an the un erreach effect only occurs f or faults in the following line section(s), it is not usually necessary to a jus t Zone 2 impe ance settings to compensate. However, Zone 3elements are inten e to provi e backup protection to a jacent line sections an hence the un er-reac hing effect must be allowe for in the impe ance calculations. 13.2.2.3 Behaviou r of istance relays with earth faults on the protecte fee er When an earth fau lt occurs in the system, the voltage applie to the earth fault element of the r elay in one circuit inclu es an in uce voltage proportional to the zero sequenc e current in the other circuit. • 13 • • 204 •
Network Protection & Automation Gui e
Z S1,Z SO IB Line B IA Line A n ZL1 ZL0 ZM0 Z S1 , Z SO
These currents are expresse below in terms of the line an source parameters:
I B 0 I A0 = ′′ ′ (2 − n ) ZSO + (1 − n ) (ZSO + Z L 0 + Z M 0 ) (2 − n )ZS′′1 + (1 − n )(Z 1 ) I I A1 = 1 2 (ZS1 + ZS1 ) + Z L1 ′ ′′ I A0 = ′′ ′ (2 − n )ZSO + (1 − n )(ZSO + Z L 0 + ) I 0 2 (ZSO + ZSO ) + Z L 0 + Z M 0 ′ ′′ nZSO − (1 − n )ZSO ′′ ′ F ult Rel y R loc tion ( ) Single line di gr m IB1 Z' S1 IA1 nZL1 R F1 (1 n)ZL1 ZL1 Z' ' S1 IB0 Z' nZM0 S0 R IA0 (ZLO ZMO) (1 n)ZMO Z'' S0 F0 I0 (1 n)(ZLO ZMO) n(ZLO ZMO)
ZM0 = zero sequence mutu l imped nce etween the two circuits NOTE: For e rth f n tory fro ults I1 = I0 All sym ols in the ove expressions re either selfexpl e, f m Figure 13.3 or h ve een introduced in Ch pter 11. Using the ove formul milies of re ch curves m y e constructed, of which Figure 13.4 is typic l. In this figure, n’ is th ff ctiv p r unit r ach of a r lay s t to prot ct 80% of t h lin . It has b n assum d that an infinit busbar is locat d at ach lin nd , that is, Z’S1 and Z’’S1 ar both z ro. A family of curv s of constant n’ has b n plot t d for variations in th sourc z ro s qu nc imp danc s Z’S0 and Z’’S0. 100 50 Limit of n n (c) Z ro S qu nc n twork Figur 13.3: G n ral parall l circuit f d from both nds ( I B 0 I A0 ) M wh r : M =ZM 0 Z L1
Z A = nZ L1
1 +
2 ( I A1 I A 0 ) + K
1 ...Equation 13.1 0.5 Limit of wh n y x 0.1 0.5 1 Z x= ZLO 5 10 50 0 • Th tru imp danc to th fault is nZL1 wh r n is th p r unit fault position m asur d from R and ZL1 is th positiv s qu nc imp danc of a singl circuit. T h rror in m asur m nt is d t rmin d from th fraction insid th brack t; th is vari s with th positiv and z ro s qu nc curr nts in circuit A and th z ro s qu nc curr nt in circuit B. Figur 13.4: Typical r ach curv s illustrating th ff ct of mutual coupling It can b s n from Figur 13.4 that r lay R can und rr ach or ov r-r ach, accor
ding to th r lativ valu s of th z ro s qu nc sourc to lin imp danc ratios ; th N twork Prot ction & Automation Guid • 205 • n n = 0.9 As th curr nt distribution in th two circuits is unaff ct d by th pr s nc of mutual coupling, no similar variation in th curr nt appli d to th r lay l m nt tak s plac and, cons qu ntly, th r lay m asur s th imp danc to th fault incorr ctly. Wh th r th appar nt imp danc to th fault is gr at r or l ss than th actual imp danc d p nds on th dir ction of th curr nt flow in th h alth y circuit. For th common cas of two circuits, A and B, conn ct d at th local and r mot busbars, as shown in Figur 13.3, th imp danc of Lin A m asur d by a distanc r lay, with th normal z ro s qu nc curr nt comp nsation from its o wn f d r, is giv n by: n n = 0.7 10 5 Z
y = SO ZLO
n n • P rot ction of Compl x Transmission Circuits 13 • I1 (b) Positiv s qu nc n twork and
xtr m ff ctiv p r unit r ach s for th r lay ar 0.67 and 1. R lay ov r-r ac h is not a probl m, as th condition b ing xamin d is a fault in th prot ct d f d r, for which r lay op ration is d sirabl . It can also b s n from Figur 13.4 that r lay R is mor lik ly to und r-r ach. How v r th r lay locat d at th opposit lin nd will t nd to ov r-r ach. As a r sult, th Zon 1 charact ris tic of th r lays at both nds of th f d r will ov rlap for an arth fault any wh r in th f d r – s S ction 13.2.3.5 for mor d tails. Satisfactory prot cti on can b obtain d with a transf r trip, und r-r ach typ distanc sch m . Furth r, comp nsation for th ff ct of z ro s qu nc mutual imp danc is not n c ssa ry unl ss a distanc -to-fault facility is provid d. Som manufactur rs comp nsat for th ff ct of th mutual imp danc in th distanc r lay l m nts, whil o th rs may r strict th application of comp nsation to th distanc -to-fault func tion only. Th latt r is asy to impl m nt in softwar for a digital/num rical r lay but is impractical in r lays using old r t chnologi s. Comp nsation is achi v d by inj cting a proportion of th z ro s qu nc curr nt flowing in th paral l l f d r into th r lay. How v r, som Utiliti s will not p rmit this du to t h pot ntial hazards associat d with f ding a r lay prot cting on circuit from a CT locat d in a diff r nt circuit. For th r lay to m asur th lin imp danc accurat ly, th following condition must b m t: VR = Z L1 IR Thus: KR = KM =
Z L 0 − Z L1 Z L1 ZM 0 Z L1 13.2.3.4 Dist nce rel y eh viour with e rth f ults on the p r llel feeder Altho ugh dist nce rel ys with mutu l compens tion me sure the correct dist nce to the f ult, they m y not oper te correctly if the f ult occurs in the dj cent feede r. D vison nd Wright [13.1] h ve shown th t, while dist nce rel ys without mutu l compens tion will not over re ch for f ults outside the protected feeder, the rel ys m y see f ults in the dj cent feeder if mutu l compens tion is provided . With reference to Figure 13.3, the mount of over re ch is highest when Z’’S1=Z’’S2=Z’’S0 Und r th s conditions, faults occurring in th first 43% of f d r A will app ar to th distanc r lay in f d r B to b within its Zon 1 r ach. Th solution is to limit th mutual comp nsation appli d to 150% of th z ro s qu nc comp n sation. 13.2.3.5 Distanc r lay b haviour with singl -circuit op ration If only on of th parall l f d rs is in s rvic , th prot ction in th r maining f d r m asur s th fault imp danc corr ctly, xc pt wh n th f d r that is not in s rvic is arth d at both nds. In this cas , th z ro s qu nc imp danc n two rk is as shown in Figur 13.5. Humpag and Kandil [13.2] hav shown that th app ar nt imp danc pr s nt d to th r lay und r th s conditions is giv n by: Z R = Z L1 − 2 I A0 Z M 0 I RZ L0 P rotection of Complex Tr nsmission Circuits For solid ph se to e rth f ult t the theoretic l re ch of the rely, the volt ge nd current in the f ulty ph se t the rel ying point re given y: • 13 • V A = I A1Z L1 + I A 2 Z L 2 + I A 0 Z L 0 + I B 0 Z M 0 A 0 …Equ tion 13.2 ...Equ tion 13.4
I A = I A1 + I A 2 + I
where: IR is the current fed into the rel y = IA + KRIA0 The volt ge nd current fed into the rel y re given y: FO IO Z'SO IGO Rel y loc tion IHO nZLO mZLO ZLO (1 n)ZLO Z'' SO V R =V A I R = I A + K R I A0 + K M I B 0 ...Equ tion 13.3 (1 n)ZMO where: KR is the residu l compens tion f ctor KM is the mutu l compens tion f ct or NO Figure 13.5: Zero sequence imped nce network during single circuit oper tion • 206 • Network Protection & Autom tion Guide
The r tio IA0/IR vries with the system conditions, re ching m ximum when the system is e rthed ehind the rel y with no gener tion t th t end. In this c se, the r tio IA0/IR is equ l to ZL1/ZL0 , nd the pp rent imped nce presented to the rel y is: protectionof two ended feeders c n lso e used for multi ended feeders. Howeve r, the pro lems involved in the pplic tion of these schemes to multi ended feed ers re much more complex nd require speci l ttention. The protection schemes th t c n e used with multi ended feeders re unit protection nd dist nce schem es. E ch uses some form of sign lling ch nnel, such s fi re optic c le, power line c rrier or pilot wires. The specific pro lems th t m y e met when pplying these protections to multi ended feeders re discussed in the following section s. 13.3.1 A.C. Pilot Wire Protection Z R = Z L1
1 −
2 ZM 0 2 Z L0
Typic l v lues of zero sequence line imped nces for HV lines in the United Kingd om re given in T le 13.1, where the m ximum per unit over re ch error (ZM0/ZL0 )2 e noted th t the over re ch v lues quoted in this t is lso given. It should le re m xim , nd will e found only in r re c ses. In most c ses, there will e gener tion t oth ends of the feeder nd the mount of over re ch will ther efore e reduced. In the c lcul tions c rried out y Hump ge nd K ndil, with mo re re listic conditions, the m ximum error found in 400kV dou le circuit line w s 18.6%. Conductor size Line volt ge 32kV 275kV 400kV (sq.in) 0.4 2 x 0.4 4 x 0.4 Metric (sq.mm) equiv lent 258 516 1032 Zero sequence mutu l imped nce ZMO ohms/mile 0.3 + j0.81 0.18+j0.69 0.135+j0.6 ohms/km 0.19+j0.5 0.11+j0.43 0.80+j0.37 Zero sequ ence line imped nce ZLO ohms/mile ohms/km 0.264 0.292 0.2666 Per unit over re ch error (ZMO/ZLO)2 The limit tions of pilot wire rel ys for pl in feeder protection lso pply. The length of feeder th t c n e protected is limited y the ch r cteristics of the pilot wires. The protection sees incre sing pilot wire resist nce s tending to n open circuit nd shunt c p cit nce s n .c. short circuit cross the pilot s. The protection will h ve limiting v lues for e ch of these qu ntities, nd wh en these re exceeded, loss of sensitivity for intern l f ults nd m loper tion for extern l f ults m y occur. For tee’d f d rs, th curr nts for an xt rnal ar th fault will not usually b th sam . Th prot ction must b lin ar for any cur r nt up to th maximum through-fault valu . As a r sult, th voltag in th pilo ts during fault conditions cannot b k pt to low valu s, and pilot wir s with 25 0V insulation grad ar r quir d. 13.3.2 Balanc d Voltag Sch m s for T ’d Circui ts In this s ction two typ s of old r balanc d voltag sch m s still found in ma ny locations ar d scrib d. 13.3.2.1 ‘Translay’ balanc d voltag prot ction This is a modification of th balanc d voltag sch m d scrib d in S ction 10.7.1. Sinc it is n c ssary to maintain lin arity in th balancing circuit, though not in t h s nding l m nt, th voltag r f r nc is d riv d from s parat quadratur tr ansform rs, as shown in Figur 13.6. Th s ar auxiliary units with summation wi ndings n rgiz d by th main curr nt transform rs in s ri s with th upp r l ct romagn ts of th s nsing l m nts. Th s condary windings of th quadratur curr nt transform rs at all nds ar int rconn ct d by th pilots in a s ri s circui t that also includ s th low r l ctromagn ts of th r lays. S condary windings on th r lay l m nts ar not us d, but th s l m nts ar fitt d with bias loop s in th usual way. Th plain f d r s ttings ar incr as d in th t d sch m by 50% for on t and 75% for two. 0.41+j1.61 0.25+j1.0 0.24+j1.3 0.15+j0.81 0.16+j1.18 0.1+j0.73
• Tabl 13.1: Maximum ov r-r ach rrors found during singl circuit working 13.3 MULTI-ENDED FEEDERS – UNIT PROTECTION SCHEMES A multi- nd d f d r is d fin d as on having thr or mor t rminals, with ith r load or g n ration, or both, at any t rminal. Thos t rminals with load only ar usually known as ’taps’. Th si mpl st multi-t rminal f d rs ar thr - nd d, and ar g n rally known as t ’d f d rs. This is th typ most commonly found in practic . Th prot ction sch m s d scrib d pr viously for th N twork Prot ction & Automation Guid • 207 • P rot ction of Compl x Transmission Circuits 13 • It is appar nt from th abov formula that th r lay has a t nd ncy to ov r-r a ch. Car should b tak n wh n Zon 1 s ttings ar s l ct d for th distanc prot ction of lin s in which this condition may b ncount r d. In ord r to ov rcom this possibl ov r-r aching ff ct, som Utiliti s r duc th r ach of arth fa ult r lays to around 0.65ZL1 wh n lin s ar tak n out of s rvic . How v r, th p robability of having a fault on th first s ction of th following lin whil on lin is out of s rvic is v ry small, and many Utiliti s do not r duc th s t ting und r this condition. It should b not d that th us of mutual comp nsatio n would not ov rcom th ov r-r aching ff ct sinc arthing clamps ar normally plac d on th lin sid of th curr nt transform rs. A.C. pilot wir r lays provid a low-cost fast prot ction; th y ar ins nsitiv to pow r swings and, owing to th ir r lativ simplicity, th ir r liability is x c ll nt.
End A End B 13.3.3 Pow r Lin Carri r Phas Comparison Sch m s A B C Quadratur CT A1 1 A1 A N A1 C C1 N N S1 S2 A 1 C C1 S1 S2 R lay S1 S2 Pilots P rot ction of Compl x Transmission Circuits Figur 13.6: Balanc d voltag T ’d f d r sch m 13.3.2.2 High - sp d prot ction typ DSB7 This typ is of high r sp d and is s hown in Figur 13.7. Summation quadratur transform rs ar us d to provid th a nalogu quantity, which is balanc d in a s ri s loop through a pilot circuit. S parat s condary windings on th quadratur curr nt transform rs ar conn ct d t o full-wav r ctifi rs, th outputs of which ar conn ct d in s ri s in a s cond pilot loop, so that th l ctromotiv forc s summat arithm tically. Th m asur ing r lay is a doubl -wound moving coil typ , on coil b ing n rgiz d from th v ctorial summation loop; th oth r r c iv s bias from th scalar summation in t h s cond loop proportional to th sum of th curr nts in th s v ral lin t rmi nals, th valu b ing adjust d by th inclusion of an appropriat valu of r sis tanc . Sinc th op rating and biasing quantiti s ar both d riv d by summation, th r lays at th diff r nt t rminals all b hav alik , ith r to op rat or to r strain as appropriat . Sp cial f atur s ar includ d to nsur stability, bot h in th pr s nc of transform r inrush curr nt flowing through th f d r zon and also with a 2-1-1 distribution of fault curr nt caus d by a short circuit on th s condary sid of a star-d lta transform r. End A End C End B A B C Th op rating principl of th s prot ction sch m s has alr ady b n cov r d in d tail in S ction 10.9. It involv s comparing th phas angl s of signals d riv
d from a combination of th s qu nc curr nts at ach nd of th f d r. Wh n th phas angl diff r nc xc ds a pr -s t valu , th ‘trip angl ’, a trip signal is s nt to th corr sponding circuit br ak rs. In ord r to pr v nt incorr ct op ra tion for xt rnal faults, two diff r nt d t ctors, s t at diff r nt l v ls, ar us d. Th low-s t d t ctor starts th transmission of carri r signal, whil th high-s t d t ctor is us d to control th trip output. Without this saf guard, th sch m could op rat incorr ctly for xt rnal faults b caus of op rating tol ranc s of th quipm nt and th capacitiv curr nt of th prot ct d f d r. This condition is wors with multi-t rminal f d rs, sinc th curr nts at th f d r t rminals can b v ry dissimilar for an xt rnal fault. In th cas of th thr -t rminal f d r in Figur 13.8, if incorr ct op ration is to b avoid d, it i s n c ssary to mak c rtain that th low-s t d t ctor at nd A or nd B is n rg iz d wh n th curr nt at nd C is high nough to op rat th high-s t d t ctor a t that nd. As only on low-s t start r, at nd A or nd B, n ds to b n rgiz d for corr ct op ration, th most unfavourabl condition will b wh n curr nts I A and IB ar qual. To maintain stability und r this condition, th high-s t to low-s t s tting ratio of th fault d t ctors n ds to b twic as larg as that r quir d wh n th sch m is appli d to a plain f d r. This r sults in a loss of s nsitivity, which may mak th quipm nt unsuitabl if th minimum fault l v l of th pow r syst m is low. A IA IC C T • 13 • IB Fault B Figur 13.8: Ext rnal fault conditions P4 Quadratur CT D E D Op rating coil E R straints coil Bias pilots A furth r unfavourabl condition is that illustrat d in Figur 13.9. If an int r nal fault occurs n ar on of th nds of th f d r ( nd B in Figur 13.9) and t h r is littl or no g n ration at nd C, th curr nt at this nd may b flowing outwards. Th prot ction is th n pr v nt d from op rating, sinc th fault curr nt distribution is similar to that for an xt rnal fault; s Figur 13.8. Th fault can b cl ar d only by th backup prot ction and, if high sp d of op rati on is r quir d, an alt rnativ typ of primary prot ction must b us d. • 208 • N twork Prot ction & Automation Guid Figur 13.7: Typ DSB7 fast t ’d f d r prot ction
A C IA T IC Trip Diff r ntial curr nt Idiff IB Fault B Idiff = K Ibias R strain IS Figur 13.9: Int rnal fault with curr nt flowing out at on lin nd Bias curr nt Ibias Figur 13.11: P rc ntag bias d diff r ntial prot ction charact ristic If IA, IB, IC ar th curr nt v ctor signals at lin nds A, B, C, th n for a h althy circuit: IA + IB + IC = 0 Th basic principl s of op ration of th syst m ar that ach r lay m asur s its local thr phas curr nts and s nds its valu s to th oth r r lays. Each r lay th n calculat s, for ach phas , a r sultant di ff r ntial curr nt and also a bias curr nt, which is us d to r strain th r lay in th mann r conv ntional for bias d diff r ntial unit prot ction. Th bias f a tur is n c ssary in this sch m b caus it is d sign d to op rat from conv nti onal curr nt transform rs that ar subj ct to transi nt transformation rrors. T h two quantiti s ar : 13.3.4 Diff r ntial R lay using Optical Fibr Signalling Curr nt diff r ntial r lays can provid unit prot ction for multi- nd d circuits without th r strictio ns associat d with oth r forms of prot ction. In S ction 8.6.5, th charact rist ics of optical fibr cabl s and th ir us in prot ction signalling ar outlin d. Th ir us in a thr - nd d syst m is shown in Figur 13.10, wh r th r lays at ach lin nd ar digital/num rical r lays int rconn ct d by optical fibr link s so that ach can s nd information to th oth rs. In practic th optical fibr links can b d dicat d to th prot ction syst m or multipl x d, in which cas m ultipl xing quipm nt, not shown in Figur 13.10, will b us d to t rminat th fibr s. Optical fibr signalling chann ls RA RB I diff > I A + I B + I C I bias = 1 I A + I B + IC 2 ( ) Figur 13.11 shows th p rc ntag bias d diff r ntial charact ristic us d, th t ripping crit ria b ing: I diff > K I bias • and I diff > I S A IA IB B
wh r : K = p rc ntag bias s tting IS = minimum diff r ntial curr nt s tting If th magnitud s of th diff r ntial curr nts indicat that a fault has occurr d, th r lays trip th ir local circuit br ak r. RC C IC Figur 13.10: Curr nt diff r ntial prot ction for t ’d f d rs using optical fibr signalling N twork Prot ction & Automation Guid • 209 • P rot ction of Compl x Transmission Circuits 13 • A point that should also b consid r d wh n applying this sch m is th att nuat ion of carri r signal at th t junctions. This att nuation is a function of th r lativ imp danc s of th branch s of th f d r at th carri r fr qu ncy, including th imp danc of th r c iving quipm nt. Wh n th imp danc s of th s cond and third t rminals ar qual, a pow r loss of 50% tak s plac . In oth r w ords, th carri r signal s nt from t rminal A to t rminal B is att nuat d by 3dB by th xist nc of th third t rminal C. If th imp danc s of th two branch s corr sponding to t rminal B to C ar not qual, th att nuation may b ith r g r at r or l ss than 3dB.
Th r lays also continuously monitor th communication chann l p rformanc and c arry out s lf-t sting and diagnostic op rations. Th syst m m asur s individual phas curr nts and so singl phas tripping can b us d wh n r quir d. R lays ar provid d with softwar to r configur th prot ction b tw n two and thr t r minal lin s, so that modification of th syst m from two t rminals to thr t rm inals do s not r quir r lay r plac m nt. Furth r, loss of a singl communicatio ns link only d grad s sch m p rformanc slightly. Th r lays can r cognis this and us alt rnat communications paths. Only if all communication paths from a r lay fail do s th sch m hav to r v rt to backup prot ction. th r lay in this cas can b xpr ss d in t rms of th sourc imp danc s as fol lows: Z A = Z LA + Z LB + (ZSB + Z LB ) Z (ZSC + Z LC ) LB P rot ction of Compl x Transmission Circuits 13.4 MULTI-ENDED FEEDERS - DISTANCE RELAYS Distanc prot ction is wid ly us d at pr s nt for t d f d r prot ction. How v r, its application is not straightfo rward, r quiring car ful consid ration and syst matic ch cking of all th condit ions d scrib d lat r in this s ction. Most of th probl ms found wh n applying d istanc prot ction to t ’d f d rs ar common to all sch m s. A pr liminary discu ssion of th s probl ms will assist in th ass ssm nt of th p rformanc of th diff r nt typ s of distanc sch m s. 13.4.1 Appar nt Imp danc s n by Distanc R lays Th imp danc s n by th distanc r lays is aff ct d by th curr nt inf ds in th branch s of th f d rs. R f rring to Figur 13.12, for a fault at th busbars of th substation B, th voltag VA at busbar A is giv n by: VA = IAZL A + IBZLB so th imp danc ZA s n by th distanc r lay at t rminal A is giv n by: V I Z A = A = Z LA + B Z LB IA IA Th magnitud of th third t rm in this xpr ssion is a function of th total im p danc s of th branch s A and B and can r ach a r lativ ly high valu wh n th fault curr nt contribution of branch C is much larg r than that of branch A. Fig ur 13.13 illustrat s how a distanc r lay with a mho charact ristic locat d at A with a Zon 2 l m nt s t to 120% of th prot ct d f d r AB, fails to s a f ault at th r mot busbar B. Th ’t ’ point T in this xampl is halfway b tw n su bstations A and B (ZLA = ZLB) and th fault curr nts IA and IC hav b n assum d to b id ntical in magnitud and phas angl . With th s conditions, th fault app ars to th r lay to b locat d at B inst ad of at B - i. . th r lay app ar s to und r-r ach. ZSA A IA ZLA ZLC IC C ZSC IB ZLB Fault B ZSB T Figur 13.12: Fault at substation B busbars X B • 13 • B or I Z A = Z LA + B Z LB IA
or Z A = Z LA + Z LB + IC Z LB IA T ...Equation 13.5 A R Figur 13.13: Appar nt imp danc pr s nt d to th r lay at substation A for a fa ult at substation B busbars Th appar nt imp danc pr s nt d to th r lay has b n modifi d by th t rm (IC /IA)ZLB. If th pr -fault load is z ro, th curr nts IA and IC ar in phas and th ir ratio is a r al numb r. Th appar nt imp danc pr s nt d to Th und r-r aching ff ct in th sak of simplicity, th or
t ’d f d rs can b found for any kind of fault. For quations and xampl s m ntion d so far hav b n f
• 210 • N twork Prot ction & Automation Guid
balanc d faults only. For unbalanc d faults, sp cially thos involving arth, t h quations b com som what mor complicat d, as th ratios of th s qu nc fau lt curr nt contributions at t rminals A and C may not b th sam . An xtr m x ampl of this condition is found wh n th third t rminal is a tap with no g n ra tion but with th star point of th primary winding of th transform r conn ct d dir ctly to arth, as shown in Figur 13.14. Th corr sponding s qu nc n twork s ar illustrat d in Figur 13.15. ZSA A B T ZLA ZLC ZT ZLB Phas A to ground fault ZSB comp nsat for th r duction in z ro s qu nc curr nt. How v r, th solution has two possibl limitations: i. ov r-r ach will occur wh n th transform r is not conn ct d, and h nc op ration for faults outsid th prot ct d zon may occur i i. th inh r nt possibility of malop ration of th arth fault l m nts for art h faults b hind th r lay location is incr as d 13.4.2 Eff ct of Pr -fault Load In all th pr vious discussions it has b n assum d that th pow r transf r b tw n t rminals of th f d r imm diat ly b for th fault occurr d was z ro. If t his is not th cas , th fault curr nts IA and IC in Figur 13.12 may not b in phas , and th factor IC /IA in th quation for th imp danc s n by th r lay at A, will b a compl x quantity with a positiv or a n gativ phas angl acco rding to wh th r th curr nt IC l ads or lags th curr nt IA. For th fault cond ition pr viously consid r d in Figur s 13.12 and 13.13, th pr -fault load curr nt may displac th imp danc s n by th distanc r lay to points such as B’1 or B’2, shown in Figur 13.16, according to th phas angl and th magnitud of th pr -fault load curr nt. Humpag and L wis [13.3] hav analys d th ff ct of pr -fault load on th imp danc s s n by distanc r lays for typical cas s. Th ir r sults and conclusions point out som of th limitations of c rtain r lay charac t ristics and sch m s. X B 2 B 1 IA C M Load Figur 13.14: Transform r tap with primary winding solidly arth d ZSA1 A1 EA ZLA1 T1 ZLB1 B1 ZSH1 IA1 Z LJ1 ZT1 ZM1 EB ZSA2 G1 ZLA2 T2 ZLB2 B2 ZSB2 IA2 ZLC2 ZT2 ZM2
B ZSA0 A0 ZLA0 IC0 ZT0 T0 ZLB0 B0 ZSB0 IA0 ZLC0 T • A Figur 13.15: S qu nc n tworks for a phas A to ground fault at busbar B in th syst m shown in Figur 13.14 Figur 13.16: ff cts of th pr -fault load on th appar nt imp danc pr s nt d to th r lay R It can b s n from Figur 13.15 that th pr s nc of th tap has littl ff ct in th positiv and n gativ s qu nc n tworks. How v r, th z ro s qu nc imp d anc of th branch actually shunts th z ro s qu nc curr nt in branch A. As a r sult, th distanc r lay locat d at t rminal A t nds to und r-r ach. On soluti on to th probl m is to incr as th r sidual curr nt comp nsating factor in th distanc r lay, to N twork Prot ction & Automation Guid 13.4.3 Eff ct of th Fault Curr nt Flowing Outwards at On T rminal Up to this p oint it has b n assum d that th fault curr nts at t rminals A and C flow into th f d r for a fault at th busbar B. Und r som conditions, how v r, th curr nt at on of th s t rminals may flow outwards inst ad of inwards. A typical ca s is illustrat d in Figur 13.17; that of a parall l tapp d f d r with on of th nds of th parall l circuit op n at t rminal A. • 211 • P rot ction of Compl x Transmission Circuits 13 •
A IA ZA T ZB IB B I B IC I C Fault C und r r v rs unbalanc d fault conditions if th curr nt flowing through th r l ay is high and th r lay s tting r lativ ly larg . Th s conditions aris princi pally from arth faults. Th r lay s tting and th r v rs fault curr nt ar now r lat d, th first b ing a function of th maximum lin l ngth and th s cond d p nding mainly on th imp danc of th short st f d r and th fault l v l at t hat t rminal. For instanc , r f rring to Figur 13.19, th s tting of th r lay at t rminal A will d p nd on th imp danc (ZA + ZB) and th fault curr nt inf d IC, for a fault at B, whil th fault curr nt IA for a r v rs fault may b qu it larg if th T point is n ar th t rminals A and C. A B P rot ction of Compl x Transmission Circuits Figur 13.17: Int rnal Fault at busbar B with curr nt flowing out at t rminal C IA ZA Fault T ZB ZC IC IB A IA T IB B Fault C I B IC I C Figur 13.19: Ext rnal fault b hind h r lay at t rminal A C A summary of th main probl ms m t in th application of distanc prot ction to t d f d rs is giv n in Tabl 13.2.
Cas D scription 1 Und r-r aching ff ct for int rnal faults du to curr nt inf d at th T point Eff ct of pr -fault load on th imp danc s n by th r lay O v r-r aching ff ct for xt rnal faults, du to curr nt flowing outwards at on t rminal Failur to op rat for an int rnal fault, du to curr nt flowing out at on t rminal Incorr ct op ration for an xt rnal fault, du to high curr nt f d from n ar st t rminal R l vant figur numb r 13.12 to 13.15 13.16 13.17 13.18 1 3.19 Figur 13.18: Int rnal fault n ar busbar B with curr nt flowing out at t rminal C 2 3 4 5 • 13 • As th curr nts IA and IC now hav diff r nt signs, th factor IC /IA b com s n gativ . Cons qu ntly, th distanc r lay at t rminal A s s an imp danc small r than that of th prot ct d f d r, (ZA + ZB), and th r for has a t nd ncy to o v r-r ach. In som cas s th appar nt imp danc pr s nt d to th r lay may b as low as 50% of th imp danc of th prot ct d f d r, and v n low r if oth r li n s xist b tw n t rminals B and C. If th fault is int rnal to th f d r and clos to th busbars B, as shown in Figur 13.18, th curr nt at t rminal C may still flow outwards. As a r sult, th fault app ars as an xt rnal fault to th distanc r lay at t rminal C, which fails to op rat . 13.4.4 Malop ration with R v rs Faults Earth fault distanc r lays with a dir ctional charact ristic t nd to los th ir dir ctional prop rti s Tabl 13.2: Main probl ms m t in th application of distanc prot ction to t d f d rs. 13.5 MULTI-ENDED FEEDERS – APPLICATION OF DISTANCE PROTECTION SCHEMES Th sch m s that hav b n d scrib d in Chapt r 12 for th prot ction of plain f d rs may a lso b us d for t d f d r prot ction. How v r, th applications of som of th s sch m s ar much mor limit d in this cas . Distanc sch m s can b subdivid d into two main groups; transf r trip sch m s and blocking sch m s. Th usual c onsid rations wh n comparing th s sch m s ar s curity, that is, no op ration f or xt rnal faults, and • 212 • N twork Prot ction & Automation Guid
d p ndability, that is, assur d op ration for int rnal faults. In addition, it s hould b born in mind that transf r trip sch m s r quir fault curr nt inf d a t all th t rminals to achi v high-sp d prot ction for any fault in th f d r . This is not th cas with blocking sch m s. Whil it is rar to find a plain f d r in high voltag syst ms wh r th r is curr nt inf d at on nd only, it is not difficult to nvisag a t ’d f d r with no curr nt inf d at on nd, for xampl wh n th t ’d f d r is op rating as a plain f d r with th circuit br ak r at on of th t rminals op n. N v rth l ss, transf r trip sch m s ar also us d for t ’d f d r prot ction, as th y off r som advantag s und r c rtain cond itions. 13.5.1 Transf r Trip Und r-R ach Sch m s Th main r quir m nt for transf r trip und r-r ach sch m s is that th Zon 1 of th prot ction, at on nd at l ast, shall s a fault in th f d r. In ord r to m t this r quir m nt, th Z on 1 charact ristics of th r lays at diff r nt nds must ov rlap, ith r th t hr of th m or in pairs. Cas s 1, 2 and 3 in Tabl 13.2 should b ch ck d wh n th s ttings for th Zon 1 charact ristics ar s l ct d. If th conditions m nt ion d in cas 4 ar found, dir ct transf r trip may b us d to cl ar th fault; th alt rnativ is s qu ntially at nd C wh n th fault curr nt IC r v rs s aft r th circuit br ak r at t rminal B has op n d; s Figur 13.18. Transf r trip sch m s may b appli d to f d rs that hav branch s of similar l ngth. If on o r two of th branch s ar v ry short, and this is oft n th cas in t d f d r s, it may b difficult or impossibl to mak th Zon 1 charact ristics ov rlap. Alt rnativ sch m s ar th n r quir d. Anoth r cas for which und r-r ach sch m s may b advantag ous is th prot ction of tapp d f d rs, mainly wh n th tap is short and is not n ar on of th main t rminals. Ov rlap of th Zon 1 charac t ristics is th n asily achi v d, and th tap do s not r quir prot ction appli d to th t rminal. 13.5.2 Transf r Trip Ov r-R ach Sch m s For corr ct op ratio n wh n int rnal faults occur, th r lays at th thr nds should s a fault at any point in th f d r. This condition is oft n difficult to m t, sinc th i mp danc s n by th r lays for faults at on of th r mot nds of th f d r m ay b too larg , as in cas 1 in Tabl 13.2, incr asing th possibility of malop ration for r v rs faults, cas 5 in Tabl 13.2. In addition, th r lay charact ristic might ncroach on th load imp danc . Th s consid rations, in addition to th signalling chann l r quir m nts m ntion d lat r on, mak transf r trip ov r-r ach sch m s unattractiv for multi- nd d f d r prot ction. 13.5.3 Blocking Sch m s Blocking sch m s ar particularly sui t d to th prot ction of multi- nd d f d rs, sinc high-sp d op ration can b obtain d with no fault curr nt inf d at on or mor t rminals. Th only disadva ntag is wh n th r is fault curr nt outf d from a t rminal, as shown in Figur 13.18. This is cas 4 in Tabl 13.2. Th prot ction units at that t rminal may s th fault as an xt rnal fault and s nd a blocking signal to th r mot t rm inals. D p nding on th sch m logic ith r r lay op ration will b block d, or cl aranc will b in Zon 2 tim . Th s tting of th dir ctional unit should b such that no malop ration can occur for faults in th r v rs dir ction; cas 5 in Tabl 13.2. 13.5.4 Signalling Chann l Consid rations Th minimum numb r of si gnalling chann ls r quir d d p nds on th typ of sch m us d. With und r-r ach and blocking sch m s, only on chann l is r quir d, wh r as a p rmissiv ov r-r ach sch m r q-uir s as many chann ls as th r ar f d r nds. Th signalling c hann l quipm nt at ach t rminal should includ on transmitt r and (N-1) r c i v rs, wh r N is th total numb r of f d r nds. This may not b a probl m if f ibr -optic cabl s ar us d, but could l ad to probl ms oth rwis . If fr qu ncy s hift chann ls ar us d to improv th r liability of th prot ction sch m s, mai nly with transf r trip sch m s, N additional fr qu nci s ar r quir d for th pu rpos . Probl ms of signal att nuation and imp danc matching should also b car fully consid r d wh n pow r lin carri r fr qu ncy chann ls ar us d. 13.5.5 Dir ctional Comparison Blocking Sch m s Th principl of op ration of th s sch m s is th sam as that of th distanc blocking sch m s d scrib d in th pr vious s ction. Th main advantag of dir ctional comparison sch m s ov r distanc sch m s is th ir gr at r capability to d t ct high-r sistanc arth faults. Th r li ability of th s sch m s, in t rms of stability for through faults, is low r tha
n that of distanc blocking sch m s. How v r, with th incr asing r liability of mod rn signalling chann ls, dir ctional comparison blocking sch m s s m to off r good solutions to th many and difficult probl ms ncount r d in th • N twork Prot ction & Automation Guid • 213 • P rot ction of Compl x Transmission Circuits 13 •
prot ction of multi- nd d f d rs. Mod rn r lays impl m nt th r quir d f atur s in diff r nt ways – for furth r information s Chapt r 12 and sp cific r lay man uals. 13.6 PROTECTION OF SERIES COMPENSATED LINES Figur 13.20 d picts th basic pow r transf r quation. It can b s n from this quation that transmitt d pow r is proportional to th syst m voltag l v l and load angl whilst b ing inv r s ly proportional to syst m imp danc . S ri s comp nsat d lin s ar us d in tran smission n tworks wh r th r quir d l v l of transmitt d pow r can not b m t, ith r from a load r quir m nt or syst m stability r quir m nt. S ri s comp nsat d transmission lin s introduc a s ri s conn ct d capacitor, which has th n t r sult of r ducing th ov rall inductiv imp danc of th lin , h nc incr asing th prosp ctiv , pow r flow. Typical l v ls of comp nsation ar 35%, 50% and 70 %, wh r th p rc ntag l v l dictat s th capacitor imp danc compar d to th t ransmission lin it is associat d with. EA Bus A Bus B E VF jXS -jXC E XS>XC IF VF Z< IF Figur 13.21: Voltag inv rsion on a transmission lin P rot ction of Compl x Transmission Circuits ZT EB PT a EA EB sin d ZT Figur 13.20: Pow r transf r in a transmission lin A s cond probl m is that of curr nt inv rsion which is d monstrat d in Figur 13 .22. In this cas , th ov rall fault imp danc is tak n to b capacitiv . Th fa ult curr nt th r for l ads th syst m .m.f. by 90° whilst th m asur d fault vol tag r mains in phas with syst m .m.f.. Again this condition can giv ris to dir ctional stability probl ms for a vari ty of prot ction d vic s. Practically, th cas of curr nt inv rsion is difficult to obtain. In ord r to prot ct capac itors from high ov r voltag s during fault conditions som form of voltag limit ing d vic (usually in th form of MOV’s) is install d to bypass th capacitor at a s t curr nt l v l. In th cas of curr nt inv rsion, th ov rall fault imp dan c has to b capacitiv and will g n rally b small. This l ads to high l v ls o f fault curr nt, which will trigg r th MOV’s and bypass th capacitors, h nc l a ving an inductiv fault imp danc and pr v nting th curr nt inv rsion. VF jXS -jXC VF jIFXS XS< XC IF Figur 13.22: Curr nt inv rsion in a transmission lin • 13 • Th introduction of a capacitiv imp danc to a n twork can giv ris to s v ral r laying probl ms. Th most common of th s is th situation of voltag inv rsi on, which is shown in Figur 13.21. In this cas a fault occurs on th prot ct d lin . Th ov rall fault imp danc is inductiv and h nc th fault curr nt is i nductiv (shown lagging th syst m .m.f. by 90 d gr s in this cas ). How v r, th voltag m asur d by th r lay is that across th capacitor and will th r for
lag th fault curr nt by 90 d gr s. Th n t r sult is that th voltag m asur d by th r lay is in anti-phas to th syst m .m.f.. Whilst this vi w is highl y simplistic, it ad quat ly d monstrat s pot ntial r lay probl ms, in that any p rot ction r liant upon making a dir ctional d cision bas s its d cision on an in ductiv syst m i. . on wh r a forward fault is indicat d by fault curr nt lagg ing th m asur d voltag . A good xampl of this is a distanc r lay, which assu m s th transmission lin is an v nly distribut d inductiv imp danc . Pr s nti ng th r lay with a capacitiv voltag (imp danc ) can l ad th r lay to mak an incorr ct dir ctional d cision. E Z< IF E In g n ral, th application of prot ctiv r lays to a s ri s comp nsat d pow r s yst m n ds car ful valuation. Th probl ms associat d with th introduction of a s ri s capacitor can b ov rcom by a vari ty of r laying t chniqu s so it is important to nsur th suitability of th chos n prot ction. Each particular a pplication r quir s car ful inv stigation to d t rmin th most appropriat solu tion in r sp ct of prot ction – th r ar no g n ral guid lin s that can b giv n. N twork Prot ction & Automation Guid • 214 •
13.7 EXAMPLES In this s ction, an xampl calculation illustrating th solution to a probl m m ntion d in this Chapt r is giv n. 13.7.1 Distanc R lay appli d t o Parall l Circuits Th syst m diagram shown in Figur 13.23 indicat s a simpl 110kV n twork suppli d from a 220kV grid through two auto-transform rs. Th foll owing xampl shows th calculations n c ssary to ch ck th suitability of thr zon distanc prot ction to th two parall l f d rs int rconn cting substation s A and B, Lin 1 b ing s l ct d for this purpos . All r l vant data for this x rcis ar giv n in th diagram. Th MiCOM P441 r lay with quadrilat ral charact ristics is us d to provid th r lay data for th xampl . R lay quantiti s us d in th xampl ar list d in Tabl 13.3, and calculations ar carri d out in t rms of actual syst m imp danc s in ohms, rath r than CT s condary quantiti s. T his simplifi s th calculations, and nabl s th xampl to b simplifi d by xc luding consid rations of CT ratios. Most mod rn distanc r lays p rmit s ttings to b sp cifi d in syst m quantiti s rath r than CT s condary quantiti s, but ol d r r lays may r quir th syst m quantiti s to b conv rt d to imp danc s as s n by th r lay. TZ3 R1G R2G R3G Tim d lay - Zon 3 Ground fault r sistiv r ach valu - Zon 1 Ground fault r s istiv r ach valu - Zon 2 Ground fault r sistiv r ach valu - Zon 3 0.45 84.8 84.8 84.8 s Ω Ω Ω Table 13.3: Distance relay settings 13.7.1.1 Resi ual compensation Gri supply at 220kV Maximum fault level 7500MVA Minimum fault level MVA 220/110/11kV transformers XHL=0.15; XHT=0.35; XLT=0.25 XH=0.125; =0.225 on 120MVA 110kV Substation P Current transformer ratio 600/1A 5MVA 132/33kV transformers XT=0.125 Line 2 Voltage transformer ratio
2500MVA 120 XL=0.025 XT T5 Line 1 4 110kV/110V
T3 T4
The relaysuse are calibrate in terms of the positive sequence impe ance of th e protecte line. Since the earth faultimpe ance of Line 1 is ifferent from th e positive sequence impe ance,the impe ance seen by the relay in the case of a fault involving earth will be ifferent to that seen for a phase fault. Hence, t he reach of the earth fault elements of the relay nee s to be ifferent. For the relay use , this a justment is provi e by the resi ual (or neutral) compensati on factor Kzo, set equal to: K ZO = (Zo − Z1 ) 3 Z1 •
110kV Su st tion Q T7 33kV us rs 45MVA 132/33kV tr nsformers XT=0.125 Line 4 K ZO = For Lines 1 nd 2, (Zo − Z1 ) 3 Z1
T6 Line 3 Z L1 = 0.177 + j0.402 Ω (0.439 o
66.236 Ω)
110kV transmission lines: Z1=0.177+j0.40Ω/km ZO=0.354+j1.022Ω/km Length of line: 1, 2 =50km 3 =100km 4 =40km Z LO = 0.354 + j1.022 Ω (1.082 70.895 Ω) o Hence,
Figure 13.23: Example network for istance relay setting calculation K ZO = 0.490 • 215 •
K ZO =7.8 o
Network Protection & Automation Gui e P rotection of Complex Transmission Circuits 13 • Relay Parameter ZL1 (mag) ZL1 (ang) ZL0 (mag) ZL0 (ang) KZ0 (mag) KZ0 (ang) Z1 ( mag) Z1 (ang) Z2 (mag) Z2 (ang) Z3 (mag) Z3 (ang) R1ph R2ph R3ph KZ1 (mag) KZ1 ( ang) KZ2 (mag) KZ2 (ang) TZ1 TZ2 Parameter Parameter Description Value Line positive sequence impe ance (magnitu e) 21.95 66.236 Line zero sequenc Line positive sequence impe ance (phase angle) e impe ance(magnitu e) 54.1 Line zero sequence impe ance (phase angle) 70.895 D efault resi ual compensation factor (magnitu e) 0.49 Default resi ual compensati on factor (phase angle) 7.8 Zone 1 reach impe ance setting (magnitu e) 17.56 Zon e 1 reach impe ance setting (phase angle) 66.3 Zone 2 reach impe ance setting (m agnitu e) 30.73 Zone 2 reach impe ance setting (phase angle) 66.3 Zone 3 reach i mpe ance setting (magnitu e) 131.8 Zone 3 reach impe ance setting (phase angle) 66.3 Phase fault resistive reach value - Zone 1 84.8 Phase fault resistive reach value - Zone 2 84.8 Phase fault resistive reach value- Zone 3 84.8 Zone 1 resi ual compensation factor (magnitu e) 0.426 Zone 1 resi ual compensation factor ( phaseangle) 9.2 Zone 2 resi ual compensation factor (magnitu e) not use Zone 2 resiual compensation factor (phase angle) not use 0 Time elay - Zone 1 0.25 Time elay - Zone 2 Units Ω eg Ω eg eg Ω eg Ω eg Ω eg Ω Ω Ω eg eg s s
13.7.1.2 Zone impe ance reach settings – phase faults Firstly, the impe ance reach es for the three relay zones are calculate . 13.7.1.3 Zone 1 reach Zone 1 impe a nce is set to 80% of the impe ance of the protecte line. Hence, 13.7.1.6 Zone Time Delay Settings Proper co-or ination of the istance relay set tings with those of other relays is require . In epen ent timers are available f or the three zones to trippingis normal. ensure this. For Zone 1, instantaneous A time elay is use only in cases where large .c. offsets occur an ol circu it breakers, incapable of breaking the instantaneous .c. component, areinvolve . The Zone 2 element has to gra e with the relays protecting Lines 3 an 4 sinc e the Zone 2 element covers part of these lines. Assuming that Lines3/4 have i , the time stance, unit or instantaneous high-set overcurrent protection applie elay require is that to cover the total clearance time of the ownstream relay s. To this must be a e the reset time for the Zone 2 elements following cleara nce A typical time of a fault on an a jacent line, an a suitable safety margin. elay is 250ms, an the normal range is 200-300ms. The consi erationsfor the Z one 3 element are the same as for the Zone 2 element, except that the ownstream fault clearance time is that for the Zone 2 element of a istance relay or IDMT overcurrent protection. Assuming istance relays are use , a typical time is 45 0ms. In summary: TZ1 = 0ms (instantaneous) TZ2 = 250msTZ3 = 450ms 13.7.1.7 Phas e Fault Resistive Reach Settings With the use of a qua rilateral characteristic, the resistive reach settings for each zone can be set in epen ently of the impe ance reach settings. The resistive reach setting represents the maximum amount of a itional fault resistance (in excess of the line impe ance) for which a zon e will trip, regar less of the fault within the zone. Two constraints are impose upon the settings, as follows: (i) it must be greater than the maximum expecte phase-phase fault resistance (principally that of the fault arc) (ii) it must be less than the apparent resistance measure ue to the heaviest loa on the li ne The minimum fault current at Substation B is of the or er of 1.5kA, lea ing t o a typical arc resistance Rarc using the van Warrington formula (equation 11.6) of 9Ω. Using the current transformer ratio on Line 1 as a gui e to the maximum ex pecte loa current, the minimum loa impe ance Zlmin will be 106Ω. Typically, the resistive reaches will be setto avoi the minimum loa impe ance by a 20% marg in for the phase elements, lea ing to a maximum resistive reach setting of 84.8.Ω. Z1 = 0.8 ×50 × 0.439 66.236 o Ω = 0.8 ×21.95 66.236 o Ω =17.56 66.236 o Ω Use a value of 17.56 66.3 o Ω 13.7.1.4 Zone 2 reach Zone 2 impe ance reach is set to cover the maximum of: (i) 120% of Line 1 length (ii) Line 1 + 50% of shortest l ine from Substation B i.e. 50% of Line 4 From the line impe ances given, (i) 1.2 ×21.95 66.236 o = 26.34 66.236 o Ω o (ii) 21.95 66.236 + ( ) P rotection of Complex Transmission Circuits 0.5 × 40 × 0.439 66.236 o Ω It is clear that con ition (ii) governs the setting, an therefore the initial Z one 2 reach setting is: Z2 = 30.73 66.3 o Ω • 13 •
The effect of parallel Line 2is to make relay 1 un erreach for faults on a jace nt line sections, as iscusse in Section 11.9.3. This is not a problem for the phase fault elements because Line 1 will always be protecte . 13.7.1.5 Zone 3 re
ach The function of Zone 3 is to provi e backup protection for uncleare faults in a jacent line sections. The criterion use is that the relay shoul be set to cover 120% of the impe ance between the relay location an the en of the longe st a jacent line, taking account of any possible fault infee from other circuit s or parallel in the relay un paths. In this case, faults in Line 3 will result er-reaching ue to the parallel Lines 1 an 2, so the impe ance of Line 3 shoul be ouble to take this effect into account. Therefore, 21.95
66.3 o
Z3 =1.2 ×
Ω
+100 ×2 ×0.439 66.3 o
• 216 •
Network Protection & Automation Gui e
=131.8
66.3 o Ω
Therefore, the resistive reach setting lies between 9Ω an 84.8Ω. While each zone ca n have its own resistive reach setting,for this simple example, all of the resi stive reach settings can be set equal ( epen ing on the particular istance prot ection scheme use an the nee to inclu e Power Swing Blocking, thisnee not a lways be the case). Suitable settings are chosen to be 80% of the loa resistanc e: R3 ph = 84.8 Ω R2 ph = 84.8 Ω R1 ph = 84.8 Ω 13.7.1.8 Earth Fault Reach Settings By efault, the resi ual compensation factor as calculate in section 13.7.1.1is use toa just the phase fault reach setting in the case of earth faults, an is applie to t all zones. However, it is also possible to apply this compensation o zones in ivi ually. Two cases in particular require consi eration, an are cov ere in this example. 13.7.1.9 Zone 1 earth fault reach Where istance protectio n is applie to parallel lines (as in this example), the Zone 1 earth fault elem ents may sometimes over-reach an therefore operate when one line is out of serv ice an earthe at both en s The solution is to re uce the earth fault reach of the Zone 1 element to typically 80% of the efault setting. Hence: where: Za j = impe ance of a jacent line covere by Zone 2 I fltp = fault current in pa rallel line I flt = total fault current since the two parallel lines are i entical, an hence, for Lines 1 an 2, Un er - reach = 8.78 66.3 o × 0.5 = 4.39 66.3 o Ω % Un er-reach = % Un er-reach = 14.3% This is not significant an hence amount of un er-reach an no a justment nee be ma e. If a justment is require , this can be achieve by using the KZ2 relay setting,increasing it over the KZ0 setting by the percenta ge un er-reach. When this is one, care must also be taken that the percentage o ver-reach uring single circuit operation is notexcessive – if it is then use can be ma e of the alternative setting groups provi e in most mo ern istance rela ys to change the relay settings accor ing to the number of circuits in operation . 13.7.1 11 Groun fault resistive reach settings The same settings can be use as for the phase fault resistive reaches. Hence, R3G = 84.8Ω R2G = 84.8Ω R1G = 84.8Ω T his completes the setting of the relay. Table 13.3 also shows the settings calcu late . 13.8 REFERENCES 13.1 Some factors affecting the accuracy of istance type protective equipment un er earth fault con itions. Davison, E.B. an Wright, A. Proc. IEE Vol. 110, No. 9, Sept. 1963, pp. 1678-1688. 13.2 Distance protection performance un er con itionsof single-circuit working in oublecircuit transmis sion lines. Humpage, W.D. an Kan il, M.S. Proc.IEE. Vol. 117. No. 4, April197 0, pp. 766-770. 13.3 Distance protection of tee circuits. Humpage, W.A. an Le wis, D.W. Proc. IEE, Vol. 114, No. 10, Oct. 1967, pp. 1483-1498. K Z1 = 0.8 × K ZO = 0.8 ×0.532 = 0.426 In practice, the setting is selecte by using analternative setting group, sele cte when the parallel line is out of service an earthe . 13.7.1.10 Zone 2 eart h fault reach With parallel circuits, the Zone 2 element will ten to un er-reac h ue to the zero sequence mutual coupling between the lines. Maloperation may o ccur, particularly for earth faults occurring on the remote busbar. The effect c an be countere by increasing the Zone 2 earth fault reach setting, but first it is necessary to calculate the amount of un er-reach that occurs. •
Un erreach = Za j × I fltp I flt
Network Protection & Automation Gui e
• 217 • P rotection of Complex Transmission Circuits 13 • Un er-reach Reach of protecte zone
• 14 • Auto-Reclosing Intro uction Application of auto-reclosing Auto-reclosing on HV istribution net works Factors influencing HV auto-reclose schemes Auto-reclosing on EHV transmis sion lines High spee auto-reclosing on EHV systems Single-phase auto-reclosing High spee auto-reclosing on lines employing istance schemes Delaye auto-reclo sing on EHV systems Operating features of auto-reclose schemes Auto-close circui ts Examples of auto-reclose applications 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10 14.11 14.12
• 14 • Auto-Reclosing 14.1 INTRODUCTION Faults on overhea lines fall into one of three categories: a. transient b. semi-permanent c. permanent 80-90% of faults on any overhea line network are transient in nature. The remaining 10%-20% of faults are either semi -permanent or permanent. Transient faults are commonly cause by lightning an t emporary contact with foreign objects. The imme iate tripping of one or more cir cuit breakers clears the fault. Subsequent re-energisationof the line is usuall y successful. A small tree branch falling on the line coul causea semi-permane nt fault. The cause of the fault woul not be remove by theimme iate tripping of the circuit, but coul be burnt away uring a time- elaye trip. HV overhea lines in forest areas are prone to this typeof fault. Permanent faults, such as broken con uctors, an faults on un ergroun cable sections, must be locate an repaire before the supply can be restore . Use of an auto-reclose scheme to r e-energise the line after a fault trip permits successful re-energisation of the line. Sufficient time must be allowe after tripping for the fault arc to e-en ergise prior to reclosing otherwise the arc will re-strike. Such schemes have be en the cause of a substantial improvement in continuity of supply. A furtherben efit, particularly to EHV systems, is the maintenance of system stability an sy nchronism. A typical single-shot auto-reclose scheme is shown in Figures 14.1 an 14.2. Figure 14.1 shows a successful reclosure in the event of a transient fau lt, an Figure 14.2 an unsuccessful reclosure followe by lockout of the circuit breaker if the fault is permanent. 14.2 APPLICATION OFAUTO-RECLOSING The most important parameters of an auto-reclose scheme are: 1. ea time 2. reclaim time 3. single or multi-shot These parameters are influence by: a. type of protecti on b. type of switchgear c. possible stability problems . effects on the variou s types of consumer loa s Network Protection & Automation Gui e • 219 •
Instant of fault Protection Operates Resets Transient fault Circuit breaker
Operating time Arc Trip coil Contacts Contacts energise separate extinguishe f ully open Closing circuit energise Contacts Contacts make fully close Opening Arcing time time Operating time Closing time Dea time Relay rea y to respon to further fault inci ents (after successful reclosure) System isturbance time Reclose initiate by protection Auto-reclose relay Dea time Closing pulse time Reclaim time Time Figure 14.1: Single-shot auto-reclose scheme operation for a transient fault Operates Protection Resets Reclose on to fault Operates Resets Permanent fault Circuit breaker
Operating time Trip coil Contacts Arc Contacts energise separate extinguishe f ully open Closing circuit Contacts energise make Arc Contacts Contacts fully close separate Extinguishe Contacts fully open Auto-Reclosing
Opening Arcingtime time Operating time Reclose initiate by protection Auto-rec lose relay Dea time Closing time Dea time Trip coil energise Relay locks out for protection re-operation before reclaim t ime has elapse Closing pulse time Reclaim time starts Reclaim time resets Time •
14 • The weighting given to the above factors is ifferent for HV istribution networ ks an EHV transmission systems an therefore it is convenient to iscuss them u n er separatehea ings. Sections 14.3 an 14.4 cover the application of auto-rec losing to HV istribution networks while Sections 14.5-14.9 cover EHV schemes. T he rapi expansion in the use of auto-reclosing has le to the existence ofa va riety of ifferent control schemes. The various features in common use are iscu sse in Section 14.10. The relate subject of auto-closing, that is, the automat ic closing of normally open circuit breakers, is ealt with in Section 14.11. 14 .3 AUTO-RECLOSING ON HV DISTRIBUTION NETWORKS On HV istribution networks, auto reclosing is applie Figure 14.2: Operation of single-shot auto-reclose scheme on a permanent fault mainly to ra ial fee erswhereproblems of system stability o not arise, an th e main a vantages to be erive from its use can be summarise as follows: a. re uction to a minimum of the interruptions of supply to the consumer b. instantan eous fault clearance can be intro uce , with the accompanying benefits of shorte rfault uration, less fault amage, an fewer permanent faults As 80% of overhe a line faults are transient, elimination of loss of supply from this cause by t he intro uction of auto-reclosing gives obvious benefits through: a. improve su pply continuity b. re uction of substation visits Instantaneous tripping re uces the uration of the Network Protection & Automation Gui e • 220 •
power arc resulting from an overhea line to a minimum. The chance of perm fault anent amage occurring to the line is re uce . The application of instantaneous protection may result in non-selective tripping of a number of circuit breakers an an ensuing loss of supply to a numberof healthy sections. Auto-reclosing al lows these circuit breakers to be reclose within a few secon s. With transient faults, the overall effect woul be loss of supply for a very short time but aff ecting a larger number of consumers. If only time-gra e protection without auto -reclose was use , a smaller number of consumers might be affecte , but for a lo nger time perio . When instantaneous protection is use with autoreclosing, the scheme is normally arrange to inhibit the instantaneous protection after the fi rst trip. For a permanent fault, the time-gra e protection will give iscrimina tive tripping after reclosure, resulting in the isolation of the faulte section . Some schemes allow a number of reclosures an time-gra e trips after the firs t instantaneous trip, which may result in the burning out an clearance of semipermanent faults. A further benefit of instantaneous tripping is a re uction in circuit breaker maintenance by re ucing pre-arc heating when clearing transient faults. When consi un erg ering fee ers that are partly overhea line an partly roun cable, any ecision to install auto-reclosing woul be influence by any ata known on the frequency oftransient faults. Where a significant proportion o f faults are permanent, the a vantages of auto-reclosing are small, particularly since reclosing on to a faulty cable is likely to aggravate the amage. 14.4 FA CTORS SCHEMES The factors that influence INFLUENCING HV AUTO-RECLOSE the choice of ea time, reclaim time, an the number of shots are now iscusse . 14.4.1 De a Time Several factors affect the selection of system ea time as follows: a. system stability an synchronism b. type of loa c. CB characteristics . fault path e-ionisation time e. protection reset time These in factors are iscusse the following sections. 14.4.1.1 System stability an synchronism In or erto re close without loss of synchronism after a fault on the interconnecting fee er, t he ea time must be kept to the minimum permissible consistent with e-ionisati on Network Protection & Automation Gui e of the fault arc. Other time elays that contribute to the total system isturba nce time must also be kept as short as possible. The problem arises onlyon ist ribution canbe fe into b networks with more than one power source, where power oth en s of an inter-connecting line. A typical example is embe e generation ( see Chapter 17), or where asmall centre of population with a local iesel gener ating plant may be connecte to the rest of the supply system by a single tie-li ne. The use of high-spee protection, such as unit protection or istance scheme s, with operating times of less than 0.05s is essential. The circuit breakers mu st have times an then be able to reclosethe circuit after very short operation a ea time of the or er of 0.3s-0.6s to allow for fault-arc e-ionisation. It may be esirable in some cases to use synchronism check logic, so that auto-recl osing isprevente if the phase angle has move outsi e specifie limits. The ma tter is ealt with more fully in Section 14.9 on EHV systems. 14.4.1.2 Type of l oa On HV systems, the main problem to be consi ere in relation to ea time is the effect on various types of consumer loa . a. in ustrial consumers Most in u strial consumers operate mixe loa s comprising in uction motors, lighting, proc ess control an static loa s. Synchronous motors may also be use . Dea time has to be long enough to to trip out on loos allow motor circuits ofsupply. Once t he supply is restore , restarting of rives can then occur un er irection of th e process control system in a safe an programme manner, an can often be fast enough to ensure no significant loss of pro uction or prouct quality b. omesti c consumers It that expensive is improbable processes or angerous con itions wi ll be involve with omestic consumers an the main consi eration is thatof inc onvenience an compensation for supply interruption. A ea time of secon s or a few minutes is of little importance compare with the loss of cooking facilitie s, central heating, lightan au io/visual entertainment resulting from a longer supply failure that coul occur without auto-reclosing 14.4.1.3Circuit breaker characteristics The time elays impose by the circuit breaker uring a trippin
g an reclosing operation must be taken into consi eration, especially when asse ssing the possibility of applying high spee auto-reclosing. a. mechanism resett ing time Most circuit breakers are ‘trip free’, which means that the breaker can be trippe uring the closing stroke. • • 221 • Auto-Reclosing 14 •
After tripping, a time of the or er of 0.2s must be allowe for thetrip-free me chanism to impulse. Where high spee reclosing i reset before applying a closing s require , a latch check interlock is esirable in the reclosing circuit b. clo sing time This is the time interval between the energisation of the closing mech anism an the making of the contacts. Owing to the time constant of the solenoi an the inertia of the plunger, a solenoi closing mechanism may take 0.3s to c lose. A spring-operate breaker, on the other han , can close in less than 0.2s. Mo ern vacuum circuit breakers may have aclosing time of less than 0.1s The ci rcuit breaker mechanism imposes a minimum ea time ma e up from the sum of (a) an (b) above. Figure 14.3 illustrates the performance of mo ern HV circuit brea kers in this respect. Ol er circuit breakers may require longer times than those shown. Arc extinguishe Breaker fully open: closing circuit energise Time (s) Contacts make Breaker fully lly close t6 Air 380kV 0.035 0.045 0.235 0.065 0.3 0.02 SF6 132kV 0.04 0.07 0.03 0.08 0.11 0.12 SF6 380kV 0.02 0.05 0.01 0.06 0.07 0.04 when on maximum time setting, an ea times of at least this value may be requi re . When short ea times are require , the protection relays must resetalmost instantaneously, a requirement that is easily met by the use of static, igital an numerical I.D.M.T. relays. 14.4.2 Reclaim Time Factors affecting the settin g of the reclaim time are iscusse in the following sections. 14.4.2.1 Type of protection The reclaim time must be long enough to allow the protection relays t o operate when the circuit breakeris reclose on to a permanentfault. The most common formsof protection applie to HV lines are I.D.M.T. or efinite time ov er-current an earth-fault relays. The maximum operating time for the former wit h very low fault levels coul be up to 30 secon s, while for fault levels of sev eral times rating the operating time may be 10 secon s or less. In the case of efinite time protection, settings of 3 secon s or less are common, with 10 secon s as an absolute maximum. It has been common practice to use reclaim times of 3 0 secon s on HV auto-reclose schemes. However, there is a anger with a setting of this length that uring a thun erstorm, when the inci ence of transient fault sis high, the breaker may reclose successfully after one fault, an then trip a n lock out for a secon fault within this time. Use of a shorter reclaim time o f, say, 15 secon s may enable the secon fault to be treate as a separate inci ent, with a further successful reclosure. Where fault levels are low, it may be ifficult to select I.D.M.T. time settings to give satisfactory gra ing with an operating time limit of 15 secon s, an the matter becomes a question of selecti ng a reclaim time compatible with I.D.M.T. requirements. It is common to fit sen sitive earth-fault protection to supplement the normal protection in or er to e tect high resistance earth faults. This protection cannot possibly be stable on through faults, an is therefore set to have an operating time longer than that of themain protection. This longer time may have to be taken into consi eration when eci ing ona reclaim time. A broken overhea con uctor in contact with r y groun or a woo fence may cause this type of fault. It is rarely if ever tran sient an may be a anger to the public. It is therefore common practice to use a contact on the sensitive earth faultrelay to block auto-reclosing an lock ou t the circuit breaker. Where high-spee protection is use , reclaim times of 1 s econ or less woul be a equate. However, such short Network Protection & Automation Gui e Contacts separate Trip p initiation t1 t2 t3 Oil 11kV 0.06 0.1 0.08 0.16 0.24 0.02 Vacuum 15kV 0.038 0.053 0.023 0.048 0.2 8 0.07 t5 Oil 132kV 0.03 0.06 0.2 0.35 0.55 0.01 t4 Auto-Reclosing
t1 t2 t4 t5 t6 3 Figure 14.3: Typical circuit breaker trip-close operation times • 14 •
14.4.1.4 De-ionisation of fault path As mentione above, successful high spee r eclosure requires the interruption of the fault by the circuitbreaker to be fol lowe by atime elay long enough to allow the ionise air to isperse. This tim e is epen ent on the system voltage, cause of fault,weather con itions an so on, but at voltages up to 66kV, 0.1s-0.2s shoul be a equate. On HV systems, the refore, fault eionisation time is of less importance than circuit breaker time elays. 14.4.1.5 Protectionreset time If time elaye protection is use , it is essential thatthe timing evice shall fully reset uring the ea time, so tha t correct time iscrimination will be maintaine after reclosure on to a fault. The reset time of the electromechanical I.D.M.T. relay is 10 secon s or more • 222 •
times are rarely use in practice, to relieve the uty on the circuit breaker. 1 4.4.2.2 Spring win ing time The reclaim time of motor-woun spring-close breake rs must be at least as long as the spring win ing time, to ensure that the break er is not subjecte to a further reclosing operating with a partly woun spring. 14.4.3 Number of Shots There are no efinite rules for efining the number of s hots for any particular auto-reclose application, but a number of factors must b e taken into account. 14.4.3.1 Circuit breaker limitations Important consi erati ons are the ability of the circuit breaker to perform several trip an close ope rationsin quick successionan the effect of these operations on the maintenanc e perio . Maintenance perio s vary accor ing to the type of circuit breaker use an the fault current broken when clearing each fault. Use of mo ern numerical relays can assist, as they often have a CB con ition-monitoring feature inclu e that can be arrange to in icate to a Control Centre when maintenance isrequir e . Auto-reclose may then be locke out until maintenance has been carrie out. 14.4.3.2 System con itions If statistical information on a particularsystem sho ws a mo erate percentage of semi-permanent faults that coul be burne out urin g 2 or 3 time- elaye trips, a multi-shot scheme may be justifie . This is often the case in forest areas. Another situation is where fuse ‘tees’ are use an the fault level is low, since the fusing time may not iscriminate with the main I.D .M.T. relay. The use of several shots will heat the fuse tosuch an extent that it woul eventually blow before the main protection operate . 14.5 AUTO-RECLOSIN G ON EHV TRANSMISSION LINES The most important consi eration in the application of auto-reclosing to EHV transmission lines isthe maintenance of system stabili ty an synchronism. The problems involve are epen ent on whether the transmiss ion system is weak or strong. With a weak system, loss of a transmission link ma y lea quickly to an excessive phase angle across the CB use for re-closure, th us preventing a successful re-closure. In a relatively strong system, the rate o f change of phase angle will be slow, so that elaye auto-reclose can be succes sfully applie . An illustration is the interconnector between two power systems as shown in Figure 14.4. Un er healthy Network Protection & Automation Gui e con itions, the amount of synchronising power transmitte , P, crosses the power/ angle curve OAB at point X, showing that the phase isplacement between the two systems is θo. Under fault conditions, the curve OCB is applicable, and the operat ing point changes to Y. Assuming constant power input to both ends of the line, there is now an accelerating power XY. As a result, the operating point moves to Z, with an increased phase displacement, θ1, between the two systems. At this poi nt the circuit breakers trip and break the connection. The phase displacement co ntinues to increase at a rate dependent on the inertia of the two power sources. To maintain synchronism, the circuit breaker must be reclosed in a time short e nough to prevent the phase angle exceeding θ2. This angle is such that the area (2 ) stays greater than the area (1), which is the condition for maintenance of syn chronism. Fault Loads Loads A Normal system condition 2 Input line Power P X Z Y 1 C Fault condition θ2
0 θ0 θ1 B Phase displacement Figure 14.4: Effect of high speed three phase auto reclosing on system stability for a weak system This example, for a weak system, shows that the successful application of auto r eclosing in such a case needs high speed protection and circuit breakers, and a short dead time. On strong systems, synchronism is unlikely to be lost by the tr ipping out of a single line. For such systems, an alternative policy of delayed autoreclosing may be adopted. This enables the power swings on the system result ing from the fault to decay before reclosure is attempted. The various factors t o be considered when using EHV auto reclose schemes are now dealt with. High spe ed and delayed auto reclose schemes are discussed separately. 14.6 HIGH SPEED AU TO RECLOSING ON EHV SYSTEMS The first reuirement for the application of high sp eed auto reclosing is knowledge of the system disturbance • • 223 • Auto Reclosing 14 •
time that can be tolerated without loss of system stability. This will normally reuire transient stability studies to be conducted for a defined set of power s ystem configurations and fault conditions. With knowledge of protection and circ uit breaker operating characteristics and fault arc de ionisation times, the fea sibility of high speed auto reclosing can then be assessed. These factors are no w discussed. 14.6.1 Protection Characteristics The use of high speed protection euipment, such as distance or unit protection schemes, giving operating times o f less than 50ms, is essential. In conjunction with fast operating circuit break ers, high speed protection reduces the duration of the fault arc and thus the to tal system disturbance time. It is important that the circuit breakers at both e nds of a fault line should be tripped as rapidly as possible. The time that the line is still being fed from one end represents an effective reduction in the de ad time, and may well jeopardise the chances of a successful reclosure. When dis tance protection is used, and the fault occurs near one end of the line, special measures have to be adopted to ensure simultaneous tripping at each end. These are described in Section 14.8. 14.6.2 De Ionisation of Fault Arc It is important to know the time that must be allowed for complete de ionisation of the arc, to prevent the arc restriking when the voltage is re applied. The de ionisation ti me of an uncontrolled arc, in free air depends on the circuit voltage, conductor spacing, fault currents, fault duration, wind speed and capacitive coupling fro m adjacent conductors. Of these, the circuit voltage is the most important, and as a general rule, the higher the voltage the longer the time reuired for deion isation. Typical values are given in Table 14.1. Line voltage (kV) 66 110 132 220 275 400 525 Table 14.1: Fault arc de ionisation times Minimum de energisation time (seconds) 0.2 0.28 0.3 0.35 0.38 0.45 0.55 the dead time reuired. This is a particular problem on long distance EHV transm ission lines. 14.6.3 Circuit Breaker Characteristics The high fault levels invol ved in EHV systems imposes a very severe duty on the circuit breakers used in hi ghspeed auto reclose schemes. The accepted breaker cycle of break make break re uires the circuit breaker to interrupt the fault current, reclose the circuit af ter a time delay of upwards of 0.2s and then break the fault current again if th e fault persists. The types of circuit breaker commonly used on EHV systems are oil, air blast and SF6 types. 14.6.3.1 Oil circuit breakers Oil circuit breakers are used for transmission voltages up to 300kV, and can be subdivided into the two types: ‘bulk oil’ and ‘small oil volum ’. Th latt r is a d sign aim d at r ducing t h fir hazard associat d with th larg volum of oil contain d in th bulk oil br ak r. Th op rating m chanisms of oil circuit br ak rs ar of two typ s, ‘fix d trip’ and ‘trip fr ’, of which th latt r is th most common. With trip-fr typ s, th r closing cycl must allow tim for th m chanism to r s t aft r tripping b for applying th closing impuls . Sp cial m ans hav to b adopt d to obtain t h short d ad tim s r quir d for high-sp d auto-r closing. Various typ s of tri pping m chanism hav b n d v lop d to m t this r quir m nt. Th thr typ s of closing m chanism fitt d to oil circuit br ak rs ar : i. sol noid ii. spring ii i. pn umatic CB’s with sol noid closing ar not suitabl for highsp d auto-r clos du to th long tim constant involv d. Spring, hydraulic or pn umatic closing m chanisms ar univ rsal at th upp r nd of th EHV rang and giv th fast st closing tim . Figur 14.3 shows th op ration tim s for various typ s of EHV ci rcuit br ak rs, including th d ad tim that can b attain d. 14.6.3.2 Air blast circuit br ak rs Air blast br ak rs hav b n d v lop d for voltag s up to th high st at pr s nt in us on transmission lin s. Th y fall into two cat gori s: a. pr ssuris d h ad circuit br ak rs b. non-pr ssuris d h ad circuit br ak rs In pr ssuris d h ad circuit br ak rs, compr ss d air is maintain d in th chamb r surrounding th main contacts. Wh n a tripping signal is r c iv d, an auxiliary N twork Prot ction & Automation Guid • Auto-R closing 14 •
If singl -phas tripping and auto-r closing is us d, capacitiv coupling b tw n th h althy phas s and th faulty phas t nds to maintain th arc and h nc xt nd • 224 •
air syst m s parat s th main contacts and allows compr ss d air to blast throug h th gap to th atmosph r , xtinguishing th arc. With th contacts fully op n , compr ss d air is maintain d in th chamb r. Loss of air pr ssur could r sult in th contacts r closing, or, if a m chanical latch is mploy d, r striking of th arc in th d -pr ssuris d chamb r. For this r ason, s qu ntial s ri s isola tors, which isolat th main contacts aft r tripping, ar commonly us d with air blast br ak rs. Sinc th s ar comparativ ly slow in op ning, th ir op ration must b inhibit d wh n auto-r closing is r quir d. A contact on th auto-r clos r lay is mad availabl for this purpos . Non-pr ssuris d h ad circuit br ak rs ar slow r in op ration than th pr ssuris d h ad typ and ar not usually appl i d in high-sp d r closing sch m s. 14.6.3.3 SF6 circuit br ak rs Most EHV circ uit br ak r d signs now manufactur d us SF6 gas as an insulating and arc-qu nch ing m dium. Th basic d sign of such circuit br ak rs is in many ways similar to that of pr ssuris d h ad air blast circuit br ak rs, and normally r tain all, o r almost all, of th ir voltag withstand capability, v n if th SF6 pr ssur l v l falls to atmosph ric pr ssur . S qu ntial s ri s isolators ar th r for not normally us d, but th y ar som tim s sp cifi d to pr v nt damag to th circui t br ak r in th v nt of a lightning strik on an op n nd d conductor. Provisi on should th r for b mad to inhibit s qu ntial s ri s isolation during an aut o-r clos cycl . 14.6.4 Choic of D ad Tim At voltag s of 220kV and abov , th d -ionisation tim will probably dictat th minimum d ad tim , rath r than any circuit br ak r limitations. This can b d duc d from Tabl 14.1. Th d ad tim s tting on a high-sp d auto-r clos r lay should b long nough to nsur compl t d -ionisation of th arc. On EHV syst ms, an unsucc ssful r closur is mor d trim ntal to th syst m than no r closur at all. 14.6.5 Choic of R claim Tim Wh r EHV oil circuit br ak rs ar conc rn d, th r claim tim should tak acc ount of th tim n d d for th closing m chanism to r s t r ady for th n xt r closing op ration. 14.6.6 Numb r of Shots High-sp d auto-r closing on EHV syst ms is invariably singl shot. R p at d r closur att mpts with high fault l v ls would hav s rious ff cts on syst m stability, so N twork Prot ction & Automation Guid th circuit br ak rs ar lock d out aft r on unsucc ssful att mpt. Also, th in cid nc of s mi-p rman nt faults which can b cl ar d by r p at d r closur s is l ss lik ly than on HV syst ms. 14.7 SINGLE-PHASE AUTO-RECLOSING Singl phas to arth faults account for th majority of ov rh ad lin faults. Wh n thr -phas auto-r closing is appli d to singl circuit int rconn ctors b tw n two pow r s yst ms, th tripping of all thr phas s may caus th two syst ms to drift apar t in phas , as d scrib d in S ction 14.5. No int rchang of synchronising pow r can tak plac during th d ad tim . If only th faulty phas is tripp d, synchr onising pow r can still b int rchang d through th h althy phas s. Any diff r n c in phas b tw n th two syst ms will b corr spondingly l ss, l ading to a r duction in th disturbanc on th syst m wh n th circuit br ak r r clos s. For singl -phas auto-r closing ach circuit br ak r pol must b provid d with its own closing and tripping m chanism; this is normal with EHV air blast and SF6 b r ak rs. Th associat d tripping and r closing circuitry is th r for mor compl icat d, and, xc pt in distanc sch m s, th prot ction may n d th addition of phas s l ction logic. On th occurr nc of a phas - arth fault, singl -phas a uto-r clos sch m s trip and r clos only th corr sponding pol of th circuit br ak r. Th autor clos function in a r lay th r for has thr s parat l m n ts, on for ach phas . Op ration of any l m nt n rgis s th corr sponding d a d tim r, which in turn initiat s a closing puls for th appropriat pol of th circuit br ak r. A succ ssful r closur r sults in th autor clos logic r s tt ing at th nd of th r claim tim , r ady to r spond to a furth r fault incid nt . If th fault is p rsist nt and r closur is unsucc ssful, it is usual to trip and lock out all thr pol s of th circuit br ak r. Th abov d scrib s only on of many variants. Oth r possibiliti s ar : a. thr -phas trip and lockout for phas -phas or 3phas faults, or if ith r of th r maining phas s should d v l op a fault during th d ad tim b. us of a s l ctor switch to giv a choic of singl or thr -phas r closing c. combin d singl and thr -phas auto-r closin
g; singl phas to arth faults initiat singl -phas tripping and r closur , an d phas -phas faults initiat thr -phas tripping and r closur Mod rn num rica l r lays oft n incorporat th logic for all of th abov sch m s, for th us r to s l ct as r quir d. Us can b mad of any us r-d finabl logic • • 225 • Auto-R closing 14 •
f atur in a num rical r lay to impl m nt oth r sch m s that may b r quir d. Th advantag s of singl -phas auto-r closing ar : a. th maint nanc of syst m in t grity b. on multipl arth syst ms, n gligibl int rf r nc with th transmiss ion of load. This is b caus th curr nt in th fault d phas can flow through arth via th various arthing points until th fault is cl ar d and th faulty p has r stor d Th main disadvantag is th long r d -ionisation tim r sulting f rom capacitiv coupling b tw n th faulty and h althy lin s. This l ads to a lo ng r d ad tim b ing r quir d. Malop ration of arth fault r lays on doubl circ uit lin s owing to th flow of z ro s qu nc curr nts may also occur. Th s ar induc d by mutual induction b tw n faulty and h althy lin s (s Chapt r 13 for d tails). 14.8 HIGH-SPEED AUTO-RECLOSING ON LINES EMPLOYING DISTANCE SCHEMES Th importanc of rapid tripping of th circuit br ak rs at ach nd of a fault d lin wh r high-sp d autor closing is mploy d has alr ady b n cov r d in S ct ion 14.6. Simpl distanc prot ction pr s nts som difficulti s in this r sp ct. r closing appli d to th circuit br ak rs at ach nd of th f d r could r sult ith r in no d ad tim or in a d ad tim insuffici nt to allow d -ionisation of th fault arc. A transi nt fault could th r for b s n as a p rman nt on , r sulting in th locking out of both circuit br ak rs. Two m thods ar availabl f or ov rcoming this difficulty. Firstly, on of th transf r-trip or blocking sch m s that involv s th us of an int rtrip signal b tw n th two nds of th li n can b us d. Alt rnativ ly, a Zon 1 xt nsion sch m may b us d to giv ins tantan ous tripping ov r th whol lin l ngth. Furth r d tails of th s sch m s ar giv n in Chapt r 12, but a bri f d scription of how th y ar us d in conjun ction with an auto-r clos sch m is giv n b low. 14.8.1 Transf r-Trip or Blocki ng Sch m s This involv s us of a signalling chann l b tw n th two nds of th lin . Tripping occurs rapidly at both nds of th faulty lin , nabling th us of high-sp d autor clos . Som complication occurs if singl -phas autor clos is us d, as th signalling chann l must id ntify which phas should b tripp d, but this probl m do s not xist if a mod rn num rical r lay is us d. Irr sp cti v of th sch m us d, it is customary to provid an auto-r clos blocking r lay to pr v nt th circuit br ak rs auto-r closing for faults s n by th distanc r lay in Zon s 2 and 3. 14.8.2 Zon 1 Ext nsion In this sch m , th r ach of Zon 1 is normally xt nd d to 120% of th lin l ngth and is r s t to 80% wh n a c ommand from th auto-r clos logic is r c iv d. This auto-r clos logic signal s hould occur b for a closing puls is appli d to th circuit br ak r and r main op rat d until th nd of th r claim tim . Th logic signal should also b pr s nt wh n auto-r clos is out of s rvic . 14.9 DELAYED AUTO-RECLOSING ON EHV SYST EMS On highly int rconn ct d transmission syst ms, wh r th loss of a singl li n is unlik ly to caus two s ctions of th syst m to drift apart significantly and los synchronism, d lay d auto-r closing can b mploy d. D ad tim s of th ord r of 5s-60s ar commonly us d. No probl ms ar pr s nt d by fault arc d -ion isation tim s and circuit br ak r op rating charact ristics, and pow r swings on th syst m d cay b for r closing. In addition, all tripping and r clos sch m s can b thr -phas only, simplifying control circuits in comparison with singl phas sch m s. In syst ms on which d lay d autor closing is p rmissibl , th ch anc s of a r closur b ing N twork Prot ction & Automation Guid Auto-R closing Owing to th rrors involv d in d t rmining th ohmic s tting of th distanc r lays, it is not possibl to s t Zon 1 of a distanc r lay to cov r 100% of th prot ct d lin – s Chapt r 11 for mor d tails. Zon 1 is s t to cov r 80-85% of th lin l ngth, with th r maind r of th lin cov r d by tim -d lay d Zon 2 prot ction. Zon 3(J) Zon 3(G) Middl Zon End Zon G Zon 1(H) Zon 2(K) Zon 3(K) Zon 3( H) Zon 1(G) End Zon H J Zon 1(K) Zon 2 (G) Zon 1(J) K Zon 2(J) •
14 • Zon 2(H) Figur 14.5: Typical thr zon distanc sch m Figur 14.5 illustrat s this for a typical thr -zon distanc sch m cov ring t wo transmission lin s. For this r ason, a fault occurring in an nd zon would b cl ar d instantan ously, by th prot ction at on nd of th f d r. How v r, th CB at th oth r nd op ns in 0.3-0.4 s conds (Zon 2 tim ). High-sp d auto• 226 •
succ ssful ar som what gr at r with d lay d r closing than would b th cas wi th high-sp d r closing. 14.9.1 Sch m Op ration Th s qu nc of op rations of a d lay d auto-r clos sch m can b b st und rstood by r f r nc to Figur 14.6. This shows a transmission lin conn cting two substations A and B, with th cir cuit b ak rs at A and B tripping out in th v nt of a lin fault. Synchronism i s unlik ly to b lost in a syst m that mploys d lay d autor clos . How v r, th transf r of pow r through th r maining ti -lin s on th syst m could r sult in th d v lopm nt of an xc ssiv phas diff r nc b tw n th voltag s at points A and B. Th r sult, if r closur tak s plac , is an unacc ptabl shock to th syst m. It is th r for usual practic to incorporat a synchronism ch ck r lay into th r closing syst m to d t rmin wh th r auto-r closing should tak plac . A (a) N twork diagram Protn. op rat d (local or int rtrip) AR lockout CB clos d & S Q R Q If for any r ason th lin fails to ‘d ad lin charg ’ from nd A, r closur from n d B would tak plac aft r 15 s conds. Th circuit br ak r at A would th n b gi v n th opportunity to r clos with a synchronism ch ck. 14.9.2 Synchronism Ch c k R lays Th synchronism ch ck r lay l m nt commonly provid s a thr -fold ch c k: i. phas angl diff r nc ii. voltag iii. fr qu ncy diff r nc Th phas ang l s tting is usually s t to b tw n 20o–45o, and r closur is inhibit d if th ph as diff r nc xc ds this valu . Th sch m waits for a r closing opportunity with th phas angl within th s t valu , but locks out if r closur do s not o ccur within a d fin d p riod, typically 5s. A voltag ch ck is incorporat d to p r v nt r closur und r various circumstanc s. A numb r of diff r nt mod s may b availabl . Th s ar typically und rvoltag on ith r of th two m asur d volta g s, diff r ntial voltag , or both of th s conditions. B 1 AR in progr ss 1 0 ti & tR 0 CB op n Protn. r s t CB h althy Syst m h althy & D ad tim td 0 & S Q R Q CB clos command tR: r claim tim ti: inhibit tim (b) Autor clos logic for ach CB td: d ad tim
Figur 14.6: D lay d auto-r clos sch m logic Aft r tripping on a fault, it is normal proc dur to r clos th br ak r at on nd first, a proc ss known as ‘liv bus/d ad lin charging’. R closing at th oth r and is th n und r th control of a synchronism ch ck r lay l m nt for what is k nown as ‘liv bus/liv lin r closing’. For xampl , if it w r d cid d to charg th lin initially from station A, th d ad tim in th auto-r clos r lay at A wo uld b s t at, say, 5 s conds, whil th corr sponding tim r in th auto-r clos r lay at B would b s t at, say, 15 s conds. Th circuit b ak r at A would th n r clos aft r 5 s conds provid d that voltag monitoring r lays at A indicat d that th busbars w r aliv and th lin d ad. With th lin r charg d, th circ uit br ak r at B would th n r clos with a synchronism ch ck, aft r a 2 s cond d lay impos d by th synchronism ch ck r lay l m nt. N twork Prot ction & Automation Guid As w ll as ‘liv bus/d ad lin ’ and ‘liv bus/liv lin ’ r closing, som tim s ‘liv lin /d ad bus’ r closing may n d to b impl m nt d. A num rical r lay will typically al low any combination of th s mod s to b impl m nt d. Th voltag s ttings for d istinguishing b tw n ‘liv ’ and ‘d ad’ conditions must b car fully chos n. In addition , th locations of th VT’s must b known and ch ck d so that th corr ct voltag signals ar conn ct d to th ‘lin ’ and ‘bus’ inputs. 14.10 OPERATING FEATURES OF AUTO-R ECLOSE SCHEMES Th xt nsiv us of auto-r closing has r sult d in th xist nc of a wid vari ty of diff r nt control sch m s. Som of th mor important vari ations in th f atur s provid d ar d scrib d b low. • • 227 • Auto-R closing 14 • AR inhibit tim R claim tim r Th logic also incorporat s a fr qu ncy diff r nc ch ck, ith r by dir ct m asu r m nt or by using a tim r in conjunction with th phas angl ch ck. In th lat t r cas , if a 2 s cond tim r is mploy d, th logic giv s an output only if th phas diff r nc do s not xc d th phas angl s tting ov r a p riod of 2 s c onds. This limits th fr qu ncy diff r nc (in th cas of a phas angl s tting of 20o) to a maximum of 0.11% of 50Hz, corr sponding to a phas swing from +20o to -20o ov r th m asur d 2 s conds. Whil a significant fr qu ncy diff r nc i s unlik ly to aris during a d lay d autor clos s qu nc , th tim availabl al lows this ch ck to b carri d out as an additional saf guard.
14.10.1 Initiation Mod rn auto-r closing sch m s ar invariably initiat d by th tripping command of a prot ction r lay function. Som old r sch m s may mploy a contact on th circuit br ak r. Mod rn digital or num rical r lays oft n incor porat a compr h nsiv auto-r clos facility within th r lay, thus liminating th n d for a s parat autor clos r lay and any start r r lays. 14.10.2 Typ o f Prot ction On HV distribution syst ms, advantag is oft n tak n of auto-r clos ing to us instantan ous prot ction for th first trip, follow d by I.D.M.T. for subs qu nt trips in a singl fault incid nt. In such cas s, th auto-r clos r lay must provid a m ans of isolating th instantan ous r lay aft r th first tr ip. In old r sch m s, this may b don with a normally clos d contact on th aut o-r clos starting l m nt wir d into th conn ction b tw n th instantan ous r lay contact and th circuit br ak r trip coil. With digital or num rical r lays with in-built auto-r clos faciliti s, int rnal logic faciliti s will normally b us d. With c rtain supply authoriti s, it is th rul to fit tripping r lays to v ry circuit br ak r. If auto-r closing is r quir d, l ctrically r s t trip ping r lays must b us d, and a contact must b provid d ith r in th autor clo s logic or by s parat trip r lay r s tting sch m to n rgis th r s t coil b for r closing can tak plac . 14.10.3 D ad Tim r This will hav a rang of s t tings to cov r th sp cifi d high-sp d or d lay d r closing duty. Any int rlock s that ar n d d to hold up r closing until conditions ar suitabl can b conn ct d into th d ad tim r circuit. S ction 14.12.1 provid s an xampl of this a ppli d to transform r f d rs. 14.10.4 R closing Impuls Th duration of th r c losing impuls must b r lat d to th r quir m nts of th circuit br ak r closin g m chanism. On auto-r clos sch m s using spring-clos d br ak rs, it is suffici nt to op rat a contact at th nd of th d ad tim to n rgis th latch r l a s coil on th spring-closing m chanism. A circuit br ak r auxiliary switch can b us d to canc l th closing puls and r s t th auto-r clos r lay. With sol n oid op rat d br ak rs, it is usual to provid a closing puls of th ord r of 12 s conds, so as to hold th sol noid n rgis d for a short tim aft r th main contacts hav clos d. This nsur s that th m chanism s ttl s in th fully latch d-in position. Th pn umatic or hydraulic closing m chanisms fitt d to oil, air blast and SF6 circuit br ak rs us a circuit br ak r auxiliar y switch for t rminating th closing puls appli d by th auto-r clos r lay. 14 .10.5 Anti-Pumping D vic s Th function of an anti-pumping d vic is to pr v nt th circuit br ak r closing and op ning s v ral tim s in quick succ ssion. This might b caus d by th application of a closing puls whil th circuit br ak r is b ing tripp d via th prot ction r lays. Alt rnativ ly, it may occur if th c ircuit br ak r is clos d on to a fault and th closing puls is long r than th sum of prot ction r lay and circuit br ak r op rating tim s. Circuit br ak rs wi th trip fr m chanisms do not r quir this f atur . 14.10.6 R claim Tim r El ct rom chanical, static or softwar -bas d tim rs ar us d to provid th r claim ti m , d p nding on th r lay t chnology us d. If l ctrom chanical tim rs ar us d , it is conv ni nt to mploy two ind p nd ntly adjustabl tim d contacts to obta in both th d ad tim and th r claim tim on on tim r. With static and softwar bas d tim rs, s parat tim r l m nts ar g n rally provid d. 14.10.7 CB Lockou t If r closur is unsucc ssful th auto-r clos r lay locks out th circuit br a k r. Som sch m s provid a lockout r lay with a flag, with provision of a conta ct for r mot alarm. Th circuit br ak r can th n only b clos d by hand; this a ction can b arrang d to r s t th autor clos r lay l m nt automatically. Alt rnativ ly, most mod rn r lays can b configur d such that a lockout condition ca n b r s t only by op rator action. Circuit br ak r manufactur rs stat th maxi mum numb r of op rations allow d b for maint nanc is r quir d. A numb r of sch m s provid a fault trip counting function and giv a warning wh n th total ap proach s th manufactur r s r comm ndation. Th s sch m s will lock out wh n th total numb r of fault trips has r ach d th maximum valu allow d. 14.10.8 Manu al Closing It is und sirabl to p rmit auto-r closing if circuit br ak r closing is manually initiat d. Auto-r clos sch m s includ th facility to inhibit aut o-r clos initiation for a s t tim following manual CB closur . Th tim is typ ically in th rang of 2-5 s conds.
N twork Prot ction & Automation Guid • Auto-R closing 14 • • 228 •
14.10.9 Multi-Shot Sch m s Sch m s providing up to thr or four shots us timin g circuits ar oft n includ d in an auto-r clos r lay to provid diff r nt, ind p nd ntly adjustabl , d ad tim s for ach shot. Instantan ous prot ction can b us d for th first trip, sinc ach sch m provid s a signal to inhibit instant an ous tripping aft r a s t numb r of trips and s l cts I.D.M.T. prot ction for subs qu nt on s. Th sch m r s ts if r closur is succ ssful within th chos n numb r of shots, r ady to r spond to furth r fault incid nts. 14.11 AUTO-CLOSE S CHEMES Auto-clos sch m s ar mploy d to clos automatically circuit br ak rs t hat ar normally op n wh n th supply n twork is h althy. This may occur for a v ari ty of r asons, for instanc th fault l v l may b xc ssiv if th CB’s w r normally clos d. Th circuits involv d ar v ry similar to thos us d for auto-r closing. Two typical applications ar d scrib d in th following s ctions. 14.1 1.1 Standby Transform rs Figur 14.7 shows a busbar station f d by thr transfo rm rs, T1, T2 and T3. Th loss of on transform r might caus s rious ov rloadin g of th r maining two. How v r, conn ction of a furth r transform r to ov rcom this may incr as th fault l v l to an unacc ptabl valu . th standby transform r. Som sch m s mploy an auto-tripping r lay, so that wh n th faulty transform r is r turn d to s rvic , th standby is automatically di sconn ct d. 14.11.2 Bus Coupl r or Bus S ction Br ak r If all four pow r transfo rm rs ar normally in s rvic for th syst m of Figur 14.7, and th bus s ction s ar int rconn ct d by a normally-op n bus s ction br ak r inst ad of th isola tor, th bus s ction br ak r should b auto-clos d in th v nt of th loss of o n transform r, to spr ad th load ov r th r maining transform rs. This, of cou rs , is subj ct to th fault l v l b ing acc ptabl with th bus-s ction br ak r clos d. Starting and auto-trip circuits ar mploy d as in th stand-by sch m . Th auto-clos r lay us d in practic is a variant of on of th standard auto r clos r lays. 14.12 EXAMPLES OF AUTO-RECLOSE APPLICATIONS Auto-r clos facilit i s in common us for a numb r of standard substation configurations ar d scrib d in th following s ctions. 14.12.1 Doubl Busbar Substation A typical doubl busbar station is illustrat d in Figur 14.8. Each of th six EHV transmission l in s brought into th station is und r th control of a circuit br ak r, CB1 to CB6 inclusiv , and ach transmission lin can b conn ct d ith r to th main or to th r s rv busbars by manually op rat d isolators. Lin 1 Lin 2 Lin 3 CB1A T1 T2 CB2A Bus C EHV Busbars Main BC R s rv L1 T1 T2 T3 T4 (Standby) Lin 4 Lin 5 Lin 6 L4 L2 L3 L5 L6 CB1 CB2
CB3 CB4 with auto-closing IT1 IT2 CB1 CB2 CB3 CB4 CB5 CB6 • Figur 14.7: Standby transform r with auto-closing Th solution is to hav a standby transform r T4 p rman ntly n rgis d from th primary sid and arrang d to b switch d into s rvic if on of th oth rs trips on fault. Th starting circuits for br ak r CB4 monitor th op ration of transf orm r prot ction on any of th transform rs T1, T2 and T3 tog th r with th trip ping of an associat d circuit br ak r CB1-CB3. In th v nt of a fault, th auto -clos circuit is initiat d and circuit br ak r CB4 clos s, aft r a short tim d lay, to switch in N twork Prot ction & Automation Guid Figur 14.8: Doubl busbar substation Bus s ction isolators nabl s ctions of busbar to b isolat d in th v nt of f ault, and bus coupl r br ak r BC p rmits s ctions of main and r s rv bars to b int rconn ct d. 14.12.1.1 Basic sch m – bank d transform rs omitt d Each lin ci rcuit br ak r is provid d with an auto-r clos r lay that r clos s th appropria t circuit br ak rs in th • 229 • Auto-R closing 14 •
v nt of a lin fault. For a fault on Lin 1, this would r quir op ning of CB1 and th corr sponding CB at th r mot nd of th lin . Th op ration of ith r th busbar prot ction or a VT Buchholz r lay is arrang d to lock out th auto-r closing s qu nc . In th v nt of a p rsist nt fault on Lin 1, th lin circuit br ak rs trip and lock out aft r on att mpt at r closur . 14.12.1.2 Sch m wit h bank d transform rs Som utiliti s us a variation of th basic sch m in whic h Transform rs T1 and T2 ar bank d off Lin s 1 and 2, as shown in Figur 14.8. This provid s som conomy in th numb r of circuit br ak rs r quir d. Th corr sponding transform r circuits 1 and 2 ar t d off Lin s 1 and 2 r sp ctiv ly. Th transform r s condari s ar conn ct d to a s parat HV busbar syst m via cir cuit br ak rs CB1A and CB2A. Auto-r clos faciliti s can b xt nd d to cov r th circuits for bank d transform rs wh r th s ar us d. For xampl , a fault on lin 1 would caus th tripping of circuit br ak rs CB1, CB1A and th r mot li n circuit br ak r. Wh n Lin 1 is r - n rgis d, ith r by auto-r closur of CB1 or by th r mot circuit br ak r, which v r is s t to r clos first, transform r T1 is also n rgis d. CB1A will not r clos until th app aranc of transform r s condary voltag , as monitor d by th s condary VT; it th n r clos s on to th HV busbars aft r a short tim d lay, with a synchronism ch ck if r quir d. In th v nt of a fault on transform r T1, th local and r mot lin circuit br ak rs and br ak r CB1A trip to isolat th fault. Automatic op ning of th motoris d transform r isolator IT1 follows this. Th lin circuit br ak rs th n r clos in th normal mann r and circuit br ak r CB1A locks out. A shortcoming of this s ch m is that this r sults in h althy transform r T1 b ing isolat d from th sys t m; also, isolator L1 must b op n d manually b for circuit br ak rs CB1 and C B1A, can b clos d to r - stablish supply to th HV busbars via th transform r. A variant of this sch m is d sign d to instruct isolator L1 to op n automatica lly following a p rsist nt fault on Lin 1 and provid a s cond auto-r closur o f CB1 and CB1A. Th supply to Bus C is th r by r stor d without manual int rv nt ion. 14.12.2 Singl Switch Substation Th arrang m nt shown in Figur 14.9 consi sts basically of two transform r f d rs int rconn ct d by a singl circuit br a k r 120. Each transform r th r for has an alt rnativ sourc of supply in th v nt of loss of on or oth r of th f d rs. • 230 • Bus A EHV Lin 1 103 120 203 EHV Lin 2 113 T1 T2 213 Bus B B1 B2 Figur 14.9: Singl switch substation For xampl , a transi nt fault on Lin 1 caus s tripping of circuit br ak rs 120 and B1 follow d by r closur of CB 120. If th r closur is succ ssful, Transfo rm r T1 is r - n rgis d and circuit br ak r B1 r clos s aft r a short tim d lay . If th lin fault is p rsist nt, 120 trips again and th motoris d lin isolat or 103 is automatically op n d. Circuit br ak r 120 r clos s again, follow d by B1, so that both transform rs T1 and T2 ar th n suppli d from Lin 2. A transfo
rm r fault caus s th automatic op ning of th appropriat transform r isolator, lock-out of th transform r s condary circuit br ak r and r closur of circuit br ak r 120. Faciliti s for d ad lin charging or r closur with synchronism ch ck ar provid d for ach circuit br ak r. 14.12.3 Four-Switch M sh Substation Th m sh substation illustrat d in Figur 14.10 is xt nsiv ly us d by som utilit i s, ith r in full or part. Th basic m sh has a f d r at ach corn r, as show n at m sh corn rs MC2, MC3 and MC4. On or two transform rs may also b bank d a t a m sh corn r, as shown at MC1. M sh corn r prot ction is r quir d if mor tha n on circuit is f d from a m sh corn r, irr sp ctiv of th CT locations – s Ch apt r 15 for mor d tails. • Auto-R closing 14 • G1A T1A 113A G1B T1B 403 Lin 1 MC1 420 m sh corn r MC4 Lin 4 120 320 MC3 303 Lin 3 Lin 2 220 Figur 14.10: Four-switch m sh substation N twork Prot ction & Automation Guid
Consid rabl probl ms can ar ncount r d in th application of auto-r closing t o th m sh substation. For xampl , circuit br ak rs 120 and 420 in Figur 14.10 ar tripp d out for a vari ty of diff r nt typ s of fault associat d with m sh corn r 1 (MC1), and ach r quir s diff r nt tr atm nt as far as auto-r closing i s conc rn d. Furth r variations occur if th faults ar p rsist nt. Following no rmal practic , circuit br ak rs must b r clos d in s qu nc , so s qu ncing circ uits ar n c ssary for th four m sh br ak rs. Closing priority may b in any or d r, but is normally 120, 220, 320, and 420. A summary of faciliti s is now giv n, bas d on m sh corn r MC1 to show th inclusion of bank d transform rs; facili ti s at oth r corn rs ar similar but omit th op ration of quipm nt sol ly ass ociat d with th bank d transform rs. 14.12.3.1 Transi nt fault on Lin 1 Trippi ng of circuit br ak rs 120, 420, G1A and G1B is follow d by r closur of 120 to giv d ad lin charging of Lin 1. Br ak r 420 r clos s in s qu nc , with a sync hronism ch ck. Br ak rs G1A, G1B r clos with a synchronism ch ck if n c ssary. 14.12.3.2 P rsist nt fault on Lin 1 Circuit br ak r 120 trips again aft r th f irst r closur and isolator 103 is automatically op n d to isolat th fault d l in . Br ak rs 120, 420, G1A and G1B th n r clos in s qu nc as abov . 14.12.3.3 Transform r fault (local transform r 1A) Automatic op ning of isolator 113A to isolat th fault d transform r follows tripping of circuit br ak rs 120, 420, G 1A and G1B. Br ak rs 120, 420 and G1B th n r clos in s qu nc , and br ak r G1A is lock d out. 14.12.3.4 Transform r fault (r mot transform r) For a r mot tra nsform r fault, an int rtrip signal is r c iv d at th local station to trip br ak rs 120, 420, G1A and G1B and inhibit auto-r closing until th fault d transfo rm r has b n isolat d at th r mot station. If th int rtrip p rsists for 60 s conds it is assum d that th fault cannot b isolat d at th r mot station. Is olator 103 is th n automatically op n d and circuit br ak rs 120, 420, G1A and G 1B ar r clos d in s qu nc . 14.12.3.5 Transi nt m sh corn r fault Any fault cov r d by th m sh corn r prot ction zon , shown in Figur 14.10, r sults in tripp ing of circuit br ak rs 120, 420, G1A and G1B. Th s ar th n r clos d in s qu n c . Th r may b circumstanc s in which r closur onto a p rsist nt fault is not p rmitt d – cl arly it is not known in advanc of r closur if th fault is p rsist nt or not. In th s circumstanc s, sch m logic inhibits r closur and locks out th circuit br ak rs. 14.12.3.6 P rsist nt m sh corn r fault Th s qu nc d scrib in S ction 14.12.3.5 is foll ow d initially. Wh n CB 120 is r clos d, it will trip again du to th fault and lock out. At this point, th logic inhibits th r closur of CB’s 420, G1A and G1 B and locks out th s CB’s. Lin isolator 103 is automatically op n d to isolat t h fault from th r mot station. • N twork Prot ction & Automation Guid • 231 • Auto-R closing 14 •
• 15 • Busbar Prot ction Introduction Busbar faults Prot ction r quir m nts Typ s of prot ction syst m Sy st m prot ction sch m s Fram - arth prot ction (Howard prot ction) Diff r ntial prot ction principl s High imp danc diff r ntial prot ction Low imp danc bias d diff r ntial prot ction Num rical busbar prot ction R f r nc s 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11
• 15 • Busbar P rot ction 15.1 INTRODUCTION Th prot ction sch m for a pow r syst m should cov r th whol syst m against all probabl typ s of fault. Unr strict d forms of lin prot ct ion, such as ov rcurr nt and distanc syst ms, m t this r quir m nt, although f aults in th busbar zon ar cl ar d only aft r som tim d lay. But if unit pro t ction is appli d to f d rs and plant, th busbars ar not inh r ntly prot ct d. Busbars hav oft n b n l ft without sp cific prot ction, for on or mor of th following r asons: a. th busbars and switchg ar hav a high d gr of r lia bility, to th point of b ing r gard d as intrinsically saf b. it was f ar d th at accid ntal op ration of busbar prot ction might caus wid spr ad dislocation of th pow r syst m, which, if not quickly cl ar d, would caus mor loss than w ould th v ry infr qu nt actual bus faults c. it was hop d that syst m prot ctio n or back-up prot ction would provid suffici nt bus prot ction if n d d It is tru that th risk of a fault occurring on mod rn m tal-clad g ar is v ry small, but it cannot b ntir ly ignor d. How v r, th damag r sulting from on uncl ar d fault, b caus of th conc ntration of fault MVA, may b v ry xt nsiv ind d, up to th compl t loss of th station by fir . S rious damag to or d stru ction of th installation would probably r sult in wid spr ad and prolong d supp ly int rruption. Finally, syst m prot ction will fr qu ntly not provid th cov r r quir d. Such prot ction may b good nough for small distribution substation s, but not for important stations. Ev n if distanc prot ction is appli d to all f d rs, th busbar will li in th s cond zon of all th distanc prot ctions , so a bus fault will b cl ar d r lativ ly slowly, and th r sultant duration o f th voltag dip impos d on th r st of th syst m may not b tol rabl . With o utdoor switchg ar th cas is l ss cl ar sinc , although th lik lihood of a fau lt is high r, th risk of wid spr ad damag r sulting is much l ss. In g n ral t h n, busbar prot ction is r quir d wh n th syst m prot ction do s not cov r th busbars, or wh n, in ord r N twork Prot ction & Automation Guid • 233 •
to maintain pow r syst m stability, high-sp d fault cl aranc is n c ssary. Uni t busbar prot ction provid s this, with th furth r advantag that if th busbar s ar s ctionalis d, on s ction only n d b isolat d to cl ar a fault. Th cas for unit busbar prot ction is in fact strong st wh n th r is s ctionalisation . 1 5 . 2 B U S B A R F A U LT S Th majority of bus faults involv on phas an d arth, but faults aris from many caus s and a significant numb r ar int rpha s cl ar of arth. In fact, a larg proportion of busbar faults r sult from huma n rror rath r than th failur of switchg ar compon nts. With fully phas -s gr gat d m talclad g ar, only arth faults ar possibl , and a prot ction sch m n d hav arth fault s nsitivity only. In oth r cas s, an ability to r spond to p has faults cl ar of arth is an advantag , although th phas fault s nsitivity n d not b v ry high. 1 5 . 3 P R OT E C T I O N R E Q U I R E M E N T S Altho ugh not basically diff r nt from oth r circuit prot ction, th k y position of t h busbar int nsifi s th mphasis put on th ss ntial r quir m nts of sp d an d stability. Th sp cial f atur s of busbar prot ction ar discuss d b low. incid nc , amounting to no mor than an av rag of on fault p r busbar in tw nt y y ars, it is cl ar that unl ss th stability of th prot ction is absolut , th d gr of disturbanc to which th pow r syst m is lik ly to b subj ct d may b incr as d by th installation of bus prot ction. Th possibility of incorr ct op ration has, in th past, l d to h sitation in applying bus prot ction and ha s also r sult d in application of som v ry compl x syst ms. Incr as d und rstan ding of th r spons of diff r ntial syst ms to transi nt curr nts nabl s such syst ms to b appli d with confid nc in th ir fundam ntal stability. Th th ory of diff r ntial prot ction is giv n lat r in S ction 15.7. Notwithstanding th compl t stability of a corr ctly appli d prot ction syst m, dang rs xist in pr actic for a numb r of r asons. Th s ar : a. int rruption of th s condary circ uit of a curr nt transform r will produc an unbalanc , which might caus trippi ng on load d p nding on th r lativ valu s of circuit load and ff ctiv s ttin g. It would c rtainly do so during a through fault, producing substantial fault curr nt in th circuit in qu stion b. a m chanical shock of suffici nt s v rity may caus op ration, although th lik lihood of this occurring with mod rn num r ical sch m s is r duc d c. accid ntal int rf r nc with th r lay, arising from a mistak during maint nanc t sting, may l ad to op ration In ord r to maintain th high ord r of int grity n d d for busbar prot ction, it is an almost invar iabl practic to mak tripping d p nd on two ind p nd nt m asur m nts of fault quantiti s. Mor ov r, if th tripping of all th br ak rs within a zon is d riv d from common m asuring r lays, two s parat l m nts must b op rat d at ach stag to compl t a tripping op ration. Although not curr nt practic , in many c as s th r lays ar s parat d by about 2 m tr s so that no r asonabl accid ntal m chanical int rf r nc to both r lays simultan ously is possibl . Th two m as ur m nts may b mad by two similar diff r ntial syst ms, or on diff r ntial sy st m may b ch ck d by a fram - arth syst m, by arth fault r lays n rgis d by curr nt transform rs in th transform r n utral- arth conductors or by ov rcurr nt r lays. Alt rnativ ly, a fram - arth syst m may b ch ck d by arth fault r l ays. If two syst ms of th unit or oth r similar typ ar us d, th y should b n rgis d by s parat curr nt transform rs in th cas of high imp danc unbias d diff r ntial sch m s. Th duplicat ring CT cor s may b mount d on a common pr imary conductor but N twork Prot ction & Automation Guid Busbar P rot ction 15.3.1 Sp d Busbar prot ction is primarily conc rn d with: a. limitation of con s qu ntial damag b. r moval of busbar faults in l ss tim than could b achi v d by back-up lin prot ction, with th obj ct of maintaining syst m stability So m arly busbar prot ction sch m s us d a low imp danc diff r ntial syst m havi ng a r lativ ly long op ration tim , of up to 0.5 s conds. Th basis of most mod rn sch m s is a diff r ntial syst m using ith r low imp danc bias d or high i mp danc unbias d r lays capabl of op rating in a tim of th ord r of on cycl
at a v ry mod rat multipl of fault s tting. To this must b add d th op rat ing tim of th tripping r lays, but an ov rall tripping tim of l ss than two c ycl s can b achi v d. With high-sp d circuit br ak rs, compl t fault cl aranc may b obtain d in approximat ly 0.1 s conds. Wh n a fram - arth syst m is us d, th op rating sp d is comparabl . 15.3.2 Stability Th stability of bus prot ction is of paramount importanc . B aring in mind th low rat of fault • 15 • • 234 •
ind p nd nc must b maintain d throughout th s condary circuit. In th cas of low imp danc , bias d diff r ntial sch m s that cat r for un qual ratio CT s, t h sch m can b n rgis d from ith r on or two s parat s ts of main curr nt transform rs. Th crit ria of doubl f atur op ration b for tripping can b ma intain d by th provision of two s ts of ratio matching int rposing CT s p r cir cuit. Wh n multi-contact tripping r lays ar us d, th s ar also duplicat d, on b ing n rgis d from ach discriminating r lay; th contacts of th tripping r lay ar th n s ri s-conn ct d in pairs to provid tripping outputs. S parat tr ipping r lays, ach controlling on br ak r only, ar usually pr f rr d. Th imp ortanc of such r lays is th n no mor than that of normal circuit prot ction, s o no duplication is r quir d at this stag . Not l ast among th advantag s of us ing individual tripping r lays is th simplification of trip circuit wiring, com par d with taking all trip circuits associat d with a giv n bus s ction through a common multi-contact tripping r lay. In doubl busbar installations, a s parat prot ction syst m is appli d to ach s ction of ach busbar; an ov rall ch ck syst m is provid d, cov ring all s ctions of both busbars. Th s parat zon s ar arrang d to ov rlap th busbar s ction switch s, so that a fault on th s ctio n switch trips both th adjac nt zon s. This has som tim s b n avoid d in th p ast by giving th s ction switch a tim advantag ; th s ction switch is tripp d first and th r maining br ak rs d lay d by 0.5 s conds. Only th zon on th f aulty sid of th s ction switch will r main op rat d and trip, th oth r zon r s tting and r taining that s ction in s rvic . This gain, applicabl only to v ry infr qu nt s ction switch faults, is obtain d at th xp ns of s riously d l aying th bus prot ction for all oth r faults. This practic is th r for not g n rally favour d. Som variations ar d alt with lat r und r th mor d tail d s ch m d scriptions. Th r ar many combinations possibl , but th ss ntial prin cipl is that no singl accid ntal incid nt of a s condary natur shall b capab l of causing an unn c ssary trip of a bus s ction. S curity against malop ratio n is only achi v d by incr asing th amount of quipm nt that is r quir d to fun ction to compl t an op ration; and this in vitably incr as s th statistical ri sk that a tripping op ration du to a fault may fail. Such a failur , l aving as id th qu stion of cons qu ntial damag , may r sult in disruption of th pow r syst m to an xt nt as gr at, or gr at r, than would b caus d by an unwant d tr ip. Th r lativ risk of failur of this kind may b slight, but it has b n tho ught worthwhil in som instanc s to provid a guard in this r sp ct as w ll. S curity of both stability and op ration is obtain d by providing thr ind p nd nt chann ls (say X, Y and Z) whos outputs ar arrang d in a ‘two-out-of thr ’ vot ing arrang m nt, as shown in Figur 15.1. + X Y Z Y Z X Trip circuits _ Figur 15.1: Two-out-of-thr principl 1 5 . 4 T Y P E S O F P R OT E C T I O N S Y S T E M A numb r of busbar prot cti on syst ms hav b n d vis d: a. syst m prot ction us d to cov r busbars b. fram - arth prot ction c. diff r ntial prot ction d. phas comparison prot ction . dir ctional blocking prot ction Of th s , (a) is suitabl for small substations only, whil (d) and ( ) ar obsol t . D tail d discussion of typ s (b) and (c) o ccupi s most of this chapt r. Early forms of bias d diff r ntial prot ction for busbars, such as v rsions of Translay prot ction and also a sch m using harmo nic r straint, w r sup rs d d by unbias d high imp danc diff r ntial prot ctio n. Th r lativ simplicity of th latt r, and mor importantly th r lativ as with which its p rformanc can b calculat d, hav nsur d its succ ss up to th pr s nt day. But mor r c ntly th advanc s in s miconductor t chnology, coupl d with a mor pr ssing n d to b abl to accommodat CT s of un qual ratio, ha v l d to th r -introduction of bias d sch m s, g n rally using static r lay d signs, particularly for th most xt nsiv and on rous applications. Fram - arth
prot ction syst ms hav b n in us for many y ars, mainly associat d with smal l r busbar prot ction sch m s at distribution voltag s and for m talclad busbars ( .g. SF6 insulat d busbars). How v r, it has oft n b n quit common for a uni t prot ction sch m to b us d in addition, to provid two s parat m ans of fau lt d t ction. Th diff r nt typ s of prot ction ar d scrib d in th following s ctions. • N twork Prot ction & Automation Guid • 235 • Busbar P rot ction 15 •
1 5 . 5 S Y S T E M P R OT E C T I O N S C H E M E S Syst m prot ction that incl ud s ov rcurr nt or distanc syst ms will inh r ntly giv prot ction cov r to th busbars. Ov rcurr nt prot ction will only b appli d to r lativ ly simpl dist ribution syst ms, or as a back-up prot ction, s t to giv a consid rabl tim d lay. Distanc prot ction will provid cov r for busbar faults with its s cond an d possibly subs qu nt zon s. In both cas s th busbar prot ction obtain d is slo w and suitabl only for limiting th cons qu ntial damag . Th only xc ption is th cas of a m sh-conn ct d substation, in which th curr nt transform rs ar locat d at th circuit br ak rs. H r , th busbars ar includ d, in s ctions, in th individual zon s of th main circuit prot ction, wh th r this is of unit ty p or not. In th sp cial cas wh n th curr nt transform rs ar locat d on th lin sid of th m sh, th circuit prot ction will not cov r th busbars in th instantan ous zon and s parat busbar prot ction, known as m sh-corn r prot cti on, is g n rally us d – s S ction 15.7.2.1 for d tails. 1 5 . 6 F R A M E - E A R T H P R OT E C T I O N ( H O WA R D P R OT E C T I O N ) Fram l akag prot ct ion has b n xt nsiv ly us d in th past in many diff r nt situations. Th r ar s v ral variations of fram l akag sch m s availabl , providing busbar prot c tion sch m s with diff r nt capabiliti s. Th following s ctions sch m s hav th us b n r tain d for historical and g n ral r f r nc purpos s. A consid rabl n umb r of sch m s ar still in s rvic and fram l akag may provid an acc ptabl solution in particular circumstanc s. How v r, th n d to insulat th switch board fram and provid cabl gland insulation and th availability of alt rnati v sch m s using num rical r lays, has contribut d to a d clin in us of fram l akag syst ms. 15.6.1 Singl -Busbar Fram -Earth Prot ction This is pur ly an arth fault syst m and, in principl , involv s simply m asuring th fault curr nt flowing from th switchg ar fram to arth. A curr nt transform r is mount d on th arthing conductor and is us d to n rgiz a simpl instantan ous r lay as shown in Figur 15.2. No oth r arth conn ctions of any typ , including incid nt al conn ctions to structural st lwork ar allow d. This r quir m nt is so that: a. th principal arth conn ction and curr nt transform r ar not shunt d, th r by raising th ff ctiv s tting. An incr as d ff ctiv s tting giv s ris to th possibility of r lay malop ration. This risk is small in practic G H J K Fram - arth fault r lay N utral ch ck r lay > I > I + Trip all circuit br ak r Figur 15.2: Singl zon fram - arth prot ction b. arth curr nt flowing to a fault ls wh r on th syst m cannot flow into or out of th switchg ar fram via two arth conn ctions, as this might l ad to a s purious op ration Th switchg ar must b insulat d as a whol , usually by standi ng it on concr t . Car must b tak n that th foundation bolts do not touch th st l r inforc m nt; suffici nt concr t must b cut away at ach hol to p rmi
t grouting-in with no risk of touching m talwork. Th insulation to arth finall y achi v d will not b high, a valu of 10 ohms b ing satisfactory. Wh n plannin g th arthing arrang m nts of a fram l akag sch m , th us of on common l c trod for both th switchg ar fram and th pow r syst m n utral point is pr f r r d, b caus th fault path would oth rwis includ th two arthing l ctrod s in s ri s. If ith r or both of th s ar of high r sistanc or hav inad quat curr nt carrying capacity, th fault curr nt may b limit d to such an xt nt th at th prot ction quipm nt b com s inop rativ . In addition, if th l ctrod arthing th switchg ar fram is th off nd r, th pot ntial of th fram may b rais d to a dang rous valu . Th us of a common arthing l ctrod of ad quat rating and low r sistanc nsur s suffici nt curr nt for sch m op ration and li mits th ris in fram pot ntial. Wh n th syst m is r sistanc arth d, th ar thing conn ction from th switchg ar fram is mad b tw n th bottom of th ar thing r sistor and th arthing l ctrod . Figur 15.3 illustrat s why a low r l imit of 10 ohms insulation r sistanc b tw n fram and arth is n c ssary. N twork Prot ction & Automation Guid • Busbar P rot ction 15 • • 236 •
Outgoing f d r Switchg ar fram Fram -l akag curr nt transform r Switchg ar fram bonding bar Zon G G n rator K Insulation barri rs IF = I1 + I2 Zon H L Zon J M Earth bar Syst m arning r sistor I1 + I2 I1 I2 Earthing l ctrod r sistanc I > Zon G fram l akag r lay I > I1 Fram insulation r sistanc to arth Zon H fram l akag r lay Trip r lays K L1 L2 M Figur 15.3: Curr nt distribution for xt rnal fault Trip K Trip L Trip M All cabl glands must b insulat d, to pr v nt th circulation of spurious curr nt through th fram and arthing syst m by any voltag s induc d in th cabl sh ath. Pr f rably, th gland insulation should b provid d in two lay rs or stag s, with an int rposing lay r of m tal, to facilitat th t sting of th gland in sulation. A t st l v l of 5kV from ach sid is suitabl . 15.6.2 Fram -Earth Pro t ction - S ction d Busbars S ction 15.6.1 cov r d th basic r quir m nts for a syst m to prot ct switchg ar as a whol . Wh n th busbar is divid d into s ction s, th s can b prot ct d s parat ly, provid d th fram is also sub-divid d, th s ctions mutually insulat d, and ach provid d with a s parat arth conductor , curr nt transform r and r lay. Id ally, th s ction switch should b tr at d a s a s parat zon , as shown in Figur 15.4, and provid d with ith r a s parat r lay or two s condari s on th fram -l akag curr nt transform r, with an arran g m nt to trip both adjac nt zon s. Th individual zon r lays trip th ir r sp c
tiv zon and th s ction switch. Trip r lays Insulation barri r Zon G K J Zon H L • Zon G I > Zon H I > J K1 K2 L Trip J Trip K Trip L Figur 15.5: Fram - arth sch m : bus s ction br ak r insulat d on on sid only N twork Prot ction & Automation Guid • 237 • Busbar P rot ction 15 • Und r xt rnal fault conditions, th curr nt I1 flows through th fram -l akag curr nt transform r. If th insulation r sistanc is too low, suffici nt curr nt may flow to op rat th fram -l akag r lay, and, as th ch ck f atur is unr s trict d, this will also op rat to compl t th trip circuit. Th arth r sistan c b tw n th arthing l ctrod and tru arth is s ldom gr at r than 1Ω, so wit h 10Ω insulation resistance the current I1 is limite to 10% of the total earth fa ult current I1 an I2. For this reason, the recommen e minimum setting for the scheme is about 30% of the minimum earth fault current. Figure 15.4: Three zone frame earth scheme
If it is inconvenient to insulate the section switch frame on one si e, this swi tch may be inclu e in that zone. It is then necessary to intertrip the other zo ne after approximately 0.5 secon sif a fault persists after the zone inclu ing the section switch has been trippe . This is illustrate in Figure 15.5.
For the it is necessary to have a least one infee or above schemes to function earthe source of supply, an in the latter case it isessential that this sourc e of supply be connecte to the si e of the switchboar not containing the secti on switch. Further, if possible, it is preferable that an earthe source of supp ly be provi e on both si es of the switchboar , in orer to ensure that any fau lts that may evelop between the insulating barrier an the section switch will continue to be fe with fault current after the isolation of the first half of t he switchboar , an thus allow the fault to be remove . Of the two arrangements, the first is the one normally recommen e , since it provi es instantaneous clea rance of busbar faults on all sections of the switchboar . 15.6.3 Frame-Earth Sc heme - Double Bus Substation It is not generally feasible to separately insulate the metal enclosures of the main an auxiliary busbars. Protectionis therefore generally provi e as for single bus installations, but with the a itional fea ture that circuits connecte to the auxiliary bus are trippe for all faults, as shown in Figure 15.6. Insulation barriers Zone J Zone G j1 K > M L h1 H j2 as operation ue to mechanical shock or mistakes ma e by personnel. Faults in th e low voltage auxiliary wiring must also be prevente from causing operation by passing current to earth through the switchgear frame. A useful check is provi e by a relay energise by the system neutral current, or resi ual current. If th e neutral check cannot be provi e , the frame-earth relays shoul have a short t ime elay. When a check system is use , instantaneousrelays can be use , with a setting of 30% of the minimum earth fault current an an operating time at five times setting of 15 millisecon s or less. Figure 15.7 shows a frame-leakage sch emefor a metalcla switchgear installation similar to that shown in Figure 15.4 an incorporating a neutral current check obtaine from a suitable zero sequenc e current source, such as that shown in Figure 15.2. + In Out 64A-1 GH CSS-G 64B-1 CSS-H Trip relays K L1 L2 M L5 64CH-1 _ 64CH-2 64A-2 64B-2 I > Busbar P rotection g 74-1 74-2 I Zone H relay I > _ In Out L3 L4
Zone G relay + CSS-G L3 L4 L6 g1 j1 K M1 M2 L1 L2 Tripping relays CSS-H • 15 • 74 Alarm cancellation relay CSS Control selector switch protection in/protection out L3 Busbar protection in service lamp L4 Busbar protection out of service la mp L5 Tripping supply healthy lamp L6 Alarm an in ication supply healthy lamp F igure 15.7: Typical tripping an alarm circuits for a frame-leakage scheme h1 GH D.C. Zone bus wires j2 N Busbar isolator auxiliary switches Figure 15.6: Frame-earth scheme for ouble busbar substation 15.6.4 Frame-Earth Protection -Check System On all but the smallest equipments, a check system shoul be provi e to guar against such contingencies The protection relays use for the iscriminating an check functions are of the attracte armature type, with two normally open The trippi self reset contacts. ng circuits cannot be complete unless both the iscriminating an check relays o perate; this is because the iscriminating an check relay contacts are connecte in series. The tripping relays are of the attracte armature type. Network Protection & Automation Gui e • 238 •
It is operation of the protection scheme wit usual to supervise the satisfactory h au ible an visual alarms an in ications for the following: a. busbar faults b. busbar protection in service c. busbar protection out of service . tripping supply healthy e. alarm supply healthy Toenablethe protection equipment of eac h zone to be taken out of service in epen ently uring maintenance perio s, isol ating switches one switch per zone are provi e in the trip supply circuits an an alarm cancellation relay is use . 1 5 . 7 D I F F E R E N T I A L P R OT E C T I O N P R I N C I P L E S The Merz-Price principle is applicable to a mult i-terminal zone such as a busbar. The principle is a irect application of Kirch hoff s first law. Usually, the circulating current arrangement is use , in which the current transformers an interconnections form an analogue of the busbar an circuit connections. A relay connecte across the CT bus wires represents a fa ult path in the primary system in the analogue an hence is not energise until a fault occurs on the busbar; it then receives an input that, in principle at le ast, represents the fault current. The scheme may consist of a single relay connecte to the bus wires connecting a ll the current transformers in parallel, one set per circuit, associate with a particular zone, as shown in Figure 15.8(a). This will give earth fault protecti on for the busbar. This arrangementhas often been thought to be a equate. If th e current transformers are connecte as a balance group for each phase together with a three-element relay, as shown in Figure 15.8(b), a itional protection f or phase faults can be obtaine . The phase an earth fault settings are i entica l, an this scheme is recommen e for its ease of application an goo performan ce. 15.7.1 Differential Protection for Sectionalise an Duplicate Busbars Each section of a ivi e bus is provi e with a separate circulating current system. The zones so forme are over-lappe across the section switches, sothat a faul t on the latter will trip the two a jacent zones. This is illustrate in Figure 15.9.Tripping two zones for a section switch fault can be avoi e by using the time- elaye technique of Section 15.6.2. However instantaneous operation is the preferre choice. Zone A G H J K I > B Differential f relay Zone B BS BC • Zone C G
H Typical fee er circuits Figure 15.9: Zones of protection for ouble bus statio n A B C N Differential relay I I> I > For ouble bus installation, the two busbars will be treate as separate zones. The auxiliary busbar zone will overlap the appropriate main busbar zone at the b us coupler. Since any circuit may be transferre from one busbar to the other by isolator switches, these an the associate tripping circuit must also be switc he to the appropriate b) Phase an earth fault circulating current scheme using three-element relay Fi gure 15.8: Circulating current scheme Network Protection & Automation Gui e
• 239 • Busbar P rotection 15 •
zone by early make an late break auxiliary contacts. This is to ensure that when the isolators are closing, the auxiliary switches makebefore the main con tacts of the isolator, an that when the isolators are opene , their main contac ts part before the auxiliary switches open. The result is that the secon ary cir cuits of the two zones concerne are briefly parallele while the circuit is bei ng transferre ; these two zones have in any case been unite through the circuit isolators uring the transfer operation. 15.7.2 Location of Current Transformer s I eally, the separate iscriminating zones shoul overlap each other an also the in ivi ual circuit protections. The overlap shoul occur across a circuit br eaker, so that the latter lies in both zones.For this arrangement it is necessa ry to install current transformers on both si es of the circuit breakers, which is economically possible with many but not all types of switchgear. With both th e circuit an the bus protection current transformers on the same si e of the ci rcuit breakers, the zones may be overlappe at the current transformers, but a f ault between the CT location an the circuit breaker will not be completely isol ate . This matter is important in all switchgear to which these con itions apply , an is particularly important in the case of out oor switchgear where separate ly mounte , multi-secon ary current transformers are generally use . The con iti ons are shown in Figure 15.10. (a) (b) Figure 15.10(a) shows the i eal arrangement in which both the circuit an busbar zones are overlappe leaving no region of the primary circuit unprotecte. Figu re 15.10(b) shows how mounting all current transformers on the circuitsi e of t he breaker results in a small region of the primary circuit unprotecte . This un protecte region is typically referre to as the ‘short zone’. The fault shown will cause operation of the busbar protection, tripping the circuit breaker, but the fault will continue to be fe from the circuit, if a source of power is present. It is necessary for the bus protection to intertrip the far en of the circuit protection, if the latter is of the unit type.With reference to Figure 15.10(b) , special ‘short zone’ protection can be provi e to etectthat the circuit breaker has opene but that the fault current is still flowing. Un er these con itions, the protection can initiate an intertrip to the remote en of the circuit. This technique may be use , particularly when the circuit inclu es a generator. In t his case the intertrip proves that the fault is in theswitchgear connections an not in the generator; thelatter is therefore but not shu trippe electrically t own on the mechanical si e so as to be imme iately rea y for further service if the fault can be cleare . 15.7.2.1 CT locations for mesh-connecte substation s The protection of busbars in mesh connecte substations gives rise to a ition al consi erations in respect of CT location. A single mesh corner is shown in Fi gure Mesh corner (Note 1) Busbar P rotection Bus protection
Line protection relay Note 1: Only 1 connection to the mesh corner permitte (a) CT arrangements for protection inclu ing mesh corner Fault • 15 • Transformer protection Mesh corner (Note 2)
Circuit protection Line protection a. Current transformers mounte on both si es transformers mounte on circuit si of breaker -no unprotecte region b. Current e only ofbreaker -fault shown not cleare by circuit protection Figure 15.10: Unprotecte zone with current transformers mounte on one si e of the circuit br
eaker only • 240 •
Mesh corner protection Note 2: Multiple circuits may be connecte to the mesh co rner (b) CT arrangements for protection a itional mesh corner protection requir e Figure 15.11: Mesh-corner protection Network Protection & Automation Gui e
15.11(a). Where only one connection to the mesh is ma e at a corner, CT’s locate as shown of the mesh will provi e protection not only to the line but the corner inclu e between them. However, this arrangement cannot be use where more than one connection is ma e to a mesh corner. This is because a fault on any of the connecte circuits woul result in isconnection of them all, without any means of etermining the faulte connection. Protection CT’s must therefore be locate o n each connection, as the corner of the me shown in Figure15.11(b). This leaves sh unprotecte , so a itional CT’s an a relay to provi e mesh-corner protection a re a e , as also shown in Figure 15.11(b). 15.8 HIGH IMPEDANCE D I F F E R E N T I A L P R OT E C T I O N This form of protection isstill in common use. The c onsi erations that have to be taken into account are etaile in the following s ections. 15.8.1 Stability The inci ence of fault current with an initial unilate ral transient component causes an abnormal buil -up of flux in a current transfo rmer, as escribe in Section 6.4.10. When through-fault current traverses a zon e protecte by a ifferential system, the transient flux pro uce in the current transformers is not etrimental as long as it remains within the substantially linear range of current of appreciabl the magnetising characteristic. With fault e magnitu e an long transient time constant, the flux ensity will pass into th e saturate region of the characteristic; this will not in itself pro uce a spil l output from a pair of balancing current transformers provi e that these are i entical an equally bur ene . A group of current transformers, though they may be of the same esign, will not be completely i entical, but a more important fa ctor is inequality of bur en. In the case of a ifferential system for a busbar, an external fault may be fe through a single circuit,the current being suppli e to the busbar The faulte circuit is many times m through all other circuits. ore heavily loa e than the others an the correspon ing current transformers ar e likely to be heavily saturate , while those of the other circuits are not. Sev ere unbalance is therefore probable, which, with a relay of normal bur en, coul excee any acceptable For this reason such systems were at one current setting. time always provi e with a time elay. This practice is, however, no longer ac ceptable. It is not feasible to calculate the spill current that may occur, but, fortunately, this is not necessary; an alternative approach provi es both the n ecessary information an the technique require to obtain a high performance. An equivalent circuit, as in Figure 15.12, can represent a circulating current s ystem. G H RCTG RLG RLH RCTH
R R ZEG I > ZEH Figure 15.12: Equivalent circuit of circulating current system Saturation has the effect of lowering the exciting impe ance, an is assume to H until, at the limit, the shunt impe take place severely in current transformer ance becomes zero an the CT can pro uce no output. This con ition is represent e by a short circuit, shown in broken line, across the exciting impe ance. It s houl be note that this is not the equivalent of a physical short circuit, sinc e it is behin the win ing resistance . Applying the Thévenin metho of solution, the voltage evelope across the relay will be given by: IR= Vf R R + R LH + R C
TH • ...Equation 15.1 The current through the relay is given by: = I f ( R LH + R CTH ) R R + R LH + R CTH ...Equation 15.2 IfRR is small, IR will approximate to IF, which is unacceptable. On the other h an , if RR is large IR is re uce . Equation 15.2 can be written, with little err or, as follows: Network Protection & Automation Gui e • 241 • Busbar P rotection 15 •
The current transformers are replace in the iagram by i eal current transforme rs that represents the magnetising lossesan seco fee ing an equivalent circuit n ary win ing resistance, an also the resistance of the connecting lea s. These circuits can then be interconnecte as shown, with a relay connecte to the jun ction points to form the complete equivalent circuit.
IR I Vf = = RR f (R LH + R CTH ) …Equation 15.3 RL + RCT = lea + CT win ing resistance K = factor epen ing on relay esign (ra nge 0.7 - 2.0) RR or alternatively: I RRR = V f = I f ( R LH + R CTH ) …Equation 15.4 It remains to be shown that the setting chosen is suitable. The current transfor mers will have an excitation curve which has not so far been relate to the rela y setting voltage, the latter being equal to the maximum nominal voltage rop ac ross the lea loop an the CT secon ary win ing resistance, with the maximum sec on ary fault current flowing through them. Uner in-zone fault con itions it is necessary for the current transformers t to pro uce sufficient output to operate he relay. This willbe achieve provi e the CT knee-point voltage excee s the r elay setting. In or erto cater for errors, it is usual to specify that the curr ent transformers shoul have a knee-point e.m.f. of at least twice the necessary setting voltage; a higher multiple is of a vantage in ensuring a high spee of operation. 15.8.2 Effective Setting or Primary Operating Current The minimum pri mary operating current is a further criterion of the esign of a ifferential sy stem. The secon ary effective setting is the sum of the relay minimum operating current an the excitation losses in all parallel connecte current transformers , whether carrying primary current or not. This summation shoul strictly speaki ng be vectorial, but is usually one arithmetically. It can be expresse as: IR = IS +nIeS where: IR = effective setting IS = relay circuit setting current IeS = CT excitation current atrelay setting voltage n = number of parallel - connec te CT’s Having establishe the relay setting voltage from stability consi eration s, as shown in Section 15.8.1, an knowing the excitation characteristic of the current transformers, the effective setting can be compute . The secon ary setti ng is converte to the primary operating current by multiplying by the turns rat io of the current transformers. The operating current so etermine shoul be co nsi ere in terms of the con itions of the application. For a phase an earth fa ult scheme the for mini setting can be base on the fault current to be expecte mum plant an maximum system outage con itions. However, it shoul be remembere that: • 242 • Network Protection & Automation Gui e It is clear that, by increasing RR, the spill current IR can be re uce below an y specifie relay setting. RR is frequently increase by the a ition of a serie s-connecte resistor which is known as the stabilising resistor. It can also be seen from Equation 15.4 that it is only the voltage rop in the relay circuit at setting current that is important. The relay can be esigne as a voltage measu ring evice consuming negligible current; an provi e its setting voltage excee
s the value Vf of Equation 15.4, the system will be stable. In fact, the settin g voltage nee not excee Vf, since the erivation of Equation 15.4 involves an extreme con ition of unbalance between the G an H current transformers that is not completely realise . So a safety margin is built-in if the voltage setting i sma e equal to Vf. It is necessary to realise that the value of If to be insert e in Equation 15.4 is the complete function of the fault current an the spill current IR through the relay, in the limiting con ition, will be of the same for m. If the relay requires more time to operate than the effective uration of the .c. transient component, or has been esigne with special features to block t he .c. component, then this factor can be ignore an only the symmetrical valu e of the fault current nee be entere in Equation 15.4. If the relay setting vo ltage, Vs, is ma e equal to Vf, that is, If (RL + RCT), an inherent safety facto r of the or er of twowill exist. In the case of a faster relay, capable of oper ating in one cycle an with no special features to block the .c. component, it is the r.m.s. value of the first offset wave that is significant. This value, fo r a fully offset waveform _ with no .c. ecrement, is √3If. If settings are then chosen in terms of_ the symmetrical component of the fault current, the √3 factor which has been ignore will take up most of the basic safety factor, leaving onl y a very small margin. Finally, if a truly instantaneous relay were use , the re levant value of If woul be the maximum offset peak. In this case, the factor ha s become less than unity, possibly as low as 0.7. It is therefore possible to re write Equation 15.4 as: I SL = where: ISL VS = stability of scheme = relay circu it voltage setting K × VS R L + R CT Busbar P rotection ...Equation 15.6 • 15 • …Equation 15.5
a. phase-phasefaults give only 86% of the threephase fault current b. fault arc resistance an earth path resistance re uce fault currents somewhat c. a reason able margin shoul be allowe to ensure that relays operate quickly an ecisive ly It is esirable that the primary effective setting shoul not excee 30% of t he prospective minimum fault current. In the case of a scheme exclusively for ea rth fault protection, the minimumearth fault current shoul be consi ere , taki ng into account any earthing impe ance that might be present as well. Furthermor e, in the event of a ouble phase to earth fault, regar less of the interphase c urrents, only 50% of the system e.m.f. is available in the earth path, causing a further re uction in the earth fault current. The primary operating current mus t therefore earth fault curr be not greater than 30% of the minimum single-phase ent. In or er to achieve high-spee operation, it is esirable that settings sho ul be still lower, particularly in the case of the soli ly earthe power system . The in conjunction with transient component of the fault current unfavourable resi ual flux in the CT can cause a high egree of saturation an loss of output , possibly lea ing to a elay of several cycles a itional to the natural operat ing time of the element. This will not happen to any large egree if the fault current is a larger multip le of setting; for example, if the fault current is five times the scheme primar y operating current an the CT knee-point e.m.f. is three times the relay settin g voltage, the a itional elay is unlikely to excee one cycle. The primary ope ratingcurrent is sometimes esigne to excee the maximum expecte circuit loa in or er to re uce the possibility of false operation un er loa current as a r esult of a broken CT lea . Desirable as this safeguar may be, it will be seen t hat it is better not to increase the effective current setting too much, as this will sacrifice some spee ; the check feature in any case, maintainsstability. An overall earth fault scheme for a large istribution boar may be ifficult to esign because of the large number of current transformers parallele together, which may lea to an excessive setting. It may be a vantageous in such a case t o provi e a three-element phase an earth fault scheme, mainly to re uce the num ber of current transformers parallele into one group. Extra-high-voltage substa tions usually present no such problem. Using the voltage-calibrate relay, the c urrent consumption can be very small. Asimplification can be achieve by provi ing one relay per circuit, all connecte to the CT paralleling buswires. Zone R c1 c2 D a1 Zone M1 E F Zone M2 b1 G H • c1 c Zone M2 Zone R Bus wires Check zone Bus wires 95 CHX-2 B C A B C N
Zone relay same as check M1 First main busbar M2 Secon main busbar R Reserve bu sbar Zone M1 relay same as check + _
Zone M2 relay same as check Stabilising Resistor I High Impe ance g p Circulatin g Current Relay I > I Supervision Relay Metrosil o (non-linear resistor) Figure 15.13: A.C. cir cuits for high impe ance circulating current scheme for uplicate busbars Network Protection & Automation Gui e • 243 •
Busbar P rotection 15 •
+ In Out CSS-M1 CSS-M2 CSS-R 87M1-1 87M2-1 87R-1 M1 M2 R 96 D2 96 D1 96 E 96 F1 96 G 96 H1 87CH-1 a1 c1 b1 c2 96 F2 D.C. Buswires 96 H2 80 T 87M1-2 87M2-2 87R-2 87CH-2 30 M2 95 M1X 95 RX 74 30 M1 30 R 95 M2X 95 CHX 95M1-1 95M2-1 95R-1 95CH-1 30M1-1 30M2-1 30R-1 95M1X-1 95M2X-1 95RX-1 95CHX-1 In Out 74-1 74-2 Busbar P rotection L1 L2 CSS-M1 L1 L2 CSS-M2 L1 L2 • 15 • 30 74 80 87 95 CSS-R
I Zone in icating relay Alarm cancellation relay D.C. volts supervision relay Hi gh impe ance circulating current relay Bus wires supervision relay 80 95X CSS L1 L2
Zone bus wiresshorting relay Control selector switch In icating lamp protection in service In icating lamp protection out of service Figure 15.14: D.C. circuits for high impe ance circulating current scheme
This enablesthe trip circuits to be confine to the least area an re uces the risk of acci ental operation. 15.8.3 Check Feature Schemes for earth faults only can be checke by a frameearth system, applie to the switchboar as a whole, n o sub ivision being necessary. For the check will usually be phase fault schemes, a similar type of scheme applie to the switchboar as a single overall zone. A set ofcurrent transformers separate from those use in the iscriminating zones shoul be provi e . No CT switching is require an no current transformers are • 244 •
Network Protection & Automation Gui e
nee e for the check zone in bus-coupler an bussection breakers. 15.8.4 Supervi sion of CT Secon ary Circuits Any interruption of a CT secon ary circuit up to t he paralleling will cause an unbalance in the system, equivalen interconnections t to the loa being carrie by the relevant primary circuit. Even though this e gree of spurious output is below the effective setting the con ition cannot be i gnore , since it is likely to lea to instability un er any through fault con it ion. Supervision can be carrie out to etect such con itions by connecting a se nsitive alarm relay across the bus wires of each zone. For a phase an earth fau lt scheme, an internal three-phase rectifier can be use to effect a summation o f the bus wire voltages on to a single alarmelement; see Figures 15.13 an 15.1 4. The alarm relay operation oes not occur with the protection s is set so that ystem healthy un er normal loa .Subject to this proviso, the alarm relay is ma e as sensitive as possible; the esire effective setting is 125 primary amperes or 10% of the lowest circuit rating, whichever is the greater. Since a relay of this or er of sensitivity islikely to operate uring through faults, a time e lay, typically of three secon s, is applie to avoi unnecessary alarm signals. 15.8.5 Arrangement of CT Connections It is 15.4 how the settin shown in Equation g voltage for a given stability level is irectly relate to the resistance of t he CT secon ary lea s. This shoul therefore be kept to a practical minimum. Tak ing into account the practical physical laying of auxiliary cables, the CT busw ires are best arrange in the form aroun the switchgear site. In a o of a ring uble bus installation, the CT lea s shoul be taken irectly to the isolator sel ection switches. The usual routing of cables on a ouble bus site is as follows: a. current transformers to marshalling kiosk b. marshalling kiosk to bus select ion isolator auxiliary switches c. interconnections between marshalling kiosks t o form a close ring The relay for each zone is connecte to one point of the ri ng bus wire. For convenience of cabling, the main zone relays will be connecte through a multicore cable between the relay panel an the bus section-switch mar shalling cubicle. The reserve bar zone an the check zone relays will be connect e together by a cable running to the bus coupler circuit breaker marshalling Network Protection & Automation Gui e cubicle. Itis possible that special circumstances involving onerous con itions may an make connection to some other part of the rin over-ri e this convenience g esirable. Connecting lea s will usually be not less than 7/0.67mm (2.5mm2), b ut for large sites or in other ifficult circumstances it may be necessary to us e cables of, for example 7/1.04mm (6mm2) for the bus wire ring an the CT connec tions to it. The cable from the ring to the relay nee not be of the larger sect ion. When thereserve bar is split by bus section isolators an the two portions are protecte as separate zones, it is necessary to common the bus wires by mea ns of auxiliary contacts, thereby making these two zones into one when the secti on isolators are close . 15.8.6Summary of Practical Details This section provi es a summary of practical consi erations when implementing a high-impe ance busb ar protection scheme. 15.8.6.1 Designe stability level For normal circumstances , the stability level shoul be esigne to correspon to the switchgear rating; even if the available short-circuit power in the system ismuch less than this figure, it can be expecte that the system will be evelope up to thelimit of rating. 15.8.6.2 Current transformers Current transformers must have i entical t urns ratios, but a turns error of one in 400 is recognise as areasonable manuf acturing tolerance. Also, they shoul preferably be of similar esign; where thi s is not possible the magnetising characteristics shoul be reasonably matche . Current transformers for use with high impe ance protection schemes shoul meet the requirements of Class PX of IEC 60044-1. 15.8.6.3 Setting voltage The settin g voltage is given by theequation Vs > If (RL + RCT) where: Vs = relay circuit voltage setting If = stea y-state through fault current RL = CT lea loop resist ence RCT = CT secon ary win ing resistance • • 245 •
Busbar P rotection 15 •
15.8.6.4 Knee-point voltage of current transformers This is given by the formula VK 2Vs 15.8.6.5 Effective setting (secon ary) The effective setting of the rela y is given by IR = IS + nIeSIR where: IS = relay circuit current setting IeS = C T excitation current at voltage setting n = number of CT’s in parallel For the pri mary fault setting multiply IR by the CT turns ratio. 15.8.6.6 Current transform er secon ary rating Itis clear from Equations 15.4 an 15.6 that it is a vantag eous to keep the secon ary fault current low; this is one by making the CT turn s ratio high. It is common practice to use current transformers with a secon ary rating of 1A. It can be shown that there is an optimum turns ratio for the curr ent transformers; this value epen s on all the application parameters but is ge nerally about 2000/1. Although a lower ratio, for instance 400/1,is often emplo ye , the use of the optimum ratio can result in a consi erable reuction in the physical size of the current transformers.15.8.6.7 Peak voltage evelope by cu rrent transformers Un er in-zone fault con itions, a high impe ance relay consti tutes an excessive bur en to the current transformers, lea ing to the evelopmen t of a high voltage; the voltage waveform will be highly istorte but the peak value may be many times the nominal saturation voltage. When the bur en resistan ce is finite although high, an approximate formula for the peak voltage is: V P = 2 2 V K (V F − V K where:VP = pe k volt ge developed VK = knee point volt ge VF = prospective volt ge in sence of s tur tion This formul does not hold for t he open circuit condition nd is in ccur te for very high urden resist nces th t pproxim te to n open circuit, ec use simplifying ssumptions used in the de riv tion of the formul re not v lid for the extreme condition. Another ppro c h pplic le to the open circuit second ry condition is: VP = where: If = f ult current Iek = exciting current t knee point volt ge VK = knee point volt ge Any urden connected cross the second ry will reduce the volt ge, ut the v lue c nnot e deduced from simple com in tion of urden nd exciting imped nces. These formul e re therefore to e reg rded only s guide to the possi le pe k volt ge. With l rge current tr nsformers, p rticul rly those with low second ry current r ting, thevolt ge m y e very high, ove suit leinsul tion volt ge. The volt ge c n e limited without detriment to the scheme y connecting cer mic non line r resistor in p r llel with the rel y h ving ch r cteristic given y: V = CIβ where C is con st nt depending on dimensions nd β is const nt in the r nge 0.2 0.25. The curre nt p ssed y the non line r resistor t the rel y volt ge setting depends on the v lue of C; in order to keep the shunting effect to minimum it is recommended to use non line r resistor with v lue of C of 450 for rel y volt ges up to 175V nd one with v lue of C of 900 for setting volt ges up to 325V. 15.8.6.8 High imped nce rel y Inst nt neous ttr cted rm ture rel ys re used.Simple f st oper ting rel ys would h ve low s fety f ctor const nt in the st ility equ Equ tion 15.5, s discussed in Section 15.8.1. The perform nce is improve tion, d y series tuning the rel y coil, there y m king the circuit resistive in effec t. Inductive rect nce would tend to reduce st ility, where s the ction of c p cit nce is to lock the unidirection l tr nsient component of f ult current nd so r ise the st ility const nt. An ltern tive technique used in some rel ys i s to pply the limited spill volt ge principle shown in Equ tion 15.4. A tuned e lement is connected vi plug ridge to ch in of resistors; nd the rel y is c li r ted in terms of volt ge. 15.9 LOW IMPEDANCE BIASED D I F F E R E N T I A L P R OT E C T I O N The principles of low imped nce differenti l protection h v e een descri ed in Section 10.4, including the principle dv nt ges to e g ine d y the use of i s 2 If I ek VK ...Equ tion 15.8 •
Bus r P rotection 15 • )
...Equ tion 15.7 • 246 • Network Protection & Autom tion Guide
technique. Most modern us r protection schemes use this technique. The princip les of check zone, zone selection, nd tripping rr ngements c n still e ppl to ied. Current tr nsformer second ry circuits re not switched directly y isol r cont cts ut inste d y isol tor repe t rel ys fter second ry st ge of curr ent tr nsform tion. These switching rel ys form replic of the us r within t he protection nd providethe complete selection logic. 15.9.1 St ility With so me i sed rel ys, the st ility is not ssured y the through current i s fe tu re lone, ut is enh nced y the ddition of st ilising resistor, h ving v lue which m y e c lcul ted s follows. The through current will incre se the ef fective rel y minimum oper ting current for i sed rel y s follows: IR = IS + BIF where: IR = effective minimum opr ting current IS = rel y setting current I F = through f ult current B = percent ge restr int As IF is gener lly much gre t er th n IS, the rel yeffective current, IR = BIF pproxim tely. From Equ tion 1 5.4, the v lue of st ilising resistor is given y: RR = I f It must e recognised though th t the use of ny technique for inhiiting oper t ion, to improvest ility perform nce for through f ults, must not e llowed to diminish the ility of the rel y to respond to intern l f ults. 15.9.2 Effecti ve Setting or Prim ry Oper ting Current For n intern l f ult, nd with no throu gh f ult current flowing, the effective setting (IR) is r ised ove the sic r el y setting (IS) y wh tever i sing effect is produced y the sum of the CT m gnetising currents flowing through the i scircuit. With low imped nce i sed d ifferenti l schemes p rticul rly where the us r inst ll tion h s rel tively fe w circuits, these m gnetising currents m y e negligi le, depending on the v lue of IS. The sic rel y setting current w s formerly defined s the minimum curr ent required solely in the differenti l circuit to c use oper tion – Figure 15.15( ). This ppro ch simplified n lysis of perform nce, ut w s considered to e u nre listic, s in pr ctice ny current flowing in the differenti l circuit must flow in t le st one h lf of the rel y i s circuit c using the pr ctic l minimu m oper ting current lw ys to e higher th n the nomin l sic setting current. As result, l terdefinition, s shown in Figure 15.15( ) w s developed. Conv ersely, it needs to e ppreci ted th t pplying the lter definition of rel y s etting current, which flows through t le st h lf the i s circuit, the notion l t less, minimum oper tion current in the differenti l circuit lone is somewh s shown in Figure 15.15( ). Using the definition presently pplic le, the effec tive minimum prim ry oper ting current where: Iop (R LH + R CTH ) = N I S + B ∑ I eS N = CT r tio Iop [ ] IR =
R LH + R CTH B It is interesting to note th t the v lue of the st ilising resis t nce is independent of current level, nd th t there would ppe r to e no limi t to the through f ults st ility level. This h s een identified [15.1] s ‘The P rinciple of Infinite St ility’. Th stabilising r sistor still constitut s a sign ificant burd n on th curr nt transform rs during int rnal faults. An alt rnativ t chniqu , us d by th MBCZ syst m d scrib d in S ction 15.9.6, is to block th
diff r ntial m asur m nt during th portion of th cycl that a curr nt transf orm r is saturat d. If this is achi v d by mom ntarily short-circuiting th diff r ntial path, a v ry low burd n is plac d on th curr nt transform rs. In this way th diff r ntial circuit of th r lay is pr v nt d from r sponding to th sp ill curr nt. N twork Prot ction & Automation Guid • 247 • • IS IB Lin (B% ) IS I S IS Bi in as L Bia s B% ( ) IB IB IS IR = S I + BIB IR = I + I = I B I S (a) Sup rs d d d finition Figur 15.15: D finitions of r lay s tting curr nt for bias d r lays (b) Curr nt d finition Busbar P rot ction 15 •
Unl ss th minimum ff ctiv op rating curr nt of a sch m has b n rais d d lib rat ly to som pr f rr d valu , it will usually b d t rmin d by th ch ck zon , wh n pr s nt, as th latt r may b xp ct d to involv th gr at st numb r of curr nt transform rs in parall l. A slightly mor on rous condition may aris wh n two discriminating zon s ar coupl d, transi ntly or oth rwis , by th closin g of primary isolators. It is g n rally d sirabl to attain an ff ctiv primary op rating curr nt that is just gr at r than th maximum load curr nt, to pr v n t th busbar prot ction from op rating spuriously from load curr nt should a s c ondary circuit wiring fault d v lop. This consid ration is particularly importan t wh r th ch ck f atur is ith r not us d or is f d from common main CT s. 15 .9.3 Ch ck F atur For som low imp danc sch m s, only on s t of main CT s is r quir d. This s ms to contradict th g n ral principl of all busbar prot ctio n syst ms with a ch ck f atur that compl t duplication of all quipm nt is r q uir d, but it is claim d that th spirit of th ch cking principl is m t by mak ing op ration of th prot ction d p nd nt on two diff r nt crit ria such as dir ctional and diff r ntial m asur m nts. Th usual solution is to rout all th CT s condary circuits back to th prot ct ion pan l or cubicl to auxiliary CT s. It is th n th s condary circuits of th auxiliary CT’s that ar switch d as n c ssary. So auxiliary CT s may b includ d for this function v n wh n th ratio matching is not in qu stion. In static pro t ction quipm nt it is und sirabl to us isolator auxiliary contacts dir ctly for th switching without som form of insulation barri r. Position transduc rs that follow th op ning and closing of th isolators may provid th latt r. Alt rnativ ly, a simpl r arrang m nt may b provid d on multipl busbar syst ms wh r th isolators switch th auxiliary curr nt transform r s condary circuits via auxiliary r lays within th prot ction. Th s r lays form a r plica of th busb ar and p rform th n c ssary logic. It is th r for n c ssary to rout all th c urr nt transform r s condary circuits to th r lay to nabl th m to b conn ct d into this busbar r plica. Som installations hav only on s t of curr nt tran sform rs availabl p r circuit. Wh r th facility of a ch ck zon is still r qu ir d, this can still b achi v d with th low imp danc bias d prot ction by con n cting th auxiliary curr nt transform rs at th input of th main and ch ck zo n s in s ri s, as shown in Figur 15.16. Busbar P rot ction In th MBCZ sch m , d scrib d in S ction 15.9.6, th provision of auxiliary CT s as standard for ratio matching also provid s a r ady m ans for introducing th ch ck f atur duplication at th auxiliary CT s and onwards to th r lays. This may b an attractiv compromis wh n only on s t of main CT s is availabl . 15. 9.4 Sup rvision of CT S condary Circuits In low imp danc sch m s th int grity of th CT s condary circuits can also b monitor d. A curr nt op rat d auxiliary r lay, or l m nt of th main prot ction quipm nt, may b appli d to d t ct an y unbalanc d s condary curr nts and giv an alarm aft r a tim d lay. For optimu m discrimination, th curr nt s tting of this sup rvision r lay must b l ss tha n that of th main diff r ntial prot ction. In mod rn busbar prot ction sch m s, th sup rvision of th s condary circuits typically forms only a part of a comp r h nsiv sup rvision facility. 15.9.5 Arrang m nt of CT conn ctions It is a com mon mod rn r quir m nt of low imp danc sch m s that non of th main CT s conda ry circuits should b switch d, in th pr viously conv ntional mann r, to match th switching of primary circuit isolators. Main zon Main zon Ch ck zon Ch ck zon
• 15 • Figur 15.16: Alt rnativ CT conn ctions 15.9.6 Static Low Imp danc Bias d Diff r ntial Prot ction - Typ MBCZ Th Typ MBCZ sch m conforms in g n ral to th principl s outlin d arli r and compris s a syst m of standard modul s that can b ass mbl d to suit a particular busbar installation. Additional modul s can b add d at any tim as th busbar is xt n d d. A s parat modul is us d for ach circuit br ak r and also on for ach zo n of prot ction. In addition to th s th r is a common alarm modul and a numb r of pow r supply units. Ratio corr ction faciliti s ar provid d within ach d iff r ntial modul to accommodat a wid rang of CT mismatch. N twork Prot ction & Automation Guid • 248 •
Zon 1 Zon 2 Zon 3a Zon 3b Bus coupl r 1 F d r 1 Z1 Z3a F d r 2 Bus s ction F d r 3 Z2 Z3b Ch ck F d r 4 zon Bus coupl r 2 Int rmodul plug-in buswir conn ctions Figur 15.17: Typ MBCZ busbar prot ctio n showing corr lation b tw n circuit br ak rs and prot ction modul s Figur 15.17 shows th corr lation b tw n th circuit br ak rs and th prot cti on modul s for a typical doubl busbar installation. In practic th modul s ar mount d in a multi-ti r rack or cubicl . Th modul s ar int rconn ct d via a m ulticor cabl that is plugg d into th back of th modul s. Th r ar fiv main groups of buswir s, allocat d for: i. prot ction for main busbar ii. prot ction for r s rv busbar iii. prot ction for th transf r busbar. Wh n th r s rv bu sbar is also us d as a transf r bar th n this group of buswir s is us d iv. auxi liary conn ctions us d by th prot ction to combin modul s for som of th mor compl x busbar configurations v. prot ction for th ch ck zon On xtra modul , not shown in this diagram, is plugg d into th multicor bus. This is th alar m modul , which contains th common alarm circuits and th bias r sistors. Th p ow r suppli s ar also f d in through this modul . 15.9.6.1 Bias All zon s of m asur m nt ar bias d by th total curr nt flowing to or from th busbar syst m v ia th f d rs. This nsur s that all zon s of m asur m nt will hav similar fau lt s nsitivity und r all load conditions. Th bias is d riv d from th ch ck zon and fix d at 20% with a charact ristic g n rally as shown in Figur 15.15(b). Thus som ratio mismatch is tol rabl . 15.9.6.2 Stability with saturat d curr nt transform rs Th traditional m thod fo r stabilising a diff r ntial r lay is to add a r sistor to th diff r ntial path . Whilst this improv s stability it incr as s th burd n on th curr nt transfor m r for int rnal faults. Th t chniqu us d in th MBCZ sch m ov rcom s this pr obl m. Th MBCZ d sign d t cts wh n a CT is saturat d and short-circuits th dif f r ntial path for th portion of th cycl for which saturation occurs. Th r s ultant spill curr nt do s not th n flow through th m asuring circuit and stabil ity is assur d. This principl allows a v ry low imp danc diff r ntial circuit to b d v lop d that will op rat succ ssfully with r lativ ly small CT s. 15.9. 6.3 Op ration for int rnal faults If th CT s carrying fault curr nt ar not sat urat d th r will b ampl curr nt in th diff r ntial circuit to op rat th di ff r ntial r lay quickly for fault curr nts xc ding th minimum op rating l v l, which is adjustabl b tw n 20%-200% rat d curr nt. Wh n th only CT(s) carry ing int rnal fault curr nt b com saturat d, it might b suppos d that th CT sa turation d t ctors may compl t ly inhibit op ration by short-circuiting th diff r ntial circuit. How v r, th r sulting inhibit puls s r mov only an insignifi cant portion of th diff r ntial curr nt, so op ration of th r lay is th r for
virtually unaff ct d. • N twork Prot ction & Automation Guid • 249 • Busbar P rot ction 15 •
Sup rvision c = Ch ck m = Main r = R s rv t = Transf r Alarm Curr nt Buswir S l ction Links =1 1 CT Fault Diff r ntial c m r t Sup rvision OR Enabl 1 Prot ction fault Bias d Diff r ntial Enabl Trip Buswir S l ction Links c m r Trip Bias Bias d Diff r ntial =1 1 Trip t Out of s rvic Figur 15.18: Block diagram of m asuring unit 15.9.6.4 Discr pancy alarm f atur As shown in Figur 15.18, ach m asuring modu l contains duplicat d bias d diff r ntial l m nts and also a pair of sup rvisi on l m nts, which ar a part of a compr h nsiv sup rvision facility. This arra ng m nt provid s sup rvision of CT s condary circuits for both op n circuit cond itions and any impairm nt of th l m nt to op rat for an int rnal fault, witho ut waiting for an actual syst m fault condition to show this up. For a zon to o p rat it is n c ssary for both th diff r ntial sup rvision l m nt and th bia s d diff r ntial l m nt to op rat . For a circuit br ak r to b tripp d it r qu ir s th associat d main zon to b op rat d and also th ov rall ch ck zon , as shown in Figur 15.19. Main zon S1 + v S2 D1 D2 Ch ck zon S1 S2 D1 D2 Trip Busbar P rot ction to op rat th two busbar s ctions as a singl bar. Th fault curr nt will th n divid b tw n th two m asuring l m nts in th ratio of th ir imp danc s. If b oth of th two m asuring l m nts ar of low and qual imp danc th ff ctiv m inimum op rating curr nt of th sch m will b doubl d. This is avoid d by using a mast r/follow r arrang m nt. By making th imp danc of on of th m asurin g l m nts v ry much high r than th oth r it is possibl to nsur that on of
th r lays r tains its original minimum op ration curr nt. Th n to nsur that b oth th parall lconn ct d zon s ar tripp d th trip circuits of th two zon s a r conn ct d in parall l. Any m asuring unit can hav th rol of mast r or f ollow r as it is s l ctabl by m ans of a switch on th front of th modul . 15 .9.6.6 Transf r tripping for br ak r failur S rious damag may r sult, and v n dang r to lif , if a circuit br ak r fails to op n wh n call d upon to do so. T o r duc this risk br ak r fail prot ction sch m s w r d v lop d som y ars ago . Th s sch m s ar g n rally bas d on th assumption that if curr nt is still f lowing through th circuit br ak r a s t tim aft r th trip command has b n is su d, th n it has fail d to function. Th circuit br ak rs in th n xt stag bac k in th syst m ar th n automatically tripp d. For a bus coupl r or s ction br ak r this would involv tripping all th inf ds to th adjac nt zon , a facilit y that is includ d in th busbar prot ction sch m . N twork Prot ction & Automation Guid • 15 • Figur 15.19: Busbar prot ction trip logic 15.9.6.5 Mast r/follow r m asuring ct d tog th r by isolators it will t d in parall l wh n th isolators • 250 •
units Wh n two s ctions of a busbar ar conn r sult in two m asuring l m nts b ing conn c ar clos d
F d r 1 F d r 2 CT PU CB PU CT CB PU CT CB CT CB Fibr optic link P rsonal Comput r PU C ntral Unit CU Syst m Communication N twork PU: P riph ral Unit CU: C ntral Unit Figur 15.20: Archit ctur for num rical prot ction sch m 1 5 . 10 N U M E R I C A L B U S B A R P R OT E C T I O N SCHEMES Th applicatio n of num rical r lay t chnology to busbar prot ction has lagg d b hind that of o th r prot ction functions. Static t chnology is still usual for such sch m s, bu t num rical t chnology is now r adily availabl . Th v ry lat st d v lopm nts in th t chnology ar includ d, such as xt nsiv us of a data bus to link th va rious units involv d, and fault tol ranc against loss of a particular link by p roviding multipl communications paths. Th d v lopm nt proc ss has b n v ry ri gorous, b caus th r quir m nts for busbar prot ction in r sp ct of immunity to malop ration ar v ry high. Th philosophy adopt d is on of distribut d proc s sing of th m asur d valu s, as shown in Figur 15.20. F d rs ach hav th ir o wn proc ssing unit, which coll cts tog th r information on th stat of th f d r (curr nts, voltag s, CB and isolator status, tc.) and communicat s it ov r h igh-sp d fibr -optic data links to a c ntral unit. For larg substations, mor than on c ntral unit may b us d, whil in th cas of small installations, all of th units can b co-locat d, l ading to th app aranc of a traditional c nt ralis d archit ctur . For simpl f d rs, int rfac units at a bay may b us d w ith th data transmitt d to a singl c ntrally locat d p riph ral unit. Th c nt ral unit p rforms th calculations r quir d for th prot ction functions. Availa bl prot ction functions ar : a. prot ction b. backup ov rcurr nt prot ction c. br ak r failur N twork Prot ction & Automation Guid d. d ad zon prot ction In addition, monitoring functions such as CB and isolato r monitoring, disturbanc r cording and transform r sup rvision ar provid d. B caus of th distribut d topology us d, synchronisation of th m asur m nts tak n by th p riph ral units is of vital importanc . A high stability num rically-c ontroll d oscillator is fitt d in ach of th c ntral and p riph ral units, with tim synchronisation b tw n th m. In th v nt of loss of th synchronisation signal, th high stability of th oscillator in th aff ct d f d r unit(s) nab l s proc ssing of th incoming data to continu without significant rrors until synchronisation can b r stor d. Th p riph ral units hav r sponsibility for c oll cting th r quir d data, such as voltag s and curr nts, and proc ssing it in to digital form for onwards transmission to th c ntral unit. Mod lling of th C T r spons is includ d, to liminat rrors caus d by ff cts such as CT saturat ion. Disturbanc r cording for th monitor d f d r is impl m nt d, for lat r do wnload as r quir d. B caus ach p riph ral unit is conc rn d only with an indiv idual f d r, th prot ction algorithms must r sid in th c ntral unit. Th dif f r ntial prot ction algorithm can b much mor sophisticat d than with arli r t chnology, du to improv m nts in proc ssing pow r. In addition to calculating th sum of th m asur d curr nts, th algorithm can also valuat diff r nc s b tw n succ ssiv curr nt sampl s, sinc a larg chang abov a thr shold may ind icat a fault – th thr shold b ing chos n such that normal load chang s, apart fr om inrush conditions do not xc d th thr shold. Th sam
• • 251 • Busbar P rot ction 15 •
consid rations can also b appli d to th phas angl s of curr nts, and incr m n tal chang s in th m. On advantag gain d from th us of num rical t chnology i s th ability to asily r -configur th prot ction to cat r for chang s in conf iguration of th substation. For xampl , addition of an xtra f d r involv s t h addition of an xtra p riph ral unit, th fibr -optic conn ction to th c ntr al unit and ntry via th MMI of th n w configuration into th c ntral unit. Fi gur 15.21 illustrat s th lat st num rical t chnology mploy d. 15.10.1 R liabi lity Consid rations In consid ring th introduction of num rical busbar prot cti on sch m s, us rs hav b n conc rn d with r liability issu s such as s curity a nd availability. Conv ntional high imp danc sch m s hav b n on of th main p rot ction sch m s us d for busbar prot ction. Th basic m asuring l m nt is sim pl in conc pt and has f w compon nts. Calculation of stability limits and oth r s tting param t rs is straightforward and sch m p rformanc can b pr dict d w ithout th n d for costly t sting. Practically, high imp danc sch m s hav pro v d to b a v ry r liabl form of prot ction. In contrast, mod rn num rical sch m s ar mor compl x with a much gr at r rang of faciliti s and a much high com pon nt count. Bas d on low imp danc bias t chniqu s, and with a gr at r rang o f faciliti s to s t, s tting calculations can also b mor compl x. How v r, studi s of th comparativ r liability of conv ntional high imp danc s ch m s and mod rn num rical sch m s hav shown that ass ssing r lativ r liabili ty is not quit so simpl as it might app ar. Th num rical sch m has two advan tag s ov r its old r count rpart: a. th r is a r duction in th numb r of xt r nal compon nts such as switching and oth r auxiliary r lays, many of th functio ns of which ar p rform d int rnally within th softwar algorithms b. num rical sch m s includ sophisticat d monitoring f atur s which provid alarm faciliti s if th sch m is faulty. In c rtain cas s, simulation of th sch m functions can b p rform d on lin from th CT inputs through to th tripping outputs and thus sch m functions can b ch ck d on a r gular basis to nsur a full op rati onal mod is availabl at all tim s R liability analys s using fault tr analys is m thods hav xamin d issu s of d p ndability ( .g. th ability to op rat wh n r quir d) and s curity ( .g. th ability not to provid spurious/indiscrimina t op ration). Th s analys s hav shown that: a. d p ndability of num rical sch m s is b tt r than conv ntional high imp danc sch m s b. s curity of num rical and conv ntional high imp danc sch m s ar comparabl In addition, an importan t f atur of num rical sch m s is th in-built monitoring syst m. This consid ra bly improv s th pot ntial availability of num rical sch m s compar d to conv nt ional sch m s as faults within th quipm nt and its op rational stat can b d t ct d and alarm d. With th conv ntional sch m , failur to r instat th sch m corr ctly aft r maint nanc may not b d t ct d until th sch m is r quir d t o op rat . In this situation, its ff ctiv availability is z ro until it is d t ct d and r pair d. 1 5 . 11 R E F E R E N C E S 15.1 Th B haviour of Curr nt T ransform rs subj ct d to Transi nt Asymm tric Curr nts and th Eff cts on Associ at d Prot ctiv R lays. J.W. Hodgkiss. CIGRE Pap r Numb r 329, S ssion 15-25 Jun 1960. • Busbar P rot ction 15 • Figur 15.21: Busbar prot ction r lay using th lat st num rical t chnology (MiC OM P740 rang ) • 252 • N twork Prot ction & Automation Guid
• 16 • Transform r and Transform r-f d r Prot ction Introduction Winding faults Magn tising inrush Transform r ov rh ating Transform r prot ction – ov rvi w Transform r ov rcurr nt prot ction R strict d arth fault prot ction Diff r ntial prot ction Stabilisation of diff r ntial prot ction dur ing magn tising inrush conditions Combin d diff r ntial and r strict d arth fau lt sch m s Earthing transform r prot ction Auto-transform r prot ction Ov rfluxi ng prot ction Tank- arth prot ction Oil and gas d vic s Transform r-f d r prot ction Int rtripping Condition monitoring of transform rs Exampl s of transform r prot ction 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 16.10 16.11 16.12 16.13 16.14 16.15 16.16 16.17 16.18 16.19
• 16 • Transform r and Transform r-F d r P rot ction 16.1 INTRODUCTION Th d v lopm nt of mod rn pow r syst ms has b n r fl ct d in th advanc s in transform r d sign. This has r sult d in a wid rang of transfo rm rs with siz s ranging from a f w kVA to s v ral hundr d MVA b ing availabl f or us in a wid vari ty of applications. Th consid rations for a transform r p rot ction packag vary with th application and importanc of th transform r. T o r duc th ff cts of th rmal str ss and l ctrodynamic forc s, it is advisabl to nsur that th prot ction packag us d minimis s th tim for disconn ctio n in th v nt of a fault occurring within th transform r. Small distribution t ransform rs can b prot ct d satisfactorily, from both t chnical and conomic co nsid rations, by th us of fus s or ov rcurr nt r lays. This r sults in tim -d lay d prot ction du to downstr am co-ordination r quir m nts. How v r, tim -d l ay d fault cl aranc is unacc ptabl on larg r pow r transform rs us d in distri bution, transmission and g n rator applications, du to syst m op ration/stabili ty and cost of r pair/l ngth of outag consid rations. Transform r faults ar g n rally classifi d into six cat gori s: a. winding and t rminal faults b. cor f aults c. tank and transform r acc ssory faults d. on–load tap chang r faults . ab normal op rating conditions f. sustain d or uncl ar d xt rnal faults For faults originating in th transform r its lf, th approximat proportion of faults du to ach of th caus s list d abov is shown in Figur 16.1. Winding and t rminal Cor Tank and acc ssori s OLTC Figur 16.1: Transform r fault statistics N twork Prot ction & Automation Guid • 255 •
1 6 . 2 W I N D I N G F A U LT S A fault on a transform r winding is controll d in magnitud by th following factors: i. sourc imp danc ii. n utral arthing imp danc iii. transform r l akag r actanc iv. fault voltag v. winding conn c tion S v ral distinct cas s aris and ar xamin d b low. 16.2.2 Star-conn ct d winding with N utral Point Solidly Earth d Th fault curr nt is controll d mainly by th l akag r actanc of th winding, which vari s in a compl x mann r with th position of th fault. Th variabl fault point volta g is also an important factor, as in th cas of imp danc arthing. For faults clos to th n utral nd of th winding, th r actanc is v ry low, and r sults in th high st fault curr nts. Th variation of curr nt with fault position is shown in Figur 16.3. 20 Transform r and Transform r-F d r P rot ction 16.2.1 Star-Conn ct d Winding with N utral Point Earth d through an Imp danc Curr nt (p r unit) Th winding arth fault curr nt d p nds on th arthing imp danc valu and is a lso proportional to th distanc of th fault from th n utral point, sinc th fault voltag will b dir ctly proportional to this distanc . For a fault on a t ransform r s condary winding, th corr sponding primary curr nt will d p nd on t h transformation ratio b tw n th primary winding and th short-circuit d s co ndary turns. This also vari s with th position of th fault, so that th fault curr nt in th transform r primary winding is proportional to th squar of th fraction of th winding that is shortcircuit d. Th ff ct is shown in Figur 16 .2. Faults in th low r third of th winding produc v ry littl curr nt in th primary winding, making fault d t ction by primary curr nt m asur m nt difficult . 100 P rc ntag of r sp ctiv maximum singl -phas arth fault curr nt 15 Fault curr nt 10 5 Primary curr nt 0 10 20 30 40 50 60 70 80 90 100 Distanc of fault from n utral (p rc ntag of winding) Figur 16.3 Earth fault curr nt in solidly arth d star winding 90 80 70 60 50 40 30 20 10 Fault curr nt (IF) For s condary winding faults, th primary winding fault curr nt is d t rmin d by th variabl transformation ratio; as th s condary fault curr nt magnitud sta ys high throughout th winding, th primary fault curr nt is larg for most poin ts along th winding. 16.2.3 D lta-conn ct d Winding No part of a d lta-conn ct d winding op rat s with a voltag to arth of l ss than 50% of th phas voltag . Th rang of fault curr nt magnitud is th r for l ss than for a star winding . Th actual valu of fault curr nt will still d p nd on th m thod of syst m a rthing; it should also b r m mb r d that th imp danc of a d lta winding is pa rticularly high to fault curr nts flowing to a c ntrally plac d fault on on l g . Th imp danc can b xp ct d to b b tw n 25% and 50%, bas d on th transfor m r rating, r gardl ss of th normal balanc d through-curr nt imp danc . As th pr fault voltag to arth at this point is half th normal phas voltag , th a rth fault curr nt may b no mor than th rat d curr nt, or v n l ss than this valu if th sourc or syst m arthing imp danc is appr ciabl . Th curr nt wil l flow to th fault from ach sid through th two half windings, and will b di vid d b tw n two
• 16 • p) 0 10 20 30 40 50 60 70 80 90 100 (p rc ntag of winding) Ip IF Figur 16.2 Earth fault curr nt in r sistanc - arth d star winding • 256 • N twork Prot ction & Automation Guid
phas s of th syst m. Th individual phas curr nts may th r for b r lativ ly low, r sulting in difficulti s in providing prot ction. 16.2.4 Phas to Phas Fa ults Faults b tw n phas s within a transform r ar r lativ ly rar ; if such a f ault do s occur it will giv ris to a substantial curr nt comparabl to th ar th fault curr nts discuss d in S ction 16.2.2. 16.2.5 Int rturn Faults In low vo ltag transform rs, int rturn insulation br akdown is unlik ly to occur unl ss t h m chanical forc on th winding du to xt rnal short circuits has caus d ins ulation d gradation, or insulating oil (if us d) has b com contaminat d by mois tur . A high voltag transform r conn ct d to an ov rh ad transmission syst m wi ll b subj ct d to st p front d impuls voltag s, arising from lightning strik s, faults and switching op rations. A lin surg , which may b of s v ral tim s th rat d syst m voltag , will conc ntrat on th nd turns of th winding b cau s of th high quival nt fr qu ncy of th surg front. Part-winding r sonanc , involving voltag s up to 20 tim s rat d voltag may occur. Th int rturn insulat ion of th nd turns is r inforc d, but cannot b incr as d in proportion to th insulation to arth, which is r lativ ly gr at. Partial winding flashov r is th r for mor lik ly. Th subs qu nt progr ss of th fault, if not d t ct d in th arli st stag , may w ll d stroy th vid nc of th tru caus . A short circu it of a f w turns of th winding will giv ris to a h avy fault curr nt in th short-circuit d loop, but th t rminal curr nts will b v ry small, b caus of t h high ratio of transformation b tw n th whol winding and th short-circuit d turns. 100 Fault curr nt (multipl s of rat d curr nt) Th graph in Figur 16.4 shows th corr sponding data for a typical transform r of 3.25% imp danc with th short-circuit d turns symm trically locat d in th c ntr of th winding. 16.2.6 Cor Faults A conducting bridg across th laminat d structur s of th cor can p rmit suffici nt ddy-curr nt to flow to caus s r ious ov rh ating. Th bolts that clamp th cor tog th r ar always insulat d to avoid this troubl . If any portion of th cor insulation b com s d f ctiv , th r sultant h ating may r ach a magnitud suffici nt to damag th winding. 16.2.7 Tank Faults Loss of oil through tank l aks will ultimat ly produc a dang rous condition, ith r b caus of a r duction in winding insulation or b caus of ov rh ating on load du to th loss of cooling. Ov rh ating may also occur du to prolong d ov rloading, block d cooling ducts du to oil sludging or failur of th forc d cooling syst m, if fitt d. 16.2.8 Ext rnally Appli d Conditions S ourc s of abnormal str ss in a transform r ar : a. ov rload b. syst m faults c. ov rvoltag d. r duc d syst m fr qu ncy 16.2.8.1 Ov rload Ov rload caus s incr a s d copp r loss and a cons qu nt t mp ratur ris . Ov rloads can b carri d fo r limit d p riods and r comm ndations for oil-imm rs d transform rs ar giv n in IEC 60354. Th th rmal tim constant of naturally cool d transform rs li s b tw n 2.5-5 hours. Short r tim constants apply in th cas of forc -cool d transf orm rs. 10 Fault curr nt in short circuit d turns Primary curr nt (multipl s of rat d curr nt) • 80 8 60 Primary input curr nt 40 6 4
20 2 0 5 10 15 20 25 Turns short-circuit d (p rc ntag of winding) Figur 16.4 Int rturn fault curr nt/numb r of turns short-circuit d N twork Prot ction & Automation Guid • 257 • Transform r and Transform r-F d r P rot ction 16 • Th additional cor loss, although causing s v r local h ating, will not produc a notic abl chang in input curr nt and could not b d t ct d by th normal l ctrical prot ction; it is n v rth l ss highly d sirabl that th condition sho uld b d t ct d b for a major fault has b n cr at d. In an oil-imm rs d transf orm r, cor h ating suffici nt to caus winding insulation damag will also caus br akdown of som of th oil with an accompanying volution of gas. This gas w ill scap to th cons rvator, and is us d to op rat a m chanical r lay; s S ction 16.15.3.
16.2.8.2 Syst m faults Syst m short circuits produc a r lativ ly int ns rat o f h ating of th f ding transform rs, th copp r loss incr asing in proportion to th squar of th p r unit fault curr nt. Th typical duration of xt rnal sh ort circuits that a transform r can sustain without damag if th curr nt is lim it d only by th s lf-r actanc is shown in Tabl 16.1. IEC 60076 provid s furth r guidanc on short-circuit withstand l v ls. Transform r r actanc (%) 4 5 6 7 Tabl 16.1: Fault withstand l v ls Fault curr nt (Multipl of rating) 25 20 16.6 14.2 P rmitt d fault duration (s conds) 2 2 2 2 fr qu ncy, but op ration must not b continu d with a high voltag input at a lo w fr qu ncy. Op ration cannot b sustain d wh n th ratio of voltag to fr qu nc y, with th s quantiti s giv n valu s in p r unit of th ir rat d valu s, xc ds unity by mor than a small amount, for instanc if V/f >1.1. If a substantial r is in syst m voltag has b n cat r d for in th d sign, th bas of unit volt ag should b tak n as th high st voltag for which th transform r is d sign d. 16.3 MAGNETISING INRUSH Th ph nom non of magn tising inrush is a transi nt c ondition that occurs primarily wh n a transform r is n rgis d. It is not a faul t condition, and th r for transform r prot ction must r main stabl during th inrush transi nt. Transform r and Transform r-F d r P rot ction Maximum m chanical str ss on windings occurs during th first cycl of th fault . Avoidanc of damag is a matt r of transform r d sign. 16.2.8.3 Ov rvoltag s O v rvoltag conditions ar of two kinds: i. transi nt surg voltag s ii. pow r fr qu ncy ov rvoltag Transi nt ov rvoltag s aris from faults, switching, and lig htning disturbanc s and ar liabl to caus int rturn faults, as d scrib d in S ction 16.2.5. Th s ov rvoltag s ar usually limit d by shunting th high voltag t rminals to arth ith r with a plain rod gap or by surg div rt rs, which co mpris a stack of short gaps in s ri s with a non-lin ar r sistor. Th surg div rt r, in contrast to th rod gap, has th advantag of xtinguishing th flow o f pow r curr nt aft r discharging a surg , in this way avoiding subs qu nt isola tion of th transform r. Pow r fr qu ncy ov rvoltag caus s both an incr as in str ss on th insulation and a proportionat incr as in th working flux. Th l att r ff ct caus s an incr as in th iron loss and a disproportionat ly larg incr as in magn tising curr nt. In addition, flux is div rt d from th laminat d cor into structural st l parts. Th cor bolts, which normally carry littl flux, may b subj ct d to a larg flux div rt d from th highly saturat d r gion of cor alongsid . This l ads to a rapid t mp ratur ris in th bolts, d stroy ing th ir insulation and damaging coil insulation if th condition continu s. 16 .2.8.4 R duc d syst m fr qu ncy R duction of syst m fr qu ncy has an ff ct with r gard to flux d nsity, similar to that of ov rvoltag . It follows that a trans form r can op rat with som d gr of ov rvoltag with a corr sponding incr as in Voltag and flux Flux Normal p ak flux Magn tising curr nt (a) Typical magn tising charact ristic Transi nt flux 80% r sidual at switching Transi nt flux no r sidual at switching St ady flux stat Voltag Tim (b) St ady and maximum offs t flux s • 16 •
Slow d cr m nt Z ro axis (c) Typical inrush curr nt Z ro axis (d) Inrush without offs t, du to yok saturation Figur 16.5: Transform r magn tising inrush Figur 16.5(a) shows a transform r magn tising charact ristic. To minimis mat r ial costs, w ight and siz , transform rs ar g n rally op rat d n ar to th ‘kn point’ of th magn tising charact ristic. • 258 • N twork Prot ction & Automation Guid
Cons qu ntly, only a small incr as in cor flux abov normal op rating l v ls w ill r sult in a high magn tising curr nt. Und r normal st ady-stat conditions, th magn tising curr nt associat d with th op rating flux l v l is r lativ ly s mall (Figur 16.5(b)). How v r, if a transform r winding is n rgis d at a volta g z ro, with no r man nt flux, th flux l v l during th first voltag cycl (2 x normal flux) will r sult in cor saturation and a high non-sinusoidal magn ti sing curr nt wav form – s Figur 16.5(c). This curr nt is r f rr d to as magn ti sing inrush curr nt and may p rsist for s v ral cycl s. A numb r of factors aff ct th magnitud and duration of th magn tising curr nt inrush: a. r sidual flu x – worst-cas conditions r sult in th flux p ak valu attaining 280% of normal v alu b. point on wav switching c. numb r of bank d transform rs d. transform r d sign and rating . syst m fault l v l Th v ry high flux d nsiti s quot d abov ar so far b yond th normal working rang that th incr m ntal r lativ p rm ability of th cor approximat s to unity and th inductanc of th winding fall s to a valu n ar that of th air-cor d inductanc . Th curr nt wav , starting from z ro, incr as s slowly at first, th flux having a valu just abov th r sidual valu and th p rm ability of th cor b ing mod rat ly high. As th flux pass s th normal working valu and nt rs th highly saturat d portion of th magn tising charact ristic, th inductanc falls and th curr nt ris s rapidly t o a p ak that may b 500% of th st ady stat magn tising curr nt. Wh n th p ak is pass d at th n xt voltag z ro, th following n gativ half cycl of th vo ltag wav r duc s th flux to th starting valu , th curr nt falling symm tric ally to z ro. Th curr nt wav is th r for fully offs t and is only r stor d to th st ady stat condition by th circuit loss s. Th tim constant of th tran si nt has a rang b tw n 0.1 s cond (for a 100kVA transform r) to 1.0 s cond (f or a larg unit). As th magn tising charact ristic is nonlin ar, th nv lop o f th transi nt curr nt is not strictly of xpon ntial form; th magn tising cur r nt can b obs rv d to b still changing up to 30 minut s aft r switching on. A lthough corr ct choic of th point on th wav for a singl –phas transform r wil l r sult in no transi nt inrush, mutual ff cts nsur that a transi nt inrush o ccurs in all phas s for thr -phas transform rs. 16.3.1 Harmonic Cont nt of Inrush Wav form Th wav form of transform r magn tisi ng curr nt contains a proportion of harmonics that incr as s as th p ak flux d nsity is rais d to th saturating condition. Th magn tising curr nt of a transf orm r contains a third harmonic and progr ssiv ly small r amounts of fifth and h igh r harmonics. If th d gr of saturation is progr ssiv ly incr as d, not onl y will th harmonic cont nt incr as as a whol , but th r lativ proportion of fifth harmonic will incr as and v ntually xc d th third harmonic. At a stil l high r l v l th s v nth would ov rtak th fifth harmonic but this involv s a d gr of saturation that will not b xp ri nc d with pow r transform rs. 1 6 . 4 T R A N S F O R M E R O V E R H E AT I N G Th rating of a transform r i s bas d on th t mp ratur ris abov an assum d maximum ambi nt t mp ratur ; un d r this condition no sustain d ov rload is usually p rmissibl . At a low r ambi nt t mp ratur som d gr of sustain d ov rload can b saf ly appli d. Short-t rm ov rloads ar also p rmissibl to an xt nt d p nd nt on th pr vious loadin g conditions. IEC 60354 provid s guidanc in this r sp ct. Th only c rtain stat m nt is that th winding must not ov rh at; a t mp ratur of about 95°C is consid r d to b th normal maximum working valu b yond which a furth r ris of 8°C-10°C, if sustain d, will halv th insulation lif of th unit. Prot ction against ov rload is th r for bas d on winding t mp ratur , which is usually m asur d by a th rmal imag t chniqu . Prot ction is arrang d to trip th transform r if xc ssiv t mp ratur is r ach d. Th trip signal is usually rout d via a digital in put of a prot ction r lay on on sid of th transform r, with both alarm and tr ip faciliti s mad availabl through programmabl logic in th r lay. Int rtripp ing b tw n th r lays on th two sid s of th transform r is usually appli d to nsur total disconn ction of th transform r. Winding t mp ratur prot ction m ay b includ d as a part of a compl t monitoring packag – s S ction 16.18 for mor d tails.
• N twork Prot ction & Automation Guid • 259 • Transform r and Transform r-F d r P rot ction 16 • Th n rgising conditions that r sult in an offs t inrush curr nt produc a wav form that is asymm trical. Such a wav typically contains both v n and odd harm onics. Typical inrush curr nts contain substantial amounts of s cond and third h armonics and diminishing amounts of high r ord rs. As with th st ady stat wav , th proportion of harmonics vari s with th d gr of saturation, so that as a s v r inrush transi nt d cays, th harmonic mak up of th curr nt pass s throu gh a rang of conditions.
16.5 TRANSFORMER PROTECTION – OVERVIEW Th probl ms r lating to transform rs d scr ib d in S ctions 16.2-4 abov r quir som m ans of prot ction. Tabl 16.2 summa ris s th probl ms and th possibl forms of prot ction that may b us d. Th fo llowing s ctions provid mor d tail on th individual prot ction m thods. It is normal for a mod rn r lay to provid all of th r quir d prot ction functions i n a singl packag , in contrast to l ctrom chanical typ s that would r quir s v ral r lays compl t with int rconn ctions and high r ov rall CT burd ns. Fault Typ Primary winding Phas -phas fault Primary winding Phas - arth fault S condary winding Phas -phas fault S condary winding Phas - arth fault Int rturn Fault Cor Fault Tank Fault Ov rfluxing Ov rh ating Tabl 16.2: Transform r fau lts/prot ction Prot ction Us d Diff r ntial; Ov rcurr nt Diff r ntial; Ov rcurr nt Diff r ntial Diff r ntial; R strict d Earth Fault Diff r ntial, Buchholz Diff r ntial, Buchholz Diff r ntial, Buchholz; Tank-Earth Ov rfluxing Th rmal kVA 10 0 200 315 500 1000 Transform r rating Full load curr nt (A) 5.25 10.5 15.8 26.2 52.5 Fus Rat d curr nt (A) 16 25 36 50 90 Op rating tim at 3 x rating(s) 3.0 3.0 10 .0 20.0 30.0 Tabl 16.3: Typical fus ratings Transform r and Transform r-F d r P rot ction This tabl should b tak n only as a typical xampl ; consid rabl diff r nc s xist in th tim charact ristic of diff r nt typ s of HRC fus s. Furth rmor gra ding with prot ction on th s condary sid has not b n consid r d. 16.6.2 Ov rc urr nt r lays With th adv nt of ring main units incorporating SF6 circuit br ak rs and isolators, prot ction of distribution transform rs can now b provid d b y ov rcurr nt trips ( .g. tripping controll d by tim limit fus s conn ct d acro ss th s condary windings of in-built curr nt transform rs) or by r lays conn ct d to curr nt transform rs locat d on th transform r primary sid . Ov rcurr nt r lays ar also us d on larg r transform rs provid d with standard circuit br ak r control. Improv m nt in prot ction is obtain d in two ways; th xc ssiv d l ays of th HRC fus for low r fault curr nts ar avoid d and an arth-fault trip ping l m nt is provid d in addition to th ov rcurr nt f atur . Th tim d lay charact ristic should b chos n to discriminat with circuit prot ction on th s condary sid . A high-s t instantan ous r lay l m nt is oft n provid d, th cur r nt s tting b ing chos n to avoid op ration for a s condary short circuit. This nabl s high-sp d cl aranc of primary t rminal short circuits. 16.7 RESTRICTE D EARTH FAULT PROTECTION Conv ntional arth fault prot ction using ov rcurr nt l m nts fails to provid ad quat prot ction for transform r windings. This is p articularly th cas for a star-conn ct d winding with an imp danc - arth d n ut ral, as consid r d in S ction 16.2.1. Th d gr of prot ction is v ry much impr ov d by th application of r strict d arth fault prot ction (or REF prot ction) . This is a unit prot ction sch m for on winding of th transform r. It can b of th high imp danc typ as shown in Figur 16.6, or of th bias d lowimp dan c typ . For th high-imp danc typ , th r sidual curr nt of thr lin curr nt transform rs is balanc d against th output of a curr nt transform r in th 16.6 TRANSFORMER OVERCURRENT PROTECTION Fus s may ad quat ly prot ct small trans form rs, but larg r on s r quir ov rcurr nt prot ction using a r lay and CB, as fus s do not hav th r quir d fault br aking capacity. 16.6.1 Fus s Fus s comm only prot ct small distribution transform rs typically up to ratings of 1MVA at distribution voltag s. In many cas s no circuit br ak r is provid d, making fus prot ction th only availabl m ans of automatic isolation. Th fus must hav a rating w ll abov th maximum transform r load curr nt in ord r to withstand t h short duration ov rloads that may occur. Also, th fus s must withstand th m agn tising inrush curr nts drawn wh n pow r transform rs ar n rgis d. High Rup turing Capacity (HRC) fus s, although v ry fast in op ration with larg fault cu
rr nts, ar xtr m ly slow with curr nts of l ss than thr tim s th ir rat d va lu . It follows that such fus s will do littl to prot ct th transform r, s rvi ng only to prot ct th syst m by disconn cting a faulty transform r aft r th fa ult has r ach d an advanc d stag . Tabl 16.3 shows typical ratings of fus s for us with 11kV transform rs. • 16 • • 260 • N twork Prot ction & Automation Guid
n utral conductor. In th bias d low-imp danc v rsion, th thr phas curr nts and th n utral curr nt b com th bias inputs to a diff r ntial l m nt. Th s yst m is op rativ for faults within th r gion b tw n curr nt transform rs, th at is, for faults on th star winding in qu stion. Th syst m will r main stabl for all faults outsid this zon . cov r th compl t transform r; this is possibl b caus of th high ffici ncy of transform r op ration, and th clos quival nc of amp r -turns d v lop d on th primary and s condary windings. Figur 16.7 illustrat s th principl . Curr nt transform rs on th primary and s condary sid s ar conn ct d to form a circ ulating curr nt syst m. Id> I High imp danc r lay > Figur 16.7: Principl of transform r diff r ntial prot ction Figur 16.6: R strict d arth fault prot ction for a star winding 16.8.1 Basic Consid rations for Transform r Diff r ntial Prot ction In applying th principl s of diff r ntial prot ction to transform rs, a vari ty of consid r ations hav to b tak n into account. Th s includ : a. corr ction for possibl phas shift across th transform r windings (phas corr ction) b. th ff cts of th vari ty of arthing and winding arrang m nts (filt ring of z ro s qu nc cu rr nts) c. corr ction for possibl unbalanc of signals from curr nt transform r s on ith r sid of th windings (ratio corr ction) d. th ff ct of magn tising inrush during initial n rgisation . th possibl occurr nc of ov rfluxing In traditional transform r diff r ntial sch m s, th r quir m nts for phas and ra tio corr ction w r m t by th application of xt rnal int rposing curr nt trans form rs (ICT’s), as a s condary r plica of th main winding conn ctions, or by a d lta conn ction of th main CT’s to provid phas corr ction only. Digital/num ric al r lays impl m nt ratio and phas corr ction in th r lay softwar inst ad, th us nabling most combinations of transform r winding arrang m nts to b cat r d for, irr sp ctiv of th winding conn ctions of th primary CT’s. This avoids th additional spac and cost r quir m nts of hardwar int rposing CT’s. 16.8.2 Lin C urr nt Transform r Primary Ratings Lin curr nt transform rs hav primary rating s s l ct d to b approximat ly qual to th rat d curr nts of th Th gain in prot ction p rformanc com s not only from using an instantan ous r lay with a low s tting, but also b caus th whol fault curr nt is m asur d, no t m r ly th transform d compon nt in th HV primary winding (if th star windin g is a s condary winding). H nc , although th prosp ctiv curr nt l v l d cr as s as fault positions progr ssiv ly n ar r th n utral nd of th winding ar co nsid r d, th squar law which controls th primary lin curr nt is not applicab l , and with a low ff ctiv s tting, a larg p rc ntag of th winding can b c ov r d. R strict d arth fault prot ction is oft n appli d v n wh n th n utral is solidly arth d. Sinc fault curr nt th n r mains at a high valu v n to th last turn of th winding (Figur 16.2), virtually compl t cov r for arth fau lts is obtain d. This is an improv m nt compar d with th p rformanc of syst ms that do not m asur th n utral conductor curr nt. Earth fault prot ction appli d to a d lta-conn ct d or un arth d star winding is inh r ntly r strict d, sinc no z ro s qu nc compon nts can b transmitt d through th transform r to th oth r windings. Both windings of a transform r can b prot ct d s parat ly with r strict d arth fault prot ction, th r by providing high-sp d prot ction again st arth faults for th whol transform r with r lativ ly simpl quipm nt. A hi gh imp danc r lay is us d, giving fast op ration and phas fault stability. 16. 8 DIFFERENTIAL PROTECTION
• Th r strict d arth fault sch m s d scrib d abov in S ction 16.7 d p nd ntir ly on th Kirchhoff principl that th sum of th curr nts flowing into a conducting n twork is z ro. A diff r ntial syst m can b arr ang d to N twork Prot ction & Automation Guid • 261 • Transform r and Transform r-F d r P rot ction 16 •
transform r windings to which th y ar appli d. Primary ratings will usually b limit d to thos of availabl standard ratio CT’s. 16.8.3 Phas Corr ction Corr ct op ration of transform r diff r ntial prot ction r quir s that th transform r primary and s condary curr nts, as m asur d by th r lay, ar in phas . If th t ransform r is conn ct d d lta/star, as shown in Figur 16.8, balanc d thr -phas through curr nt suff rs a phas chang of 30°. If l ft uncorr ct d, this phas d iff r nc would l ad to th r lay s ing through curr nt as an unbalanc d fault curr nt, and r sult in r lay op ration. Phas corr ction must b impl m nt d. d signation. Phas comp nsation is th n p rform d automatically. Caution is r qu ir d if such a r lay is us d to r plac an xisting l ctrom chanical or static r lay, as th primary and s condary lin CT’s may not hav th sam winding config uration. Phas comp nsation and associat d r lay data ntry r quir s mor d tail d consid ration in such circumstanc s. Rar ly, th availabl phas comp nsation faciliti s cannot accommodat th transform r winding conn ction, and in such c as s int rposing CT’s must b us d. 16.8.4 Filt ring of Z ro S qu nc Curr nts As d scrib d in Chapt r 10.8, it is ss ntial to provid som form of z ro s qu nc filt ring wh r a transform r winding can pass z ro s qu nc curr nt to an xt rnal arth fault. This is to nsur that out-of-zon arth faults ar not s n b y th transform r prot ction as an in-zon fault. This is achi v d by us of d l ta-conn ct d lin CT’s or int rposing CT’s for old r r lays, and h nc th winding c onn ction of th lin and/or int rposing CT’s must tak this into account, in addi tion to any phas comp nsation n c ssary. For digital/num rical r lays, th r qu ir d filt ring is appli d in th r lay softwar . Tabl 16.4 summaris s th phas comp nsation and z ro s qu nc filt ring r quir m nts. An xampl of an incorr ct choic of ICT conn ction is giv n in S ction 16.19.1. 16.8.5 Ratio Corr ction Corr ct op ration of th diff r ntial l m nt r quir s that curr nts in th dif f r ntial l m nt balanc und r load and through fault conditions. As th primar y and s condary lin CT ratios may not xactly match th transform r rat d windi ng curr nts, digital/num rical r lays ar provid d with ratio corr ction factors for ach of th CT inputs. Th corr ction factors may b HV Z ro s qu nc filt ring Y s Y s LV Z ro s qu nc filt ring Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Y s Transform r and Transform r-F d r P rot ction A B C Id> Id> Id> Figur 16.8: Diff r ntial prot ction for two-winding d lta/star transform r El ctrom chanical and static r lays us appropriat CT/ICT conn ctions to nsur that th primary and s condary curr nts appli d to th r lay ar in phas . For digital and num rical r lays, it is common to us starconn ct d lin CT’s on all w indings of th transform r and comp nsat for th winding phas shift in softwar . D p nding on r lay d sign, th only data r quir d in such circumstanc s may b th transform r v ctor group Transform r conn ction Transform r phas shift Clock fac v ctor • 16 • Phas comp nsation r quir d
Yy0 Zd0 0° 0 0° Dz0 Dd0 Yz1 Zy1 -30° 1 30° Yd1 Dy1 Yy6 Zd6 -180° 6 180° Dz6 Dd6 Yz11 Zy11 3 11 -30° Yd11 Dy11 YyH YzH YdH ZdH (H / 12) x 360° Hour H -(H / 12) x 360° DzH DyH Dd H H : phas displac m nt clock numb r , according to IEC 60076-1 Tabl 16.4: C urr nt transform r conn ctions for pow r transform rs of various v ctor groups • 262 • N twork Prot ction & Automation Guid
calculat d automatically by th r lay from knowl dg of th lin CT ratios and t h transform r MVA rating. How v r, if int rposing CT’s ar us d, ratio corr ction may not b such an asy task and may n d to tak into _ account a factor of √3 i f d lta-conn ct d CT’s or ICT’s ar involv d. If th transform r is fitt d with a ta p chang r, lin CT ratios and corr ction factors ar normally chos n to achi v curr nt balanc at th mid tap of th transform r. It is n c ssary to nsur tha t curr nt mismatch du to off-nominal tap op ration will not caus spurious op r ation. Th xampl in S ction 16.19.2 provid s an illustration of how ratio corr ction factors ar us d, and that of S ction 16.9.3 shows how to s t th ratio c orr ction factors for a transform r with an unsymm trical tap rang . 16.8.6 Bias S tting Bias is appli d to transform r diff r ntial prot ction for th sam r a sons as any unit prot ction sch m – to nsur stability for xt rnal faults whil allowing s nsitiv s ttings to pick up int rnal faults. Th situation is slight ly complicat d if a tap chang r is pr s nt. With lin CT/ICT ratios and corr cti on factors s t to achi v curr nt balanc at nominal tap, an off-nominal tap may b s n by th diff r ntial prot ction as an int rnal fault. By s l cting th m inimum bias to b gr at r than sum of th maximum tap of th transform r and pos sibl CT rrors, malop ration du to this caus is avoid d. Som r lays us a bi as charact ristic with thr s ctions, as shown in Figur 16.9. Th first s ctio n is s t high r than th transform r magn tising curr nt. Th s cond s ction is s t to allow for off-nominal tap s ttings, whil th third has a larg r bias slo p b ginning w ll abov rat d curr nt to cat r for h avy through-fault condition s. Wh n th pow r transform r has only on of its thr windings conn ct d to a sou rc of supply, with th oth r two windings f ding loads, a r lay with only two s ts of CT inputs can b us d, conn ct d as shown in Figur 16.10(a). Th s para t load curr nts ar summat d in th CT s condary circuits, and will balanc wit h th inf d curr nt on th supply sid . Wh n mor than on sourc of fault curr nt inf d xists, th r is a dang r in th sch m of Figur 16.10(a) of curr nt circulating b tw n th two parall l d s ts of curr nt transform rs without pro ducing any bias. It is th r for important a r lay is us d with s parat CT inpu ts for th two s condari s - Figur 16.10(b). Sourc Loads Id> (a) Thr winding transform r (on pow r sourc ) Sourc Possibl fault inf d Id> (b) Thr winding transform r (thr pow r sourc s) Sourc Possibl fault in f d Diff r ntial curr nt ( Id) 3 Id> • 2 Op rat (c) Thr winding transform r with unload d d lta t rtiary 1 30% R strain slop
70% slop S tting rang (0.1 - 0.5Id) 0 Figur 16.10 Diff r ntial prot ction arrang m nts for thr -winding transform rs (shown singl phas for simplicity) 1 2 3 4 Eff ctiv bias (x In) 5 6 Figur 16.9: Typical bias charact ristic 16.8.7 Transform rs with Multipl Windings Th unit prot ction principl r mains valid for a syst m having mor than two conn ctions, so a transform r with thr or mor windings can still b prot ct d by th application of th abov princi pl s. N twork Prot ction & Automation Guid 16.9 DIFFERENTIAL PROTECTION STABILISATION DURING MAGNETISING INRUSH CONDITIONS Th magn tising inrush ph nom non d scrib d in S ction 16.3 produc s curr nt inp ut to th n rgis d winding which has no quival nt on th oth r windings. Th w hol of th inrush curr nt app ars, th r for , as unbalanc and th diff r ntial prot ction is unabl to • 263 • Transform r and Transform r-F d r P rot ction 16 • Wh n th third winding consists of a d lta-conn ct d t rtiary with no conn ction s brought out, th transform r may b r gard d as a two winding transform r for prot ction purpos s and prot ct d as shown in Figur 16.10(c).
distinguish it from curr nt du to an int rnal fault. Th bias s tting is not f f ctiv and an incr as in th prot ction s tting to a valu that would avoid op ration would mak th prot ction of littl valu . M thods of d laying, r strain ing or blocking of th diff r ntial l m nt must th r for b us d to pr v nt ma lop ration of th prot ction. 16.9.1 Tim D lay Sinc th ph nom non is transi n t, stability can b maintain d by providing a small tim d lay. How v r, b caus this tim d lay also d lays op ration of th r lay in th v nt of a fault occu rring at switch-on, th m thod is no long r us d. ov rcom th op rating t nd ncy du to th whol of th inrush curr nt that flow s in th op rating circuit. By this m ans a s nsitiv and high-sp d syst m can b obtain d. 16.9.3 Inrush D t ction Blocking – Gap D t ction T chniqu Anoth r f atur that charact riz s an inrush curr nt can b s n from Figur 16.5 wh r th two wav forms (c) and (d) hav p riods in th cycl wh r th curr nt is z ro. Th minimum duration of this z ro p riod is th or tically on quart r of th cy cl and is asily _ 1 d t ct d by a simpl tim r t1 that is s t to 4f s conds. F igur 16.11 shows th circuit in block diagram form. Tim r t1 produc s an output only if th curr nt is z ro for _ 1 a tim xc ding 4f s conds. It is r s t wh n th instantan ous valu of th diff r ntial curr nt xc ds th s tting r f r nc . Bias Diff r ntial Thr shold Diff r ntial Inhibit comparator Tim r 1 t1 = 1 4f In hibit Tim r 2 t2 = 1 f Trip Transform r and Transform r-F d r P rot ction 16.9.2 Harmonic R straint Th inrush curr nt, although g n rally r s mbling an i nzon fault curr nt, diff rs gr atly wh n th wav forms ar compar d. Th diff r nc in th wav forms can b us d to distinguish b tw n th conditions. As stat d b for , th inrush curr nt contains all harmonic ord rs, but th s ar not al l qually suitabl for providing bias. In practic , only th s cond harmonic is us d. This compon nt is pr s nt in all inrush wav forms. It is typical of wav fo rms in which succ ssiv half p riod portions do not r p at with r v rsal of pola rity but in which mirrorimag symm try can b found about c rtain ordinat s. Th proportion of s cond harmonic vari s som what with th d gr of saturation of th cor , but is always pr s nt as long as th uni-dir ctional compon nt of flux xists. Th amount vari s according to factors in th transform r d sign. Norma l fault curr nts do not contain s cond or oth r v n harmonics, nor do distort d curr nts flowing in saturat d iron cor d coils und r st ady stat conditions. T h output curr nt of a curr nt transform r that is n rgis d into st ady stat s aturation will contain odd harmonics but not v n harmonics. How v r, should th curr nt transform r b saturat d by th transi nt compon nt of th fault curr n t, th r sulting saturation is not symm trical and v n harmonics ar introduc d into th output curr nt. This can hav th advantag of improving th through f ault stability p rformanc of a diff r ntial r lay. faults. Th s cond harmonic is th r for an attractiv basis for a stabilising bias against inrush ff cts, but car must b tak n to nsur that th curr nt transform rs ar suffici ntly larg so that th harmonics produc d by transi nt saturation do not d lay normal op ration of th r lay. Th diff r ntial curr nt is pass d through a filt r tha t xtracts th s cond harmonic; this compon nt is th n appli d to produc a r st raining quantity suffici nt to Figur 16.11: Block diagram to show wav form gap-d t cting principl • 16 • As th z ro in th inrush curr nt occurs towards th nd of th cycl , it is ssary to d lay op ration of th _ diff r ntial r lay by 1 s conds to nsur t th z ro f condition can b d t ct d if pr s nt. This is achi v d by using
n c tha a s
cond tim r t2 that is h ld r s t by an output from tim r t1. _ 1 Wh n no curr n t is flowing for a tim xc ding 4f s conds, tim r t2 is h ld r s t and th dif f r ntial r lay that may b controll d by th s tim rs is block d. Wh n a diff r ntial curr nt xc ding th s tting of th r lay flows, tim r t1 is r s t and t im r t2 tim s out to giv a _ trip signal in 1 s conds. If th diff r ntial curr nt is f charact ristic of transform r inrush th n tim r t2 will b r s t on ac h cycl and th trip signal is block d. Som num rical r lays may us a combinat ion of th harmonic r straint and gap d t ction t chniqu s for magn tising inrus h d t ction. 16.10 COMBINED DIFFERENTIAL AND RESTRICTED EARTH FAULT SCHEMES Th advantag s to b obtain d by th us of r strict d arth fault prot ction, discu ss d in S ction 16.7, l ad to th syst m b ing fr qu ntly us d in conjunction wi th an ov rall diff r ntial syst m. Th importanc of this is shown in Figur 16. 12 from which it will b s n that if th n utral of a star-conn ct d winding is arth d through a r sistanc of on p r unit, an ov rall diff r ntial syst m ha ving an ff ctiv s tting of 20% will d t ct faults in only 42% of th winding f rom th lin nd. N twork Prot ction & Automation Guid • 264 •
100 80 Impl m ntation of a combin d diff r ntial/REF prot ction sch m is mad asy if a num rical r lay with softwar ratio/phas comp nsation is us d. All comp nsati on is mad int rnally in th r lay. io n Primary op rating curr nt (p rc ntag of rat d curr nt) ict 40
d a r th fa u lt 60 20 f Dif r ial nt t c pro 0 100 80 R tio str n Wh r softwar ratio/phas corr ction is not availabl , ith r a summation trans form r or auxiliary CT’s can b us d. Th conn ctions ar shown in Figur s 16.13 a nd 16.14 r sp ctiv ly. Car must b tak n in calculating th s ttings, but th o nly significant disadvantag of th Combin d Diff r ntial/REF sch m is that th REF l m nt is lik ly to op rat for h avy int rnal faults as w ll as th diff r ntial l m nts, thus making subs qu nt fault analysis som what confusing. How v r, th saving in CT’s outw ighs this disadvantag . 60
pr ot ct 40 20 0 P rc ntag of winding prot ct d Figur 16.12: Amount of winding prot ct d wh n transform r is r sistanc arth d and ratings of transform r and r sistor ar qual R strict d arth fault r lay I I d> Id> I d> Diff r ntial r lay Figur 16.13 Combin d diff r ntial and arth fault prot ction using summation cu rr nt transform r • R strict d arth fault r lay I > Phas corr cting auxiliary curr nt transform rs Id> Id> Id> Diff r ntial r lay Figur 16.14: Combin d diff r ntial and r strict d arth fault prot ction using auxiliary CT’s N twork Prot ction & Automation Guid • 265 • Transform r and Transform r-F d r P rot ction 16 •
16.10.1 Application wh n an Earthing Transform r is conn ct d within th Prot ct d Zon A d lta-conn ct d winding cannot d liv r any z ro s qu nc curr nt to an arth fault on th conn ct d syst m, any curr nt that do s flow is in cons qu n c of an arth d n utral ls wh r on th syst m and will hav a 2-1-1 patt rn o f curr nt distribution b tw n phas s. Wh n th transform r in qu stion r pr s n ts a major pow r f d, th syst m may b arth d at that point by an arthing tr ansform r or arthing r actor. Th y ar fr qu ntly conn ct d to th syst m, clos to th main supply transform r and within th transform r prot ction zon . Z r o s qu nc curr nt that flows through th arthing transform r during syst m ar th faults will flow through th lin curr nt transform rs on this sid , and, withou t an quival nt curr nt in th balancing curr nt transform rs, will caus unwant d op ration of th r lays. Th probl m can b ov rcom by subtracting th appro priat compon nt of curr nt from th main CT output. Th arthing transform r n utral curr nt is us d for this purpos . As this r pr s nts thr tim s th z ro s qu nc curr nt flowing, ratio corr ction is r quir d. This can tak th form o f int rposing CT’s of ratio 1/0.333, arrang d to subtract th ir output from that o f th lin curr nt transform rs in ach phas , as shown in Figur 16.15. Th z r o s qu nc compon nt is canc ll d, r storing balanc to th diff r ntial syst m. Transform r and Transform r-F d r P rot ction A B C Earthing transform r 1/0.333 Diff r ntial r lay Id> Id> Id> I > R strict d arth fault r lay Figur 16.15: Diff r ntial prot ction with in-zon arthing transform r, with r strict d arth fault r lay • 16 • A B C Earthing transform r Diff r ntial r lay Id> Id> Id> Figur 16.16: Diff r ntial prot ction with in-zon arthing transform r; no art h fault r lay
• 266 • N twork Prot ction & Automation Guid
A B C I > Earthing transform r Diff r ntial r lay Id> Id> Id> Figur 16.17: Diff r ntial prot ction with in-zon arthing transform r, with al t rnativ arrang m nt of r strict d arth fault r lay Alt rnativ ly, num rical r lays may us softwar to p rform th subtraction, hav ing calculat d th z ro s qu nc compon nt int rnally. A high imp danc r lay l m nt can b conn ct d in th n utral l ad b tw n curr nt transform rs and diff r ntial r lays to provid r strict d arth fault prot ction to th winding. As an alt rnativ to th abov sch m , th circulating curr nt syst m can b compl t d via a thr -phas group of int rposing transform rs that ar provid d with t rtiary windings conn ct d in d lta. This winding ff ctiv ly short-circuits th z ro s qu nc compon nt and th r by r mov s it from th balancing quantiti s in th r lay circuit; s Figur 16.16. Provid d r strict d arth fault prot ction is not r quir d, th sch m shown in Figur 16.16 has th advantag of not r qu iring a curr nt transform r, with its associat d mounting and cabling r quir m n ts, in th n utral- arth conductor. Th sch m can also b conn ct d as shown in Figur 16.17 wh n r strict d arth fault prot ction is n d d. 16.11 EARTHING T RANSFORMER PROTECTION Earthing transform rs not prot ct d by oth r m ans can us th sch m shown in Figur 16.18. Th d ltaconn ct d curr nt transform rs ar c onn ct d to an ov rcurr nt r lay having thr phas -fault l m nts. Th normal a ction of th arthing transform r is to pass z ro s qu nc curr nt. Th transfor m r quival nt curr nt circulat s in th d lta form d by th CT s condari s with out n rgising th r lay. Th latt r may th r for b s t to giv fast and s nsi tiv prot ction against faults in th arthing transform r its lf. A B C I> Earthing transform r Figur 16.18: Earthing transform r prot ction 16.12 AUTOTRANSFORMER PROTECTION Autotransform rs ar us d to coupl EHV pow r n tworks if th ratio of th ir voltag s is mod rat . An alt rnativ to Diff r nti al Prot ction that can b appli d to autotransform rs is prot ction bas d on th application of Kirchhoff s law to a conducting n twork, nam ly that th sum of th curr nts flowing into all xt rnal conn ctions to th n twork is z ro. A cir culating curr nt syst m is arrang d b tw n qual ratio curr nt transform rs in th two groups of lin conn ctions and th n utral nd conn ctions. If on n utr al curr nt transform r is us d, this and all th lin curr nt transform rs can b conn ct d in parall l to a singl l m nt r lay, thus providing a sch m r spo nsiv to arth faults only; s Figur 16.19(a). If curr nt transform rs ar fit t d in ach phas at th n utral nd of th windings and a thr - l m nt r lay i s
• N twork Prot ction & Automation Guid • 267 • Transform r and Transform r-F d r P rot ction 16 •
us d, a diff r ntial syst m can b provid d, giving full prot ction against phas and arth faults; s Figur 16.19(b). This provid s high-sp d s nsitiv prot ction. It is unaff ct d by ratio chang s on th transform r du to tap-changing and is immun to th ff cts of magn tising inrush curr nt. a. high syst m voltag b. low syst m fr qu ncy c. g omagn tic disturbanc s Th l att r r sults in low fr qu ncy arth curr nts circulating through a transmission syst m. Sinc mom ntary syst m disturbanc s can caus transi nt ov rfluxing tha t is not dang rous, tim d lay d tripping is r quir d. Th normal prot ction is an IDMT or d finit tim charact ristic, initiat d if a d fin d V/f thr shold is xc d d. Oft n s parat alarm and trip l m nts ar provid d. Th alarm functi on would b d finit tim -d lay d and th trip function would b an IDMT charact ristic. A typical charact ristic is shown in Figur 16.20. G omagn tic disturba nc s may r sult in ov rfluxing without th V/f thr shold b ing xc d d. Som r lays provid a 5th harmonic d t ction f atur , which can b us d to d t ct such a condition, as l v ls of this harmonic ris und r ov rfluxing conditions. Op rating tim (s) 1000 t= 0.8 + 0.18 x K (M-1) 2 A B C Transform r and Transform r-F d r P rot ction High Id> imp danc r lay (a) Earth fault sch m A B C A B C I d> N (b) Phas and arth fault sch m I> Id> 100 10 1 =63 =40 K= K 20 =5 =1 1 1.1 1.2 M= Figur 16.19: Prot ction of auto-transform r by high imp danc diff r ntial r la ys 1.3 V/f S tting 1.4 1.5 1.6 • 16 • It do s not r spond to int rturn faults, a d fici ncy that is s rious in vi w of th high statistical risk quot d in S ction 16.1. Such faults, unl ss oth rwis cl ar d, will b l ft to d v lop into arth faults, by which tim consid rably mor damag to th transform r will hav occurr d. In addition, this sch m do s not r spond to any fault in a t rtiary winding. Unload d d lta-conn ct d t rtia ry windings ar oft n not prot ct d; alt rnativ ly, th d lta winding can b ar th d at on point through a curr nt transform r that n rgis s an instantan ous r lay. This syst m should b s parat from th main winding prot ction. If th t rtiary winding arthing l ad is conn ct d to th main winding n utral abov th n utral curr nt transform r in an att mpt to mak a combin d syst m, th r may b ‘blind spots’ which th prot ction cannot cov r. 16.13 OVERFLUXING PROTECTION Th ff cts of xc ssiv flux d nsity ar d scrib d in S ction 16.2.8. Ov rfluxing aris s principally from th following syst m conditions:
Figur 16.20: Typical IDMT charact ristic for ov rfluxing prot ction 16.14 TANK-EARTH PROTECTION This is also known as Howard prot ction. If th tran sform r tank is nominally insulat d from arth (an insulation r sistanc of 10 o hms b ing suffici nt) arth fault prot ction can b provid d by conn cting a r l ay to th s condary of a curr nt transform r th primary of which is conn ct d b tw n th tank and arth. This sch m is similar to th fram - arth fault busba r prot ction d scrib d in Chapt r 15. 16.15 OIL AND GAS DEVICES All faults b low oil in an oil-imm rs d transform r r sult in localis d h ating and br akdown of th oil; som d gr of arcing will always tak plac in a winding fault and th r sulting d composition of th oil will r l as gas s. Wh n th fault is of a v ry minor typ , such as a hot joint, gas is r l as d slowly, but a major fault involving s v r N twork Prot ction & Automation Guid • 268 •
arcing caus s a v ry rapid r l as of larg volum s of gas as w ll as oil vapour . Th action is so viol nt that th gas and vapour do not hav tim to scap bu t inst ad build up pr ssur and bodily displac th oil. Wh n such faults occur in transform rs having oil cons rvators, th fault caus s a blast of oil to pass up th r li f pip to th cons rvator. A Buchholz r lay is us d to prot ct agai nst such conditions. D vic s r sponding to abnormally high oil pr ssur or rat of-ris of oil pr ssur ar also availabl and may b us d in conjunction with a Buchholz r lay. 16.15.1 Oil Pr ssur R li f D vic s transform rs fitt d with a cons rvator. Th Buchholz r lay is contain d in a cas t housing which is conn ct d in th pip to th cons rvator, as in Figur 16.21. 3 x Int rnal pip diam t r (min) 5 x Int rnal pip diam t r (min) Cons rvator Transform r Figur 16.21: Buchholz r lay mounting arrang m nt Th surg of oil caus d by a s rious fault bursts th disc, so allowing th oil to discharg rapidly. R li ving and limiting th pr ssur ris avoids xplosiv ruptur of th tank and cons qu nt fir risk. Outdoor oil-imm rs d transform rs ar usually mount d in a catchm nt pit to coll ct and contain spilt oil (from wh at v r caus ), th r by minimising th possibility of pollution. A drawback of th frangibl disc is that th oil r maining in th tank is l ft xpos d to th at mosph r aft r ruptur . This is avoid d in a mor ff ctiv d vic , th sudd n p r ssur r li f valv , which op ns to allow discharg of oil if th pr ssur xc ds a s t l v l, but clos s automatically as soon as th int rnal pr ssur falls b low this l v l. If th abnormal pr ssur is r lativ ly high, th valv can op rat within a f w millis conds, and provid fast tripping wh n suitabl contact s ar fitt d. Th d vic is commonly fitt d to pow r transform rs rat d at 2MVA or high r, but may b appli d to distribution transform rs rat d as low as 200kV A, particularly thos in hazardous ar as. 16.15.2 Rapid Pr ssur Ris R lay This d vic d t cts rapid ris of pr ssur rath r than absolut pr ssur and th r by can r spond v n quick r than th pr ssur r li f valv to sudd n abnormally hi gh pr ssur s. S nsitiviti s as low as 0.07bar/s ar attainabl , but wh n fitt d to forc d-cool d transform rs th op rating sp d of th d vic may hav to b s low d d lib rat ly to avoid spurious tripping during circulation pump starts. 16 .15.3 Buchholz Prot ction Buchholz prot ction is normally provid d on all N twork Prot ction & Automation Guid A typical Buchholz r lay will hav two s ts of contacts. On is arrang d to op r at for slow accumulations of gas, th oth r for bulk displac m nt of oil in th v nt of a h avy int rnal fault. An alarm is g n rat d for th form r, but th latt r is usually dir ct-wir d to th CB trip r lay. Th d vic will th r for g iv an alarm for th following fault conditions, all of which ar of a low ord r of urg ncy. a. hot spots on th cor du to short circuit of lamination insulat ion b. cor bolt insulation failur c. faulty joints d. int rturn faults or oth r winding faults involving only low r pow r inf ds . loss of oil du to l akag Wh n a major winding fault occurs, this caus s a surg of oil, which displac s th low r float and thus caus s isolation of th transform r. This action will tak plac for: i. all s v r winding faults, ith r to arth or int rphas ii. loss of oil if allow d to continu to a dang rous d gr An insp ction window is usually provid d on ith r sid of th gas coll ction spac . Visibl whit or y llow gas indicat s that insulation has b n burnt, whil black or gr y gas indi cat s th pr s nc of, dissociat d oil. In th s cas s th gas will probably b inflammabl , wh r as r l as d air will not. A v nt valv is provid d on th top of th housing for th gas to b r l as d or •
• 269 • Transform r and Transform r-F d r P rot ction 16 • Th simpl st form of pr ssur r li f d vic is th wid ly us d ‘frangibl disc’ that is normally locat d at th nd of an oil r li f pip protruding from th top of th transform r tank. 76mm typical
coll ct d for analysis. Transform rs with forc d oil circulation may xp ri nc oil flow to/from th cons rvator on starting/stopping of th pumps. Th Buchholz r lay must not op rat in this circumstanc . Cl aning op rations may caus a ra tion of th oil. Und r such conditions, tripping of th transform r du to Buchh olz op ration should b inhibit d for a suitabl p riod. B caus of its univ rsa l r spons to faults within th transform r, som of which ar difficult to d t ct by oth r m ans, th Buchholz r lay is invaluabl , wh th r r gard d as a main prot ction or as a suppl m nt to oth r prot ction sch m s. T sts carri d out by striking a high voltag arc in a transform r tank fill d with oil, hav shown th at op ration tim s of 0.05s-0.1s ar possibl . El ctrical prot ction is g n rall y us d as w ll, ith r to obtain fast r op ration for h avy faults, or b caus B uchholz r lays hav to b pr v nt d from tripping during oil maint nanc p riods . Cons rvators ar fitt d to oil-cool d transform rs abov 1000kVA rating, xc p t thos to North Am rican d sign practic that us a diff r nt t chniqu . 16.16 TRANSFORMER-FEEDER PROTECTION A transform r-f d r compris s a transform r dir c tly conn ct d to a transmission circuit without th int rv ntion of switchg ar. Exampl s ar shown in Figur 16.22. HV LV prot ct d as a singl zon or b provid d with s parat prot ctions for th f d r and th transform r. In th latt r cas , th s parat prot ctions can both b unit typ syst ms. An ad quat alt rnativ is th combination of unit transform r prot ction with an unr strict d syst m of f d r prot ction, plus an int rtri pping f atur . 16.16.1 Non-Unit Sch m s Th following s ctions d scrib how nonunit sch m s ar appli d to prot ct transform r-f d rs against various typ s of fault. 16.16.1.1 F d r phas and arth faults High-sp d prot ction against ph as and arth faults can b provid d by distanc r lays locat d at th nd of th f d r r mot from th transform r. Th transform r constitut s an appr ciabl lump d imp danc . It is th r for possibl to s t a distanc r lay zon to cov r th whol f d r and r ach part way into th transform r imp danc . With a nor mal tol ranc on s tting thus allow d for, it is possibl for fast Zon 1 prot c tion to cov r th whol of th f d r with c rtainty without risk of ov r-r achi ng to a fault on th low voltag sid . Although th distanc zon is d scrib d a s b ing s t ’half way into th transform r’, it must not b thought that half th tr ansform r winding will b prot ct d. Th ff cts of auto-transform r action and variations in th ff ctiv imp danc of th winding with fault position pr v nt this, making th amount of winding b yond th t rminals which is prot ct d v ry small. Th valu of th syst m is confin d to th f d r, which, as stat d abov , r c iv s high-sp d prot ction throughout. 16.16.1.2 F d r phas faults A di stanc sch m is not, for all practical purpos s, aff ct d by varying fault l v ls on th high voltag busbars and is th r for th b st sch m to apply if th fault l v l may vary wid ly. In cas s wh r th fault l v l is r asonably consta nt, similar prot ction can b obtain d using high s t instantan ous ov rcurr nt r lays. Th s should hav a low transi nt ov r-r ach, d fin d as: IS − IF × 100% IF Tr nsformer nd Tr nsformer Feeder P rotection LV HV HV LV • 16 • HV LV Figure 16.22: Typic l tr nsformer feeder circuits. The s ving in switchge r so chieved is offset y incre sed complic tion in the
necess ry protection. The prim ry requirement is intertripping, since the feeder protection remote from will not respond to the low current f ul the tr nsformer t conditions th t c n e detected y restricted e rth fult nd Buchholz protect ions. Either unrestricted or restricted protection c n e pplied; moreover, the tr nsformer feeder c n e where: IS = setting current IF = ste dy st te r.m.s. v lue of f ult current wh ich whenfully offset just oper tes the rel y The inst nt neous overcurrent rel ys must e set without risk of them oper ting for f ults on the remote side of t he tr nsformer. Network Protection & Autom tion Guide • 270 •
~ ZS ZL ZT I>> IF1 IF2 Setting r tio r = IS IF2 Tr nsient over re ch (%) 0.25 0.5 ZT ZS + Z L 1.0 2.0 4.0 8.0 5 1.01 0.84 0.63 0.42 0.25 0.14 25 1.20 1.00 0.75 0.50 0.30 0.17 50 1.44 1.20 0.90 0.60 0.36 0.20 100 1.92 1.60 1.20 0.80 0.48 0.27 x= Is = Rel y setting = 1.2(1 + t)IF2 t = Tr nsient over re ch (p.u.) Figure 16.23: Over re ch consider tions in the pplic tion of tr nsformer feeder protection Referring to Figure 16.23, the required setting to ensure th t the rel y will no t oper te for fully offset f ult IF2 is given y: IS = 1.2 (1 + t) IF2 where I F2 is the f ult current under m ximum source conditions, th t is, when ZS is min imum, nd the f ctor of 1.2 covers possi le errors in the system imped nce det i ls used for c lcul tion of IF2 , together with rel y nd CT errors. As it is des ir le for the inst nt neous overcurrent protection to cle r ll ph se f ults n ywhere within the feeder under v rying system oper ting conditions, it is necess ry to h ve rel y setting less th n IF1 in order to ensure f st nd reli le o per tion. Let the setting r tio resulting from setting IS e I r = S I F1 Theref ore, rIF1 = 1.2(1 + t)IF2 Hence, ZS + Z L r = 1.2 (1 + t ) ZS + Z L + ZT r = 1.2 (1 + t ) = 1.2 (1 + t ) 1+x ZS + Z L (1 + x )( Z S + Z L ) where: x = ZT ZS + Z L It c n e seen th t for given tr nsformer size, the most sensitive protection for the line will e o t ined y using rel ys with the lowest tr nsient overre c h. It should e noted th t where r is gre ter th n 1, the protection will not co ver the whole line. Also, ny incre se in source imped nce ove the minimum v l ue will incre se the effective setting r tios ove those shown. The inst nt neo us protection is usu lly pplied with time del yed overcurrent element h ving lower current setting. In this w y, inst nt neous protection is provided for t he feeder, with the time del yed element covering f ults on the tr nsformer. Whe n the power c n flow in thetr nsformer feeder in either direction, overcurrent rel ys will e required t oth ends. In the c se of p r llel tr nsformer feeder
s, it is essenti l th t the overcurrent rel ys on the low volt ge side e direct ion l, oper ting only for f ult current fed into the tr nsformer feeder, s desc ri ed in Section 9.14.3. 16.16.1.3 E rth f ults Inst nt neous restricted e rth f ult protection is norm lly provided. When the high volt ge winding is delt con nected, rel y in the residu l circuit of the line current tr nsformers gives e rth f ult protection which • Network Protection & Autom tion Guide • 271 • Tr nsformer nd Tr nsformer Feeder P rotection 16 •
is fund ment lly limited to the feeder nd the ssoci ted delt connected tr nsf ormer winding. The l tter is un le to tr nsmit ny zero sequence current to t hrough e rth f ult. When the feeder is ssoci ted with n erthed st rconnected winding, norml restricted e rth f ult protection s descri ed in Section 16.7 i s not pplic le ec use of the remoteness of the tr nsformer neutr l. Restricte d protection c n e pplied using directionl e rth f ult rel y. A simple sens itive nd high speed direction l element c n e used, ut ttention must e p id to the tr nsient st ility of the element. Altern tively, direction l IDMT re l y m y e used, the time multiplier eing set low. The slight inverse time del y in oper tion will ensure th t unw nted tr nsient oper tion is voided. When th e supply source is on the high volt ge st r side, n ltern tive scheme th t doe s not require volt ge tr nsformer c n e used. The scheme is shown in Figure 1 6.24. For the circuit re ker to trip, oth rel ys A nd B must oper te, which w ill occur for e rth f ults on the feeder or tr nsformer winding. Extern l e rth f ults c use the tr nsformer to deliver zero sequence current only, which will c ircul te in the closed delt connection of the second ry windings of the three uxili ry current tr nsformers. No output is v il le to rel y B. Through ph se f ults will oper te rel y B, ut not the residu l rel y A. Rel y B must h ve s etting A B C point is l ove them ximum lo d. As the e rthing of the neutr l t receiving ikely to e solid nd the e rth f ult current will therefore e comp r le with the ph se f ult current, high settings re not seriouslimit tion. E rth f ult protection of the low volt ge winding will e provided y restricted e rth f ult system using either three or four current tr nsformers, ccording to whether the winding is delt or st r connected, s descri ed in Section 16.7. 16.16.1.4 In zone c p cit nce The feeder portion of the tr nsformer feeder will h ve n ppreci le c p cit nce etween e ch conductor nd e rth. During n extern l e rt h f ult the neutr l will e displ ced, nd the resulting zero sequence component of volt ge will produce corresponding component of zero sequence c p cit nce current. In the limiting c se of full neutr l displ cement, this zero sequence c urrent will e equ l in v lue to the norm l positive sequence current. The resul ting residu l current is equ l to three times the zero sequence current nd henc e to three times the norml line ch rging current. The v lue of this component o f in zone current should e considered when est lishing the effective setting o f e rth f ult rel ys. 16.16.2 Unit Schemes The sic differences etween the req uirements of feeder Tr nsformer nd Tr nsformer Feeder P rotection • 16 • Rel y A I > Rel y B I> I> I> + A
B B B Trip circuit Figure 16.24: Inst nt neous protection of tr nsformer feeder • 272 • Network Protection & Autom tion Guide
Althoughtechnic lly superior, the use of sep r te protection systems is seldom justifi le when comp red with n over ll system or com in tion of non unit fe eder protection nd unit tr nsformer system. An over ll unit system must t ke into ccount the f ct th t zero sequence current on one side of tr nsformer m y not e reproduced in ny form on the other side. This represents little diffic ulty to modern numeric l rel y using softw re ph se/zero sequence compens tion nd digit l communic tions to tr nsmit full inform tion on the ph se nd e rth currents from one rel y to the other. However, it does represent more difficul t pro lem for rel ys using older technology. The line current tr nsformers c n e connected to summ tion tr nsformer with unequ l t ps, s shown in Figure 16. 25( ). This rr ngement produces n output for ph se f ults nd lso some respon se for A nd B ph se e rth f ults. However, the resulting settings will e simil r to those for ph se f ults nd no protection will e given for C ph se e rth f ults. An ltern tive technique is shown in Figure 16.25( ). The B ph se is t ke n through sepr te winding on nother tr nsformer or rel y electrom gnet, to p rovide nother l ncing system. The two tr nsformers re interconnected with th eir counterp rts t the other end of the feeder tr nsformer y four pilot wires. Oper tion with three pilot cores is possi le ut four re prefer le, involving little incre se in pilot cost. 16.17 INTERTRIPPING In order to ensure th t oth the high nd low volt ge circuit re kers oper te for f ults within the tr nsfo rmer nd feeder, it is necess ry to oper te oth circuit re kers from protectio n norm lly ssoci ted with one. The technique for doing this is known s intertr ipping. Network Protection & Autom tion Guide Sever l methods re v il le for intertripping; these re discussed in Ch pter 8. 16.17.1 Neutr l Displ cement An ltern tive to intertripping is to detect the condition y me suring the residu l volt ge on the feeder. An e rth f ult occur ring on the feeder connected to n une rthed tr nsformer winding should e cle r ed y the feeder circuit, ut if there is lso source of supply on the second ry side of the tr nsformer, the feeder m y e still live. The feeder will then e loc l une rthed system, nd, if the e rth f ult continues in n rcing condi tion, d ngerous overvolt ges m y occur. A volt ge rel y is energised from the r oken delt connected second ry winding of volt ge tr nsformer on the high volt ge line, nd receives n input proportion l to the zero sequence volt ge of the line, th t is, to ny displ cement ofthe neutr l point; see Figure 16.26. The rel y norm lly receives zero volt ge, ut, in the presence of n e rth f ult, th e roken delt volt ge will rise to three times the ph se volt ge. E rth f ults elsewhere in the system m y lso result in displ cement of the neutr l nd hence discrimin tion is chieved using definite or inverse time ch r cteristics. 16.1 8 CONDITION MONITORING OF TRANSFORMERS It is possi le to provide tr nsformers wi th me suring devices to detect e rly signs of degr d tion in v rious • • 273 • Tr nsformer nd Tr nsformer Feeder P rotection 16 • lie in the limit tion imposed on the tr nsfer of e r nd tr nsformer protections th f ult current y the tr nsformer nd the need for high sensitivity in the tr nsformer protection, suggesting th t the two components of tr nsformer feeder should e protected sep r tely. This involves mounting current tr nsformers dj cent to, or on, the high volt ge termin ls of the tr nsformer. Sep r te current tr nsformers re desir le for the feeder nd tr nsformer protections so th t th ese c n e rr nged intwo sep r te overl pping zones. The use of common current tr nsformers is possi le, ut m y involve the use of uxili ry current tr nsfor mers, or speci l winding nd connection rr ngements of the rel ys. Intertrippin g of the remote circuit re ker from the tr nsformer protection will e necess r y, ut this c n e done using the communic tion f cilities of the feeder protect
ion rel ys. The necessity for intertripping on tr nsformer feeders rises from the f ct th t cert in types of f ult produce insufficient current to oper te the protection ssoci ted with one of the circuit re kers. These f ults re: . f ults in the t r nsformer th t oper te the Buchholz rel y nd trip the loc l low volt ge circui t re ker, while f iling to produce enough f ult current tooper te the protecti on ssoci ted with the remote high volt gecircuit re ker . e rth f ults on th e st r winding of the tr nsformer, which, ec use of the position of the f ult i n the winding, g in produce insufficient current for rel y oper tion t the rem ote circuit re ker c. e rth f ults on the feeder or high volt ge delt connected winding which trip the high volt ge circuit re ker only, le ving the tr nsform er energised form the low volt ge side nd with two high volt ge ph ses t ne r line to line volt ge ove e rth. Intermittent rcing m y follow nd there is possi ility of tr nsient overvolt ge occurring nd c using further re kdown o f insul tion
Feeder A B C D D Tr nsformer nd Tr nsformer Feeder P rotection E E D Bi s winding E Oper ting winding Differenti l rel ys ( ) Circul ting current system A B C • 16 • Pilots
Rel y electrom gnets ( i s inherent) ( ) B l nced volt ge system Figure 16.25: Methods of protection for tr nsformer feeders using electromech ni c l st tic technology • 274 • Network Protection & Autom tion Guide
A B C Volt ge tr nsformer
oper tor c n m ke etter judgement s to the frequency of m inten nce, nd det ect e rly signs of deterior tion th t, if ignored, would le d to n intern l f u lt occurring. Such techniques re n enh ncement to, ut re not repl cement f or, the protection pplied to tr nsformer. The extent to which condition monit oring is pplied to tr nsformers on system will dependon m ny f ctors, mongs t which will e the policy of the sset owner, the suit ility of the design (ex isting trnsformers m y require modific tions involving period out of service – this m y e costly nd not justified), the import nce of the sset to system ope r tion, nd the gener l record of reli ility. Therefore, it should not e expec ted th t ll tr nsformers would e, or need to e, so fitted. A typic l conditio n monitoring system for n oilimmersed tr nsformer is c p le of monitoring the condition of v rious tr nsformer components s shown in T le 16.5. There c n e some overl p with the me surements v il le from digit l/numeric l rel y. By the use of softw re to store nd perform trend n lysis of the me sured d t , t he oper tor c n e presented with inform tion on the st te of he lth of the tr n sformer, nd l rms r ised when me sured v lues exceed ppropri te limits. This will norm lly provide the He lth Inform tion Insul tion qu lity Lo ding Permissi le overlo d r ting Hot spot temper ture Insul tion qu lity Hot spot temper ture Permissi le overlo d r ting Oil qu lity Winding insul tion condition Oil qu lit y Winding insul tion condition Frequency of use of e ch t p position OLTC he lth OLTC cont ct we r OLTC he lth Cooler efficiency Cooling pl nt he lth T nk integ rity Ursd > Residu l volt ge rel y Figure 16.26: Neutr l displ cement detection using volt ge tr nsformer. components nd provide w rning to the oper tor in order to void lengthy nd e xpensive out ge due to f ilure. The technique, which c n e pplied to other pl nt s well s tr nsformers, is c lled condition monitoring, s the intent is to provide the oper tor with regul r inform tion on the condition of the tr nsforme r. By reviewing the trends in the inform tion provided, the Monitored Equipment Me sured Qu ntity Volt ge P rti l disch rge me surement (wide nd volt ge) Bushings Lo d current Oil pressure Oil temper ture T nk G s in oil content Buchh olz g s content Moisture in oil content Position Drive power consumption Tot l s witched lo d current OLTC oil temper ture Oil temper ture difference Cooling ir temper ture Am ient temper ture Pump st tus Oil level • T p ch nger Coolers Conserv tor T le 16.5: Condition monitoring for tr nsformers Network Protection & Autom tion Guide • 275 • Tr nsformer nd Tr nsformer Feeder P rotection 16 •
oper tor with e rly w rning of degr dtion within one or more components of the tr nsformer, en ling m inten nce to e scheduled to correct the pro lem prior t o f ilure occurring. The m inten nce c n o viously e pl nned to suit systemcon ditions, provided the r te of degr d tion is not excessive. As sset owners eco me more conscious of the costs of n unpl nned out ge, nd electric supply netwo rks re utilised closer to c p city for long periods of time, the usefulness of this technique c n e expected to grow. 16.19 EXAMPLES OF TRANSFORMER PROTECTION This section provides three ex mples of the pplic tion of modern rel ys to tr nsformer protection. The l test MiCOM P630 series rel y provides dv nced softw re to simplify the c lcul tions, so n e rlier AREVA type KBCH rel y is used to illustr te the complexity of the required c lcul tions. 16.19.1 Provision of Zer o Sequence Filtering Figure 16.27 shows delt st r tr nsformer to e protected using unit protection scheme. With m in winding connection of Dyn11, suit le choices of prim ry nd second ry CT winding rr ngements, nd softw re ph se compens tion re to e m de. With the KBCH rel y, ph se compens tion is selected y the user in the form of softw re implemented ICT’s. +30° or on th s condary sid having a phas shift of –30°. Th r is a wid combinatio n of primary and s condary ICT winding arrang m nts that can provid this, such as Yd10 (+60°) on th primary and Yd3 (-90°) on th s condary. Anoth r possibility i s Yd11 (+30°) on th primary and Yy0 (0°) on th s condary. It is usual to choos th simpl st arrang m nts possibl , and th r for th latt r of th abov two poss ibiliti s might b s l ct d. How v r, th distribution of curr nt in th primary and s condary windings of th transform r du to an xt rnal arth fault on th s condary sid of th transform r must now b consid r d. Th transform r has a n arth conn ction on th s condary winding, so it can d liv r z ro s qu nc cur r nt to th fault. Us of star conn ct d main CT’s and Yy0 conn ct d ICT’s provid s a path for th z ro s qu nc curr nt to r ach th prot ction r lay. On th prima ry sid of th transform r, th d lta conn ct d main primary winding caus s z ro -s qu nc curr nt to circulat round th d lta and h nc will not b s n by th primary sid main CT’s. Th prot ction r lay will th r for not s any z ro-s qu nc curr nt on th primary sid , and h nc d t cts th s condary sid z ro s qu nc curr nt incorr ctly as an in-zon fault. Th solution is to provid th ICT’s on th s condary sid of th transform r with a d lta winding, so that th z ro -s qu nc curr nt circulat s round th d lta and is not s n by th r lay. Th r for , a rul can b d v lop d that a transform r winding with a conn ction to a rth must hav a d lta-conn ct d main or ICT for unit prot ction to op rat corr ctly. S l ction of Yy0 conn ction for th primary sid ICT’s and Yd1 (–30°o) for th s condary sid ICT’s provid s th 10MVA 33/11kV Z=10% Dyn11 Transform r and Transform r-F d r P rot ction Primary CT s Dyn 11 S condary CT s • 16 • Primary ICT s Id > Unit prot ction r lay S condary ICT s Primary CT s Yy0, 250/1 S condary CT s Yy0, 600/1
FLC = 175A FLC = 525A Figur 16.27: Transform r z ro s qu nc filt ring xampl 600/1 With th Dyn11 conn ction, th s condary voltag s and curr nts ar displac d by +30° from th primary. Th r for , th combination of primary, s condary and phas corr ction must provid a phas shift of –30° of th s condary quantiti s r lativ t o th primary. For simplicity, th CT’s on th primary and s condary windings of t h transform r ar conn ct d in star. Th r quir d phas shift can b achi v d ith r by us of ICT conn ctions on th primary sid having a phas shift of Primary ICT s Yy0 R=1000 A Rstab Id> Unit Prot ction R lay S condary ICT s Yd1 Figur 16.28: Transform r unit prot ction xampl • 276 • N twork Prot ction & Automation Guid
r quir d phas shift and th z ro-s qu nc trap on th s condary sid . 16.19.2 U nit Prot ction of a D lta-Star Transform r Figur 16.28 shows a d lta-star trans form r to which unit prot ction is to b appli d, including r strict d arth fau lt prot ction to th star winding. R f rring to th figur , th ICT’s hav alr ady b n corr ctly s l ct d, and ar conv ni ntly appli d in softwar . It th r for r mains to calculat suitabl ratio comp nsation (it is assum d that th transf orm r has no taps), transform r diff r ntial prot ction s ttings and r strict d arth fault s ttings. 16.19.2.1 Ratio comp nsation Transform r HV full load curr nt on s condary of main CT’s is: 175/250 = 0.7 Ratio comp nsation = 1/0.7 = 1.428 S l ct n ar st valu = 1.43 LV s condary curr nt = 525/600 = 0.875 Ratio comp n sation = 1/0.875 = 1.14 16.9.2.2 Transform r unit prot ction s ttings A curr nt s tting of 20% of th rat d r lay curr nt is r comm nd d. This quat s to 35A pr imary curr nt. Th KBCH r lay has a dual slop bias charact ristic with fix d bi as slop s ttings of 20% up to rat d curr nt and 80% abov that l v l. Th corr sponding charact ristic is shown in Figur 16.29. 600 primary arth fault curr nt of 25% rat d arth fault curr nt (i. . 250A). Th pr im task in calculating s ttings is to calculat th valu of th stabilising r sistor Rstab and stability factor K. A stabilising r sistor is r quir d to nsur through fault stability wh n on of th s condary CT’s saturat s whil th oth r s do not. Th r quir m nts can b xpr ss d as: VS wh r : VS VK K IS Rct Rl RB = stability voltag s tting = CT kn point voltag = r lay curr nt s tting = CT winding r sistanc = CT s condary l ad r sistanc = r sistanc of any oth r comp on nts in th r lay circuit = r lay stability factor = ISRstab and VS > KIf (Rct + 2Rl + RB ) Rstab = stabilising r sistor For this xampl : VK Rct Rl = 97V = 3.7Ω = 0.057Ω For the relay use , the various factors are relate by the graph of Figure 16.30 . 0.1 70 60 50 40
Overall operationtime - millisecon s 500 Differential current (A) 400 Operate 300 200 100 0 • 0.2 K Factor 0.3 30 20 Stable 10 0 Unstable K Factor Overall op time 0.4 0.5 0.6 0.7 0.8 0.9 1 10 Restrain 0 200 Effective bias (A) 400
600 800 ifferential current
Figure 16.29: Transformer unit protection characteristic 1 2 3 4 VK VS 6 7 8 9
16.9.2.3 Restricte earth fault protection The KBCH relay implements high-impe a nce Restricte Earth Fault (REF) protection. Operation is require for a Figure 16.30: REF operating characteristic for KBCH relay Network Protection & Automation Gui e • 277 •
Transformer an Transformer-Fee er P rotection 16 •
Starting with the esire operating time, the VK/VS ratio an K factorcan be fo un . An operating of 40ms (2 cycles at 50Hz) is usually acceptable, an hence, f rom Figure 16.30, VK/VS K =4 = 0.5 an substituting values, Unit VP = 544V. Thus a Metrosil is not require . 16.9.3 Protection for On-Loa Tap Changing Transformer The previous example eals with a transformer having no taps. most transformers have a range of ta In practice, ps tocater for ifferent loa ing con itions. While most transformers have an of f-loa tap-changer, transformers use for voltage control in a network are fitte with an on-loa tap-changer. The protection settings must then take the variat ion of tap-change position into account to avoi the possibility of spurious tri ps at extreme tap positions. For this example, the same transformer as in Sectio n 16.19.2 will be use , but with an on-loa tapping range of +5% to -15%. The ta p-changer is locate on theprimary win ing, while the tap-step usually oes not matter. The stagesinvolve in the calculation are as follows: a. etermine rat io correction at mi -tap an resulting secon ary currents b. etermine HV curren ts at tap extremities with correction c. etermine the ifferential curren ratio t at the tap extremities . etermine bias current at tap extremities e. check f or sufficient marginbetween ifferential an operating currents 16.19.3.1Ratio correction In accor ance with Section 16.8.4, the mi -tap position isuse to c alculate the ratio correction factors. The mi tap position is –5%, an at this ta p position: Primary voltage to give rate secon ary voltage: = 33 x 0.95 = 31.35 kV an Rate Primary Current = 184A Transformer HV full loa current on secon ar yof main CT’s is: 184/250 = 0.737 Ratio compensation Select nearest value LV seco n ary current Ratio compensation = 1/0.737 = 1.357 = 1.36 = 525/600 = 0.875 = 1/ 0.875 = 1.14 The maximum earth fault current is limite by the earthing resistor to 1000A (pr imary). The maximum phase fault current can be estimate by assuming the source impe ance to be zero, so it is limite only by transformer impe ance to 5250A, o r 10A secon ary after takingaccount of the ratio compensation. Hence the stabil ity voltage can be calculate as Transformer an Transformer-Fee er P rotection VS = 0.5 x 10( 3.7 + 2 x 0.057) = 19.07V Hence, Calculate VK = 4 x 19.07 = 76.2 8V However, Actual VK = 91V an VK/VS = 4.77 Thus from Figure16.30, with K = 0. 5, the protection is unstable. By a opting an iterative proce ure for values of VK/VS an K, a final acceptable result of VK/VS = 4.55, K = 0.6, is obtaine . Th is results in an operating time of 40ms. The require earth fault setting curren t Iop is 250A. The chosen E/F CT has an exciting current Ie of 1%, an hence usi ng the equation: Iop = CT ratio x (IS + nIe) where: n = no of CT’s in parallel (=4 ) = 0.377, use 0.38 nearest settable value. IS • 16 •
The stabilising resistance Rstab can be calculate as 60.21Ω. The relay can only w ithstan a maximum of 3kV peak un er fault con itions. A check is require to se e if this voltage isexcee e – if it is,a non-linear resistor, known as a Metros il, must be connecte across the relay an stabilising resistor. The peak voltag e is estimate using the formula: V P = 2 2 V K (V F − V K where: VF = If (Rct + 2 Rl + Rst ) nd If = f ult current in second ry of CT circuit )
Both of the ove v lues c n e set in the rel y. • 278 • Network Protection & Autom tion Guide
16.19.3.2 HV currents t t p extremities At the +5% t p, the HV full lo d curren t will e: 16.19.3.5 M rgin etween differenti l nd oper ting currents The oper ting curre nt of the rel y is given y the formul Iop = IS + 0.2I i s Hence, t the +5% t p, with IS = 0.2 Iopt1 = 0.2 + (0.2x 0.952) = 0.3904A At the –15% t p, Iop = IS + 0.2 +(I i s 1) x 0.8 (since the i s >1.0) Iopt2 = 0.2 + 0.2 +(1.059 1) x 0 .8 For s tisf ctory oper tion of the rel y, the oper ting current should e no g re ter th n 90% of the differenti l current t the t p extremities. For the +5% t p, the differenti l current is 24% of the oper ting current, nd t the –15% t p , the differenti l current is 27% of the oper ting current. Therefore, setting of IS is s tisf ctory. 10 33 × 1.05 × 3 =166.6A prim ry Hence, the second ry current with r tio correction: 166.6 × 1.36 = 250 = 0.906A At the 15% t p, the HV full lo d current on the prim ry of the CT’s: 10 = 33 × 0.85 × = 205.8 A 3 H nc , th s condary curr nt with ratio corr ction: = 205.8 × 1.36 250 = 1.12 A 16.19.3.3 D t rmin diff r ntial curr nt at tap xtr miti s Th full load curr n t s n by th r lay, aft r ratio corr ction is 0.875 x 1.14 = 0.998A. At th +5% tap, th diff r ntial curr nt Idifft2 = 0.998 - 0.906 = 0.092A At th –15% tap, I difft2 = 1.12 - 0.998 = 0.122A 16.19.3.4 D t rmin bias curr nts at tap xtr mit i s Th bias curr nt is giv n by th formula: I bias = wh r : ( I RHV + I RLV 2 ) • IRHV = r lay HV curr nt IRLV = r lay LV curr nt H nc , I biast1 = (0.998 + 0.906 ) 2 = 0.952A and I biast 2 = (0.998 + 1.12 ) 2
= 1.059A N twork Prot ction & Automation Guid • 279 • Transform r and Transform r-F d r P rot ction 16 • = 0.4472A
• 17 • G n rator and G n rator Transform r Prot ction Introduction G n rator arthing Stator winding faults Stator winding prot ction Diff r ntial prot ction of dir ct-conn ct d g n rators Diff r ntial prot ction o f g n rator –transform r units Ov rcurr nt prot ction Stator arth fault prot ctio n Ov rvoltag prot ction Low forward pow r/r v rs pow r prot ction 17.1 17.2 17 .3 17.4 17.5 17.6 17.7 17.8 17.9 Und rvoltag prot ction 17.10 17.11 Unbalanc d loading 17.12 Prot ction against inadv rt nt n rgisation 17.13 Und r /Ov rfr qu ncy/Ov rfluxing prot ction 17.14 Rotor faults 17.15 Loss of xcitatio n prot ction 17.16 Pol slipping prot ction 17.17 Ov rh ating 17.18 M chanical f aults 17.19 Compl t g n rator prot ction sch m s 17.20 Emb dd d g n ration 17.2 1 Exampl s of g n rator prot ction s ttings 17.22
• 17 • G n rator and G n rator-Transform r P rot ction 17.1 INTRODUCTION Th cor of an l ctric pow r syst m is th g n ration. With t h xc ption of m rging fu l c ll and solar-c ll t chnology for pow r syst ms, th conv rsion of th fundam ntal n rgy into its l ctrical quival nt normally r quir s a prim mov r to d v lop m chanical pow r as an int rm diat stag . Th natur of this machin d p nds upon th sourc of n rgy and in turn this ha s som b aring on th d sign of th g n rator. G n rators bas d on st am, gas, w at r or wind turbin s, and r ciprocating combustion ngin s ar all in us . El c trical ratings xt nd from a f w hundr d kVA (or v n l ss) for r ciprocating n gin and r n wabl n rgy s ts, up to st am turbin s ts xc ding 1200MVA. Smal l and m dium siz d s ts may b dir ctly conn ct d to a pow r distribution syst m . A larg r s t may b associat d with an individual transform r, through which i t is coupl d to th EHV primary transmission syst m. Switchg ar may or may not b provid d b tw n th g n rator and transform r. In som cas s, op rational and conomic advantag s can b attain d by providing a g n rator circuit br ak r in addition to a high voltag circuit br ak r, but sp cial d mands will b plac d on th g n rator circuit br ak r for int rruption of g n rator fault curr nt wav forms that do not hav an arly z ro crossing. A unit transform r may b tapp d off th int rconn ction b tw n g n rator and transform r for th supply of pow r to auxiliary plant, as shown in Figur 17.1. Th unit transform r could b of th ord r of 10% of th unit rating for a larg fossil-fu ll d st am s t with a dditional flu -gas d sulphurisation plant, but it may only b of th ord r of 1% of unit rating for a hydro s t. N twork Prot ction & Automation Guid • 281 •
G n rator Main transform r r quir d. Th amount of prot ction appli d will b gov rn d by conomic consid r ations, taking into account th valu of th machin , and th valu of its outpu t to th plant own r. Th following probl ms r quir consid ration from th poin t of vi w of applying prot ction: a. stator l ctrical faults b. ov rload c. ov rvoltag d. unbalanc d loading . ov rfluxing f. inadv rt nt n rgisation . rot or l ctrical faults f. loss of xcitation g. loss of synchronism h. failur of prim mov r j. lubrication oil failur l. ov rsp ding m. rotor distortion n. di ff r nc in xpansion b tw n rotating and stationary parts PCC HV busbars Unit transform r Auxiliary suppli s switchboard Figur 17.1: G n rator-transform r unit G n rator and G n rator-Transfor m r P rot ction Industrial or comm rcial plants with a r quir m nt for st am/hot wat r now oft n includ g n rating plant utilising or producing st am to improv ov rall conom ics, as a Combin d H at and Pow r (CHP) sch m . Th plant will typically hav a conn ction to th public Utility distribution syst m, and such g n ration is r f rr d to as ‘ mb dd d’ g n ration. Th g n rating plant may b capabl of xport of surplus pow r, or simply r duc th import of pow r from th Utility. This is sh own in Figur 17.2. Utility o. xc ssiv vibration p. cor lamination faults 17.2 GENERATOR EARTHING G n rator Rating: yMW Industrial plant main busbar Th n utral point of a g n rator is usually arth d to facilitat prot ction of th stator winding and associat d syst m. Earthing also pr v nts damaging transi nt ov rvoltag s in th v nt of an arcing arth fault or f rror sonanc . For HV g n rators, imp danc is usually ins rt d in th stator arthing conn ction to limit th magnitud of arth fault curr nt. Th r is a wid variation in th ar th fault curr nt chos n, common valu s b ing: 1. rat d curr nt 2. 200A-400A (low imp danc arthing) 3. 10A-20A (high imp danc arthing) Th main m thods of im p danc - arthing a g n rator ar shown in Figur 17.3. Low valu s of arth fault curr nt may limit th damag caus d from a fault, but th y simultan ously mak d t ction of a fault towards th stator winding star point mor difficult. Exc p t for sp cial applications, such as marin , LV g n rators ar normally solidly arth d to comply with saf ty r quir m nts. Wh r a st p-up transform r is appli d, • 17 • Plant f d rs - total d mand: xMW PCC: Point of Common Coupling Wh n plant g n r ator is running: If y>x, Plant may xport to Utility across PCC If x>y, Plant ma x d mand from Utility is r duc d Figur 17.2: Emb dd d g n ration A mod rn g n rating unit is a compl x syst m comprising th g n rator stator win
ding, associat d transform r and unit transform r (if pr s nt), th rotor with i ts fi ld winding and xcitation syst m, and th prim mov r with its associat d auxiliari s. Faults of many kinds can occur within this syst m for which div rs forms of l ctrical and m chanical prot ction ar • 282 • N twork Prot ction & Automation Guid
th g n rator and th low r voltag winding of th transform r can b tr at d as an isolat d syst m that is not influ nc d by th arthing r quir m nts of th p ow r syst m. suffici nt that th transform r b d sign d to hav a primary winding kn -point .m.f. qual to 1.3 tim s th g n rator rat d lin voltag . 17.3 STATOR WINDING FAULTS Failur of th stator windings or conn ction insulation can r sult in s v r damag to th windings and stator cor . Th xt nt of th damag will d p n d on th magnitud and duration of th fault curr nt. 17.3.1 Earth Faults (a) Dir ct arthing Typical s tting (% of arthing r sistor rating) I>> I> (b) R sistanc arthing 10 5 Loading r sistor U> (c) Distribution transform r arthing with ov rvoltag r lay. Loading r sistor I> (d) Distribution transform r arthing with ov rcurr nt r lay Figur 17.3: M thods of g n rator arthing An arthing transform r or a s ri s imp danc can b us d as th imp danc . If a n arthing transform r is us d, th continuous rating is usually in th rang 52 50kVA. Th s condary winding is load d with a r sistor of a valu which, wh n r f rr d through th transform r turns ratio, will pass th chos n short-tim art h-fault curr nt. This is typically in th rang of 5-20A. Th r sistor pr v nts th production of high transi nt ov rvoltag s in th v nt of an arcing arth fa ult, which it do s by discharging th bound charg in th circuit capacitanc . F or this r ason, th r sistiv compon nt of fault curr nt should not b l ss than th r sidual capacitanc curr nt. This is th basis of th d sign, and in pract ic valu s of b tw n 3-5 Ico ar us d. It is important that th arthing transf orm r n v r b com s saturat d; oth rwis a v ry und sirabl condition of f rror sonanc may occur. Th normal ris of th g n rat d voltag abov th rat d valu caus d by a sudd n loss of load or by fi ld forcing must b consid r d, as w l l as flux doubling in th transform r du to th point-on-wav of voltag applic ation. It is An arth fault involving th stator cor r sults in burning of th iron at th p oint of fault and w lds laminations tog th r. R plac m nt of th faulty conducto r may not b a v ry s rious matt r (d p nd nt on s t rating/voltag /construction ) but th damag to th cor cannot b ignor d, sinc th w lding of laminations may r sult in local ov rh ating. Th damag d ar a can som tim s b r pair d, bu t if s v r damag has occurr d, a partial cor r build will b n c ssary. A fla shov r is mor lik ly to occur in th nd-winding r gion, wh r l ctrical str s s s ar high st. Th r sultant forc s on th conductors would b v ry larg and th y may r sult in xt nsiv damag , r quiring th partial or total r winding of th g n rator. Apart from burning th cor , th gr at st dang r arising from fa ilur to quickly d al with a fault is fir . A larg portion of th insulating ma t rial is inflammabl , and in th cas of an air-cool d machin , th forc d v nt ilation can quickly caus an arc flam to spr ad around th winding. Fir will n ot occur in a hydrog n-cool d machin , provid d th stator syst m r mains s al d . In any cas , th l ngth of an outag may b consid rabl , r sulting in major f inancial impact from loss of g n ration r v nu and/or import of additional n r gy. 17.3.2 Phas -Phas Faults Phas -phas faults cl ar of arth ar l ss common; th y may occur on th nd portion of stator coils or in th slots if th windin g involv s two coil sid s in th sam slot. In th latt r cas , th fault will i nvolv arth in a v ry short tim . Phas fault curr nt is not limit d by th m t hod of arthing th n utral point. 17.3.3 Int rturn Faults Int rturn faults ar rar , but a significant fault-loop curr nt can aris wh r such a fault do s occ
ur. • N twork Prot ction & Automation Guid • 283 • G n rator and G n rator-Transfor m r P rot ction 17 • Th most probabl mod of insulation failur is phas to arth. Us of an arthi ng imp danc limits th arth fault curr nt and h nc stator damag .
Conv ntional g n rator prot ction syst ms would b blind to an int rturn fault, but th xtra cost and complication of providing d t ction of a pur ly int rturn fault is not usually justifi d. In this cas , an int rturn fault must d v lop i nto an arth fault b for it can b cl ar d. An xc ption may b wh r a machin has an abnormally complicat d or multipl winding arrang m nt, wh r th probab ility of an int rturn fault might b incr as d. 17.4 STATOR WINDING PROTECTION T o r spond quickly to a phas fault with damaging h avy curr nt, s nsitiv , highsp d diff r ntial prot ction is normally appli d to g n rators rat d in xc ss of 1MVA. For larg g n rating units, fast fault cl aranc will also maintain sta bility of th main pow r syst m. Th zon of diff r ntial prot ction can b xt nd d to includ an associat d st p-up transform r. For small r g n rators, IDMT/ instantan ous ov rcurr nt prot ction is usually th only phas fault prot ction appli d. S ctions 17.5-17.8 d tail th various m thods that ar availabl for st ator winding prot ction. 17.5 DIFFERENTIAL PROTECTION OF DIRECT CONNECTED GENERA TORS Th th ory of circulating curr nt diff r ntial prot ction is discuss d full y in S ction 10.4. Stator A B C calculation, aft r m asur m nt of th individual CT s condary curr nts. In such r lay d signs, th r is full galvanic s paration of th n utral-tail and t rmina l CT s condary circuits, as indicat d in Figur 17.5(a). This is not th cas fo r th application of high-imp danc diff r ntial prot ction. This diff r nc can impos som sp cial r lay d sign r quir m nts to achi v stability for bias d d iff r ntial prot ction in som applications. 17.5.1 Bias d Diff r ntial Prot cti on Th r lay conn ctions for this form of prot ction ar shown in Figur 17.5(a) and a typical bias charact ristic is shown in Figur 17.5(b). Th diff r ntial curr nt thr shold s tting Is1 can b s t as low as 5% of rat d g n rator curr nt , to provid prot ction for as much of th winding as possibl . Th bias slop b r ak-point thr shold s tting Is2 would typically b s t to a valu abov g n rat or rat d curr nt, say 120%, to achi v xt rnal fault stability in th v nt of transi nt asymm tric CT saturation. Bias slop K2 s tting would typically b s t at 150%. I1 I2 G n rator and G n rator-Transfor m r P rot ction (a): R lay conn ctions for bias d diff r ntial prot ction Idiff = I1+I2 I Op rat K2 • 17 • IS1 Id> Id> Id> K1 IS2 R strain IBIAS = I1+ 2
(b) Bias d diff r ntial op rating charact ristic Figur 17.4: Stator diff r ntial prot ction Figur 17.5: Typical g n rator bias d diff r ntial prot ction High-sp d phas fault prot ction is provid d, by us of th conn ctions shown i n Figur 17.4. This d picts th d rivation of diff r ntial curr nt through CT s condary circuit conn ctions. This prot ction may also off r arth fault prot cti on for som mod rat imp danc - arth d applications. Eith r bias d diff r ntial or high imp danc diff r ntial t chniqu s can b appli d. A subtl diff r nc wi th mod rn, bias d, num rical g n rator prot ction r lays is that th y usually d riv th diff r ntial curr nts and biasing curr nts by algorithmic 17.5.2 High Imp danc Diff r ntial Prot ction This diff rs from bias d diff r nt ial prot ction by th mann r in which r lay stability is achi v d for xt rnal f aults and by th fact that th diff r ntial curr nt must b attain d through th l ctrical conn ctions of CT s condary circuits. If th imp danc of ach r lay in Figur 17.4 is high, th v nt of on CT b coming saturat d by th through f ault curr nt (l ading to a • 284 • N twork Prot ction & Automation Guid
r lativ ly low CT imp danc ), will allow th curr nt from th unsaturat d CT to flow mainly through th saturat d CT rath r than through th r lay. This provid s th r quir d prot ction stability wh r a tun d r lay l m nt is mploy d. In practic , xt rnal r sistanc is add d to th r lay circuit to provid th n c s sary high imp danc . Th principl of high-imp danc prot ction application is i llustrat d in Figur 17.6, tog th r with a summary of th calculations r quir d to d t rmin th valu of xt rnal stabilising r sistanc . To calculat th primary op rating curr nt, th following xpr ssion is us d: Io p = N x (Is1 + nI ) wh r : Iop = primary op rating curr nt N = CT ratio Is1 = r lay s tting n = numb r of CT’s in parall l with r lay l m nt I = CT magn tising curr nt at Vs H althy CT Zm RCT1 RL1 Prot ct d zon Saturat d CT RCT2 If Rst Vr RL4 RL3 RL2 Id> It can b s n from th abov that th calculations for th application of high imp danc diff r ntial prot ction ar mor compl x than for bias d diff r ntial prot ction. How v r, th prot ction sch m is actually quit simpl and it off r s a high l v l of stability for through faults and xt rnal switching v nts. Wi th th adv nt of multi-function num rical r lays and with a d sir to disp ns w ith xt rnal compon nts, high imp danc diff r ntial prot ction is not as popula r as bias d diff r ntial prot ction in mod rn r laying practic . 17.5.3 CT R qui r m nts Th CT r quir m nts for diff r ntial prot ction will vary according to t h r lay us d. Mod rn num rical r lays may not r quir CT’s sp cifically d sign d for diff r ntial prot ction to IEC 60044-1 class PX (or BS 3938 class X). How v r, r quir m nts in r sp ct of CT kn -point voltag will still hav to b ch ck d for th sp cific r lays us d. High imp danc diff r ntial prot ction may b mo r on rous in this r sp ct than bias d diff r ntial prot ction. Many factors aff ct this, including th oth r prot ction functions f d by th CT’s and th kn -po int r quir m nts of th particular r lay conc rn d. R lay manufactur rs ar abl to provid d tail d guidanc on this matt r. Voltag across r lay circuit Vr = If (RCT + 2RL) and Vs = KVr wh r 1.0
17.6 DIFFERENTIAL PROTECTION OF GENERATOR-TRANSFORMERS A common conn ction arran g m nt for larg g n rators is to op rat th g n rator and associat d st p-up t ransform r as a unit without any int rv ning circuit br ak r. Th unit transform r supplying th g n rator auxiliari s is tapp d off th conn ction b tw n Diff r ntial g n rator and st p-up transform r. prot ction can b arrang d as follow s. Rst NLR = Non-lin ar r sistanc (M trosil) Figur 17.7: R lay conn ctions for high imp danc diff r ntial prot ction N twork Prot ction & Automation Guid • 285 • G n rator and G n rator-Transfor m r P rot ction 17 • Is1 is typically s t to 5% of g n rator rat d s condary curr nt.
17.6.1 G n rator/St p-up Transform r Diff r ntial Prot ction Th g n rator stato r and st p-up transform r can b prot ct d by a singl zon of ov rall diff r nt ial prot ction (Figur 17.8). This will b in addition to diff r ntial prot ctio n appli d to th g n rator only. Th curr nt transform rs should b locat d in t h g n rator n utral conn ctions and in th transform r HV conn ctions. Alt rnat iv ly, CT’s within th HV switchyard may b mploy d if th distanc is not t chni cally prohibitiv . Ev n wh r th r is a g n rator circuit br ak r, ov rall diff r ntial prot ction can still b provid d if d sir d. G n rator and G n rator-Transfor m r P rot ction G n rator Main transform r transform r rating is xtr m ly low in r lation to th g n rator rating, .g. fo r som hydro applications. Th location of th third s t of curr nt transform rs is normally on th primary sid of th unit transform r. If locat d on s condar y sid of th unit transform r, th y would hav to b of an xc ptionally high r atio, or xc ptionally high ratio int rposing CT’s would hav to b us d. Thus, th us of s condary sid CT’s is not to b r comm nd d. On advantag is that unit transform r faults would b within th zon of prot ction of th g n rator. How v r, th s nsitivity of th g n rator prot ction to unit transform r phas fault s would b consid r d inad quat , du to th r lativ ly low rating of th transf orm r in r lation to that of th g n rator. Thus, th unit transform r should ha v its own diff r ntial prot ction sch m . Prot ction for th unit transform r i s cov r d in Chapt r 16, including m thods for stabilising th prot ction agains t magn tising inrush conditions. 17.7 OVERCURRENT PROTECTION Ov rcurr nt prot ct ion of g n rators may tak two forms. Plain ov rcurr nt prot ction may b us d a s th principl form of prot ction for small g n rators, and back-up prot ction for larg r on s wh r diff r ntial prot ction is us d as th primary m thod of g n rator stator winding prot ction. Voltag d p nd nt ov rcurr nt prot ction may b appli d wh r diff r ntial prot ction is not justifi d on larg r g n rators, or wh r probl ms ar m t in applying plain ov rcurr nt prot ction. 17.7.1 Plai n Ov rcurr nt Prot ction It is usual to apply tim -d lay d plain ov rcurr nt pro t ction to g n rators. For g n rators rat d l ss than 1MVA, this will form th p rincipal stator winding prot ction for phas faults. For larg r g n rators, ov r curr nt prot ction can b appli d as r mot back-up prot ction, to disconn ct th unit from any uncl ar d xt rnal fault. Wh r th r is only on s t of diff r ntial main prot ction, for a small r g n rator, th ov rcurr nt prot ction will also provid local back-up prot ction for th prot ct d plant, in th v nt that th main prot ction fails to op rat . Th g n ral principl s of s tting ov rcur r nt r lays ar giv n in Chapt r 9. In th cas of a singl g n rator f ding an isolat d syst m, curr nt transform rs at th n utral nd of th machin should n rgis th ov rcurr nt prot ction, to allow a r spons to winding fault condit ions. R lay charact ristics should b s l ct d to tak into account th fault cu rr nt d cr m nt b haviour of th g n rator, with allowanc for th p rformanc o f th xcitation Prot ct d zon Id> HV busbars Figur 17.8: Ov rall g n rator-transform r diff r ntial prot ction • 17 • Th curr nt transform rs should b rat d according to S ction 16.8.2. Sinc a po w r transform r is includ d within th zon of prot ction, bias d transform r di ff r ntial prot ction, with magn tising inrush r straint should b appli d, as d
iscuss d in S ction 16.8.5. Transi nt ov rfluxing of th g n rator transform r m ay aris du to ov rvoltag following g n rator load r j ction. In som applicat ions, this may thr at n th stability of th diff r ntial prot ction. In such ca s s, consid ration should b giv n to applying prot ction with transi nt ov rflu xing r straint/blocking ( .g. bas d on a 5th harmonic diff r ntial curr nt thr s hold). Prot ction against sustain d ov rfluxing is cov r d in S ction 17.14. 17. 6.2 Unit Transform r Diff r ntial Prot ction Th curr nt tak n by th unit trans form r must b allow d for by arranging th g n rator diff r ntial prot ction as a thr - nd d sch m . Unit transform r curr nt transform rs ar usually appli d to balanc th g n rator diff r ntial prot ction and pr v nt th unit transform r through curr nt b ing s n as diff r ntial curr nt. An xc ption might b wh r th unit • 286 • N twork Prot ction & Automation Guid
In th mor usual cas of a g n rator that op rat s in parall l with oth rs and which forms part of an xt nsiv int rconn ct d syst m, back-up phas fault prot ction for a g n rator and its transform r will b provid d by HV ov rcurr nt pr ot ction. This will r spond to th high rl v l backf d from th pow r syst m to a unit fault. Oth r g n rators in parall l would supply this curr nt and, b ing stabilis d by th syst m imp danc , it will not suff r a major d cr m nt. This prot ction is usually a r quir m nt of th pow r syst m op rator. S ttings must b chos n to pr v nt op ration for xt rnal faults f d by th g n rator. It is c ommon for th HV ov rcurr nt prot ction r lay to provid both tim -d lay d and i nstantan ous high-s t l m nts. Th tim -d lay d l m nts should b s t to nsur that th prot ct d it ms of plant cannot pass l v ls of through fault curr nt in xc ss of th ir short-tim withstand limits. Th instantan ous l m nts shoul d b s t abov th maximum possibl fault curr nt that th g n rator can supply, but l ss than th syst m-suppli d fault curr nt in th v nt of a g n rator win ding fault. This back-up prot ction will minimis plant damag in th v nt of m ain prot ction failur for a g n rating plant fault and instantan ous tripping f or an HV-sid fault will aid th r cov ry of th pow r syst m and parall l g n r ation. 17.7.2 Voltag -D p nd nt Ov rcurr nt Prot ction Th plain ov rcurr nt pro t ction s tting difficulty r f rr d to in th pr vious s ction aris s b caus al lowanc has to b mad both for th d cr m nt of th g n rator fault curr nt wit h tim and for th passag of full load curr nt. To ov rcom th difficulty of d iscrimination, th g n rator t rminal voltag can b m asur d and us d to dynami cally modify th basic r lay curr nt/tim ov rcurr nt charact ristic for faults clos to th g n rating plant. Th r ar two basic alt rnativ s for th applicat ion of voltag -d p nd nt ov rcurr nt prot ction, which ar discuss d in th foll owing s ctions. 2. und r clos -up fault conditions, th busbar voltag must fall b low th volta g thr shold so that th s cond prot ction charact ristic will b s l ct d. This charact ristic should b s t to allow r lay op ration with fault curr nt d cr m nt for a clos -up fault at th g n rator t rminals or at th HV busbars. Th pr ot ction should also tim -grad with xt rnal circuit prot ction. Th r may b a dditional inf ds to an xt rnal circuit fault that will assist with grading Typ ical charact ristics ar shown in Figur 17.9. Curr nt pick-up l v l I> KI> • Vs Voltag l v l Figur 17.9: Voltag controll d r lay charact ristics 17.7.2.2 Voltag r strain d ov rcurr nt prot ction Th alt rnativ t chniqu is to continuously vary th r lay l m nt pickup s tting with g n rator voltag var iation b tw n upp r and low r limits. Th voltag is said to r strain th op ra tion of th curr nt l m nt. Th ff ct is to provid a dynamic I.D.M.T. prot ct ion charact ristic, according to th voltag at th machin N twork Prot ction & Automation Guid • 287 • G n rator and G n rator-Transfor m r P rot ction 17 •
syst m and its fi ld-forcing capability. Without th provision of fault curr nt compounding from g n rator CT’s, an xcitation syst m that is pow r d from an xci tation transform r at th g n rator t rminals will xhibit a pronounc d fault cu rr nt d cr m nt for a t rminal fault. With failur to consid r this ff ct, th pot ntial xists for th initial high fault curr nt to d cay to a valu b low th ov rcurr nt prot ction pick-up s tting b for a r lay l m nt can op rat , unl ss a low curr nt s tting and/or tim s tting is appli d. Th prot ction would t h n fail to trip th g n rator. Th s ttings chos n must b th b st compromis b tw n assur d op ration in th for going circumstanc s and discrimination with th syst m prot ction and passag of normal load curr nt, but this can b impos sibl with plain ov rcurr nt prot ction. Th choic d p nds upon th pow r syst m charact ristics and l v l of prot ction to b provid d. Voltag -d p nd nt ov rcurr nt r lays ar oft n found appli d to g n rators us d on industrial syst ms as an alt rnativ to full diff r ntial pr ot ction. 17.7.2.1 Voltag controll d ov rcurr nt prot ction Voltag controll d ov rcurr nt prot ction has two tim /curr nt charact ristics which ar s l ct d a ccording to th status of a g n rator t rminal voltag m asuring l m nt. Th vo ltag thr shold s tting for th switching l m nt is chos n according to th fol lowing crit ria. 1. during ov rloads, wh n th syst m voltag is sustain d n ar normal, th ov rcurr nt prot ction should hav a curr nt s tting abov full load curr nt and an op rating tim charact ristic that will pr v nt th g n rating p lant from passing curr nt to a r mot xt rnal fault for a p riod in xc ss of t h plant shorttim withstand limits
t rminals. Alt rnativ ly, th r lay l m nt may b r gard d as an imp danc typ with a long d p nd nt tim d lay. In cons qu nc , for a giv n fault condition, th r lay continu s to op rat mor or l ss ind p nd ntly of curr nt d cr m nt i n th machin . A typical charact ristic is shown in Figur 17.10. consid rations. 17.8.1.2 S nsitiv arth fault prot ction This m thod is us d in th following situations: a. dir ct-conn ct d g n rators op rating in parall l b. g n rators with high-imp danc n utral arthing, th arth fault curr nt b in g limit d to a f w t ns of amps c. installations wh r th r sistanc of th gro und fault path is v ry high, du to th natur of th ground In th s cas s, con v ntional arth fault prot ction as d scrib d in S ction 17.8.1.1 is of littl u s . Curr nt pick-up l v l I> G n rator and G n rator-Transfor m r P rot ction KI> VS2 VS1 Voltag l v l Figur 17.10: Voltag r strain d r lay charact ristics 17.8 STATOR EARTH FAULT PROTECTION Earth fault prot ction must b appli d wh r imp danc arthing is mploy d that limits th arth fault curr nt to l ss than th pick-up thr shold of th ov rcurr nt and/or diff r ntial prot ction for a fa ult locat d down to th bottom 5% of th stator winding from th starpoint. Th typ of prot ction r quir d will d p nd on th m thod of arthing and conn ction of th g n rator to th pow r syst m. 17.8.1 Dir ct-Conn ct d G n rators A sing l dir ct-conn ct d g n rator op rating on an isolat d syst m will normally b d ir ctly arth d. How v r, if s v ral dir ct-conn ct d g n rators ar op rat d in parall l, only on g n rator is normally arth d at a tim . For th un arth d g n rators, a simpl m asur m nt of th n utral curr nt is not possibl , and oth r m thods of prot ction must b us d. Th following s ctions d scrib th m thod s availabl . 17.8.1.1 N utral ov rcurr nt prot ction With this form of prot ctio n, a curr nt transform r in th n utral- arth conn ction n rgis s an ov rcurr n t r lay l m nt. This provid s unr strict d arth-fault prot ction and so it mus t b grad d with f d r prot ction. Th r lay l m nt will th r for hav a tim d lay d op rating charact ristic. Grading must b carri d out in accordanc with th principl s d tail d in Chapt r 9. Th s tting should not b mor than 33% o f th maximum arth fault curr nt of th g n rator, and a low r s tting would b pr f rabl , d p nding on grading Th principl s of s nsitiv arth fault prot ction ar d scrib d in S ctions 9.1 7.1, 9.18 and 9.19. Th arth fault (r sidual) curr nt can b obtain d from r si dual conn ction of lin CT’s, a lin -conn ct d CBCT, or a CT in th g n rator n ut ral. Th latt r is not possibl if dir ctional prot ction is us d. Th polarisin g voltag is usually th n utral voltag displac m nt input to th r lay, or th r sidual of th thr phas voltag s, so a suitabl VT must b us d. For P t rs n Coil arthing, a wattm tric t chniqu (S ction 9.19) can also b us d. For di r ct conn ct d g n rators op rating in parall l, dir ctional s nsitiv arth fau lt prot ction may b n c ssary. This is to nsur that a fault d g n rator will b tripp d b for th r is any possibility of th n utral ov rcurr nt prot ction tripping a parall l h althy g n rator. Wh n b ing driv n by r sidually-conn ct
d phas CT’s, th prot ction must b stabilis d against incorr ct tripping with tr ansi nt spill curr nt in th v nt of asymm tric CT saturation wh n phas fault or magn tising inrush curr nt is b ing pass d. Stabilising t chniqu s includ th addition of r lay circuit imp danc and/or th application of a tim d lay. Wh r th r quir d s tting of th prot ction is v ry low in comparison to th rat d curr nt of th phas CT’s, it would b n c ssary to mploy a singl CBCT for th arth fault prot ction to nsur transi nt stability. Sinc any g n rator in th parall l d group may b arth d, all g n rators will r quir to b fitt d with both n utral ov rcurr nt prot ction and s nsitiv dir ctional arth fault prot ction. Th s tting of th s nsitiv dir ctional arth fault prot ction is chos n to co-ordinat with g n rator diff r ntial prot ction and/or n utral voltag di splac m nt prot ction to nsur that 95% of th stator winding is prot ct d. Fig ur 17.11 illustrat s th compl t sch m , including optional blocking signals w h r difficulti s in co-ordinating th g n rator and downstr am f d r arth-fau lt prot ction occur. • 17 • • 288 • N twork Prot ction & Automation Guid
F d r I > I >> * Optional int rlock d arth-fault prot ction if grading probl ms xist sum r sidually. As th prot ction is still unr strict d, th voltag s tting of th r lay must b gr at r than th ff ctiv s tting of any downstr am arth-fau lt prot ction. It must also b tim -d lay d to co-ordinat with such prot ction. Som tim s, a s cond high-s t l m nt with short tim d lay is us d to provid f ast-acting prot ction against major winding arth-faults. Figur 17.12 illustrat s th possibl conn ctions that may b us d. I > Ursd I > Block* I > Ursd > I > Block* Op n R R Minimum arth fault l v l = IF V R 2 1 Va Vb c Figur 17.11: Compr h nsiv arth-fault prot ction sch m for dir ct-conn ct d g n rators op rating in parall l 3 For cas s (b) and (c) abov , it is not n c ssary to us a dir ctional facility. Car must b tak n to us th corr ct RCA s tting – for instanc if th arthing i mp danc is mainly r sistiv , this should b 0°. On insulat d or v ry high imp dan c arth d syst ms, an RCA of -90° would b us d, as th arth fault curr nt is pr dominat ly capacitiv . Dir ctional s nsitiv arth-fault prot ction can also b us d for d t cting winding arth faults. In this cas , th r lay l m nt is app li d to th t rminals of th g n rator and is s t to r spond to faults only with in th machin windings. H nc arth faults on th xt rnal syst m do not r sult in r lay op ration. How v r, curr nt flowing from th syst m into a winding ar th fault caus s r lay op ration. It will not op rat on th arth d machin , so that oth r typ s of arth fault prot ction must also b appli d. All g n rators must b so fitt d, sinc any can b op rat d as th arth d machin . 17.8.1.3 N utral voltag displac m nt prot ction In a balanc d n twork, th addition of th thr phas arth voltag s produc s a nominally z ro r sidual voltag , sinc th r would b littl z ro s qu nc voltag pr s nt. Any arth fault will s t up a z ro s qu nc syst m voltag , which will giv ris to a non-z ro r sidual voltag . This can b m asur d by a suitabl r lay l m nt. Th voltag signal must b d riv d from a VT that is suitabl – i. . it must b capabl of transforming z ros qu nc voltag , so 3-limb typ s and thos without a primary arth conn ction a r not suitabl . This unbalanc voltag provid s a m ans of d t cting arth faul
ts. Th r lay l m nt must b ins nsitiv to third harmonic voltag s that may b pr s nt in th syst m voltag wav forms, as th s will Vn 1 D riv d from phas n utral voltag s 2 M asur d from arth imp danc 3 M asu r d from brok n d lta VT Figur 17.12: N utral voltag displac m nt prot ction 17.8.2 Indir ctly-Conn ct d G n rators As not d in S ction 17.2, a dir ctly- arth d g n ratortransform r unit cannot in t rchang z ro-s qu nc curr nt with th r maind r of th n twork, and h nc an arth fault prot ction grading probl m do s not xist. Th following s ctions d tail th prot ction m thods for th various forms of imp danc arthing of g n r ators. 17.8.2.1 High r sistanc arthing – n utral ov rcurr nt prot ction A curr n t transform r mount d on th n utral- arth conductor can driv an instantan ous and/or tim d lay d ov rcurr nt r lay l m nt, as shown in Figur 17.13. It is i mpossibl to provid prot ction for th whol of th winding, and Figur 17.13 a lso d tails how th p rc ntag of winding cov r d can b calculat d. For a r lay l m nt with an instantan ous s tting, prot ction is typically limit d to 90% o f th winding. This is to nsur that th prot ction will not malop rat with z ro s qu nc curr nt during op ration of a primary fus for a VT arth fault or w ith any transi nt surg curr nts that could flow through th int rwinding capaci tanc of th st p-up transform r for an HV syst m arth fault. A tim -d lay d r lay is mor s cur in this r sp ct, and it may hav a s tting to cov r 95% of th stator winding. Sinc th g n rating units und r consid ration ar usually lar g , instantan ous and tim d lay d r lay l m nts ar • N twork Prot ction & Automation Guid • 289 • G n rator and G n rator-Transfor m r P rot ction 17 •
oft n appli d, with s ttings of 10% and 5% of maximum arth fault curr nt r sp c tiv ly; this is th optimum compromis in p rformanc . Th portion of th windin g l ft unprot ct d for an arth fault is at th n utral nd. Sinc th voltag t o arth at this nd of th winding is low, th probability of an arth fault occ urring is also low. H nc additional prot ction is oft n not appli d. Loading r sistor V I> G n rator and G n rator-Transfor m r P rot ction (a) Prot ction using a curr nt l m nt a If Is R If = aV R Loading r sistor U> IsR V %cov r d 1-a amin = in ) x 1100% (b) Prot ction using a voltag l m nt g n rator stator winding using a curr nt l m nt Figur 17.13: Earth fault prot ction of high-r sistanc arth d g n rator stator winding using a curr nt l m nt Figur 17.14: G n rator winding arth-fault prot ction - distribution transform r arthing 17.8.2.2 Distribution transform r arthing using a curr nt l m nt In this arran g m nt, shown in Figur 17.14(a), th g n rator is arth d via th primary windi ng of a distribution transform r. Th s condary winding is fitt d with a loading r sistor to limit th arth fault curr nt. An ov rcurr nt r lay l m nt n rgis d from a curr nt transform r conn ct d in th r sistor circuit is us d to m asu r s condary arth fault curr nt. Th r lay should hav an ff ctiv s tting qu ival nt to 5% of th maximum arth fault curr nt at rat d g n rator voltag , in ord r to prot ct 95% of th stator winding. Th r lay l m nt r spons to third harmonic curr nt should b limit d to pr v nt incorr ct op ration wh n a s nsiti v s tting is appli d. As discuss d in S ction 17.8.2.1 for n utral ov rcurr nt prot ction, th prot ction should b tim d lay d wh n a s nsitiv s tting is ap pli d, in ord r to pr v nt malop ration und r transi nt conditions. It also must grad with g n rator VT primary prot ction (for a VT primary arth fault). An o p ration tim in th rang 0.5s-3s is usual. L ss s nsitiv instantan ous prot c tion can also b appli d to provid fast tripping for a h avi r arth fault cond ition. 17.8.2.3 Distribution transform r arthing using a voltag l m nt Earth fault p
rot ction can also b provid d using a voltag m asuring l m nt in th s condary circuit inst ad. Th s tting consid rations would b similar to thos for th c urr nt op rat d prot ction, but transpos d to voltag . Th circuit diagram is sh own in Figur 17.14(b). Application of both voltag and curr nt op rat d l m nt s to a g n rator with distribution transform r arthing provid s som advantag s . Th curr nt op rat d function will continu to op rat in th v nt of a short -circuit d loading r sistor and th voltag prot ction still functions in th v nt of an op ncircuit d r sistor. How v r, n ith r sch m will op rat in th v nt of a flashov r on th primary t rminals of th transform r or of th n utral cabl b tw n th g n rator and th transform r during an arth fault. A CT cou ld b add d in th n utral conn ction clos to th g n rator, to n rgis a high -s t ov rcurr nt l m nt to d t ct such a fault, but th fault curr nt would pro bably b high nough to op rat th phas diff r ntial prot ction. 17.8.2.4 N ut ral voltag displac m nt prot ction This can b appli d in th sam mann r as fo r dir ctconn ct d g n rators (S ction 17.8.1.3). Th only N twork Prot ction & Automation Guid • 17 • • 290 •
diff r nc is that th ar no grading probl ms as th prot ction is inh r ntly r strict d. A s nsitiv s tting can th r for b us d, nabling cov r of up to 95 % of th stator winding to b achi v d. 17.8.3 R strict d Earth Fault Prot ction This t chniqu can b us d on small g n rators not fitt d with diff r ntial pro t ction to provid fast acting arth fault prot ction within a d fin d zon that ncompass s th g n rator. It is ch ap r than full diff r ntial prot ction but only provid s prot ction against arth faults. Th principl is that us d for tr ansform r REF prot ction, as d tail d in S ction 16.7. How v r, in contrast to t ransform r REF prot ction, both bias d lowimp danc and high-imp danc t chniqu s can b us d. 17.8.3.1 Low-imp danc bias d REF prot ction This is shown in Fig ur 17.15. Th main advantag is that th n utral CT can also b us d in a mod r n r lay to provid conv ntional arth-fault prot ction and no xt rnal r sistors ar us d. R lay bias is r quir d, as d scrib d in S ction 10.4.2, but th formu la for calculating th bias is slightly diff r nt and also shown in Figur 17.15 . Phas CT ratio 1000/1 Phas A Phas B Phas C prot ction of a g n rator, using thr r sidually conn ct d phas CT’s balanc d ag ainst a similar singl CT in th n utral conn ction. S ttings of th ord r of 5% of maximum arth fault curr nt at th g n rator t rminals ar typical. Th usua l r quir m nts in r sp ct of stabilising r sistor and non-lin ar r sistor to gua rd against xc ssiv voltag across th r lay must b tak n, wh r n c ssary. 17 .8.4 Earth Fault Prot ction for th Entir Stator Winding All of th m thods for arth fault prot ction d tail d so far l av part of th winding unprot ct d. I n most cas s, this is of no cons qu nc as th probability of a fault occurring in th 5% of th winding n ar st th n utral conn ction is v ry low, du to th r duc d phas to arth voltag . How v r, a fault can occur anywh r along th st ator windings in th v nt of insulation failur du to localis d h ating from a cor fault. In cas s wh r prot ction for th ntir winding is r quir d, p rha ps for alarm only, th r ar various m thods availabl . 17.8.4.1 M asur m nt of third harmonic voltag On m thod is to m asur th int rnally g n rat d third h armonic voltag that app ars across th arthing imp danc du to th flow of th ird harmonic curr nts through th shunt capacitanc of th stator windings tc. Wh n a fault occurs in th part of th stator winding n ar st th n utral nd, t h third harmonic voltag drops to n ar z ro, and h nc a r lay l m nt that r s ponds to third harmonic voltag can b us d to d t ct th condition. As th faul t location mov s progr ssiv ly away from th n utral nd, th drop in third harm onic voltag from h althy conditions b com s l ss, so that at around 20-30% of t h winding distanc , it no long r b com s possibl to discriminat b tw n a h a lthy and a faulty winding. H nc , a conv ntional arth-fault sch m should b us d in conjunction with a third harmonic sch m , to provid ov rlapping cov r of th ntir stator winding. Th m asur m nt of third harmonic voltag can b tak n ith r from a star-point VT or th g n rator lin VT. In th latt r cas , th VT must b capabl of carrying r sidual flux, and this pr v nts th us of 3-lim b typ s. If th third harmonic voltag is m asur d at th g n rator star point, an und rvoltag charact ristic is us d. An ov rvoltag charact ristic is us d if th m asur m nt is tak n from th g n rator lin VT. For ff ctiv application of this form of prot ction, th r should b at l ast 1% third harmonic voltag a cross th g n rator n utral arthing imp danc und r all op rating conditions. A probl m ncount r d is that th l v l of third harmonic voltag g n rat d is r lat d to th output of th g n rator. Th voltag is low wh n g n rator output N utral CT ratio 200/1 IBIAS = (high st of IA B,
I 2 Nx scaling factor) 200 = = 0.2 1000 I ) N wh r scaling factor = IDIFF = IA IB IC (scaling factor • Figur 17.15: Low imp danc bias d REF prot ction of a g n rator Th initial bias slop is commonly s t to 0% to provid maximum s nsitivity, and appli d up to th rat d curr nt of th g n rator. It may b incr as d to count r th ff cts of CT mismatch. Th bias slop abov g n rator rat d curr nt is ty pically s t to 150% of rat d valu . Th initial curr nt s tting is typically 5% of th minimum arth fault curr nt for a fault at th machin t rminals. 17.8.3. 2 High Imp danc REF prot ction Th principl of high imp danc diff r ntial pro t ction is giv n in Chapt r 10 and also d scrib d furth r in S ction 17.5.2. Th sam t chniqu can b us d for arth-fault N twork Prot ction & Automation Guid • 291 • G n rator and G n rator-Transfor m r P rot ction 17 •
is low. In ord r to avoid malop ration wh n op rating at low pow r output, th r lay l m nt can b inhibit d using an ov rcurr nt or pow r l m nt (kW, kvar or kVA) and int rnal programmabl logic. 17.8.4.2 Us of low-fr qu ncy voltag inj ction Anoth r m thod for prot cting th ntir stator winding of a g n rator is to d ploy signal inj ction quipm nt to inj ct a low fr qu ncy voltag b tw n th stator star point and arth. An arth fault at any winding location will r s ult in th flow of a m asurabl inj ction curr nt to caus prot ction op ration. This form of prot ction can provid arth fault prot ction wh n th g n rator i s at standstill, prior to run-up. It is also an appropriat m thod to apply to v ariabl sp d synchronous machin s. Such machin s may b mploy d for variabl s p d motoring in pump d-storag g n ration sch m s or for starting a larg gas t urbin prim mov r. 17.9 OVERVOLTAGE PROTECTION Ov rvoltag s on a g n rator may occur du to transi nt surg s on th n twork, or prolong d pow r fr qu ncy ov rv oltag s may aris from a vari ty of conditions. Surg arr stors may b r quir d to prot ct against transi nt ov rvoltag s, but r lay prot ction may b us d to p rot ct against pow r fr qu ncy ov rvoltag s. A sustain d ov rvoltag condition s hould not occur for a machin with a h althy voltag r gulator, but it may b ca us d by th following conting nci s: a. d f ctiv op ration of th automatic vol tag r gulator wh n th machin is in isolat d op ration b. op ration und r manu al control with th voltag r gulator out of s rvic . A sudd n variation of th load, in particular th r activ pow r compon nt, will giv ris to a substantia l chang in voltag b caus of th larg voltag r gulation inh r nt in a typica l alt rnator c. sudd n loss of load (du to tripping of outgoing f d rs, l avin g th s t isolat d or f ding a v ry small load) may caus a sudd n ris in t rm inal voltag du to th trapp d fi ld flux and/or ov rsp d Sudd n loss of load should only caus a transi nt ov rvoltag whil th voltag r gulator and gov rn or act to corr ct th situation. A maladjust d voltag r gulator may trip to man ual, maintaining xcitation at th valu prior to load loss whil th g n rator suppli s littl or no load. Th t rminal voltag will incr as substantially, an d in s v r cas s it would b limit d only by th saturation charact ristic of t h g n rator. A ris in sp d simply compounds th probl m. If load that is s ns itiv to ov rvoltag s r mains conn ct d, th cons qu nc s in t rms of quipm nt damag and lost r v nu can b s v r . Prolong d ov rvoltag s may also occur on isolat d n tworks, or on s with w ak int rconn ctions, du to th fault conditio ns list d arli r. For th s r asons, it is prud nt to provid pow r fr qu ncy o v rvoltag prot ction, in th form of a tim d lay d l m nt, ith r IDMT or d fi nit tim . Th tim d lay should b long nough to pr v nt op ration during norm al r gulator action, and th r for should tak account of th typ of AVR fitt d and its transi nt r spons . Som tim s a high-s t l m nt is provid d as w ll, w ith a v ry short d finit -tim d lay or instantan ous s tting to provid a rapid trip in xtr m circumstanc s. Th us fuln ss of this is qu stionabl for g n r ators fitt d with an xcitation syst m oth r than a static typ , b caus th xc itation will d cay in accordanc with th op n-circuit tim constant of th fi l d winding. This d cay can last s v ral s conds. Th r lay l m nt is arrang d to trip both th main circuit br ak r (if not alr ady op n) and th xcitation; tr ipping th main circuit br ak r alon is not suffici nt. 17.10 UNDERVOLTAGE PROT ECTION Und rvoltag prot ction is rar ly fitt d to g n rators. It is som tim s u s d as an int rlock l m nt for anoth r prot ction function or sch m , such as f i ld failur prot ction or inadv rt nt n rgisation prot ction, wh r th abnorm ality to b d t ct d l ads dir ctly or indir ctly to an und rvoltag condition. A transmission syst m und rvoltag condition may aris wh n th r is insuffici n t r activ pow r g n ration to maintain th syst m voltag profil and th condi tion must b addr ss d to avoid th possibl ph nom non of syst m voltag collap s . How v r, it should b addr ss d by th d ploym nt of ’syst m prot ction’ sch m s . Th g n ration should not b tripp d. Th gr at st cas for und rvoltag prot ction b ing r quir d would b for a g n rator supplying an isolat d pow r syst m or to m t Utility d mands for conn ction of mb dd d g n ration (s S ction 1 7.21). In th cas of g n rators f ding an isolat d syst m, und rvoltag may oc cur for s v ral r asons, typically ov rloading or failur of th AVR. In som ca
s s, th p rformanc of g n rator auxiliary plant f d via a unit transform r fro m th g n rator t rminals could b adv rs ly aff ct d by prolong d und rvoltag . Wh r und rvoltag prot ction is r quir d, it should compris an und rvoltag l m nt and an associat d tim d lay. S ttings must b chos n to avoid malop rati on during th in vitabl voltag dips during pow r syst m fault cl aranc or ass ociat d with motor starting. Transi nt r ductions in voltag down to 80% or l ss may b ncount r d during motor starting. N twork Prot ction & Automation Guid • G n rator and G n rator-Transfor m r P rot ction 17 • • 292 •
17.11 LOW FORWARD POWER/REVERSE POWER PROTECTION Low forward pow r or r v rs po w r prot ction may b r quir d for som g n rators to prot ct th prim mov r. P arts of th prim mov r may not b d sign d to xp ri nc r v rs torqu or th y may b com damag d through continu d rotation aft r th prim mov r has suff r d som form of failur . 17.11.1 Low Forward Pow r Prot ction Low forward pow r p rot ction is oft n us d as an int rlocking function to nabl op ning of th mai n circuit br ak r for non-urg nt trips – .g. for a stator arth fault on a high-i mp danc arth d g n rator, or wh n a normal shutdown of a s t is taking plac . This is to minimis th risk of plant ov rsp ding wh n th l ctrical load is r mov d from a high-sp d cylindrical rotor g n rator. Th rotor of this typ of g n rator is highly str ss d m chanically and cannot tol rat much ov rsp d. Wh il th gov rnor should control ov rsp d conditions, it is not good practic to op n th main circuit br ak r simultan ously with tripping of th prim mov r f or non-urg nt trips. For a st am turbin , for xampl , th r is a risk of ov rsp ding du to n rgy storag in th trapp d st am, aft r st am valv tripping, o r in th v nt that th st am valv (s) do not fully clos for som r ason. For u rg nt trip conditions, such as stator diff r ntial prot ction op ration, th ris k involv d in simultan ous prim mov r and g n rator br ak r tripping must b ac c pt d. 17.11.2 R v rs Pow r Prot ction Prim Mov r Motoring Pow r (% of rat d) 5-25 10-15 (split shaft) >50% (singl sh aft) 0.2-2 (blad s out of wat r) >2 (blad s in wat r) 0.5-6 Possibl Damag Fir / xplosion du to unburnt fu l M chanical damag to g arbox/shafts g arbox damag 50% of motoring pow r Prot ction S tting wh r a prot ction s nsitivity of b tt r than 3% is r quir d, a m t ring class C T should b mploy d to avoid incorr ct prot ction b haviour du to CT phas ang l rrors wh n th g n rator suppli s a significant l v l of r activ pow r at c los to z ro pow r factor. Th r v rs pow r prot ction should b provid d with a d finit tim d lay on op ration to pr v nt spurious op ration with transi nt pow r swings that may aris following synchronisation or in th v nt of a pow r transmission syst m disturbanc . 17.12 UNBALANCED LOADING 17.12.1 Eff ct of N gativ S qu nc Curr nt Th n gativ s qu nc compon nt is s imilar to th positiv s qu nc syst m, xc pt that th r sulting r action fi ld rotat s in th opposit dir ction to th d.c. fi ld syst m. H nc , a flux is pr oduc d which cuts th rotor at twic th rotational v locity, th r by inducing d oubl fr qu ncy curr nts in th fi ld syst m and in th rotor body. Th r sultin g ddy-curr nts ar v ry larg and caus s v r h ating of th rotor. So s v r is this ff ct that a singl -phas load qual to th normal thr -phas rat d cu rr nt can quickly h at th rotor slot w dg s to th soft ning point. Th y may th n b xtrud d und r c ntrifugal forc until th y stand abov th rotor surfac , wh n it is possibl that th y may strik th stator cor . A g n rator is assign d a continuous n gativ s qu nc rating. For turbo-g n rators this rating is lo w; standard valu s of 10% and 15% of th g n rator continuous rating hav b n a dopt d. Th low r rating appli s wh n th mor int nsiv cooling t chniqu s ar appli d, for xampl hydrog n-cooling with gas ducts in th rotor to facilitat dir ct cooling of th winding. Short tim h ating is of int r st during syst m f ault conditions and it is usual in d t rmining th g n rator n gativ s qu nc w ithstand capability to assum that th h at dissipation during such p riods is n gligibl . Using this approximation it is possibl to xpr ss th h ating by th law: 2 I 2t = K Di s l Engin • Gas Turbin Hydro
blad and runn r cavitation turbin blad damag g arbox damag on g ar d s ts St am Turbin Tabl 17.1: G n rator r v rs pow r probl ms R v rs pow r prot ction is appli d to pr v nt damag to m chanical plant it ms in th v nt of failur of th prim mov r. Tabl 17.1 giv s d tails of th pot ntial probl ms for various prim mov r typ s and th typical s ttings for r v rs pow r prot ction. For applications N twork Prot ction & Automation Guid • 293 • G n rator and G n rator-Transfor m r P rot ction 17 • A thr -phas balanc d load produc s a r action fi ld that, to a first approxima tion, is constant and rotat s synchronously with th rotor fi ld syst m. Any unb alanc d condition can b r solv d into positiv , n gativ and z ro s qu nc comp on nts. Th positiv s qu nc compon nt is similar to th normal balanc d load. Th z ro s qu nc compon nt produc s no main armatur r action.
wh r : I2R = n gativ s qu nc compon nt (p r unit of MCR) t = tim (s conds) K = constant proportional to th th rmal capacity of th g n rator rotor For h ati ng ov r a p riod of mor than a f w s conds, it is n c ssary to allow for th h at dissipat d. From a combination of th continuous and short tim ratings, th ov rall h ating charact ristic can b d duc d to b : s qu nc capacity and may not r quir prot ction. Mod rn num rical r lays d riv th n gativ s qu nc curr nt l v l by calculation, with no n d for sp cial ci rcuits to xtract th n gativ s qu nc compon nt. A tru th rmal r plica approa ch is oft n follow d, to allow for: a. standing l v ls of n gativ s qu nc curr nt b low th continuous withstand capability. This has th ff ct of short ning th tim to r ach th critical t mp ratur aft r an incr as in n gativ s qu n c curr nt abov th continuous withstand capability b. cooling ff cts wh n n g ativ s qu nc curr nt l v ls ar b low th continuous withstand capability Th advantag of this approach is that cooling ff cts ar mod ll d mor accurat ly, but th disadvantag is that th tripping charact ristic may not follow th wit hstand charact ristic sp cifi d by th manufactur r accurat ly. Th typical r la y l m nt charact ristic tak s th form of 2 I 2 s t t = − 2 log e 1 − I 2 set I2 Gener tor nd Gener tor Tr nsfor mer P rotection I M = 2 = I2R where: − I2 t 1 − e ( 2R ) 1 K I2R = neg tive ph se sequence continuous r ting in per unit of MCR The he ting c h r cteristics of v rious designs of gener tor re shown in Figure 17.16. K 10000 …Equ tion 17.1 where: 1000 t = time to trip 2
I flc
K = K g ×
Ip
100 Indirectly cooled ( ir) Indirectly cooled (H2) 350MW direct cooled Time (sec) I flc I 2 set = I 2 cmr ×
Ip
×I n 10 660MW direct cooled 1000MW direct cooled
Kg K g = neg tive sequence withst nd = neg tive sequence withst nd coefficient coef ficient (Figure 17.16) (Figure 17.16) • 17 • 1 Using I2 t model 2 Using true therm l model I2cmr = gener tor m ximum m ximum continuous I 2 I 2cmr = gener tor continuous I 2 withst nd Iflc Ip IN = gener tor r ted prim ry current withst nd = CT prim ry current r ted prim ry current I flc = gener tor = rel y I p = CT prim ry current r ted current 0.1 0.01 0.01 0.1 1 10 Neg tive sequence current (p.u.) Figure 17.16: Typic l neg tive ph se sequence current withst nd of cylindric l r otor gener tors 17.12.2 Neg tive Ph se Sequence Protection This protection is pplied to prevent overhe ting due to neg tive sequence currents. Sm ll s lient pole gener tors h ve proportion tely l rger neg tive Figure 17.16 lso =showsr ted current replic time I n rel y the therm l ch r ct eristic descried y Equ tion 17.1, from which it will e seen th t signific n t g in in c p ility is chieved t low levels of neg tive sequence current. Suc h protection element will lso respond to ph see rth nd ph se ph se f ults wh ere sufficient neg tive sequence current rises. Gr ding with downstre m power s ystem protection rel ys is therefore required. A definite minimum time setting m ust e pplied to the neg tive sequence rel y element to ensure correct gr ding. A m ximum trip time setting m y lso e used to ensure correct tripping when th e neg tive sequence Network Protection & Autom tion Guide • 294 •
current level is only slightly in excess of the continuous withst nd c p ility nd hence the trip time from the therm l model m y dep rt signific ntly from the rotor withst nd limits. 17.13 PROTECTION AGAINST INADVERTENT ENERGISATION Accid ent l energis tion of gener tor when it is not running m y cuse severe d m ge to it. With the gener tor t st ndstill, closing the circuit re ker results in the gener tor cting s n induction motor; the field winding (if closed) nd t he rotor solid iron/d mper circuits cting s rotor circuits. Very high currents re induced in these rotor components, nd lso occur in the st tor, with resul t nt r pid overhe ting nd d m ge. Protection g inst this condition is therefor e desir le. A com in tion of st tor undervolt ge nd overcurrent c n e used to detect this condition. An inst nt neous overcurrent element is used, nd g ted with threeph se undervolt ge element (fed from VT on the gener tor side of t he circuit re ker) to provide the protection. The overcurrent element c n h ve low setting, s oper tion is locked when the gener tor is oper ting norm lly. The volt ge setting should e low enough to ensure th t oper tion c nnot occur for tr nsient f ults. A setting of out 50% of r ted volt ge is typic l. VTf i lure c n c use m loper tion of the protection, so the element should e inhi ite d under these conditions. 17.14 UNDER/OVERFREQUENCY/ OVERFLUXING PROTECTION Thes e conditions re grouped together ec use these pro lems often occur due to de p rture from synchronous speed. 17.14.1 Overfluxing Overfluxing occurs when the r tio of volt ge to frequency is too high. The iron s tur tes owing to the high flux density nd results in str y flux occurring in components not designed toc rry it. Overhe ting c n then occur, resulting in d m ge. The pro lem ffects o connected th direct nd indirectly gener tors. Either excessive volt ge, or low frequency, or com in tion of oth c n result in overfluxing, volt ge to freq uency r tio in excess of 1.05p.u. norm lly eing indic tive of this condition. E xcessive flux c n rise tr nsiently, which is not pro lem for the gener tor. F or ex mple, gener tor c n e su jected to tr nsiently high power frequency v olt ge, t nomin l frequency, immedi tely fter full lod rejection. Since the c ondition would not e sust ined, it only presents pro lem for the st ility Network Protection & Autom tion Guide of the tr nsformer differenti l protection schemes pplied t the power st tion (see Ch pter 16 for tr nsformer protection). Sust ined overfluxing c n rise dur ing run up, if excit tion is pplied too e rly with the AVR in service, or if th e gener tor is run down, with the excit tion still pplied. Other overfluxing in st nces h ve occurred from loss of the AVR volt gefeed ck sign l, due to ref erence VT pro lem. Such sust ined conditions must e detected y dedic ted ove rfluxing protection function th t will r ise n l rm nd possi ly force n imme di te reduction in excit tion. Most AVRs’ hav an ov rfluxing prot ction facility includ d. This may only b op rativ wh n th g n rator is on op n circuit, and h nc fail to d t ct ov rfluxing conditions du to abnormally low syst m fr qu n cy. How v r, this facility is not ngin r d to prot ction r lay standards, and should not b sol ly r li d upon to provid ov rfluxing prot ction. A s parat r lay l m nt is th r for d sirabl and provid d in most mod rn r lays. It is us ual to provid a d finit tim -d lay d alarm s tting and an instantan ous or inv rs tim -d lay d trip s tting, to match th withstand charact ristics of th pr ot ct d g n rator and transform r. It is v ry important that th VT r f r nc fo r ov rfluxing prot ction is not th sam as that us d for th AVR. 17.14.2 Und r /Ov rfr qu ncy Th gov rnor fitt d to th prim mov r normally provid s prot cti on against ov rfr qu ncy. Und rfr qu ncy may occur as a r sult of ov rload of g n rators op rating on an isolat d syst m, or a s rious fault on th pow r syst m that r sults in a d ficit of g n ration compar d to load. This may occur if a g rid syst m suff rs a major fault on transmission lin s linking two parts of th syst m, and th syst m th n splits into two. It is lik ly that on part will hav an xc ss of g n ration ov r load, and th oth r will hav a corr sponding d f icit. Fr qu ncy will fall fairly rapidly in th latt r part, and th normal r sp ons is load sh dding, ith r by load sh dding r lays or op rator action. How v r, prim mov rs may hav to b prot ct d against xc ssiv ly low fr qu ncy by tr ipping of th g n rators conc rn d. With som prim mov rs, op ration in narrow
fr qu ncy bands that li clos to normal running sp d ( ith r abov or b low) m ay only b p rmitt d for short p riods, tog th r with a cumulativ lif tim dura tion of op ration in such fr qu ncy bands. This typically occurs du to th pr s nc of rotor torsional fr qu nci s in such fr qu ncy bands. In such cas s, moni toring of th p riod of tim sp nt in th s fr qu ncy bands is r quir d. A sp ci al r lay is fitt d in such cas s, arrang d to provid alarm and trip faciliti s if ith r an individual or • • 295 • G n rator and G n rator-Transfor m r P rot ction 17 •
cumulativ p riod xc ds a s t tim . 17.15 ROTOR FAULTS Th fi ld circuit of a g n rator, comprising th fi ld winding of th g n rator and th armatur of th xcit r, tog th r with any associat d fi ld circuit br ak r if it xists, is an isolat d d.c. circuit which is not normally arth d. If an arth fault occurs, th r will b no st adystat fault curr nt and th n d for action will not b vid nt. Dang r aris s if a s cond arth fault occurs at a s parat point in th fi ld syst m, to caus th high fi ld curr nt to b div rt d, in part at l ast, from th int rv ning turns. S rious damag to th conductors and possibly th ro tor can occur v ry rapidly und r th s conditions. Mor damag may b caus d m c hanically. If a larg portion of th winding is short-circuit d, th flux may ad opt a patt rn such as that shown in Figur 17.17. Th attracting forc at th su rfac of th rotor is giv n by: produc a balancing forc on this axis. Th r sult is an unbalanc d forc that i n a larg machin may b of th ord r of 50-100 tons. A viol nt vibration is s t up that may damag b aring surfac s or v n displac th rotor by an amount suf fici nt to caus it to foul th stator. 17.15.1 Rotor Earth-Fault Prot ction Two m thods ar availabl to d t ct this typ of fault. Th first m thod is suitabl for g n rators that incorporat brush s in th main g n rator fi ld winding. T h s cond m thod r quir s at l ast a slip-ring conn ction to th fi ld circuit: a. pot ntiom t r m thod b. a.c. inj ction m thod 17.15.1.1 Pot ntiom t r m thod This is a sch m that was fitt d to old r g n rators, and it is illustrat d in F igur 17.18. An arth fault on th fi ld winding would produc a voltag across th r lay, th maximum voltag occurring for faults at th nds of th winding. A ‘blind spot would xist at th c ntr of th fi ld winding. To avoid a fault at this location r maining und t ct d, th tapping point on th pot ntiom t r coul d b vari d by a pushbutton or switch. Th r lay s tting is typically about 5% o f th xcit r voltag . G n rator and G n rator-Transfor m r P rot ction F= wh r : A = ar a B = flux d nsity B2A 8π Fiel Win ing Short Circuit Fiel in ing I > Exciter • 17 • 17.15.1.2 Injection metho s Figure 17.18: Earth fault rotection of fiel circuit by otentiometer metho Figure 17.17: Flux istribution on rotor ith artial in ing short circuit Itillbe seenfrom Figure 17.17 that the flux is concentrate on one ole but i ely iserse over the other an intervening surfaces. The attracting force i s in consequence large on one ole but very eak on the oosite one, hile flux on the qua rature axis ill
To metho s are in common use. The first is base on lo frequency signal inject ion, ith series filtering, It comrises an injecti as sho n in Figure 17.19(a). on source that is connecte bet een earth an one si e of the fiel circuit, thr ough an the measurement circuit. The fiel circuit is subje ca acitive cou ling cte to an alternating otentialat substantially the same level throughout. An earth fault any here in the fiel system ill give rise to a current that is et ecte as an equivalent voltage across the a justable resistor by the relay. The reventing the ischar caacitive cou ling blocks the normal .c. fiel voltage, ge of a large irect current through the rotection scheme. The combination • 296 •
Netork Protection & Automation Gui e
of series caacitor an reactor forms a lo ass tune circuit, the intention be ing to filter higher frequency of re rotor currents that may occur for a variety eance asons. Other schemes are base on oer frequency signal injection. An im relay element is use , a fiel in ing earth fault re ucing the ime ance seen by the relay. These suffer the ra back of being susce tible to static excitati on system harmonic currents hen there is significant fiel in ing an excitati on system shunt caacitance. Greater immunity for such systems isoffere by ca acitively cou ling the rotection scheme to both en s of the fiel in ing, her e brush or sli ring access is ossible (Figure 17.19(b)). The lo–frequency injec tion scheme is also a vantageous in that the current flo through the fiel in ing shunt ca acitance ill be lo er than for a o er frequency scheme. Such curr ent oul flo through the machine bearings to cause erosion of the bearingsurf ace. For o er frequency schemes, a solution is to insulate the bearings an ro vi e an earthing brush for the shaft. 17.15.2 Rotor Earth Fault Protection for Brushless Generators A brushless genera tor hasan excitation system consisting of: 1. a main exciter ith rotatingarma ture an stationary fiel in ings 2. a rotating rectifier assembly, carrie on the main shaft line out 3. acontrolle rectifier ro ucing the .c. fiel volta ge for the main exciter fiel from an a.c. source (often a small ‘ilot’ xcit r) 17.15.3 Rotor Short d Turn Prot ction As d tail d in S ction 17.15 a short d s c tion of fi ld winding will r sult in an unsymm trical rotor flux patt rn and in pot ntially damaging rotor vibration. D t ction of such an l ctrical fault is p ossibl using a prob consisting of a coil plac d in th airgap. Th flux patt r n of th positiv and n gativ pol s is m asur d and any significant diff r nc in flux patt rn b tw n th pol s is indicativ of a short d turn or turns. Auto mat d wav form comparison t chniqu s can b us d to provid a prot ction sch m , or th wav form can b insp ct d visually at r gular int rvals. An imm diat sh utdown is not normally r quir d unl ss th ff cts of th fault ar s v r . Th fault can b k pt und r obs rvation until a suitabl shutdown for r pair can b arrang d. R pair will tak som tim , sinc it m ans unthr ading th rotor and d ismantling th winding. Sinc short-circuit d turns on th rotor may caus damag ing vibration and th d t ction of fi ld faults for all d gr s of abnormality i s difficult, th provision of a vibration a d t ction sch m is d sirabl – this f orms part of th m chanical prot ction of th g n rator. 17.15.4 Prot ction agai nst Diod Failur A short-circuit d diod will produc an a.c. rippl in th xc it r fi ld circuit. This can b d t ct d by a r lay monitoring th curr nt in th xcit r fi ld circuit, how v r such syst ms hav prov d to b unr liabl . Th r lay would n d to b tim d lay d to pr v nt an alarm b ing issu d with normal fi ld forcing during a pow r syst m fault. A d lay of 5-10 s conds may b n c s sary. G n rator fi ld winding Excit r L.F. inj ction supply ∼ ∼ U> (a) Low fr qu ncy a.c. voltag inj ction - curr nt m asur m nt • G n rator fi ld winding Excit r
Inj ction supply < Z< (b) Pow r fr qu ncy a.c. voltag inj ction imp danc m asur m nt Figur 17.19: Earth fault prot ction of fi ld circuit by a.c. inj ction N twork Prot ction & Automation Guid • 297 • G n rator and G n rator-Transfor m r P rot ction 17 • H nc , no brush s ar r quir d in th g n rator fi ld circuit. All control is ca rri d out in th fi ld circuit of th main xcit r. D t ction of a rotor circuit arth fault is still n c ssary, but this must b bas d on a d dicat d rotor-mou nt d syst m that has a t l m try link to provid an alarm/data.
Fus s to disconn ct th faulty diod aft r failur may b fitt d. Th fus s ar of th indicating typ , and an insp ction window can b fitt d ov r th diod wh l to nabl diod h alth to b monitor d manually. A diod that fails op n-cir cuit occurs l ss oft n. If th r is mor than on diod in parall l for ach arm of th diod bridg , th only impact is to r strict th maximum continuous xci tation possibl . If only a singl diod p r bridg arm is fitt d, som rippl wi ll b pr s nt on th main fi ld supply but th inductanc of th circuit will sm ooth this to a d gr and again th main ff ct is to r strict th maximum conti nuous xcitation. Th s t can b k pt running until a conv ni nt shutdown can b arrang d. 17.15.5 Fi ld Suppr ssion Th n d to rapidly suppr ss th fi ld of a machin in which a fault has d v lop d should b obvious, b caus as long as th xcitation is maintain d, th machin will f d its own fault v n though isol at d from th pow r syst m. Any d lay in th d cay of rotor flux will xt nd th fault damag . Braking th rotor is no solution, b caus of its larg kin tic n rgy. Th fi ld winding curr nt cannot b int rrupt d instantan ously as it flow s in a highly inductiv circuit. Cons qu ntly, th flux n rgy must b dissipat d to pr v nt an xc ssiv inductiv voltag ris in th fi ld circuit. For machi n s of mod rat siz , it is satisfactory to op n th fi ld circuit with an air-b r ak circuit br ak r without arc blow-out coils. Such a br ak r p rmits only a m od rat arc voltag , which is n v rth l ss high nough to suppr ss th fi ld cur r nt fairly rapidly. Th inductiv n rgy is dissipat d partly in th arc and pa rtly in ddy-curr nts in th rotor cor and damp r windings. With g n rators abo v about 5MVA rating, it is b tt r to provid a mor d finit m ans of absorbing th n rgy without incurring damag . Conn cting a ‘fi ld discharg r sistor’ in par all l with th rotor winding b for op ning th fi ld circuit br ak r will achi v this obj ctiv . Th r sistor, which may hav a r sistanc valu of approximat ly fiv tim s th rotor winding r sistanc , is conn ct d by an auxiliary contac t on th fi ld circuit br ak r. Th br ak r duty is th r by r duc d to that of o p ning a circuit with a low L/R ratio. Aft r th br ak r has op n d, th fi ld c urr nt flows through th discharg r sistanc and di s down harml ssly. Th us of a fairly high valu of discharg r sistanc r duc s th fi ld tim constant t o an acc ptably low valu , though it may still b mor than on s cond. Alt rnat iv ly, g n rators fitt d with static xcitation syst ms may t mporarily inv rt t h appli d fi ld voltag to r duc xcitation curr nt rapidly to z ro b for th xcitation syst m is tripp d. 17.16 LOSS OF EXCITATION PROTECTION Loss of xcitation may occur for a vari ty o f r asons. If th g n rator was initially op rating at only 20%-30% of rat d pow r, it may s ttl to run sup r-synchronously as an induction g n rator, at a low l v l of slip. In doing so, it will draw r activ curr nt from th pow r syst m for rotor xcitation. This form of r spons is particularly tru of sali nt pol g n rators. In th s circumstanc s, th g n rator may b abl to run for s v r al minut s without r quiring to b tripp d. Th r may b suffici nt tim for r m dial action to r stor th xcitation, but th r activ pow r d mand of th mac hin during th failur may s v r ly d pr ss th pow r syst m voltag to an unac c ptabl l v l. For op ration at high initial pow r output, th rotor sp d may ris to approximat ly 105% of rat d sp d, wh r th r would b low pow r output and wh r a high r activ curr nt of up to 2.0p.u. may b drawn from th supply . Rapid automatic disconn ction is th n r quir d to prot ct th stator windings from xc ssiv curr nt and to prot ct th rotor from damag caus d by induc d sl ip fr qu ncy curr nts. 17.16.1 Prot ction against Loss of Excitation Th prot ct ion us d vari s according to th siz of g n rator b ing prot ct d. 17.16.1.1 Sm all g n rators On th small r machin s, prot ction against asynchronous running has t nd d to b optional, but it may now b availabl by d fault, wh r th fun ctionality is availabl within a mod rn num rical g n rator prot ction packag . If fitt d, it is arrang d ith r to provid an alarm or to trip th g n rator. I f th g n rator fi ld curr nt can b m asur d, a r lay l m nt can b arrang d t o op rat wh n this drops b low a pr s t valu . How v r, d p nding on th g n ra tor d sign and siz r lativ to th syst m, it may w ll b that th machin woul d b r quir d to op rat synchronously with littl or no xcitation und r c rtai
n syst m conditions. Th fi ld und rcurr nt r lay must hav a s tting b low th minimum xciting curr nt, which may b 8% of that corr sponding to th MCR of th machin . Tim d lay r lays ar us d to stabilis th prot ction against malop ration in r spons to transi nt conditions and to nsur that fi ld curr nt fluc tuations du to pol slipping do not caus th prot ction to r s t. If th g n r ator fi ld curr nt is not m asurabl , th n th t chniqu d tail d in th followi ng s ction is utilis d. 17.16.1.2 Larg g n rators (>5MVA) For g n rators abov about 5MVA rating, prot ction against loss of xcitation and pol slipping condi tions is normally appli d. N twork Prot ction & Automation Guid • G n rator and G n rator-Transfor m r P rot ction 17 • • 298 •
Consid r a g n rator conn ct d to n twork, as shown in Figur 17.20. On loss of xcitation, th t rminal voltag will b gin to d cr as and th stator curr nt w ill incr as , r sulting in a d cr as of imp danc vi w d from th g n rator t r minals and also a chang in pow r factor. XG A +jX ZS D XG+ T+ZS Th g n ral cas can b r pr s nt d by a syst m of circl s with c ntr s on th l in CD; s Figur 17.21. Also shown is a typical machin t rminal imp danc loc us during loss of xcitation conditions. +jX EG =1.5 ES EG XT ZS ES 1.8 2.0 2.5 5.0 D Load point Loss of fi ld locus EG 1 ES -R XT ZR A θ +R
R EG =1 ES +R C XG 0.5 0.6 C 0.7 jX
jX Figure 17.21: Swing curves and loss of synchronism locus Figure 17.20: Basic interconnected system A relay to detect loss of synchronism can be located at point A. It can be shown that the impedance presented to the relay under loss of synchronism conditions (phase swinging or pole slipping) is given by: ZR = The special cases of EG=ES and EG=0 result in a straight line locus that is the right angled bisector of CD, and in a circular locus that is shrunk to point C, respectively. When excitation is removed from a generator operating synchronousl y the flux dies away slowly, during which period the ratio of EG/ES is decreasin g, and the rotor angle of the machine is increasing. The operating condition plo tted on an impedance diagram therefore travels along a locus that crosses the po wer swing circles. At the same time, it progresses in the direction of increasin g rotor angle. After passing the anti phase position, the locus bends round as t he internal e.m.f. collapses, condensing on an impedance value eual to the mach
ine reactance. The locus is illustrated in Figure 17.21. The relay location is d isplaced from point C by the generator reactance XG. One problem in determining the position of these loci relative to the relay location is that the value of m achine impedance varies with the rate of slip. At zero slip XG is eual to Xd, t he synchronous reactance, and at 100% slip XG is eual to X’’d, th subtransi nt r a ctanc . Th imp danc in a typical cas has b n shown to b qual to X’d, th tra nsi nt r actanc , at 50% slip, and to 2X’d with a slip of 0.33%. Th slip lik ly t o b xp ri nc d with asynchronous running is ( X G + X T + Z S )n (n − cos θ − j sin θ) (n − cos θ) 2 + sin 2 θ −XG …Euation 17.2 • where: n = EG = generated voltage system ES θ = angle by which EG leads Es If the generator and system voltages are eual, the above expression becomes: ZR = ( X G + X T + Z S )(1 − j cotθ 2 ) − X 2 G Network Protection & Automation Guide • 299 • Generator and Generator Transfor mer P rotection 17 •
low, perhaps 1%, so that for the purpose of assessing the power swing locus it i s sufficient to take the value XG=2X’d. This consid ration has assum d a singl va lu for XG. How v r, th r actanc Xq on th quadratur axis diff rs from th di r ct-axis valu , th ratio of Xd/Xg b ing known as th sali ncy factor. This fac tor vari s with th slip sp d. Th ff ct of this factor during asynchronous op ration is to caus XG to vary at slip sp d. In cons qu nc , th loss of xcita tion imp danc locus do s not s ttl at a singl point, but it continu s to d sc rib a small orbit about a m an point. G n rator and G n rator-Transfor m r P rot ction A prot ction sch m for loss of xcitation must op rat d cisiv ly for this cond ition, but its charact ristic must not inhibit stabl op ration of th g n rator . On limit of op ration corr sponds to th maximum practicabl rotor angl , tak n to b at 120°. Th locus of op ration can b r pr s nt d as a circl on th imp danc plan , as shown in Figur 17.22, stabl op ration conditions lying outsid th circl . +jX ZS -R d 2X d XT +R Locus of constant MVA sch m for loss of xcitation could b bas d on imp danc m asur m nt. Th imp d anc charact ristic must b appropriat ly s t or shap d to nsur d cisiv op ra tion for loss of xcitation whilst p rmitting stabl g n rator op ration within allowabl limits. On or two offs t mho und r imp danc l m nts (s Chapt r 11 for th principl s of op ration) ar id ally suit d for providing loss of xcit ation prot ction as long as a g n rator op rating at low pow r output (20-30%Pn) do s not s ttl down to op rat as an induction g n rator. Th charact ristics of a typical two-stag loss of xcitation prot ction sch m ar illustrat d in F igur 17.23. Th first stag , consisting of s ttings Xa1 and Xb1 can b appli d to provid d t ction of loss of xcitation v n wh r a g n rator initially op r ating at low pow r output (20-30%Pn) might s ttl down to op rat as an inductio n g n rator. X Normal machin op rating imp danc -Xa2 X -Xa1 X R Alarm angl Xb2 Xb1 Xd Limiting g n ration point Figur 17.23: Loss of xcitation prot ction charact ristics R lay • 17 • Locus of constant load angl -jX Diam t r = d/ 2 Figur 17.22: Locus of limiting op rating conditions of synchronous machin On th sam diagram th full load imp danc locus for on p r unit pow r can b drawn. Part of this circl r pr s nts a condition that is not f asibl , but th point of int rs ction with th maximum rotor angl curv can b tak n as a limit ing op rating condition for s tting imp danc -bas d loss of xcitation prot ctio n. 17.16.2 Imp danc -Bas d Prot ction Charact ristics Figur 17.21 allud s to th
possibility that a prot ction Pick-up and drop-off tim d lays td1 and tdo1 ar associat d with this imp danc l m nt. Tim r td1 is us d to pr v nt op ration during stabl pow r swings that may caus th imp danc locus of th g n rator to transi ntly nt r th locus o f op ration s t by Xb1. How v r, th valu must short nough to pr v nt damag a s a r sult of loss of xcitation occurring. If pol -slipping prot ction is not r quir d (s S ction 17.17.2), tim r tdo1 can b s t to giv instantan ous r s t . Th s cond fi ld failur l m nt, comprising s ttings Xa2, Xb2, and associat d tim rs td2 and tdo2 can b us d to giv instantan ous tripping following loss o f xcitation und r full load conditions. 17.16.3 Prot ction S ttings Th typical s tting valu s for th two l m nts vary according to th xcitation syst m and op rating r gim of th g n rator conc rn d, sinc th s aff ct th g n rator i mp danc s n by th r lay und r normal and abnormal conditions. For a g n rator that is n v r N twork Prot ction & Automation Guid • 300 •
op rat d at l ading pow r factor, or at load angl s in xc ss of 90° th typical s ttings ar : imp danc l m nt diam t r Xb1 = Xd imp danc l m nt offs t Xa1 = -0.5X’d tim d lay on pick-up, td1 = 0.5s – 10s tim d lay on drop-off, tdo1 = 0s If a fast xcitation syst m is mploy d, allowing load angl s of up to 120° to b us d, th imp danc diam t r must b r duc d to tak account of th r duc d g n ra tor imp danc s n und r such conditions. Th offs t also n ds r vising. In th s circumstanc s, typical s ttings would b : imp danc l m nt diam t r Xb1 = 0. 5Xd imp danc l m nt offs t Xa1 = -0.75X’d tim d lay on pick-up, td1 = 0.5s – 10s tim d lay on drop-off, tdo1 = 0s Th typical imp danc s ttings for th s cond l m nt, if us d, ar : imp danc l m nt diam t r Xb2 = 17.17.1 Prot ction using R v rs Pow r El m nt During pol -slipping, th r will b p riods wh r th dir ction of activ pow r flow will b in th r v rs dir c tion, so a r v rs pow r r lay l m nt can b us d to d t ct this, if not us d f or oth r purpos s. How v r, sinc th r v rs pow r conditions ar cyclical, th l m nt will r s t during th forward pow r part of th cycl unl ss ith r a v ry short pick-up tim d lay and/or a suitabl drop-off tim d lay is us d to l iminat r s tting. Th main advantag of this m thod is that a r v rs pow r l m nt is oft n alr ady pr s nt, so no additional r lay l m nts ar r quir d. Th main disadvantag s ar th tim tak n for tripping and th inability to control th syst m angl at which th g n rator br ak r trip command would b issu d, i f it is a r quir m nt to limit th br ak r curr nt int rruption duty. Th r is a lso th difficulty of d t rmining suitabl s ttings. D t rmination of s ttings i n th fi ld, from a d lib rat pol -slipping t st is not possibl and analytical studi s may not discov r all conditions und r which pol slipping will occur. 17 .17.2 Prot ction using an Und r Imp danc kV 2 MVA El m nt Xa2 = -0.5X’d Th tim d lay s ttings td2 and tdo2 ar s t to z ro to giv instant an ous op ration and r s t. 17.17 POLE SLIPPING PROTECTION A g n rator may pol slip, or fall out of synchronism with th pow r syst m for a numb r of r asons. Th principal caus s ar prolong d cl aranc of a h avy fault on th pow r syst m, wh n th g n rator is op rating at a high load angl clos to th stability l imit, or partial or compl t loss of xcitation. W ak transmission links b tw n th g n rator and th bulk of th pow r syst m aggravat th situation. It can also occur with mb dd d g n rators running in parall l with a strong Utility n twork if th tim for a fault cl aranc on th Utility n twork slow, p rhaps b c aus only IDMT r lays ar provid d. Pol slipping is charact ris d by larg and rapid oscillations in activ and r activ pow r. Rapid disconn ction of th g n rator from th n twork is r quir d to nsur that damag to th g n rator is avo id d and that loads suppli d by th n twork ar not aff ct d for v ry long. Prot ction can b provid d using s v ral m thods. Th choic of m thod will d p nd o n th probability of pol slipping occurring and on th cons qu nc s should it o ccur. N twork Prot ction & Automation Guid With r f r nc to Figur 17.21, a loss of xcitation und r imp danc charact ris tic may also b capabl of d t cting loss of synchronism, in applications wh r th l ctrical c ntr of th pow r syst m and th g n rator li s ‘b hind’ th r layi ng point. This would typically b th cas for a r lativ ly small g n rator that is conn ct d to a pow r transmission syst m (XG >> (XT + XS)). With r f r nc t o Figur 17.23; if pol -slipping prot ction r spons is r quir d, th drop-off t im r tdo1 of th larg r diam t r imp danc m asuring l m nt should b s t to pr v nt its r s t of in ach slip cycl , until th td1 trip tim d lay has xpir d . As with r v rs pow r prot ction, this would b an l m ntary form of pol -sli pping prot ction. It may not b suitabl for larg machin s wh r rapid tripping is r quir d during th first slip cycl and wh r som control is r quir d for
th syst m angl at which th g n rator circuit br ak r trip command is giv n. W h r prot ction against pol -slipping must b guarant d, a mor sophisticat d m thod of prot ction should b us d. A typical r s t tim r d lay for pol -slippin g prot ction might b 0.6s. For g n rator transform r units, th additional imp danc in front of th r laying point may tak th syst m imp danc outsid th u nd r imp danc r lay charact ristic r quir d for loss of xcitation prot ction. Th r for , th acc ptability of this pol slipping prot ction sch m will b d p nd nt on th application. • • 301 • G n rator and G n rator-Transfor m r P rot ction 17 •
17.17.3 D dicat d Pol -Slipping Prot ction Larg g n rator-transform r units dir ctly conn ct d to grid syst ms oft n r quir a d dicat d pol -slipping prot cti on sch m to nsur rapid tripping and with syst m angl control. Historically, d dicat d prot ction sch m s hav usually b n bas d on an ohm-typ imp danc m asur m nt charact ristic. 17.17.3.1 Pol slipping prot ction by imp danc m asur m nt Although a mho typ l m nt for d t cting th chang in imp danc during p ol -slipping can b us d in som applications, but with p rformanc limits, a st raight lin ohm charact ristic is mor suitabl . Th prot ction principl is tha t of d t cting th passag of th g n rator imp danc through a zon d fin d by two such imp danc charact ristics, as shown in Figur 17.24. Th charact ristic is divid d into thr zon s, A, B, and C. Normal op ration of th g n rator li s in zon A. Wh n a pol -slip occurs, th imp danc trav rs s zon s B and C, and tripping occurs wh n th imp danc charact ristic nt rs zon C. +jX 17.17.3.2 Us of l nticular charact ristic A mor sophisticat d approach is to m asur th imp danc of th g n rator and us a l nticular imp danc charact ris tic to d t rmin if a pol -slipping condition xists. Th l nticular charact ris tic is shown in Figur 17.25. Th charact ristic is divid d into two halv s by a straight lin , call d th blind r. Th inclination, θ, of the lens and blinder is determined by the angle of the total system impedance. The impedance of the sys tem and generator transformer determines the forward reach of the lens, ZA, and the transient reactance of the generator determines the reverse reach ZB. Blinder X P ZA Generator and Generator Transfor mer P rotection P' α θ R ZS Relaying point ing Lens ZB T XG Slip locus EG=ES C R B A +R Figure 17.25: Pole slipping protection using lenticular characteristic and blind er • 17 •
jX Ohm relay 2 Ohm relay 1 Figure 17.24: Pole slipping detection by ohm relays The xis g a of
width of the lens is set by the angle α nd the line PP’, p rp ndicular to th a of th l ns, is us d to d t rmin if th c ntr of th imp danc swing durin transi nt is locat d in th g n rator or pow r syst m. Op ration in th cas a g n rator is as follows. Th charact ristic is divid d into 4 zon s and 2
r gions, as shown in Figur 17.26. Normal op ration is with th m asur d imp dan c in zon R1. If a pol slip d v lops, th imp danc locus will trav rs though zon s R2, R3, and R4. Wh n nt ring zon R4, a trip signal is issu d, provid d th imp danc li s b low r actanc lin PP’ and h nc th locus of swing li s with in or clos to th g n rator – i. . th g n rator is pol slipping with r sp ct to th r st of th syst m. Tripping only occurs if all zon s ar trav rs d s qu ntially. Pow r syst m fault s should r sult in th zon s not b ing fully trav rs d so that tripping will not b initiat d. Th s curity of this typ of prot ction sch m is normally nhanc d by th addition of a plain und r imp danc control l m nt (circl about th origin of th imp danc diagram) that is s t to pr v nt tripping for imp danc t raj ctori s for r mot pow r syst m faults. S tting of th ohm l m nts is such that th y li parall l to th total syst m imp danc v ctor, and nclos it, as shown in Figur 17.24. • 302 • N twork Prot ction & Automation Guid
X L ft-l ns A P Z windings and to issu an alarm or trip to pr v nt damag . Right-l ns B Pow r Swing In Syst m O ZS P R4 T2 R3 S XT R2 a 1 M R1 Pol Slipping Charact ristic Although curr nt-op rat d th rmal r plica prot ction cannot tak into account th ff cts of ambi nt t mp ratur or un v n h at distribution, it is oft n appli d as a back-up to dir ct stator t mp ratur m asuring d vic s to pr v nt ov rh a ting du to high stator curr nt. With som r lays, th th rmal r plica t mp ratu r stimat can b mad mor accurat through th int gration of dir ct m asurin g r sistanc t mp ratur d vic s. Irr sp ctiv of wh th r curr nt-op rat d th rm al r plica prot ction is appli d or not, it is a r quir m nt to monitor th stat or t mp ratur of a larg g n rator in ord r to d t ct ov rh ating from what v r caus . T mp ratur s nsitiv l m nts, usually of th r sistanc typ , ar mb dd d in th stator winding at hot-spot locations nvisag d by th manufactur r, th numb r us d b ing suffici nt to cov r all variations. Th l m nts ar conn ct d to a t mp ratur s nsing r lay l m nt arrang d to provid alarm and trip o utputs. Th s ttings will d p nd on th typ of stator winding insulation and on its p rmitt d t mp ratur ris . 17.19 MECHANICAL FAULTS Various faults may occu r on th m chanical sid of a g n rating s t. Th following s ctions d tail th mor important on s from an l ctrical point of vi w. 17.19.1 Failur of th Pri m Mov r Wh n a g n rator op rating in parall l with oth rs los s its pow r inpu t, it r mains in synchronism with th syst m and continu s to run as a synchrono us motor, drawing suffici nt pow r to driv th prim mov r. This condition may not app ar to b dang rous and in som circumstanc s will not b so. How v r, th r is a dang r of furth r damag b ing caus d. Tabl 17.1 lists som typical pr obl ms that may occur. Prot ction is provid d by a low forward pow r/r v rs pow r r lay, as d tail d in S ction 17.11 17.19.2 Ov rsp d Th sp d of a turbo-g n rator s t ris s wh n th st am input is in xc ss of that r quir d to driv th load at nominal fr qu ncy. Th sp d gov rnor can normally control th sp d, and, in any cas , a s t running in parall l with oth rs in an int rconn ct d sys t m cannot acc l rat much ind p nd ntly v n if synchronism is lost. How v r, i f load is sudd nly lost wh n th HV circuit br ak r is tripp d, th s t will b g in to acc l rat X ZB Stabl Pow r Swing Blind r Figur 17.26: D finition of zon s for l nticular charact ristic If th imp danc locus li s abov lin PP’, th swing li s far out in th pow r sy st m – i. . on part of th pow r syst m, including th prot ct d g n rator, is sw inging against th r st of th n twork. Tripping may still occur, but only if sw
inging is prolong d – m aning that th pow r syst m is in dang r of compl t br ak -up. Furth r confid nc ch cks ar introduc d by r quiring that th imp danc lo cus sp nds a minimum tim within ach zon for th pol -slipping condition to b valid. Th trip signal may also b d lay d for a numb r of slip cycl s v n if a g n rator pol -slip occurs – this is to both provid confirmation of a pol -slip ping condition and allow tim for oth r r lays to op rat if th caus of th po l slip li s som wh r in th pow r syst m. Should th imp danc locus trav rs th zon s in any oth r s qu nc , tripping is block d. 17.18 STATOR OVERHEATING O v rh ating of th stator may r sult from: i. ov rload ii. failur of th cooling syst m iii. ov rfluxing iv. cor faults Accid ntal ov rloading might occur thro ugh th combination of full activ load curr nt compon nt, gov rn d by th prim mov r output and an abnormally high r activ curr nt compon nt, gov rn d by th l v l of rotor xcitation and/or st p-up transform r tap. With a mod rn prot ct ion r lay, it is r lativ ly simpl to provid a curr nt-op rat d th rmal r plica prot ction l m nt to stimat th th rmal stat of th stator N twork Prot ction & Automation Guid • • 303 • G n rator and G n rator-Transfor m r P rot ction 17 •
rapidly. Th sp d gov rnor is d sign d to pr v nt a dang rous sp d ris v n w ith a 100% load r j ction, but n v rth l ss an additional c ntrifugal ov rsp d trip d vic is provid d to initiat an m rg ncy m chanical shutdown if th ov r sp d xc ds 10%. To minimis ov rsp d on load r j ction and h nc th m chani cal str ss s on th rotor, th following s qu nc is us d wh n v r l ctrical tr ipping is not urg ntly r quir d: i. trip prim mov r or gradually r duc pow r i nput to z ro ii. allow g n rat d pow r to d cay towards z ro iii. trip g n rator circuit br ak r only wh n g n rat d pow r is clos to z ro or wh n th pow r fl ow starts to r v rs , to driv th idl turbin 17.19.3 Loss of Vacuum A failur of th cond ns r vacuum in a st am turbin driv n g n rator r sults in h ating of th tub s. This th n produc s strain in th tub s, and a ris in t mp ratur of th low-pr ssur nd of th turbin . Vacuum pr ssur d vic s initiat progr s siv unloading of th s t and, if v ntually n c ssary, tripping of th turbin valv s follow d by th high voltag circuit br ak r. Th s t must not b allow d to motor in th El ctrical trip of gov rnor
v nt of loss of vacuum, as this would caus rapid ov rh ating of th low-pr ssu r turbin blad s. 17.20 COMPLETE GENERATOR PROTECTION SCHEMES G n rator and G n rator-Transfor m r P rot ction From th pr c ding s ctions, it is obvious that th prot ction sch m for a g n rator has to tak account of many possibl faults and plant d sign variations. D t rmination of th typ s of prot ction us d for a particular g n rator will d p nd on th natur of th plant and upon conomic consid rations, which in turn i s aff ct d by s t siz . Fortunat ly, mod rn, multifunction, num rical r lays ar suffici ntly v rsatil to includ all of th commonly r quir d prot ction funct ions in a singl packag , thus simplifying th d cisions to b mad . Th followi ng s ctions provid illustrations of typical prot ction sch m s for g n rators c onn ct d to a grid n twork, but not all possibiliti s ar illustrat d, du to th wid variation in g n rator siz s and typ s. 17.20.1 Dir ct-Conn ct d G n rato r A typical prot ction sch m for a dir ct-conn ct d g n rator is shown in Figur 17.27. It compris s th following prot ction functions: Gov rnor trip Em rg ncy push button Stator diff r ntial (bias d/high imp danc ) Stator E/F (or n utral voltag displ ac m nt) Back-up ov rcurr nt (or voltag d p nd nt O/C) Lubricating oil failur M chanical faults (urg nt) R v rs /low forward pow r Und rfr qu ncy Pol slippin g Ov rfluxing Inadv rt nt n rgisation • 17 • Loss of xcitation Stator winding t mp ratur Unbalanc d loading Und r/ov rvolta g M chanical faults (non-urg nt) N.B. Alarms and tim d lays omitt d for simpli city Low pow r int rlock Excitation circuit br ak r G n rator circuit br ak r Figur 17.27: Typical prot ction arrang m nt for a dir ct-conn ct d g n rator • 304 • N twork Prot ction & Automation Guid
1. stator diff r ntial prot ction 2. ov rcurr nt prot ction – conv ntional or volt ag d p nd nt 3. stator arth fault prot ction 4. ov rvoltag prot ction 5. und rvoltag prot ction 6. ov rload/low forward pow r/ r v rs pow r prot ction (acc ording to prim mov r typ ) 7. unbalanc d loading 8. ov rh ating 9. pol slippin g 10. loss of xcitation 11. und rfr qu ncy 12. inadv rt nt n rgisation 13. ov rfluxing 14. m chanical faults Figur 17.27 illustrat s which trips r quir an instantan ous l ctrical trip and which can b tim d lay d until l ctrical pow r has b n r duc d to a low valu . Th faults that r quir tripping of th prim mov r as w ll as th g n rator circuit br ak r ar also shown. 17.20.2 G n rat or-Transform r Units Th s units ar g n rally of high r output than dir ctconn ct d g n rators, and h nc mor compr h nsiv prot ction is warrant d. In additi on, th g n rator transform r also r quir s prot ction, for which th prot ction d tail d in Chapt r 16 is appropriat Figur 17.28 shows a typical ov rall sch m . El ctrical trip of gov rnor Gov rnor trip Em rg ncy push button Stator diff r ntial (bias d/high imp danc ) Stator E/F (or n utral voltag displ ac m nt) Back-up ov rcurr nt (or voltag d p nd nt O/C) Lubricating oil failur M chanical faults (urg nt) R v rs /low forward pow r Und rfr qu ncy Pol slippin g Ov rfluxing Inadv rt nt n rgisation • Ov rall diff r ntial (transform r diff r ntial) Buchholz HV ov rcurr nt HV r str ict d E/F Transform r winding t mp ratur Loss of xcitation Stator winding t mp ratur Unbalanc d loading Und r/ov rvoltag M chanical faults (non-urg nt) N.B. Alarms and tim d lays omitt d for simplicity Low pow r int rlock Excitation circuit br ak r G n rator circuit br ak r Figur 17.28: Typical tripping arrang m nts for g n rator-transform r unit N twork Prot ction & Automation Guid • 305 • G n rator and G n rator-Transfor m r P rot ction 17 • Ov rall bias d g n rator/g n rator transform r diff r ntial prot ction is common ly appli d in addition, or inst ad of, diff r ntial prot ction for th transform r alon . A singl prot ction r lay may incorporat all of th r quir d function s, or th prot ction of th transform r (including ov rall g n rator/g n rator t ransform r diff r ntial prot ction) may utilis a s parat r lay.
17.21 EMBEDDED GENERATION In r c nt y ars, through d -r gulation of th l ctric ity supply industry and th nsuing comm rcial comp tition, many l ctricity us rs conn ct d to MV pow r distribution syst ms hav install d g n rating s ts to op rat in parall l with th public supply. Th int ntion is ith r to utilis s urplus n rgy from oth r sourc s, or to us wast h at or st am from th prim m ov r for oth r purpos s. Parall l conn ction of g n rators to distribution syst ms did occur b for d r gulation, but only wh r th r was a n t pow r import fr om th Utility. Pow r xport to Utility distribution syst ms was a r lativ ly n w asp ct. Sinc g n ration of this typ can now b locat d within a Utility dist ribution syst m, as oppos d to b ing c ntrally dispatch d g n ration conn ct d t o a transmission syst m, th t rm ‘Emb dd d G n ration’ is oft n appli d. Figur 17. 2 illustrat s such an arrang m nt. D p nding on siz , th mb dd d g n rator(s) may b synchronous or asynchronous typ s, and th y may b conn ct d at any volta g appropriat to th siz of plant b ing consid r d. Th impact of conn cting g n ration to a Utility distribution syst m that was originally ngin r d only f or downward pow r distribution must b consid r d, particularly in th ar a of p rot ction r quir m nts. In this r sp ct, it is not important wh th r th mb dd d g n rator is normally capabl of xport to th Utility distribution syst m or not, sinc th r may xist fault conditions wh n this occurs irr sp ctiv of th d sign int nt. If plant op ration wh n disconn ct d from th Utility supply is r quir d, und rfr qu ncy prot ction (S ction 17.4.2) will b com an important f atur of th in-plant pow r syst m. During isolat d op ration, it may b r lativ ly asy to ov rload th availabl g n ration, such that som form of load manag m nt syst m may b r quir d. Similarly, wh n running in parall l with th Utili ty, consid ration n ds to b giv n to th mod of g n rator op ration if r acti v pow r import is to b controll d. Th impact on th control sch m of a sudd n br ak in th Utility conn ction to th plant main busbar also r quir s analysi s. Wh r th in-plant g n ration is run using constant pow r factor or constant r activ pow r control, automatic r v rsion to voltag control wh n th Utility conn ction is lost is ss ntial to pr v nt plant loads b ing subj ct d to a volt ag outsid acc ptabl limits. Limits may b plac d by th Utility on th amount of pow r/r activ pow r import/ xport. Th s may d mand th us of an in-plant Pow r Manag m nt Syst m to control th mb dd d g n ration and plant loads accor dingly. Som Utiliti s may insist on automatic tripping of th int rconn cting c ircuit br ak rs if th r is a significant d partur outsid p rmissibl l v ls o f fr qu ncy and voltag , or for oth r r asons. From a Utility standpoint, th conn ction of mb dd d g n ration may caus probl ms with voltag control and incr a s d fault l v ls. Th s ttings for prot ction r lays in th vicinity of th plan t may r quir adjustm nt with th m rg nc of mb dd d g n ration. It must also b nsur d that th saf ty, s curity and quality of supply of th Utility distr ibution syst m is not compromis d. Th mb dd d g n ration must not b p rmitt d to supply any Utility custom rs in isolation, sinc th Utility supply is norma lly th m ans of r gulating th syst m voltag and fr qu ncy within th p rmitt d limits. It also normally provid s th only syst m arth conn ction(s), to nsu r th corr ct p rformanc of syst m prot ction in r spons to arth faults. If th Utility pow r inf d fails, it is also important to disconn ct th mb dd d g n ration b for th r is any risk of th Utility pow r supply r turning on to unsynchronis d machin s. In practic this g n rally r quir s th following prot ction functions to b appli d at th ‘Point of Common Coupling’ (PCC) to trip th co upling circuit br ak r: a. ov rvoltag b. und rvoltag c. ov rfr qu ncy d. und r fr qu ncy . loss of Utility supply In addition, particular circumstanc s may r quir additional prot ction functions: f. n utral voltag displac m nt g. r v rs pow r h. dir ctional ov rcurr nt In practic , it can b difficult to m t th prot ction s ttings or p rformanc d mand d by th Utility without a high risk o f nuisanc tripping caus d by lack of coordination with normal pow r syst m faul ts and disturbanc s that do not n c ssitat tripping of th mb dd d g n ration. This is sp cially tru wh n applying prot ction sp cifically to d t ct loss of th Utility supply (also call d ‘loss of mains’) to cat r for op rating conditions
wh r th r would b no imm diat xcursion in voltag or fr qu ncy to caus op ration of conv ntional prot ction functions. 17.21.1 Prot ction Against Loss of Utility Supply If th normal pow r inf d to a distribution syst m, or to th pa rt of it containing mb dd d g n ration is lost, th ff cts may b as follows: a. mb dd d g n ration may b ov rload d, l ading to g n rator und rvoltag /und rfr qu ncy N twork Prot ction & Automation Guid • G n rator and G n rator-Transfor m r P rot ction 17 • • 306 •
b. mb dd d g n ration may b und rload d, l ading to ov rvoltag /ov rfr qu ncy c. littl chang to th absolut l v ls of voltag or fr qu ncy if th r is litt l r sulting chang to th load flow through th PCC Th first two ff cts ar c ov r d by conv ntional voltag and fr qu ncy prot ction. How v r, if condition ( c) occurs, conv ntional prot ction may not d t ct th loss of Utility supply con dition or it may b too slow to do so within th short st possibl auto-r clos d ad-tim s that may b appli d in association with Utility ov rh ad lin prot ct ion. D t ction of condition (c) must b achi v d if th r quir m nts of th Util ity ar to b m t. Many possibl m thods hav b n sugg st d, but th on most o ft n us d is th Rat of Chang of Fr qu ncy (ROCOF) r lay. Its application is b as d on th fact that th rat of chang of small chang s in absolut fr qu ncy, in r spons to in vitabl small load chang s, will b fast r with th g n ratio n isolat d than wh n th g n ration is in parall l with th public, int rconn ct d pow r syst m. How v r, probl ms with nuisanc tripping in r spons to nationa l pow r syst m v nts, wh r th syst m is subj ct to significant fr qu ncy xcu rsions following th loss of a larg g n rator or a major pow r int rconn ctor, hav occurr d. This is particularly tru for g ographically island d pow r syst ms, such as thos of th British Isl s. An alt rnativ to ROCOF prot ction is a t chniqu som tim s r f rr d to as ‘voltag v ctor shift’ prot ction. In this t chni qu th rat of phas chang b tw n th dir ctly m asur d g n rator bus voltag is compar d with a m moris d a.c. bus voltag r f r nc . Sourc s of mb dd d g n ration ar not normally arth d, which pr s nts a pot ntial saf ty hazard. In th v nt of an Utility syst m arth fault, th Utility prot ction should op rat to r mov th Utility pow r inf d. In th ory, this should also r sult in r mo val of th mb dd d g n ration, through th action of th stipulat d voltag /fr qu ncy prot ction and d p ndabl ‘loss of mains’ prot ction. How v r, in vi w of saf ty consid rations ( .g. fall n ov rh ad lin conductors in public ar as), an ad ditional form of arth fault prot ction may also b d mand d to pr v nt th back f d of an arth fault by mb dd d g n ration. Th only way of d t cting an art h fault und r th s conditions is to us n utral voltag displac m nt prot ction . Th additional r quir m nt is only lik ly to aris for mb dd d g n ration rat d abov 150kVA, sinc th risk of th small mb dd d g n rators not b ing cl ar d by oth r m ans is n gligibl . N twork Prot ction & Automation Guid 17.21.2 ROCOF R lay D scription A ROCOF r lay d t cts th rat of chang of fr q u ncy in xc ss of a d fin d s tpoint. Th signal is obtain d from a voltag tra nsform r conn ct d clos to th Point of Common Coupling (PCC). Th principal m thod us d is to m asur th tim p riod b tw n succ ssiv z rocrossings to d t rmin th av rag fr qu ncy for ach half-cycl and h nc th rat of chang of fr qu ncy. Th r sult is usually av rag d ov r a numb r of cycl s. 17.21.3 Volta g V ctor Shift R lay D scription 17.21.4 S tting Guid lin s Should loss of th Utility supply occur, it is xtr m ly unlik ly that th r will b an xact match b tw n th output of th mb dd d g n rator(s) and th conn ct d load. A small fr qu ncy chang or voltag phas angl chang will th r for occur, to which can b add d any chang s du to th small natural variations in loading of an isolat d g n rator with tim . Onc th rat of chang of fr qu ncy xc ds th s tting of th ROCOF r lay for a s t t im , or onc th voltag phas angl drift xc ds th s t angl , tripping occur s to op n th conn ction b tw n th in-plant and Utility n tworks. Whil it is possibl to stimat th rat of chang of fr qu ncy from knowl dg of th g n r ator s t in rtia and MVA rating, this is not an accurat m thod for s tting a RO COF r lay b caus th rotational in rtia of th compl t n twork b ing f d by th mb dd d g n ration is r quir d. For xampl , th r may b oth r mb dd d g n rators to consid r. As a r sult, it is invariably th cas that th r lay s ttin gs ar d t rmin d at sit during commissioning. This is to nsur that th Utili ty r quir m nts ar m t whil r ducing th possibility of a spurious trip und r th various op rating sc narios nvisag d. How v r, it is v ry difficult to d t rmin wh th r a giv n rat of chang of fr qu ncy will b du to a ‘loss of mains’ i
ncid nt or a load/fr qu ncy chang on th public pow r n twork, and h nc spurio us trips ar impossibl to liminat . Thus th provision of Loss of Utility Supp ly prot ction to m t pow r distribution Utility int rfac prot ction • • 307 • G n rator and G n rator-Transfor m r P rot ction 17 • A voltag v ctor shift r lay d t cts th drift in voltag phas angl b yond a d fin d s tpoint as long as it tak s plac within a s t p riod. Again, th voltag signal is obtain d from a voltag transform r conn ct d clos to th Point of Common Coupling (PCC). Th principal m thod us d is to m asur th tim p riod b tw n succ ssiv z ro-crossings to d t rmin th duration of ach half-cycl , a nd th n to compar th durations with th m moris d av rag duration of arli r half-cycl s in ord r to d t rmin th phas angl drift.
G n rator and G n rator-Transfor m r P rot ction r quir m nts, may actually conflict with th int r sts of th national pow r sys t m op rator. With th growing contribution of non-dispatch d mb dd d g n ratio n to th aggr gat national pow r d mand, th loss of th mb dd d g n ration fo llowing a transmission syst m incid nt that may alr ady chall ng th s curity o f th syst m can only aggravat th probl m. Th r hav b n claims that voltag v ctor shift prot ction might off r b tt r s curity, but it will hav op ration tim s that vary with th rat of chang of fr qu ncy. As a r sult, d p nding on th s ttings us d, op ration tim s might not comply with Utility r quir m nts u nd r all circumstanc s. R f r nc 17.1 provid s furth r d tails of th op ration of ROCOF r lays and th probl ms that may b ncount r d. N v rth l ss, b caus such prot ction is a common r quir m nt of som Utiliti s, th ‘loss of mains’ prot ction may hav to b provid d and th possibility of spurious trips will hav t o b acc pt d in thos cas s. Sit m asur m nts ov r a p riod of tim of th typ ical rat s of fr qu ncy chang occurring may assist in n gotiations of th s tti ngs with th Utility, and with th fin -tuning of th prot ction that may alr ad y b commission d. 17.22 EXAMPLES OF GENERATOR PROTECTION SETTINGS This s ction giv s xampl s of th calculations r quir d for g n rator prot ction. Th first is for a typical small g n rator install d on an industrial syst m that runs in parall l with th Utility supply. Th s cond is for a larg r g n rator-transform r unit conn ct d to a grid syst m. 17.22.1 Prot ction S ttings of a Small Indus trial G n rator G n rator Data kVA 6250 kW 5000 PF 0.8 Rat d voltag 11000 Rat d Rat d curr nt f r qu ncy 328 50 Rat d Prim Mov r sp d typ 1500 St am Turbin Sali nt d tails of th g n rator, n twork and prot ction r quir d ar giv n in T abl 17.2. Th xampl calculations ar bas d on a MiCOM P343 r lay in r sp ct o f s tting rang s, tc. 17.22.1.1 Diff r ntial prot ction Bias d diff r ntial pro t ction involv s th d t rmination of valu s for four s tting valu s: Is1, Is2, K1 and K2 in Figur 17.5. Is1 can b s t at 5% of th g n rator rating, in accor danc with th r comm ndations for th r lay, and similarly th valu s of Is2 (1 20%) and K2 (150%) of g n rator rating. It r mains for th valu of K1 to b d t rmin d. Th r comm nd d valu is g n rally 0%, but this only appli s wh r CT’s t hat conform to IEC 60044-1 class PX (or th sup rs d d BS 3938 Class X) ar us d – i. . CT’s sp cifically d sign d for us in diff r ntial prot ction sch m s. In th is application, th CT’s ar conv ntional class 5P CT’s that m t th r lay r quir m nts in r sp ct of kn -point voltag , tc. Wh r n utral tail and t rminal CT’s c an saturat at diff r nt tim s du to transi ntly offs t magn tising inrush or m otor starting curr nt wav forms with an r.m.s. l v l clos to rat d curr nt and wh r th r is a high L/R tim constant for th offs t, th us of a 0% bias slo p may giv ris to malop ration. Such wav forms can b ncount r d wh n plant o f similar rating to th g n rator is b ing n rgis d or start d. Diff r nc s b t w n CT d signs or diff ring r man nt flux l v ls can l ad to asymm tric saturat ion and th production of a diff r ntial spill curr nt. Th r for , it is appropr iat to s l ct a non-z ro s tting for K1, and a valu of 5% is usual in th s ci rcumstanc s. 17.22.1.2 Voltag controll d ov rcurr nt prot ction This prot ction is appli d as r mot backup to th downstr am ov rcurr nt prot ction in th v nt of prot ction or br ak r failur conditions. This nsur s that th g n rator will not continu to supply th fault und r th s conditions. At normal voltag , th curr nt s tting must b gr at r than th maximum g n rator load curr nt of 328A. A margin must b allow d for r s tting of th r lay at this curr nt (r s t ratio = 95%) and for th m asur m nt tol ranc s of th r lay (5% of Is und r r f r nc conditions), th r for th curr nt s tting is calculat d as: I vcs t > 328 ×1.05 0.95 • 17 •
G n rator typ Sali nt Pol Xd p.u. 2.349 G n rator Param t rs X’d p.u. CT Ratio 0.297 500/1 N twork Data Minimum phas faul t curr nt 145A VT Ratio 11000/110 Earthing r sistor 31.7Ω Maximum earth fault current 200A Maximum ownstream phase fault current 850A > 362.5 A CT Ratio 200/1 Existing Protection Overcurrent Settings Characteristic Setting 144A SI TMS 0.17 6 Earth Fault Settings TMS 0.15 The nearest settable value is 365A, or 0.73In. The minimum phase-phase voltage f or a close-up singlephase to earth fault is 57%, so the voltage setting Vs must be less than this. A value of 30% is typically use , giving Vs = 33V. The curren t setting multiplying factor • 308 • Characteristic Setting SI 48A Table 17.2: Data for small generator protection example Network Protection & Automation Gui e
K must be chosen such that KIS isless than 50% of the generator stea y-state cu rrent contribution to an uncleare remote fault. This information is not availab le (missing ata being common in protection stu ies). However, the maximum susta ine close-up phase fault current (neglecting AVR action) is 145A, so that a set ting chosen to be significantly below this value will suffice. A value of 87.5A (60% of the close-up sustaine phase fault current) is thereforechosen, an hen ce K = 0.6. This is consi ere to be appropriate base on knowle ge of the syste m circuit impe ances. The TMS setting is chosen to co-or inate with the ownstre am fee er protection such that: 1. for a close-up fee er three-phase fault, that results in almost total voltage collapse as seen by the relay 2. for a fault at the next ownstream relay location, if the relay voltage is less than the switc hing voltage It shoul also be chosen so that the generator cannot be subjecte to fault or overloa current in excess of the stator short-time current limits. A curve shoul be provi e by the manufacturer, but IEC 60034-1 eman s that an AC generator shoul be able to pass 1.5 times rate current for at least 30 seco n s. The operating time of the ownstream protection for a three-phase fault cur rent of 850A is 0.682s, so the voltage controlle relay element shoul have a mi nimum operating time of 1.09s (0.4s gra ing margin use as the relay technology use for the ownstream relay is not state – see Table 9.2). With a current setti ng of 87.5A, the operating time of the voltage controlle relay element at a TMS of 1.0 is: 0.14 850 87.5 0.02 an operation time of not less than 1.13s. At a TMS of 1.0, the generator protect ion relay operating time will be: 0.14
s 0.02 200
−1
20
( )
1.13 = 0.38 . 2.97 Use setting of 0.4, ne rest v il le setting. =2.97s, so the required TMS is 17.22.1.4 Neutr l volt ge displ cement protection This protection is provided s ck up e rth f ult protection for the gener tor nd downstre m system (direct connected gener tor). It must therefore h ve se tting th t gr des with the downstre m protection. The protection is driven from the gener tor st r connected VT, while the downstre m protection is current oper ted. It is therefore necess ry to tr nsl te the current setting of the downstre m setting of the current oper ted e rth f ult protection into the equiv lent vo lt ge for the NVD protection. The equiv lent volt ge is found from the formul : V eff = = ( I pe × Z e ) × 3 VT r tio 48 × 31.7 × 3 100 = 45.6 V where: Veff = effective volt ge setting Ipe = downstre m e rth f ult current set ting Ze = e rthing resist nce Hence setting of 48V is ccept le. Time gr ding is required, with minimum oper ting time of the NVD protection of 1.13s t n e rth f ult current of 200A. Using the expression for the oper tion time of the NVD element: t = K/(M 1) sec where: = 3.01s −1 Therefore TMS of: 1.09 = 0.362 3.01
is required. Use 0.375, ne rest v il le setting. 17.22.1.3 St tor e rth f ult protection The m ximum e rthf ult current, from T le 17.2, is 200A. Protection for 95% of the winding c n e provided if the rel y is set to detect prim ry e rth f ult current of 16.4A, nd this equ tes to CT second ry current of 0.03 3A. The ne rest rel y setting is 0.04A, providing protection for 90% of the wind ing. The protection must gr de with the downstre m e rth f ult protection, the s ettings of which re lso given in T le 17.2. At n e rth f ult current of 200A , the downstre m protection h s n oper tion time of 0.73s. The gener tor e rth f ult protection must therefore h ve • V M = V snvd nd V = volt ge seen y rel y Vsnvd = rel y setting volt ge the v lue of K c n e c lcul ted s 3.34. The ne rest sett le v lue is 3.5, giving n oper tion tim e of 1.18s. Network Protection & Autom tion Guide • 309 • Gener tor nd Gener tor Tr nsfor mer P rotection 17 •
17.22.1.5 Loss of excit tion protection Loss of excit tion is detected y mho imped nce rel y element, s det iled in Section 17.16.2. The st nd rd settings f or the P340 series rel y re: X = 0.5X’d x (CT ratio/VTratio) (in s condary quan titi s) = -0.5 x 0.297 x 19.36 x 500/100 = -14.5Ω Xb = X x (CT ratio/VT ratio) = 2.349Ω x 19.36 x (500/100) =227Ω The nearest settings provi e by the relay are Xa = 14.5Ω Xb = 227Ω. The time elay t 1 shoul be setto avoi relay element operation on power swings an a typical setting of 3s is use . This value may nee to be mo ifie in the light of operating experience. To prevent cyclical pick-up of th e relay element without tripping, such as might occur uring pole-slipping con i tions, a rop-off time elay t o1 is provi e an set to 0.5s.17.22.1.6 Negativ e phase sequence current protection This protection is require to guar against excessive heating from negative phase sequence currents, whatever the cause. Th e generator is of salient pole esign, so from IEC 60034-1, the continuous withs tan is 8% of rating 2 an the I 2t value is 20s. Using Equation 17.1, the requi re relay settings can foun as I2>> = 0.05 an K = 8.6s. The nearest available values are I2>> = 0.05 an K = 8.6s. The relay also has a cooling time constant Kreset that is normally set equal to the value of K. To coor inate with clearanc e of heavy asymmetric system faults, that might otherwise cause oper unnecessary ation of this protection, a minimum operation time tmin shoul be applie . It is recommen e to set this to a value of 1. Similarly, a maximum time can be appli e to ensure that the thermal rating of the generator is not excee e (as this i s uncertain, ata not available) an to take account of the fact that the P343 c haracteristic is not i entical with that specifie in IEC 60034. The recommen e setting for tmax is 600s. 17.22.1.7 Overvoltage protection This is require to guar against various failure mo es, e.g. AVR failure, resulting in excessive st ator voltage. A two-stageprotection is available, the first being a lowset time - elaye stage that shoul be set to gra e with transient overvoltages that can be tolerate following loa rejection. The secon is a high-set stage use for i nstantaneous tripping in the event of an intolerable overvoltage con ition arisi ng. Generators can normally withstan 105% of rate voltage continuously, so the low-set stage shoul be set higher than this value. A setting of 117.7V in seco n ary quantities (correspon use , wi ing to 107% of rate stator voltage) is typically th a efinite time elay of 10s to allow for transients ue to loa switchoff/re jection, overvoltages on recovery from faults or motor starting, etc. The secon element provi es protection in the event of a largeovervoltage, by tripping ex citation an the generator circuit breaker (if close ). This must be set below t he maximum stator voltage possible, taking into account saturation. As the open circuit characteristic of the generator is not available, typical values must be use . Saturation will normally limit the maximumovervoltage on this type of ge nerator to 130%, so a setting of 120% (132V secon ary) is typically use . Instan taneous operation is require . Generator manufacturers are normally able to prov i e recommen ations for the relay settings. For embe e generators, the require ments of the local Utility may also have to be taken into account. For both elem ents, a variety of voltage measurement mo es are available to take account of po ssible VT connections (single or threephase, etc.), an con itions to be protect e against. In this example, a three-phase VT connection is use , an overvoltag es on any phase are to be etecte , so a selection of ‘Any’ is use for this setting . 17.22.1.8 Un erfrequency protection This is require to protect the generator from sustaine overloa con itions uring perio s of operation isolate from the Utility supply. The generating set manufacturer will normallyprovi e the etai ls of machine short-time capabilities. The example relay provi es four stages of un erfrequency protection. In this case, the first stage is use for alarm purp oses an a secon stage woul be applie to trip the set. The alarm stage might typically be set to 49Hz, with a time elay of 20s, to avoi an alarm being rais e un er transient con itions, e.g. uring plant motor starting. The trip stage might be set to 48Hz, with a time elay of 0.5s, to avoi tripping for transient , but recoverable, ips in frequency below this value. 17.22.1.9 Reverse power p rotection The relay setting is 5% of rate power.
0.05 ×5 ×10 6
setting =
CT ratio ×VT ratio
0.05 ×5 ×10 6
=
500 ×100
•
Generator an Generator-Transfor mer P rotection 17 • This value can be set in the relay. A time elay is require toguar against po wer swingswhile generating at low power levels, so use a time elay of 5s. No r eset time elay is require . Network Protection & Automation Gui e • 310 •
Protection
Quantity Is1 Is2 K1 K2 Ise TMS Vsnv K Xa Xb t 1 tDO1 Ivcset Vs K TMS I2>> K Kre set tmin tmax V> meas mo e V> operatemo e V>1 setting V>1 function V>1 time el ay V>2 setting V>2 function V>2 time elay F<1 setting F<1 time elay F<2 settin g F<2 time elay P1 function P1 setting P1 time elay P1 DO time Value 5% 120% 5% 150% 0.04 0.4 48V 3.5 -14.5Ω 227Ω 3s 0.5s 0.73 33 0.6 0.375 0.05 8. 6s 8.6s 1.5s 600s three-phase any 107% DT 10s 120% DT 0sec 49Hz 20s 48Hz 0.5s re verse power 5W 5s 0s Parameter Value Unit MVA MW kV pu pu pu pu pu pu pu s pu Differential protection
Stator earth fault Neutral voltage isplacement Loss of excitation Negative phase sequence Overvoltage
Table 17.4: System ata for large generator protection example Un erfrequency 17.22.2.2 Voltage restraine overcurrent protection The setting current Iset has to be greater thanthe fullloa currentof the generator (6019A). A suitable ma rgin must be allowe for operation at re uce voltage, so use a multiplying fact or of 1.2. The nearest settable value is 7200A. The factor K is calculate so th at the operating current is less than the current for a remote en three phase f ault. The stea y-state current an voltage at the generator for a remote-en thr ee-phase fault are given by the expressions: Reverse Power Table 17.3: Small generator protection example – relay settings 17.22.2 Large Generator Transformer Unit Protection The ata for this unit are g iven in Table 17.4. It is fitte with two main protection systems to ensure secu rity of tripping in the event of a fault. To economise on space, the setting cal culations for only one system, that using a MiCOM P343 relayare given. Settings are given in primary quantities throughout. 17.22.2.1 Biase ifferential prote ction The settings follow the gui elines previously state . As 100% stator win i ng earth-fault protection is provi e , high sensitivity is not require an henc e Is1 can be set to 10% of generator rate current. This equates to 602A, an th e nearest settable value on the relay is 640A (= 0.08 of rate CT current). The settings for K1, Is2 an K2 follow the gui elines in the relay manual. Network Protection & Automation Gui e • 311 • • I flt = where:
VN ( nR f ) + ( X 2
+ X t + nX f ) 2
where : I f = minimum generator primary If = minimum generator primary current for a cu rrent fee er-en fault multi-phase for a multi − ph se feeder − end f ult VN = no lo d ph se neutr l gener tor volt ge Xd =V N = no −lo d ph se − neutr l re ct nce gener tor d xis synchronous g ener tor volt ge Xt = gener tor tr nsformer re ct nce rf = X d = gener tor d − xis synchronous feeder resist nce re ct nce Xf = feederr e ct n ce n = X t = gener tor tr nsformer re ct n ce num er of p r llel gener t ors r f = feeder resist n ce X f = feeder re ct n ce n = num er of p r llel g ener t ors Gener tor nd Gener tor Tr nsfor mer P rotection 17 • Volt ge controlled overcurrent Gener tor MVA r ting 187.65 Gener tor MW r ting 160 Gener tor volt ge 18 Synchro nous re ct nce 1.93 Direct xis tr nsient re ct nce 0.189 Minimum oper ting volt ge 0.8 Gener tor neg tive sequence c p ility 0.08 Gener tor neg tive sequence f ctor, Kg 10 Gener tor third h rmonic volt ge under lo d 0.02 Gener tor motorin g power 0.02 l rm 1.1 Gener tor overvolt ge time del y 5 trip 1.3 Gener tor und ervolt ge not required M x pole slipping frequency 10 Gener tor tr nsformer r ti ng 360 Gener tor tr nsformer le k ge re ct nce 0.244 Gener tor tr nsformer overf lux l rm 1.1 Gener tor tr nsformer overflux l rm 1.2 Network resist nce (refer red to 18kV) 0.56 Network re ct nce (referred to 18kV) 0.0199 System imped nce ngle (estim ted) 80 Minimum lo d resist nce 0.8 Gener tor CT r tio 8000/1 Gener tor VT r tio 18000/120 Num er of gener tors in p r llel 2 Hz MVA pu pu pu mΩ Ω eg Ω
hence, Iflt = 2893A = 0.361N an A TMS value of 10 is selecte , to match the withstan curve supplie by the manu facturer. 17.22.2.6 100% Stator earth fault protection This is provi e by a com bination of neutral voltage isplacement an thir harmonic un ervoltage protect ion. For the neutral voltage isplacement protection to cover 90% of the stator win ing, the minimum voltage allowing for generator operation at a minimum of 92 % of rate voltage is: 0.92 ×18 kV ×0.1 3 = 956.1V V flt = V N 3(( nR f ) 2 + ( X t + nX f ) 2 ) ( nR f ) 2 + ( X
+ X t + nX f ) 2
=1304 V = 0.07U N A suitable value of K is therefore 0.3611.2 = 0.3 . Generator an Generator-Transfor mer P rotection A suitable value of V2set is 120% of Vflt, giving a value of 1565V. The nearest settable value is 3000V, minimum allowable relay setting. The value of V1set is require to be above the minimum voltage for a close-up ph seen by the generator ase-earthfault. A value of 80% of rate voltage is use for V1set, 14400V. 17.2 2.2.3 Ina vertentenergisation protection This protection is a combination of ov ercurrent with un ervoltage, the voltage signal beingobtaine from a VT on the generator si e of the system. The current setting use is that of rate generato r current of 6019A, in accor ance with IEEE C37.102 as the generator is for inst allation in the USA. Use 6000A nearest settable value. The voltage setting canno t be more than 85% of the generator rate voltage to ensure operation oes not o ccur un er normal operation. For this application, a value of 50% of rate volta ge is chosen. The generator has a m 17.22.2.4 Negative phase sequence protection aximum stea y-state capability of 8% of rating, an a value of Kg of10. Setting s of I2cmr = 0.06 (=480A) an Kg = 10 are therefore use . Minimum an maximumti me elays of 1s an 1300s are use to co-or inate with external protection an e nsure tripping at low levels of negative sequence current are use .17.22.2.5 Ov erfluxing protection The generator-transformer manufacturer supplie the followi ng characteristics: Alarm: V f >1.1 Trip: V Use a value of 935.3V, nearest settable value that ensures 90% of the win ing is time elay covere . A 0.5s efinite is use to prevent spurious trips. The thir harmonic voltage un er normal con itions is 2% of rate voltage, giving a valu e of: 18 kV ×0.02 3 = 207.8 V The setting of the thir harmonic un ervoltage protection must be below this val ue, a factor of 80% being acceptable. Use a value of 166.3V. A time elay of 0.5 s isuse . Inhibition of the element at low generator output requires eterminat ion uring commissioning. 17.22.2.7 Loss of excitation protection The client req uires a two-stage loss ofexcitation protection function. The first is alarm onl y, while the secon provi es tripping un er high loa con itions. To achieve thi s, the firstimpe ance elementof the P343 loss of excitation protection can be set in accor ance with the gui elines of Section 17.16.3 for a generator operati ng at rotor angles up to 120o, as follows: Xb1 = 0.5X = 1.666Ω Xa1 = 0.75X’ = 0.24 5Ω Use nearestsettable values of 1.669Ω an 0.253Ω. A time elay of 5s is use to pre vent alarms un er transient con itions. For the trip stage, settings for high lo a as given in Section 17.16.3 are use : X b2 = 18 2 kV 2 = =1.727 Ω MVA 187.65 •
17 • f >1.2 , inverse time characteristic time characteristic Hencethe alarm settingis 18000 ×1.05 60 =315 V Hz . A time elay of 5s is use to avoi alarms ue to transient con itions. The trip setting is 18000 ×1.2 60 = 360 V Hz . X a 2 = −0.75 X d = −0.1406 Ω ′ The nearest settable value for Xb2 is 1.725Ω. A time • 312 • Network Protection & Automation Gui e
elay of 0.5s is use .
17.22.2.8 Reverse power protection The manufacturer-supplie value for motoring power is 2% of rate power. The recommen e setting is therefore 1.6MW. An instr umentation class CT is use in conjunction with the relay for this protection, t o ensure accuracy of measurement. A time elay of 0.5s is use . The settings sho ul be checke at the commissioning stage. 17.22.2.9 Over/un er-frequency protec tion For un er-frequency protection, the client has specifie the following char acteristics: Alarm: 59.3Hz, 0.5s time elay 1st stage trip: 58.7Hz, 100s time e lay 2n stage trip: 58.2Hz, 1s time elay Similarly, the overfrequency is requir e to be set as follows: Alarm: 62Hz, 30s time elay Trip: 63.5Hz, 10s time ela y These characteristics can be set in the relay irectly. 17.22.2.10 Overvoltage protection The generator manufacturers’ recommen ation is: Alarm: 110% voltage fo r 5s Trip: 130% voltage, instantaneous This translates into thefollowing relay settings: Alarm: 19800V, 5s time elayTrip: 23400V, 0.1s time elay 17.22.2.11 Pole slipping protection This is provi e by the metho escribe in Section 17. 7.3.2. Detection at a maximum slip frequency of 10Hz is require . The setting a ta, accor ing to the relay manual, is as follows. Forwar reach, ZA = Zn + Zt = 0.02 + 0.22 = 0.24Ω Reverse reach, ZB = ZGen = 2 x X’ = 0.652Ω Reactance line, ZC = 0 .9 x Z = 0.9 x 0.22 = 0.198Ω where: Z1 = generator transformer leakage impe ance Z n = network impe ance Protection
Quantity Is1 Is2 K1 K2 Vn3H< Vn3H elay Vsnv Time Delay Xa1Xb1 t 1 Xa2 Xb2t 2 tDO1 Iset K V1set V2set I2>> Kg Kreset tmin tmax V> meas mo e V> operatemo e V >1 setting V>1 function V>1 time elay V>2 settingV>2 functionV>2 time elay P 1 function P1 setting P1 time elay P1 DO time Dea Mach I> Dea Mach V< Za Zb Z c α θ T1 T2 F>1 setting F>1 time delay F>2 setting F>2 time delay P1 function P1 set ting P1 time delay P1 DO time F<1 setting F<1 time delay F<2 setting F<2 time de lay F<3 setting F<3 time delay Value 8% 100% 0% 150% 166.3V 0.5s 935.3V 0.5s 0.245Ω 1.666Ω 5s -0.1406Ω 1.725Ω 0.5s 0s 7200A 3 14400V 3000V 0.06 10 10 1s 1300s three-phase any 19800V DT 5s 23400V DT 0.1s reverse power 1.6MW 0.5s 0s 6000A 9000V 0.243Ω 0.656Ω 0.206Ω 90° 80° 15ms 15ms 62Hz 3 0s 63.5Hz 10s reverse power 1.6MW 0.5s 0s 59.3Hz 0.5s 58.7Hz 100s 58.2Hz 1s Differential protection
100% Stator earth fault Neutral voltage isplacement Loss of excitation Voltage controlle overcurrent Negative phase sequence Overvoltage Un erfrequency Reverse Power Ina vertent energisation Pole Slipping Protection • Reverse Power Overfrequency
Un erfrequency Table 17.5: Relay settings for large generator protection example Network Protection & Automation Gui e • 313 •
Generator an Generator-Transfor mer P rotection 17 •
The nearest settable values are 0.243Ω, 0.656Ω, an 0.206Ω respectively. The lens angl e setting, α, is found from the equ tion: 1.54 − R l min α min =180 o −2 t n −1 (Z A + Z B ) nd, su stituting v lues, αmin = 62.5° Use the minimum sett le v lue of 90°. The lin der ngle, θ, is estimated to be 80°, and re uires checking during commissioning. Ti mers T1 and T2 are set to 15ms as experience has shown that these settings are s atisfactory to detect pole slipping freuencies up to 10Hz. This completes the s ettings reuired for the generator, and the relay settings are given in Table 17 .5. Of course, additional protection is reuired for the generator transformer, according to the principles described in Chapter 16. Generator and Generator Transfor mer P rotection 17.23 REFERENCES 17.1 Survey of Rate Of Change of Freuency Relays and Voltage P hase Shift Relays for Loss of Mains Protection. ERA Report 95 0712R, 1995. ERA T echnology Ltd. • 17 • • 314 • Network Protection & Automation Guide
• 18 • Industrial and Commercial Power System Protection Introduction Busbar arrangement Discrimination HRC fuses Industrial circuit brea kers Protection relays Co ordination problems Fault current contribution from in duction motors Automatic changeover systems Voltage and phase reversal protectio n Power factor correction and protection of capacitors Examples References 18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8 18.9 18.10 18.11 18.12 18.13
• 18 • Industrial and Commercial Power System P rotection 18.1 INTRODUCTION As industrial and commercial operations processes and plants h ave become more complex and extensive (Figure 18.1), the reuirement for improve d reliability of electrical power supplies has also increased. The potential cos ts of outage time following a failure of the power supply to a plant have risen dramatically as well. The introduction of automation techniues into industry an d commerce has naturally led to a demand for the deployment of more power system automation, to improve reliability and efficiency. Figure 18.1: Large modern industrial plant The protection and control of industrial power supply systems must be given care ful attention. Many of the techniues that have been evolved for EHV power syste ms may be applied to lower voltage systems also, but typically on a reduced scal e. However, industrial systems have many special problems that have warranted in dividual attention and the development of specific solutions. Many industrial pl ants have their own generation installed. Sometimes it is for emergency use only , feeding a limited number of busbars and with limited capacity. This arrangemen t is often adopted to ensure safe shutdown of process plant and personnel safety . In Network Protection & Automation Guide • 317 •
other plants, the nature of the process allows production of a substantial uant ity of electricity, perhaps allowing export of any surplus to the public supply system – at either at sub transmission or distribution voltage levels. Plants that run generation in parallel with the public supply distribution network are ofte n referred to as cogeneration or embedded generation. Special protection arrange ments may be demanded for the point of connection between the private and public Utility plant (see Chapter 17 for further details). Industrial systems typicall y comprise numerous cable feeders and transformers. Chapter 16 covers the protec tion of transformers and Chapters 9/10 the protection of feeders. 18.2 BUSBAR AR RANGEMENT The arrangement of the busbar system is obviously very important, and it can be uite complex for some very large industrial systems. However, in most systems a single busbar divided into sections by a bus section circuit breaker is common, as illustrated in Figure 18.2. Main and standby drives for a particul ar item of process euipment will be fed from different sections of the switchbo ard, or sometimes from different switchboards. of the standby generator facility. A standby generator is usually of the turbo c harged diesel driven type. On detection of loss of incoming supply at any switch board with an emergency section, the generator is automatically started. The app ropriate circuit breakers will close once the generating set is up to speed and rated voltage to restore supply to the Essential Services sections of the switch boards affected, provided that the normal incoming supply is absent for a typi cal diesel generator set, the emergency supply would be available within 10 20 s econds from the start seuence command being issued. Industrial and Commercial Power System Protection 110kV * NO 33kV A * NO B 6kV EDG NO HV supply 1 HV supply 2 A A * NO NO B B NC C 6kV 0.4kV * Transformer 1 2 out of 3 mechanical or electrical interlock Transformer 2 A NO * NO
NC B C 0.4kV NO • 18 • * A NO Bus section C Essential supplies EDG Emergency generator * Two out of three interlock B NC C 0.4kV Figure 18.2: Typical switchboard configuration for an industrial plant Figure 18.3: Typical industrial power system The main power system design criterion is that single outages on the electrical network within the plant will not cause loss of both the main and standby drives simultaneously. Considering a medium sized industrial supply system, illustrate d in Figure 18.3, in more detail, it will be seen that not only are duplicate su pplies and transformers used, but also certain important loads are segregated an d fed from ‘Essential Services Board(s)’ (also known as ‘Em rg ncy’ boards), distribut d throughout th plant. This nabl s maximum utilisation Th Ess ntial S rvic s Boards ar us d to f d quipm nt that is ss ntial for t h saf shut down, limit d op ration or pr s rvation of th plant and for th sa f ty of p rsonn l. This will cov r proc ss driv s ss ntial for saf shutdown, v nting syst ms, UPS loads f ding m rg ncy lighting, proc ss control comput rs, tc. Th m rg ncy g n rator may rang in siz from a singl unit rat d 2030kW in a small plant up to s v ral units of 2-10MW rating in a larg oil r fin ry or similar plant. Larg N twork Prot ction & Automation Guid • 318 •
financial trading institutions may also hav standby pow r r quir m nts of s v r al MW to maintain comput r s rvic s. 18.3 DISCRIMINATION Prot ction quipm nt wo rks in conjunction with switchg ar. For a typical industrial syst m, f d rs and plant will b prot ct d mainly by circuit br ak rs of various typ s and by fus d contactors. Circuit br ak rs will hav th ir associat d ov rcurr nt and arth fault r lays. A contactor may also b quipp d with a prot ction d vic ( .g. mo tor prot ction), but associat d fus s ar provid d to br ak fault curr nts in x c ss of th contactor int rrupting capability. Th rating of fus s and s l ction of r lay s ttings is carri d out to nsur that discrimination is achi v d – i. . th ability to s l ct and isolat only th faulty part of th syst m. 18.4 HRC FUSES Th prot ction d vic n ar st to th actual point of pow r utilisation is most lik ly to b a fus or a syst m of fus s and it is important that consid ra tion is giv n to th corr ct application of this important d vic . Th HRC fus is a k y fault cl aranc d vic for prot ction in industrial and comm rcial inst allations, wh th r mount d in a distribution fus board or as part of a contactor or fus -switch. Th latt r is r gard d as a vital part of LV circuit prot ction , combining saf circuit making and br aking with an isolating capability achi v d in conjunction with th r liabl short-circuit prot ction of th HRC fus . Fu s s combin th charact ristics of conomy and r liability; factors that ar mos t important in industrial applications. HRC fus s r main consist nt and stabl i n th ir br aking charact ristics in s rvic without calibration and maint nanc . This is on of th most significant factors for maintaining fault cl aranc dis crimination. Lack of discrimination through incorr ct fus grading will r sult i n unn c ssary disconn ction of suppli s, but if both th major and minor fus s a r HRC d vic s of prop r d sign and manufactur , this n d not ndang r p rsonn l or cabl s associat d with th plant. 18.4.1 Fus Charact ristics Th tim r qu ir d for m lting th fusibl l m nt d p nds on th magnitud of curr nt. This t im is known as th ‘pr -arcing’ tim of th fus . Vaporisation of th l m nt occur s on m lting and th r is fusion b tw n th vapour and th filling powd r l adi ng to rapid arc xtinction. Fus s hav a valuabl charact ristic known as ‘cut-off’, illustrat d in Figur 18.4. Wh n an unprot ct d circuit is subj ct d to a short circuit fault, th r.m.s. curr nt ris s towards a ‘prosp ctiv ’ (or maximum) valu . Th fus usually int rrupts th short circuit curr nt b for it can r ach th pr osp ctiv valu , in th first quart r to half cycl of th short circuit. Th ri sing curr nt is int rrupt d by th m lting of th fusibl l m nt, subs qu ntly dying away dying away to z ro during th arcing p riod. Curv of asymm trical prosp ctiv short-circuit curr nt Curr nt trac Ip Tim Start of short-circuit Pr -arcing tim Arcing tim 1 Cycl Figur 18.4: HRC fus cut-off f atur Total cl aranc tim f atur Sinc th l ctromagn tic forc s on busbars and conn ctions carrying sho rt circuit curr nt ar r lat d to th squar of th curr nt, it will b appr cia t d that ‘cut-off’ significantly r duc s th m chanical forc s produc d by th fault curr nt and which may distort th busbars and conn ctions if not corr ctly rat d. A typical xampl of ‘cut-off’ curr nt charact ristics is illustrat d in Figur 1 8.5. 1000 100
Cut off curr nt (p ak kA) 10 1250A 710A 800A 500A 630A 400A 200A 315A 125A 80A 50A 35A 25A 16A 6A 2A • 1.0 0.1 0.1 1.0 10 100 500 Prosp ctiv curr nt (kA r.m.s. symm trical) Figur 18.5: Typical fus cut-off curr nt charact ristics N twork Prot ction & Automation Guid • 319 • Industrial and Comm rcial Pow r Syst m Prot ction 18 •
It is possibl to us this charact ristic during th d sign stag of a proj ct t o utilis quipm nt with a low r fault withstand rating downstr am of th fus , than would b th cas if ‘cut-off’ was ignor d. This may sav on costs, but appropr iat docum ntation and maint nanc controls ar r quir d to nsur that only r p lac m nt fus s of v ry similar charact ristics ar us d throughout th lif tim of th plant conc rn d – oth rwis a saf ty hazard may aris . 18.4.2 Discriminatio n B tw n Fus s Fus s ar oft n conn ct d in s ri s l ctrically and it is ss n tial that th y should b abl to discriminat with ach oth r at all curr nt l v ls. Discrimination is obtain d wh n th larg r (‘major’) fus r mains unaff ct d by fault curr nts that ar cl ar d by th small r (‘minor’) fus . Th fus op rating t im can b consid r d in two parts: i. th tim tak n for fault curr nt to m lt th l m nt, known as th ‘pr -arcing tim ’ ii. th tim tak n by th arc produc d i nsid th fus to xtinguish and isolat th circuit, known as th ‘arcing tim ’ Th total n rgy dissipat d in a fus during its op ration consists of ‘pr -arcing n rgy’ and ‘arc n rgy’. Th valu s ar usually xpr ss d in t rms of I2t, wh r I is t h curr nt passing through th fus and t is th tim in s conds. Expr ssing th quantiti s in this mann r provid s an ass ssm nt of th h ating ff ct that th fus impos s on associat d quipm nt during its op ration und r fault condition s. To obtain positiv discrimination b tw n fus s, th total I2t valu of th m inor fus must not xc d th pr arcing I2t valu of th major fus . In practic , this m ans that th major fus will hav to hav a rating significantly high r than that of th minor fus , and this may giv ris to probl ms of discriminati on. Typically, th major fus must hav a rating of at l ast 160% of th minor f us for discrimination to b obtain d. 18.4.3 Prot ction of Cabl s by Fus s PVC cabl is allow d to b load d to its full nominal rating only if it has ‘clos xc ss curr nt prot ction’. This d gr of prot ction can b giv n by m ans of a fus link having a ‘fusing factor’ not xc ding 1.5, wh r : Fusing factor = 18.4.4 Eff ct of Ambi nt T mp ratur High ambi nt t mp ratur s can influ nc th capability of HRC fus s. Most fus s ar suitabl for us in ambi nt t mp ratur s up to 35°C, but for som fus ratings, d rating may b n c ssary at high r ambi nt t mp ratur s. Manufactur rs publish d lit ratur should b consult d for th d -rating factor to b appli d. 18.4.5 Prot ction of Motors Th manufactur rs lit ratur should also b consult d wh n fus s ar to b appli d to motor circui ts. In this application, th fus provid s short circuit prot ction but must b s l ct d to withstand th starting curr nt (possibly up to 8 tim s full load cur r nt), and also carry th normal full load curr nt continuously without d t rior ation. Tabl s of r comm nd d fus siz s for both ‘dir ct on lin ’ and ‘assist d start’ m otor applications ar usually giv n. Exampl s of prot ction using fus s ar giv n in S ction 18.12.1. 18.5 INDUSTRIAL CIRCUIT BREAKERS Som parts of an industri al pow r syst m ar most ff ctiv ly prot ct d by HRC fus s, but th r plac m nt of blown fus links can b particularly inconv ni nt in oth rs. In th s locati ons, circuit br ak rs ar us d inst ad, th r quir m nt b ing for th br ak r to int rrupt th maximum possibl fault curr nt succ ssfully without damag to its lf. In addition to fault curr nt int rruption, th br ak r must quickly disp rs th r sulting ionis d gas away from th br ak r contacts, to pr v nt r -striki ng of th arc, and away from oth r liv parts of quipm nt to pr v nt br akdown. Th br ak r, its cabl or busbar conn ctions, and th br ak r housing, must all b construct d to withstand th m chanical forc s r sulting from th magn tic f i lds and int rnal arc gas pr ssur produc d by th high st l v ls of fault curr nt to b ncount r d. Th typ s of circuit br ak r most fr qu ntly ncount r d in industrial syst m ar d scrib d in th following s ctions. 18.5.1 Miniatur C ircuit Br ak rs (MCB’s) MCB’s ar small circuit br ak rs, both in physical siz but mor importantly, in ratings. Th basic singl pol unit is a small, manually cl os d, l ctrically or manually op n d switch hous d in a mould d plastic casing. Th y ar suitabl for us on 230V a.c. singl -phas /400V a.c. thr -phas syst ms and for d.c. auxiliary supply syst ms, with curr nt ratings of up to 125A. Co ntain d within ach unit is a th rmal l m nt, in which a bim tal N twork Prot ction & Automation Guid
• Industrial and Comm rcial Pow r Syst m Prot ction 18 • Minimum Fusing Curr nt Curr nt Rating Cabl s construct d using oth r insulating mat rials ( .g. pap r, XLPE) hav no s p cial r quir m nts in this r sp ct. • 320 •
strip will trip th switch wh n xc ssiv curr nt pass s through it. This l m n t op rat s with a pr d t rmin d inv rs -tim /curr nt charact ristic. High r curr nts, typically thos xc ding 3-10 tim s rat d curr nt, trip th circuit br ak r without int ntional d lay by actuating a magn tic trip ov rcurr nt l m nt. T h op rating tim charact ristics of MCB’s ar not adjustabl . Europ an Standard E N 60898-2 d fin s th instantan ous trip charact ristics, whil th manufactur r can d fin th inv rs tim th rmal trip charact ristic. Th r for , a typical t ripping charact ristic do s not xist. Th maximum a.c. br aking curr nt p rmitt d by th standard is 25kA. Singl -pol units may b coupl d m chanically in gro ups to form 2, 3 or 4 pol units, wh n r quir d, by ass mbly on to a rail in a d istribution board. Th availabl ratings mak MCB s suitabl for industrial, com m rcial or dom stic applications, for prot cting quipm nt such as cabl s, light ing and h ating circuits, and also for th control and prot ction of low pow r m otor circuits. Th y may b us d inst ad of fus s on individual circuits, and th y ar usually ‘back d-up’ by a d vic of high r fault int rrupting capacity. Various acc ssory units, such as isolators, tim rs, and und rvoltag or shunt trip r l as units may b combin d with an MCB to suit th particular circuit to b contr oll d and prot ct d. Wh n p rsonn l or fir prot ction is r quir d, a r sidual c urr nt d vic (RCD) may b combin d with th MCB. Th RCD contains a miniatur c or balanc curr nt transform r that mbrac s all of th phas and n utral condu ctors to provid s nsitivity to arth faults within a typical rang of 0.05% to 1.5% of rat d curr nt, d p nd nt on th RCD s l ct d. Th cor balanc CT n rgi s s a common magn tic trip actuator for th MCB ass mbly. It is also possibl to obtain curr nt-limiting MCB’s. Th s typ s op n prior to th prosp ctiv fault cu rr nt b ing r ach d, and th r for hav similar prop rti s to HRC fus s. It is c laim d that th xtra initial cost is outw igh d by lif tim savings in r plac m nt costs aft r a fault has occurr d, plus th advantag of providing improv d p rot ction against l ctric shock if an RCD is us d. As a r sult of th incr as d saf ty provid d by MCB’s fitt d with an RCD d vic , th y ar t nding to r plac f us s, sp cially in n w installations. 18.5.2 Mould d Cas Circuit Br ak rs (MCC B’s) Th s circuit br ak rs ar broadly similar to MCB’s but hav th following impo rtant diff r nc s: a. th maximum ratings ar high r, with voltag ratings up to 1000V a.c./1200V d.c. Curr nt ratings of 2.5kA continuous/180kA r.m.s br ak ar possibl , d p nd nt upon pow r factor N twork Prot ction & Automation Guid b. th br ak rs ar larg r, comm nsurat with th l v l of ratings. Although ava ilabl as singl , doubl or tripl pol units, th multipl pol units hav a co mmon housing for all th pol s. Wh r fitt d, th switch for th n utral circuit is usually a s parat d vic , coupl d to th multi-pol MCCB c. th op rating l v ls of th magn tic and th rmal prot ction l m nts may b adjustabl , particu larly in th larg r MCCB’s d. b caus of th ir high r ratings, MCCB’s ar usually po sition d in th pow r distribution syst m n ar r to th pow r sourc than th MC B’s Car must b tak n in th short-circuit ratings of MCCB’s. MCCB’s ar giv n two br a king capaciti s, th high r of which is its ultimat br aking capacity. Th sign ificanc of this is that aft r br aking such a curr nt, th MCCB may not b fit for continu d us . Th low r, or s rvic , short circuit br aking capacity p rmit s continu d us without furth r d tail d xamination of th d vic . Th standard p rmits a s rvic br aking capacity of as littl as 25% of th ultimat br akin g capacity. Whil th r is no obj ction to us of MCCB’s to br ak short-circuit cu rr nts b tw n th s rvic and ultimat valu s, th insp ction r quir d aft r su ch a trip r duc s th us fuln ss of th d vic in such circumstanc s. It is also cl arly difficult to d t rmin if th magnitud of th fault curr nt was in xc ss of th s rvic rating. Th tim -d lay charact ristics of th magn tic or th rmal tim d trip, tog th r with th n c ssity for, or siz of, a back-up d vic v ari s with mak and siz of br ak r. Som MCCB’s ar fitt d with microproc ssorcon troll d programmabl trip charact ristics off ring a wid rang of such charact ristics. Tim –d lay d ov rcurr nt charact ristics may not b th sam as th stand
ard charact ristics for d p nd nt-tim prot ction stat d in IEC 60255-3. H nc , discrimination with oth r prot ction must b consid r d car fully. Th r can b probl ms wh r two or mor MCB’s or MCCB’s ar l ctrically in s ri s, as obtaining s l ctivity b tw n th m may b difficult. Th r may b a r quir m nt that th m ajor d vic should hav a rating of k tim s th minor d vic to allow discrimina tion, in a similar mann r to fus s – th manufactur r should b consult d as to va lu of k. Car ful xamination of manufactur rs’ lit ratur is always r quir d at t h d sign stag to d t rmin any such limitations that may b impos d by particu lar mak s and typ s of MCCB’s. An xampl of co-ordination b tw n MCCB’s, fus s and r lays is giv n in S ction 18.12.2. • • 321 • Industrial and Comm rcial Pow r Syst m Prot ction 18 •
. th appropriat Europ an sp cification is EN 60947-2
18.5.3 Air Circuit Br ak rs (ACB’s) Air circuit br ak rs ar fr qu ntly ncount r d on industrial syst ms rat d at 3.3kV and b low. Mod rn LV ACB’s ar availabl in curr nt ratings of up to 6.3kA with maximum br aking capaciti s in th rang of 85kA120kA r.m.s., d p nding on syst m voltag . This typ of br ak r op rat s on th principl that th arc produc d wh n th main contacts op n is controll d b y dir cting it into an arc chut . H r , th arc r sistanc is incr as d and h nc th curr nt r duc d to th point wh r th circuit voltag cannot maintain th arc and th curr nt r duc s to z ro. To assist in th qu nching of low curr nt arcs, an air cylind r may b fitt d to ach pol to dir ct a blast of air across th contact fac s as th br ak r op ns, so r ducing contact rosion. Air circui t br ak rs for industrial us ar usually withdrawabl and ar construct d with a flush front plat making th m id al for inclusion tog th r with fus switch s and MCB’s/MCCB’s in modular multi-ti r distribution switchboards, so maximising th numb r of circuits within a giv n floor ar a. Old r typ s using a manual or d p nd nt manual closing m chanism ar r gard d as b ing a saf ty hazard. This aris s und r conditions of closing th CB wh n a fault xists on th circuit b ing co ntroll d. During th clos trip op ration, th r is a dang r of gr ss of th arc from th casing of th CB, with a cons qu nt risk of injury to th op rator. Su ch typ s may b r quir d to b r plac d with mod rn quival nts. Industrial and Comm rcial Pow r Syst m Prot ction ACB’s ar normally fitt d with int gral ov rcurr nt prot ction, thus avoiding th n d for s parat prot ction d vic s. How v r, th op rating tim charact ristic s of th int gral prot ction ar oft n d sign d to mak discrimination with MCB’s/ MCCB’s/fus s asi r and so th y may not b in accordanc with th standard d p nd nt tim charact ristics giv n in IEC 60255-3. Th r for , probl ms in co-ordinati on with discr t prot ction r lays may still aris , but mod rn num rical r lays hav mor fl xibl charact ristics to all viat such difficulti s. ACB’s will also hav faciliti s for acc pting an xt rnal trip signal, and this can b us d in conjunction with an xt rnal r lay if d sir d. Figur 18.6 illustrat s th typic al tripping charact ristics availabl . 18.5.4 Oil Circuit Br ak rs (OCB’s) Oil cir cuit br ak rs hav b n v ry popular for many y ars for industrial supply syst m s at voltag s of 3.3kV and abov . Th y ar found in both ‘bulk oil’ and ‘minimum oil’ ty p s, th only significant diff r nc b ing th volum of oil in th tank. In thi s typ of br ak r, th main contacts ar hous d in an oil-fill d tank, with th oil acting as th both th insulation and th arc-qu nching m dium. Th arc prod uc d during contact s paration und r fault conditions caus s dissociation of th hydrocarbon insulating oil into hydrog n and carbon. Th hydrog n xtinguish s th arc. Th carbon produc d mix s with th oil. As th carbon is conductiv , th oil must b chang d aft r a pr scrib d numb r of fault cl aranc s, wh n th d gr of contamination r ach s an unacc ptabl l v l. B caus of th fir risk in volv d with oil, pr cautions such as th construction of fir /blast walls may ha v to b tak n wh n OCB’s ar install d. 18.5.5 Vacuum Circuit Br ak rs (VCB’s) Inv rs V ry Inv rs Ultra Inv rs Short Circuit • 18 • 1000 100 10 Tim (s) In r c nt y ars, this typ of circuit br ak r, along with CB’s using SF6, has r pl
ac d OCB’s for n w installations in industrial/comm rcial syst ms at voltag s of 3 .3kV and abov . Compar d with oil circuit br ak rs, vacuum br ak rs hav no fir risk and th y hav high r liability with long maint nanc fr p riods. A varia tion is th vacuum contactor with HRC fus s, us d in HV motor start r applicatio ns. 1 Curr nt (multipl of s tting) 10 20 1 0.1 0.01 18.5.6 SF6 Circuit Br ak rs In som countri s, circuit br ak rs using SF6 gas as th arc-qu nching m dium ar pr f rr d to VCB’s as th • 322 • Figur 18.6: Typical tripping charact ristics of an ACB N twork Prot ction & Automation Guid
CT conn ctions A (a) B C Phas l m nts R sidualcurr nt l m nts Syst m Typ of fault Not s 3Ph. 3w Ph. - Ph. P t rs n coil and un arth d syst ms. A (b) B C 3Ph. 3w (i) Ph. - Ph. (ii) Ph. - E* (c) 3Ph. 4w (i) Ph. - Ph. (ii) Ph. - E* (iii) Ph. - N A B C Phas l m nts must b in sam phas s at all stations. Earth-fault s ttings ma y b l ss than full load (d) 3Ph. 3w (i) Ph. - Ph. (ii) Ph. - E A ( ) B C 3Ph. 3w (i) Ph. - Ph. (ii) Ph. - E Earth-fault s ttings may b l ss than full load Earth-fault s ttings may b l ss than full load, but must b gr at r than larg st Ph. - N load (f) 3Ph. 4w (i) Ph. - Ph. (ii) Ph. - E (iii) Ph. - N
A B C N (g) 3Ph. 4w (i) Ph. - Ph. (ii) Ph. - E (iii) Ph. - N Earth-fault s ttings may b l ss than full load • A B C N (h) 3Ph. 3w or 3Ph. 4w Ph. - E Earth-fault s ttings may b l ss than full load Ph. = phas ; w = wir ; E = arth ; N = n utral Figur 18.7: Ov rcurr nt and arth fault r lay conn ctions N twork Prot ction & Automation Guid • 323 • Industrial and Comm rcial Pow r Syst m Prot ction 18 • * Earth-fault prot ction only if arth-fault curr nt is not l ss than twic prim ary op rating curr nt
r plac m nt for air- and oil-insulat d CB’s. Som mod rn typ s of switchg ar cubic l s nabl th us of ith r VCB’s or SF6-insulat d CB’s according to custom r r qui r m nts. Ratings of up to 31.5kA r.m.s. fault br ak at 36kV and 40kA at 24kV ar typical. SF6-insulat d CB’s also hav advantag s of r liability and maint nanc i nt rvals compar d to air- or oil-insulat d CB’s and ar of similar siz to VCB’s for th sam rating. 18.6 PROTECTION RELAYS Wh n th circuit br ak r its lf do s no t hav int gral prot ction, th n a suitabl xt rnal r lay will hav to b provi d d. For an industrial syst m, th most common prot ction r lays ar tim -d lay d ov rcurr nt and arth fault r lays. Chapt r 9 provid s d tails of th applicat ion of ov rcurr nt r lays. Traditionally, for thr wir syst ms, ov rcurr nt r lays hav oft n b n appli d to two phas s only for r lay l m nt conomy. Ev n with mod rn multi- l m nt r lay d signs, conomy is still a consid ration in t r ms of th numb r of analogu curr nt inputs that hav to b provid d. Two ov rcu rr nt l m nts will d t ct any int rphas fault, so it is conv ntional to apply two l m nts on th sam phas s at all r lay locations. Th phas CT r sidual cu rr nt conn ctions for an arth fault r lay l m nt ar unaff ct d by this conv n tion. Figur 18.7 illustrat s th possibl r lay conn ctions and limitations on s ttings. 18.7 CO-ORDINATION PROBLEMS Th r ar a numb r of probl ms that common ly occur in industrial and comm rcial n tworks that ar cov r d in th following s ctions. 18.7.1.1 Earth Fault prot ction with r sidually-conn ct d CT’s For four -wir syst ms, th r sidual conn ction of thr phas CT’s to an arth fault r lay l m nt will off r arth fault prot ction, but th arth fault r lay l m nt mu st b s t abov th high st singl -phas load curr nt to avoid nuisanc tripping . Harmonic curr nts (which may sum in th n utral conductor) may also r sult in spurious tripping. Th arth fault r lay l m nt will also r spond to a phas -n utral fault for th phas that is not cov r d by an ov rcurr nt l m nt wh r on ly two ov rcurr nt l m nts ar appli d. Wh r it is r quir d that th arth fau lt prot ction should r spond only to arth fault curr nt, th prot ction l m nt must b r sidually conn ct d to thr phas CT’s and to a n utral CT or to a cor -balanc CT. In this cas , ov rcurr nt prot ction must b appli d to all thr p has s to nsur that all phas -n utral faults will b d t ct d by ov rcurr nt pr ot ction. Placing a CT in th n utral arthing conn ction to driv an arth fault r lay provid s arth fault prot ction at th sourc of supply for a 4-wir syst m. If th n utral CT is omitt d, n utral curr nt is s n by th r lay as arth fault curr nt and th r lay s tting would hav to b incr as d to pr v nt tripping und r normal load conditions. Wh n an arth fault r lay is driv n from r sidually conn ct d CT’s, th r lay curr nt and tim s ttings must b such that that th prot ction will b stabl during th passag of transi nt CT spill curr nt through th r lay. Such spill curr nt can flow in th v nt of transi nt, asymm tric CT saturation during th passag of offs t f ault curr nt, inrush curr nt or motor starting curr nt. Th risk of such nuisanc tripping is gr at r with th d ploym nt of low imp danc l ctronic r lays rat h r than l ctrom chanical arth fault r lays which pr s nt d significant r lay circuit imp danc . En rgising a r lay from a cor balanc typ CT g n rally nab l s mor s nsitiv s ttings to b obtain d without th risk of nuisanc tripping with r sidually conn ct d phas CT’s. Wh n this m thod is appli d to a four-wir syst m, it is ss ntial that both th phas and n utral conductors ar pass d th rough th cor balanc CT ap rtur . For a 3wir syst m, car must b tak n with th arrang m nt of th cabl sh ath, oth rwis cabl faults involving th sh ath may not r sult in r lay op ration (Figur 18.8). Cabl gland Cabl box Industrial and Comm rcial Pow r Syst m Prot ction Cabl gland /sh ath ground conn ction I > •
18 • I > R lay do s not op rat (a) Incorr ct I > (b) Corr ct R lay op rat s Figur 18.8: CBCT conn ction for four-wir syst m • 324 • N twork Prot ction & Automation Guid
18.7.2 Four-Wir Dual-F d Substations Th co-ordination of arth fault r lays pr ot cting fourwir syst ms r quir s sp cial consid ration in th cas of low volt ag , dual-f d installations. Horch r [18.1] has sugg st d various m thods of ach i ving optimum coordination. Probl ms in achi ving optimum prot ction for common configurations ar d scrib d b low. 18.7.2.1 Us of 3-pol CB’s Wh n both n utral s ar arth d at th transform rs and all circuit br ak rs ar of th 3-pol typ , th n utral busbar in th switchg ar cr at s a doubl n utral to arth conn c tion, as shown in Figur 18.9. In th v nt of an uncl ar d f d r arth fault o r busbar arth fault, with both th incoming supply br ak rs clos d and th bus s ction br ak r op n, th arth fault curr nt will divid b tw n th two arth conn ctions. Earth fault r lay RE2 may op rat , tripping th supply to th h alt hy s ction of th switchboard as w ll as r lay RE1 tripping th supply to th fa ult d s ction. 18.7.2.2 Us of singl arth l ctrod A configuration som tim s adopt d with fo ur-wir dualf d substations wh r only a 3-pol bus s ction CB is us d is to us a singl arth l ctrod conn ct d to th mid-point of th n utral busbar in th switchg ar, as shown in Figur 18.10. Wh n op rating with both incoming main c ircuit br ak rs and th bus s ction br ak r clos d, th bus s ction br ak r must b op n d first should an arth fault occur, in ord r to achi v discrimination . Th co-ordination tim b tw n th arth fault r lays RF and RE should b sta blish d at fault l v l F2 for a substation with both incoming supply br ak rs an d bus s ction br ak r clos d. I RS1 Supply 1 F1 > I > RS2 I E > Supply 2 N IF/2 RE1 IF RE2 IF/2 IF/2 I RF > F2 Supply 1 IF/2 N utral busbar Bus s ction CB Supply 2 Figur 18.10: Dual f d four-wir syst ms: us of singl point n utral arthing IF Figur 18.9: Dual f d four-wir syst ms: us of 3-pol CB’s If only on incoming supply br ak r is clos d, th arth fault r lay on th n r gis d sid will s only a proportion of th fault curr nt flowing in th n utra l busbar. This not only significantly incr as s th r lay op rating tim but als
o r duc s its s nsitivity to lowl v l arth faults. Th solution to this probl m is to utilis 4-pol CB’s that switch th n utral as w ll as th thr phas s. Th n th r is only a singl arth fault path and r lay op ration is not compromis d. N twork Prot ction & Automation Guid Wh n th substation is op rat d with th bus s ction switch clos d and ith r on or both of th incoming supply br ak rs clos d, it is possibl for unbalanc d n utral busbar load curr nt caus d by singl phas loading to op rat r lay RS1 and/or RS2 and inadv rt ntly trip th incoming br ak r. Int rlocking th trip ci rcuit of ach RS r lay with normally clos d auxiliary contacts on th bus s ctio n br ak r can pr v nt this. How v r, should an arth fault occur on on sid of th busbar wh n r lays RS ar alr ady op rat d, it is possibl for a contact rac to occur. Wh n th bus s ction br ak r op ns, its br ak contact may clos b fo r th RS r lay trip contact on th h althy sid can op n (r s t). Raising th p ick-up l v l of r lays RS1 and RS2 abov th maximum unbalanc d n utral curr nt may pr v nt th tripping of both supply br ak rs in this cas . How v r, th b st solution is to us 4-pol circuit br ak rs, and • • 325 • Industrial and Comm rcial Pow r Syst m Prot ction 18 •
ind p nd ntly arth both sid s of th busbar. If, during a busbar arth fault or uncl ar d f d r arth fault, th bus s ction br ak r fails to op n wh n r quir d, th int rlocking br ak auxiliary contact will also b inop rativ . This will pr v nt r lays RS1 and RS2 from op rating and providing back-up prot ction, wit h th r sult that th fault must b cl ar d v ntually by slow r phas ov rcurr nt r lays. An alt rnativ m thod of obtaining back-up prot ction could b to con n ct a s cond r lay R’E, in s ri s with r lay RE, having an op ration tim s t lon g r than that of r lays RS1 and RS2. But sinc th additional r lay must b arra ng d to trip both of th incoming supply br ak rs, back-up prot ction would b o btain d but busbar s l ctivity would b lost. An xampl of prot ction of a typi cal dual-f d switchboard is giv n in S ction 18.12.3. 18.8 FAULT CURRENT CONTRIB UTION FROM INDUCTION MOTORS Wh n an industrial syst m contains motor loads, th motors will contribut fault curr nt for a short tim . Th y contribut to th to tal fault curr nt via th following m chanism. Wh n an induction motor is runnin g, a flux, g n rat d by th stator winding, rotat s at synchronous sp d and int racts with th rotor. If a larg r duction in th stator voltag occurs for any r ason, th flux in th motor cannot chang instantan ously and th m chanical in rtia of th machin will t nd to inhibit sp d r duction ov r th first f w c ycl s of fault duration. Th trapp d flux in th rotor g n rat s a stator voltag qual initially to th back .m.f. induc d in th stator b for th fault and d caying according to th X/R ratio of th associat d flux and curr nt paths. Th induction motor th r for acts as a g n rator r sulting in a contribution of c urr nt having both a.c. and d.c. compon nts d caying xpon ntially. Typical 50Hz motor a.c. tim constants li in th rang 10ms-60ms for LV motors and 60-200ms for HV motors. This motor contribution has oft n b n n gl ct d in th calculat ion of fault l v ls. Industrial syst ms usually contain a larg compon nt of mot or load, so this approach is incorr ct. Th contribution from motors to th tota l fault curr nt may w ll b a significant fraction of th total in syst ms havin g a larg compon nt of motor load. Standards r lating to fault l v l calculation s, such as IEC 60909, r quir th ff ct of motor contribution to b includ d wh r appropriat . Th y d tail th conditions und r which this should b don , and th calculation m thod to b us d. Guidanc is provid d on typical motor fault curr nt contribution for both HV and LV motors if th r quir d data is not known . Th r for , it is now r lativ ly asy, using appropriat calculation softwar , to d t rmin th magnit ud and duration of th motor contribution, so nabling a mor accurat ass ssm nt of th fault l v l for: a. discrimination in r lay co-ordination b. d t rmina tion of th r quir d switchg ar/busbar fault rating For prot ction calculations, motor fault l v l contribution is not an issu that is g n rally is important. In industrial n tworks, fault cl aranc tim is oft n assum d to occur at 5 cycl s aft r fault occurr nc , and at this tim , th motor fault l v l contribution is much l ss than just aft r fault occurr nc . In rar cas s, it may hav to b tak n into consid ration for corr ct tim grading for through-fault prot ction c onsid rations, and in th calculation of p ak voltag for high-imp danc diff r ntial prot ction sch m s. It is mor important to tak motor contribution into a ccount wh n consid ring th fault rating of quipm nt (busbars, cabl s, switchg ar, tc.). In g n ral, th initial a.c. compon nt of curr nt from a motor at th instant of fault is of similar magnitud to th dir ct-on-lin starting curr nt of th motor. For LV motors, 5xFLC is oft n assum d as th typical fault curr n t contribution (aft r taking into account th ff ct of motor cabl imp danc ), with 5.5xFLC for HV motors, unl ss it is known that low starting curr nt HV moto rs ar us d. It is also acc pt d that similar motors conn ct d to a busbar can b lump d tog th r as on quival nt motor. In doing so, motor rat d sp d may n d to b tak n into consid ration, as 2 or 4 pol motors hav a long r fault cur r nt d cay than motors with a gr at r numb r of pol s. Th kVA rating of th sin gl quival nt motor is tak n as th sum of th kVA ratings of th individual mo tors consid r d. It is still possibl for motor contribution to b n gl ct d in cas s wh r th motor load on a busbar is small in comparison to th total load (again IEC 60909 provid s guidanc in this r sp ct). How v r, larg LV motor loa
ds and all HV motors should b consid r d wh n calculating fault l v ls. 18.9 AU TOMATIC CHANGEOVER SYSTEMS Induction motors ar oft n us d to driv critical loa ds. In som industrial applications, such as thos involving th pumping of flui ds and gas s, this has l d to th n d for a pow r supply control sch m in whic h motor and oth r loads ar transf rr d automatically on loss of th normal supp ly to an alt rnativ supply. A quick chang ov r, nabling th motor load to b r -acc l rat d, r duc s th possibility of a proc ss trip occurring. Such sch m s ar commonly appli d for larg g n rating units to transf r unit loads from th unit transform r to th station supply/start-up transform r. N twork Prot ction & Automation Guid • Industrial and Comm rcial Pow r Syst m Prot ction 18 • • 326 •
Two m thods of automatic transf r ar us d: a. in-phas transf r syst m b. r sid ual voltag syst m Pr f rr d f d r Standby f d r Phas angl r lay ϕ< High speed CB Two eeders are used, supplying two busbar sections connected by a normally open bus section breaker. Each eeder is capable o carrying the total busbar load. Each bus section voltage is monitored and loss o supply on either section cause s the relevant incomer CB to open. Provided there are no protection operations t o indicate the presence o a busbar ault, the bus section breaker is closed aut omatically to restore the supply to the unpowered section o busbar a ter the re sidual voltage generated by the motors running down on that section has allen t o a an acceptable level. This is between 25% and 40%, o nominal voltage, depend ent on the characteristics o the power system. The choice o residual voltage s etting will in luence the reacceleration current a ter the bus section breaker c loses. For example, a setting o 25% may be expected to result in an inrush curr ent o around 125% o the starting current at ull voltage. Alternatively, a tim e delay could be used as a substitute or residual voltage measurement, which wo uld be set with knowledge o the plant to ensure that the residual voltage would have decayed su iciently be ore trans er is initiated. The protection relay se ttings or the switchboard must take account o the total load current and the v oltage dip during the re acceleration period in order to avoid spurious tripping during this time. This time can be several seconds where large inertia HV drive s are involved. 18.10 VOLTAGE AND PHASE REVERSAL PROTECTION Voltage relays have been widely used in industrial power supply systems. The principle purposes are to detect undervoltage and/or overvoltage conditions at switchboards to disconne ct supplies be ore damage can be caused rom these conditions or to provide inte rlocking checks. Prolonged overvoltage may cause damage to voltage sensitive equ ipment (e.g. electronics), while undervoltage may cause excessive current to be M (a) In phase trans er method Feeder No.1 Feeder No.2 • Ursd < Ursd < M M (b) Residual voltage method M M Figure 18.11: Auto trans er systems Network Protection & Automation Guide • 327 • Industrial and Commercial Power System Protection 18 • When the normal supply ails, induction motors that remain connected to the busb
ar slow down and the trapped rotor lux generates a residual voltage that decays exponentially. All motors connected to a busbar will tend to decelerate at the same rate when the supply is lost i they remain connected to the busbar. This i s because the motors will exchange energy between themselves, so that they tend to stay ‘synchronised’ to ach oth r. As a r sult, th r sidual voltag s of all th motors d cay at n arly th sam rat . Th magnitud of this voltag and its phas displac m nt with r sp ct to th h althy alt rnativ supply voltag is a funct ion of tim and th sp d of th motors. Th angular displac m nt b tw n th r sidual motor voltag and th incoming voltag will b 180° at som instant. If th h althy alt rnativ supply is switch d on to motors which ar running down und r th s conditions, v ry high inrush curr nts may r sult, producing str ss s whi ch could b of suffici nt magnitud to caus m chanical damag , as w ll as a s v r dip in th alt rnativ supply voltag . Th in-phas transf r m thod is illustrat d in Figur 18.11(a). Normal and stand by f d rs from th sam pow r sourc ar us d. Phas angl m asur m nt is us d to s ns th r lativ phas angl b tw n th standby f d r voltag and th mot or busbar voltag . Wh n th voltag s ar approximat ly in phas , or just prior t o this condition through pr diction, a high-sp d circuit br ak r is us d to com pl t th transf r. This m thod is r strict d to larg high in rtia driv s wh r th gradual run down charact ristic upon loss of normal f d r supply can b pr dict d accurat ly. Figur 18.11(b) illustrat s th r sidual voltag m thod, whi ch is mor common, sp cially in th p troch mical industry.
drawn by motor loads. Motors ar provid d with th rmal ov rload prot ction to pr v nt damag with xc ssiv curr nt, but und rvoltag prot ction is commonly app li d to disconn ct motors aft r a prolong d voltag dip. With a voltag dip caus d by a sourc syst m fault, a group of motors could d c l rat to such a d gr that th ir aggr gat r -acc l ration curr nts might k p th r cov ry voltag d pr ss d to a l v l wh r th machin s might stall. Mod rn num rical motor prot ction r lays typically incorporat voltag prot ction functions, thus r moving t h n d for discr t und rvoltag r lays for this purpos (s Chapt r 19). Old r installations may still utilis discr t und rvoltag r lays, but th s tting crit ria r main th sam . Capacitor kvar kW ϕ1 kV A V ϕ2 kVA lo 2 a with co d current mpensa tion Industrial and Commercial Power System Protection Reverse phase sequence voltage protection should be applied where it may be dang erous or a motor to be started with rotation in the opposite direction to that intended. Incorrect rotation due to reverse phase sequence might be set up ollo wing some error a ter power system maintenance or repairs, e.g. to a supply cabl e. Older motor control boards might have been itted with discrete relays to det ect this condition. Modern motor protection relays may incorporate this unction . I reverse phase sequence is detected, motor starting can be blocked. I rever se phase sequence voltage protection is not provided, the high set negative phas e sequence current protection in the relay would quickly detect the condition on ce the starting device is closed – ut initi l reverse rot tion of the motor could not e prevented. 18.11 POWER FACTOR CORRECTION AND PROTECTION OF CAPACITORS Lo ds such s induction motors dr w signific nt re ctive power from the supply sys tem, nd poor over ll power f ctor m y result. The flow of re ctive power incr e ses the volt ge drops through series re ct nces such s tr nsformers nd re ct ors, it uses up some of the current c rrying c p city of power system pl nt nd it incre ses the resistive losses in the power system. To offset the losses nd restrictions in pl nt c p city they incur nd to ssist with volt ge regul tion, Utilities usu lly pply t riff pen lties to l rge industri l or commerci l cust omers is t for running their pl nt t excessively low power f ctor. The customer here y induced to improve the power f ctor of his system nd it m y e cost effe ctive to inst ll fixed or v ri le power f ctor correction equipment to r ise or regul te the pl nt power f ctor to n ccept le level. Shunt c p citors re of ten used to improve power f ctor. The sis for compens tion is illustr ted in F igure 18.12, where ϕ1 represents the uncorrected power actor angle and ϕ2 the angle r elating to the desired power actor, a ter correction. Figure 18.12: Power actor correction principle The ollowing may be deduced rom this vector diagram: Uncorrected power actor = = cos ϕ1 Corrected power actor = = cos ϕ2 Reduction in kVA = kVA1 kVA2 I the kW lo ad and uncorrected power actors are known, then the capacitor rating in kvar to achieve a given degree o correction may be calculated rom: Capacitor kvar = k W x (tan cos ϕ1 tan cos ϕ2) A spreadsheet can easily be constructed to calculate the req uired amount o compensation to achieve a desired power actor. 18.11.1 Capacito r Control Where the plant load or the plant power actor varies considerably, it is necessary to control the power actor correction, since over correction will result in excessive system voltage and unnecessary losses. In a ew industrial systems, capacitors are switched in manually when required, but automatic contro llers are standard practice. A controller provides automatic power actor correc tion, by comparing the running power actor with
kW kVA 2 kW kVA 1 • 18 • • 328 • Network Protection & Automation Guide Compensating kvar Magnetising kvar 1 lo co ad c mp ur en ren sa t w tio n itho ut
From incoming trans ormer Metering Trip P1 11kV Lockout I >> I >> I> I > Metering U> U< PFC/V Controller P2 Id> Capacitor bank • I> I> I> * I>> * I>> * I>> * Element uses Network Protection & Automation Guide • 329 • Figure 18.13: Protection o capacitor banks Industrial and Commercial Power System Protection 18 • the target value. Based on the available groupings, an appropriate amount o cap acitance is switched in or out to maintain an optimum average power actor. The controller is itted with a ’loss of voltag ’ r lay l m nt to nsur that all s l c t d capacitors ar disconn ct d instantan ously if th r is a supply voltag int rruption. Wh n th supply voltag is r stor d, th capacitors ar r conn ct d p rogr ssiv ly as th plant starts up. To nsur that capacitor groups d grad at roughly th sam rat , th controll r usually rotat s s l ction or randomly s l cts groups of th sam siz in ord r to v n out th conn ct d tim . Th provisi on of ov rvoltag prot ction to trip th capacitor bank is also d sirabl in som applications. This would b to pr v nt a s v r syst m ov rvoltag if th pow r factor corr ction (PFC) controll r fails to tak fast corr ctiv action. Th d sign of PFC installations must r cognis that man y industrial loads g n rat harmonic voltag s, with th r sult that th PFC capa citors may sink significant harmonic curr nts. A harmonic study may b n c ssary to d t rmin th capacitor th rmal ratings or wh th r s ri s filt rs ar r quir d. 18.11.2 Motor P.F. Corr ction Wh n d aling with pow r factor corr ction of m otor loads, group corr ction is not always th most
conomical m thod. Som industrial consum rs apply capacitors to s l ct d motor substations rath r than applying all of th corr ction at th main incoming subs tation busbars. Som tim s, pow r factor corr ction may v n b appli d to indivi dual motors, r sulting in optimum pow r factor b ing obtain d und r all conditio ns of aggr gat motor load. In som instanc s, b tt r motor starting may also r sult, from th improv m nt in th voltag r gulation du to th capacitor. Motor capacitors ar oft n six-t rminal units, and a capacitor may b conv ni ntly co nn ct d dir ctly across ach motor phas winding. Capacitor sizing is important, such that a l ading pow r factor do s not occur und r any load condition. If x c ss capacitanc is appli d to a motor, it may b possibl for s lf- xcitation t o occur wh n th motor is switch d off or suff rs a supply failur . This can r s ult in th production of a high voltag or in m chanical damag if th r is a su dd n r storation of supply. Sinc most star/d lta or auto-transform r start rs o th r than th ‘Korndorff r’ typ s involv a transitional br ak in supply, it is g n rally r comm nd d that th capacitor rating should not xc d 85% of th motor m agn tising r activ pow r. 18.11.3 Capacitor Prot ction Wh n consid ring prot ct ion for capacitors, allowanc should b mad for th transi nt inrush curr nt oc curring on switch-on, sinc this can r ach p ak valu s of around 20 tim s normal curr nt. Switchg ar for us with capacitors is usually d -rat d consid rably to allow for this. Inrush curr nts may b limit d by a r sistor in s ri s with ac h capacitor or bank of capacitors. Prot ction quipm nt is r quir d to pr v nt r uptur of th capacitor du to an int rnal fault and also to prot ct th cabl s and associat d quipm nt from damag in cas of a capacitor failur . If fus pro t ction is cont mplat d for a thr -phas capacitor, HRC fus s should b mploy d with a curr nt rating of not l ss than 1.5 tim s th rat d capacitor curr nt. M dium voltag capacitor banks can b prot ct d by th sch m shown in Figur 18 .13. Sinc harmonics incr as capacitor curr nt, th r lay will r spond mor cor r ctly if it do s not hav in-built tuning for harmonic r j ction. Doubl star c apacitor banks ar mploy d at m dium voltag . As shown in Figur 18.14, a curr nt transform r in th int r star-point conn ction can b us d to driv a prot ct ion r lay to d t ct th out-of-balanc curr nts that will flow wh n capacitor l m nts b com shortcircuit d or op n-circuit d. Th r lay will hav adjustabl c urr nt s ttings, and it might contain a bias circuit, f d from an xt rnal volta g transform r, that can b adjust d to comp nsat for st ady-stat spill curr n t in th int r star-point conn ction. A B C Capacitor bank Industrial and Comm rcial Pow r Syst m Prot ction IU > Alarm Trip Figur 18.14: Prot ction of doubl star capacitor banks Som industrial loads such as arc furnac s involv larg inductiv compon nts an d corr ction is oft n appli d using v ry larg high voltag capacitors in variou s configurations. Anoth r high voltag capacitor configuration is th ‘split phas ’ arrang m nt wh r th l m nts making up ach phas of th capacitor ar split i nto two parall l paths. Figur 18.15 shows two possibl conn ction m thods for t h r lay. A diff r ntial r lay can b appli d with a curr nt transform r for ac h parall l branch. Th r lay compar s th curr nt in th split phas s, using s n sitiv curr nt s ttings but also adjustabl comp nsation for th unbalanc curr nts arising from initial capacitor mismatch. 18.12 EXAMPLES In this s ction, xa mpl s of th topics d alt with in th Chapt r ar consid r d. 18.12.1 Fus Co-or
dination An xampl of th application of fus s is bas d on th arrang m nt in F igur 18.16(a). This shows an unsatisfactory sch m with commonly ncount r d sh ortcomings. It can b s n that fus s B, C and D will discriminat with fus A, but th 400A sub-circuit fus E may not discriminat , with th 500A sub-circuit fus D at high r l v ls of fault curr nt. • 18 • • 330 • N twork Prot ction & Automation Guid
A Rating 1000A A I> Rating 500A B Rating 500A C D Rating 500A B I> Rating 400A E Rating 30A ach F I> (a) Incorr ct layout giving ris to probl ms in discrimination Rating 1000A A Alarm (a) A Trip Rating 400A E Rating 100A B Rating 500A C Rating 500A D I> Rating 30A B F I> (b) Corr ct layout and discrimination Figur 18.16: Fus prot ction: ff ct of layout on discrimination C I> Alarm (b)
Trip How v r, th r ar industrial applications wh r discrimination is a s condary f actor. In th application shown in Figur 18.17, a contactor having a fault rati ng of 20kA controls th load in on sub-circuit. A fus rating of 630A is s l ct d for th minor fus in th contactor circuit to giv prot ction within th thr oughfault capacity of th contactor. Th major fus of 800A is chos n, as th mi nimum rating that is gr at r than th total load curr nt on th switchboard. Dis crimination b tw n th two fus s is not obtain d, as th pr -arcing I2t of th 800A fus is l ss than th total I2t of th 630A fus . Th r for , th major fus will blow as w ll as th minor on , for most faults so that all oth r loads f d from th switchboard will b lost. This may b acc ptabl in som cas s. In mos t cas s, how v r, loss of th compl t switchboard for a fault on a singl outgo ing circuit will not b acc ptabl , and th d sign will hav to b r vis d. • Figur 18.15: Diff r ntial prot ction of split phas capacitor banks Th solution, illustrat d in Figur 18.16(b), is to f d th 400A circuit E dir ct from th busbars. Th sub-circuit fus D may now hav its rating r duc d from 500A to a valu , of say 100A, appropriat to th r maining subcircuit. This arr ang m nt now provid s a discriminating fus distribution sch m satisfactory for an industrial syst m. N twork Prot ction & Automation Guid • 331 • Industrial and Comm rcial Pow r Syst m Prot ction 18 • C
800A 400V 630A Fus d contactor With th CT ratio of 2000/1A and a r lay r s t ratio of 95% of th nominal curr nt s tting, a curr nt s tting of at l ast 80% would b satisfactory, to avoid tr ipping and/or failur to r s t with th transform r carrying full load curr nt. How v r, choic of a valu at th low r nd of this curr nt s tting rang would mov th r lay charact ristic towards that of th MCCB and discrimination may b lost at low fault curr nts. It is th r for prud nt to s l ct initially a r lay curr nt s tting of 100%. 18.12.2.2 R lay charact ristic and tim multipli r s l ction An EI charact ristic is s l ct d for th r lay to nsur discrimination w ith th fus (s Chapt r 9 for d tails). From Figur 18.19, it may b s n that at th fault l v l of 40kA th fus will op rat in l ss than 0.01s and th MCC B op rat s in approximat ly 0.014s. Using a fix d grading margin of 0.4s, th r quir d r lay op rating tim b com s 0.4 + 0.014 = 0.414s. With a CT ratio of 200 0/1A, a r lay curr nt s tting of 100%, and a r lay TMS s tting of 1.0, th xtr m ly inv rs curv giv s a r lay op rating tim of 0.2s at a fault curr nt of 40 kA. This is too fast to giv ad quat discrimination and indicat s that th EI c urv is too s v r for this application. Turning to th VI r lay charact ristic, th r lay op ration tim is found to b 0.71s at a TMS of 1.0. To obtain th r quir d r lay op rating tim of 0.414s: TMS s tting = 0.414 0.71 Industrial and Comm rcial Pow r Syst m Prot ction Auxiliary circuits Figur 18.17: Exampl of back-up prot ction 18.12.2 Grading of Fus s/MCCB’s/ Ov rcurr nt R lays An xampl of an application i nvolving a mould d cas circuit br ak r, fus and a prot ction r lay is shown in Figur 18.18. A 1MVA 3.3kV/400V transform r f ds th LV board via a circuit br ak r, which is quipp d with a MiCOM P141 num rical r lay having a s tting rang of 8-400% of rat d curr nt and f d from 2000/1A CT’s. Fus 1MVA 2000/1A = 0.583 10.0 LV board fault l v l = 30kA MCCB 400A 3300/415V I>> I> Charact ristic for r lay Figur 18.18: N twork diagram for prot ction co-ordination xampl –fus /MCCB/r la y 18.12.2.1 D t rmination of r lay curr nt s tting Th r lay curr nt s tting chos n must not b l ss than th full load curr nt l v l and must hav nough margin to allow th r lay to r s t with full load curr nt flowing. Th latt r may b d t rmin d from th transform r rating: FLC = = kVA kV x 3 1000 0.4 × 3 = 1443 A Op rating tim (s)
• 18 • 1.0 Discrimination is r quir d b tw n th r lay and both th fus and MCCB up to th 40kA fault rating of th board. To b gin with, th tim /curr nt charact ristic s of both th 400A fus and th MCCB ar plott d in Figur 18.19. Fus 0.1 MCCB 0.01 1000 10,000 100,000 Op rating curr nt (A) to 415V bas R vis d r lay charact ristic O riginal r lay charact ristic Figur 18.19: Grading curv s for Fus /MCCB/r lay grading xampl • 332 • N twork Prot ction & Automation Guid
Us a TMS of 0.6, n ar st availabl s tting. Th us of a diff r nt form of inv rs tim charact ristic mak s it advisabl to ch ck discrimination at th low r curr nt l v ls also at this stag . At a fault curr nt of 4kA, th r lay will op rat in 8.1s, which do s not giv discrimination with th MCCB. A r lay op ratio n tim of 8.3s is r quir d. To ov rcom this, th r lay charact ristic n ds to b mov d away from th MCCB charact ristic, a chang that may b achi v d by usi ng a TMS of 0.625. Th r vis d r lay charact ristic is also shown in Figur 18.1 9. 18.12.3 Prot ction of a Dual-F d Substation As an xampl of how num rical pr ot ction r lays can b us d in an industrial syst m, consid r th typical larg industrial substation of Figur 18.20. Two 1.6MVA, 11/0.4kV transform rs f ding a busbar whos buss ction CB is normally op n. Th LV syst m is solidly arth d . Th larg st outgoing f d r is to a motor rat d 160kW, 193kVA, and a starting curr nt of 7 x FLC. amount of motor load. Th contribution of motor load to th fault l v l at th s witchboard is usually larg r than that from a singl inf ding transform r, as t h transform r r stricts th amount of fault curr nt inf d from th primary sid . Th thr -phas br ak fault l v l at th switchboard und r th s conditions i s assum d to b 40kA rms. R lays C ar not r quir d to hav dir ctional charact ristics (s S ction 9.14.3) as all thr circuit br ak rs ar only clos d mom n tarily during transf r from a singl inf ding transform r to two inf ding tran sform rs configuration. This transf r is normally an automat d s qu nc , and th chanc of a fault occurring during th short p riod (of th ord r of 1s) wh n a ll thr CB’s ar clos d is tak n to b n gligibly small. Similarly, although this configuration giv s th larg st fault l v l at th switchboard, it is not consi d r d from ith r a switchboard fault rating or prot ction vi wpoint. It is assu m d that mod rn num rical r lays ar us d. For simplicity, a fix d grading margi n of 0.3s is us d. 18.12.3.2 Motor prot ction r lay s ttings From th motor char act ristics giv n, th ov rcurr nt r lay s ttings (R lay A) can b found using t h guid lin s s t out in Chapt r 19 as: Th rmal l m nt: 1.6 MVA 11/0.4kV Z=6.25% R lay C1 > I >> 2500/1 2500/1 NO 2500/1 I >> R lay C2 curr nt s tting: 300A tim constant: 20 mins Instantan ous l m nt: curr nt s tt ing: 2.32kA Th s ar th only s ttings r l vant to th upstr am r lays. 18.12.3 .3 R lay B s ttings R lay B s ttings ar d riv d from consid ration of th loadi ng and fault l v ls with th bus-s ction br ak r b tw n busbars A1 and A2 clos d. No information is giv n about th load split b tw n th two busbars, but it can b assum d in th abs nc of d finitiv information that ach busbar is capa bl of supplying th total load of 1.6MVA. With fix d tap transform rs, th bus voltag may fall to 95% of nominal und r th s conditions, l ading to a load cur r nt of 2430A. Th IDMT curr nt s tting must b gr at r than this, to avoid r la y op ration on normal load curr nts and (id ally) with aggr gat starting/r -acc l ration curr nts. If th ntir load on th busbar was motor load, an aggr gat starting curr nt in xc ss of 13kA would occur, but a curr nt s tting of this ord r would b xc ssiv ly high and l ad to grading probl ms furth r upstr am. I t is unlik ly that th ntir load is motor load (though this do s occur, sp ci ally wh r a supply voltag of 690V is chos n for motors – an incr asingly common practic ) or that all A2 0.4kV 50kA rms >> I >> I> R lay B Trip 300/1
> I >> R lay A Motor cabl M 160kW • Figur 18.20: R lay grading xampl for dual-f d switchboard Th transform r imp danc is to IEC standards. Th LV switchg ar and bus bars ar fault rat d at 50kA rms. To simplify th analysis, only th phas -fault LV pro t ction is consid r d. 18.12.3.1 G n ral consid rations Analysis of many substat ions configur d as in Figur 18.20 shows that th maximum fault l v l and f d r load curr nt is obtain d with th bus-s ction circuit br ak r clos d and on of th inf ding CB’s op n. This appli s so long as th switchboard has a significan t N twork Prot ction & Automation Guid • 333 • Industrial and Comm rcial Pow r Syst m Prot ction 18 •
Industrial and Comm rcial Pow r Syst m Prot ction Th high-s t s tting n ds to b abov th full load curr nt and motor starting/ r -acc l ration transi nt curr nt, but l ss than th fault curr nt by a suitabl margin. A s tting of 12.5kA is initially s l ct d. A tim d lay of 0.3s has to us d to nsur grading with r lay A at high fault curr nt l v ls; both r lays A and B may s a curr nt in xc ss of 25kA for faults on th cabl sid of th CB f ding th 160kW motor. Th r lay curv s ar illustrat d in Figur 18.21. Tim (s) motors ar start d simultan ously (but simultan ous r acc l ration may w ll occu r). What is ss ntial is that r lay B do s not issu a trip command und r th s circumstanc s –i. . th r lay curr nt/tim charact ristic is in xc ss of th curr nt/tim charact ristic of th worst-cas starting/r -acc l ration condition. It is th r for assum d that 50% of th total bus load is motor load, with an av r ag starting curr nt of 600% of full load curr nt (= 6930A), and that r -acc l r ation tak s 3s. A curr nt s tting of 3000A is th r for initially us d. Th SI c haract ristic is us d for grading th r lay, as co-ordination with fus s is not r quir d. Th TMS is r quir d to b s t to grad with th th rmal prot ction of r lay A und r ‘cold’ conditions, as this giv s th long st op ration tim of R lay A , and th r acc l ration conditions. A TMS valu of 0.41 is found to provid sat isfactory grading, b ing dictat d by th motor starting/r -acc l ration transi n t. Adjustm nt of both curr nt and TMS s ttings may b r quir d d p nding on th xact r -acc l ration conditions. Not that low r curr nt and TMS s ttings could b us d if motor starting/r -acc l ration did not n d to b consid r d. Th curr nt s tting has to b abov that for r lay B to achi v full co-ordinati on, and a valu of 3250A is suitabl . Th TMS s tting using th SI charact risti c is chos n to grad with that of r lay B at a curr nt of 12.5kA (r lay B instan tan ous s tting), and is found to b 0.45. Th high-s t l m nt must grad with that of r lay B, so a tim d lay of 0.62s c is r quir d. Th curr nt s tting mus t b high r than that of r lay B, so us a valu of 15kA. Th final r lay gradin g curv s and s ttings ar illustrat d in Figur 18.22. R lay A R lay B R lay C R lay A lay R lay B lay R lay C Valu Param t r Valu 300A Tim const 1200s TMS 0.175 0.25 2750A TMS R -acc l ration R lay A s tting R lay B s tting g R lay C s tting dinst dinst I> I> I>> 15000 tdinst Valu 0 0.32s 0.62s (a) R lay s ttings 1000 100 10 1 I> I> I>> I>> 0.1 0.01 100 10000 1000 Curr nt (A) r f rr d to 0.4kV (b) Grading curv s 100000 Figur 18.22: Final r lay grading curv s 18.12.3.5 Comm nts on grading
R lay A R lay B R -acc l ration R lay A s tting lay g R lay B s tting 1000 • 18 • 100 Tim (s) 10 1 0.1 0.01 100 1000 10000 100000 Curr nt (A) r f rr d to 0.4kV Figur 18.21: Grading of r lays A and B 18.12.3.4 R lays C has to b suitabl iting condition is for r lays C.
s ttings Th s tting of th IDMT l m nt of r lays C1 and C2 for prot cting th busbar whil grading with r lay B. Th lim grading with r lay B, as this giv s th long st op ration tim
Whil th abov grading may app ar satisfactory, th prot ction on th primary s id of th transform r has not b n consid r d. IDMT prot ction at this point wi ll hav to grad with r lays C and with th through-fault shorttim withstand cu rv s of th transform r and cabling. This may r sult in xc ssiv ly long op rati on tim s. Ev n if th op ration tim at th 11kV l v l is satisfactory, th r is probably a Utility inf d to consid r, which will involv a furth r s t of r la ys and anoth r stag of tim grading, and th fault cl aranc tim at th utilit y inf d will almost c rtainly b xc ssiv . On solution is to acc pt a total l oss of supply to th 0.4kV bus und r conditions of a singl inf d and bus s cti on CB clos d. This is achi v d by s tting r lays C such that grading with r lay B do s not occur at all curr nt l v ls, or omitting r lay B from th prot ction sch m . Th argum nt for this is that n twork op ration policy is to nsur loss of supply to both s ctions of th switchboard do s not occur for singl conting nci s. As singl inf d op ration is not normal, a conting ncy (wh th r fault o r maint nanc ) has alr ady occurr d, so that a furth r fault causing total loss of supply to th switchboard N twork Prot ction & Automation Guid • 334 •
through tripping of on of r lays B is a s cond conting ncy. Total loss of suppl y is th r for acc ptabl . Th alt rnativ is to acc pt a lack of discrimination at som point on th syst m, as alr ady not d in Chapt r 9. Anoth r solution is to mploy partial diff r ntial prot ction to r mov th n d for R lay A, but t his is s ldom us d. Th strat gy adopt d will d p nd on th individual circumsta nc s. 18.13 REFERENCES 18.1 Ov rcurr nt R lay Co-ordination for Doubl End d Sub stations. G org R Horch r. IEEE. Vol. 1A-14 No. 6 1978. • N twork Prot ction & Automation Guid • 335 • Industrial and Comm rcial Pow r Syst m Prot ction 18 •
• 19 • A.C. Motor Prot ction Introduction Mod rn r lay d sign Th rmal (Ov rload) prot ction Start/Stall prot ction Short circuit prot ction Earth fault prot ction N gativ phas s qu nc pr ot ction Wound rotor induction motor prot ction RTD t mp ratur d t ction B arin g failur s Und rvoltag prot ction Loss-of-load prot ction Additional prot ction for synchronous motors Motor prot ction xampl s 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 19.10 19.11 19.12 19.13 19.14
• 19 • A.C. Motor P rot ction 19.1 INTRODUCTION Th r ar a wid rang of a.c. motors and motor charact ristic s in xist nc , b caus of th num rous duti s for which th y ar us d. All moto rs n d prot ction, but fortunat ly, th mor fundam ntal probl ms aff cting th choic of prot ction ar ind p nd nt of th typ of motor and th typ of load to which it is conn ct d. Th r ar som important diff r nc s b tw n th prot ction of induction motors and synchronous motors, and th s ar fully d alt with in th appropriat s ction. Motor charact ristics must b car fully consid r d wh n applying prot ction; whil this may b r gard d as stating th obvious, it is mphasis d b caus it appli s mor to motors than to oth r it ms of pow r sys t m plant. For xampl , th starting and stalling curr nts/tim s must b known w h n applying ov rload prot ction, and furth rmor th th rmal withstand of th m achin und r balanc d and unbalanc d loading must b cl arly d fin d. Th condit ions for which motor prot ction is r quir d can b divid d into two broad cat go ri s: impos d xt rnal conditions and int rnal faults. Tabl 19.1 provid s d tai ls of all lik ly faults that r quir prot ction. Ext rnal Faults Unbalanc d suppli s Und rvoltag s Singl phasing R v rs phas s qu nc Tabl 19.1: Caus s of motor failur s Int rnal faults B aring failur s Wi nding faults Ov rloads 1 9 . 2 M O D E R N R E L AY D E S I G N Th d sign of a mod rn motor prot ction r lay must b ad quat to cat r for th prot ction n ds of any on of th vast rang of motor d signs in s rvic , many of th d signs having no p rmissibl al lowanc for ov rloads. A r lay off ring compr h nsiv prot ction will hav th f ollowing s t of f atur s: a. th rmal prot ction b. xt nd d start prot ction c. stalling prot ction N twork Prot ction & Automation Guid • 337 •
d. numb r of starts limitation . short circuit prot ction f. arth fault prot c tion g. winding RTD m asur m nt/trip h. n gativ s qu nc curr nt d t ction i. u nd rvoltag prot ction j. loss-of-load prot ction k. out-of-st p prot ction l. l oss of supply prot ction m. auxiliary supply sup rvision (it ms k and l apply to synchronous motors only) In addition, r lays may off r options such as circuit br ak r condition monitoring as an aid to maint nanc . Manufactur rs may also of f r r lays that impl m nt a r duc d functionality to that giv n abov wh r l ss compr h nsiv prot ction is warrant d ( .g. induction motors of low rating). Th following s ctions xamin ach of th possibl failur mod s of a motor and d iscuss how prot ction may b appli d to d t ct that mod . h at at a rat proportional to t mp ratur ris . This is th principl b hind th ‘th rmal r plica’ mod l of a motor us d for ov rload prot ction. Th t mp ratur T where: Tmax = final s eady at any instant is giv n by: T = Tmax (1 - -t/τ) s a e empera ure τ = hea ing ime cons an Tempera ure rise is propor ional o he cur ren squared: 2 T = KI R (1 − e − τ )
where: if flowing coninuously, produces empera IR = curren which, ure Tmax in he mo or ha , for any overload curren I , he per Therefore, i can beshown missible ime for his curren o flow is: A.C. Mo or Pro ec ion 1 9 . 3 T H E R M A L ( O V E R L O A D ) P R OT E C T I O N The majori yof win ding failures are ei her indirec ly or direc ly causedby overloading (ei her pr olonged or cyclic), opera ion on unbalanced or single phasing, w supply vol age, hich all lead hrough excessive hea ing o he de eriora ion of he winding insu la ion un il an elec rical faul occurs. The generally accep ed rule is ha ins ula ion life is halved for each 10° C rise in empera ure above he ra edvalue, m odified by he leng h of ime spen a he higher empera ure. As an elec rical machine has a relaivelylarge hea s orage capaci y, i follows ha infrequen overloads of shor dura ion may no adversely affec hemachine. However, sus ained overloads of only a few percen may in prema resul ure ageingand insula ion failure. Fur hermore, he hermal wi hs and capabili y of he mo or isaffec ed by hea ing in he winding prior o a faul . I is herefore impor an ha he relay charac eris ic akes accoun of he ex remes of zero and full load pre faul curren known respec ively as he 'Cold' and 'Ho ' condi ions. The varie y of mo or designs, diverse applica ions, varie y of possible abnormal opera ing condi ions and resul resul in acomplex hermal relaionsh ing modes offailure ip. ha is accura e is herefore impossible o cre A generic ma hema ical model ae. However, i is possible o develop an approxima e model if i is assumed h a he mo or is a homogeneous body, crea ing and dissipa ing 1 − (I R I ) In general, hesupply o which a mo or is 1 = τ loge 2 y con ain componen s, and bo h posiive and nega ive sequence bo h componen s of curren give rise o hea ing in he mo or. Therefore, he shoul hermal replica d ake in o accoun bo h of hese componen s, a ypical equa ion for he equival en curren being: { } I eq = where (I
2 1 2 + KI 2 )
I1 = posi ive sequence curren I2 = nega ive sequence curren and nega ive seque nce ro or resis ance K= ——————————————————— —posi ive sequence roor resis ance a e ofK is3. Finally, he replica model needs o ake ino accoun he f hermal ac ha he mo or will end o cool down during periods of ligh load, and he ini ial sa e of he mo or. The mo or will have a cooling ime cons an , τr , ha defines he ra e of cooling. Hence, he final hermal model can be expressed as : • 19 • = τ log e k 2 − A 2 ( ) (k 2 −1 )
…Equa ion 19.1 • 338 • Ne work Pro ec ion & Au oma ion Guide
where: 1 9 . 4 S TA R T / S TA L L P R OT E C T I O N τ k
= hea ing ime cons an = I eq I h A 2 = ini ial s a e ofmo or (cold or ho ) I h = hermal se ing curren Equa ion 19.1 akes in o accoun he ‘cold’ and ‘hot’ charact ristics d fin d in IEC 602 55, part 8. Som r lays may us a dual slop charact ristic for th h ating tim constant, and h nc two valu s of th h ating tim constant ar r quir d. Switc hing b tw n th two valu s tak s plac at a pr -d fin d motor curr nt. This may b us d to obtain b tt r tripping p rformanc during starting on motors that us a stard lta start r. During starting, th motor windings carry full lin curr nt, whil in th ‘run’ condition, th y carry only 57% of th curr nt s n by th r l ay. Similarly, wh n th motor is disconn ct d from th supply, th h ating tim constant τ is se ime cons an τr. Since he equal o he cooling relay should idea lly be ma ched o he pro ec ed mo or and be capable of close sus ained overload pro ec ion, a wide range of relay adjus men is desirable oge her wi h good ac curacy and low hermal overshoo . Typical relay se ing curves are shown in Figu re 19.1. 100 000 When a mo raing or is s ar ed,i draws a curren well in excess of full load hroughou heperiod ha he mo or akes o run up o speed. While he mo ors ar ing curren reduces somewha as mo or speed increases, i is normal inpro ec ion prac ice o assume ha he mo or curren remains cons an hroughou he s ar ing period. The s ar ing curren will vary depending on he design of he mo or and me hod of s ar ing. For mo ors s ar ed DOL (direc on line), he nominal saring curren can be 4 8 imes when as ar del a full load curren . However, s ar er is used, he line curren will only be 1 DOL s ar ing curren . 3 of he Should o s ar , due o excessive loading, a mo or s all whils running, or fail he mo or will draw a curren equal o i s’ lock d rotor curr nt. It is not th r for possibl to distinguish b tw n a stall condition and a h althy start sol l y on th basis of th curr nt drawn. Discrimination b tw n th two conditions m ust b mad bas d on th duration of th curr nt drawn. For motors wh r th sta rting tim is l ss than th saf stall tim of th motor, prot ction is asy to arrang . 19.4.1 Exc ssiv Start Tim /Lock d Rotor Prot ction A motor may fail to acc l ra t from r st for a numb r of r asons: 10 000 T 1 =60min 2 T 1 T 2=54min T 1=T 2=48min T T =T =42min 1 2=36min loss of a supply phas m chanical probl ms low supply voltag xc ssiv load tor qu tc. A larg curr nt will b drawn from th supply, and caus xtr m ly high t mp ratur s to b g n rat d within th motor. This is mad wors by th fact t hat th motor is not rotating, and h nc no cooling du to rotation is availabl . Winding damag will occur v ry quickly – ith r to th stator or rotor windings d p nding on th th rmal limitations of th particular d sign (motors ar said t o b stator or rotor limit d in this r sp ct). Th m thod of prot ction vari s d p nding on wh th r th starting tim is l ss than or gr at r than th saf stal l tim . In both cas s, initiation of th start may b s ns d by d t ction of th closur of th switch in th motor f d r (contactor or CB) and optionally curr nt rising abov a starting curr nt thr shold valu – typically
1000 • 100 Op rating tim (s conds) 10 T =T 2=30min T2 T 1=T 2=24min T T 1 1 =12min T 1 2=6min min T 1 2 =1min 0 1 th rmal thr shold Iθ> Ie in terms of the current 10 Figure 19.1: Thermal overload characteristic curves Cold curves. Initial thermal state 0% Network Protection & Automation Guide • 339 • A.C. Motor Protection 19 • However, where motors are used to drive high inertia loads, the stall withstand time can be less than the starting time. In these cases, an additional means mus t be provided to enable discrimination between the two conditions to be achieved .
200% of motor rated current. For the case of both conditions being sensed, they may have to occur within a narrow aperture of time for a start to be recognised. Special reuirements may exist for certain types of motors installed in hazardo us areas (e.g. motors with type of protection EEx ‘e’) and th s tting of th r lay must tak th s into account. Som tim s a p rmissiv int rlock for machin pr ss urisation (on EEx ‘p’ machin s) may b r quir d, and this can b conv ni ntly achi v d by us of a r lay digital input and th in-built logic capabiliti s. 19.4.1.1 Start tim < saf stall tim Prot ction is achi v d by us of a d finit tim o v rcurr nt charact ristic, th curr nt s tting b ing gr at r than full load curr nt but l ss than th starting curr nt of th machin . Th tim s tting should b a littl long r than th start tim , but l ss than th p rmitt d saf starting tim of th motor. Figur 19.2 illustrat s th principl of op ration for a suc c ssful start. Figur 100 19.2. R lay s tting for succ ssful start: start tim stall tim 1 CB Clos d Curr nt Sp d Switch Information Trip Command 0 1 0 1 0 1 0 Tim Tim Tim (a) Succ ssful start CB Clos d Curr nt Sp d Switch Information Trip Command 1 0 1 0 1 0 1 0 Stall tim s tting Tim A.C. Motor Prot ction Tim Tim (b) Unsucc ssful start Tim (s) Figur 19.3: Motor start prot ction Start tim > Saf stall tim 19.4.2 Stall Prot ction 1 • 19 • 0.1 0.1 1 Curr nt (p.u. ) 10 Should a motor stall wh n running or b unabl to start b caus of xc ssiv loa d, it will draw a curr nt from th supply quival nt to th lock d rotor curr nt . It is obviously d sirabl to avoid damag by disconn cting th machin as quic kly as possibl if this condition aris s. Motor stalling can b r cognis d by th motor curr nt xc ding th start curr nt thr shold aft r a succ ssful start – i . . a motor start has b n d t ct d and th motor curr nt has dropp d b low th start curr nt thr shold within th motor saf start tim . A subs qu nt ris in m
otor curr nt abov th motor starting curr nt thr shold is th n indicativ of a stall condition, and tripping will occur if this condition p rsists for gr at r than th s tting of th stall tim r. An instantan ous ov rcurr nt r lay l m nt provid s prot ction. In many syst ms, transi nt supply voltag loss (typically u p to 2 s conds) do s not r sult in tripping of d signat d motors. Th y ar allow d to r -acc l rat upon r storation of th supply. During r -acc l ration, th y Figur 19.2: Motor start prot ction start tim < saf stall tim 19.4.1.2 Start tim => saf stall tim For this condition, a d finit tim ov rc urr nt charact ristic by its lf is not suffici nt, sinc th tim d lay r quir d is long r than th maximum tim that th motor can b allow d to carry starting curr nt saf ly. An additional m ans of d t ction of rotor mov m nt, indicating a saf start, is r quir d. A sp d-s nsing switch usually provid s this function . D t ction of a • 340 • N twork Prot ction & Automation Guid
Motor start d t ction 1 0 Tim No. of starts 1 0 Sup rvising tim Sup rvising tim Start lockout 1 Tim 0 Tim Inhib. start tim Figur 19.4: Start lockout information. draw a curr nt similar to th starting curr nt for a p riod that may b s v ral s conds. It is thus abov th motor stall r lay l m nt curr nt thr shold. Th s tall prot ction would b xp ct d to op rat and d f at th obj ct of th r -acc l ration sch m . A motor prot ction r lay will th r for r cognis th pr s nc of a voltag dip and r cov ry, and inhibit stall prot ction for a d fin d p rio d. Th und rvoltag prot ction l m nt (S ction 19.11) can b us d to d t ct th pr s nc of th voltag dip and inhibit stall prot ction for a s t p riod aft r voltag r cov ry. Prot ction against stall d motors in cas of an unsucc ssful r -acc l ration is th r for maintain d. Th tim d lay s tting is d p nd nt on th r acc l ration sch m adopt d and th charact ristics of individual motors. It should b stablish d aft r p rforming a transi nt stability study for th r acc l ration sch m propos d. 19.4.3 Numb r of Starts Limitation Any motor has a r striction on th numb r of starts that ar allow d in a d fin d p riod without th p rmitt d winding, tc. t mp ratur s b ing xc d d. Starting should b block d if th p rmitt d numb r of starts is xc d d. Th situation is complicat d by th fact th numb r of p rmitt d ‘hot’ starts in a giv n p riod is l ss than th numb r of ‘cold’ starts, du to th diff ring initial t mp ratur s of th motor. Th r lay must maintain a s par at count of ‘cold’ and ‘hot’ starts. By making us of th data h ld in th motor th rma l r plica, ‘hot’ and ‘cold’ starts can b distinguish d. To allow th motor to cool down b tw n starts, a tim d lay may b sp cifi d b tw n cons cutiv starts (again distinguishing b tw n ‘hot’ and ‘cold’ starts). Th start inhibit is r l as d aft r a tim d t rmin d by th motor sp cification. Th ov rall prot ction function is i llustrat d in Figur 19.4. • N twork Prot ction & Automation Guid • 341 • A.C. Motor Prot ction 19 •
1 9 . 5 S H O R T- C I R C U I T P R OT E C T I O N Motor short-circuit prot cti on is oft n provid d to cat r for major stator winding faults and t rminal flash ov rs. B caus of th r lativ ly gr at r amount of insulation b tw n phas wind ings, faults b tw n phas s s ldom occur. As th stator windings ar compl t ly nclos d in ground d m tal, th fault would v ry quickly involv arth, which wo uld th n op rat th instantan ous arth fault prot ction. A singl d finit tim ov rcurr nt r lay l m nt is all that is r quir d for this purpos , s t to abo ut 125% of motor starting curr nt. Th tim d lay is r quir d to pr v nt spuriou s op ration du to CT spill curr nts, and is typically s t at 100ms. If th moto r is f d from a fus d contactor, co-ordination is r quir d with th fus , and th is will probably involv us of a long tim d lay for th r lay l m nt. Sinc t h obj ct of th prot ction is to provid rapid fault cl aranc to minimis dama g caus d by th fault, th prot ction is ff ctiv ly worthl ss in th s circums tanc s. It is th r for only provid d on motors f d via circuit br ak rs. Diff r ntial (unit) prot ction may b provid d on larg r HV motors f d via circuit br ak rs to prot ct against phas phas and phas - arth faults, particularly wh r t h pow r syst m is r sistanc - arth d. Damag to th motor in cas of a fault oc curring is minimis d, as th diff r ntial prot ction can b mad quit s nsitiv and h nc d t cts faults in th ir arly stag s. Th normal d finit tim ov rcu rr nt prot ction would not b suffici ntly s nsitiv , and s nsitiv arth fault prot ction may not b provid d. Th us r may wish to avoid th d tail d calculat ions r quir d of capacitanc curr nt in ord r to s t s nsitiv non-dir ctional arth fault ov rcurr nt prot ction corr ctly on HV syst ms (Chapt r 9) or th r m ay b no provision for a VT to allow application of dir ctional s nsitiv arth fault prot ction. Th r is still a low r limit to th s tting that can b appli d, du to spill curr nts from CT saturation during starting, whil on som motor s, n utral curr nt has b n found to flow during starting, v n with balanc d su pply voltag s, that would caus th diff r ntial prot ction to op rat . For d ta ils on th application of diff r ntial prot ction, r f r to Chapt r 10. How v r, non-dir ctional arth fault ov rcurr nt prot ction will normally b ch ap r in cas s wh r ad quat s nsitivity can b provid d. 1 9 . 6 E A R T H F A U LT P R OT E C T I O N On of th most common faults to occur on a motor is a stator wi nding fault. What v r th initial form of th fault (phas -phas , tc.) or th c aus (cyclic ov rh ating, tc.), th pr s nc of th surrounding m tallic fram and casing will nsur that it rapidly d v lops into a fault involving arth. Th r for , provision of arth fault prot ction is v ry important. Th typ and s n sitivity of prot ction provid d d p nds larg ly on th syst m arthing, so th v arious typ s will b d alt with in turn. It is common, how v r, to provid both instantan ous and tim -d lay d r lay l m nts to cat r for major and slowly d v loping faults. 19.6.1 Solidly-Earth d Sys t m Most LV syst ms fall into this cat gory, for r asons of p rsonn l saf ty. Tw o typ s of arth fault prot ction ar commonly found – d p nding on th s nsitivit y r quir d. For applications wh r a s nsitivity of > 20% of motor continuous ra t d curr nt is acc ptabl , conv ntional arth fault prot ction using th r sidua l CT conn ction of Figur 19.5 can b us d. A low r limit is impos d on th s tt ing by possibl load unbalanc and/or (for HV syst ms) syst m capacitiv curr nt s. Figur 19.5. prot ction c Upstr am a R sidual CT conn ction for arth fault b c Ia a Flow of curr nt b
A.C. Motor Prot ction Ib Ic Ia+Ib+Ic MiCOM P241 Downstr am Figur 19.5: R sidual CT conn ction for arth fault prot ction • 19 • Car must b tak n to nsur that th r lay do s not op rat from th spill curr nt r sulting from un qual CT saturation during motor starting, wh r th high c urr nts involv d will almost c rtainly saturat th motor CT’s. It is common to us a stabilising r sistor in s ri s with th r lay, with th valu b ing calculat d using th formula: R stab = I st IO ( Rct + kR l + R r ) …Equation 19.2 wh r : Ist I0 Rstab Rct Rl = starting curr nt r f rr d to CT s condary = r lay arth fault s tting (A) = st abilising r sistor valu (ohms) = d.c. r sistanc of CT s condary (ohms) = CT si ngl l ad r stistanc (ohms) • 342 • N twork Prot ction & Automation Guid
k = CT conn ction factor (= 1 for star pt at CT = 2 for star pt at r lay) Rr = r lay input r stistanc (ohms) Th ff ct of th stabilising r sistor is to incr as th ff ctiv s tting of th r lay und r th s conditions, and h nc d lay t ripping. Wh n a stabilising r sistor is us d, th tripping charact ristic should normally b instantan ous. An alt rnativ t chniqu , avoiding th us of a stab ilising r sistor is to us a d finit tim d lay charact ristic. Th tim d lay us d will normally hav to b found by trial and rror, as it must b long noug h to pr v nt malop ration during a motor start, but short nough to provid ff ctiv prot ction in cas of a fault. Co-ordination with oth r d vic s must also b consid r d. A common m ans of supplying a motor is via a fus d contactor. Th contactor its lf is not capabl of br aking fault curr nt b yond a c rtain valu , which will normally b b low th maximum syst m fault curr nt – r lianc is pla c d on th fus in th s circumstanc s. As a trip command from th r lay instruc ts th contactor to op n, car must b tak n to nsur that this do s not occur until th fus has had tim to op rat . Figur 19.6(a) illustrat s incorr ct gra ding of th r lay with th fus , th r lay op rating first for a rang of fault curr nts in xc ss of th contactor br aking capacity. Figur 19.6(b) illustrat s corr ct grading. To achi v this, it may r quir th us of an int ntional d f init tim d lay in th r lay. If a mor s nsitiv r lay s tting is r quir d, it is n c ssary to us a cor -bal anc CT. This is a ring typ CT, through which all phas s of th supply to th m otor ar pass d, plus th n utral on a four-wir syst m. Th turns ratio of th CT is no long r r lat d to th normal lin curr nt xp ct d to flow, so can b c hos n to optimis th pickup curr nt r quir d. Magn tising curr nt r quir m nts ar also r duc d, with only a singl CT cor to b magn tis d inst ad of thr , thus nabling low s ttings to b us d. Figur 19.7 illustrat s th application o f a cor -balanc CT, including th routing of th cabl sh ath to nsur corr ct op ration in cas of cor -sh ath cabl faults. Cabl gland Cabl box Cabl gland /sh ath ground conn ction (a) Conn ction SEF No op ration SEF (b) Incorr ct wiring Tim Fus Contactor br aking capacity E/F r lay • Curr nt (a) Incorr ct Op ration SEF (c) Corr ct wiring Tim Fus Contactor br aking capacity E/F r lay Figur 19.7: Application of cor -balanc CT 19.6.2 R sistanc -Earth d Syst ms Th s ar commonly found on HV syst ms, wh r th int ntion is to limit damag caus d by arth faults through limiting th ar th fault curr nt that can flow. Two m thods of r sistanc arthing ar commonly us d: • 343 • Curr nt (b) Corr ct
Figur 19.6: Grading of r lay with fus d contactor N twork Prot ction & Automation Guid A.C. Motor Prot ction 19 •
19.6.2.1 Low r sistanc arthing In this m thod, th valu of r sistanc is chos n to limit th fault curr nt to a f w hundr d amps – valu s of 200A-400A b ing ty pical. With a r sidual conn ction of lin CT’s, th minimum s nsitivity possibl i s about 10% of CT rat d primary curr nt, du to th possibility of CT saturation during starting. For a cor -balanc CT, th s nsitivity that is possibl using a simpl non-dir ctional arth fault r lay l m nt is limit d to thr tim s th st ady-stat charging curr nt of th f d r. Th s tting should not b gr at r than about 30% of th minimum arth fault cur r nt xp ct d. Oth r than this, th consid rations in r sp ct of s ttings and ti m d lays ar as for solidly arth d syst ms. Ia1 Ib1 IR1 -jXc1 A.C. Motor Prot ction IH1 Ia2 Ib2 IR2 -jXc2 • 19 • IH2 Ia3 Ib3 IH1+IH2+IH3 IR3 -jXc3 IR3=IH1+ IH2+ IH3-IH3:IR3= IH1+ IH2 IH3 IH1+IH2 Figur 19.8: Curr nt distribution in insulat d- arth syst m for phas - arth faul t • 344 • N twork Prot ction & Automation Guid
19.6.2.2 High r sistanc arthing In som HV syst ms, high r sistanc arthing i s us d to limit th arth fault curr nt to a f w amps. In this cas , th syst m capacitiv charging curr nt will normally pr v nt conv ntional s nsitiv arth f ault prot ction b ing appli d, as th magnitud of th charging curr nt will b comparabl with th arth fault curr nt in th v nt of a fault. Th solution is to us a s nsitiv dir ctional arth fault r lay. A cor balanc CT is us d in conjunction with a VT m asuring th r sidual voltag of th syst m, with a r lay charact ristic angl s tting of +45° (s Chapt r 9 for d tails). Th VT must b suitabl for th r lay and th r for th r lay manufactur r should b consult d ov r suitabl typ s – som r lays r quir that th VT must b abl to carry r sidu al flux and this rul s out us of a 3-limb, 3-phas VT. A s tting of 125% of th singl phas capacitiv charging curr nt for th whol syst m is possibl using this m thod. Th tim d lay us d is not critical but must b fast nough to dis conn ct quipm nt rapidly in th v nt of a s cond arth fault occurring imm dia t ly aft r th first. Minimal damag is caus d by th first fault, but th s con d ff ctiv ly r mov s th curr nt limiting r sistanc from th fault path l adin g to v ry larg fault curr nts. An alt rnativ t chniqu using r sidual voltag d t ction is also possibl , and is d scrib d in th n xt s ction. 19.6.3 Insulat d Earth Syst m Earth fault d t ction pr s nts probl ms on th s syst ms sinc n o arth fault curr nt flows for a singl arth fault. How v r, d t ction is stil l ss ntial as ov rvoltag s occur on sound phas s and it is n c ssary to locat and cl ar th fault b for a s cond occurs. Two m thods ar possibl , d t ction of th r sulting unbalanc in syst m charging curr nts and r sidual ov rvoltag . 19.6.3.1 Syst m charging curr nt unbalanc S nsitiv arth fault prot ction usi ng a cor -balanc CT is r quir d for this sch m . Th principl is that d tail d in S ction 9.16.2, xc pt that th voltag is phas shift d by +90° inst ad of -9 0°. To illustrat this, Figur 19.8 shows th curr nt distribution in an Insulat d syst m subj ct d to a C-phas to arth fault and Figur 19.9 th r lay v ctor d iagram for this condition. Th r sidual curr nt d t ct d by th r lay is th sum of th charging curr nts flowing in th h althy part of th syst m plus th h a lthy phas charging curr nts on th fault d f d r – i. . thr tim s th p r phas charging curr nt of th h althy part of th syst m. A r lay s tting of 30% of this valu can b us d to provid prot ction without th risk of a trip du to h althy syst m capacitiv charging curr nts. As th r is no arth fault curr nt, it is also possibl to s t th r lay at sit aft r d lib rat ly applying arth faults at various parts of th syst m and m asuring th r sulting r sidual curr nts. If it is possibl to s t th r lay to a valu b tw n th ch arging curr nt on th f d r b ing prot ct d and th charging curr nt for th r st of th syst m, th dir ctional facility is not r quir d and th VT can b dis p ns d with. Th comm nts mad in arli r s ctions on grading with fus d contact ors also apply. Vaf IR1 Vapf Ib1 Op rat Vbf Vcpf Vr s (=-3Vo) V Vbpf R strain Ia1 An RCA s tting of +90° shifts th MTA to h r IR3 H1+ IH2) Figur 19.9: R lay v ctor diagram 19.6.3.2 R sidual voltag m thod A singl arth fault r sults in a ris in th v oltag b tw n syst m n utral and arth, which may b d t ct d by a r lay m asur ing th r sidual voltag of th syst m (normally z ro for a p rf ctly balanc d, h althy syst m). Thus, no CT’s ar r quir d, and th t chniqu may b us ful wh r provision of an xt nsiv numb r of cor -balanc CT’s is impossibl or difficult,
du to physical constraints or on cost grounds. Th VT’s us d must b suitabl fo r th duty, thus 3-limb, 3-phas VT’s ar not suitabl , and th r lay usually has alarm and trip s ttings, ach with adjustabl tim d lays. Th s tting voltag m ust b calculat d from knowl dg of syst m arthing and imp danc s, an xampl f or a r sistanc arth d syst m is shown in Figur 19.10. Grading of th r lays mu st b carri d out with car , as th r sidual voltag will b d t ct d by all r l ays in th aff ct d s ction of th syst m. Grading has to b carri d out with th is in mind, and will g n rally b on a tim basis for providing alarms (1st stag ), with a high s t d finit tim trip s cond stag to provid backup. • N twork Prot ction & Automation Guid • 345 • A.C. Motor Prot ction 19 •
19.6.4 P t rs n Coil Earth d Syst m Earthing of a HV pow r syst m using a r acto r qual to th syst m shunt capacitanc is known as P t rs n Coil (or r sonant c oil) arthing. With this m thod, a singl arth fault r sults in z ro arth faul t curr nt flowing (for p rf ct balanc b tw n th arthing inductanc and syst m shunt capacitanc ), and h nc th syst m can b run in this stat for a substa ntial p riod of tim whil th fault is locat d and corr ct d. Th d tail d th o ry and prot ction m thod is xplain d in S ction 9.17. 19.7 NEGATIVE PHASE SEQUE NCE PROTECTION N gativ phas s qu nc curr nt is g n rat d from any unbalanc d voltag condition, such as unbalanc d loading, loss of a singl phas , or singl -phas faults. Th latt r will normall y b d t ct d by arth fault prot ction, how v r, a fault location in a motor wi nding may not r sult in th arth fault prot ction op rating unl ss it is of th s nsitiv vari ty. Th actual valu of th n gativ s qu nc curr nt d p nds on th d gr of unbalanc in th supply voltag and th ratio of th n gativ to th positiv s qu nc imp danc of th machin . Th d gr of unbalanc d p nds on many factors, but th n gativ s qu nc imp danc is mor asily d t rmin d. Consid ring th classical induction motor quival nt circuit with magn tising im p danc n gl ct d of Figur 19.11: E N Z S Z S R Z L F E A-G A.C. Motor Prot ction S V A-G S R V S R G,F G,F A-G G,F V V V C-G B-G C-G V
V B-G C-G V B-G • 19 • V RES V V RES B-G V V B-G RES V B-G V A-G V V A-G V V C-G A-G C-G V C-G V Z = 2Z S1 SO
+3Z L1 E RES x3E LO +Z SO +2Z +Z +3Z E Figur 19.10: R sidual voltag arth fault prot ction for r sistanc - arth d sys t m. • 346 • N twork Prot ction & Automation Guid
Motor positiv s qu nc imp danc at slip s =
R1 p + R 2 p
( (2 − s ) ) ( 2 ' + X 1p + X 2 p ) 2
0.5
leading o excessive heaing. For he same mo or, nega ive sequence volages in excess of 17% willresul in a nega ive sequence curren larger han ra ed full load curren . Nega ive sequence curren is a wice supply frequency. Skin effec in he ro or means ha he hea ing effec in he ro or of a given nega ive se quence curren is larger han he same posi ive sequence curren . Thus, nega ive sequencecurren may resul in rapid hea ing of he mo or. Larger mo ors are mo re suscep ible in his respec , as he ro or resis ance of such machines ends o be higher.Pro ecion curren s is herefore essenia agains nega ive sequence l. Modern mo or pro ec ion relays have a nega ive sequence curren measuremen c apabili y, in order o providesuch pro ec ion. The level of nega ive sequence u phase a s nbalance depends largely upon he ype of faul . For loss of a single curren will ar , he nega ive sequence be 50%of he normal s ar ing curren . I is more difficul o provide an es ima e of he nega ive sequence curren if loss of a phase occurs while running. This is because he impac on he mo or ma y vary widely, from increased hea ing o s alling due o he reduced orque avai lable. A ypicalse ing for nega ive sequence curren pro ec ion mus ake in o accoun he fac ha he mo or circui pro ec ed by he relay may no be he s ive sequence curren. Time should be allowed for ource of he nega he appropria e pro ec ion o clear he source of he nega ive sequence curren wi hou in ro This indica ducing risk of overhea ing o he mo or being considered. esa wo s age ripping charac eris ic, similar in principle o overcurren pro ec ion. A low se defini e ime delay elemen can be used o provide an alarm, wi h an IDM T elemen used o rip he mo or in hecase of higher levels of nega ive sequen ce curren , such as loss of phase condi ions a s ar , occurring. Typical se in gs migh be 20% of CTra ed primary curren for he defini e ime elemen and 50 % for he IDMT elemen . The IDMT ime delay has o be chosen o pro ec he mo o r while, if possible, grading wi h o her nega ive sequence relays on he sys em. Some relaysmay no incorpora e wo elemen s, in which case he single elemen should be se o pro ec he mo or, wi h grading being a secondary considera ion . 1 9 . 8 F A ULT S I N ROTO RW I ND I NG S On wound ro or machines, somed egree of pro faul sin he ro or winding ec ion agains canbegiven by an ins a n aneous s a or curren overcurren relay elemen . As he s ar ing curren is no rmally limi ed by resis ance o a maximum of wice full load, he ins an aneous uni can safely be se o abou hree imes full load if a sligh Hence, a s ands ill (s=1.0), impedance ' = R1 p + R 2 p (
) + (X 2 1p ' + X2p ) 2
0.5
The mo or nega ive sequence impedance a slip s ' = R1n + R 2 n s ( ) + (X 2 1n ' + X 2n ) 2 0.5
and, a normal running speed, he impedance ' = R1n + R 2 n 2 ( ) + (X 2 1n ' + X 2n ) 2 0.5
where: suffix p indica es posi ive sequence quan i ies and suffix n indica es ne ga ive sequence quan i ies R1 + R'2 j(X1 + X' ) 2
[(1 s)/s] x R'2 (a) Posi ive phase sequence equivalen circui R1 + R'2 j(X1 + X' ) 2 [(s 1)/(2 s)] x R'2 (b) Nega ive phase sequence equivalen circui Figure 19.11: Induc ion mo or equivalen circui •
Now, as he resis ance is small compared if resis ance isneglec ed (jus ifiable o he reac ance), i can be seen ha henega ive sequencereac ance a runni ng speed reac ance is approxima ely equal o he posi ive sequence a s ands ill . An al erna ive more meaningful way of expressing his is: posi iveseq. impeda nce s ar ing curren — = —————— — nega ive seq. impedance ra — ——————————— a a ypical LV mo or s ar ing curren is 6xFLC. Therefore, a 5% nega ive sequen ce vol age (due o, say, unbalanced loads on he sys em) would produce a 30% neg a ive sequence curren in he machine, Ne work Pro ec ion & Au oma ion Guide • 347 • A.C. Mo or Pro ec ion 19 •
ed. ime delay of approxima ely 30 milliseconds is incorpora I should be no ed ha faul soccurring in he ro orwinding would no be de ec ed by any differen ial pro ec ion applied o he s a or. 1 9 . 9 R T D T E M P E R AT U R E D E T E C T I O N RTD’s ar us d to m asur t mp ratur s of motor windings or shaft b ar ings. A ris in t mp ratur may d not ov rloading of th machin , or th b ginn ing of a fault in th aff ct d part. A motor prot ction r lay will th r for usu ally hav th capability of acc pting a numb r of RTD inputs and int rnal logic to initiat an alarm and/or trip wh n th t mp ratur xc ds th appropriat s tpoint(s). Occasionally, HV motors ar f d via a unit transform r, and in th s circumstanc s, som of th motor prot ction r lay RTD inputs may b assign d to th transform r winding t mp ratur RTD’s, thus providing ov rt mp ratur prot cti on for th transform r without th us of a s parat r lay. 1 9 . 10 B E A R I N G F A I L U R E S Motors f d by contactors hav inh r nt und rvoltag prot ction, unl ss a latch d contactor is us d. Wh r a sp cific und rvoltag trip is r quir d, a d finit t im und rvoltag l m nt is us d. If two l m nts ar provid d, alarm and trip s ttings can b us d. An int rlock with th motor start r is r quir d to block r lay op ration wh n th starting d vic is op n, oth rwis a start will n v r b p rmitt d. Th voltag and tim d lay s ttings will b syst m and motor d p nd n t. Th y must allow for all voltag dips lik ly to occur on th syst m during tra nsi nt faults, starting of motors, tc. to avoid spurious trips. As motor starti ng can r sult in a voltag d pr ssion to 80% of nominal, th voltag s tting is lik ly to b b low this valu . R -acc l ration is normally possibl for voltag dips lasting b tw n 0.5-2 s conds, d p nding on syst m, motor and driv charact ristics, and th r for th tim d lay will b s t b aring th s factors in mind . 1 9 . 1 2 L O S S - O F - L O A D P R OT E C T I O N Loss-of-load prot ction h as a numb r of possibl functions. It can b us d to prot ct a pump against b co ming unprim d, or to stop a motor in cas of a failur in a m chanical transmiss ion ( .g. conv yor b lt), or it can b us d with synchronous motors to prot ct a gainst loss-of-supply conditions. Impl m ntation of th function is by a low for ward pow r r lay l m nt, int rlock d with th motor starting d vic to pr v nt op ration wh n th motor is tripp d and thus pr v nting a motor start. Wh r sta rting is against a v ry low load ( .g. a compr ssor), th function may also n d to b inhibit d for th duration of th start, to pr v nt malop ration. Th s t ting will b influ nc d by th function to b p rform d by th r lay. A tim d l ay may b r quir d aft r pickup of th l m nt to pr v nt op ration during syst m transi nts. This is sp cially important for synchronous motor loss-of supply prot ction. 1 9 . 1 3 A D D I T I O N A L P R OT E C T I O N F O R S Y N C H R O N O U S M OTO R S Th diff r nc s in construction and op rational charact risti cs of synchronous motors m an that additional prot ction is r quir d for th s t yp s of motor. This additional prot ction is discuss d in th following s ctions . 19.13.1 Out-of-St p Prot ction A synchronous motor may d c l rat and los syn chronism (fall out-of-st p) if a m chanical ov rload xc ding th p ak motor to rqu occurs. Oth r conditions that may caus this condition ar a fall in th N twork Prot ction & Automation Guid A.C. Motor Prot ction • Th r ar two typ s of b arings to b consid r d: th anti-friction b aring (bal l or roll r), us d mainly on small motors (up to around 350kW), and th sl v b aring, us d mainly on larg motors. Th failur of ball or roll r b arings usua lly occurs v ry quickly, causing th motor to com to a standstill as pi c s of th damag d roll r g t ntangl d with th oth rs. Th r is th r for v ry littl chanc that any r lay op rating from th input curr nt can d t ct b aring failu r s of this typ b for th b aring is compl t ly d stroy d. Th r for , prot cti on is limit d to disconn cting th stall d motor rapidly to avoid cons qu ntial damag . R f r to S ction 19.2 on stall prot ction for d tails of suitabl prot c tion. Failur of a sl v b aring can b d t ct d by m ans of a ris in b aring
t mp ratur . Th normal th rmal ov rload r lays cannot giv prot ction to th b aring its lf but will op rat to prot ct th motor from xc ssiv damag . Us of RTD t mp ratur d t ction, as not d in S ction 19.9, can provid suitabl prot ction, allowing inv stigation into th caus of th b aring running hot prior to compl t failur . 1 9 . 11 U N D E R V O LTA G E P R OT E C T I O N Motors may stall wh n subj ct d to prolong d und rvoltag conditions. Transi nt und rvoltag s will g n rally allow a motor to r cov r wh n th voltag is r stor d, unl ss th supply is w ak. 19 • • 348 •
appli d voltag to stator or fi ld windings. Such a fall may not n d to b prol ong d, a voltag dip of a f w s conds may b all that is r quir d. An out-of-st p condition caus s th motor to draw xc ssiv curr nt and g n rat a pulsating torqu . Ev n if th caus is r mov d promptly, th motor will probably not r cov r synchronism, but v ntually stall. H nc , it must b disconn ct d from th su pply. Th curr nt drawn during an out-of-st p condition is at a v ry low pow r f actor. H nc a r lay l m nt that r sponds to low pow r factor can b us d to pr ovid prot ction. Th l m nt must b inhibit d during starting, wh n a similar low pow r factor condition occurs. This can conv ni ntly b achi v d by us of a d finit tim d lay, s t to a valu slightly in xc ss of th motor start tim . Th pow r factor s tting will vary d p nding on th rat d pow r factor of th m otor. It would typically b 0.1 l ss than th motor rat d pow r factor i. . for a motor rat d at 0.85 pow r factor, th s tting would b 0.75. 19.13.2 Prot ctio n against Sudd n R storation of Supply If th supply to a synchronous motor is i nt rrupt d, it is ss ntial that th motor br ak r b tripp d as quickly as poss ibl if th r is any possibility of th supply b ing r stor d automatically or w ithout th machin op rator’s knowl dg . This is n c ssary in ord r to pr v nt th supply b ing r stor d out of phas with th motor g n rat d voltag . Two m thod s ar g n rally us d to d t ct this condition, in ord r to cov r diff r nt op ra ting mod s of th motor. 19.13.2.1 Und rfr qu ncy prot ction Th und rfr qu ncy r lay l m nt will op rat in th cas of th supply failing wh n th motor is o n load, which caus s th motor to d c l rat quickly. Typically, two l m nts ar provid d, for alarm and trip indications. Th und rfr qu ncy s tting valu n ds to consid r th pow r syst m charact ristics. In som pow r syst ms, l ngthy p riods of op ration at fr qu nci s substantially b low normal occur, and should not r sult in a motor trip. Th minimum saf op rating fr qu ncy of th motor u nd r load conditions must th r for b d t rmin d, along with minimum syst m fr qu ncy. 19.13.2.2 Low-forward-pow r prot ction This can b appli d in conjunctio n with a tim d lay to d t ct a loss-of-supply condition wh n th motor may shar a busbar with oth r loads. Th motor may att mpt to supply th oth r loads wit h pow r from th stor d kin tic n rgy of rotation. A low forward pow r r lay can d t ct this condition. S S ction 19.12 for d tai ls. A tim d lay will b r quir d to pr v nt op ration during syst m transi nts l ading to mom ntary r v rs pow r flow in th motor. 1 9 . 1 4 M OTO R P R OT E C T I O N E X A M P L E S This s ction giv s xampl s of th prot ction of HV a nd LV induction motors. 19.14.1 Prot ction of a HV Motor Tabl 19.2 giv s r l va nt param t rs of a HV induction motor to b prot ct d. Using a MiCOM P241 motor prot ction r lay, th important prot ction s ttings ar calculat d in th follow ing s ctions. Quantity Rat d output Rat d Voltag Rat d fr qu ncy Rat d pow r factor/ ffici nc y Stall withstand tim cold/hot Starting curr nt P rmitt d starts cold/hot CT ra tio Start tim @100% voltag Start tim @ 80% voltag H ating/cooling tim constan t Syst m arthing Control d vic Tabl 19.2: Motor data for xampl Valu 1000kW CMR 3.3kV 50Hz 0.9/0.92 20/7s 550% DOL 3/2 250/1 4s 5.5s 25/75 min Solid Circui t Br ak r 19.14.1.1 Th rmal prot ction Th curr nt s tting ITH is s t qual to th motor f ull load curr nt, as it is a CMR rat d motor. Motor full load curr nt can b cal culat d as 211A, th r for (in s condary quantiti s): • I TH = 211 250 = 0.844 Us a valu of 0.85, n ar st availabl s tting. Th r lay has a param t r, K, to
allow for th incr as d h ating ff ct of n gativ s qu nc curr nts. In th ab s nc of any sp cific information, us K=3. a Two thrmal hating tim constants r provid d, τ1 and τ2. τ2 is used for s ar ing me hods o her han DOL, o herwise i i s se equal o τ1. τ1 is se o he hea ing ime cons an , hence τ1=τ2=25mins. Cooling ime cons an τr is se as a mul iple of τ1. Wi h a cooling ime cons an of 75mins, τr = 3 x τ1 Ne work Pro ec ion & Au oma ion Guide • 349 • A.C. Mo or Pro ec ion 19 •
19.14.1.2 ionsof Secion 19.5 Shor circui pro ec ion Following he recommenda , wi has ar ing curren of 550% of fullload curren , he shor circui elemen isse o 1.25 x 5.5 x 211A = 1450A. In erms of he relay nominal curren , he se ing value is 1450/250 = 5.8IN. There is a minimum ime of 100ms forc delay urren s up o 120%of se ing o allow for ransien CT sa ura ion during s ar i ng and 40ms above his curren value. These se ings are sa isfac ory. 19.14.1.3 ear h faul pro ec ion I is assumed ha no CBCT is fi ed. A ypical se ing of 30% of mo or ra ed curren is used, leading o an ear h faul relay se ing o f 0.3 x 211/250 = 0.25IN. A s abilising resis or is required, calcula edin acco rdance wi h Equa ion 19.2 o preven malopera ion due o CT spill curren during s ar ing as he CT’s may saturat . With th stabilising r sistor pr s nt, instant an ous tripping is p rmitt d. Th alt rnativ is to omit th stabilising r sisto r and us a d finit tim d lay in association with th arth fault l m nt. How v r, th tim d lay must b found by trial and rror during commissioning. r sulting charact ristic is shown in Figur 19.13. Th motor th rmal prot ction, as it utilis s a n gativ s qu nc compon nt, is us d for prot ction of th mot or at low l v ls of n gativ s qu nc curr nt. Cold Th rmal Hot Th rmal S/C Lock d Rotor Stall Start Curr nt (100%V) Start Curr nt (80%V) 1000 100 10 1 0.1 0.01 Motor tripping charact ristics. Tim (s c) 0.01 1 Ith/I (pu) 10 A.C. Motor Prot ction 19.14.1.4 Lock d rotor/Exc ssiv start tim prot ction Th curr nt l m nt must b s t in xc ss of th rat d curr nt of th motor, but w ll b low th starting curr nt of th motor to nsur that a start condition is r cognis d (this could also b achi v d by us of an auxiliary contact on th motor CB wir d to th r l ay). A s tting of 500A (2 x IN) is suitabl . Th associat d tim d lay n ds to b s t to long r than th start tim , but l ss than th cold stall tim . Us a v alu of 15s. 19.14.1.5 Stall prot ction Th sam curr nt s tting as for lock d r otor prot ction can b us d – 500A. Th tim d lay has to b l ss than th hot sta ll tim of 7s but gr at r than th start tim by a suffici nt margin to avoid a spurious trip if th start tim happ ns to b a littl long r than anticipat d. Us a valu of 6.5s. Th prot ction charact ristics for S ctions 19.14.1.1-5 ar shown in Figur 19.12. 19.14.1.6 N gativ phas s qu nc prot ction Two prot ct ion l m nts ar provid d, th first is d finit tim -d lay d to provid an alar m. Th s cond is an IDMT l m nt us d to trip th motor on high l v ls of n gati v s qu nc curr nt, such as would occur on a loss of phas condition at startin g. In accordanc with S ction 19.7, us a s tting of 20% with a tim d lay of 30 s for th d finit tim l m nt and 50% with a TMS of 1.0 for th IDMT l m nt. Th 10 Figur 19.12: Prot ction charact ristics for motor prot ction xampl Tim (s c) 1 0.1
10 Curr nt (A) 10000 • 19 • Figur 19.13: Motor prot ction xampl n gativ s qu nc prot ction charact risti c 19.14.1.7 Oth r prot ction consid rations If th r lay can b suppli d with a su itabl voltag signal, stall prot ction can b inhibit d during r -acc l ration aft r a voltag dip using th und rvoltag l m nt (s t to 80-85% of rat d volta g ). Und rvoltag prot ction (s t to approximat ly 80% voltag with a tim d lay of up to s v ral s conds, d p nd nt on syst m charact ristics) and r v rs phas prot ction can also b impl m nt d to provid xtra prot ction. Unl ss th dri v is critical to th proc ss, it is not justifiabl to provid a VT sp cially t o nabl th s f atur s to b impl m nt d. 19.14.2 Prot ction of an LV Motor LV motors ar commonly f d via fus d contactors and th r for th tripping tim s of a prot ction r lay for • 350 • N twork Prot ction & Automation Guid
ov rcurr nt must b car fully co-ordinat d with th fus to nsur that th cont actor do s not att mpt to br ak a curr nt in xc ss of its rating. Tabl 19.3(a) giv s d tails of an LV motor and associat d fus d contactor. A MiCOM P211 motor prot ction r lay is us d to provid th prot ction. Param t r Valu Standard IEC 60034 Motor Voltag 400 Motor kW 75 Motor kVA 91.45 Motor FLC 132 Starting Curr nt 670 Starting Tim 4.5 Contactor rating 300 650 C ontactor br aking capacity Fus rating 250 (a) LV motor xampl data Symbol Valu Param t r Ib I>t I2 I2>t
(b) R lay trip charact ristic trip tim start curr nt Figur 19.14: Motor prot ction xampl contactor-f d motor 19.14.2.4 N gativ s qu nc (phas unbalanc ) prot ction Th motor is built to I EC standards, which p rmit a n gativ s qu nc (unbalanc ) voltag of 1% on a co ntinuous basis. This would l ad to approximat ly 7% n gativ s qu nc curr nt in th motor (S ction 19.7). As th r lay is fitt d only with a d finit tim r la y l m nt, a s tting of 20% (from S ction 19.7) is appropriat , with a tim d la y of 25s to allow for short high-l v l n gativ s qu nc transi nts arising from oth r caus s. 19.14.2.5 Loss of phas prot ction Th r lay has a s parat l m nt for this prot ction. Loss of a phas giv s ris to larg n gativ s qu nc cu rr nts, and th r for a much short r tim d lay is r quir d. A d finit tim d l ay of 5s is consid r d appropriat . Th r lay s ttings ar summaris d in Tabl 1 9.3(b). • • 351 • A.C. Motor Prot ction 19 • 100 Tim
• 20 • Prot ction of A.C. El ctrifi d Railways Introduction Prot ction philosophy Classical singl -phas f ding Cat nary th rm al prot ction Cat nary backup prot ction Autotransform r f ding F d r substati on prot ction Exampl of classical syst m prot ction 20.1 20.2 20.3 20.4 20.5 20 .6 20.7 20.8
• 20 • P rot ction of A.C. El ctrifi d Railways 20.1 INTRODUCTION Many r gional, urban and high-sp d int r-urban rail n tworks worldwid ar l ctrifi d, to provid th motiv pow r for trains (Figur 20.1). Figur 20.1: Mod rn high-sp d a.c. l ctric int r-urban train Th l ctrification syst m s rv s as th contact int rfac for curr nt coll ctio n by ach train, and in a.c. l ctrifi d railways as th m ans to distribut pow r. In g n ral, on of two philosophi s ar follow d: an ov rh ad cat nary abov th track, with pow r coll ction by a pantograph; or conductor-rail l ctrifica tion, with curr nt coll ction via contact sho s on a surfac of a sp cial m tall ic conductor laid clos to th running rails. Th latt r arrang m nt is most com monly us d for d.c. traction, whil th form r arrang m nt is us d for a.c. and d.c. traction. Som rail rout s hav dual ov rh ad and conductor-rail l ctrific ation to facilitat rout sharing by diff r nt rail op rators. Ov rh ad cat nari s ar g n rally consid r d to b saf r, as th y ar abov th track, out of r a ch of rail p rsonn l and th public. Th y ar th only way in which a traction f d at high voltag s can b ngin r d. Th y provid a singl -phas a.c. supply with a voltag in th rang of 11kV-50kV with r sp ct to th running rails, alth ough 1.5kV and 3kV d.c. cat nari s ar pr dominant in som N twork Prot ction & Automation Guid • 353 •
countri s. Wh n a conductor-rail syst m is us d, th supply voltag is g n rally 600V to 1700V d.c. This Chapt r cov rs prot ction associat d with HV ov rh ad a .c. cat nary l ctrification. Du to th natur of many rail rout s and th limi t d l ctrical cl aranc s ( sp cially wh r an xisting non- l ctrifi d rout is to b l ctrifi d), cat nary faults ar common. A typical fault rat is on fau lt p r y ar p r rout kilom tr of track. Th r lativ ly high fault rat , coupl d with th high m chanical t nsion in th contact wir (typically 620kN) d mands fast fault cl aranc . Should a fault not b cl ar d quickly, th conductors tha t form th cat nary may br ak du to int ns ov rh ating, with th cons qu nt ri sk of furth r s v r damag caus d by moving trains and l ngthy disruption to tr ain s rvic s. 20.2 PROTECTION PHILOSOPHY b com th standard. Figur 20.2 illustrat s classical 25kV f ding with boost r transform rs (BT). Th boost r transform rs ar us d to forc th traction r tu rn curr nt to flow in an a rially mount d r turn conductor, anchor d to th back of th supporting masts (Figur 20.3). This arrang m nt limits traction curr nt r turning through th rails and arth in a larg crosss ctional loop, th r by r ducing l ctromagn tic int rf r nc with adjac nt t l communication circuits. A st p-down transform r conn ct d phas to phas across th Utility grid is g n r ally th sourc of th traction supply. Th l ctrical f d to th train is via th ov rh ad cat nary, with th r turn curr nt flowing via th rails and th n th rough th r turn conductor. Supply transform r Path of traction curr nt BT R turn conductor BT Cat nary 25kV (nominal) Rails BT: Boost r transform r Figur 20.2: Classical 25kV f ding wit h boost r transform rs P rot ction of A.C. El ctrifi d Railways +• Th application of prot ction to l ctrical pow r transmission sch m s is bias d towards s curity whilst nsuring d p ndability only for th most s v r faults within th prot ct d circuit. B ing too adv nturous with th application of r mo t back-up prot ction should b avoid d, sinc th cons qu nc s of unwant d trip ping ar s rious. In th cas of l ctrifi d railways, th r is a high probabili ty that sustain d l ctrical faults of any typ (high r sistanc , r mot br ak r /prot ction failur tc.) may b associat d with ov rh ad wir damag or a fault y traction unit. Fall n liv wir s caus d by m chanical damag or accid nt r pr s nt a gr at r saf ty hazard with railways, du to th high r probability of p o pl b ing clos by (railway p rsonn l working on th track, or pass ng rs). Trac tion unit faults ar a fir hazard and a saf ty risk to pass ng rs, sp cially i n tunn ls. For th s r asons, th r will b a bias towards d p ndability of back -up prot ction at th xp ns of s curity. Th cons qu nc s of an occasional unw ant d trip ar far mor acc ptabl (th control c ntr simply r clos s th tripp d CB, som trains ar d lay d whil th control c ntr nsur s it is saf to r clos ) than th cons qu nc s of a failur to trip for a fall n wir or a tractio n unit fault. 20.3 CLASSICAL SINGLE-PHASE FEEDING Classical singl -phas a.c. ra ilway l ctrification has b n us d sinc th 1920’s. Earli r syst ms us d low fr qu ncy suppli s and in many countri s, l ctrification syst ms using 162/3Hz and 25Hz suppli s ar in us . Th cost of conv rsion of an xt nsiv n twork, with a r quir m nt for through working of locomotiv s, throughout th n c ssary chang ov r p riod, is usually prohibitiv . Starting from W st rn Europ and with th influ nc spr ading worldwid , singl -phas a.c. l ctrification at th standard pow r syst m fr qu ncy of 50/60Hz, has 20 • Figur 20.3: Classical ov rh ad lin construction As th running rails ar bond d to arth at r gular int rvals, th y ar nominall y at arth pot ntial. A singl pol circuit br ak r is all that is r quir d to di
sconn ct th supply to th cat nary in th v nt of a fault. 20.3.1 Classical Sy st m - F ding Diagram In practic , singl -track railway lin s ar rar , and two or four parall l tracks ar mor common. Th ov rh ad lin quipm nt is th n co mpris d of two or four l ctrically ind p nd nt cat nari s, running in parall l. Figur 20.4 N twork Prot ction & Automation Guid • 354 •
shows th f ding diagram for a typical two-track railway using a classical l c trification syst m. Th inf d to th tracks in th ‘northbound’ dir ction is via gr id transform r T1 at th F d r Station (FS). Th pow r is th n distribut d via cat nari s A and B abov th northbound and southbound tracks. At int rvals, it is usual to parall l th two cat nari s at parall ling/subs ctioning substations , as illustrat d in th Figur 20.4. Load curr nt can th n flow in th parall l paths, which r duc s th imp danc to th load and h nc th lin voltag drops. As th substation t rminology impli s, th provision of circuit br ak rs for a ch of th outgoing f ds to th cat nari s also allows subs ctioning – i. . th ab ility to disconn ct supply from s ctions of cat nary, in th v nt of a fault, o r to allow for maint nanc . For a fault on cat nary ‘A’ in Figur 20.4, circuit br a k rs A at th f d r station and at SS1 would b tripp d to isolat th fault d cat nary. Th supply to th h althy s ctions of cat nary B, C, D, E and F would b maintain d. F d r Station, th y ar locat d at v ry point wh r l ctrical isolation facil iti s ar provid d. 20.3.2 Classical Syst m - Prot ction Philosophy Th grid inf d transform rs ar typically rat d at 10 to 25MVA, with a r actanc of around 10% (or 2.5 wh n r f rr d to th 25kV winding). Thus, v n for a fault at th F d r Station busbar, th maximum prosp ctiv short circuit curr nt is low in com parison to a Utility syst m (typically only 10 tim s th rating of a singl cat nary). If a fault occurs furth r down th track, th r will b th additional im p danc of th cat nary and r turn conductor to b add d to th imp danc of th fault loop. A typical loop imp danc would b 0.6 /km (1 /mil ). Account may ha v to b tak n of un qual cat nary imp danc s – for instanc on a four-track railw ay, th cat nari s for th two c ntr tracks hav a high r imp danc than thos for th out r tracks du to mutual coupling ff cts. For a fault at th r mot nd of a prot ct d s ction ( .g. Cat nary s ction ‘A’ in Figur 20.4), th curr nt m asur d at th upstr am circuit br ak r location (CB A at th FS) may b twic ra t d curr nt. Thus at F d r Stations, ov rcurr nt prot ction can b appli d, as th r is a suffici nt margin b tw n th maximum continuous load curr nt and th fault curr nt at th r mot nds of cat nary s ctions. How v r, ov rcurr nt pro t ction is oft n us d only as tim -d lay d back-up prot ction on railways, for t h following r asons: a. th prot ction n ds to b discriminativ , to nsur th at only th two circuit br ak rs associat d with th fault d lin s ction ar tr ipp d. This d mands that th prot ction should b dir ctional, to r spond only t o fault curr nt flowing into th s ction. At location SS1, for xampl , th prot ction for cat nari s A and B would hav to look back towards th grid inf d. F or a fault clos to th FS on cat nary A, th r mot nd prot ction will m asur only th proportion of fault curr nt that flows via h althy cat nary B, along t h ‘hairpin’ path to SS1 and back along cat nary A to th location of th fault. Thi s fault curr nt contribution may b l ss than rat d load curr nt (s Figur 20. 5) SS1 A Cat nary s ction A Grid supply T2 T1 F d r station north A B F ding South SS1 Dir ction of trav l SS2 C E D Dir ction of trav l F MPSS BS2 BS: Bus s ction FS: F d r station SS: Parall ling/Sub-s ctioning substation MPS S: Mid Point substation NS: N utral s ction Figur 20.4: Classical 25kV f ding diagram
Th inf d from T1 g n rally f ds only as far as th normally op n bus s ction circuit br ak r (BS2) at th mid-point substation (MPSS). B yond th MPSS th r is a mirror imag of th l ctrical arrang m nts T1 to BS2 shown in Figur 20.4, with th r mot nd f d r station oft n 40-60km distant from T1. BS2 must r ma in op n during normal f ding, to pr v nt Utility pow r transf r via th singl phas cat nary, or to avoid parall lling suppli s that may b d riv d from diff r nt phas pairs of th Utility grid – .g. Phas A-B at T1, and B-C at th n xt F S to th north. Th sam is tru for BS1, which normally r mains op n, as th T1 and T2 f ds ar g n rally from diff r nt phas pairs, in an att mpt to balanc th loading on th thr phas Utility grid. Th n utral s ction (NS) is a nonconducting s ction of cat nary us d to provid continuity of th cat nary for th pantographs of motiv pow r units whil isolating l ctrically th s ctions of track. Whil only two (on p r rail track) ar shown for simplicity, s parating th tracks f d by T1 and T2 at th N twork Prot ction & Automation Guid • 355 • • Fault curr nt contribution F d r F substation via CB A Fault curr nt contribution via s ction B Cat nary s ction B B Figur 20.5 ‘Hairpin’ fault curr nt contribution P rot ction of A.C. El ctrifi d Railways 20 •
b. th prosp ctiv fault curr nt l v ls at SS1, SS2 and MPSS ar progr ssiv ly s mall r, and th m asur d fault curr nts at th s locations may b low r than rat d curr nt c. during outag s of grid supply transform rs, alt rnativ f ding ma y b n c ssary. On possibl arrang m nt is to xt nd th normal f ding by clos ing th bus s ction circuit br ak r at th MPSS. Th prosp ctiv curr nt l v ls for faults b yond th MPSS will b much low r than normal Ov rcurr nt prot ction is d tail d in S ction 20.5. In addition to prot ction against faults, th rmal prot ction of th cat nary is r quir d to pr v nt xc ssiv contact wir sag, l ading to possibl d wir m nts. S ction 20.4 d tails th principl s of cat nary t h rmal prot ction. zon s. Thr zon s of prot ction (shown as Z1, Z2, Z3) ar commonly appli d. For ach zon , th forward and r sistiv imp danc r ach s ttings must b optimis d to avoid tripping for load curr nt, but to off r th r quir d cat nary fault co v rag . All fault imp danc r ach s for distanc zon s ar calculat d in polar f orm, Z , wh r Z is th r ach in ohms, and is th lin angl s tting in d gr s. For railway syst ms, wh r all cat nari s hav a similar fault imp danc angl , it is oft n conv ni nt to add and subtract s ction imp danc s alg braically and tr at Z as a scalar quantity. X Typical solid fault imp danc charact ristic Max normal load ar a Z3 Z2 P rot ction of A.C. El ctrifi d Railways Distanc prot ction has b n th most prov n m thod of prot cting railway cat na ri s, du to its inh r nt ability to r main stabl for h avy load curr nt, whils t b ing abl to discriminativ ly trip for quit low l v ls of fault curr nt. For g n ral d tails of distanc prot ction, s Chapt r 11. Figur 20.5 shows how t h fault curr nt g n rally lags th syst m voltag by a gr at r phas angl than is usual und r load conditions, and thus th imp danc phas angl m asur m nt is an important attribut of distanc r lays for discriminating b tw n minimum load imp danc and maximum r mot fault imp danc . 20.3.3 Distanc Prot ction Zo n R ach s Distanc r lays appli d to a classical singl -phas l ctrifi d railw ay syst m hav two m asur m nt inputs: a. a cat nary to rail voltag signal d ri v d from a lin or busbar conn ct d voltag transform r b. a track f d r curr n t signal d riv d from a curr nt transform r for th circuit br ak r f ding th prot ct d s ction Distanc r lays p rform a v ctor division of voltag by curr n t to d t rmin th prot ct d circuit loop imp danc (Z). Typical r lay charact r istics ar shown in th R+j X imp danc plan , Figur 20.6. Solid faults on th cat nary will pr s nt imp danc s to th r lay along th dott d lin in Figur 20 .6. Th illustrat d quadrilat ral distanc r lay op rating zon s hav b n s t w ith charact ristic angl s to match th cat nary solid-fault imp danc angl , whi ch is usually 70 to 75 d gr s. Two of th zon s of op ration hav b n s t as d ir ctional, with th third b ing s midir ctional to provid back-up prot ction. Th m asur d fault imp danc will b low r for a fault clos r to th r lay locat ion, and th r lay mak s a trip d cision wh n th m asur d fault imp danc falls within its tripping Z1 R Max r g n rativ load ar a Figur 20.6: Polar imp danc plot of typical tri p charact ristics R lays at all of th track s ctioning substations (SS1, tc.) will s th r v r s -looking load and r g n ration ar as in addition to thos in th forward dir c tion shown in Figur 20.6. Th r v rs -looking zon s, which ar mirror imag s of th forward-looking zon s, hav b n omitt d from th diagram for clarity. 20.3 .3.1 Zon 1 Th Zon 1 l m nt of a distanc r lay is usually s t to prot ct as much of th imm diat cat nary s ction as possibl , without picking-up for fault s that li outsid of th s ction. In such applications Zon 1 tripping do s not n d to b tim -grad d with th op ration of oth r prot ction, as th Zon 1 r ach (Z1) cannot r spond to faults b yond th prot ct d cat nary s ction. Zon 1 tripping can b instantan ous (i. . no int ntional tim d lay). For an und r-r a
ching application, th Zon 1 r ach must th r for b s t to account for any pos sibl ov rr aching rrors. Th s rrors com from th r lay, th VT’s and CT’s and i naccurat cat nary imp danc data. It is th r for r comm nd d that th r ach of th Zon 1 l m nt is r strict d to 85% of th prot ct d cat nary imp danc , wi th th Zon 2 l m nt s t to cov r th final 15%. 20.3.3.2 Zon 2 To allow for u nd r-r aching rrors, th Zon 2 r ach (Z2) N twork Prot ction & Automation Guid +• 20 • • 356 •
should b s t to a minimum of 115% of th prot ct d cat nary imp danc for all f ault conditions. This is to guarant cov rag of th r mot nd not cov r d by Zon 1. It is oft n b n ficial to s t Zon 2 to r ach furth r than this minimum, in ord r to provid fast r back-up prot ction for uncl ar d downstr am faults. A constraining r quir m nt is that Zon 2 do s not r ach b yond th Zon 1 r ach of downstr am cat nary prot ction. This principl is illustrat d in Figur 20.7 , for a four-track syst m, wh r th local br ak r for s ction H has fail d to t rip. R lay A FS Z< A B C D 70% H CB fail d clos d A = Prot ct d s ction imp danc H = Short st following s ction F SS1 E F G H CB op n SS2 Z = imp danc of s ctions A, B, C, D in parall l Th possibility of curr nt foll owing out and back along a hairpin path to a fault has alr ady b n discuss d an d it is ss ntial that th r lay do s not ov rr ach und r th s conditions. Th f ding sc nario is shown in Figur 20.8. R lay A FS Z< A SS1 B Hairpin C 70% D CB op n D CB fail d clos d f ding F A = Prot ct d s ction imp danc D = Short st Hairpin F d s ction Figur 20.8 : Fault sc nario for maximum Zon 2 r ach (Hairpin F ding) Figur 20.7: Fault sc nario for Zon 2 r ach constraint (Normal F ding) In ord r to calculat Z2 for th FS circuit br ak r of prot ct d cat nary ‘A’, a fau lt is imagin d to occur at 70% of th short st following s ction. This is th cl os st location that unwant d ov rlap could occur with Z2 main prot ction for cat nary H. Th valu of 70% is d t rmin d by subtracting a suitabl margin for m a sur m nt rrors (15%) from th nominal 85% Z1 r ach for cat nary H prot ction. T h appar nt imp danc of th fault, as vi w d from r lay A at location FS is th n calculat d, noting that any fault imp danc b yond SS1 app ars to b approxima t ly four tim s its actual ohmic imp danc , du to th fault curr nt parall llin g along four adjac nt tracks. Th s tting appli d to th r lay is th r sult of this calculation, with a furth r 15% subtract d to allow for accommodat any m a sur m nt rrors at r lay A location. Th quation for th maximum Zon 2 r ach b com s: Figur 20.8 d picts a fault that has b n cl ar d at on nd only, with th r mo t nd br ak r for s ction D failing to trip. Th fault is assum d to b on th low st imp danc cat nary, which is an important consid ration wh n th r ar mo r than two tracks. In a four-track syst m, it is usual for mutual induction to caus inn r (middl ) track cat nari s to hav a charact ristic imp danc that is 13% high r than for th outsid tracks. Th calculation principl is similar to that for normal f ding, xc pt that now th fault curr nt is parall lling alon g thr (= numb r of tracks minus on ) adjac nt tracks. Th thr cat nari s con c rn d ar th prot ct d cat nary A, and th r maind r of th h althy cat nari s (R), i. . cat nari s B and C. Th quation for th maximum hairpin Zon 2 r ach b com s: • ( Z + 0.7 D ) × Z2 = wh r : 1.15 ( A + R) R …Equation 20.2 ( Z + 0.7 H ) × Z2 = wh r : 1.15
( A + R) R …Equation 20.1 H = imp danc of short st following s ction A = imp danc of prot ct d s ction R = imp danc of s ctions B, C, D in parall l N twork Prot ction & Automation Guid • 357 • D = imp danc of short st hairpin f d s ction A = imp danc of prot ct d s ction R = imp danc of s ctions B and C in parall l Z = imp danc of s ctions A, B, C , D in parall l P rot ction of A.C. El ctrifi d Railways 20 •
To avoid ov rr aching for both normal f ding and hairpin f d faults, th low r of th two calculat d imp danc s is us d as th Zon 2 r ach s tting. 20.3.3.3 Z on 3 Th Zon 3 l m nt would usually b us d to provid ov rall back-up prot c tion for downstr am cat nary s ctions. Th Zon 3 r ach (Z3) should typically b s t to at l ast 115% of th combin d appar nt imp danc of th prot ct d cat na ry plus th long st downstr am cat nary. Figur 20.9 shows th f ding consid r d: R lay A FS Z< A B SS1 SS2 furth r, to offs t th ff cts of trains with r g n rativ braking, which would provid an additional curr nt inf d to th fault. An additional 5% r ach incr a s would g n rally b suffici nt to allow for r g n rativ und rr ach. R lay A FS Z< A B Hairpin F ding SS1 C 100% D CB op n A = Prot ct d s ction imp danc CB op n F D CB fail d clos d P rot ction of A.C. El ctrifi d Railways C D 100% E CB fail d clos d A = Prot ct d s ction imp danc E = Long st followin g s ction F D = Long st Hairpin F d s ction Figur 20.10: Fault sc nario for Zon 3 minimum r ach (Hairpin F ding) 20.3.3.4 R v rs R aching Zon s An imp danc m asur m nt zon with r v rs r ach is typically appli d to provid back-up prot ction for th local busbar at a pa rall ling/s ctionalising substation. A typical r v rs r ach is 25% of th Zon 1 r ach of th r lay. Typically Zon 3 is s t with a r v rs offs t to provid t his prot ction and also so that th Zon 3 l m nt will satisfy th r quir m nt for Switch-on-to Fault (SOTF) prot ction. 20.3.3.5 Distanc zon tim d lay s tt ings Th Zon 1 tim d lay (tZ1) is g n rally s t to z ro, giving instantan ous op ration. Th Zon 2 tim d lay (tZ2) should b s t to co-ordinat with Zon 1 fault cl aranc tim for downstr am cat nari s. Th total fault cl aranc tim w ill consist of th downstr am Zon 1 op rating tim plus th associat d br ak r op rating tim . Allowanc must also b mad for th Zon 2 l m nts to r s t fol lowing cl aranc of an adjac nt lin fault and also for a saf ty margin. A typic al minimum Zon 2 tim d lay is of th ord r of 150-200ms. This tim may hav to b adjust d wh r th r lay is r quir d to grad with oth r Zon 2 prot ction o r slow r forms of back-up prot ction for downstr am circuits. Th Zon 3 tim d lay (tZ3) is typically s t with th sam consid rations mad for th Zon 2 tim d lay, xc pt that th d lay n ds to co-ordinat with th downstr am Zon 2 fa ult cl aranc . A typical minimum Zon 3 op rating tim would b in th r gion of 400ms. Again, this may n d to b modifi d to co-ordinat with slow r forms of back-up prot ction for adjac nt circuits. N twork Prot ction & Automation Guid Figur 20.9: Fault sc nario for Zon 3 minimum r ach (Normal F ding) Th quation for th minimum Zon 3 r ach (normal f ding) for R lay A b com s: Z 3 = 1.15 × ( Z + E ) × wh r : E = imp danc of lon st following s ction A = prot c t d s ction imp danc R = imp danc of s ctions B, C, D in parall l Z = imp danc of s ctions A, B, C, D in parall l It can b appr ciat d that hairpin f ding sc narios too must b consid r d, and this is d pict d in Figur 20.10: Th qua tion for th minimum Zon 3 r ach (hairpin f ding) b com s: Z 3 = 1.15 × ( Z + D ) × wh r : D = imp danc of long st hairpin f d s ction To avoid und r-r aching fo r both normal f ding and hairpin f d faults, th high r of th two calculat d i mp danc s is us d as th Zon 3 r ach s tting. Occasionally th Zon 3 r ach r q
uir m nt may b rais d ( A + R) R …Equation 20.3 +• 20 • ( A + R) R …Equation 20.4 • 358 •
20.3.4 Load Avoidanc Figur 20.4 shows how th distanc r lay trip charact rist ics must avoid r gions of th polar plot wh r th traction load may b pr s nt. This has historically b n achi v d by using shap d trip charact ristics, such as th l nticular charact ristic. Comm ncing around 1990, th b n fits of applyi ng quadrilat ral charact ristics w r r alis d with th introduction of int grat d circuit r lays. A quadrilat ral charact ristic p rmits th r sistiv r ach to b s t ind p nd ntly of th r quir d forward zon r ach, which d t rmin s th p osition of th top lin of th quadrilat ral l m nt. Th r sistiv r ach s ttin g is th n s t m r ly to avoid th traction load imp danc by a saf margin and t o provid acc ptabl r sistiv fault cov rag . Figur 20.11 shows how th r sist iv r ach s ttings ar d t rmin d: by CT’s, VT’s tc. will b mor pronounc d. It is th r for common to s t th r sist iv r ach s progr ssiv ly marginally small r for zon s with long r r ach s. A pr actical s tting constraint to nsur that zon s with long r ach s ar not too na rrow, and not ov rly aff ct d by angl m asur m nt tol ranc s, is for th r sist iv r ach not to b l ss than 14% of th zon r ach. 20.3.5 Enhanc d Mod rn R la y Charact ristics Figur 20.12 illustrat s th polygonal distanc r lay charact ristics of a mod rn num rical railway distanc r lay. Introduction of a s tting modifi s th basic quadrilat ral charact ristic into a polygonal on , in ord r t o optimis fault imp danc cov rag and load avoidanc for mod rn railway applic ations. Figur 20.12: Polygon distanc charact ristics Figur 20.11: R sistiv r ach s ttings for load avoidanc For all quadrilat ral charact ristics, imp danc point B is th critical loading to avoid. Th magnitud of th imp danc is calculat d from Z = V/I taking th minimum op rational cat nary voltag and th maximum short-t rm cat nary curr nt . Th cat nary voltag is p rmitt d to fall to 80% of nominal or l ss at th tra in location und r normal op rating conditions, and th short t rm curr nt loadin g to ris to 160% of nominal – th s worst-cas m asur d valu s should b us d wh n aiming to find th low st load imp danc . Th phas angl of point B with r sp ct to th r sistiv axis is d t rmin d as: = Cos-1 (max lagging pow r factor) T h diagram shows how r sistiv r ach E-F for Zon 1 has b n chos n to avoid th worst-cas loading by a suitabl margin of 10%-20%. Zon s 2 and 3 r ach furth r , thus th ff ct of any angular rrors introduc d N twork Prot ction & Automation Guid Th us of th s tting allows a load avoidanc notch to b plac d within th rig ht-hand r sistiv r ach lin of th quadrilat ral. is chos n to b around 10 d g r s gr at r than th worst-cas pow r factor load angl , limiting th r sistiv r ach to Rg to avoid all load imp danc s. For imp danc angl s gr at r than , t h zon r sistiv r ach R appli s, and th fault arc r sistiv cov rag is impro v d. This is sp cially b n ficial for Zon 3 back-up prot ction of adjac nt cat nari s, wh r th appar nt l v l of arc r sistanc will b rais d through th ff ct of parall l circuit inf ds at th int rv ning substation. 20.3.6 Impact o f Trains with R g n rativ Braking It is common for th Zon 1 charact ristic to apply to th forward dir ction only. How v r, oth r zon s may b s t to hav a r v rs r ach – s S ction 20.3.3.4 for d tails. Anoth r cas wh r r v rs -r ach ing zon s may b r quir d is wh r trains having r g n rativ braking ar us d. Such trains usually r g n rat at a l ading pow r factor • • 359 • P rot ction of A.C. El ctrifi d Railways
20 •
to avoid th cr ation of ov rvoltag s on th cat nary. Wh r a r g n rating trai n contribut s to fault curr nt, th fault imp danc m asur d by distanc r lays may shift up to 10° gr at r than . Som railway administrations r quir that th f ault imp danc r mains within th trip charact ristic, and do s not stray outsid th top l ft hand r sistiv boundary of th polygon. This can b obtain d by s tting th r v rs r sistiv r ach (Rbw) to b gr at r than th forward r sistiv r ach (Rfw). 20.3.7 Oth r R lay Charact ristics R c nt r lay t chnology d v lo pm nts also allow th us of d t ctors for rat of chang of curr nt and voltag (di/dt and dv/dt). Th s d t ctors ar us d to control th tim d lays associat d with tim -d lay d Zon s 2 and 3, and h nc obtain b tt r discrimination b tw n load and fault imp danc s. Th t chniqu is still in its infancy, but shows s ignificant pot ntial for th futur . 20.4 CATENARY THERMAL PROTECTION It is ss ntial that railway cat nari s r main in th corr ct position r lativ to th tra ck, thus nsuring good curr nt coll ction by train pantographs. Th cat nary is d sign d to op rat continuously at a t mp ratur corr sponding to its full load rating, wh r h at g n rat d is balanc d with h at dissipat d by radiation tc. Ov rt mp ratur conditions th r for occur wh n curr nts in xc ss of rating ar allow d to flow for a p riod of tim . Economic cat nary d sign d mands that th cat nary rating b that of th maximum av rag continuous load xp ct d. P aks in loading du to p ak-hour tim tabl s, or trains starting or acc l rating simu ltan ously ar accommodat d using th th rmal capacity of th cat nary - in much th sam way as us is mad of transform r ov rload capacity to cat r for p ak loading. It can b shown that th t mp ratur s during h ating follow xpon ntial tim constants and a similar xpon ntial d cr as of t mp ratur occurs during cooling. It is important that th cat nary is not allow d to ov rh at, as this w ill l ad to contact wir supporting arms moving b yond acc ptabl limits, and lo ss of th corr ct alignm nt with r sp ct to th track. Th p riod of tim for wh ich th cat nary can b ov rload d is th r for a function of th rmal history of th cat nary, d gr of ov rload, and ambi nt t mp ratur . Th t nsion in th c at nary is oft n maintain d by balanc w ights, susp nd d at ach nd of t nsion l ngths of contact wir . Ov rt mp ratur will caus th cat nary to str tch, wi th th balanc w ights v ntually touching th ground. Furth r h ating will th n r sult in a loss of contact wir t nsion, and xc ssiv sagging of th contact wir . To provid prot ction against such conditions, cat nary th rma l prot ction is provid d. 20.4.1 Cat nary Th rmal Prot ction M thod Cat nary th rmal prot ction typically us s a curr nt bas d th rmal r plica, using load curr nt to mod l h ating and cooling of th prot ct d cat nary. Th l m nt can b s t with both alarm (warning) and trip stag s. Th h at g n rat d within th cat n ary is th r sistiv loss (I2Rxt). Thus, th th rmal tim charact ristic us d in th r lay is th r for bas d on curr nt squar d, int grat d ov r tim . Th h at ing l ads to a t mp ratur ris abov ambi nt t mp ratur , so in ord r to calcul at th actual cat nary t mp ratur , th r lay must know th ambi nt t mp ratur along its’ l ngth. This can b ith r s t as an assum d ‘d fault’ ambi nt t mp ratur , or m asur d, typically using a t mp ratur prob mount d xt rnally to th sub station building. How v r, th t nsion l ngth of a contact wir may b ov r 1km, and trav rs cuttings and tunn ls - with r sulting significant chang s in th l ocal ambi nt t mp ratur . Th r for , th prob should id ally b mount d in a lo cation that most accurat ly mod ls th coolant air around th cat nary for th m ajority of th prot ct d s ction: a. if xpos d to dir ct sunlight, th n th pro b should b mount d to fac th sun b. if shad d from sunlight, such as running in a tunn l, th n th prob should b mount d on an xt rior wall facing away f rom th sun c. if running in a cutting, shi ld d from wind, th prob should b mount d in th l of th substation d. if xpos d to th wind, th prob should also b mount d on an xpos d wall It is virtually impossibl to sit th prob such as to xactly mod l th ambi nt conditions along th prot ct d s ction, an d thus a typical rror in th allowabl t mp ratur ris of b tw n 1°C and 3°C will r sult (for w ll-sit d and poorly-sit d prob s, r sp ctiv ly). RTD and CT rror s, along with r lay tol ranc s may also introduc furth r rrors of up to 1°C in t h th rmal mod l. Ov rall, th rror in th t mp ratur r ading abov th 20°C rat
d ambi nt could b 4°C. Th r for , r lays may hav a s tting to comp nsat for su ch m asur m nt tol ranc s, to nsur that th trip will not occur too lat to pr v nt m chanical damag . Som r lays may hav an option to xpr ss th abov tol ranc as a p rc ntag of th t mp ratur at which a trip is r quir d, rath r th an in absolut t rms. N twork Prot ction & Automation Guid +• P rot ction of A.C. El ctrifi d Railways 20 • • 360 •
20.5 CATENARY BACKUP PROTECTION Railway syst ms oft n us ov rcurr nt prot ction as tim -d lay d back-up prot ction for th main distanc prot ction. Two diff r nt philosophi s for ov rcurr nt prot ction ar typical: a. d finit -tim ov rcu rr nt prot ction (DTOC) b. back-up ov rcurr nt prot ction (BUOC) 20.5.1 D finit -tim Ov rcurr nt Prot ction (DTOC) This form of prot ction is continually in s rvic , in parall l to th distanc r lay l m nts, ith r includ d within th sa m r lay as th distanc function, or as a s parat r lay. Th latt r approach i s curr ntly mor common for installations at F d r Stations. This is du to th p rc iv d incr as in s curity and r liability obtain d from th r dundancy of s parat d vic s. How v r, th tr nds vid nt in oth r prot ction applications t o provid mor functionality within a singl r lay will in tim sur ly apply to this ar a as w ll. It op rat s on th basis of conv ntional d finit -tim ov rcu rr nt prot ction, as d scrib d in Chapt r 9. Th tim s ttings ar chos n to ns ur that th distanc r lay l m nts should op rat first, thus th ov rcurr nt l m nts only op rat if th distanc l m nts fail, or if th y ar out of s rvi c for som r ason. 20.5.2 Back-up Ov rcurr nt Prot ction (BUOC) This form of ba ck-up prot ction is switch d in s rvic only during p riods wh n th distanc pr ot ction is out of s rvic . A typical xampl is wh r VT sup rvision or a m asu ring circuit monitoring function d t cts a blown VT fus or an MCB trip. In such instanc s th distanc prot ction is automatically block d, and th BUOC l m n ts can b automatically brought into s rvic , such that cat nary prot ction is n ot lost. M thods of s tting ov rcurr nt prot ction ar cov r d in Chapt r 9. An xampl of using ov rcurr nt prot ction is giv n in S ction 20.8. 20.6 AUTOTRANS FORMER FEEDING High-sp d rail lin s, with maximum sp ds in xc ss of 200km/h ( 125mph) hav much high r traction pow r d mands. This is not only to cop with t h p ak pow r r quir d for rapid acc l ration to high sp d, but also to cop wi th th st p r gradi nts that ar commonly ncount r d along such rout s. Th to tal traction pow r p r train may amount to 12-16MW, comprising two or mor pow r cars p r unit and oft n two units coupl d tog th r to form a compl t train. Th h avy load curr nts drawn may caus significant voltag drops across th N twork Prot ction & Automation Guid cat nary f ding imp danc with a classical f ding arrang m nt – d p nding on th s ction l ngth b ing f d and th traffic fr qu ncy (in both dir ctions). To avo id a d cr as in train p rformanc , f d r stations and parall lling substations for classical syst ms would hav to b sit d at prohibitiv ly short int rvals. In such circumstanc s, sp cially wh r th rout involv s n w construction, aut otransform r f ding is normally favour d. 20.6.1 D scription of Autotransform r F ding Autotransform r f ding us s a high voltag syst m comprising of a c nt r -tapp d supply transform r, cat nary wir and a f d r wir . Th f d r wir i s a rially mount d on insulators along th back of th ov rh ad lin masts. Th running rails ar conn ct d to th c ntr tap of th supply transform r, and h n c a train s s only half of th syst m voltag . Autotransform rs locat d at int rvals along th tracks nsur division of load curr nt b tw n cat nary and f d r wir s that minimis s th voltag drop b tw n th supply transform r and th train. Figur 20.13 shows autotransform r f ding for th typical 25-0-25kV sys t m found in W st rn Europ . Supply pp y transform r F d r Ic AT Cat nary Rails AT: Autotransform r : 1 unit of load curr nt Figur 20.13: 25-0-25kV autot ransform r f ding • Th us of autotransform rs (AT) r sults in distribution loss s that ar low r t han for classical 25kV f ding, and th r for can support th us of high pow r 25kV traction units. F d r substation spacing can also b much gr at r than if a classical f ding syst m is us d. F w r substations m ans l ss maint nanc and
r duc d op rating costs. Two-pol switchg ar is normally us d to isolat both t h f d r and cat nary wir s in th v nt of a fault on ith r wir . How v r, so m autotransform r syst ms allow singl wir tripping, wh r s parat distanc p rot ction is provid d for ach wir . Th prot ction would th n monitor th two ‘ha lv s’ of th syst m ind p nd ntly, with Prot ction Zon s 1 and 2 typically s t to 85% and 120% of th prot ct d • 361 • P rot ction of A.C. El ctrifi d Railways 20 •
25-0-25 kV F d r substation I> Z< as * as * * I> Z< I> S ction switch prot ction Z< as * I> * Z< To NO NO F C C To Track via B NO P rot ction of A.C. El ctrifi d Railways Up F - f d r F A.E.C - A rial arth conductor NO Figur 20.14: Autotransform r-f d syst m on -lin diagram showing prot ction 1.20 Cat nary wir AEC Contact wir F d r wir 6.30 5.50 5.08 1.34 3.57 3.25 750 4.50 Rail l v l CL Down CL Up 0.3 Buri d arth conductor (B.E.C)
Figur 20.15: Typical autotransform r–f d and cat nary layout • 362 • N twork Prot ction & Automation Guid 7.25 +• 20 • A rial arth conductor (AEC)
circuit imp danc - similar to th prot ction of a classical cat nary syst m. Fi gur 20.13 also illustrat s th distribution of load curr nt for a train situat d midway b tw n AT locations. Th topology of th AT syst m is oft n similar to th classical syst m shown in Figur 20.4, xc pt that th grid supply transfor m r 50kV s condary winding is wound as a c ntr -tapp d AT winding, and AT’s ar co nn ct d cat nary-rail-f d r at ach downstr am substation and at int rv ning lo cations. Figur 20.14 shows a typical prot ction on -lin diagram for an autotra nsform r-f d syst m, whil Figur 20.15 shows th construction of th cat nary s yst m. 20.6.2 Autotransform r Syst m Prot ction Philosophy From Figur 20.13 it can b s n that that th summation (Ic - If) at any location will b qual to t h downstr am traction load curr nt. Th sam is tru for fault curr nt, and so physically p rforming this curr nt summation, through th parall l conn ction of f d r and cat nary CT s condary windings, or math matically summating within a prot ction r lay, can b th basis for autotransform r circuit prot ction. To d iscriminat b tw n normal load curr nt and f d r wir or cat nary faults, dist anc prot ction is commonly appli d, with (Ic - If) b ing th m asur d curr nt. Th m asur d voltag is g n rally th cat nary to rail voltag . Th r lativ ly l ow r actanc of th AT’s – typically 1% on a 10MVA bas – nsur s that any fault volta g drop on th cat nary will b proportional to th f d r wir voltag drop. Wh n applying zon s of distanc prot ction to AT syst ms, with doubl -pol trippin g, it should b appr ciat d that it is not usually possibl to provid fully dis criminativ prot ction. Wh n th cat nary and f d r curr nts ar combin d, th r lationship b tw n imp danc and distanc -to-fault is non-lin ar. Cons qu ntly , it is mor difficult to s t Zon 1 to b und r-r aching and Zon 2 to b ov rr aching in th normal mann r. Th approach that is normally adopt d is to s t th F d r Station distanc prot ction to d t ct all faults along any track, up to , but not b yond, th MidPoint Substation. It can b arrang d that op ration of any distanc r lay will trip all F d r Station br ak rs. In th v nt of any fa ult up to th MPSS, simultan ous tripping of all th track f d r circuit br ak rs at th FS will cut suppli s to all tracks. Wh r this sch m is adopt d, th application of auto-r closing is ss ntial to r stor suppli s to all but th p rman ntly fault d s ction of cat nary and f d r. Th mom ntum of moving trains will nsur that littl sp d is lost during th d ad tim of th auto-r clos s qu nc s. Consid rations r lating to th application of autoN twork Prot ction & Automation Guid r closur ar d tail d in S ction 20.5.5. With high sp d lin s g n rally b ing b tt r f nc d, and having f w r ov rbridg s and gr at r l ctrical cl aranc s co mpar d to classical syst ms, th infr qu nt loss s of supply caus f w op ration al probl ms. As tripping of circuit br ak rs at th FS isolat s all lin faults, th r is th n no n d to hav switchg ar at downstr am substations rat d to int rrupt fault curr nt. For conomy, loadbr aking switch s ar us d inst ad of br ak rs at SS1 and SS2 in Figur 20.4. 20.6.3 Distanc Prot ction Zon R ach s Fig ur 20.16 illustrat s th typical locus of imp danc m asur d at th FS, for a c at nary to arth fault, at a variabl location upstr am of SS2, for any on trac k. Whil a similar ff ct xists for classically-f d syst ms, it is small by com parison and normally ignor d. Th imp danc m asur d is d fin d as: Z = V cat na ry (I cat nary I f d r ) Zmax B Zmin Solid lin shows Z m asur d
Fault imp danc (ohms) Z 12 11 9 8 7 6 5 4 3 2 0 FS 2 C A SS2 4 6 8 10 12 14 Distanc to fault (km) 16 18 Figur 20.16: Variation of imp danc m asur m nt with fault location along track For clarity, only th imp danc s m asur d for a cat nary to arth fault locat d upstr am of SS2 ar plott d. Th hump-lik imp danc locus in Figur 20:16 has a numb r of id ntifiabl tr nds: a. th initial slop of th locus, in ohms/km, s hown as lin ‘A’. This is according to th cat nary-to-rail loop imp danc of (th 2 5kV loop in Figur 20.13), sinc th fault curr nt flows almost ntir ly in th cat nary-rail loop for faults clos to a f ding point b. at AT locations, slop ‘B’ shows how th ff ctiv ohms/km tr nd is l ss than half th cat nary-tof d r l oop imp danc (th 50kV loop in Figur 20.13) du to th m thod of imp danc m a sur m nt and du to th fault curr nt distribution. For a cat nary- arth fault l ocat d at an autotransform r, th fault curr nt will circulat • • 363 • P rot ction of A.C. El ctrifi d Railways 20 •
almost ntir ly in th cat nary-f d r loop rath r than in th cat nary-rail loo p. Additionally, th imp danc of th cat nary-f d r loop is low r than that of th cat nary-rail loop, as th f d r cabl is a b tt r conductor than th rail s c. b yond SS1, th ff ct of parall l f ding from oth r circuits b tw n th FS and SS1 m ans that slop ‘C’ for a singl circuit b yond SS1 is gr at r than slop ‘A’. With r f r nc to Figur 20.12, th syst m simulat d is four track, thus th gradi nt of ‘C’ will b approximat ly four tim s that of ‘A’ (marginally high r than fou r for th inn r tracks, and l ss than four for out r tracks) Consid rations for th s tting of distanc r lay r ach s ar d tail d in th following s ctions. 20 .6.3.1 Zon 1 prot ction for faults b yond th MPSS, or with a long r r ach to cov r instanc s wh r AT’s ar switch d out of s rvic , such that th ff ctiv normal f ding im p danc b com s high r. 20.6.4 Distanc Zon Tim D lay S ttings and Load Avoida nc Th principl s us d ar id ntical to thos for classical f ding, with on xc ption. A short tim d lay of th ord r of 50ms may b us d with th Zon 1 l m nt if a r lay without magn tising inrush r straint is us d. Th r lay us s (I c - If), which is m asuring th combin d load curr nt of all trains at th ir pan tographs. Th r for , th load imp danc to avoid is that m asur d from cat nary to rail (th ‘25kV’ imp danc in Figur 20.11). 20.6.5 Implications of using Two-Pol Switching and Auto-R closur A full discussion of op rational implications is b yond th scop of this Chapt r, thus only th important points ar list d: a. it is usual to r mov all parall lling b tw n tracks prior to any br ak r r clo sing. This avoids r p titiv r -tripping of h althy cat nary s ctions as multipl track f d r circuit br ak rs ar b ing r clos d aft r cl aranc of a fault on on f d r. Parall ling is r mov d by op ning th motoris d isolators at all SS and MPSS locations. Following f d r br ak r r closur , th tracks will b radi ally f d. A p rsist nt fault would only r sult in r tripping of th fault d trac k circuit br ak rs b. in th p riod wh r tracks ar b ing radially-f d, th r l ays at th FS should only trip th ir own track circuit br ak rs. Cross-tripping of parall l track circuit br ak rs should b inhibit d c. prot ction at th FS c an trip for an AT fault. Sinc th r would typically b no circuit br ak rs at t h SS and MPSS autotransform r locations, AT prot ction should wait for loss of lin voltag during th d ad tim of FS circuit br ak rs b for initiating th o p ning of a local motoris d disconn ctor switch. This action should tak plac w ithin th d ad tim so that th fault d AT will hav b n disconn ct d b for r closur of th FS br ak rs d. with radially f d tracks, multipl shot autor clos ing, is oft n appli d to dislodg any d bris (wildlif or oth r stray mat rial) that may hav caus d a s mi-p rman nt fault. B for th last auto-r clos shot, it is common to disconn ct all AT’s downstr am of th FS. With all AT’s and N twork Prot ction & Automation Guid P rot ction of A.C. El ctrifi d Railways +• Th Zon 1 l m nts of any FS distanc r lay should not ov rr ach and trip for f aults b yond th MPSS, wh n th mid-point bus s ction br ak r is clos d. If it i s known that th MPSS is d finit ly op n, th n th r is no r al r ach constraint for distanc prot ction. How v r, if th mid-point br ak r is clos d, or no sta tus information is communicat d to th prot ction to control ov rr ach, through r v rsion to an alt rnativ s tting group, th n th r lay must not trip for th low st imp danc for a fault at th MPSS busbar. R f rring to Figur 20.16, this fault imp danc would b Zmin along slop B (to 15km and 7.5 ). Th appli d Zon 1 s tting should b r strict d to 85% of this imp danc , to allow for all m as ur m nt and imp danc data tol ranc s. A low r r ach s tting might b n c ssary to pr v nt unwant d tripping with aggr gat magn tising inrush curr nts followin g circuit n rgisation. This will d p nd on th r spons of th r lay l m nts t o inrush curr nt and to th numb r of AT’s appli d. For r lays that hav magn tisi ng inrush r straint or som m ans of providing immunity or r duc d s nsitivity t o inrush curr nts such a constraint may not apply. 20.6.3.2 Zon 2 Allowing for
und r-r aching rrors, th Zon 2 r ach (Z2) should b s t in xc ss of 115% of th prot ct d lin imp danc for all fault conditions. Th r l vant imp danc in Figur 20.16 would b th Zmax p ak b tw n SS2 and MPSS. A typical valu of Zm ax would b approximat ly 11.5 at 13km distanc from th f d r station. If trai ns with r g n rativ braking ar in s rvic along th prot ct d track a 20% addi tional r ach margin would typically b appli d. With th stat d Zon 1 and Zon 2 s tting policy, r lays at th F d r Station provid compl t track prot ction up to th MPSS. 20.6.3.3 Zon 3 Zon 3 may b appli d to provid r mot back-up 20 • • 364 •
parall ling r mov d th fault d circuit distanc r lays would th n s a lin ar r lationship b tw n th imp danc m asur d and th distanc to fault. Th r sul ts obtain d from a conv ntional, int gral fault location algorithms would th n o ff r r ctification cr ws a fairly accurat stimat of wh r th p rman nt fault might b locat d . it may b n c ssary to automatically incr as th Zon r ac h s of distanc r lay l m nts b for th final auto-r clos att mpt to allow fo r th high r cat nary to rail fault loop imp danc up to th MPSS rath r than th low r cat nary-f d r loop imp danc . This may b achi v d by switching to an alt rnativ s tting group with Z2 s t high r than pr viously 20.6.6 Backup Prot ction Backup prot ction consid rations for autotransform r f d syst ms ar simil ar, in principl , to thos for classical syst ms, as d scrib d in S ction 20.5. 20.7 FEEDER SUBSTATION PROTECTION Each f d r substation compris s transform rs, busbars, cabl s, switchg ar, tc. All of th s it ms r quir prot ction. Du to th much high r fr qu ncy of faults on th cat nary syst m, sp cial att ntion m ust b giv n to nsuring that th substation prot ction r mains stabl for cat n ary faults, whilst off ring d p ndabl prot ction for substation faults. Oth r t han this, th r ar no sp cial r quir m nts for th prot ction of f d r substat ion quipm nt and th forms of prot ction d tail d in Chapt rs 9-16 ar dir ctly applicabl , on a singl phas basis. 20.8 EXAMPLE OF CLASSICAL SYSTEM PROTECTIO N Busbar VT 26.4/11kV 275/26kV Zt=10% 88° TF1 600/1 2 TF2 S1 12.2km S2 13.7km S3 4 Z< Substation S1 ar to b calculat d. Th inputs to th r lay ar d riv d from th track f d r CT adjac nt to th circuit br ak r, and from a s ction busbar VT a t busbar S1 (a cat nary-sid VT would b qually suitabl ). Th syst m data is g iv n in Tabl 20.1. A MiCOM P438 r lay is us d in th xampl . Equipm nt Cat nary Imp danc Boost r Transform r Imp danc Boost r Transform r S pacing Maximum Load Curr nt CB Typ CB trip tim Max Zon 1 prot ction trip tim Cat nary Th rmal Prot ction Cat nary d sign t mp ratur rang for corr ct t nsi on Typical assum d max. wint r t mp ratur (610A rating) Typical assum d max. sp ring/autumn t mp (540A rating) Typical assum d summ r t mp ratur (515A rating) Worst-cas assum d hott st ambi nt T mp ratur for Balanc W ights to touch grou nd T mp ratur at which 20% loss of t nsion, train sp ds must b r strict d T m p ratur at which possibl damag du to clashing of supports at ov rlaps occurs H ating tim constant - daytim Cooling tim constant - nightim Tabl 20.1: El ctrifi d railway syst m data Data 0.26+j0.68 /km 0.051+j0.21 v ry 3km 900A Vac uum 0.065 s 0.045 s -18°C to 38°C 10°C 20°C 23°C 28°C 38°C 48°C 56°C 5 mins 7 mins 20.8.1 S ction Imp danc Data Th first st p is to calculat th primary imp dan c for th cat nary s ctions to b prot ct d. Zon 1 for th r lay associat d wi th f d r TF-1 prot cts s ction 1, how v r th backup prot ction off r d by Zon s 2 and 3 must discriminat with downstr am r lays and so th imp danc of s cti ons 2, 3 and 4 n ds to b calculat d too. In this xampl ach pair of cat nari s runs b tw n th common substations, and so th imp danc of adjac nt s ction s will b id ntical. Th r ar situations wh r this is not th cas , of which a . th s ctions to b prot ct d consist of four tracks b. th two tracks follow d iff r nt rout s du to th g ography of th rout and h nc may not b of th sa m l ngth • 1 3 c. if th r is a junction within a s ction ar thr xampl s. Th quival nt s ction imp danc p r kilom tr is giv n by th formula: Zs ct/km = lin imp danc /km + (BT imp danc /BT spacing) Off-load voltag 26kV
Figur 20.17: N twork Diagram – xampl calculation (0.26 + j 0.68 ) + (0.051 + 3 j 0.21 ) Figur 20.17 d picts a typical 25kV syst m, wh r th s ttings for th r lay pro t cting track f d r TF-1 at = 0.277 + j0.75 /km = 0.8 69.7° /km N twork Prot ction & Automation Guid • 365 • P rot ction of A.C. El ctrifi d Railways 20 •
This will b round d up to 70° as th n ar st s ttabl valu of th common charact ristic lin angl of th r lay, . Distanc prot ction r lays ar oft n s t and inj ctiont st d in t rms of th imp danc on th s condary sid of th CT’s/VT’s us d. Th r for , it is h lpful for t sting if th primary imp danc s on th syst m ar conv rt d to s condary quantiti s. Th quation to b us d is: Z s c t = Z s c t × wh r : Zs ct = syst m imp danc r f rr d to primary Z’s ct = syst m imp danc r f rr d to s condary CT ratio VT ratio Th s tting r quir d is th low st of th abov two configurations. 20.8.4.1 ‘Foll ow-on’ configuration Figur 20.7 shows th condition to consid r, with two track f ding only for th ar a f d by Substation S1. Equation 20.1 is us d to calculat th r ach: ( Z + 0.7 E ) × Z2 = wh r : 1.15 ( A + R) R Z = imp danc of s ctions 1 and 2 in parall l A = th track s ction of int r st, s ction 1 R = parall l fault curr nt path (s ction 2) E = short st following s ction (3 or 4) H nc , Z2 = (12.2 + 0.7 × 27.4 ) 24.4 + 24.4 24.4 1.15 2 1.15 P rot ction of A.C. El ctrifi d Railways H nc , 600 Z s c t = Z s c t × 26400 1 110 = Z s c t × 2.5 20.8.2 S ction Imp danc Calculations Th s ction imp danc s can b calculat d as follows: 20.8.2.1 S ctions 1 and 2 Th imp danc s for s ctions 1 and 2 ar : Zs ct = 12.2 x 0.8 = 9.76 Z’s ct = 9.76 x 2.5 = 24.4 20.8.2.2 S ctions 3 and 4 Th imp danc s for s ctions 3 and 4 ar : Zs ct = 13.7 x 0.8 = 10.96 Z’s c t = 10.96 x 2.5 = 27.4 20.8.3 Zon 1 R ach Calculation for TF-1 Th Zon 1 forwa rd r ach is s t to b 85% of th s ction 1 imp danc , r f rr d to th s condary of th r lay. H nc , th forward r ach is calculat d as Z1fw = 24.4 x 0.85 = 20. 75 Zon 1 is not r quir d to op rat in th r v rs dir ction, so th s tting Z1 bw is s t to Block d. 20.8.4 Zon 2 R ach Calculation for TF-1 Two configuration s hav to b consid r d in th s tting of th Zon 2 r ach. Th s ar : a. th ‘fol low-on’ configuration of Figur 20.7 b. th ‘Hairpin’ f ding configuration of Figur 20.8 × = (12.2 + 0.7 × 27.4 ) × = 54.6 Notic how for two track f ding, (A + R)/R abov b com s 2, du to a fault curr nt split b tw n two id ntical parall l paths. 20.8.4.2 ‘Hairpin’ f ding configura tion R f rring to Figur 20.8, it is appar nt that with only two tracks, inn r t racks B and C ar not pr s nt. Onc circuit br ak r TF-2 at substation S1 is op n, th imp danc to th fault is m r ly 170% tim s th imp danc of track s ctio n 1 or 2. Thus, from Equation 20.2: Z2 = 24.4 + = 36.1 For Zon 2 it is always t h low r of th two calculat d r sults that is us d. Th r for , us a s tting of : Forward r ach Z2fw = 36.1 Th R v rs R ach, Z2bw, is s t to Block d, as only forward dir ctional op ration is r quir d. • 366 • N twork Prot ction & Automation Guid +• 20 • (0.7 × 24.4 )
1.15
20.8.5 Zon 3 R ach Calculation for TF-1 In similar fashion to th Zon 2 r ach, th ‘follow-on’ and ‘Hairpin’ fault configurations hav to b consid r d. As Zon 3 mus t t nd to ov rr ach rath r than und rr ach, 120% of th fault imp danc calculat d is us d as th s tting and th high r of th two possibl s ttings is us d. 2 0.8.5.1 ‘Follow-On’ fault configuration Figur 20.9 shows th configuration for a fo llow-on fault with two tracks: It is appar nt that th calculation is xactly as for Zon 2 follow-on, xc pt that th multipli r of 0.7 (70%) is r plac d by 1 (100%). Z3 = (12.2 + 27.4) x 2 x 1.2 = 95.1 20.8.5.2 ‘Hairpin f ding’ fault configu ration R p ating th sam for hairpin f ding (Figur 20.10, Equation 20.4): Z3 = (24.4 + 24.4) x 1.2 = 58.6 H nc , us a s tting of: Forward r ach Z3fw = 95.1 For Zon 3, a r v rs r ach is r quir d to act a backup to th upstr am prot cti on. Th usual s tting is 25% of th Zon 1 forward r ach. Th r for , us a s tti ng of: R v rs r ach Z3bw = 0.25 x 20.75 = 5.2 20.8.6 Zon Tim D lays Th Zon 1 tim d lay will b s t to instantan ous op ration (t1 = 0) – it is not common pr actic to tim grad this zon with th primary prot ction fitt d on board th tr ains. Zon 2 (t2) should b d lay d as follows: t2 = CB max trip tim + R lay ma x trip tim + 50ms margin H nc , t2 = 65 + 45 + 50 = 160ms As all of th prot ct ion and circuit br ak rs ar id ntical, this valu can b us d for t2. If th do wnstr am r lays w r l ctrom chanical (typically 40-70ms slow r than num rical) , or th circuit br ak rs w r oil insulat d (OCB’s, typically 40 to 60ms slow r t han VCB’s), th n th t2 d lay would n d to b xt nd d accordingly. Th 50ms marg in allows for th r s t tim of th Z2 l m nt. N twork Prot ction & Automation Guid Th Zon 3 tim d lay can typically b s t doubl th minimum calculat d abov . How v r, as Zon 3 is oft n most at risk of unwant d pickup du to train startin g curr nts or mom ntary ov rloads, a long r s tting of t3 = 500ms is appli d. 20 .8.7 Ov rcurr nt Prot ction Ov rcurr nt prot ction can b appli d to th 25kV sy st m in Figur 20.17. For railway applications, nondir ctional ov rcurr nt prot ction is normal. Th simpl st application is for track f d rs at F d r Station s, such as TF-1. At this location and with normal f ding, any fault curr nt wil l naturally b flowing away from th busbar, and so no r v rs op ration can occ ur. At downstr am substations it will not b possibl to apply ov rcurr nt prot ction in a similar way, and any l m nts nabl d would t nd to b s t with long tim d lays to nsur that all of th distanc prot ction zon s ar giv n suffic i nt tim to trip b for hand. 20.8.7.1 Back-Up ov rcurr nt (BUOC) at f d r stat ions Should th distanc prot ction b out of s rvic , two BUOC ov rcurr nt l m nts could b s t. Firstly a high s t ov rcurr nt l m nt is s t to und rr ach t h prot ct d s ction, mimicking Zon 1 op ration. This can b s t for instantan ous tripping. S condly, a low r-s t ov rcurr nt l m nt can b appli d to compl t prot ction for th TB-1 s ction, to ov rr ach th nd of th prot ct d s ctio n at S2. Th ov rcurr nt l m nt of th r lay would b s t accordingly and with a d finit tim d lay. 20.8.7.2 Calculation of fault curr nt In ord r to d t rmi n th ov rcurr nt s ttings, th fault curr nt m asur d by TF-1 CT for a fault a djac nt to th S2 busbar n ds to b calculat d. Th r ar two possibl configur ations to consid r: a. fault curr nt for a fault at th nd of s ction 1, with t wo tracks in-s rvic b. curr nt for a fault at th nd of s ction 1, with s ctio n 2 isolat d for maint nanc For th first configuration, th fault curr nt p r track can b calculat d as I f1= wh r : E = sourc voltag = 26.4kV Zt = transfo rm r imp danc = 4.5 88° Zsp = parall l imp danc of s ctions 1 and 2 = 9.76 70° /2 E 2 × Z t +Z sp ) • ( • 367 • P rot ction of A.C. El ctrifi d Railways
20 •
Not that th fault curr nt splits into two parall l paths, f d via TF-1 and TF2. H nc , th division by 2 in th quation for calculating th p r-track curr n t m asur d by th prot ction. H nc , I f 1 = 1.4kA For th s cond configuration, I f2= wh r : Zs1 = imp danc of s ction 1 H nc , I f 2 = 1.84kA 20.8.7.3 Ov rcu rr nt s tting for BUOC instantan ous stag To pr v nt ov rr ach, s t at l ast 20 % abov th high r of th two fault sc narios: Iinst = 1840 x 1.2 = 2200A Th s condary curr nt s tting on th r lay is found by dividing by th CT ratio: I i nst = 2200 600 =3.68 A E Z t +Z s1 Idtinst wh r : 1.5 x Iflc Iflc = full load curr nt of f d r H nc , Idtinst = 1.5 x 600 = 900A R f rr d to th s condary sid of th CT, I dtinst =900 600 =1.5 A Th tim d lay appli d must b long r than th t3 distanc zon d lay, so tI’dtins t would b acc ptabl . 20.8.8 Th rmal Prot ction Th th rmal data for th cat na ry ar also giv n in Tabl 20.1. Th calculation of th th rmal prot ction s tti ngs is giv n in th following s ctions. 20.8.8.1 Th rmal r f r nc curr nt/ t mp ratur Th P438 r quir s a th rmal rat d curr nt or r f r nc curr nt, Ir f, to b s t that corr sponds to full load curr nt. Th ambi nt t mp ratur at which this appli s qualifi s this rat d curr nt. Th r f r nc curr nt r f rr d to th CT primary is giv n in Tabl 20.1 as: Ir fp = 540A Th r lay s tting is in t rm s of th s condary curr nt. H nc , th s condary curr nt s tting on th r lay is found by dividing by th CT ratio: I r fs = 540 =0.9 A 600 P rot ction of A.C. El ctrifi d Railways 20.8.7.4 Ov rcurr nt s tting for BUOC d finit -tim d lay d stag To nsur comp l t cov r for short circuits in th prot ct d s ction, th s tting should b no gr at r than 80% of th low r of th two fault sc narios: I oc 1400 x 0.8 = 110 0A In t rms of s condary quantiti s, Th ambi nt t mp ratur tamb at which Ir fp occurs is s t at 20°C. 20.8.8.2 M chan ical damag prot ction Th cat nary t mp ratur at which m chanical damag may b gin to occur is 56°C. This must corr spond to th P438 th rmal trip command, and so: tcatmax = 56°C Account must b tak n of th m asur m nt rrors d scrib d in S ction 20.4.1. Th P438 r lay s tting, trip, must allow for th s rrors, which a r tak n to b 4°C. H nc , = (56 - 4)°C = 52°C To avoid chatt ring of contacts wh n th load curr nt is clos to th trip thr shold, a hyst r sis s tting is provid d on r s t. Typically th hyst r sis is s t to 2%, such that following a trip, th th rmal mod l must cool by 2% b for th trip contacts will r s t. trip N twork Prot ction & Automation Guid +• 20 • I
oc =1100
600 =1.86 A A tim s tting no l ss than th Zon 2 distanc tim d lay would b us d, so tI’ o c = 250ms is suitabl . All ov rcurr nt prot ction must hav a pickup in xc ss o f th maximum xp ct d load curr nt. Assuming that th maximum ov rloading would n v r xc d 150% of CT rating, th I’inst and I’ oc s ttings ar acc ptabl . 20.8.
7.5 D finit Tim Ov rcurr nt (DTOC) It is not g n ral practic to s t instantan ous prot ction l m nts that ar running in parall l to th distanc zon s. Thu s oft n just on d finit tim d lay d stag is us d. This s tting can b appli d at all locations, and must b in xc ss of th maximum load and ov rload curr nt xp ct d. • 368 •
20.8.8.3 D wir m nt prot ction An alarm should b issu d to warn th rail op rat or wh n sp d r strictions ar n c ssary, to avoid th risk of d wir m nts. From Tabl 20.1, th cat nary t mp ratur at which th r is a dang r of d wir m nt i s 48°C. Th sam m asur m nt rrors apply as for th trip s tting. H nc th r lay s tting, warning, is: warning = (48 - 4)°C = 44°C 20.8.8.4 Maximum ambi nt t mp ratur It is possibl to plac a limit on th maxi mum ambi nt t mp ratur that will b us d by th th rmal mod l, to avoid ov r-r strictiv loading constraints b ing impos d. From Tabl 20.1: tambmax = 28°C 20.8. 8.5 D fault ambi nt t mp ratur If ambi nt t mp ratur comp nsation is not b ing us d, an assum d d fault coolant t mp ratur ambi nt must b chos n. Th d faul t ambi nt t mp ratur must b chos n to b suffici ntly high to minimis th dan g r of und t ct d probl ms occurring on hot days, wh n th ambi nt t mp ratur i s w ll in xc ss of th d fault valu . Similarly, it must not b so high that al arms and/or trips occur unn c ssarily. A d fault ambi nt t mp ratur , tambd f, o f 20°C, would provid ad quat prot ction, xc pt for a calculat d risk on c rtain hot summ r days. Not that th rat d th rmal curr nt at this ambi nt is Ir fs. 20.8.8.6 Th rmal tim constants Th cat nary th rmal mod l r quir s h ating and cooling tim constants to b sp cifi d. For most cat nari s, th h ating and coo ling tim constants would b xp ct d to b qual. How v r, this may not always b th cas , for xampl th cooling tim constant at night may b long r than t hat applicabl during th day. Th r lay can accommodat diff r nt s ttings wh r r quir d. Cons rvativ s ttings that assum th worst cas tim constants for h ating ( h) and cooling ( c) would b to assum a daytim h ating tim constant and nightim cooling tim constant. H nc : h c Param t r Zon 1 forward r ach Zon 1 backward r ach backward r ach Zon 3 forward r ach Zon 3 backward 2 tim d lay Zon 3 tim d lay Back-Up Ov rcurr nt ng Back-Up Ov rcurr nt IDMT curr nt s tting
Zon 2 forward r ach Zon 2 r ach Zon 1 tim d lay Zon Instantan ous curr nt s tti
Symbol Z1fw Z1bw Z2fw Z2bw Z3fw Z3bw t1 t2 t3 I inst I oc Valu 20.75 Block d 36.1 Block d 95.1 5.2 0s 160ms 500ms 3.68A 1.86A 250ms 1.5A 800ms 0.9A 20°C 52°C 44°C 28°C 20°C 5 min 7 min Tabl 20.2: El ctrifi d railway syst m xampl -r lay s ttings • = 5min = 7min Th P438 also allows th th rmal rating of th prot ction to b modifi d, bas d on signals from opto inputs. How v r, this facility is not us d in this xampl . 20.8.9 Summary of Cat nary Prot ction S ttings Th prot ction calculations for th cat nary ar now compl t . Th r lay s ttings ar summaris d in Tabl 20.2. N twork Prot ction & Automation Guid • 369 • P rot ction of A.C. El ctrifi d Railways Back-Up Ov rcurr nt IDMT tim d lay s tting tI oc D finit Tim Ov rcurr nt prot ction curr nt s tting I dtinst D finit Tim Ov rcurr nt prot ction tim d lay s tting tI dtinst Th rmal Prot ction r f r nc curr nt Ir fs Ambi nt t mp ratur
r f r nc tamb Th rmal trip t mp ratur trip Th rmal warning t mp ratur warnin g Maximum ambi nt t mp ratur tambmax D fault ambi nt t mp ratur tambd f H atin g tim constant - daytim h Cooling tim constant - nightim c 20 •
• 21 • R lay T sting and Commissioning Introduction El ctrical typ t sts El ctromagn tic compatibility t sts Product s af ty typ t sts Environm ntal typ t sts Softwar typ t sts Dynamic validation typ t sting Production t sting Commissioning t sts S condary inj ction t st q uipm nt S condary inj ction t sting 21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 21.9 21.10 21.11 Primary inj ction t sting 21.12 T sting of prot ction sch m logic 21.13 Trippin g and alarm annunciation t sts 21.14 P riodic maint nanc t sts 21.15 Prot ction sch m d sign for maint nanc 21.16 R f r nc s 21.17
• 21 • R lay T sting and Commissioning 21.1 INTRODUCTION Th t sting of prot ction quipm nt sch m s pr s nts a numb r of probl ms. This is b caus th main function of prot ction quipm nt is sol ly conc rn d with op ration und r syst m fault conditions, and cannot r adily b t st d und r normal syst m op rating conditions. This situation is aggravat d by th incr asing compl xity of prot ction sch m s and us of r lays containing sof twar . Th t sting of prot ction quipm nt may b divid d into four stag s: i. t yp t sts ii. routin factory production t sts iii. commissioning t sts iv. p ri odic maint nanc t sts 21.1.1 Typ T sts Typ t sts ar r quir d to prov that a r lay m ts th publish d sp cification and compli s with all r l vant standard s. Sinc th principal function of a prot ction r lay is to op rat corr ctly un d r abnormal pow r conditions, it is ss ntial that th p rformanc b ass ss d und r such conditions. Compr h nsiv typ t sts simulating th op rational condi tions ar th r for conduct d at th manufactur r s works during th d v lopm nt and c rtification of th quipm nt. Th standards that cov r most asp cts of r lay p rformanc ar IEC 60255 and ANSI C37.90. How v r complianc may also invol v consid ration of th r quir m nts of IEC 61000, 60068 and 60529, whil produc ts int nd d for us in th EEC also hav to comply with th r quir m nts of Dir ctiv s 89/336/EEC and 73/23/EEC. Sinc typ t sting of a digital or num rical r lay involv s t sting of softwar as w ll as hardwar , th typ t sting proc ss i s v ry complicat d and mor involv d than a static or l ctrom chanical r lay. N twork Prot ction & Automation Guid • 371 •
21.1.2 Routin Factory Production T sts Th s ar conduct d to prov that r lays ar fr from d f cts during manufactur . T sting will tak plac at s v ral st ag s during manufactur , to nsur probl ms ar discov r d at th arli st possi bl tim and h nc minimis r m dial work. Th xt nt of t sting will b d t rmi n d by th compl xity of th r lay and past manufacturing xp ri nc . 21.1.3 Com missioning T sts Th s t sts ar d sign d to prov that a particular prot ction sch m has b n install d corr ctly prior to s tting to work. All asp cts of th sch m ar thoroughly ch ck d, from installation of th corr ct quipm nt throu gh wiring ch cks and op ration ch cks of th individual it ms of quipm nt, fini shing with t sting of th compl t sch m . would tak 4 y ars to writ th functional typ -t st sp cifications, 30 y ars to p rform th t sts and s v ral y ars to writ th t st r ports that r sult. Auto mat d t chniqu s/ quipm nt ar cl arly r quir d, and ar cov r d in S ction 21. 7.2. Rang 0.08 - 4.00In 0.08 - 32In Forward/R v rs /Non-dir ctional -95° to +95° DT/IDMT 0 - 100s IEC Standard Inv rs IEC V ry Inv rs IEC IDMT Tim D lay IEC Extr m l y Inv rs UK Long Tim Inv rs Tim Multipli r S tting (TMS) 0.025 - 1.2 IEEE Mo d rat ly Inv rs IEEE V ry Inv rs IEEE IDMT Tim D lay IEEE Extr m ly Inv rs U S-CO8 Inv rs US-CO2 Short Tim Inv rs Tim Dial (TD) 0.5 - 15 IEC R s t Tim ( DT only) 0 - 100s IEEE R s t Tim IDMT/DT IEEE DT R s t Tim 0 - 100s IEEE Mod r at ly Inv rs IEEE V ry Inv rs IEEE IDMT R s t Tim IEEE Extr m ly Inv rs US-C O8 Inv rs US-CO2 Short Tim Inv rs Tabl 21.1: Ov rcurr nt r lay l m nt sp ci fication El m nt I>1 I>2 Dir ctionality RCA Charact ristic D finit Tim D lay S t p Siz 0.01In 0.01In 1° 0.01s 0.025 R l a y T s t i n g a n d C o m m i s s i o n i n g 21.1.4 P riodic Maint nanc Ch cks Th s ar r quir d to id ntify quipm nt fail ur s and d gradation in s rvic , so that corr ctiv action can b tak n. B caus a prot ction sch m only op rat s und r fault conditions, d f cts may not b r v al d for a significant p riod of tim , until a fault occurs. R gular t sting a ssists in d t cting faults that would oth rwis r main und t ct d until a fault occurs. 21.2 ELECTRICAL TYPE TESTS Various l ctrical typ t sts must b p rform d, as follows: 0.1 0.01s 0.01s T st 1 Thr phas non-dir ctional pick up and drop off accuracy ov r compl t curr nt s tting rang for both stag s Thr phas dir ctional pick up and drop off accur acy ov r compl t RCA s tting rang in th forward dir ction, curr nt angl sw p Thr phas dir ctional pick up and drop off accuracy ov r compl t RCA s ttin g rang in th r v rs dir ction, curr nt angl sw p Thr phas dir ctional pi ck up and drop off accuracy ov r compl t RCA s tting rang in th forward dir c tion, voltag angl sw p Thr phas dir ctional pick up and drop off accuracy ov r compl t RCA s tting rang in th r v rs dir ction, voltag angl sw p Th r phas polarising voltag thr shold t st Accuracy of DT tim r ov r compl t s tting rang Accuracy of IDMT curv s ov r claim d accuracy rang Accuracy of IDM T TMS/TD Eff ct of changing fault curr nt on IDMT op rating tim s Minimum Pick-U p of Starts and Trips for IDMT curv s Accuracy of r s t tim rs Eff ct of any blo cking signals, opto inputs, VTS, Autor clos Voltag polarisation m mory 21.2.1 Functional T sts Th functional t sts consist of applying th appropriat inputs to th r lay und r t st and m asuring th p rformanc to d t rmin if it m ts th sp cification. Th y ar usually carri d out und r controll d nvironm ntal conditions. Th t sting may b xt nsiv , v n wh r only a simpl r lay f
unction is b ing t st d., as can b r alis d by consid ring th simpl ov rcurr nt r lay l m nt of Tabl 21.1. To d t rmin complianc with th sp cification, th t sts list d in Tabl 21.2 ar r quir d to b carri d out. This is a tim co nsuming task, involving many ngin rs and t chnicians. H nc it is xp nsiv . W h n a mod rn num rical r lay with many functions is consid r d, ach of which ha s to b typ -t st d, th functional typ -t sting involv d is a major issu . In t h cas of a r c nt r lay d v lopm nt proj ct, it was calculat d that if on p r son had to do all th work, it T st 2 • 21 • T st 3 T st 4 T st 5 T st 6 T st 7 T st 8 T st 9 T st 10 T st 11 T st 12 T st 13 T st 14 Tabl 21.2: Ov rcurr nt r lay l m nt functional typ t sts • 372 • N twork Prot ction & Automation Guid
21.2.2 Rating T sts Rating typ t sts ar conduct d to nsur that compon nts ar us d within th ir sp cifi d ratings and that th r ar no fir or l ctric sho ck hazards und r a normal load or fault condition of th pow r syst m. This is i n addition to ch cking that th product compli s with its t chnical sp cificatio n. Th following ar amongst th rating typ t sts conduct d on prot ction r lay s, th sp cifi d param t rs ar normally to IEC 60255-6. 21.2.3 Th rmal Withstan d Th th rmal withstand of VT’s, CT’s and output contact circuits is d t rmin d to nsur complianc with th sp cifi d continuous and short-t rm ov rload condition s. In addition to functional v rification, th pass crit rion is that th r is n o d trim ntal ff ct on th r lay ass mbly, or circuit compon nts, wh n th prod uct is subj ct d to ov rload conditions that may b xp ct d in s rvic . Th rmal withstand is ass ss d ov r a tim p riod of 1s for CT’s and 10s for VT’s. 21.2.4 R lay Burd n Th burd ns of th auxiliary supply, optically isolat d inputs, VT’s an d CT’s ar m asur d to ch ck that th product compli s with its sp cification. Th burd n of products with a high numb r of input/output circuits is application s p cific i. . it incr as s according to th numb r of optically isolat d input an d output contact ports which ar n rgis d und r normal pow r syst m load condit ions. It is usually nvisag d that not mor than 50% of such ports will b n rg is d in any application. 21.2.5 R lay Inputs R lay inputs ar t st d ov r th sp cifi d rang s. Inputs includ thos for auxiliary voltag , VT, CT, fr qu ncy, o ptically isolat d digital inputs and communication circuits. 21.2.6 R lay Output Contacts Prot ction r lay output contacts ar typ t st d to nsur that th y c omply with th product sp cification. Particular withstand and nduranc typ t sts hav to b carri d out using d.c., sinc th normal supply is via a station batt ry. 21.2.7 Insulation R sistanc Th insulation r sistanc t st is carri d out according to IEC 60255-5, i. . 500V d.c. 10%, for a minimum of 5 N twork Prot ction & Automation Guid s conds. This is carri d out b tw n all circuits and cas arth, b tw n all in d p nd nt circuits and across normally op n contacts. Th acc ptanc crit rion f or a product in n w condition is a minimum of 100MΩ. After a amp heat test the pa ss criterion is a minimum of 10MΩ. 21.2.7 Auxiliary Supplies Digital an numerical protection relays normally require an auxiliary supply to provi e power to the on-boar microprocessor circuitry an the interfacing optoisolate input circuit s an output relays. The auxiliary supply can be either a.c. or .c., supplie f rom a number of sources or safe supplies- i.e. batteries, UPS’, generators, etc., all of which may be subjectto voltage ips, short interruptions an voltage va riations. Relays are esigne to ensure that operation is maintaine an no ama ge occurs uring a isturbance of the auxiliary supply. Tests are carrie out fo rboth a.c. an .c. auxiliary supplies an inclu e mains variation both above a n below the nominalrating, supply interruptions erive by open circuit an sh ort circuit, supply ips as a percentage of the nominal supply, repetitive start s. The uration of the interruptions an supply ips range from 2ms to 60s inter vals. A short supply interruption or ip up to 20ms, possibly longer, shoul not cause any malfunction of the relay. Malfunctions inclu e the operation of outpu t relays an watch og contacts, the reset of microprocessors, alarm or trip in i cation, acceptance of corrupte ata over the communication link an the corrupt ion of store ata or settings. For a longer supply interruption, or ip in exce ss of 20ms, the relay self recovers without the loss of any function, ata, sett ings or corruption of ata. No operator intervention is require to restore oper ation after an Many relays have a specificati interruption or ip in the supply. on that excee sthis requirement, tolerating ips of up to 50ms without operatio n being affecte . In a ition to the above, the relay is subjecte to a number o f repetitive starts or a sequenceof supply interruptions. Again the relay is te ste to ensure that no amage or ata corruption has occurre uring the repetit ive tests. Specific tests carrie out on .c. auxiliary supplies inclu e reverse polarity, a.c. waveform superimpose on the .c. supply an the effect of a ris ing an ecaying auxiliary voltage. All tests are carrie out at various levels of loa ing of the relay auxiliary supply. 21.3 ELECTROMAGNETIC COMPATIBILITY TES TS There are numerous tests that are carrie out to etermine the ability of rel
ays to withstan the electrical • • 373 • R e l a y Te s t i n g a n
C o m m i s s i o n i n g 21 •
environment in which they are installe . The substation environment is a very se vere environment in terms of the electrical an electromagnetic interference tha t can arise. There are many sources ofinterference within a substation, some or iginating internally, others beingcon ucte along the overhea lines or cables into the substation from external isturbances. The most common sources are: a. switching operations b. system faults c. lightning strikes . con uctor flashove r e. telecommunication operations e.g. mobile phones A whole suite of tests are performe to simulate these types of interference, an they fall un er the broa umbrella of what is known as EMC, or Electromagnetic Compatibility tests. that the relay can withstan an interruption in the auxiliary supply without eenergising, e.g. switching off, an that when this time is excee e an it oes transiently switch off, that no maloperation occurs. It simulates the effect of a loose fuse in the battery circuit, or a short circuit in the common .c. suppl y, interrupte by a fuse. Another source of .c. interruption isif there is a p ower system fault an the battery is supplying both the relay an the circuit br eaker trip coils. When the battery energises the coils to initiate the circuit b reaker trip, the voltage may fall below the require level for operation of the relay an hence a .c. interrupt occurs. The test is specifie in IEC 60255-11 a n comprises a interruptions of 2, 5, 10, 20, 50, 100 an 200ms. For interruptio ns lasting up to an inclu ing 20ms, the relay must not e-energise of maloperat e, while for longer interruptions it must not maloperate. The relay is powere f rom a battery supply, an both short circuit an open circuit interruptions are carrie out. Each interruption is applie 10 times, an for auxiliary power supp lies with a large operating range, the tests are performe at minimum, maximum, an other voltages across this range, to ensure compliance overthe complete ran ge. 21.3.2 A.C. Ripple on D.C. Supply This test (IEC 60255-11) etermines thatt he relay is able to operate correctly with a superimpose a.c. voltage on the . c. supply. This is cause by the station battery being charge by the battery ch arger, an the relevant waveform is shown in Figure 21.1. It consists of a 12% p eak-to-peak ripple superimpose on the .c. supply voltage. 60.00 50.00 40.00 Voltage (V) R e l a y Te s t i n g a n C o m m i s s i o n i n g Broa ly speaking, EMC can be efine as: ‘The ability of equipment to co-exist in the same electromagnetic environment’ It is not a new subject an has been teste for by the military ever sincethe a vent of electronic equipment. EMC can cause real an serious problems, an oes nee to be taken into account when esignin g electronic equipment. EMC tests etermine the impact on the relay un er test o f high-frequency electrical isturbances of various kin s. Relays manufacture o r inten e for use in the EEC have to comply with EEC Directive 89/336/EEC in th is respect. To achieve this, in a ition to esigning forstatutorycompliance t o this Directive, the following range of tests a. .c. interrup are carrie out: t test b. a.c. ripple on .c. supply test c. .c. ramp test . high frequency i sturbance test e. fast transient test f. surge immunity test g. power frequency interference test h. electrostatic ischarge test i. con ucte an ra iate emis sionstests j. con ucte an ra iate immunity tests k. power frequency magnetic fiel tests 21.3.1 D.C Interrupt Test This is a test to etermine the maximum l ength of time • 21 • 30.00 20.00 10.00 0.00 1 88 175 262 349 436 523 610 697 784 871 958 1045 1132 1219 1306 1393 Time (ms) Figure 21.1: A.C. ripple superimpose on .c. supply test
For auxiliary power supplies with a large operating range, the tests are perform e at minimum, maximum, an other voltages across this range, to ensure complian ce for the complete range. The interference is applie using a full wave rectifi er network, connecte in parallel with the battery supply. The relay must contin ue to operate without malfunction uring the test. Network Protection & Automation Gui e • 374 •
21.3.3 D.C. Ramp Down/Ramp Up This test simulates a faile station battery charg er, which woul result in the auxiliary voltage to the relay slowly ramping own . The ramp up part simulates the battery being recharge ischarging. The after relay must power up cleanly when the voltage is applie an not maloperate. Ther e is no international stan ar for this test, so in ivi ual manufacturers can e ci e if they wish to con uct such a test an what the test specification shall b e. 21.3.4 High Frequency Disturbance Test The High Frequency Disturbance Test si mulates high voltage transients that resultfrom power system faults an plant s witching operations. It consists of a 1MHz ecaying sinusoi al waveform, as show n in Figure 21.2. The interference is applie across each in epen ent circuit ( ifferential mo e) an between each in epen ent circuitan earth (common mo e) v ia an external coupling an switching network. The pro uct is energise in both normal (quiescent) an trippe mo es for this test, an must not maloperate when the interference is applie for a 2 secon uration. V Burst perio , 300 ms Burst uration (1/15 ms) t V 5 ns rise time, 50 ns pulse wi th Repetition perio Figure 21.3: Fast Transient Test waveform t
21.3.6 Surge Immunity Test The Surge Immunity Test simulates interference cause by major power system isturbances such as capacitor bank switching an lightni ng strikes on overhea lines within 5km of the substation. The testwaveform has an open circuit voltage of 4kV for common mo e surges an 2kV for ifferential mo e surges. The test waveshape consists on open circuit of a 1.2/50ms rise/fall time an a short circuit current of 8/20ms rise/fall time. The generator is cap able of provi ing a short circuit test current of up to 2kA, making this test po tentially estructive. The surges are applie sequentially un er software contro l via e icate coupling networks in both ifferential an common mo es with the pro uct energise in its normal (quiescent) state. The pro uct shall not malope rate uring the test, shall still operatewithin specification after the test se quence an shall not incur any permanent amage. 21.3.7 Power Frequency Interfer ence This test simulates the type of interference that is cause when there is a power system fault an very high levels of fault current flow in the primary co n uctors or the earth gri . This causes 50 or 60Hz interference to be in uce on to control an communications circuits. There is no international stan ar for t his test,but one use by some Utilities is: a. 500V r.m.s., common mo e b. 250V r.m.s., ifferential mo e 0 Time Figure 21.2: High Frequency Disturbance Test waveform • 21.3.5FastTransient Test The Fast Transient Test simulates the HV interference cause by isconnector operations in GIS substations or break own of the SF6 in sulation between con uctors an the earthe enclosure. This interference can eit her be in uctively couple onto relay circuits or can be irectly intro uce via the CT or VT inputs. It consists of a series of 15ms uration bursts at 300ms i ntervals, each burst consisting of a train of 50ns wi e pulses with very fast (5 ns typical) rise times (Figure 21.3), with a peak voltage magnitu e of 4kV. Network Protection & Automation Gui e • 375 •
R e l a y Te s t i n g a n C o m m i s s i o n i n g 21 • The pro uct is energise in both normal (quiescent) an trippe mo es forthis t est. It must not maloperate when the interference is applie in common mo e via the integral coupling network to each circuit in turn, for 60 secon s. Interfere nce is couple onto communications circuits, if require , using an external capa citive coupling clamp. Voltage
applie . Tests are c to circuits for which power system inputs are not connecte arrie out oneach circuit, with the relay in the following mo es of operation: 1. current an voltage applie at 90% of setting, (relay not trippe ) 2. current an voltage applie at 110% of setting, (relay trippe ) 3. main protection an communications functions are teste to etermine the effect of the interference The relay shall not maloperate uring the test, an shall still perform its main functions within the claime tolerance. 21.3.8 Electrostatic Discharge Test Thi s test simulates the type of high voltage interferencethat occurs when an opera tor touches the relay’s front panel after being charge to a high potential. This is exactly the same phenomenon as getting an electric shock when stepping out of a car or after walking on a synthetic fibre carpet. In this case the ischarge is only ever applie to the front panel of the relay, with the cover both on an off. are applie , air ischarge an contact ischarge. Two types of ischarges Air ischarges are use on surfaces that are normally insulators, an contact i scharges are use on surfaces that are normally con ucting. IEC 60255-22-2 is th e relevant stan ar this test, for whichthe test parameters are: a. cover on: C lass 4, 8kV contact ischarge, 15kV air ischarge b. cover off: Class 3, 6kV con tact ischarge,8kV air ischarge In both cases above, all the lower test levels are also teste . The ischarge current waveform is shown in Figure 21.4. 100 90 80 70 60 50 40 30 20 10 0 Current, % of Peak 1. current an voltage applie at 90% of setting, (relay not trippe ) 2. current an voltage applie at 110% of setting, an (relay trippe ) 3. main protection communications functions are teste to etermine the effect of the ischarge To pass, the relay shall not maloperate, an shall still perform its main functions within the claime tolerance. 21.3.9 Con ucte an Ra iate Emissions Tests The se tests arise primarily from the essential protection requirements of the Europ ean Community (EU) irective on EMC. These require manufacturers to ensure that any equipment to be sol in the countries comprising the European Union must not interfere with other equipment. the To achieve this it is necessary to measure emissions from the equipment an ensure that they are below the specifie limits ports an . Con ucte emissions are measure only from the equipment’s power supply are to ensure that when connecte to a mains network, the equipment oes not i nject interferenceback into the network which coul a versely affect the other equipment connecte to thenetwork. Ra iate emissions measurements are to ensur e that the interference ra iate from the equipment is not at a level that coul cause interference to other equipment. This test is normally carrie out on an OpenArea Test Site (OATS) where there are no reflecting structures or sources o f ra iation, an therefore the measurements obtaine are a true in icationof th e emission spectrum of the relay. An example of a plot obtaine uring con ucte is shown in Figure 21.5. The test arrangements for the con ucte emissions tests an ra iate emissions tests are shown in Figure 21.6. When performing these t wo tests, the relay is in a quiescent con ition, that is not trippe , with curre nts an voltages applie at 90% of the setting values. This is because for the m ajority of its life, the relay will be in the quiescentstate an theemission o f electromagnetic interferencewhenthe relayis trippe is consi ere to be of no significance. Tests are con ucte in accor ance with IEC 60255-25 an EN 5008 1-2, an are etaile in Table 21.3. Frequency Range Ra iate Specifie Limits 30 B(µV/m) at 30 - 230MHz 230 - 1000MHz 30m 37 B(µV/m) at 30m 66 B(µV) average 79 B(µV) quasi-peak 73 B(µV) quasi-peak 60 B(µV) av erageTest Limits 40 B(µV/m) at 10m 47 B(µV/m) at 10m 79 B(µV) quasi-peak 66 B(µV) avera ge 73 B(µV) quasi-peak 60 B(µV) average • R e l a y Te s t i n g a n
C o m m i s s i o n i n g 21 • Rise Time = 0.7 to 1.0 ns. Current specifie for 30 ns an 60 ns
0 10 20 30 40 50 Time, ns 60 70 80 90 Figure 21.4: ESD Current Waveform The test is performe with single ischarges repeate on each test point 10 time s with positive polarity an 10 times with negative polarity at each test level. The time interval between successive ischarges is greater than 1 secon . Tests are carrie out at each level, with the relay in the following mo es of operati on: 0.15 - 0.5MHz Con ucte 0.5 - 30MHz Table 21.3: Test criteria for Con ucte an Ra iate Emissions tests • 376 •
Network Protection & Automation Gui e
100 90 80 70 Quasi-peak limits Average limits Typical trace Emissions Level, BuV 60 50 40 30 20 10 0 0.1 1 Frequency, MHz 10 100 Figure 21.5: Con ucte Emissions Test Plot Screene room Ante-chamber E.U.T. Access panel Impe ance network Support/analysis equipment (a) Con ucte EMC emissions test arrangement 10m • Antenna E.U.T. Turntable Earth Plane (b) Ra iate Emissions test arrangement on an OATS E.U.T. - Equipment un er test Figure 21.6: EMC test arrangements Network Protection & Automation Gui e • 377 • R e l a y Te s t i n g a n
C o m m i s s i o n i n g 21 •
21.3.10 Con ucte an Ra iate Immunity Tests These tests are esigne to ensure that the equipment is immune of tolevels interference that it may be subjecte to. The two tests, con ucte an ra iate , arise from the fact that for a con u ctor to be an efficient antenna, it must have a length of at least 1/4 of the wa velength of the electromagnetic wave it is require to con uct. If a relay were to be subjecte to ra iate interference at 150kHz, then a con uctor length of a t least λ = 300 x106/(150 x 103 x 4) = 500 m wou d be needed to conduct the interf erence. Even with a the cab ing attached and with the ongest PCB track ength taken into account, it wou d be high y un ike y that the re ay wou d be ab e to conduct radiation of this frequency, and the test therefore, wou d have no effe ct. The interference has to be physica y introduced by conduction, hence the co nducted immunity test. However, at the radiated immunity ower frequency imit o f 80MHz, a conductor ength of approximate y 1.0m is required. At this frequency , radiated immunity tests can be performed with the confidence that the re ay wi conduct this interference, through a combination of the attached cab ing and the PCB tracks. A though the test standards state that a 6 faces of the equipm ent shou d be subjected to the interference, in practice this is not carried out . App ying interference to the sides and top and bottom of the re ay wou d have itt e effect as the circuitry inside is effective y screened by the earthed met a case. However, the front and rear of the re ay are not comp ete y enc osed by meta and are therefore not at a we screened, and can be regarded as an EMC ho e. E ectromagnetic interference when directed at the front and back of the r e ay can enter free y onto the PCB’s inside. When performing these two tests, the re ay is in a quiescent condition, that is not tripped, with currents and vo tag es app ied at 90% of the setting va ues. This is because for the majority of its ife, the re ay wi be in the quiescent state and the coincidence of an e ectr omagnetic disturbance and a fau t is considered to be un ike y. However, spot ch ecks are performed at se ected frequencies when the main protection and contro functions of the re ay are exercised, to ensure that it wi operate as expected , shou d it be required to do so. The frequencies for the spot checks are in gen era se ected to coincide with the radio frequency broadcast bands, and in parti cu ar, the frequencies of mobi e communications equipment used by personne work ing in the substation. This is to ensure that when working in the vicinity of a re ay, the personne shou d be ab e to operate their radios/mobi e phones without fear of re ay ma operation. IEC 60255 -22-3 specifies the radiated immunity tests to be conducted (ANSI/IEEE C37.90.2 is used for equipment bui t to US standards), with signa eve s of: 1. IEC: C a ss III, 10V/m, 80MHz -1000MHz 2. ANSI/IEEE: 35V/m 25MHz - 1000MHz with no modu a tion, and again with 100% pu se modu ation IEC 60255-22-6 is used for the conduc ted immunity test, with a test eve of: C ass III, 10V r.m.s., 150kHz - 80MHz. 21.3.11 Power Frequency Magnetic Fie d Tests These tests are designed to ensure that the equipment is immune to magnetic interference. The three tests, steady s tate, pu sed and damped osci atory magnetic fie d, arise from the fact that for different site conditions the eve and waveshape is a tered. 23.3.11.1 Steady state magnetic fie d tests These tests simu ate the magnetic fie d that wou d be experienced by a device ocated within c ose proximity of the power system. Tes ting is carried out by subjecting the re ay to a magnetic fie d generated by two induction coi s. The re ay is rotated such that in each axis it is subjected to the fu magnetic fie d strength. IEC 610004-6 is the re evant standard, using a signa eve of: Leve 5: 300A/m continuous and 1000A/m short duration The tes t arrangement is shown in Figure 21.7. R e a y Te s t i n g a n d C o m m i s s i o n i n g Induction coi • 21 •
Induction coi E.U.T. Ground p ane E.U.T. - Equipment under test Figure 21.7: Power frequency magnetic fie d set-up • 378 • Network Protection & Automation Guide
To pass the steady-state test, the re ay sha not ma operate, and sha sti p erform its main functions within the c aimed to erance. During the app ication o f the short duration test, the main protection function sha be exercised and v erified that the operating characteristics of the re ay are unaffected. 21.3.11. 2 Pu sed magnetic fie d These tests simu ate the magnetic fie d that wou d be ex perienced by a device ocated within c ose proximity of the power system during a transient fau t condition. According to IEC 61000-4-9, the generator for the i nduction coi s sha produce a 6.4/16µs waveshape with test eve 5, 100A/m with t he equipment configured as for the steady state magnetic fie d test. The re ay s ha not ma operate, and sha sti perform its main functions within the c aim ed to erance during the test. 21.3.11.3 Damped osci atory magnetic fie d These tests simu ate the magnetic fie d that wou d be experienced by a device ocated within c ose proximity of the power system during a transient fau t condition. I EC 61000-4-10 specifies that the generator for the coi sha produce an osci a tory waveshape with a frequency of 0.1MHz and 1MHz, to give a signa eve in ac cordance with Leve 5 of 100A/m, and the equipment sha be configured as in Fig ure 21.7. 21.4 PRODUCT SAFETY TYPE TESTS A number of tests are carried out to de monstrate that the product is safe when used for its intended app ication. The e ssentia requirements are that the re ay is safe and wi not cause an e ectric shock or fire hazard under norma conditions and in the presence of a sing e fau t. A number of specific tests to prove this may be carried out, as fo ows. 21. 4.1 Die ectric Vo tage Withstand Die ectric Vo tage Withstand testing is carried out as a routine test i.e. on every unit prior to despatch. The purpose of this test is to ensure that the product bui d is as intended by design. This is done by verifying the c earance in air, thus ensuring that the product is safe to op erate under norma use conditions. The fo owing tests are conducted un ess othe rwise specified in the product documentation: 1. 2.0kV r.m.s., 50/60Hz for 1 min ute between a termina s and case earth and a so between independent circuits, in accordance with IEC 60255-5. Some communication circuits are exc uded from th is test, or have modified test requirements e.g. those using D-type connectors 2 . 1.5kV r.m.s., 50/60Hz for 1 minute across norma y Network Protection & Automation Guide open contacts intended for connection to tripping circuits, in accordance with A NSI/IEEE C37.90 3. 1.0kV r.m.s., 50/60Hz for 1 minute across the norma y open c ontacts of watchdog or changeover output re ays, in accordance with IEC 60255-5 The routine die ectric vo tage withstand test time may be shorter than for the 1 minute type test time, to a ow a reasonab e production throughput, e.g. for a minimum of 1 second at 110% of the vo tage specified for 1 minute. 21.4.2 Insu a tion Withstand for Overvo tages The purpose of the High Vo tage Impu se Withstan d type test is to ensure that circuits and their components wi withstand overv o tages on the power system caused by ightning. Three positive and three negati ve high vo tage impu ses, 5kV peak, are app ied between a circuits and the cas e earth and a so between the termina s of independent circuits (but not across n orma y open contacts). As before, different requirements app y in the case of c ircuits using D-type connectors. The test generator characteristics are as speci fied in IEC 60255-5 and are shown in Figure 21.8. No disruptive discharge (i.e. f ashover or puncture) is a owed. If it is necessary to repeat either the Die e ctric Vo tage or High Vo tage Impu se Withstand tests these shou d be carried ou t at 75% of the specified eve , in accordance with IEC 60255-5, to avoid overst ressing insu ation and components. Vo tage • 5kV peak Rise time (10 % to 90 %) = 1.2 s Duration (50 %) = 50 s Time
Figure 21.8: Test generator characteristics for insu ation withstand test 21.4.3 Sing e Fau t Condition Assessment An assessment is made of whether a sing e fau t condition such as an over oad, or an open or short circuit, app ied to the product may cause an e ectric shock or fire • 379 • R e a y Te s t i n g a n d C o m m i s s i o n i n g 21 •
hazard. In the case of doubt, type testing is carried out to ensure that the pro duct is safe. 21.4.4 Earth Bonding Impedance C ass 1 products that re y on a pro tective earth connection for safety are subjected to an earth bonding impedance (EBI) type test. This ensures that the earth path between the protective earth c onnection and any accessib e earthed part is sufficient y ow to avoid damage in the event of a sing e fau t occurring. The test is conducted using a test vo ta ge of 12V maximum and a test current of twice the recommended maximum protective fuse rating. After 1 minute with the current f owing in the circuit under test, the EBI sha not exceed 0.1Ω. 21.4.5 CE Marking 21.5.2 Humi ity Test The humi ity test is performe to ensure that the pro uct w ill withstan an operate correctly when subjecte to 93% relative humi ity at a constant temperature of 40°C for 56 ays. Tests are performe to ensure that the pro uct functions correctly within specification after 21 an 56 ays. After the test, visual inspections are ma e for any signs of unacceptable corrosion an m oul growth. 21.5.3 Cyclic Temperature/Humi ity Test This is a short-term test t hat stressesthe relay by subjecting it to temperature cycling in conjunction wi th high humi ity. The test oes not replace the 56 ay humi ity test, but is use for testing extension to ranges orminor mo ifications to prove that the esig n is unaffecte .The applicable stan ar is IEC 60068-2-30 an test con itions o f: +25°C 3°C an 95% relative humi ity/+55°C 2°C an 95% relative humi ity are use , over the 24 hour cycle shown in Figure 21.9. Relative humi ity % R e l a y Te s t i n g a n C o m m i s s i o n i n g A CE mark on the pro uct, or its packaging, shows that compliance is claime aga inst relevant European Community irectives e.g. Low Voltage Directive 73/23/EEC an Electromagnetic Compatibility (EMC) Directive 89/336/EEC. 21.5 ENVIRONMENTA L TYPE TESTS Various tests have to be con ucte to prove that a relay can withst an the effects of the environment in which it is expecte to work. They consist of: the following tests: 1. temperature 2. humi ity 3. enclosure protection 4. mechanical These tests are escribe in the following sections. 21.5.1 Temperatu re Test Temperature tests are performe to ensure that a pro uct can withstan e xtremes in temperatures, both hot an col , uring transit, storage an operatin g con itions. Storage range an transit con itions are efine as a temperature of –25°C to +70°C an operating as–25°C to +55°C. Dry heat withstan tests are performe a 70°C for 96 hours with the relay e-energise . Col withstan tests are performe at –40°C for 96 hours with the relay eenergise . Operating range tests are carrie out with the pro uct energise , checking all main functions operate within tole rance over the specifie working temperature range –25°C to +55°C. 100 90 80 70
96% 95% 90% 15min En of temperature rise 95% 80% temperature Time +55°C Ambient Temperature °C
0.5h 3h 12h 0.5h 24h 3h 6h • 21 • +28°C +22°C Time +25°C
Figure 21.9: Cyclic temperature/humi ity test profile
For these tests the relay is place in a humi ity cabinet, an energise with no rmal in-service quantities for the complete uration of the tests. In practical terms this usually means energising the relay with currents an voltages such th at it is 10% from the threshol for operation. Throughout the uration of the te st the relay ismonitore to ensure that no unwante operations occur. Once the relay is remove from the humi ity cabinet, its insulation resistance is measure to ensure that it has not eteriorate to below the claime level. The relay i s then functionally teste again, an finally ismantle to Network Protection & Automation Gui e • 380 •
check for signs The acceptance criterion of component corrosion an growth. is t hatno unwante operations shall occur inclu ing transient operation of in icati ng evices.After the test the relay’s insulation resistance shoul not have signi ficantly re uce , an it shoul perform all of its main protection an communica tions functions within the claime tolerance. The relay shoul also suffer no si gnificant corrosion or growth, an photographs are usually taken of each PCB an the case as a recor of this. 21.5.4 Enclosure Protection Test Enclosure protec tion tests prove thatthe casing system an connectors on the pro uct protectag ainst the ingress of ust, moisture, water roplets (striking the case at pre ef ine angles) an other pollutants. An ‘acceptable’ level of ust or water may penetr ate the case uring testing, but must not impair normal pro uct operation, safet y or cause tracking across insulate parts of connectors. 21.5.5Mechanical Test s Mechanical tests simulate a number of ifferent mechanical con itions that the pro uct may have to en ure uring its lifetime. These fallinto two categories a. response to isturbances while energise b. response to isturbances uring t ransportation ( e-energise state) Tests in the first category are concerne wit hthe response to vibration, shock an The tests are esign seismic isturbance. e to simulate normal in-service con itions for the pro uct, for example earthqu akes. These tests are performe in all three axes, with the pro uct energise in its normal (quiescent) state. During the test, all output contacts are continua lly monitore for change using contact follower circuits. Vibration levels of 1g n, over a 10Hz-150Hz frequency sweep are use . Seismictests use excitation in a single axis, using an a test frequency of 35Hz an peak isplacements of 7.5mm 3.5mm in the x an y axes respectively below the crossover frequency an peak a ccelerations of 2.0gn an 1.0gn in these axes above the crossover frequency. The secon category consists of vibration en urance, shock withstan an bump tests . They are esigne to simulate the longer-term affects of shock an vibration t hat coul occur uring transportation. These tests are performe with the pro uc t e-energise. After these tests, the pro uct must still operate within its spe cification an show no signs of permanent mechanical amage. Equipment un ergoin g a seismic type test is shown in Figure 21.10, while the waveform for the shock /bump test is shown in Figure 21.11 Network Protection & Automation Gui e Figure 21.10: Relay un ergoing seismic test 1.2A A 0.8A Pulse shape (half sine) 0.4D 2.5D D D 2.5D
2.4D = T1 6D = T2 D - uration of nominal pulse A - peak acceleration of nominal pulse T1- minimum time for monitoring of pulse when conventional shock/bump mac hine is use T2 - as T1 when a vibration generator is use Figure 21.11: Shock/B ump Impulse waveform The test levels forshock an bump tests are: (energise ): 3 puls Shock response uration ): 3 pulses, 15g, 11ms es, each 10g, 11ms Shock withstan ( e-energise uration Bump ( e-energise ): 1000 pulses, 10g, 16ms uration 21.6 SOFTWARE TYPE TESTS Digital an numerical relays contain software to implement the protection an measurement functions of a relay. This software must bethoroughly teste , to ensure thatthe relay complies with all specifications an that isturbances of various kin s o not result in unexpecte results. Software is teste in vari ous stages: a. unit testing b. integration testing c. functional qualification t esting • • 381 •
R e l a y Te s t i n g a n +0.2A 0 -0.2A
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The purpose of or proce u unit testing is to etermine if an in ivi ual function re implemente using software, or small group of closely relate functions, is f ree of ata, logic, or stan ar s errors. It is much easier to etect these types of errors in in ivi ual units or small groups of units than it is in an integra te software architecture an /or system. Unit testing is typically performe aga inst the software etaile esign an by the eveloper of the unit(s). Integrati on testing typically focuses on these interfaces an also issues such as perform ance, timings an synchronisation that are not applicable in unit testing. Integ ration testing also focuses on ‘stressing’ the software an relate interfaces. Inte gration testing is ‘black box’ in nature, i.e. it oes not take into account the str ucture of in ivi ual units. It is typically performe against the software archi tectural an etaile esign. The specifie software requirements woul typicall y also be use as a source for some of the test cases. 21.6.1 Static Unit Testin g Static Unit Testing (or static analysis as it is often calle ) analyses the un it(s) source co e for complexity, precision tracking, initialisation checking, v alue tracking, strong type checking, macro analysis etc. While Static UnitTesti ng can be performe manually, it is a laborious an error prone process an is b est performe using a proprietary automate static unit analysis It is imp tool. ortant to ensure that any such tool is configure correctly an use consistentl y uring evelopment. 21.6.2 Dynamic Testing • 21.6.3 Unit Testing Environment Both Dynamic an Static Unit Testing are perform e in the host environment rather than the target environment. Dynamic Unit Test ing uses a test harness to execute the unit(s) concerne . The test harness is e signe such that it simulates the interfaces of the unit(s) being teste - both software-software interfaces an software-har wareinterfaces - using what are k nown as stubs. The test harness provi es the test ata to those units being test e an outputs the test results in a form un erstan able to a eveloper. There a re many commercially available testing tools to automate test harness pro uction an the execution of tests. 21.6.4 Software/Software Integration Testing Softwa re/Software Integration Testing isperforme in the host environment. It uses a test harness to simulate inputs an outputs, har ware calls an system calls (e. g. the target environment operating system). 21.6.5 Software/Har ware Integratio n Testing Software/Har ware Integration Testing is performe in the target envir onment, i.e. it uses the actual target har ware, operating system, rivers etc. It is usually performe after Software/Software Integration Testing. Testing the interfaces to the har ware is an important feature of Software/Harware Integra tionTesting. Test cases for Integration Testing are typically base on those e fine for Vali ation Testing. However the emphasis shoul be on fin ing errors a n problems. Performing a ry run of the vali ation testing often completes Inte gration Testing. 21.6.6 Vali ation Testing The purpose of Vali ation Testing (al so known as meets it Software Acceptance Testing) is to verify that the software s specifie functional requirements. Vali ation Testing is performe against the software requirementsspecification, using the target environment. In i eal cir cumstances, someone in epen ent of the software evelopment performs the tests. Vali ation Testing is ‘black box’ in nature, i.e. it oes not take into account the internal structure of the software. For relays, the non-protection functions inc lu e in the software are consi ere to be as important as the protection functi ons,an hence teste in the same manner. Each vali ation test shoul have pre e fine evaluation criteria, to be use to eci e if the test has passe or faile . The evaluation criteria shoul be explicit with no room for interpretation or ambiguity. Network Protection & Automation Gui e R e l a y Te s t i n g a n C o m m i s s i o n i n g 21 • Dynamic of the unit(s) being tes Testing is concerne with the runtime behaviour te an sotherefore, the unit(s) must be execute . Dynamic unit testing can be sub- ivi e into ‘black box’ testing an ‘white box’ testing. ‘Black box’ testing verifies
he implementation of the requirement(s) allocate to the unit(s). It takes no ac count of the internal structure of the unit(s) being teste . It is only concerne with provi ing known inputs an etermining if the outputs from the unit(s) ar e correct for those inputs. ‘White box’ testing is concerne with testing the intern al structure of the unit(s) an measuring the test coverage, i.e. how much of th e co e within the unit(s) has been execute uring the tests. The objective of t he unit testing may, for example, be to achieve 100% statement coverage, in whic h every line of the co e is execute at least once, or to execute every possible path through the unit(s) at least once. • 382 •
21.6.7 Traceability of Vali ation Tests Traceability of tests to soft vali ation ware requirements is vital. Each software requirement ocumente in the software requirements specification shoul have at least one vali ation test, an it is important to be able to prove this. 21.6.8 Software Mo ifications - Regression T esting Regression Testing is not atype test in its’ own right. It is the overall name given to the testing performe when an existing software pro uct is change . The purpose of RegressionTesting is to show that uninten e changes to the fu nctionality (i.e. errors an efects) have not been intro uce . Each change to a n existing software pro uct must be consi ere in its’ own right.It is impossible to specify a stan ar set of regression tests that can be applie as a ‘catch-all’ forintro uce errors an efects. Each change to the software must be analyse to etermine what there might be of unintentional changes to the functional risk ity being intro uce . Those areas of highest risk will nee to be regression tes te . The ultimate regression test is to perform the complete Vali ation Testing programme again, up ate to take account of the changes ma e. Regression Testing is extremely important. If it is not performe , there is a high risk of errors being foun in the fiel . Performing it will not re uce tozero the chance of an error or efect remaining in the software, but it will re uce it. Determining t he Regression Testing that is require is ma e much easier if there is traceabil ity from properly ocumente software requirements through esign (again properl y ocumente an up to ate), co ing an testing. 21.7 DYNAMIC VALIDATION TYPE T ESTING There are two possible metho s of ynamically proving the satisfactory pe rformance of protection relays orschemes; the first metho is by actually apply ing faults on the power system an the secon is to carry out comprehensive test ing is extremely unlikely to be u on a power system simulator. The former metho se – lea times are lengthy an therisk of amage occurring makes the tests very expensive. It is therefore only use on a very limite basis an the faults app lie are restricte in number an type. Because of this, a proving perio for ne w protection equipment un er service con itions has usually been require . As fa ults may occur on the power system at infrequent intervals, it can take a number of years before any possible shortcomings are iscovere , uring which time fur ther installations may have occurre. Network Protection & Automation Gui e Powersystem simulators can be ivi e into two types: a. those which use analog ue mo els of a power system b. those which mo el the power system mathematically using igital simulation techniques 21.7.1 Use of Power System Analogue Mo els For many years, relays have been teste on analogue mo els of power systems such as artificial transmission lines, or test plant capable of supplying significan t amounts of current [21.1]. However, these approaches have significant limitati ons in the current an voltage waveforms that can be generate , an are not suit able for automate , unatten e , testing programmes. While still use on a limite basis for testing an static relays, a ra ically ifferent a electromechanical pproach is require for ynamic testing of numerical relays. 21.7.2 Use of Micro processor Base Simulation Equipment The complexity ofnumerical relays, reliant on software for implementation of the functions inclu e , ictates some kin of automate test equipment. The functions of even a simple numerical overcurrent relay (inclu ing all auxiliary functions) can take several months of automate , 24 hours/ ay testing to test completely. If such test equipment was able to appl y realistic current an voltage waveforms that closely match those foun on powe r systems uring fault con itions, the equipment can be use either for type tes ting of in ivi ual relay esigns or of a complete protection scheme esigne for a specific application. of this, a new generation of power syste In recognition m simulators has been evelope , which is capable of provi ing a far more accura te simulation of power system con itions than hasbeen possible in the past. The simulator enables relays to be teste un er a wi e range of system con itions, representing the equivalent of many years of site experience. 21.7.2.1 Simulatio n har ware Equipment is now available to provi e high-spee ,highly accurate mo elling of a section of is base on istribute mic a power system. The equipment roprocessor-base har ware containing software mo els of the various elements of
a power system, an is shown in Figure 21.12. The mo ules have outputs linke t o current an voltage sources that have a similar transient capability an have suitable output levels for irect connection to the inputs of relays –i.e. 110V fo r voltage an 1A/5A for current. Inputs are also provi e to monitor the respons e of relays un er test (contact closures for tripping, etc.) an these inputs ca n be use as part of the mo el of the power • • 383 • R e l a y Te s t i n g a n
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Figure 21.12: Digital power system simulator for relay/protection scheme testing system. The software is also capable of mo elling the ynamic response of CT’s an VT’s accurately. Where it is esire to check the response of a relay or protecti on scheme to an actual power system transient, the transient can be simulate us ing sophisticate power systems analysis software an the results transferre i gitally to the simulator, or the event recor er recor ing of the transient can b e use , in either igital or analogue form as inputs to the simulator mo el. Out put signal conversion involves circuits to eliminate the quantisation steps norm ally foun in conventional D/A conversion. Analogue mo els of the system trans u cer characteristics can beinterpose between the signalprocessors an the outp ut amplifiers when require . This equipment shows many a vantages over tra ition al test equipment: a. the power system mo el is capable of repro ucing high freq uency transients such as travelling waves b. tests involving very long time cons tants can be carrie out c. it is not affecte by the harmonic content, noise an frequency variations in the a.c. supply . it is capable of representing the v ariation in the current associate with generator faults an power swings e. sat uration effects in CT’s an VT’s can be mo elle f. a set of test routines can be sp ecifie in software an then left to run unatten e (or with only occasional mon itoring) to completion, with a etaile recor of test results being available oncompletion A block schematic of the equipment is shown in Figure 21.13, is base aroun a computer which calculates an stores t he igital ata representing the system voltages an currents. The computer cont rols conversion of the igital ata into analogue signals, an it monitors an c ontrols the relaysbeing teste . 21.7.2.2 Simulation software Unlike most tra it ional software use for power systems analysis, the software use is suitable fo r the mo elling the fast transients that occur in the first few millisecon s aft er fault Two very accurate simulation programs are use , one base on inception. time omain an the other on frequency omain techniques. In both programs, sin gle an ouble circuit transmission lines are represente by fully istribute p arameter mo els. The line parameters are calculate from the physical constructi on of the line (symmetrical, asymmetrical, transpose or non-transpose ), taking into account the effect of con uctorgeometry, con uctor internal impe ance an the earth return path. It also inclu es, where appropriate, the frequency epen ence of the line parameters in the frequency omain program. The frequency epe n ent variable effects are calculate using Fast Fourier Transforms an the resu lts are converte to the time omain. Conventional current transformers an capa citor voltage transformers can be simulate . The fault can be applie at any one point in the system an can be anycombination of phase to phase or phase Network Protection & Automation Gui e • 21 • • 384 •
IA Linear interpolation circuits IB D/A conversion CT mo el Current amplifier IC VDU I/O Subsystem VA Computer Linear interpolation circuits Equipment un er test CVT mo el Voltage amplifier Keyboar VDU VB D/A conversion VC
Keyboar Storage
Contact status monitor Signalling Channel Simulation To secon RTDS Key : CT - Current transformer CVT - Capacitor voltage transformer VDU - Visual isplay unit Figure 21.13: Block iagram of microprocessor-base automate relay test system to earth, resistive, or non-linear phase to earth arcing faults. For series comp ensate lines, flashover across a series capacitor following a shortcircuit fau lt can be simulate . The frequency omain mo el is not suitable for eveloping f aults an switching sequences, therefore the wi ely use Electromagnetic Transie nt Program (EMTP), working in the time omain, is employe in such cases. In a ition to these two programs, a simulation program base on lumpe resistance an in uctance parameters is use . This simulation is use to represent systems wit h long time constants an slow system changes ue, for example, to power swings. 21.7.2.3 Simulator applications The simulator is use for checking the accuracy of calibration an performing type tests on a wi e range of protection relays uring their evelopment. It has the following a vantages over existing test meth o s: a. state of the art power system mo elling ata can be use to test relay s b. free om from frequency variations an noise or harmonic content of the a.c. supply c. the relay un er test oes notbur en the power system simulation . a ll tests are accurately repeatable e. wi e ban wi th signals can be pro uce f. a wi erange of frequencies can be repro uce g. selecte harmonics may be super impose on the Network Protection & Automation Gui e power frequency h. the use of irect couple current amplifiers allows time cons tants of any length i. capable of simulating slow system changes j. repro uces f ault currents whose peak amplitu e varies with time k. trans ucer mo els can be inclu e l. automatic testing removes the likelihoo of measurement an setting errors m. two such equipments can be linke together to simulate a system mo el with two relaying points The simulator is also use for the pro uction testing o f relays, in which most of the a vantages liste above apply. As the tests an m
easurements are ma e automatically, the quality of testing is also greatly enhan ce . Further,in cases of suspecte malfunction of a relay in the fiel un er kn own fault con itions, the simulator can be use to replicate the power system an fault con itions, an con uct a etaile investigation into the performance of the relay. Finally, complex protectionschemes can be mo elle , using both the relays inten e for use an software mo to check the els ofthem as appropriate, suitability of the propose scheme un er a wi e variety of con itions. To illus mo elle trate this, Figure 21.14(a) shows a section of a particular power system . The waveforms of Figure 21.14(b) show the three phase voltages an currents a t the primaries of VT1 an CT1 for the fault con ition in icate in Figure 21.14 (a). • • 385 • R e l a y Te s t i n g a n
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Communications link to secon RTDS (When require )
N 3G L CB3 CT3 F3 Line 2 8G F4 CT4 CB4 Infinite bus 4G 9G LR3 LR4 CB1 loa 1 loa 2 loa 3 VT1 CT1 F1 Line 1 F2 CT2 CB2 11G VT2 LR1 LR2 Relay 2 R e l a y Te s t i n g a n
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Relay 1 (a) Example power system Va Vb Vc Ia Ib Ic
Figure 21.14: Example of application stu y 0 0.08 0.16 0.24 0.32 0.4 0.48 0.56 (b) Voltages an currents at VT1/CT1 21.8 PRODUCTION TESTING Pro uction testing of protectionrelays is becoming far more eman ing as the accuracy an complexity of the pro ucts increase. Electron ic power amplifiers areuse to supply accurate voltages an currents of high st ability to the relay un er test. The inclusion of a computer in the test system allows more complex testing to be performe at an economical cost, with the a va ntage of spee 21.1 an repeatability oftests from one relayto another. Figure 5 shows a mo ern computer-controlle test bench. Thehar ware is mounte in a sp ecial rack. Each of the test system is connecte to the computer via unit an int erface bus. In ivi ual test programs for each type of relay are require , but th e interface use is stan ar for all relay types. Control of input waveforms an analogue measurements, the monitoring of output signals an the analysis of tes t ata are performe by the computer. A printout of the test results can also be pro uce if require . • 21 •
Figure 21.15: Mo ern computer-controlle test bench Because software is extensively teste at the typetesting stage, there is normal ly no nee to check the correct functioning of the software. Checks are limite to etermining that the analogue an igital I/O is functioning correctly. This is achieve for inputs by
Network Protection & Automation Gui e • 386 •
applying known voltage an current inputs to the relay un er test an checking t hat the software has capture the correct values. Similarly, igital outputs are exercise by using test softwareto actuate each output an checking that the c orrect output is energise . Provi e that appropriate proce ures are in place to ensure that only type-teste software is ownloa e , there is no nee to check the correct functioning of the software in the relay. The final step is to ownl oa the software appropriate to the relay an store it in the EPROM fitte in th e relay. 21.9 COMMISSIONING TESTS Installation of a protection scheme at site cr eates a number of possibilities for errors in the implementation of the scheme t o occur. Even if the scheme has been thoroughly teste in the factory, wiring to the CT’s an VT’s on site may be incorrectly carrie out, or the CT’s/VT’s may have bee n incorrectly installe . The impact of such errors may range from simply being a nuisance (tripping occurs repeate ly on energisation, requiringinvestigation t o locate an correct the error(s)) through to failure to trip un er fault con it ions, lea ing to major equipment amage, isruption to supplies an potential ha zar s to personnel. The strategies available to remove these risks are many, but all involve some kin of testing at site. Commissioning tests at site are there fore invariably performe before protection equipment is set to work. The aimso f commissioning tests are: 1. to ensure that the equipment has not been amage uringtransit or installation 2. to ensure that the installation work has been carrie out correctly 3. to prove the correct functioning ofthe protection sche me as a whole Thetests carrie out will normally vary accor ing to the protecti on scheme involve , the relay technology use , an thepolicy of the client. In many cases, the tests actually con ucte are etermine at the time of commissio ning by mutual agreement between the client’s representative an the commissioning team. Hence, it is not possible to provi e a efinitive list of tests that are require uringcommissioning. This section therefore escribes the tests common ly carrie out uring commissioning. The following tests are invariably carrie out, since the protection scheme willnot function correctly if faults exist. a. wiring iagram check, using circuit iagrams showing all the reference numbers of the interconnecting wiring Network Protection & Automation Gui e b. general inspection of the equipment, checking all connections, wires on relay s terminals, labels on terminal boar s, etc. c. insulation resistance measuremen t of all circuits . perform relay self-test proce ure an external communicatio ns checks on igital/numerical relays e. test main current transformers f. test main voltage transformers g. check thatprotection relay alarm/trip settings hav e been entere correctly h. tripping an alarm circuit checks to provecorrect f unctioning In a ition, the following checks may be carrie out, epen ing on th e factors note earlier. i. secon ary injection test on each relay to prove oper ation at one or more setting values k. testing of protection scheme logic This section etails the tests require to cover items (a)–(g) above. Secon ary injection test equipment is covere in Secti on 21.10 an Section 21.11 etails the secon ary injection that may becarrie o ut. Section 21.12 testing, an Section 21.13 etails th covers primary injection e checks require on any logic involve in the protection scheme. Finally, Secti on 21.14 etails the tests require on alarm/tripping circuits tripping/alarm ci rcuits. 21.9.1Insulation Tests All the eliberate earth connections on the wiri ng to be teste shoul first be remove , for example earthing links on current t ransformers, voltage transformers an .c. supplies. Some insulation testers gen erate impulses with peak voltages excee ing 5kV. In these instances any electron icequipment shoul be isconnecte while the external wiringinsulation is chec ke . The insulation resistanceshoul be measure to earth an between electrica lly separate circuits. The rea ings are recor e an compare with subsequent ro utine tests to check for any eterioration of the insulation. The insulation res istancemeasure epen s on the amount of wiringinvolve , its gra e, an the si te humi ity. Generally, if the test is restricte to one cubicle, a rea ing of s everal hun re megohms shoul be obtaine . If long lengths of site wiring are in
volve , the rea ing coul be only a few megohms. • • 387 • R e l a y Te s t i n g a n
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j.primary injection tests on each relay to prove stability for external faults an to etermine the effective current setting for internal faults (essential fo r some types of electromechanical relays)
21.9.2 Relay Self-TestProce ure Digital an numerical relays will have a self-t est proce urethatis etaile in the appropriate relay manual. These tests shou l be followe to etermine if the relay is operating correctly. This will norma lly involve checking of the relay watch og circuit, exercising all igital input s an outputs an checking that the relay analogue inputs are within calibration by applying a testcurrent or voltage. For these tests, the relay outputs are n ormally isconnecte from the remain er of the protection scheme, as it is a tes t carrie out to prove correct relay, rather than scheme, operation. Unit protec tion schemes involve relays that nee to communicate with each other. This lea s to a itional testing requirements. The communications path between the relays is teste using suitable equipment to ensure that the path is complete an that thereceive signal strength is within specification. Numerical relays may be fi tte with loopback test facilities that enable either part of or the entire comm unications link to be teste from one en . After completion of these tests, it i s usual to enter the relay settings require . This can be one manually via the relay front panel controls, or using a portable PC an suitable software. Whiche ver metho is use , a check by a secon person that the correct settings have be en use is esirable, an the settings recor e . Programmable scheme logic that is require is also entere at thisstage. 21.9.3 Current Transformer Tests The following tests are normally carrie out prior to energisation of the main circu its. 21.9.3.1 Polarity check P2 S2 S1 P1 + robust is moving coil, permanent magnet, centre-zero type. A low voltage battery use , via a single-pole pushbutton switch, to energise the primary win ing. On closing the push-button, the .c. ammeter, A, shoul give a positive flick an o n opening, a negative flick. 21.9.3.2 Magnetisation Curve Several pointsshoul be checke on each current transformer magnetisation curve. This can be one by energising the secon ary win ing from the local mains supply through a variable auto-transformer while the primary circuit remains open; see Figure 21.17. The c haracteristic is measure at suitable intervals of applie voltage, until the ma gnetising current is seen to rise very rapi ly for a small increase in voltage. This in icates the approximate knee-point orsaturation flux level of the curren t transformer. The magnetising current shoul then be recor e at similar voltag e intervals as itis re uce to zero. Care must be taken that the test equipment issuitably rate . The short-time current rating must be in excess of the CT se con ary current rating, to allow for the measurement of the saturation current. This will be in excess ofthe CT secon ary current rating. As the magnetisingcu rrentwill not be sinusoi al, a moving iron or ynamometer type ammeter shoul b e use . It is often foun that current transformers with secon ary ratings of 1A or less have a knee-point voltage higher than the local mains supply. In these cases, a step-up interposing transformer must be use to obtain the necessary vo ltage to check the magnetisation curve. Test plug isolating current transformers from relay coils A P1 S 1 To relay coil s V 250V a.c. supply R e l a y Te s t i n g a n C o m m i s s i o n i n g A B C Variable transformer 250V 8A • 21 •
_ P2 S2 Main circuit breaker open A + Step-up transformer if require _ Figure 21.17: Testing current transformer magnetising curve 21.9.4 Voltage Transformer Tests Figure 21.16: Current transformer polarity check
Voltage transformers require testing for polarity an phasing. 21.9.4.1 Polarity check The voltage transformer polarity can be checke using the metho for CT p olarity tests. Care must be taken to connect the battery supply to the primary w in ing, with Network Protection & Automation Gui e Eachcurrent transformer shoul be in ivi ually teste to verify that the primar y an secon ary polarity markings are correct; see Figure 21.16. The ammeter con necte to the secon ary of the current transformer shoul be a • 388 •
the polarity ammeter connecte to the secon ary win ing. If the voltage transfor mer is of the capacitor type,then the polarity of the transformer at the bottom of the capacitor stack shoul be checke . 21.9.4.2 Ratio check This check can b e carrie out when the main circuit is first ma e live. The voltage transformer secon ary voltage is compare with the secon ary voltage shown on the nameplate. 21.9.4.3 Phasing check The secon ary connections for a three-phase voltage tran sformer or a bank of three single-phase voltage transformers must be carefully c hecke for phasing. With the main circuit alive, the phase rotation is checke u sing a phase rotation meter connecte across the three phases, as shown in Figur e 21.18. Provi e an existing proven VTis available on the same primary system, an that secon ary earthing is employe , all that is now necessary to prove cor rect phasing is a voltage check between, say, both ‘A’ phase secon ary outputs. Ther e shoul be nominally little or no voltage if the phasing is correct. However,t his test oes not etect if the phase sequence is correct, but the phases are i splace by 120° from their correct position, i.e. phaseA occupies the position of phase C or phase B in Figure 21.18. This can be checke by removing the fuses f rom phases B an C (say) an measuring the phase-earth voltages on the secon ary of the VT. If the phasing is correct, only phase A shoul be healthy, phases B an C shoul have only a small resi ual voltage. A B C A V1 C V2 B Correct phasing shoul be further substantiate when carrying out ‘on loa ’ tests on any phase-angle sensitive relays, at the relay terminals. Loa current in a kno wn phase CT secon ary shoul be compare with the associate phaseto neutral VT secon ary voltage. The phase angle between them shoul be measure , an shoul relate to the power loa . If the three-phase voltage transf factor of the system ormer has a broken elta tertiary win ing, then acheck shoul be ma e of the vol tage across the two connections from the broken elta VN an VL, as shown in Fig ure 21.18. With the rate balance three-phase supply voltage applie to the vol tage transformer primary win ings, the broken- elta voltage shoul be below 5V w ith the rate bur en connecte . 21.9.5 Protection Relay Setting Checks At some p oint uring commissioning, the alarm an trip settings of the relay elements inv olve will require to be entere an /or checke . Where the complete scheme is en gineere an supplie by a single contractor, the settings may alrea y have been entere prior to espatch from the factory, an hence this nee not be repeate . The metho of entering settings varies accor ing to the relay technology use . For electromechanical an manual entryof the static relays, settings for each relay element is require . This metho can also be use for igital/numerical re lays. However, the amount of ata to be entere is much greater, an therefore i t is usual to use appropriate software, normally supplie by the manufacturer, f or this purpose. The software also makes the essential task of making a recor o f the ata entere much easier. Once the ata has been entere , it shoul be che cke for compliance with the recommen e settings ascalculate from the protect ion setting stu y. Where appropriate software is use for ata entry, the checks can be consi ere complete if the ata is checke prior to ownloa of the sett ings to the relay. Otherwise, a check may require subsequent to ataentry by i nspection an recor ing of the relay settings, or it may be consi ere a equate to o this at the time of ata entry. The recor e settings form an essential pa rt of the commissioning ocumentation provi e to the client. 21.10 SECONDARY IN JECTION TEST EQUIPMENT • VN V VL V2 V1 A B C Phase rotation meter Secon ary injection tests are always one prior to primary injection tests. The
purpose of secon ary injection testing is to prove the correct operation of the protection scheme that is ownstream from the inputs to the protection relay(s). Secon ary injection tests are always one prior to primary injection tests. Thi s is Figure 21.18: Voltage transformer phasing check Network Protection & Automation Gui e • 389 • R e l a y Te s t i n g a n C o m m i s s i o n i n g 21 •
because the risks uring initial testing to the LV si e of the equipmentun er t est are minimise . The primary (HV) sie of the equipment is isconnecte , so th at no amage can occur. These tests an the equipment necessary to perform them are generally escribe in the manufacturer s manuals for the relays, but brief etails are given below for the main types of protection relays. 21.10.1 Test Bl ocks/Plugs for Secon ary Injection Equipment It is common practice to provie te st blocks or test sockets in the relaycircuits so that connections can rea ily be ma e to the test equipment without isturbing wiring. Test plugs of either mu lti-finger or single-finger esign (for monitoring the currentin one CT secon a rycircuit) are use to connect test equipment to the relay un er test. The top an bottom contact of each test plug finger is separate by an insulating strip, so that the relay circuits canbe completely isolate from the switchgear wirin g when the test plug is inserte . To avoi open-circuiting CT secon ary terminal s, it is therefore essential that CT shorting jumper links are fitte across all appropriate ‘live si e’ terminals of the test plug BEFORE it is inserte . With the test plug inserte inposition, all the test circuitry can now be connecte to t he isolate th ‘relay si e’ test plug terminals. Some mo ern test blocks incorporate e live-si e jumper links within the block an these can be set to the ‘close ’ or ‘ope n’ position as appropriate, either manually prior to removing the cover an insert ing the test plug, or automatically upon removal of the cover. Removal of the co ver also exposes the colour-co e face-plate of the block, clearly in icating th at the protection scheme is not in service, an may also isconnect any .c. aux iliary supplies use for powering relay tripping outputs. With rawing the test p lug imme iately restores the connections to the main current transformers an vo ltage transformers an removes the test connections. Replacement of the test blo ck cover then removes the short circuits that ha been applie to the main CT se con ary circuits. Where several relays are use in a protection scheme, one or m ore test blocks may be fitte on the relay panel enabling the whole scheme to be teste , rather than justone relay at a time. Test blocks usually offer facilit ies for the monitoring an secon ary injection testing of any power system prote ction scheme. The test block may be use either witha multi-fingere test plug to allow isolation an monitoring of all the selecte con uctor paths, or with a single finger test plug that allows the currents on in ivi ual con uctors to be monitore . A moern test block an test plugs are illustrate in Figure 21.19. Figure 21.19: Mo ern test block/plugs 21.10.2 Secon ary Injection Test Sets The type of the relay to be teste etermi nes the type of equipment use to provi e the secon ary injection currentsan v oltages. Many electromechanical relays have a non-linear current coil impe ance when the relay operates an this can causethe test current waveform to be isto rte if the injection supply voltage is fe irectly to the coil. The presence o f harmonics in the current waveform may affect the torque of electromechanical r elays an give unreliable test results, so some injection test sets use an a jus table series reactance to control the current. This keeps the power issipation small an the equipment light an compact. Many test sets are portable an inclu e precision ammeters an voltmeters an timing equipment. Test sets may have bo th voltage an current outputs. The former are high-voltage, low current outputs for use with relay elements that require signal inputs from a VT as well as a C T. The current outputs are high-current, low voltage to connect to relay CT inpu ts. It is important, however, to ensure that the test setcurrent outputs are tr ue current sources, an hence are not affecte by the loa impe ance of a relay element current coil. Use of a test set with a current output that is essentiall y a voltage source can give riseto serious problems when testing electromechani cal relays. Any significant impe ance mismatch between the output of the test se t an the relay currentcoil uring relay operation will give rise to a variatio n in current from that esire an possibleerror in the test results. The relay operation time may be greater than expecte (never less than expecte ) or relay ‘chatter’ may occur. It is quitecommon for such errors to only be foun much later , after a fault has cause major amage to equipment through failure of the prim ary protection to operate. Failure investigation then shows that the reason for
the primary protection to operate is an incorrectly set relay, ue in turn to us e of a test set with a current output consisting of a voltage-source when the re lay was last teste . Figure 21.20 shows typical waveforms resulting from use of test set current output that is a voltage • R e l a y Te s t i n g a n
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• 390 •
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V relay/source Time
Saturation level of magnetic circuit (current) limite only by D.C. resistance o f relay coil Relay with saturation of CDG magnetic circuit (phase shift from CDG in uctive lo a shown). a) Relay current coil waveform istorte ue to use of voltage source Sinusoi al CURRENT when changing impe ance of relay is swampe out by high sourc e impe ance Time
Typical VOLTAGE waveform appearing across relay current coils with sinusoi al I above the relay setting (10 x shown). • b) Un istorte relay current coil current istorte ue to use of current source Figure 21.20: Relay current coil waveforms source – the istorte relay coilcurrent waveform gives rise to an exten e opera tion time compare to the expecte value. Mo ern test sets are computer base . T hey comprise a PC (usually a stan ar laptop PC with suitablesoftware) an a po wer amplifierthat takes the low-level outputs from the PC an amplifies them in to voltage an current signals suitable for application to the VT an CT inputs of the relay. The phase angle between voltage an current outputs will be a just able, as also will the phase angles between the in ivi ual voltages or currents making up a Network Protection & Automation Gui e 3-phase output set. Much greater precision in the setting of the magnitu es an phase angles is possible, compare to tra itional test sets. Digitalsignals to exercise the internal logic elements of the relays may also be provi e . The ala rm an trip outputs of the relay are connecte to igital inputs on the PC so th at correct operation of the relay, inclu ing accuracy of the relay tripping char acteristic can save for inclusion in repo be monitore an isplaye on-screen, rts generate later, or printe for an imme iate recor to present to the client . Optional features may inclu e GPS time synchronising equipment an remote-loca te amplifiers • 391 • R e l a y Te s t i n g a n
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to facilitate testing of unit protection schemes, an igital I/O for exercising the programmable scheme logic of mo ern relays. The software for mo ern test se ts is capable of testing the functionality of a wi e variety of relays, an con ucting a set of tests automatically. Such sets ease the task of the commissionin g engineer. The software will normally offer options for testing, ranging from a test carrie out at a particular point on the characteristic to complete eterm ination of the tripping characteristic automatically. This feature can be helpfu l if there is any reason to oubtthat the relay is operating correctly with the tripping characteristic specifie . Figure 21.21 illustrates a mo ern PC-base t est set. Tra itional test sets use an arrangement of a justable transformers an an voltage without incurring high power reactors to provi e control of current issipation. Some relays require a justment of the phase between the injecte v oltages an currents,an so phase shifting transformers may be use . Figure 21. 22 shows the circuit iagram of a tra itional test set suitable for overcurrent relay resting, while Figure 21.23 shows the circuit iagram for a test set for irectional/ istance relays. Timers are inclu e so that the response time of the relay can be measure . 21.11 SECONDARY INJECTION TESTING The purpose of secon a ry injection testing is to check that the protection scheme from the relay input terminals onwar s is functioning correctly with the settings specifie . This is achieve by applying suitable inputs from a test set to the inputs of the relay s an checking if the appropriate alarm/trip signals occur at the relay/control room/CB locations. The extent of R e l a y Te s t i n g a n C o m m i s s i o n i n g Figure 21.21: Mo ern PC-base secon ary injection test set testingwillbe largely etermine by the client specification an relay technol ogy use , an may range from a simple check of the relay characteristic at a sin gle pointto a complete verification of the trippingcharacteristics of the sche me, inclu ing the response to transient waveforms an harmonics an checking of relay bias characteristics. This may be important when the protection scheme inc lu es transformers an /or generators. • 21 • Coarse control reactor K2 250V a.c. supply Fine control variable transformer Sta rt timer Backing transformer 10% control Relay current, I = Ammeter rea ing (A) K1 x K2 Me ium control reactor Injection transformer Stop timer Relay short-circ uiting switch A Range a justing CT K1 I> I Relay coil
Figure 21.22: Circuit iagram of tra itional test set for overcurrent relays • 392 •
Network Protection & Automation Gui e
A B C N Variable transformer control Supply switch 440V 3 phase p 4 wire supply X( ) 22.5 20.0 17.5 15.0 12.5 10.0 7.5 5.0 2.5 0.0 -2.5 -5.0 -7.5 -10.0 -15.0 -1 0.0 -5.0 Fault A-N Choke Relay a justing CT A PA A V 440/110V p phase shifting g transformer PA A > voltage element Variable transformer for current control V Voltmeter A Ammeter PA Phase angle meter To other voltage elements of relay y un er test (if require ) Figure 21.23: Circuit iagram for tra itional test set for irectional/ istance relays 0.0 5.0 10.0 15.0 R( )
Figure 21.24: Distance relay zone checking using search technique an tolerance ban s Zn
21.11.1 Schemes using Digital orNumerical Relay Technology The policy for secon ary injection testing varies wi ely. In some cases, manufacturers recommen , an clients accept, that if a igital or numerical passes its’ selftest, it ca relay n be relie upon to operate at the settings use an that testing can therefore be confine to those parts of the scheme external to the relay. In such cases, s econ ary injection testing is not require at all. More often, it is require th at one element of each relay (usually the simplest) is exercise , usinga secon ary injection test set, to check that relay operation occurs at the con itions e xpecte , base on the setting of the relay element concerne . Another alternativ e is for the complete functionality of each relay to be exercise . This is rarel y require with a igital or numerical relay, probably only being carrie out in the eventof a suspecte relay malfunction. To illustrate the results that can be obtaine , Figure 21.24 shows theresults obtaine by a mo ern test set when etermining the reach settings of a istance relay using a search technique. Anot her example is the testing of the Power Swing blocking element of a istance rel ay. Figure 21.25 illustrates such a test, base on using iscrete impe ance poin ts. This kin of test may not be a equate in all cases, an test equipment may h ave the ability to generate the waveforms simulating a power swing an apply the
m to the relay (Figure 21.26). Network Protection & Automation Gui e * R PSB-Zone
Figure 21.25: Testing of power swing blocking element – iscrete points •
Figure 21.26: Simulate power swing waveform 21.11.2 Schemes using Electromechanical/Static Relay Technology Schemes using si ngle function electromechanical or static relayswill usually require each relay to be exercise . Thus a scheme with istance an back-up overcurrent elements w ill require a test on each of these functions, thereby taking up more time than if a igital or numerical relay is use . Similarly, it may be important • 393 •
R e l a y Te s t i n g a n C o m m i s s i o n i n g 21 • The testing shoul inclu e any scheme logic. If the logic isimplemente using t he programmable scheme with most logic facilities available igital or numerical relays, appropriate igital inputs may nee to be applie an outputs monitore (see Section 21.13). It is clear that a mo ern test set can facilitate such tes ts, lea ing to a re uce time require for testing. X
to check the relay characteristic over a range of input currents to confirm para meters for an overcurrent relay such as: i. the minimum current that gives opera tion at each current setting ii. the maximum current at which resetting takes pl ace iii. the operating time at suitable values of current iv. the time/current c urve at two or three points with the time multiplier setting TMS at 1 v. the res etting time at zero current with the TMS at 1 Similar consi erations apply to i stance an unit protection relays of these technologies. 21.11.3 Test Circuits f or Secon ary Injection Testing of VT’s/CT’s may not then be iscovere until either spurious tripping occurs in ser vice, or more seriously, failure to trip on a fault. This hazar is much re uce where igital/numerical relays are use , since the current an voltage measurem ent/ isplay facilities that exist in such relays enable checking of relay input values against those from other proven sources. Many connection/wiring errors ca n befoun in this way,an by isolating temporarily the relay trip outputs, unw ante trips can be avoi e . Primary injection testing is, however, the only way to provecorrect installation an operation of the whole of a protection scheme. As note in the previous section, primary injection tests are alwayscarrie ou t after secon aryinjection tests,to ensure that problems are limite to the VT’s an CT’s involve , plus associate wiring, all otherequipment in the protection scheme having been proven satisfactory from the secon ary injection tests. 21.12 .1 Test Facilities An alternator is the most useful source of power for provi in g the heavy current necessary for primary injection. Unfortunately, it is rarely available, since it requires but not only a spare alternator, also spare busbar s capable of being connecte to the alternator an circuit un er test. Therefore , primary injection is usually carrie out by means of a portable injection tran sformer (Figure 21.27), arrange to operate from the local mains supply an havi ng several low voltage, heavy current win ings. These can be connecte in series or parallel accor ing to the current require an the resistance of the primary circuit. Outputs of 10V an 1000A can be obtaine . Alternatively, mo ern PC-con trolle test sets have power amplifiers capable of injecting currents up to abou t 200A for a single unit, with higher current ratings being possible by using mu ltiple units in parallel. R e l a y Te s t i n g a n C o m m i s s i o n i n g • The test circuits use will epen on the type of relay an test set being use . Unless thetest circuits are simple an obvious, the relaycommissioning manual will give etails of the circuits to be use . Commonly use test circuits can a lso be foun in Chapter 23 of reference [21.1]. Whenusingthe circuits in this reference, suitable simplifications can easily be ma e if igital or numerical r elays are being teste , to allow for their built-in measurement capabilities – ext ernal ammeters an voltmeters may not be require . All results shoul be careful ly note an file for recor Departures from the expecte results mus purposes. t be thoroughly investigate an the cause etermine . After rectification of er rors, all tests whose results may have been affecte (even those that may have g iven correct results) shoul be repeate to ensure that the protection scheme ha s been implemente accor ing to specification. 21.12 PRIMARY INJECTION TESTS Thi stype oftest involves the entire circuit; current transformer primary an seco n ary win ings, relay coils, trip an alarm circuits, an all intervening wiring are checke . There is no nee to isturb wiring, which obviates the hazar of o pen-circuiting current transformers, an there is generally no nee for any swit ching in the current transformer or relay circuits. The rawback of such tests i s that they an expensive reliance are time consuming to organise. Increasingly, is place on all wiring an installation iagrams being correct an the install ation being carrie out as per rawings, an secon ary injection testing being c omplete satisfactorily. Un er these circumstances, the primary injection tests may be omitte . However, wiring errors between VT’s/CT’s an relays, or incorrect po larity
21 • A 250V a.c. supply Variable transformer 40A Injection transformer 250/10 + 10 + 10 + 10V 10kVA Figure 21.27: Tra itional primary injection test set • 394 •
Network Protection & Automation Gui e
win ings, these can be If the main current transformers are fitte with test use for primary injection instea of the primary win ing. The current require for primary injection is then greatly re uce an can usually be obtaine using sec on ary injection test equipment. Unfortunately, test win ings are not often prov i e , because of space limitations in the main current transformer housings or t he cost of the win ings. 21.12.2 CT Ratio Check Current is passe through the pr imary con uctors an measure on the test set ammeter, A1 in Figure 21.28. The s econ ary current is measure on the ammeter A2 or relay isplay, an the ratio o f the value on A1 to that on A2 shoul closely approximate to the ratio marke o n the current transformer nameplate. A B C the resi ual isplay, will give a rea ing of a few milliampere circuit, or relay s with rate current injecte if the current transformers are of correct polarit y. A rea ing proportional to twice the primary current will be obtaine ifthey are of wrong polarity. Because of this, ahigh-range ammeter shoul beuse init ially, for example one giving full-scale eflection for twice the rate secon ar y current. earth-fault relay with a low setting is also If an electromechanical connecte in the resi ual circuit, it is a visable to temporarily short-circuit its operating coil uring the test, to prevent possible overheating. The single phase injection shoul be carrie out for each pair of phases. Temporary three-phase short circuit 250V a.c. supply Primary injection test set A B C Relay Temporary short circuit A Figure 21.29: Polarity check on main current transformers P1 Relay S1
21.12.4 Primary Injection Testing of Relay Elements As withsecon ary injection testing, the tests to be carrie out will be those specifie bythe client, an / or those etaile in the relay commissioning manual. Digital an numerical relay s usually correct operation, an these may be r require far fewer tests to prove estricte to observations of current an voltage on the relay isplay un er norm al loa con itions. 21.13 TESTING OF PROTECTION SCHEME LOGIC Protectionschemes often involve the use of logic to etermine the con itions un er which esignate circuit breakers shoul be trippe . Simple examples of such logic can be foun in Chapters 9-14. Tra itionally, thislogic was implemente by means of iscret erelays, separate from the relays use for protection. Such implementations wou l occurwhere electromechanical or static relay technology is use . However, i gital an numerical relays normally inclu e programmable logic as part of the so ftware within the relay, together with associate igital I/O. This facility (co mmonly referre to as Programmable Scheme Logic, or PSL) offers important a vant ages to the user, by saving space an permitting mo ifications to the protection scheme logic through software if the protection scheme requirements change with time. Changes to the logic are carrie out using P2 S2 Relay or test block contact fingers A1 Primary injection test set 250V a.c. supply Figure 21.28: Current transformer ra tio check •
21.12.3 CT Polarity Check If the equipment inclu es irectional, ifferential or of the main current transformers must be check earth fault relays, the polarity e . It is not necessary to con uct the test if only overcurrent relays are use . The circuit for checking the polaritywith a single-phase test set is shown in Figure 21.29. A short circuit is place across the phases of the primary circuit on one si e of the current transformers while single-phase injection is carrie out on the other si e. The ammeterconnecte in Network Protection & Automation Gui e • 395 • R e l a y Te s t i n g a n Test plug insulation u
C o m m i s s i o n i n g 21 •
software hoste ona PC (or similar computer) an ownloa e to the relay. Use o f languages efine in IEC 61131,such as la er logic or Boolean algebra is com mon for such software, an is rea ily unerstoo by Protection Engineers. Furthe r, there are several commonly encountere protection functions that manufacturer s may schemes. Because software is supply with relays as one or more ‘ efault’ logic use , it is essential to carefully test the logic uring commissioning to ensur e correct operation. The only exception to this may beif the relevant ‘ efault’ sch emeis use . Such logicschemes will have been proven uring relay type testing, an so there is no nee for proving tests uring commissioning. However, where a customer generates the scheme logic, it is necessary to ensure that the commis sioning tests con ucte are a equateto prove the functionality of the scheme in all respects. A specific test proce ure shoul be prepare , an this proce ure shoul inclu e: Many esigns of with rawable circuit breaker can be operate while in the mainte nance position, so that unaffecte except for substation operation can continue the circuit controlle by the circuit breaker involve . In other cases, isolator scan be use to avoi the nee for busbar e-energisation if the circuit involv e is not rea y for energisation. 21.15 PERIODIC MAINTENANCE TESTS Perio ic test ing is necessary to ensure that a protection scheme continues to provi e satisfa ctory for many years after installation. All equipment is performance subject to gra ual egra ation with time, an regular testing is inten e to i entify the equipment concerne so that reme ial action can be taken before scheme maloperat ion occurs. However, ue care shoul be taken in this task, otherwise faults may be intro uce as a irect result of the reme ial work. The clearance of a fault on the system is correct only if the number of circuit breakers opene is the m inimum necessary to remove the fault. A small proportion of faults are incorrect ly cleare , the main reasons being: a. limitations in protection scheme esign b . faulty relays c. efects in the secon ary wiring . incorrect connections e. i ncorrect settings f. known application shortcomings accepte as improbable occur rences g. pilot wire faults ue to previous unreveale amage to a pilot cable h . various other causes, such as switching errors, testing errors, an relay oper ation ue to mechanical shockThe self-checking facilities of numerical relays a ssist in minimising failures ue to faulty relays. Defects in secon ary wiring a n incorrect connections are virtually eliminate if proper commissioning after scheme installation/alteration is carrie out. The possibility of incorrect sett ings is minimise by regular reviews of relay settings. Network fault levels cha nge over time, an hence setting calculations may nee to be revise . Switching an testing errors are minimise by a equate training of personnel, use of prove n software, an well esigne systematic working proce ures. All of these can be sai to be within the control of the user. The remaining three causes are not co ntrollable, while two of these three are unavoi able – engineering is not science an there will always be situations that a protection relay cannot reasonably be expecte to cover at an affor ablecost. Network Protection & Automation Gui e R e l a y Te s t i n g a n C o m m i s s i o n i n g a. checking of the scheme logic specification an iagrams to ensure that the ob jectives of the logic are achieve b. testing of the logic to ensure that the fu nctionality of the scheme is proven c. testing of the logic, as require ,to ens ure that no output occurs for the relevant input signal combinations The egree of testing of the logic will largely epen on the criticality of the applicatio n an complexity of the logic. The responsibility for ensuring that a suitable t est proce ure is pro uce for logic schemes other than the ‘ efault’ one(s) supplie lies with the specifier of the logic. Relay manufacturers cannot be expecte to take for responsibility the correct operation of logic schemes that they have n ot esigne an supplie . 21.14 TRIPPING AND ALARM ANNUNCIATION TESTS If primary an /or secon ary injection tests are not carrie out, the tripping an alarm ci rcuits will not have been checke . Even where such checks have been carrie out,
CB trip coils an /or Control Room alarm circuits may have been isolate . In suc h cases, it is essential that all of the tripping an alarm circuits are checke . This is one by closing the protection relaycontacts manually an checking th at: 1. the correct circuitbreakers are trippe 2. the alarm circuits are energi se 3. the correct flag in ications are given 4. there is no maloperation of oth er apparatus that may be connecte to the same master trip relay or circuit brea ker • 21 • • 396 •
21.15.1 Frequency shoul of Inspection an Testing Although protection equipment be in soun conition when first put into service, problemscan evelop uncheck e an unreveale because of its infrequent operation. With igital an numerica l relays, the in-built self-testing routines can be expecte to reveal an annun ciate most faults, but this oes not cover any other components that, together, comprise the protection scheme. Regular inspection an testing of a protection s cheme is therefore require . In practice, the frequency of testing may be limite by lack of staff or by the operating con itions on the power system. It is es irable to carry out maintenance on protection equipment attimes when the associ ate power apparatus is out of service. This is facilitate by co-operation betw een the maintenance staff concerne an the network operations control centre. M aintenance tests may sometimes have to be ma e when the protecte circuit is on loa . The particular equipment to be teste shoul be taken out of commission an a equate back-up protection provi e for the uration of the tests. Such back up protection may not be fully iscriminative, but shoul be sufficient to clear any fault on the apparatus whose main protection is temporarily out ofservice. Maintenance is assiste by the isplays of measure quantities provi e on igi tal an numerical relays. Incorrect isplay of a quantity is a clear in ication that something is wrong, either in the relay itself or the input circuits. 21.15 .2 Maintenance Tests Primary injection tests are normally only con ucte out ur ing initial commissioning. If scheme maloperation has occurre an the protectio n relays involve are suspect, or alterations have been ma e involving the wirin g to the relays from the VT’s/CT’s, the primary injection tests may have to be repea te . Secon ary injection tests may be carrie out at suitable intervals to check relay performance, an, if possible, the relay shoul beallowe to trip the ci rcuit breakers involve . The interval between tests will epen upon the critica lity of the circuit involve , the availability of the circuit for testing an th e technology of the relays use . Secon ary injection testing is only necessary o n the selecte relay setting an the results shoul be checke against those obt aine uring the initial commissioning of the equipment. It is better not to int erfere with relay contacts at all unless they are obviously corro e . The perfor mance of the contacts is fully checke when the relay is actuate . Insulation te sts shoul also be carrie out on the relay wiring toearth an between circuits , using a 1000V tester. These tests are necessary to etect any eterioration in the insulation resistance. Network Protection & Automation Gui e are a 21.16PROTECTION SCHEME DESIGN FORMAINTENANCE If the following principles here to as faras possible, the anger of back-fee s is lessene an fault inv estigation is ma e easier: i. test blocks shoul be use , to enable a test plug to be use , an a efective unit to be replace quickly without interrupting ser vice ii. circuits shoul be kept aselectrically separate as possible, an the u se of common wires shoul be avoi e , except where these are essential to the co rrect functioning of the circuits iii.each groupof circuits which is electrica lly separate from other circuits shoul be earthe through an in epen ent earth link iv. where a common voltage transformer or .c. supply is use for fee ing s everal circuits, each circuit shoul be fe through separate links or fuses. Wit h rawal of these shoul completely isolate the circuit concerne v. power suppli es toprotection be segregate from those supplying other equipme schemes shoul nt an provi e with fully iscriminative circuit protection vi. a single auxili ary switch shoul not be use for interrupting or closing more than one circuit vii. terminations in relay panelsrequire goo access,as these may have to be a ltere if extensions are ma e. Mo ern panels are provi e with special test faci lities, so that noconnections nee be isturbe uring routine testing viii. ju nction boxes shoul beof a equatesize an , if out oors, must be ma e waterproo f ix. all wiring shoul be ferrule for i entification an phase-coloure x. ele ctromechanical relays shoul have high operating an restraint torques an high contact pressures; jewel bearings shoul be shrou e to exclu e ust an the use of very thin wire for coils an connections shoul be avoi e . Dust-tight cases with an efficient breather are essential on these types of electromechanical el
ement xi. static, igital an numerical relays shoul have test facilities acces sible from the front to assist in fault fin ing. The relay manual shoul clearly etail the expecte results at eachtest point 21 when healthy 21.17 REFERENCES .1 Protective Relays Application Gui e, 3r e ition. AREVA Transmission an Dist ribution, Protection an Control, 1987. • • 397 • R e l a y Te s t i n g a n
C o m m i s s i o n i n g 21 •
• 22 • Power System Measurements Intro uction General characteristics Digital trans ucer technology Analogue tran s ucer technology Trans ucer selection Measurement centres Tariff metering Synch ronisers Disturbance recor ers 22.1 22.2 22.3 22.4 22.5 22.6 22.7 22.8 22.9
• 22 • Power System Measurements 22.1 INTRODUCTION The accurate measurement of the voltage, current or other para meter of a power system is a prerequisiteto any form of control, ranging from a utomatic close -loop control to the recor ing of ata for statistical purposes. Measurement of these parameters can be accomplishe in a variety of ways, inclu ing the use of irect-rea ing instruments as well as electrical measuring trans ucers. Trans ucers pro uce an accurate .c. analogue output, usually a current, which correspon s to the parameter being measure (the measuran ). They provi e electrical isolation by transformers, sometimes referre to as ‘Galvanic Isolation’, between the input an the output. This is primarily a safety feature, but also means that the cabling from the output terminals to any receiving equipment can be lightweight an have a lower insulation specification. The a vantages over i screte measuring instruments are as follows: a. mounte close to the source of t he measurement, re ucing instrument transformer bur ens an increasing safety th rough elimination of long wiring runs b. ability remo to mount isplay equipment te from trans ucer c. ability to use multiple isplay elements per trans ucer . the bur en on CT’s/VT’s is consi erably less Outputs from trans ucers may be use i n many ways – from simple presentation of measure values for an operator, to bein g utilise by a network automation scheme to etermine the control strategy. 22. 2 GENERAL CHARACTERISTICS Transucers may have single or multiple inputs an/or outputs. The inputs, outputs an any auxiliary circuits will fro all be isolate m each other. There may be more than one input quantity an the measuran may be a function of one or more of them. Network Protection & Automation Gui e • 399 •
Whatever measurement trans ucer is being use , there will usually be a choice be tween iscrete an mo ular types, the latter being plug-in units to a stan ar r ack. The location an user-preferences will ictate the choice of trans ucer typ e. 22.2.1Trans ucer Inputs The input of a trans ucer is often taken from transf ormers an these may be of many ifferent types. I eally, to obtain the best ove rall accuracy, meteringclass instrument transformers shoul be use , since the t ransformer errors will be a e , albeit algebraically, to the trans ucer errors. However, it is common to apply trans ucers to protection-class instrument trans formers an that is why trans ucers are usually characterise to be able to with stan significant short-term overloa s on their current inputs. A typical specif ication for the current input circuits of a trans ucer suitable for connection t o protection-class instrument transformers is to withstan : 22.2.3 Trans ucer Accuracy Accuracy is usually of primeimportance, but in makin gcomparisons, it shoul be note that accuracy can be efine in several ways a n may only apply un er very closely efine con itions of use. The following at tempts to clarify some of the more common terms an relate them to practical sit uations, using the terminology given in IEC 60688. The accuracy of a trans ucer will be affecte , to a greater or lesser extent, by many factors, known as influ ence quantities, over which the user has little, or no, control. Table prov 22.1 i esa complete list of influence quantities. The accuracy is checke uner an a gree set of con itions known as reference con itions. The reference con itions for each of the influence quantities can be quote as a single value (e.g. 20°C) o r a range (e.g. 10-40°C). Input current Input quantity istortion Power factor Continuous operation Intera magnetic fiel ction between measuring elements Auxiliary supply voltage External s Series mo e interference External heat Table 22.1: Trans ucer influence quant ities Input voltage Input quantity frequency Unbalance currents Output loa Amb ient temperature Auxiliary supply frequency Self heating Common mo e interferenc e Power System Measurements a. 300% of full-loa current continuously b. 2500% for three secon s c. 5000% fo r one secon The input impe ance of any current input circuit will be kept as lo w aspossible, an that for voltage inputs will be kept as high as possible. Thi s re uces errors ue to impe ance mismatch. 22.2.2 Trans ucer Outputs The output of a trans ucer is usually a current source. This the output meansthat, within voltage range (compliance voltage) of the trans ucer, a itional isplay evice s can be a e without limit an withoutany nee for a justment of the trans uc er. The value of the compliance voltage etermines the maximum loop impe ance of the output circuit, so a high value of compliance voltage facilitates remote lo cation of an in icating instrument. Where the output loop is use for control pu rposes, appropriately rate Zener io es are sometimes fitte across the termina ls of each of the evices in the series loop to guar against the possibility of their internalcircuitry becoming open circuit. This ensures that a faulty evi ce in the loop oes not cause complete failure of the output loop. The constantcurrent nature of the trans ucer outputsimply raises the voltage an continues to force the correct output signal roun the loop. • 22 •
The error etermine un er reference con itions is referre to asthe intrinsic error. All trans ucers error are groupe into a partic having the same intrinsic ular accuracy class, enote by the class in ex. The class in ex is the same as the intrinsic error expresse as a percentage (e.g. a trans ucer with an intrins ic accuracy of 0.1% of full scale has a class in ex of 0.1). The class in ex sys tem use in IEC 60688 requires that the variation for each of the influence quan
tities be strictly relate to the intrinsic error. This means that the higher th e accuracy claime by the manufacturer, the lower must be all of the variations. Because there are many influence quantities, the variations are assesse in ivi ually, whilst maintaining all the other influence quantities at reference con i tions. Thenominal range ofuse of a trans ucer is the normal operating range of the trans uceras specifie by the manufacturer. The nominal range of use will naturally be wi er than the reference value or range. Within the nominal range o f use of a trans ucer, a itional errors accumulate resulting in an a itional e rror. This a itional error is limite for any in ivi ual influence quantity to, at most, the value of the class in ex. Table 22.2 gives performance etails of a typical range of trans ucers accor ing to the stan ar . Network Protection & Automation Gui e • 400 •
Influence Quantity Input current,In Input voltage,Vn Input frequency Power facto r Unbalance current Interaction between measuring elements Continuous operation Self Heating Output loa Waveform crest factor Ambient temperature Aux. supply .c. voltage A.C. Aux. Supply frequency, fn External magnetic fiel s Output seri es mo e interference Output common mo e interference Table 22.2: Typical trans u cer performance Accuracy Class of Trans ucer: 0.5 Reference Range Max. Error- Reference Range % In=1A, 5A 20…120% Vn=50…500V 80…120% 45…65Hz Cos ϕ = 0.5…1 0…100% Current input 0…360° Cont > 6h 1…30min 10…100% 1.41 (sine wave) 0° 50° C 24…250V DC 90…110% n 0…0.4kA/m 1V 50Hz r.m. in series with output 100V 50Hz r.m.s. output to earth 0.5% 0.25% 0.5% 0.25% 0.5 % 0.25%° 0.5% 0.5% 0.25% 0.5% 0.25% 0.25% 0.5% 0.5% 0.5% Nominal Working Range 0 120% 0 120% Cos ϕ = 0…1 1.2…1.8 10°–60° C 19V-300V Max. Error- Nomina Range 0.5% 0.5% 0.5% 0.5% 1.0% 0.25% Under changing conditions of the measurand, the output signa does not fo ow th e changes instantaneous y but is time-de ayed. This is due to the fi tering requ ired to reduce ripp e or, in transducers using numerica techno ogy, prevent a i asing. The amount of the de ay is ca ed the response time. To a certain extent, ripp e and response time are interre ated. The response time can usua y be sho rtened at the expense of increased ripp e, and vice-versa. Transducers having sh orter response times than norma can be supp ied for those instances where the p ower system suffers swings, dips, and ow frequency osci ations that must be mo nitored. Transducers having a current output have a maximum output vo tage, know n as the comp iance vo tage. If the oad resistance is too high and hence the co mp iance vo tage is exceeded, the output of the transducer is no onger accurate . Certain transducers are characterised by the manufacturer for use on systems w here the waveform is not a pure sinusoid. They are common y referred to as ‘true r .m.s. sensing’ types. For these types, the distortion factor of the waveform is an inf uence quantity. Other transducers are referred to as ‘mean-sensing’ and are Network Protection & Automation Guide Once insta ed, the user expects the accuracy of a transducer to remain stab e o ver time. The use of high qua ity components and conservative power ratings wi he p to ensure ong-term stabi ity, but adverse site conditions can cause perfo rmance changes which may need to be compensated for during the ifetime of the e quipment. 22.3 DIGITAL TRANSDUCER TECHNOLOGY Digita power system transducers ma ke use of the same techno ogy as that described for digita and numerica re ays in Chapter 7. The ana ogue signa s acquired from VT’s and CT’s are fi tered to avoi d a iasing, converted to digita form using A/D conversion, and then signa proc essing is carried out to extract the information required. Basic detai s are giv en in Chapter 7. Samp e rates of 64 samp es/cyc e or greater may be used, and th e accuracy c ass is norma y 0.5. Outputs may be both digita and ana ogue. The ana ogue outputs wi be affected by the factors inf uencing accuracy as describ ed above. Digita outputs are typica y in the form of a communications ink wit h RS232 and/or RS485 types avai ab e. The response time may suffer compared to a na ogue transducers, depending on the rate at which va ues are transferred to th e communications ink and the de ay in processing data at the receiving end. In fact, a of the inf uence quantities that affect a traditiona ana ogue transdu cer a so are present in a digita transducer in some form, but • • 401 • Power System Measurements 22 • Confusion a so arises in specifying the performance under rea operating conditi
ons. The output signa is often a d.c. ana ogue of the measurand, but is obtaine d from a ternating input quantities and wi , inevitab y, contain a certain amou nt of a ternating component, or ripp e. Ripp e is defined as the peak-to-peak va ue of the a ternating component of the output signa a though some manufacturer s quote ‘mean-to-peak’ or ‘r.m.s.’ va ues. To be meaningfu , the conditions under which the va ue of the ripp e has been measured must be stated, e.g. 0.35% r.m.s. = 1. 0% peak-to-peak ripp e. adjusted to respond to the r.m.s. va ue of a pure sine wave. If the input wavefo rm becomes distorted, errors wi resu t. For examp e, the error due to third ha rmonic distortion can amount to 1% for every 3% of harmonic.
the errors resu ting may be much ess than in an ana ogue transducer and it may be more stab e over a ong period of time. The advantages of a transducer using numerica techno ogy are: 1. improved ong-term stabi ity 2. more accurate r.m.s measurements 3. improved communications faci ities 4. programmabi ity of sca in g 5. wider range of functions 6. reduced size The improved ong term stabi ity r educes costs by extending the interva s between re-ca ibration. More accurate r. m.s measurements provide the user with data of improved accuracy, especia y on supp ies with significant harmonic content. The improved communications faci iti es permit many transducers to share the same communications ink, and each trans ducer to provide severa measurements. This eads to economy in interconnecting wiring and number of transducers used. Remote or oca programmab e sca ing of t he transducer permits sca ing of the transducer in the fie d. The sca ing can be changed to ref ect changes in the network, or to be re-used e sewhere. Changes can be down oaded via the communications ink, thus removing the need for a site visit. It a so minimises the risk of the user specifying an incorrect sca ing f actor and having to return the transducer to the manufacturer for adjustment. Su pp iers can keep a wider range of transducers suitab e for a wide range of app i cations and inputs in stock, thus reducing de ivery times. Transducers are avai ab e with a much wider range of functions in one package, thus reducing space re quirements in a switchboard. Functions avai ab e inc ude harmonics up to the 31s t, energy, and maximum demand information. The atter are usefu for tariff nego tiations. 22.4 ANALOGUE TRANSDUCER TECHNOLOGY A ana ogue transducers have the fo owing essentia features: a. an input circuit having impedance Zin b. iso at ion (no e ectrica connection) between input and output c. an idea current sour ce generating an output current, I1, which is an accurate and inear function of Qin, the input quantity d. a para e output impedance, Zo. This represents the actua output impedance of the current source and shunts a sma fraction, I2, of the idea output e. an output current, Io, equa to (I1 - I2) These features are shown diagrammatica y in Figure 22.1. I1 I2 Qin Zin Zin Z0 I0 Figure 22.1: Schematic of an ana ogue transducer Power System Measurements Output ranges of 0-10mA, 0-20mA, and 4-20mA are common. Live zero (e.g. 4-20mA), suppressed zero (e.g. 0-10mA for 300-500kV) and inear inverse range (e.g. 10-0 mA for 0-15kV) transducers norma y require an auxi iary supp y. The dua -s ope type has two inear sections to its output characteristic, for examp e, an outpu t of 0-2mA for the first part of the input range, 08kV, and 2-10mA for the secon d part, 8-15kV. 22.5 TRANSDUCER SELECTION The se ection of the correct transduce r to perform a measurement function depends on many factors. These are detai ed be ow. 22.5.1 Current Transducers Current transducers are usua y connected to t he secondary of an instrument current transformer with a rated output of 1 or 5 amps. Mean-sensing and true r.m.s. types are avai ab e. If the waveform contains significant amounts of harmonics, a true r.m.s sensing type must be used for ac curate measurement of the input. They can be se f-powered, except for the true r .m.s. types, or when a ive zero output (for examp e 420mA) is required. They ar e not directiona and, therefore, are unab e to distinguish between ‘export’ and ‘impo rt’ current. To obtain a directiona signa , a vo tage input is a so required. 22. 5.2 Vo tage Transducers Connection is usua y to the secondary of an instrument vo tage transformer but may be direct if the measured quantity is of sufficient y ow vo tage. The suppressed zero type is common y used to provide an output fo r a specific range of input vo tage where measurement of zero on the input quant ity is not required. The inear inverse type is often used as an aid to synchron ising. Network Protection & Automation Guide
• 22 • • 402 •
22.5.3 Frequency Accurate measurement of frequency is of vita importance to tra nsmission system operators but not quite so important, perhaps, for the operator of a diese generator set. Accuracy specifications of 0.1% and 0.01% are avai a b e, based on percent of centre sca e frequency. This means, for examp e that a device quoted as 0.1% and having a centre sca e va ue of 50Hz wi have a maximu m error of 50mHz under reference conditions. 22.5.4 Phase Ang e Transducers for t he measurement of phase ang e are frequent y used for the disp ay of power facto r. This is achieved by sca ing the indicating instrument in a non inear fashion, fo owing the cosine aw. For digita indicators and SCADA equipment, it is nec essary for the receiving equipment to provide appropriate conversions to achieve the correct disp ay of power factor. Phase ang e transducers are avai ab e with various input ranges. When the sca ing is -180°…0°…180°, there is an ambiguous region, of about 2° at the extremes of the range. In this region, where the output is expecte d to be, for examp e, –10mA or +10mA, the output may jump sporadica y from one of the fu -sca e va ues to the other. Transducers are a so avai ab e for measurem ent of the ang e between two input vo tages. Some types of phase ang e transducer use the zero crossing point of the input waveform to obta in the phase information and are thus prone to error if the input contains signi ficant amounts of harmonics. Ca cu ating the power factor from the va ues of the outputs of a watt and a var transducer wi give a true measurement in the pres ence of harmonics. 22.5.5 Power Quantities The measurement of active power (watt s) and reactive power (vars) is genera y not quite as simp e as for the other q uantities. More care needs to be taken with the se ection of these types because of the variety of configurations. It is essentia to se ect the appropriate typ e for the system to be measured by taking into account factors such as system op erating conditions (ba anced or unba anced oad), the number of current and vo t age connections avai ab e and whether the power f ow is ike y to be ‘import’, ‘export’, or both. The range of the measurand wi need to encompass a required possibi ities of over-range under norma conditions so that the transducer and its indi cating instrument, or other receiving equipment, is not used above the upper im it of its effective range. Figure 22.2 i ustrates the connections to be used fo r the various types of measurement. Transducer Van Ia A B C N To oad 3 phase, 4 wire ba anced oad S1 P1 S2 P2 A B C Vab Vca Transducer Ia S2 P2 S1 P1 • To oad 3 phase, 3 wire ba anced oad Transducer Vab Vca S1 P1 Ic S2 P2 V Transducer Vb Vc Ia A B S1 P1 S2 P2 To oad C N S1 P1 S2 P2 S1 P1 Transducer Ic Ib A S2 P2 S1 P1 S2 P2 B C N 3 phase, 4 wire unba anced (21/ Van V cn S1 P1 Ic Ib S2 P2 S1 P1 S2 P2 S1 P1 S2 P2 To oad 2 Ia Ia A B C
To oad 3 phase, 4 wire unba anced oad 3 phase, 3 wire unba anced oad e .) oad Figure 22.2: Connections for 3-phase watt/var transducers Network Protection & Automation Guide • 403 • Power System Measurements 22 •
22.5.6 Sca ing The re ationship of the output current to the va ue of the measur and is of vita importance and needs carefu consideration. Any receiving equipm ent must, of course, be used within its rating but, if possib e, some kind of st andard shou d be estab ished. As an examp e, examine the measurement of a.c. vo tage. The primary system has a nomina va ue of 11kV and the transformer has a r atio of 11kV/110V. To specify the conversion coefficient for a 0-10mA vo tage tr ansducer to be 110V/10mA wou d not necessari y be the optimum. One of the object ives must be to have the capabi ity of monitoring the vo tage over a range of va ues so an upper imit must be se ected – for instance +20%, or 132V. Using the or igina conversion coefficient, the maximum output of the transducer is required to be 12mA. This is within the capabi ity of most 0-10mA transducers, the majori ty of which can accommodate an over-range of 25%, but it does mean any associate d ana ogue indicating instrument must have a sensitivity of 12mA. However, the s ca e required on this instrument is now 0-13.2kV, which may ead to difficu ty i n drawing the sca e in such a way as to make it readab e (and conforms to the re evant standard). In this examp e, it wou d be more straightforward to estab ish the fu -sca e indication as 15kV and to make this equiva ent to 10mA, thus mak ing the specification of the disp ay instrument much easier. The transducer wi have to be specified such that an input of 0-150V gives an output of 0-10mA. In the case of transducers with a 4-20mA output, great care is required in the out put sca ing, as there is no overrange capabi ity. The 20mA output imit is a fix ed one from a measurement point of view. Such outputs are typica y used as inpu ts to SCADA systems, and the SCADA system is norma y programmed to assume that a current magnitude in excess of 20mA represents a transducer fai ure. Thus, usi ng the above examp e, the output might be sca ed so that 20mA represents 132V an d hence the nomina 110V input resu ts in an output of 16.67mA. A more convenien t sca ing might be to use 16mA as representing110V, with 20mA output being equa to 137.5V (i.e. 25% over-range instead of the 20% required). It wou d be incorr ect to sca e the transducer so that 110V input was represented by 20mA output, a s the over-range capabi ity required wou d not be avai ab e. Simi ar considerati ons app y to current transducers and, with added comp exity, to watt transducers , where the ratios of both the vo tage and the current transformers must be take n into account. In this instance, the output wi be re ated to the primary powe r of the system. It shou d be noted that the input current corresponding to fu -sca e output may not be exact y equa to the secondary rating of the current tr ansformer but this does not matter - the manufacturer wi take this into accoun t. Some of these difficu ties do not need to be considered if the transducer is on y feeding, for examp e, a SCADA outstation. Any receiving equipment that can be programmed to app y a sca ing factor to each individua input can accommodate mo st input signa ranges. The main consideration wi be to ensure that the transd ucer is capab e of providing a signa right up to the fu -sca e va ue of the in put, that is, it does not saturate at the highest expected va ue of the measuran d. 22.5.7 Auxi iary Supp ies Many transducers do not require any auxi iary supp y. These are termed ‘se f-powered’ transducers. Of those that do need a separate sup p y, the majority have a biased, or ive zero output, such as 4-20mA. This is be cause a non-zero output cannot be obtained for zero input un ess a separate supp y is avai ab e. Transducers that require an auxi iary supp y are genera y prov ided with a separate pair of termina s for the auxi iary circuit so that the use r has the f exibi ity of connecting the auxi iary supp y input to the measured v o tage, or to a separate supp y. However, some manufacturers have standardised t heir designs such that they appear to be of the se f-powered type, but the auxi iary supp y connection is actua y interna . For a.c. measuring transducers, the use of a d.c. auxi iary supp y enab es the transducer to be operated over a wid er range of input. The range of auxi iary supp y vo tage over which a transducer can be operated is specified by the manufacturer. If the auxi iary vo tage is d erived from an input quantity, the range of measurement wi be restricted to ab out 20% of the nomina auxi iary supp y vo tage. This can give rise to prob ems w hen attempting to measure ow va ues of the input quantity. 22.6 MEASUREMENT CEN
TRES A Measurement Centre is effective y a co ection of discrete transducers mo unted in a common case. This is arge y impractica if ana ogue techno ogy for s igna processing is used, but no such imitation exists if digita or numerica techno ogy is adopted. Therefore, Measurement Centres are genera y on y found i mp emented using these techno ogies. As has been a ready noted in Chapter 7, a n umerica re ay can provide many measurements of power system quantities. Therefo re, an a ternative way of ooking at a Measurement Centre that uses numerica te chno ogy is that it is a numerica re ay, stripped of its protection functions a nd incorporating a wide range of power system parameter measurements. Network Protection & Automation Guide • Power System Measurements 22 • • 404 •
This is rather an oversimp ification of the true situation, as there are some im portant differences. A protection re ay has to provide the primary function of p rotection over a very arge range of input va ues; from perhaps 5% to 500% or gr eater of rated va ues. The accuracy of measurement, whi st important, is not req uired to be as accurate as, for instance, metering for tariff purposes. Metering does not have to cover quite such a wide range of input va ues, and therefore t he accuracy of measurement is often required to be higher than for a protection re ay. Additiona functiona ity over that provided by the measurement functions of a protection re ay is often required – for a typica set of functions provided by a measurement centre, see Tab e 22.3. On the other hand, the fundamenta meas urement process in a measurement centre based on numerica techno ogy is identic a to that of a numerica re ay, so need not be repeated here. The on y differen ces are the ranges of the input quantities and the functiona ity. The former is dea t with by suitab e design of the input signa conditioning and A/D conversio n, the atter is dea t with by the software provided. R.M.S. ine currents Neutra current Average current Negative sequence vo tage P ower (each phase and tota ) Apparent Power (each phase and tota ) Phase ang e (v o tage/current) – each phase Demand current in period Demand reactive power in per iod Demand power factor in period R.M.S. ine vo tages R.M.S. phase vo tages Ave rage vo tage Negative sequence current Reactive Power (each phase and tota ) Pow er factor (each phase and tota ) Demand time period Demand power in period Deman d VA in period Maximum demand current (each phase and tota ) since reset Energy, Wh Maximum demand (W and var) since reset Energy, varh Frequency Individua harmoni cs (to 31st) %THD (vo tage) – each phase and tota %THD (current) – each phase and t ota Programmab e mu tip e ana ogue outputs Tab e 22.3: Typica function set pro vided by a Measurement Centre Figure 22.3: Typica transducers/Measurement Centres 22.7 TARIFF METERING Tariff metering is a specia ised form of measurement, being concerned with the measurement of e ectrica power, reactive power or energy fo r the purposes of charging the consumer. As such, it must conform to the appropr iate nationa standards for such matters. Primary tariff metering is used for cu stomer bi ing purposes, and may invo ve a measurement accuracy of 0.2% of readi ng, even for readings that are 5% or ess of the nomina rated va ue. Secondary tariff metering is app ied where the user wishes to inc ude his own metering as a check on the primary tariff metering insta ed by the supp ier, or within a a rge p ant or bui ding to gain an accurate picture of the consumption of energy i n different areas, perhaps for the purpose of energy audits or interna cost a ocation. The accuracy of such metering is rather ess, an overa accuracy of 0. 5% over a wide measurement • The advantages of a Measurement Centre are that a comprehensive set of functions are provided in a sing e item of equipment, taking up very itt e extra space c ompared to a discrete transducer for a much sma er number of parameters. Theref ore, when the requisite CT’s and VT’s are avai ab e, it may we make sense to use a Measurement Centre even if not a of the functiona ity is immediate y required . History shows that more and more data is required as time passes, and incorpor ation of fu functiona ity at the outset may make sense. Figure 22.3 i ustrate s the wide variety of transducers and Measurement Centres avai ab e. Network Protection & Automation Guide • 405 • Power System Measurements 22 •
range being typica y required. As this is the overa accuracy required, each e ement in the metering chain (starting with the CT’s/VT’s) must have an accuracy rat her better than this. Carefu attention to wiring and mounting of the transducer is required to avoid errors due to interference, and the accuracy may need to b e maintained over a fair y wide frequency range. Thus a tariff metering scheme r equires carefu design of a of the equipment inc uded in the scheme. Faci itie s are norma y inc uded to provide measurements over a arge number of defined t ime periods (e.g. 24 ha f-hour periods for generator energy tariff metering) so that the exporter of the energy can produce an overa invoice for the user acco rding to the correct rates for each tariff period. The time interva s that these periods cover may change according to the time of year (winter, spring, etc.) a nd therefore f exibi ity of programming of the energy metering is required. Remo te communications are invariab y required, so that the data is transferred to th e re evant department on a regu ar basis for invoicing purposes. 22.8 SYNCHRONIS ERS be to erated without eading to excessive current/vo tage transients on CB c osu re. The check synchroniser has programmab e error imits to define the imits of acceptabi ity when making the comparison. CB c ose contro s Check synchroniser C ose Generator Network Busbar (a) App ication to generator CB c ose contro s Check synchroniser C ose Power System Measurements Synchronisers are required at points on a power system where two supp ies (eithe r generator and grid, or two grid supp ies) may need to be para e ed. They are more than just a measuring device, as they wi provide contact c osures to perm it circuit breaker c osing when conditions for para e ing (synchronising) are w ithin imits. However, they are not regarded as protection re ays, and so are in c uded in this Chapter for convenience. There are two types of synchronisers aut o-synchronisers and check synchronisers. 22.8.1 Check Synchronisers The function of a check synchroniser is to determine if two vo tages are in synchronism, or near y so, and provide outputs under these conditions. The outputs are norma y in the form of vo t-free contacts, so that they may be used in CB contro circui ts to permit or b ock CB c osing. When app ied to a power system, the check sync hroniser is used to check that it is safe to c ose a CB to connect two independe nt networks together, or a generator to a network, as in Figure 22.4. In this wa y, the check synchroniser performs a vita function in b ocking CB c osure when required. Synchronism occurs when two a.c. vo tages are of the same frequency an d magnitude, and have zero phase difference. The check synchroniser, when active , monitors these quantities and enab es CB c ose circuits when the differences a re within pre-set imits. Whi e CB c osure at the instant of perfect synchronism is the idea , this is very difficu t to obtain in practice and some mismatch in one or more of the monitored quantities can Network #2 Line A CB 1 Network #1 Busbar B (b) App ication to two networks Figure 22.4: Check synchroniser app ica tions
• 22 • The conditions under which a check synchroniser is required to provide an output are varied. Consider the situation of a check synchroniser being used as a perm issive device in the c osing contro circuit of a CB that coup es two networks t ogether at a substation – Figure 22.4(b). It is not sufficient to assume that both networks wi be ive, situations where either Line A or Busbar B may be dead m ay have to be considered, eading to the functiona ity shown in Tab e 22.4(a). Live bus/ ive ine synchronising Dead bus/ ive ine synchronising Live bus/dead ine synchronising Network supp y vo tage #1 deviation from nomina Network supp y vo tage #2 deviation Vo tage difference within imits from nomina Frequency difference within imits Phase ang e difference within imits CB c o sing advance time CB c osing pu se time Maximum number of synchronising attempts (a): Check synchroniser functiona ity Incoming supp y frequency deviation Incom ing supp y vo tage signa from nomina raise/ ower Incoming supp y vo tage raise / ower Incoming supp y frequency raise/ ower mode (pu se/continuous) mode (pu se /continuous) Incoming supp y vo tage setpoint Incoming supp y frequency setpoint Vo tage raise/ ower pu se time Frequency raise/ ower pu se time (b) Additiona functions for auto-synchroniser Tab e 22.4: Synchroniser function set • 406 • Network Protection & Automation Guide
When the c ose signa is permitted, it may be given on y for a imited period of time, to minimise the chances of a CB c ose signa remaining after the conditio ns have moved outside of imits. Simi ar y, circuits may a so be provided to b o ck c osure if the CB c ose signa from the CB c ose contro s is present prior to satisfactory conditions being present – this ensures that an operator must be mon itoring the synchronising disp ays and on y initiating c osure when synchronisin g conditions are correct, and a so detects synchronising switch contacts that ha ve become we ded together. A check synchroniser does not initiate any adjustment s if synchronising conditions are not correct, and therefore acts on y as a perm issive contro in the overa CB c osing circuit to provide a check that conditi ons are satisfactory. In a substation, check-synchronisers may be app ied indivi dua y to a required CB’s. A ternative y, a reduced number may be insta ed, tog ether with suitab e switching arrangements in the signa input/output circuits s o that a sing e device may be se ected to cover severa CB’s. 22.8.2 Auto-synchron iser An auto-synchroniser contains additiona functiona ity compared to a check synchroniser. When an autosynchroniser is p aced in service, it measures the fre quency and magnitude of the vo tages on both sides of the circuit breaker, and a utomatica y adjusts one of the vo tages if conditions are not correct. App icat ion of auto-synchronisers is norma y restricted to generators – i.e. the situatio n shown in Figure 22.4(a), rep acing the check synchroniser with an auto-synchro niser. This is because it is genera y not possib e to adjust either of the netw ork vo tages by changing the settings of one or a very few equipments in a netwo rk. When app ied to a generator, it is re ative y easy to adjust the frequency a nd magnitude of the generated vo tage by transmitting signa s to the Governor an d AVR respective y. The auto-synchroniser wi check the vo tage of the incoming generator against the network vo tage for comp iance with the fo owing (Tab e 22.4(a), (b)): a. s ip frequency within imits (i.e. difference in frequency bet ween the generator and network) b. phase difference between the vo tages within imits c. vo tage magnitude difference within imits The CB c ose command is iss ued automatica y when a three conditions are satisfied. Checks may a so be ma de that the network frequency and vo tage is within pre-set imits, and if not t he synchronising sequence is ocked out. This prevents synchronising under unusu a network conditions, when it may not be desirab e. This faci ity shou d be use d with caution, since under some Network Protection & Automation Guide emergency conditions, it cou d b ock the synchronising of a generator that was u rgent y required in service to he p assist in overcoming the condition. If (a) a bove is not within imits, signa s are sent automatica y to the governor of the generating set to adjust the speed setpoint appropriate y. In the case of (c) n ot in imits, simi ar signa s are sent to the Automatic Vo tage Regu ator to rai se or ower the setpoint. The signa s are common y in the form of pu ses to rais e or ower the setpoint, but cou d be continuous signa s if that is what the par ticu ar equipment requires. It is norma for the speed and vo tage of the genera tor to be s ight y higher than that of the network, and this can be accommodated either by initia settings on the Governor/AVR or by providing setpoint va ues in the synchroniser. This ensures stab e synchronising and export of power at a gging power factor to the network by the generator after CB c osure. The possibi ity of tripping due to reverse/ ow forward power conditions and/or fie d fai ur e/under-excitation is avoided. Use of an auto-synchroniser a so he ps avoid huma n error if manua synchronising were emp oyed – there is potentia for damage to e quipment, primari y the generator, if synchronising outside of permitted imits occurs. To ensure that the CB is c osed at the correct instant, the CB c ose tim e is norma y a required data item. The autosynchroniser ca cu ates from a know edge of this and the s ip frequency the correct time in advance of phase coincid ence to issue the CB c ose command. This ensures that the CB c oses as c ose to the instant of phase coincidence as possib e. Upon receipt of the signa indicat ing ‘CB c osed’ a further signa to raise frequency may be sent to the governor to e nsure stab e export of power is achieved. Converse y, fai ure of the CB to c ose within a set time period wi reset the auto-synchroniser, ready for another at
tempt, and if further attempts are sti unsuccessfu , the auto-synchroniser wi ock out and raise an a arm. Practice in respect of fitting of auto-synchronis ers varies wide y between Uti ities. Where po icy is f exib e, it is most common when the time to synchronise is important – i.e. emergency standby and peak oppi ng sets. Many Uti ities sti re ay on manua synchronising procedures. It is a so possib e for both an auto-synchroniser and checksynchroniser to be fitted in series. This provides protection against interna fai ure of the auto-synchronis er eading to a CB c ose command being given incorrect y. 22.9 DISTURBANCE RECOR DERS Power systems suffer from various types of disturbances. In post-fau t ana ysis, it is beneficia to have a detai ed record of a disturbance to enab e the initiating event to be distinguished from the subsequent effects. Especia y whe re the disturbance causes further prob ems (e.g. • • 407 • Power System Measurements 22 •
sing e-phase fau t deve ops into 3-phase), a detai ed recording of the fau t may be required to distinguish between cause and effect. If the effects of a fau t are spread over a wide area, records of the disturbance from a number of ocatio ns can assist in determining the ocation of the disturbance. The equipment used for this purpose is known as a disturbance, or fau t, recorder. 22.9.1 Disturba nce Recorder Features A disturbance recorder wi norma y have the fo owing ca pabi ities: a. mu ti-channe ana ogue input waveform recording b. mu ti-channe digita input recording c. storage of severa fau t records, ready for down oad/ ana ysis d. recording time of severa seconds per disturbance e. triggering from any ana ogue or digita input channe , or quantity derived from a combination o f inputs, or manua y Power system disturbances may ast from periods of a few seconds to severa minu tes. To ensure that maximum benefit is obtained from the investment, a disturban ce recorder must be ab e to capture events over a wide range of timesca es. This eads to the provision of programmab e samp ing rates, to ensure that short-ter m transients are captured with sufficient reso ution whi e a so ensuring that o nger-term ones have sufficient of the transient captured to enab e a meaningfu ana ysis to be undertaken. The record for each disturbance is divided into secti ons covering pre-fau t, fau t, and post–fau t periods, and each of these periods m ay have different samp ing rates. Time synchronisation is a so a vita feature, to enab e a recording from one recorder to be a igned with another of the same e vent from a different recorder to obtain a comp ete picture of events. Since mos t distrubance recorders are fitted in substations that are norma y unmanned, th e provision to down oad captured information is essentia . Each fau t recording wi contain a arge amount of data, and it is vita that the data is unique y i dentified in respect of recorder, fau t event, channe , etc. Standards exist in fie d to faci itate the interchange of data, of which perhaps the best known is the IEEE COMTRADE format, now a so an IEC standard. Once down oaded, the data fr om a disturbance recorder can be ana ysed by various software packages, such as WinAna yse, Eview, or TOP2000. The software wi often have the abi ity to ca cu ate the fau t ocation (distance-to-fau t), superimpose waveforms to assist in fau t ana ysis, and perform harmonic and other ana yses. Power System Measurements f. distance to fau t ocation for one or more feeders g. variab e pre/post trigg er recording ength h. time synchronisation (IRIG-B, GPS, etc.) i. programmab e samp ing rates j. standard data transfer formats (IEEE COMTRADE (now IEC 60255-2 4), etc. k. communication inks to contro centre, etc. (Ethernet, modem, etc.) . se f-monitoring/diagnostics Ana ogue channe s are provided to record the impo rtant currents and vo tages at the fau t recorder ocation. High reso ution is r equired to ensure accurate capture of the waveforms, with 14 or 16 bit A/D conve rsion being usua . Digita inputs are provided to capture signa s such as CB ope ning, protection re ay operation, intertrip signa s, etc. so that a comp ete pic ture of the sequence of events can be bui t up. The information can then be used to check that the sequence of operations post-fau t is correct, or assist in de termining the cause of an unexpected sequence of operations. To avoid oss of th e disturbance data, sufficient memory is provided to capture and store the data from severa fau ts prior to transfer of the data for ana ysis. F exibi ity in t he triggering arrangements is extreme y important, as it is point ess to insta a disturbance recorder, on y for it to miss recording events due to ack of app ropriate triggering faci ities. It is norma for triggering to be avai ab e if t he re evant thresho d is crossed on any ana ogue or digita channe , or a quanti ty that can be derived from a combination of inputs. • 22 •
• 408 • Network Protection & Automation Guide
• 23 • Power Qua ity Introduction Power Qua ity c assification Causes and impact of Power Qua ity pro b ems Power Qua ity monitoring Remedia measures Examp es 23.1 23.2 23.3 23.4 23 .5 23.6
• 23 • Power Qua ity 23.1 INTRODUCTION Over the ast thirty years or so, the amount of equipment cont aining e ectronics has increased dramatica y. Such equipment can both cause and be affected by e ectromagnetic disturbances. A disturbance that affects a proce ss contro computer in a arge industria comp ex cou d easi y resu t in shutdow n of the process. The ost production and product oss/recyc ing during start-up represents a arge cost to the business. Simi ar y, a protection re ay affected by a disturbance through conduction or radiation from nearby conductors cou d t rip a feeder or substation, causing oss of supp y to a arge number of consumer s. At the other end of the sca e, a domestic user of a PC has to re-boot the PC due to a transient vo tage dip, causing annoyance to that and other simi ar y af fected users. Therefore, transporters and users of e ectrica energy have become much more interested in the nature and frequency of disturbances in the power s upp y. The topic has become known by the tit e of Power Qua ity. 23.2 CLASSIFICA TION OF POWER SYSTEM DISTURBANCES To make the study of Power Qua ity prob ems us efu , the various types of disturbances need to be c assified by magnitude and d uration. This is especia y important for manufacturers and users of equipment t hat may be at risk. Manufacturers need to know what is expected of their equipme nt, and users, through monitoring, can determine if an equipment ma function is due to a disturbance or prob ems within the equipment itse f. Not surprising y, standards have been introduced to cover this fie d. They define the types and si zes of disturbance, and the to erance of various types of equipment to the possi b e disturbances that may be encountered. The principa standards in this fie d are IEC 61000, EN 50160, and IEEE 1159. Standards are essentia for manufacturer s and users a ike, to define what is Network Protection & Automation Guide • 411 •
reasonab e in terms of disturbances that might occur and what equipment shou d w ithstand. Tab e 23.1 provides a broad c assification of the disturbances that ma y occur on a power system, some typica causes of them and the potentia impact on equipment. From this Tab e, it wi be evident that the e ectricity supp y wa veform, often thought of as composed of pure sinusoida quantities, can suffer a wide variety of disturbances. The fo owing sections of this Chapter describe t he causes in more detai , a ong with methods of measurement and possib e remedia measures. Category Causes Loca and remote fau ts Inductive oading Switch on of arge oa ds Impacts Tripping of sensitive equipment Resetting of contro systems Motor st a ing/tripping Tripping of sensitive equipment Damage to insu ation and winding s Damage to power supp ies for e ectronic equipment Prob ems with equipment that requires constant steady-state vo tage Ma -operation of sensitive equipment and re ays Capacitor fuse or capacitor fai ures Te ephone interference Neg igib e m ost of time Motors run s ower De-tuning of harmonic fi ters F icker in: F uoresc ent amps Incandescent amps Light f icker Tripping of equipment Overheating in motors/generators Interruption of 3-phase operation Loss of supp y to customer e quipment Computer shutdowns Motor tripping A equipment without backup supp y f aci ities Contro system resetting Damage to sensitive e ectronic components Dam age to insu ation Tab e 23.2 ists the imits given in Standard EN 50160 and notes where other sta ndards have simi ar imits. Type of Vo tage disturbance Leve Vo tage 230V Variation Vo tage Dips 230V Rapid vo tage changes Short Interruptions Long Interruptions Transient Overvo tage Vo tage unba ance Undervo tage Vo tage surge Vo tage f uctuations Frequency variat ion Harmonics 230V 1kV-35kV 230V 230V 230V 230V <-10% Not specified <150% of 230 V nomina vo tage Not specified 230V 3% +/- 1% +4%, -6% THD<8% up to 40th 10 min 230V >1 min >200ms <200ms IEEE 1159 IEEE 1159 IEC 60827 Limits from EN50160 +/10% Measurement Typica Other app icab e period duration standards 95% of 1 wee k 10-1000/year 10ms –1sec IEEE 1159 Severa Short per day duration Short Per day I EEE 1159 duration 20-200 Up to 3 mins EN61000-4-11 per year 10-50 per year Not s pecified >3 mins <1ms IEEE 1159 IEEE 1159 5% to 10% <6% >99% >99% Genera y <6kV Vo tage dips Vo tage surges Capacitor switching Switch off of arge oads Phase fau ts 95% of 1 week Not specified Measured over 10s 100% of 1 week Not specified Measu red over 10s 95% of Not specified 1 week Overvo tage Load switching Capacitor switching System vo tage regu ation Industria furnaces Non- inear oads Transformers/generators Rectifier equipment Loss of generation Extreme oading conditions AC motor drives Inter-harmonic current components We ding and arc furnaces Motor starting Transformer tap changing Unba anced oads Unba anced impedances Power system fau ts Equipment fai ures Contro ma function s CB tripping Heavy network oading Loss of generation Poor power factor Lack of var support Lightning Capacitive switching Non – inear switching oads System vo tage regu ation Tab e 23.2: Power system disturbance c assification to EN 50160 Harmonics
Power Qua ity For computer equipment, a common standard that manufacturers use is the ITI (Inf ormation Techno ogy Industry) curve, i ustrated in Figure 23.1. Vo tage disturb ances that ie in the area indicated as ‘safe’ shou d not cause a ma function in any way. However, some disturbances at LV eve s that ie within the boundaries def ined by EN50160 might cause a ma function because they do not ie in the safe ar ea of the ITI curve. It may be necessary to check carefu y which standards are app icab e when considering equipment susceptibi ity. 500 450 400 350 300 250 200 Withstand disturbance 150 100 50 0 0.001 0.01 0.1 Af fected by disturbance Power frequency variation Vo tage f uctuation • 23 • Rapid vo tage change Vo tage imba ance Short and ong vo tage interruptions Percentage of nomina vo tage (r.m.s.) Affected by disturbance 1 10 100 1000 10000 100000 Duration of disturbance (ms) Figure 23.1: ITI curve for equipment susceptibi ity Undervo tage Transients Tab e 23.1: Power Qua ity issues • 412 • Network Protection & Automation Guide
23.3 CAUSES AND IMPACT OF POWER QUALITY PROBLEMS Each of the Power Qua ity distu rbance categories detai ed in Tab e 23.1 is now examined in more detai as to th e possib e causes and the impact on consumers. 23.3.1 Vo tage Dips Figure 23.2 s hows the profi e of a vo tage dip, together with the associated definitions. The major cause of vo tage dips on a supp y system is a fau t on the system, that i s sufficient y remote e ectrica y that a vo tage interruption does not occur. O ther sources are the starting of arge oads (especia y common in industria sy stems), and, occasiona y, the supp y of arge inductive oads. Vrms Nom. High Nom. Low x % be ow nomina o a PQ Standards User defined setpoint s Interruption Retained Vo tage Time Figure 23.2: Vo tage dip profi e insu ator f ashover, co isions due to birds, and excavations damaging cab es. M u tip e vo tage dips, as i ustrated in Figure 23.3, cause more prob ems for equ ipment than a sing e iso ated dip. The impact on consumers may range from the an noying (non-periodic ight f icker) to the serious (tripping of sensitive oads and sta ing of motors). Where repeated dips occur over a period of severa hour s, the repeated shutdowns of equipment can give rise to serious production prob ems. Figure 23.4 shows an actua vo tage dip, as captured by a Power Qua ity rec order. 100 80 60 40 20 0 -20 -40 -60 -80 -100 Time Figure 23.4: Recording of a vo tage dip Number of incidents/yr Vo tage dips due to the atter are usua y due to poor design of the network fee ding the consumer. A vo tage dip is the most common supp y disturbance causing i nterruption of production in an industria p ant. Fau ts on a supp y network wi a ways occur, and in industria systems, it is often practice to specify equip ment to ride-through vo tage dips of up to 0.2s. The most common exception is co ntactors, which may we drop out if the vo tage dips be ow 80% of rated vo tage for more than 50-100ms. Motor protection re ays that have an undervo tage e eme nt setting that is too sensitive is another cause. Since contactors are common y used in circuits supp ying motors, the impact of vo tage dips on motor drives, and hence the process concerned, requires consideration. Vrms Nom. High Nom. Low Typica data for undervo tage disturbances on power systems during evo ving fau ts are shown in Figure 23.5. Disturbances that ie in the front right-hand porti on of the histogram are the ones that cause most prob ems, but fortunate y these are quite rare. Number of undervo tage disturbances recorded 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 <0.5ms 0.5-1ms 1-5ms 5-10ms 10-50ms 50-100ms 100-500ms • PQ Standards User defined setpoints Retained Vo tage 91-100% 71-80% 51-60% 31-40% 11-20% 0.5-1s 1-5s 5-10s >10s Retained vo tage Interruption Time
Duration of disturbance Figure 23.5: Undervo tage disturbance histogram Figure 23.3: Mu tip e vo tage dip Other network-re ated fau t causes are weather–re ated (such as snow, ice, wind, s a t spray, dust) causing 23.3.2 Vo tage Surges/Spikes Vo tage surges/spikes are the opposite of dips – a ri se Network Protection & Automation Guide • 413 • Power Qua ity 23 • Retained vo tage 61-70% 0-10% 81-90% 41-50% 91-100% 51-60%
that may be near y instantaneous (spike) or takes p ace over a onger duration ( surge). These are most often caused by ightning strikes and arcing during switc hing operations on circuit breakers/contactors (fau t c earance, circuit switchi ng, especia y switch-off of inductive oads). Figure 23.6 shows the profi e of a vo tage surge. Vrms Nom. High Nom. Low are sufficient y high enough, protective devices may shut the equipment down to avoid damage. Some equipment, such as certain protection devices, may ma operate and cause unnecessary shutdowns. 150 100 50 User defined setpoints 0 -50 Time PQ Standards -100 -150 Figure 23.7: Supp y waveform distorted due to the presence of harmonics Interruption Time Figure 23.6: Vo tage surge profi e Equipment may suffer serious damage from these causes, ranging from insu ation d amage to destruction of sensitive e ectronic devices. The damage may be immediat e and obvious by the fact that equipment stops working, through to fai ure at a much ater date from deterioration initiated from a surge or spike of vo tage. T hese atter fai ures are very difficu t to distinguish from random fai ures due to age, minor manufacturing defects, etc. Specia provision may have to be made to fi ter harmonics from the measured sign a s in these circumstances. Interference may be caused to communication systems. Over oading of neutra conductors in LV systems has a so occurred (the harmonic s in each phase summing in the neutra conductor, not cance ing) eading to fai ure due to overheating. This is a particu ar risk in bui dings that have a arg e number of PC’s, etc., and in such cases a neutra conductor rated at up to 150% of the phase conductors has been known to be required. Busbar risers in bui ding s are a so at risk, due to harmonic-induced vibration causing joint securing bo ts, etc. to work oose. 23.3.5 Frequency Variations Frequency variations that ar e arge enough to cause prob ems are most often encountered in sma iso ated ne tworks, due to fau ty or ma adjusted governors. Other causes are serious over oa ds on a network, or governor fai ures, though on an interconnected network, a si ng e governor fai ure wi not cause widespread disturbances of this nature. Net work over oads are most common in areas with a deve oping e ectrica infrastruct ure, where a reduction in frequency may be a de iberate po icy to a eviate over oading. Serious network fau ts eading to is anding of part of an interconnecte d network can a so ead to frequency prob ems. Few prob ems are norma y caused by this prob em. Processes where product qua ity depends on motor speed contro may be at risk but such processes wi norma y have c osed- oop speed contro e rs. Motor drives wi suffer output changes, but process contro mechanisms wi norma y take care of this. Extreme under- or overfrequency may require the tri pping of generators, eading to the possibi ity of progressive network co apse through network over oading/underfrequency causes. Network Protection & Automation Guide
Power Qua ity • 23.3.3 Overvo tages Sustained overvo tages are not common. The most ike y cause s are ma adjusted vo tage regu ators on generators or on- oad tap changers, or i ncorrect y set taps on fixedtap transformers. Equipment fai ures may immediate y resu t in the case of severe overvo tages, but more ike y is acce erated degra dation eading to premature fai ure without obvious cause. Some equipment that i s particu ar y sensitive to overvo tages may have to be shut down by protective devices. 23.3.4 Harmonics This is a very common prob em in the fie d of Power Qu a ity. The main causes are Power E ectronic Devices, such as rectifiers, inverte rs, UPS systems, static var compensators, etc. Other sources are e ectric discha rge amps, arc furnaces and arc we ders. In fact, any non inear oad wi be a s ource of harmonics. Figure 23.7 i ustrates a supp y waveform that is distorted due to the presence of harmonics. Harmonics usua y ead to heating in rotating equipment (generators and motors), and transformers, eading to possib e shutdow n. Capacitors may be simi ar y affected. If harmonic eve s 23 • • 414 •
23.3.6 Vo tage F uctuations These are main y caused by oad variations, especia y arge rapid ones such as are ike y to occur in arc and induction heating fur naces, ro ing mi s, mine winders, and resistance we ders. F icker in incandesc ent amps is the most usua effect of vo tage f uctuations. It is a serious prob em, with the human eye being particu ar y sensitive to ight f icker in the fre quency range of 5-15Hz. Because of the wide use of such amps, the effects are w idespread and inevitab y give rise to a arge number of comp aints. F uorescent amps are a so affected, though to a esser extent. 23.3.7 Vo tage Unba ance Unb a anced oading of the network norma y causes vo tage unba ance. However, parts of the supp y network with unba anced impedances (such as untransposed overhead transmission ines) wi a so cause vo tage unba ance, though the effect of thi s is norma y sma . Overheating of rotating equipment resu ts from vo tage unba ance. In serious cases, tripping of the equipment occurs to protect it from dam age, eading to generation/ oad imba ance or oss of production. 23.3.8 Supp y I nterruptions Fau ts on the power system are the most common cause, irrespective of duration. Other causes are fai ures in equipment, and contro and protection ma functions. E ectrica equipment ceases to function under such conditions, wit h undervo tage protection devices eading to tripping of some oads. Short inter ruptions may be no more than an inconvenience to some consumers (e.g. domestic c onsumers), but for commercia and industria consumers (e.g. semiconductor manuf acture) may ead to engthy serious production osses with arge financia impac t. Longer interruptions wi cause production oss in most industries, as induct ion and synchronous motors cannot to erate more than 1-2 seconds interruption wi thout having to be tripped, if on y to prevent excessive current surges and resu ting arge vo tage dips on supp y restoration. On the other hand, vita compute r systems are often fed via a UPS supp y that may be capab e of supp ying power from batteries for severa hours in the event of a mains supp y fai ure. More mo dern devices such as Dynamic Vo tage Restorers can a so be used to provide conti nuity of supp y due to a supp y interruption. For interruptions asting some tim e, a standby generator can be provide a imited supp y to essentia oads, but c annot be started in time to prevent an interruption occurring. Network Protection & Automation Guide 23.3.9 Undervo tage Excessive network oading, oss of generation, incorrect y s et transformer taps and vo tage regu ator ma functions, cause undervo tage. Load s with a poor power factor (see Chapter 18 for Power Factor Correction) or a gen era ack of reactive power support on a network a so contribute. The ocation o f power factor correction devices is often important, incorrect ocation resu ti ng in itt e or no improvement. The symptoms of undervo tage prob ems are trippi ng of equipment through undervo tage trips. Lighting wi run at reduced output. Undervo tage can a so indirect y ead to over oading prob ems as equipment take s an increased current to maintain power output (e.g. motor oads). Such oads m ay then trip on overcurrent or therma protection. 23.3.10 Transients Transients on the supp y network are due to fau ts, contro and protection ma functions, ightning strikes, etc. Vo tage-sensitive devices and insu ation of e ectrica eq uipment may be damaged, as noted above for vo tage surges/spikes. Contro system s may reset. Semiconductor manufacture can be serious y affected un ess the supp ies to critica process p ant are suitab y protected. 23.4 POWER QUALITY MONITO RING If an insta ation or network is thought to be suffering from prob ems re a ted to Power Qua ity, suitab e measurements shou d to be taken to confirm the in itia diagnosis. These measurements wi a so he p quantify the extent of the pr ob em(s) and provide assistance in determining the most suitab e so utions. Fina y, fo owup measurements after insta ation wi confirm the effectiveness of the remedia measures taken. 23.4.1 Type of Insta ation Monitoring equipment fo r Power Qua ity may be suitab e for either temporary or permanent insta ation o n a supp y network. Permanent insta ation is most ike y to be used by Uti itie s for routine monitoring of parts of their networks to ensure that regu atory i mits are being comp ied with and to monitor genera trends in respect of power q ua ity issues. Consumers with sensitive oads may a so insta permanent monitor ing devices in order to monitor Power Qua ity and provide supporting evidence in
the event of a c aim for compensation being made against the supp ier if oss o ccurs due to a power qua ity prob em whose source is in the Uti ity network. • • 415 • Power Qua ity 23 •
The performance of any devices insta ed to improve Power Qua ity can a so be mo nitored. Such devices may have a data ink to a DCS or data ogger in order to p rovide historica data recording and data processing/presentation faci ities. Th ey are quite sma and are fitted in a suitab e cubic e forming part of a switch board ine-up. The data ink may be hard-wired, use a modem connection to a te e phone ine, or in the case of a uti ity with many geographica y-dispersed subst ations, radio inks for data transmission may be used. Interna data storage wi be provided to ensure effective use of the data ink. The units may be se f- o r auxi iary supp y powered, and in the case of important Uti ity substations may have battery-backed supp ies to ensure capture of vo tage interruptions. Time s ynchronisation may be required to ensure accurate identification of events. For investigation of particu ar prob ems, a portab e instrument is more suitab e. Th e same range of Power Qua ity measurement capabi ities is provided as for perman ent instrumentation. The instrument may have bui t-in ana ysis/data storage capa bi ities, but externa storage in the form of f oppy discs or a data ink to a aptop or desktop PC is commonp ace. Ana ysis/report writing software running on a PC is often avai ab e, which may be more comprehensive than that provide in th e instrument itse f. avoid signa distortion. This is especia y important if ong cab e runs are use d; this may be the case if the measuring instruments are centra ised but measure ments are being made at a number of switchboards. 23.4.3 Types of Power Qua ity Measurements Instruments for power qua ity monitoring may not offer the fu ran ge of measurements for a Power Qua ity issues. Care is therefore required that the instrument chosen is suited for the purpose. Most instruments wi provide provide measurements of current and vo tage harmonics, and capture of vo tage di ps and frequency excursions (Figure 23.9). Measurements to the common y encounte red standards may be bui t-in. For capture of surges, spikes and interruptions, more specia ised instrumentation may be required as transient high-speed wavefor m capture is required. This requires a high samp ing rate and arge memory stora ge. Most instruments designed for Power Qua ity use A/D conversion of the input waveforms. The raw waveform is stored and either transferred to a computer for a na ysis, or the instrument contains bui t-in software to carry out ana ysis of p ower qua ity in ine with accepted standards. Often the software wi have a cho ice of standards for user se ection. Figure 23.10 shows the capture of data and ana ysis for a period of one week to determine comp iance with EN 50160.More det ai ed ana ysis using the same instrument can show direct y how the resu ts compa re with this standard, as shown in Figure 23.11. To faci itate the interchange o f data between ocations and/or users, the pub ic-domain PQDIF data interchange format for Power Qua ity may be used and faci ities provided for in the software . Power Qua ity • 23.4.2 Connection to the Supp y Connection to the supp y being monitored may pre sent prob ems. For LV supp ies, the vo tage inputs are usua y taken direct y to the instrument in sing e-phase or threephase form as required. Monitoring of cu rrents may be through a current shunt or suitab e CT, depending on circuit ratin g. At higher vo tages, VT’s and CT’s a ready fitted for instrumentation/protection p urposes are used. In genera , the conventiona e ectromagnetic vo tage or curren t transformer is suitab e for use without specia considerations being required, but capacitor vo tage transformers often have a ow-pass fi ter on the output t hat has the potentia to serious y affect readings of harmonics and transient ph enomena. In such cases, the input to the monitoring device must be taken prior t o fi tering, or the fi ter characteristics must be determined and the measured s igna s processed to take account of the fi tering prior to ana ysis being undert aken. In addition, the CVT itse f may have a non- inear transfer function with r espect to frequency, though the variety of types of CVT and difficu ties of test ing make confirmation of this point virtua y impossib e at present. Where harmo
nics or high-frequency phenomena are being measured, suitab e connecting eads b etween the transducers and the measuring instrument are required to 23 • • 416 • Network Protection & Automation Guide
23.4.4 Instrument Location The ocation of the measuring instrument a so require s consideration. By carefu p acement and observing the re ative po arities, it is possib e to deduce if the source of the disturbance is on the source or oad side of the monitoring device. 23.5 REMEDIAL MEASURES There are many methods ava i ab e for correcting Power Qua ity prob ems. The most common are given in Tab e 23.3. Brief detai s of each method are given be ow, but it is emphasised that t he so ution adopted wi be tai ored specifica y to the prob em and site. Equipment UPS Earthing practices Fi ters (Active/Passive) Figure 23.9: Transient vo tage disturbance capture Energy Storage Devices App ication Vo tage variatio ns Supp y interruptions Frequency variations Harmonics Harmonics Vo tage variati ons Supp y interruptions Tab e 23.3: Power system disturbance c assification to EN 50160 A UPS system consists of the fo owing: a. an energy storage device – norma y a b attery b. a rectifier and inverter c. transfer switches The UPS may be on- ine ( continuous y in operation) or off ine (switched in when a disturbance occurs). T he former e iminates a prob ems due to vo tage surges/spikes/dips and interrup tions (within the capacity of the storage device) whi e the atter passes some o f the disturbance through, unti the supp y is transferred from the norma sourc e to the UPS. Harmonics originating in the source may be reduced, but not e imin ated in the oad, because the UPS itse f is a source of harmonics, as it contain s Power E ectronic Devices. Thus it may increase harmonic distortion on the sour ce side.The main disadvantages of UPS systems are cost and efficiency. An on- in e UPS incurs continuous osses, whi e both types require energy storage devices that can be expensive. Fastacting switches to transfer oad to the energy storag e device are required for off ine devices, whi e transfer switches to bypass the rectifier/inverter when these are undergoing maintenance may a so be required. Figure 23.12 i ustrates conceptua y both types of UPS. • 417 • Figure 23.10: Data capture for ana ysis of data to EN50160 • Figure 23.11: THD ana ysis to EN50160 Network Protection & Automation Guide Power Qua ity 23 • 23.5.1 UPS Systems
Supp y Rectifier/ Inverter Load Energy storage Figure 23.12: UPS system (vo tage source converter) techno ogies are possib e. Passive fi ters may take u p significant space, depending on the harmonics being fi tered and the connectio n vo tage. A vo tage source converter may be used instead to provide a reduced f ootprint. It can fi ter severa frequencies simu taneous y and track changes in the frequencies of the harmonics as the fundamenta frequency changes. It can be expensive when used so e y as an active fi ter, but be viab e where space is at a premium. Figure 23.14 shows the concept of an active harmonic fi ter. A dange r with fi ters is the possibi ity of resonance with part of the power system at some frequency, giving rise to prob ems that wou d not otherwise occur. Network Load Coup ing inductance 23.5.2 Dynamic Vo tage Restorer (DVR) This is a vo tage source converter and ene rgy store, connected in series (either direct y or via an injection transformer) that contro s the vo tage downstream direct y by injection of suitab e vo tage in series with the source. Ratings of up to severa MW are possib e at vo tages up to 11kV. Figure 23.13 i ustrates the concept. IGBT power section DC- ink capacitor Figure 23.14: Active harmonic fi ter concept Power Qua ity Disturbed incoming supp y Disturbance free supp y 23.5.5 Static Var Compensator (SVC) This is a shunt-connected assemb y of capaci tors, and possib y reactors, which provides reactive power to a network during d isturbances to minimise them. It is norma y app ied to transmission networks to counter vo tage dips/surges during fau ts and enhance power transmission capaci ty on ong transmission circuits. The devices are switched either in discrete st eps or made continuous y variab e through the use of PED’s. It works by providing reactive power ( eading/ agging as required) to assist in keeping the vo tage at the point of connection constant. Vo tage variations at that point are ref ecte d in var variations, so provision of reactive power of appropriate sign can redu ce the vo tage f uctuations. The STATCOM is a SVC comprised of a se f commutated static converter and capacitor energy storage. The switching of the converter i s contro ed to supp y reactive power of appropriate sign to the network. 23.5.6 Ferro-resonant Transformer This is a transformer that is designed to run high y saturated. Thus, input vo tage dips and surges have itt e effect on the output vo tage. Vo tage interruptions of very short duration resu t in the magnetic st ored energy A.C.-D.C.
Modu ar 3-phase power e ectronic inverters Energy storage system D.C.-D.C. • 23 • Figure 23.13: Dynamic Vo tage Restorer concept 23.5.3 Earthing Practices A site that suffers from prob ems with harmonics may n eed to investigate the earthing of equipment. The high neutra currents that res u t can give rise to overheating/fai ure of neutra /earth connections, whi e hig h neutra -earth impedances can give rise to commonmode vo tage prob ems. A neu tra and earth connections need to be checked to ensure they are adequate y size d and have sound joints. 23.5.4 Fi ters These are shunt-connected devices used t o e iminate harmonics. Either passive (LC or RLC) networks or active D.C.-D.C. D.C.-D.C. A.C.-D.C. • 418 • Network Protection & Automation Guide
being used up in maintaining output vo tage and current. The transformer is norm a y of 1:1 ratio, a though taps may be provided for fine adjustment of output v o tage. Appropriate shie ding of the windings enab es the impact of vo tage spik es to be reduced. It is used in LV systems, with a power output of up to a few t ens of kVA. 23.6 EXAMPLES The fo owing sections show some examp es of the measu rement of Power Qua ity prob ems, using an AREVA M720 Power Qua ity meter. 23.6. 1 F icker Detection on a LV network, using Power Qua ity Monitoring Instruments In a network known to have a high incidence of disturbances, some oca industri es were identified as the source of po ution of the e ectrica network, reducin g the eve of Power Qua ity at LV vo tages. Measurements using a Power Qua ity me ter show many vo tage dips to about 88% of the nomina vo tage, as i ustrated i n Figure 23.15. The vo tage dips were found to occur at frequencies of up to 8 d ips/second.The dips can a so be seen using the graphica viewing faci ities of t he instrument. Figure 23.16(a) shows the disp ay of the enve ope of the r.m.s. v o tage, and Figure 23.16(b), the same data magnified. The number, magnitude and frequency of the dips can be c ear y seen. Figure 23.17: Detai ed ana ysis on a sing e vo tage dip A detai ed view of one dip shows c ear y that the dips are on y just outside the norma supp y vo tage imits (Figure 23.17). Figure 23.15: Vo tage dip recording Figure 23.18: Detai ed view of vo tage dip w aveform Using the waveform capture faci ity, the prob em can be viewed in great detai , as shown in Figure 23.18. Using this information, and know edge of the operating cyc e of the industries causing the dip, the particu ar equipment responsib e f or causing the vo tage dip can be identified and remedia measures imp emented. (a) • 23.6.2 Investigation of Harmonic Po ution Prob ems on an Industria P ant An in dustria p ant was suffering Power Qua ity prob ems, and harmonic po ution was suspected as the cause. A Power Qua ity meter was insta ed at various parts of the network to determine the extent of the prob em and the equipment causing the prob em. Confirmation of the po ution as being due to harmonics was readi y ob tained. This can be seen in Figure 23.19, for the equipment identified as the so urce of the disturbance. The graphics enab e rapid and c ear identification of t he frequency and amount by which the (b) Figure 23.16: Graphica view of vo tage dip data Network Protection & Automation Guide • 419 • Power Qua ity 23 •
generated harmonics exceed the permitted imit. A Power System Ana ysis of the n etwork was then conducted to rep icate the measured resu ts, and then used for t esting the effectiveness of harmonic fi ter Figure 23.19: Harmonic po ution measurement designs. The most cost-effective fi ter design and ocation can then be se ected for imp ementation. • Power Qua ity 23 • • 420 • Network Protection & Automation Guide
• 24 • Substation Contro and Automation Introduction Topo ogy and functiona ity Hardware imp ementation Communication pr otoco s Substation automation functiona ity System configuration and testing Exa mp es of substation automation 24.1 24.2 24.3 24.4 24.5 24.6 24.7
• 24 • Substation Contro and Automation 24.1 INTRODUCTION The sometimes comp ex inter ocking and sequence contro requir ements that are to be found in a substation of any significant size end themse ves natura y to the app ication of automation. These requirements can be readi y expressed in mathematica ogic (truth tab es, boo ean a gebra, etc.) and this branch of mathematics is we -suited to the app ication of computers and associ ated software. Hence, computers have been app ied to the contro of e ectrica n etworks for many years, and examp es of them being app ied to substation contro /automation were in use in the ear y 1970’s. The first app ications were natura y in the bu k power transmission fie d, as a natura extension of a trend to cent ra ised contro rooms for such systems. The arge capita investment in such sys tems and the consequences of major system disruption made the cost of such schem es justifiab e. In the ast ten years or so, continuing cost pressures on Uti it ies and advances in computing power and software have ed to the app ication of computers to substation contro / automation on a much wider basis. This Chapter out ines the current techno ogy and provides examp es of modern practice in the fie d. 24.2 TOPOLOGY AND FUNCTIONALITY The topo ogy of a substation contro syst em is the architecture of the computer system used. The functiona ity of such a system is the comp ete set of functions that can be imp emented in the contro s ystem – but note that a particu ar substation may on y uti ise a subset of the fun ctiona ity possib e. A computer contro systems uti ise one of two basic topo ogies: a. centra ised b. distributed and the basic concepts of each are i ustra ted in Figure Network Protection & Automation Guide • 423 •
24.1. Ear y examp es of substation automation used the centra ised concept, due to imitations in techno ogy, both of processor power and communication techniqu es. Latest examp es use a distributed architecture, in that a number of Inte ig ent E ectronic Devices (IED’s) – such as microprocessor based re ays – may be inked v ia a mu tidrop seria ink to a oca processor. The oca processor may contro one or more bays in a substation. A of the oca processors are, in turn, con nected to a Human Machine Interface (or HMI), and possib y a so to a oca or re mote SCADA system for overa network monitoring/contro . I/O may inc ude digita and ana ogue I/O (for interfacing to discrete devices su ch as CB c ose/trip circuits, iso ator motors, non-microprocessor based protecti on re ays) and communications inks (seria or para e as required) to IED’s c. H uman Machine Interface (HMI). This is the principa user interface and wou d nor ma y take the form of a computer. The fami iar desktop PC is common y used, but specia ised computers are a so possib e, whi e norma y unmanned substations ma y dispense with a permanent y insta ed HMI and re y on operations/maintenance s taff bringing a portab e computer equipped with the appropriate software with th em when attendance is required. It is usua to a so provide one or more printers inked to the HMI in order to provide hard-copy records of various kinds (Seque nce of Events recorder, a arm ist, etc.) d. A communications bus or busses, in king the various devices. In a new substation, a of the e ements of the automa tion system wi norma y use the same bus, or at most two busses, to obtain cos t-effectiveness. Where a substation automation system is being retrofitted to an existing substation, it may be necessary to use existing communications busses to communicate with some existing devices. This can ead to a mu tip icity of co mmunications busses within the automation system e. A ink to a remote SCADA sys tem. This may be provided by a dedicated interface unit, be part of the HMI comp uter or part of an IED. It perhaps may not be provided at a – though since one o f the benefits of substation automation is the capabi ity of remote contro / mon itoring, this wou d be high y unusua . It may on y occur during a staged deve op ment of an automation scheme at a time when the bay operations are being automat ed but the substation is sti manned, prior to imp ementing remote contro capa bi ity 24.2.3 System Requirements A substation contro /automation scheme wi no rma y be required to possess the fo owing features: a. contro of a substati on e ectrica equipment from a centra point b. monitoring of a substation e e ctrica equipment from a centra point c. interface to remote SCADA system d. co ntro of e ectrica equipment in a bay oca y e. monitoring of e ectrica equip ment in a bay oca y Contro Centre Substation Contro and Automation Outstations (a) Centra ised topo ogy Outstation Contro centre Contro centre Ou tstation Outstation Outstation Outstation Outstation (b) Distributed topo ogy Contro centre Figure 24.1: Basic substation automation system topo ogies • 24 • 24.2.1 System E ements The main system e ements in a substation contro system a re: a. IED’s, imp ementing a specific function or functions on a circuit or busbar in a substation. The most common examp e of an IED is a microprocessor based pr otection re ay, but it cou d a so be a microprocessor based measurement device,
interface unit to o der re ays or contro , etc. b. Bay Modu e (or contro er). T his device wi norma y contain a of the software required for the contro an d inter ocking of a sing e bay (feeder, etc.) in the substation, and sufficient I/O to interface to a of the required devices required for measurement/protect ion/contro of the bay. The • 424 • Network Protection & Automation Guide
f. status monitoring of a connected substation automation equipment g. system database management h. energy management i. condition monitoring of substation e ectrica equipment (switchgear, transformers, re ays, IED’s) The system may be re quired to be fau t-to erant, imp ying that redundancy in devices and communicati on paths is provided. The extent of fau t-to erance provided wi depend on the size and critica ity of the substation to the operator, and the norma manning s tatus (manned/ unmanned). Many of the functions may be executed from a remote o cation (e.g. a System Contro Centre) in addition to the substation itse f. Cert ain of the above functions wi be required even in the most e ementary app icat ion. However, the se ection of the comp ete set of functions required for a part icu ar app ication is essentia y the responsibi ity of the enduser (Uti ity, et c.). Due to a modu ar, ‘bui ding b ock’ approach to software design, it is re ative y easy to add functiona ity at a ater stage. This often occurs through changing operators’ needs and/or e ectrica network deve opment. Compatibi ity of the unde r ying database of network data must be addressed to ensure that historica data can sti be accessed. 24.3 HARDWARE IMPLEMENTATION To form a substation contro system, the various e ements described above must be assemb ed into some form of topo ogy. Three major hardware topo ogies can be identified as being common y used, as fo ows: 24.3.1 HMI-based Topo ogy This takes the form of Figure 24.2. The software to imp ement the contro /automation functions resides in the HMI c omputer and this has direct inks to IED’s using one or more communications protoc o s. The ink to a remote SCADA system is norma y a so provided in the HMI comp uter, though a separate interface unit may be provided to off oad some of the pr ocessor requirements from the HMI computer, especia y if a proprietary communic ations protoco to the SCADA system is used. For this topo ogy, a powerfu HMI c omputer is c ear y required if arge numbers of IED’s are to be accommodated. In p ractice, costs usua y dictate the use of a standard PC, and hence there wi be imitations on substation size that it can be app ied to because of a resu ting imit to the number of IED’s that can be connected. The other important issue is one of re iabi ity and avai abi ity – there is on y one computer that can contro the substation and therefore on y oca manua contro wi be possib e if the computer fai s for any reason. Such a topo ogy i s therefore on y suited to sma MV substations where the consequences of comput er fai ure (requiring a visit from a repair crew to remedy) are acceptab e. Bay Modu es are not used, the software for contro and inter ocking of each substati on bay runs as part of the HMI computer software. SCADA interface Master c ock (GPS, radio) Remote HMI HMI Station bus Te econtro or bus interface Internet or PSTN Bus interface I/O, devices CT, VT Bus interface Legacy bus IED s Computer IED s The HMI, te econtro interface, and the bus interface cou d be: • separate equipme nt • integrated into the same PC Figure 24.2: HMI-based hardware topo ogy
24.3.2 RTU-based Topo ogy This topo ogy is an enhancement of the HMI topo ogy an d is shown in Figure 24.3. A microprocessor-based RTU is used to host the automa tion software, freeing the HMI computer for operator interface duties on y. The HMI computer can therefore be ess powerfu and usua y takes the form of a stan dard PC, or for not-norma ymanned substations, visiting personne can use a por tab e PC. SCADA interface HMI Te econtro or bus interface • Internet or PSTN RTU Bus interface Master c ock (GPS, radio) Legacy bus IED s I/O, devices CT, VT The RTU, te econtro interface and the bus interface cou d b e: • separate equipment • integrated into the same computer Figure 24.3: RTU-based topo ogy Network Protection & Automation Guide • 425 • Substation Contro and Automation 24 •
The RTU is purpose designed and can house one or more powerfu microprocessors. A greater number of I/O points can be accommodated than in the HMI topo ogy, whi e the possibi ity exists of hosting a wider variety of communication protoco s for IED’s and the remote SCADA connection. Bay Modu es are not required, the assoc iated software for inter ocking and contro sequences is part of the RTU softwar e. 24.3.3 Decentra ised Topo ogy This topo ogy is i ustrated in Figure 24.4. In it, each bay of the substation is contro ed by a Bay Modu e, which houses the contro and inter ocking software, interfaces to the various IED’s required as par t of the contro and protection for the bay, and an interface to the HMI. It is possib e to use an HMI computer to take oca contro of an individua bay for c ommissioning/testing and fau t finding purposes. The amount of data from the var ious substation I/O points dictates that a separate SCADA interface unit is prov ided (often ca ed an RTU or Gateway), whi e it is possib e to have more than on e HMI computer, the primary one being dedicated to operations and others for eng ineering use. Optiona y, a remote HMI computer may be made avai ab e via a sepa rate ink. It is a ways desirab e in such schemes to separate the rea time opera tions function from engineering tasks, which do not have the same time-critica importance. SCADA interface Master c ock (GPS, radio) HMI Te econtro or bus interface Compu ter Remote HMI Internet or PSTN Station bus Computer HMI computer Bay Modu es Bay Modu es Bay Modu es Bay Modu es (a) Star connection of bay modu es HMI computer Bay Modu es Bay Modu es Substation Contro and Automation Bay Modu es Bay Modu es (b) Ring connection of Bay Modu es Figure 24.5: Methods of hardware interconnect ion Of course, it is possib e to overcome the first prob em by dup icating inks and running the inks in physica y separate routes. However, this makes the I/O po rt prob em worse, whi e additiona design effort is required in ensuring cab e r oute diversity. An a ternative is to connect the Bay Modu es, HMI computer and S CADA gateway in a ring, as shown in Figure 24.5(b). By using a communication arc hitecture such as found in a LAN network, each device is ab e to ta k to any oth er device on the ring without any message conf icts. A sing e break in the ring does not resu t in oss of any faci ities. The detection of ring breakage and re -configuration required can be made automatica y. Thus, the avai abi ity and fa u t to erance of the network is improved. Mu tip e rings emanating from the HMI computer can be used if the number of devices exceeds the imit for a sing e rin g. It can be easier to insta on a step-by-step basis for retrofit app ications , but of course, a these advantages have a downside. The cost of such a topo o gy is higher than that of the other so utions, so this topo ogy is reserved for situations where the highest re iabi ity and avai abi ity is required - i.e. HV and EHV transmission substations. Redundancy can a so be provided at the individ ua device eve . Re ays and other IED’s may be dup icated, though this wou d not be usua un ess required for other reasons (e.g. EHV transmission ines may be r
equired to have dup icate main protections – this is not strict y speaking Bus interface Bay Modu e • 24 • Legacy bus The Bay Modu e and bus interface cou d be: • separate equipments • integrated into t he Bay Modu e IED s I/O, devices CT, VT Figure 24.4: Decentra ised topo ogy The connection between the various Bay Modu es and the HMI computer is of some i nterest. Simp est is the star arrangement of Figure 24.5(a). This is the east-c ost so ution but suffers from two disadvantages. First y, a break in the ink wi resu t in oss of remote contro of the bay affected; on y oca contro via a oca HMI computer connected to the bay is then possib e. Second y, the number of communication ports avai ab e on the HMI computer wi imit the number of B ay Modu es. • 426 • Network Protection & Automation Guide
dup ication of individua devices - which wou d require each individua main pro tection to have two identica re ays voting on a ‘1 out of 2’ basis). It is usua to have more than one operators’ HMI, either for operationa reasons or for fau t-to erance. The system computer may be dup icated on a ‘hot-standby’ or ‘dua -redundant’ ba sis, or tasks may be norma y shared between two or more system computers with e ach of them having the capabi ity of taking over the functions of one of the oth ers in the event of a fai ure. The tota I/O count in a major substation wi be come arge and it must be ensured that the computer hardware and communication inks have sufficient performance to ensure prompt processing of incoming data. O ver oad in this area can ead to one or more of the fo owing: a. undue de ay in updating the system status diagrams/events og/a arm og in response to an inci dent b. corruption of system database, so that the information presented to the operator is not an accurate representation of the state of the actua e ectrica system c. system ockup As I/O at the bay eve , both digita and ana ogue wi typica y be hand ed by inte igent re ays or specia ised IED’s, it is therefore important to ensure that these devices have sufficient I/O capacity. If addition a IED’s have to be provided so e y for ensuring adequate I/O capacity, cost and s pace requirements wi increase. There wi a so be an increase in the number of communication inks required. A practica specification for system response tim es is given in Tab e 24.1. Tab e 24.2 gives a typica specification for the maxi mum I/O capacities of a substation automation system. A significant prob em to be overcome in the imp ementation of communication ink s is the possibi ity of e ectromagnetic interference. The ow vo tage eve s tha t are used on most types of communication ink may be prone to interference as a resu t. Carefu design of the interfaces between the devices used and the commu nication bus, invo ving the use of opto-coup ers and protoco converters, is req uired to minimise the risk. Care over the arrangement of the communication cab e s is a so required. It may a so he p to use a communication protoco that incorp orates a means of error detection/correction. Whi e it may not be possib e to co rrect a errors, detection offers the opportunity to request re-transmission of the message, and a so for statistics to be gathered on error rates on various p arts of the system. An unusua y high error rate on a part of the communication system can be f agged to maintenance crews for investigation. 24.4 COMMUNICATION METHODS Digita communication between items of hardware is divided into three e ements: a. the protoco , consisting of the hardware, such as connectors, connec tor pin functions, and signa eve s b. the format, consisting of the contro of the f ow of data c. the anguage, or how the information in the data f ow is or ganised Each of these areas is covered so that an appreciation of the comp exiti es of digita communications is understood. 24.4.1 Communication Protoco s and F ormats Anyone trying to connect up the various e ements of a Hi-Fi system if the y have purchased them from different manufacturers wi be aware of the number o f different protoco s in use. The situation is the same in the industria fie d. Manufacturers of devices are often tempted to uti ise a proprietary protoco , f or no better reason sometimes than to encourage the so e use of their devices. U sers, of course, have the opposite interest; they wou d ike every manufacturer to use the same protoco so that they have the widest choice. In practice, proto co s have evo ved over time, and some protoco s are more appropriate to some com munication requirements than others. The protoco used is a so inked to the for mat used, since the number of conductors required may depend on the format used. There are two basic formats in use for data communications: a. seria b. para e • Signa Type Digita Input Ana ogue Input Digita Output Disturbance Record Fi e Response Time to/from HMI 1s 1s 0.75s 3s Tab e 24.1: Practica system response times for a substation automation scheme
I/O Type Digita Input Capacity 8196 Digita Output 2048 Ana ogue Input 2048 Ana ogue Output 512 Tab e 24.2: Typica I/O capacities for a substation automation system Network Protection & Automation Guide • 427 • Substation Contro and Automation 24 •
Seria format invo ves sending the data one bit at a time a ong the communicatio n channe . Para e format invo ves sending severa bits simu taneous y. C ear y , para e communication requires more wires than seria communication (a disadv antage) but can transmit a given amount of data faster. In practice, para e co mmunication is imited to communication over a few metres, and hence the majorit y of communications use seria format. There are a number of popu ar seria comm unication protoco s in common use in the substation automation fie d. 24.4.1.1 R S232C Protoco The RS232C protoco a ows for fu dup ex communications between two devices. The basic specification is given in Tab e 24.3. The hardware speci fication can vary – nine conductors are the minimum required for a fu imp ementa tion, whi e a 25 pin connector is common y encountered. If f ow contro of data is not required, on y three signa s are required (data transmit/receive and grou nd). Being imited to communication between two devices, this protoco is not us efu in substation automation schemes. However, it is described, because it is r egu ar y encountered in remote communication app ications, such as those between a sma substation and a contro centre using modems to transfer the data over a te ephone ine. Max. number of transmitters Max. number of receivers Connection type Mode of ope ration Maximum distance of transmission Maximum data rate Transmitter vo tage Re ceiver sensitivity Driver s ew rate Tab e 24.3: RS232C specification 1 1 25 core shie ded DC coup ing 15m 20kbit/s 5V min, 15V max 3V 30V/µsec Thus devices can be ocated throughout a substation without causing communicatio ns prob ems and significant amounts of data can be transmitted rapid y. The main drawback is that it is a ha f-dup ex system, so that communications use a kind of question and answer technique known as ‘po ing’. The equipment that needs the da ta (e.g. a substation computer or bay contro er) must ask each device in turn f or the data requested and then wait for the response prior to moving on to the n ext device. Master station IED IED IED IED Terminating resistor IED IED Substation Contro and Automation Figure 24.6: ‘Daisy-chain’ connection of RS485 devices Where devices connected to the communications channe may need to f ag a arm con ditions, this dictates continua po ing of a devices connected to the communi cations channe . If more than 31 devices need to be connected, more than one RS4 85 communications ink can be provided. 24.4.1.3 IEC 60870-5 Protoco s The two c ommon y used protoco s are IEC 60870-5-101 and IEC 60870-5-103. IEC 60870-5-101 is used for communications between devices over ong distances. A typica app ic ation wou d be communications between a substation and a Centra Contro Room (C CR). A bit seria communication technique is used, and transmission speeds of up to 64kbit/s are possib e, depending on the transmission protoco se ected from those specified in the standard. Modems can be used, and hence there is no pract ica imitation of the distance between devices. IEC60870-5-103 specifies a comm unication protoco between a master station and protection devices (e.g. protect ion re ays). The standard is based on, and is a superset of, the German VDEW com munication protoco . Either fibre optic transmission or an RS485 ink can be use d, and transmission speeds are either 9600kbit/s or 19200kbit/s. Maximum transmi ssion distance is 1000m using fibre-optic transmission . Communication is on a ‘ma ster/s ave’ basis, in which the master station continua y po s the s aves (re ay s) to determine if any information is ready to be sent by the s aves. Whi e some messages are defined by the standard, these are of •
24 • 24.4.1.2 RS485 Protoco This protoco is detai ed in Tab e 24.4, and is much mor e usefu for substation automation schemes. This is because, many devices can be attached to one data channe , the maximum distance over which communications ca n take p ace is quite arge, and the maximum bit rate is quite high. It on y req uires a simp e twisted pair connection, with a devices ‘daisy-chained’ on the ink , as shown in Figure 24.6. Max. number of transmitters Max. number of receivers Connection type Mode of ope ration Maximum distance of transmission Maximum data rate Transmitter vo tage Re ceiver sensitivity Tab e 24.4: RS485 specification 32 32 Shie ded Twisted Pair D ifferentia 1200m 10Mbit/s 1.5V min 300mV • 428 • Network Protection & Automation Guide
imited functiona ity. In addition, the standard a ows the use of manufacturerspecific ‘private’ messages. These permit much greater functiona ity, but at the sam e time hinder interoperabi ity of equipment from different manufacturers because there is no need for the format of such messages to be made pub ic. This is arg uab y the greatest drawback of the standard, since extensive use of ‘private’ messag es by manufacturers of devices essentia y turns the standard into severa propr ietary ones. 24.4.2 Network Protoco s So far, the protoco s described are usefu for imp ementing communications over a re ative y restricted geographica area. A substation automation scheme may extend over a very wide area, and hence suit ab e protoco s are needed for this situation. The most common protoco s in use c onform to the ISO 7 ayer mode of a network. This mode is internationa y recog nised as the standard for the requirements for communications between data proce ssing systems. 24.4.2.1 ISO 7- ayer mode The ISO 7- ayer mode is shown in Figu re 24.7. It represents a communications system as a number of ayers, each ayer having a specific function. This approach ensures modu arity, and hence assists in ensuring that products from different vendors that comp y with the standard wi work together. The functions of each ayer are best described by making an ana ogy with a te ephone ca , as given in Tab e 24.5. OSI Layer Physica Te ephone Ca Ana ogy Conversion of voice into e ectrica signa s. Defines type of connector, no. of pins, signa eve s, etc. Optica fibres and wires that ma ke up the physica te ephone network Message transmission, error contro and con ferencing faci ities. Words not c ear y received are requested to be re-transmit ted, using agreed procedures. For conferencing, defines how contro passes from one person to the next. Ca routing, by specifying the method of a ocating te ephone numbers and provision of dia ing faci ities. Inc udes operator faci itie s for routing to extensions. If the message is from severa sheets of paper, ens ures that a sheets have been received and are in the correct order. Monitors t ransmission qua ity and imp ements procedures if qua ity is unaceptab e - e.g. r equests both parties to hang up and one to re-dia . A so provides a mechanism to ensure that the correct persons are communicating, and searches for them (e.g. uses te ephone directory) if not. Provides faci ities for automatica y making c a s at pre-defined times, and ensures that the correct persons are present when the ca is made. A session may be interrupted and re-estab ished ater, using the same or a different network/transport connection. As ca s are ha f-dup ex, provides f ow contro procedures e.g one person says over to invite the other to speak. Data Link Network Transport Session App ication Specifies the format in which a message wi be sent when used in a specific app ication- e.g. if the app ication is to convey information about meetings attend ed by a person, wi define the format used for the p ace, time, and purpose of the meeting. Tab e 24.5: OSI 7- ayer mode – Te ephone ca ana ogy Se ects appropriate service for app ication Provides code conversion, data refor matting Co-ordinates interaction between end app ication processes Provides for end-to-end data integrity and qua ity of service Switches and routes information
Transfers unit of information to other end of physica ink Transmits bit strea m to medium App ication Presentation Session Transport Network Data Link Physica The same data item may be stored at different addresses within different devices , and hence re-programming of the c ient that receives the information is necess ary when one device is rep aced by a different one, even if the functiona ity is unchanged. It can easi y be seen how a substation equipped with a variety of de vices from different manufacturers and maybe using different protoco s for commu nication makes the prob em of app ying an automation system very difficu t and e xpensive. The major cost in such cases is deve oping the software trans ation ro utines for protoco conversion and bui ding of the required database specifying where each item of data to be acquired is he d. 24.4.2.2 Uti ity Communications Architecture protoco A recent protoco , the Uti ity Communications Architecture v2.0 (UCA v2.0), seeks to overcome these handicaps by adopting an object-orient ed approach to the data he d in a measurement/contro device, p us an internatio na y recognised protoco (ISO 9506) in the app ication ayer. Data objects and services avai ab e within a device fo ow a specified naming system. The c ient can extract a description of the data objects that a device can supp y, and serv ices that it can perform, so that it is easier to program the c ient. Sca ing fa ctors and units for data items are bui t into the se f-description, so that the effort required during commissioning is reduced. Devices are not interchangeab e , in the sense that a device from one • Figure 24.7: OSI 7- ayer interconnection mode There are a number of network protoco s that are comp iant with the OSI mode , s uch as TCP/IP, Modbus, DNP. This does not mean that the devices using different protoco s are interchangeab e, or even that devices using the same protoco are interchangeab e. Network Protection & Automation Guide • 429 • Substation Contro and Automation 24 • Presentation Removes anguage difficu ties by ensuring that the same anguage is spoken by bo th parties, or provides trans ation faci ities. A so provides encryption faci it ies for confidentia ca s.
Substation Contro and Automation manufacturer cannot be removed and rep aced by a device of simi ar functiona ity from another vendor. Rather, this protoco ensures interoperabi ity; that is th e abi ity for devices from different supp iers and of different functiona ity to communicate successfu y with each other. The transport protoco has been separ ated from the app ication protoco , so prob ems with register addresses, etc. no onger exist. A that has to be addressed is the transport protoco s used, and c ients wi norma y be ab e to communicate with devices using one of a number of common transport protoco s. This standard has an IEC equiva ent, IEC 61850. To begin with, IEC 61850 covers on y the fie d of substation automation, but wi gradua y be extended to cover the same fie ds as UCA v2.0. Manufacturers are increasing y moving away from protoco s with a proprietary e ement in them to UC A v2.0/IEC 61850. It is ike y that within a short time, most protection and con tro devices wi use one or other of these standards for communications. One im portant reason guiding this change is that these standards permit the use of the XML anguage for exchange of data between databases. As the information stored in an automation system or contro centre comprises a series of databases, infor mation exchange is therefore faci itated. 24.4.3 Languages A communications ang uage is the interpretation of the data contained in a message. The communication s anguage norma y forms part of the overa communications protoco . Obvious y , it is necessary for both transmitter and recipient of the message to use the s ame anguage. Whi e a number of communications standards attempt to specify the anguage used, there is often f exibi ity provided, eading to manufacturerspeci fic imp ementations. A popu ar work-around is for a number of organisations to a gree common standards and set up a certifying body to check for comp iance again st these standards. Thus, equipment that comp ies becomes to arge degree, inter operab e. However, the atest trend, as exemp ified by the UCA v2.0/IEC 61850 protoco , is to define the anguage very precise y at a high eve , and require such detai s to be inc uded as a part of each message so that the recipient can interpret the message without the need for any trans ator software. 24.5 SUBSTAT ION AUTOMATION FUNCTIONALITY The hardware imp ementation provides the physica m eans to imp ement the functiona ity of the substation automation scheme. The sof tware provided in the various devices is used to imp ement the functiona ity req uired. The software may be quite simp e or extreme y comp ex – Tab e 24.6 i ustra tes the functiona ity that may be provided in a arge scheme. The description of the e ectrica network and the characteristics of the various devices associate d with the network are he d within the computer as a database or set of database s. Within each database, data is organised into tab es, usua y on a ‘per device’ ba sis that ref ects the important characteristics of the device and its interre at ionship with other devices on the network. E ectrica system configuration chang es require modification of the database using an appropriate software too supp ied by the automation system vendor. The too is norma y a high eve , user-fri end y interface, so that modifications to the one- ine can be drawn direct y onscreen, with ‘pick-and–p ace’ faci ities for re ays, IED’s, etc. This work wou d norma y be done off ine on the Engineers’ workstation, if avai ab e, or as a background task on the contro computer if not. Carefu and extensive checking of the data is required, both before and after entry into the database, to ensure that no er rors have been made. Fu testing on the new configuration using a simu ator is recommended prior to use of the new database on the main contro computer to ens ure that there is minima possibi ity of errors. The software is written as a se t of we -proven, standard modu es, so there is itt e or no need for new modu e s to be written and tested for a particu ar substation. The required data for th e ca cu ations performed by the • 24 •
Functiona area Inter ocking CB s Tripping sequences CB fai ure Switching sequen ces Automatic transformer changeover Load management Load shedding Transformer s upervision OLTC contro Energy monitoring Import/export contro Switchgear monit oring AIS monitoring Equipment status Re ay status Parameter setting Re ays Acce ss contro HMI functiona ity Trend curves Interface to SCADA Tab e 24.6: Typica substation automation functiona ity Functiona ity Iso ators Contactors Intertripping Automatic busbar changeover Res toration of supp y fo owing fau t Load restoration Generator despatch Load mana gement Energy management Power factor contro GIS monitoring CB status Iso ator status Transformers Switching sequences One- ine views System views Harmonic ana ysis Remote access A arm processing 512 Simu taneous trips Network re-configuration IED configuration Event ogging Disturbance ana ysis • 430 • Network Protection & Automation Guide
software is he d in the network database. This means that adding functiona ity ater is not difficu t, so ong as the database design has considered this possib i ity. There may be prob ems if the e ectrica system configuration is a tered o r additiona functiona ity added in reading historica data prior to the change. Training of operations personne wi inevitab y be required in operation of th e system, configuration management and automation system maintenance. Automation system supp iers wi be ab e to provide configuration management and system ma intenance services under contract if required, often with defined cost schedu es and response times so financia management of the automation scheme once insta ed is we -defined. The issuing of commands to switching devices in the system has to be carefu y structured, in order to prevent commands that wou d cause a hazard from being issued. A hierarchica structure is common y used as shown in Figure 24.8, beginning with the requirement for an operator wishing to issue a c ommand to switching devices to og-in to the system using a password. Operator/ authorised person Senior authorised person Se ect user Engineer System engineer Administrator Password Password Password Password Password List of ava i ab e functions executed. At this stage, prior to execution, the operation is checked against: a . devices ocked out (i.e. prevented from operation) b. inter ocking of devices/ switching sequences to ensure that the command issued is safe to carry out. The action is cance ed and operator informed if it is not safe to proceed, otherwis e the action is carried out and the operator informed when it is comp ete. In a number of systems, some routine switching operations (e.g. transfer of a feeder from one busbar to the other in a doub e-bus substation) are automated in softwa re. The operator need on y request the ‘bustransfer’ action to be carried out on a p articu ar feeder, and the software is ab e to work out the correct switching seq uence required. This minimises the possibi ity of operator error, but at the exp ense of some extra comp exity in the software and more extensive checking at the factory test stage. However, since software is modu ar in nature, substation e ectrica topo ogy is restricted to a sma number of configurations and such seq uences are very common, the software deve opment is essentia y a one-off activi ty for any particu ar substation contro system. The deve opment cost can be spr ead over the sa e of a number of such systems, and hence the cost to any individ ua user is sma compared with the potentia benefits. 24.5.1 Future Deve opmen ts The functiona ity of a substation automation system is sti evo ving, with n ew app ications being steadi y added. Expansion of the functiona ity of such sys tems is proceeding in many areas, but two main areas current y are attracting si gnificant interest. These are condition monitoring and web-access. Condition mon itoring packages are a ready imp emented in automation systems for switchgear, w hi e stand-a one packages are avai ab e for transformers (Chapter 16). Under dev e opment are simi ar packages for generators, CT’s, VT’s, and batteries. It can be e xpected that a of these faci ities wi be offered as part of a comprehensive condition monitoring package in substation automation schemes in the near future . The advantage for the user is that the condition monitoring package can then f orm a component of the Asset Management po icy, in order to determine the schedu e for maintenance and rep acement, p us the acquisition of statistics on fai ur e rates. These can then be used in conjunction with manufacturers to enhance the design to improve avai abi ity. There has a ready been discussion on the variou s communication techniques avai ab e. Use of the Internet Figure 24.8: Hierarchica command structure Different eve s of authority, a owing for restrictions on the type and/or oca tion of switching commands capab e of being issued by a particu ar operator may be imp emented at this stage. The next eve in the hierarchy is to structure th e issuing of commands on an ‘issue/confirm/execute’ basis (Figure 24.9), so that the operator is given an opportunity to check that the command entered is correct p rior to execution. Inter ocking
• Device se ect List of avai ab e actions Action se ect Action confirm Action execute Cance Figure 24.9: Device se ection/operation The fina eve in the hierarchy is imp emented in software at the bay eve and is actioned after the operator confirms that the switching action is to be Network Protection & Automation Guide • 431 • Substation Contro and Automation 24 •
communication techniques for communications to/from a substation offers a cheap, we -proven, wide y accessib e route for this function. It a so enab es access to the data from a broader community, which may be usefu in some circumstances. However, great emphasis must be p aced on the use of secure Internet communicat ions techniques, such as those used in the financia sphere, as the opportunity for unauthorised ma icious access eading to major incidents or oss of confiden tia data is much greater. As cost is the main driver, it can be expected that a utomation systems using such communications techniques wi appear in the future , using secure communications techniques, and that users wi have to become mor e aware of the threats invo ved in order to app y suitab e countermeasures. 24.6 SYSTEM CONFIGURATION AND TESTING and the data to be disp ayed d. operator – security access eve s, a arm texts, et c. e. externa constraints – data addresses for externa database access Once a the data has been defined, the configurator too s can define the hardware config uration to provide the required functions at east cost, and the data required f or imp ementation of the automation scheme. 24.6.2 System Testing The degree of testing to be carried wi be defined by the customer and encapsu ated in a spec ification for system testing. It is norma for testing of the comp ete functiona ity of the scheme to be required prior to despatch from the manufacturer. The arger and more comp ex the automation scheme, the more important for a parties that such testing is carried out. It is accepted wisdom that the ear ier prob e ms are discovered, the cheaper and quicker it is to fix them. Remediation of pro b ems on-site during commissioning is the most expensive and time-consuming acti vity. Manua testing of a network automation scheme is on y practica for sma networks, due to the cost of testing. Simu ation too s are necessary for a oth er automation schemes. These too s fa into two categories: a. simu ator too s that re-create the network to be contro ed by the automation system. b. test ma nagement too s 24.6.2.1 Simu ator too s Simu ator too s are dedicated to the net work being tested. They wi norma y be provided with a simu ation anguage tha t the test team can use to p ay scenarios, and hence determine how the automatio n system wi react to various stimu i. Process simu ator too s may be hardware and/or software based and emu ate the response of the various devices to be cont ro ed (CB’s/iso ators/VT’s/protection re ays, etc). They must be capab e of c ose y fo owing the dynamic response of such devices under mu tip e and cascade fau t conditions. Specific too s and ibraries are deve oped as required, inc uding t he use of comp ex software such as EMTP for simu ation of the response to impu s e-type phenomena and the dynamic response of protection a gorithms. They may sim u ate the response of equipment within the contro span of the automation equipm ent, or that of equipment outside of the span of contro , in order that the resp onse of the automation system can be tested. Communications simu ator too s are used both to oad the interna communications network within the Substation Contro and Automation • These tasks, a ong with project management, are the most time consuming tasks in the process of rea ising a contro and monitoring system for an e ectrica netw ork. The strategies avai ab e for dea ing with these prob ems vary between manuf acturers, but typica approaches are as fo ows. 24.6.1 System Configuration Sof tware too s exist that assist in configuring a modern substation or network auto mation system. The extent to which the task is automated wi vary, but a requ ire as a minimum the detai s of the network to be contro ed, extending to the i ndividua device eve (circuit breaker, iso ator, disconnector, etc.). Where co mmunication to an existing SCADA system is required, data on the ogica address es expected by the SCADA system and devices contro ed remote y from the SCADA s ystem wi a so be part of the data input. Use can a so be made of existing data bases that cover pre-defined network configurations – for examp e the inter ocking equations for a substation bay. Software too s wi check the data for consiste ncy, prior to creation of: a. the required equipment that forms the automation s
cheme, together with the required interconnections b. the databases for each ind ividua device The data wi be divided into domains, according to the use made of the data: a. process – CB/iso ator position, inter ocking equation, va ues of c urrent/vo tage b. system – number of bay computers, hardware configuration of each bay computer, automated sequences c. graphica – the inks between each mimic dis p ay 24 • • 432 • Network Protection & Automation Guide
automation system to ensure that a devices are communicating correct y and tha t performance of the overa automation system is within specification during pe riods of high communications traffic. These simu ators are standardised and a si ng e simu ator may be ab e to emu ate severa items of equipment. Externa commu nications simu ators test the communications with an externa system, such as a remote contro centre. These wi norma y be customer-specific, but some standa rd simu ators may be possib e if a standard communications protoco such as IEC 60870-5-101 is used. 24.6.3 Test Strategy The strategy adopted for the testing o f the automation system must natura y satisfy c ient requirements, and genera y fo ow one of two approaches: a. a sing e test is carried out when a equipme nt for the scheme has been assemb ed, b. incrementa tests are carried out as th e automation system is bui t up, with simu ator used to represent missing equipm ent. The former so ution is quickest and cheapest, but can give rise to prob ems where it is not easy to ocate prob ems down to the device eve . It is therefo re used principa y when an upgrade to an existing system is being carried out. It is usua for a of the functiona ity to be tested, inc uding that specified for norma conditions and specified eve s of degradation within the automation system. This eads to a arge number of tests being required. Over 500 separate tests may be required for an automation system of average size in order to demon strate comp iance with the specification. 24.6.4 Management of System Tests The arge number of tests required to demonstrate the comp iance of an automation sy stem with specification makes manua techniques for management of the tests cumb ersome and time consuming. The end resu t is increased cost and timesca e. Moreo ver, each test may resu t in a arge amount of data to be ana ysed. The resu ts of the ana ysis need to be presented in an easi y understood form and stored for some time. If changes are made to software for any reason over the ifetime of the equipment, the different versions must be stored, together with a record of what the changes between versions were, and why they were made. The management o f this becomes very comp ex, and software too s are norma y used to address the issues of test schedu es, test resu t presentation, software version contro , a nd configuration management. The contro of personne working in the system test area is a so of importance, to ensure tests are unbiased. To meet this objective, test team personne are no rma y independent of those of the design team. If incrementa testing is used, it is sound practice that the fina integration test team is a so independent of the test team(s) that carried out the incrementa tests. 24.7 EXAMPLES OF SUBST ATION AUTOMATION A significant advantage to an asset-owner of using a substation automation system is the space-savings that resu t. Space costs money, and henc e minimisation of space enab es new substations to occupy a sma er physica spa ce. A ternative y, expansion of an existing substation can be undertaken making use of current y spare bays, but where there is a prob em in tight y packed re a y rooms in accommodating the extra equipment. Wa mimic RTU Contro room Marsha ing cabinets Sequence of events Protection 1 Protection 1 Protection 2 Auxi iary re aying 3 cubic es/bay Protection 2 Cubic es Auxi iary re aying (a): Current situation • Wa mimic New RTU Contro room Marsha ing cabinets Sequence of events Protection 1 Cubic es Protection 1
Protection 2 Protection 2 Auxi iary re aying Auxi iary re aying 3 cubic es/bay (b): Step 1: RTU Renovation (HW obso escence & new SCADA protoco ) Figure 24.10: Upgrade path for an existing substation Network Protection & Automation Guide • 433 • Substation Contro and Automation 24 • A common need is to update an existing substation, present y based on e ectromec hanica or e ectronic re ays, with modern devices. Figure 24.10 i ustrates how the transition to use of a substation automation system may be managed – of course , there are other possibi ities depending on the priority assigned by the assetowner. .......... .......... .......... ..........
New RTU New SOE Substation contro HMI Contro room Marsha ing cabinets was insta ed, using an AREVA ystem. The simp ified 33/11kV eneration capacity amounts to ve LV network and a number of
T&D Digita Contro System substation automation s one- ine diagram is shown in Figure 24.11. Tota g over 170MW. Not shown on the diagram is an extensi 3.3kV switchboards feeding motors.
Protection 1 Cubic es Protection 2 Auxi iary re aying 3 cubic es/bay (c): Step 2: SOE Renovation and wa mimic change New RTU Contro room New SOE Marsha ing cabinets Substation Contro and Automation Bay computer Protection 1 Cubic es Protection 2 2 cubic es/bay (d): Step 3: Progressive decentra isation and protec tion integration Contro room Bay computer Cubic es Protection 1 • 24 • Protection 2 2 cubic es/bay (e): Step 4: Fu decentra isation Figure 24.10 (cont): Upgrade path for an existing substation Examp es of automation systems on order or insta ed are given in the fo owing sections. 24.7.1 Industria Network Automation Project A arge industria networ k was significant y expanded due to the addition of extra processing faci ities. As part of the expansion, a new substation automation system .......... Protection 1 Protection 2 Auxi iary re aying Substation contro HMI ..........
.......... .......... .......... 132kV network 33kV .......... Protection 1 11kV Protection 2 Auxi iary re aying 11kV Substation contro HMI 11kV Figure 24.11: HV Sing e- ine diagram: industria system substation automation ex amp e Bay computer Protection 1 Protection 2 The system has two features that make it unusua from a contro point of view. F irst y, the generation within the system is distributed, and this resu ts in the possibi ity of severa is and networks being created in the event of a major e ectrica incident, each of which are to be run independent y unti such time as para e ing of the is ands becomes possib e. Second y, the grid system is weak, so that import has to be imited to a maximum of 40MW, even under transient dist urbances such as the simu taneous oss of two generators, each of over 30MW capa city. As a resu t of these requirements, the standard software was enhanced to a ow simu taneous contro of up to 3 autonomous is ands within the overa netwo rk, each is and having the fu range of contro faci ities inc uding circuit/de vice switching, active/reactive power contro of generators, vo tage and frequen cy contro of each is and and oad shedding. Due to the restrictions on grid imp ort, a fast oad shedding a gorithm was deve oped, as studies indicated that con ventiona under-frequency • 434 • Network Protection & Automation Guide
oad shedding did not provide the required performance. The fast oad shedding s cheme invo ves continuous ca cu ation of the amount of oad to be shed in the ev ent of oss of one or more generators and/or the grid connection, and determinat ion of which oads shou d be shed, based on operator-set priorities and actua p ower consumption. In the event of generation oss, oad that is at east equa t o the amount of ost generation is disconnected immediate y, after which a conve ntiona under-frequency/grid import oad shedding strategy is invoked to cater f or any further generation/ oad imba ance occurring. The substation automation co nfiguration is shown in Figure 24.12, whi e a samp e operator disp ay captured d uring system testing is shown in Figure 24.13. Figure 24.12: System architecture: industria system substation automation examp e Communication from the Master Station to the Bay Modu es is by a dua -redundant fibre-optic ring (EFI.P). Time synchronisation uses a GPS interface to the Maste r Station. Remote contro /monitoring faci ities are provided, both from a Remote Contro Room and a remote Network Contro Centre. The atter uses the DNP3.0 pr otoco , so that the comp ete scheme uses 4 different communication protoco s. Fi gure 24.14 i ustrates the system architecture, whi e Figures 24.15/16 show part of the 345kV and 138kV busbars respective y. 24.7.3 Substation Contro for an E ectrified Rai way A high-speed (auto-transformer fed) rai way has a route engt h of 500km. A tota of 8 traction supp y substations and 41 auto-transformer sub stations are required to provide traction power and auxi iary supp ies to the ra i ine. A of the forty-nine substations are interconnected by means of an Eth ernet OPC fibre-optic network, forming the communications spine of the system. E ach substation has a proprietary EFI.P fibre-optic ring (3.5Mbit/s) that interco nnects the Bay Modu es with the communications spine and oca operator workstat ions. The ring is composed of dua fibre-optic cab es in a sing e sheath, thus p roviding two communications channe s. Figure 24.17 i ustrates the network invo ved. • Figure 24.13: Samp e operator disp ay: industria system substation automation e xamp e 24.7.2 Uti ity Substation Automation Project This project concerns a 345/138/20k V substation. The substation consists of two 345kV ines, 2 x 345/138kV transfor mers and 2 x 345/20kV/20kV transformers. Each of the 345kV and 138kV busbars is of conventiona doub e-bus configuration, with bus coup ers connecting Network Protection & Automation Guide • 435 • Substation Contro and Automation 24 • the main and reserve busbars. Each 345kV bus is sp it into 4 sections, with bus section CB’s inking the sections. Simi ar y, the 138kV busbars are sp it into 3 s ections. The 20kV busbar is a so of doub e bus configuration. An AREVA T&D Digit a Contro System has been insta ed to provide oca and remote contro and mon itoring of the switchgear at a vo tage eve s. For the 138kV and 20kV busbars, monitoring is provided by MiCOM M301 Measurement Centres, communicating with BM 9100 or BM9200 Bay Modu es using K-Bus proprietary communications ink. Contro is exercised direct y from the Bay Modu es. Protection re ays are genera y from AREVA’s K-series and EPAC range, a so communicating with the Bay Modu us using KBus. However, ine differentia and transformer differentia re ays are from ano ther manufacturer, and communicate with the same Bay Modu es using the IEC 60870 -5-103 protoco , thus i ustrating the use of Bay Modu es to provide more than o
ne communications protoco . For the 345kV busbars, existing e ectromechanica -ty pe re ays were in use, and monitoring of these is by use of contacts on the re a ys wired back to the Bay Modu es.
Loca contro room Modem EOP-1 EOP-2 Hot stand by Modem Gateway Remote contro room HUB GPS Data acquisition Network contro centre es: Cambuci 1 & 2 2 X BM9100 345kV Line: Cabo Norte 1 & 2 345/138kV Transformers: Lado de A ta 4 X BM9100 K-bus MiCOM M301 EPAC re ays K-series re ays Substation Contro and Automation K-bus MiCOM M301 345/20/20kV Transformers 2 X BM9100 K-bus MiCOM M301 345/138kV Transformers 345/138kV Earthing transformers 2 X BM9200 K-bus MiCOM M301 K-series re ays 345kV: Bus section 1 1 X BM9100 K-bus MiCOM M301 345kV: Bus section 1 1 X BM9100 IEC 60870-5-103 Re ay PQ741 EFI.P Dua redundant Fibre optic ring 345/138kV Tran sformers: Lado de Baixa 2 X BM9100 K-bus K-bus 345kV: Bus section 2 1 X BM9100 MiCOM M301 K-series re ays 138kV Lin es: Wi son 1 & 2 2 X BM9100 345kV: Reactor 1 1 X BM9100 K-bus K-bus MiCOM M301 3 45kV: Bus coup er 1 X BM9100 138kV Lines: Ipiranga 1 & 2 2 X BM9100 MiCOM M301 E PAC re ays K-bus MiCOM M301 K-series re ays K-series re ays 20kV Busbar 2 X BM91 00 MiCOM M301 K-series re ays 345kV: Bus coup er 2 X BM9100 • 24 • K-bus MiCOM M301 K-bus MiCOM M301 EPAC re ays 138kV Bus section 2 1 X BM9100 K-bus MiCOM M301 K-bus K-series re ays MiCOM M301 K-series re ays Figure 24.14: System architectu re: Uti ity substation automation project • 436 • Network Protection & Automation Guide
345kV Busbar (Future) Line: Cabo Norte 1 Line: Cabo Norte 2 Transformer 1 345/88/138kV Section 2D Section D Section 1D 345KV Section 2C Section C Section 1C Earthing transformers Reactor 1 1B 1A 2B 2A Figure 24.15: Sing e ine diagram: Uti ity substation 345kV busbar (part) Transformer 3 345/88/138kV 138kV Busbar Transformer 2 345/88/138kV (future) • Section 3A Section 4A Section A Section B 138kV Section 3B Section 4B 138kV Line: 138kV Line: Mariana 2 Brigadeiro 2 (future) (future) 138kV Line: Ipiranga 2 (future) 138kV Line Wi son 2 Earthing transformer 2 138kV Line: Ipiranga 1 (future) Figure 24.16: Sing e ine diagram: Uti ity substation 138kV busbar (part) Network Protection & Automation Guide • 437 • Substation Contro and Automation 24 • Transformer 1 345/20/20kV Transformer 2 345/20/20kV
• Substation Contro and Automation 24 • Traction Substations 8 off Maintenance Contro Centre x3 P139 Printer xxx Dua redundant OPC server ming ine SCADA BM9200 xxx Auxi iary transformer n Printer Maintenance Workstation: Parameter settings and disturbance ana ysis BM9200 BM9200 BM9200 EFI.P ring BM9200 Bus s P921 Bus se e P921 c fi ter e xxx BM9200 Dua redundant PC: inter-substation automation BM9200 55kV ine b P438 BM9200 Gateway T103/OPC Track feeder iso ators Portab e PC for site maintenance staff Catenary/feeder BM9200
• 438 • Ethernet Network Intermediate Autotransformer Substations 34 off 4 5 4-5 per traction substation Fina Autotransformer Substation 7 off GPS OPC server PC OPC server PC xxx GPS GPS OPC server PC Intermediate Autotransformer Substation OPC server Traction autotransformer PC GPS P632 P632 P632 BM9200 Traction otransformer Traction transformer x xxx P632 xxx Traction autotransformer BM9200 BM9200 BM9200 BM9200 action ransformer Traction P P632 EFI.P ring P632 BM9200 ring BM9200 BM9200 BM9200 Auxi iary transformer Auxi iary transformer Auxi iary transformer Auxi iary Bus section Track catenary/feeder BM9200 mer Track catenary/feeder Track catenary/feeder BM920 BM9200 Harmonic fi ter Iso ators for track catenary/feeder
BM9200 BM9100 Network Protection & Automation Guide Iso ators for track catenary/feeder Figure 24.17 – Substation automation scheme – High-speed rai way ine
The substation automation scheme used is the AREVA T&D Digita Contro System. T raction substations have an incoming supp y at either 225kV or 400kV, transforme d down to 27.5kV for traction and ower vo tages for auxi iary supp ies. Redunda ncy in contro and supervision is provided through the operator at each substati on being ab e to view and contro those substations immediate y adjacent as we . There is an overa Contro Centre to monitor the comp ete system, using a Gat eway on the Ethernet spine. Approximate y 500 Bay Modu es are used, providing co ntro and measurement faci ities and a so acting as interfaces to the protection re ays. The significant aspect of this app ication is the distance over which t he automation scheme is app ied using a standard substation automation scheme. The overa ength of 500km is arge for a substation automation scheme and i ustrates the geographica span now possib e. Figure 24.18 shows the topo ogy of the substation automation equipment at a t raction substation, whi e Figures 24.19-21 show the different eve s of detai a vai ab e to a substation contro er via the HMI. Operator functions inc ude cont ro and monitoring of the substations, remote setting of a re ays and automati c retrieva of disturbance recordings from re ays for remote ana ysis. Data is r efreshed at approximate y 1 second interva s. A notab e automation feature is th e automatic reconfiguration of the power distribution network during fau ts or o ther outages to maintain continuity of traction power supp ies. • Figure 24.18: Configuration of a traction substation Network Protection & Automation Guide • 439 • Substation Contro and Automation 24 •
Substation Contro and Automation Figure 24.19: Overview of traction power supp ies • 24 • Figure 24.20: Autotransformer one- ine diagram • 440 • Network Protection & Automation Guide
Figure 24.21: Incoming supp ies at a traction substation • Network Protection & Automation Guide • 441 • Substation Contro and Automation 24 •
• 25 • Distribution System Automation Introduction Factors inf uencing the app ication of automation to distribution n etworks Primary distribution system automation Secondary distribution networks u rban areas Secondary distribution networks rura areas Communications Distributi on system automation software too s 25.1 25.2 25.3 25.4 25.5 25.6 25.7
• 25 • Distribution System Automation 25.1 INTRODUCTION Distribution systems are genera y considered to be supp y net works operating at 132kV and be ow, and to which consumers are norma y connecte d. Within a distribution system, a division into primary and secondary distribut ion systems is often made, with primary distribution systems having vo tages abo ve 22kV and secondary distribution systems vo tage be ow this va ue. Automation of distribution systems has existed for many years. The extent to which automati on has been app ied has been determined by a combination of techno ogy and cost. For many years the avai ab e techno ogy imited the app ication of automation t o those parts of the distribution system where oss of supp y had an impact on arge numbers of consumers. Techno ogy was not avai ab e to hand e the arge amou nt of geographica y dispersed data required for automation of distribution syst ems in rura areas. Even when deve opments in techno ogy began to overcome these prob ems, the cost of app ying the techno ogy was arge in re ation to the bene fits gained. Often, there was no financia incentive to app y automation in rura distribution systems, and consumers were not entit ed to compensation for oss of supp y. As re ative y few consumers wou d be affected by a fau t on a rura distribution system, compared to a simi ar fau t in an urban distribution system , the number of customer comp aints received was not a sufficient y important fa ctor to justify investment in network re iabi ity. Interruptions to consumers in rura areas were treated as being inevitab e. Recent deve opments such as priva tisation started to focus attention on the cost to the consumer of a oss in sup p y. Interruptions in supp y began to be ref ected in cost pena ties (direct y o r indirect y) to the Uti ity, thus providing a financia incentive to improve ma tters. Rura consumers gradua y became more aware of the disparity in the numbe r of supp y interruptions between rura and urban distribution networks. This e d, in conjunction with an increasing emphasis on Power Qua ity issues (see Network Protection & Automation Guide • 443 •
Chapter 23), to pressure on Uti ities to improve the situation. In addition, the popu ation in rura areas became more dependent on e ectrica equipment, and th us the consequences of a supp y outage were more significant. The term automatio n conjures up the use of microprocessors, maybe inked together over a communica tions network and running specia purpose software to execute a sequence of acti ons automatica y. Whi e such techno ogy is emp oyed and forms part of distribut ion system automation, the term automation may imp y nothing more than the abi i ty to c ose or open a switch remote y in addition to oca (hand) contro . It ma y invo ve nothing more than the addition of an actuator, and simp e on/off remote contro faci ities. Techno ogy has been app i ed to reduce the cost of such devices, thus improving the economics of their app ication. Therefore, the fie d of distribution system automation is a very broad one, and the so ution app ied to any particu ar prob em wi ref ect the partic u ar circumstances of prob em and regu atory regime of the Uti ity concerned. Fi gure 25.1 shows typica distribution systems that form the subject of this chapt er, comp ete with the e ements of the distribution system to which automation te chniques are app ied. The remaining sections of this chapter describe the variou s automation techniques avai ab e, together with typica app ications. Transmission system Distribution System Automation 110kV 110kV 110/ 11kV 110/ 33kV 110kV 110/ 33kV 110/ 11kV Large consumers 33kV RMU 33/11kV Urban distribution network Load RMU 33/11kV Load RMU 33/11kV Load • 25 • 33kV RMU 33/11kV Urban Distribution Network Load Spur ines Main circuit Rura distri bution network RMU 33/11kV Load Norma y open point RMU 33/11kV Load Spur ines Figure 25.1: E ements of a distribution system
• 444 • Network Protection & Automation Guide
25.2 FACTORS INFLUENCING THE APPLICATION OF AUTOMATION TO DISTRIBUTION NETWORKS Cost is the main driving factor in the app ication of an automation scheme to a distribution network. Regu atory pressure may a so inf uence the decision. The c ost may arise in many different ways. Savings from imp ementing distribution sys tem automation resu t from reducing: a. revenue foregone during outages b. cost of hand ing customer comp aints c. cost of contro /maintenance staff d. cost of compensation to consumers for outages Less tangib e benefits can a so be identif ied, such as deferra of system enhancement (i.e. deferra of capita expenditur e) through better know edge of network performance. The financia advantage to t he Uti ity of such benefits may be more difficu t to ca cu ate, but shou d be in corporated in any financia comparison for a proposed scheme. There are inevitab y costs incurred through use of an automation scheme: a. cost of imp ementation (capita cost) b. cost of operation c. cost of maintenance and c ear y the tota costs saved must be in excess of the tota costs of imp ementation and use to make a scheme viab e. For many years, automation has been imp emented at vo tage s above 22kV, simp y due to the number of consumers inconvenienced by a supp y o utage and the resu ting costs (in whatever form). However, in recent years, the traditiona ba ance of cost/benefit has been changed, due to: 1. increasing depe ndence by communities/industry on e ectricity 2. privatisation (in some countrie s) 3. the spread of e ectricity supp y to ever more remote areas 4. the cost of training and retaining ski ed staff 5. increasing emphasis on Power Qua ity iss ues This change has been in favour of increased automation of the distribution s ystem, inc uding system vo tages down to LV. Regu atory pressure to improve the re iabi ity and qua ity of e ectricity supp y to end-users produces an outcome t hat the associated costs are on y acceptab e if techno ogy is app ied to automat e the secondary distribution system. Therefore, automation of the secondary dist ribution system has become more widespread. At the same time, overhead ines in rura areas suffer many more fau ts eading to consumer supp y oss than urban cab e networks. These findings ar e not surprising – rura distribution networks are common y in the form of radia feeders whereas urban networks are often in the form of ring or meshed networks to minimise the chances of supp y oss to arge groups of consumers. Simi ar y, overhead ines are norma y more prone to fau ts than underground cab es. Becaus e the fau t incidence on EHV overhead ines is significant y ower than for thos e on distribution systems, it is a so arguab e that the technica standards re a ting to overhead ines on distribution networks a so require review. Therefore, deve opments in distribution system automation have concentrated arge y on app ications to the secondary distribution system. 25.3 PRIMARY DISTRIBUTION SYSTEM AUTOMATION Bu k transmission network 110kV To Primary distribution substation Large consumers 110/50kV • 50kV Feeders to secondary distribution system 50kV Feeders to secondary distribution system Figure 25.2: Primary distribution system Network Protection & Automation Guide • 445 •
Distribution System Automation 25 • The primary distribution system is genera y accepted as comprising those e emen ts of the distribution system operating at vo tages above 22kV. Distribution use s both cab e and overhead ines, and the power eve s invo ved wi enab e eithe r a arge group of domestic consumers, or severa industria p ants to be served . Very arge industria p ants may justify their own dedicated feeders from the primary distribution substation (Figure 25.2).
Automation of the primary distribution system is we estab ished, due to the im pact of supp y oss on the many and diverse consumers that it serves. In additio n, the distribution system is usua y interconnected, so that oss of supp y to consumers in the event of a circuit outage is minimised. The circuit breakers an d protection systems used in the system wi a ready be capab e of remote contro /monitoring. However, status information on a circuit may be confined to simp e on/off/open/c osed/tripped indications, and determination of the cause of a tri p wi sti require despatch of a maintenance crew to the equipment concerned. On y after the cause of a trip has been determined can fau t ocation and rectif ication take p ace. Hence modern network automation techniques can be usefu y a pp ied. App ication of such techniques brings the fo owing advantages: a. abi i ty to contro a much arger area b. provision of detai ed network performance in formation c. reduction in space requirements d. reduction in staffing 25.3.1 Con tro Area Size The modern e ectric power network has tight coup ing between the various e ements - a prob em in one area may have knock-on effects over a wide a rea. Hence, traditiona distribution contro rooms serving a imited geographica area are being rep aced by fewer (perhaps on y one for a Uti ity serving up to 10,000km2 area) and in these cases o der automation systems may not be ab e to hand e the tota I/O count. Either the upper imit on I/O points wi have been reached, or response times to an event become too s ow to be of practica use. U se of a modern automation system permits a reduction in the number of contro ce ntres used, with each centre ab e to oversee a much wider geographica area. Thu s, incidents that have an impact outside of the immediate area can be dea t with more effective y and hence resu t in a better response to the incident and fewe r customer comp aints. 25.3.2 Detai ed Network Performance Information Modern mi croprocessor-based re ays can store a wea th of information re ating to the caus e of a trip and transmit such data, when requested, to a Contro Centre. Hence, the nature and possib y the ocation of a fau t can be identified. The maintenan ce/repair crew can be provided with better information, thus shortening circuit downtime and enhancing distribution network avai abi ity. Data re ating to netwo rk oading and vo tage variations can a so be stored and down oaded at regu ar interva s and pr ovides two main benefits. First y, monitoring of Power Qua ity can be undertaken and hence customer comp aints readi y investigated. Sufficient information may we be avai ab e to estab ish the short-term actions required to correct or min imise the prob em, resu ting in fewer customer comp aints, and a possib e reduct ion in financia pena ties. Second y, a review of the oading profi e of circuit s against time can be undertaken. Using appropriate p ant therma ageing mode s, the rating of circuits can be reviewed and adjusted. This may resu t in an enha nced rating being given to circuits, and hence the postponement of capita expen diture. 25.3.3 Space Requirements Many countries have significant pressure on a nd-use for infrastructure requirements. A modern microprocessor re ay can now un dertake the functions previous y requiring severa discrete re ays, and of measu rement devices, thus e iminating numerous VT’s and CT’s, measurement transducers/ind icators, auxi iary contacts on circuit breakers, etc. Wiring between p ant items is much reduced. Use of modern communications techniques such as data transmiss ion by mobi e radio networks can simi ar y reduce wiring requirements to/from th e Contro Centre. The space requirements in a substation for housing the re ays associated with the circuits of a distribution network can be reduced, giving a significant reduction in expenditure on the bui dings associated with the substa tion. Benefits can a so be obtained from e iminating separate metering devices, reducing space provision and hence cost. 25.3.4 Staffing Leve s The reduction in the number of Contro Centres eads natura y to a reduction in the staffing re quirement for such p aces. More important y, the abi ity of inte igent re ays t o report their settings and measured va ues to a Contro Centre, and to accept r evised settings down oaded from the contro centre can ead to significant impro vements in the qua ity of supp y, whi e at the same time reducing the staffing r equired compared to a manua system. Distribution systems are subject to regu ar changes in configuration and oading, and these may require changes to protecti
on re ay settings. Manua means of determining protection re ay settings invo ve site surveys at the substations concerned to record existing settings, fo owed by further visits to carry out changes as required. Pressures on staffing may m ean that such exercises are carried out at extended interva s. A modern automate d distribution system e iminates much of the manua effort by automation of • Distribution System Automation 25 • • 446 • Network Protection & Automation Guide
the reporting and down oading of re ay settings. Whi e scope sti exists for in troducing errors into re ay setting va ues, the incidence of these is reduced. R egu ar comparison of settings against desired va ues increases the possibi ity o f incorrect settings being identified and corrected, thus minimising the resu ti ng disruption. 25.4 SECONDARY DISTRIBUTION NETWORKS URBAN AREAS A high eve of interconnection, either ring or mesh, to ensure a high degree of avai abi ity of supp y to the consumer, characterises secondary distribution networks in urban areas. Domestic, industria and commercia consumers wi suffer great inconveni ence through on y a re ative y short oss in supp y of on y a few hours, with bu siness ike y to suffer considerab e financia oss if an interruption is onger than 2-4 hours. For domestic consumers, oss of supp y for between 4-8 hours is arge y an inconvenience, though oss may resu t from spoi age of freezer conte nts, etc. and in co d weather may p ace vu nerab e sections of the community at risk. Such hazards for a privatised Uti ity give rise to the potentia for signi ficant financia oss, through c aims for compensation. A typica urban secondary distribution system is shown in Figure 25.3. There is a arge proportion of underground cab e, and fina feeders to LV distribution su bstations take the form of feeders from Ring Main Units (RMU’s). Severa RMU’s are c onnected in a oop fed from one or more substations, the oop norma y being ope n at some point. The open point is norma y chosen to equa ise oading at both e nds of the ring as far as possib e. The cab es forming the ring and a associat ed switchgear, etc., are sized for sing e-end feeding of the who e ring, to a o w for an outage affecting the ring between a substation and the first RMU, or at the substation itse f. The arrangement of an individua RMU is shown in Figure 25.4(a). For many years, on y oca operation and indications (trip/hea thy) wer e provided, so that switching operations required a visit from fie d staff. Trip s at an RMU resu ting in oss of supp y to consumers were annunciated through cu stomer comp aints, no direct indication to the contro room was provided. Ring Ring Primary distribution network Ring Main Unit Spur 33kV (a) Basic Ring Main Unit To Secondary distribution 33/11kV substation 33/11kV Ring Ring M Remote interface M I> 11kV • RMU 11/0.4kV RMU 11/0.4kV RMU 11/0.4kV RMU M Ring Main Unit Spur (b) Automated Ring Main Unit 11/0.4kV Fina distribution
RMU 11/0.4kV RMU RMU RMU 11/0.4kV RMU 11/0.4kV Figure 25.4: Ring Main Unit 11/0.4kV 11/0.4kV Fina distribution Figure 25.3: Typica urban secondary distribution system Network Protection & Automation Guide The individua items of p ant have deve oped over many years and are genera y r e iab e, taken individua y. Major fai ures of a comp ete distribution system ar e rare, and usua y stem from inadequate specification of the origina equipment , or fai ure to monitor the condition of • 447 • Distribution System Automation 25 •
equipment with time. This is especia y the case where oading and/or environmen ta conditions have changed. However, once a fau t occurs (and sooner or ater t his is inevitab e), ocation, repair and restoration of norma supp y can take t ime. In particu ar, repair of fau ts in underground cab es may take some time, a s the ocation must first be identified to within a few metres, and then the gro und excavated to effect the repair. In the centre of a arge city, excavation is not popu ar and wi be expensive. Traffic congestion wi ensure that the resp onse time for a repair crew to arrive at a substation after a fau t has been rep orted is not trivia , especia y where (in some privatised Uti ities), pena ties may be imposed for oss of supp y to consumers asting more than 60 minutes. Th e app ication of automation techniques has therefore many advantages. This wi usua y require the provision of extra features to an RMU. The most common featu res added are: compensation paid can be justification in itse f. Interrogation of re ays/(FPI’s) can then determine the feeder circuit on which the fau t has occurred, thus enab ing restoration of supp y to customers unaffected by the fau t to begin immedia te y. In some cases, it may be possib e to devise automatic sequences for this, thus re ieving the contro room operator of this duty and enab ing concentration on the task of precise fau t ocation and repair. Equipment that is used rare y may fai to operate when ca ed upon to do so. Much effort has been paid in pro tection re ay design to avoid this prob em, and digita and numerica re ays gen era y have a se f-checking function that runs regu ar y and is arranged to a ar m if the function detects an interna fau t. However, circuit breakers and other switching devices that may not operate for a considerab e period can get stuck in their norma position and thus fai to operate when commanded to. A number of major system co apses have been known to occur because of such prob ems, it be ing not a ways possib e to provide backup protection that wi operate in suffic ient time. One so ution to this prob em is to exercise such equipment on a regu ar basis. This can be done at itt e cost to the Uti ity if carried out remote y , but is prohibitive y expensive if carried out on a oca manua basis. Fina y , through an improved know edge of network performance, network enhancements may be ab e to be postponed or e iminated, which is a substantia bonus as the cost s of insta ing new cab es in urban areas can be very high. Figure 25.5 shows a modern RMU suitab e for insta ation indoors – practice varies between countries i n such matters, with outdoor insta ation a so being common. Distribution System Automation a. capabi ity for remote operation – addition of actuators for open/c ose operatio n of the various devices that are capab e of being operated from a remote ocati on b. provision of remote indications of status of the various devices c. additi on of Fau t Passage Indicators (FPI’s). An FPI is a sensor that detects passage of current in excess of a defined va ue, and therefore provides an indication that the fau t is further from the supp y point (for a radia -fed system) than the F PI d. addition of a protection re ay for phase/earth fau ts Note that once it ha s been decided to provide remote contro or indication, some form of communicati ons interface is a so required and the incrementa cost of providing both remote contro and indication instead of one or the other is very sma A typica conf iguration for an RMU with a options fitted is shown in Figure 25.4(b). Traditi ona manua operation of RMU’s can be rep aced by remote contro . Many existing de signs of RMU can be adapted in this way, whi e a new designs have this feature as standard. The remote communications feature provides the fo owing features: 1. issuing of commands to open/c ose the circuit breaker, etc. 2. provision of status avai abi ity) etc. 3. vo tage and current data Provision of remote indica tion of status to a Contro Centre enab es the response time to a fau t to be re duced. The reduction in customer comp aints and information (position, •
25 • Figure 25.5: Modern indoor RMU • 448 • Network Protection & Automation Guide
25.5 SECONDARY DISTRIBUTION NETWORKS RURAL AREAS The cha enges in network autom ation for rura areas are simi ar to those in urban areas, however the network t opo ogy may be very different. A typica conventiona network topo ogy is shown in Figure 25.6. Due to re ative y sparse popu ation, feeders are genera y radia , often with spur ines, and can be quite engthy – 60km ength of main feeder at 11kV being possib e. The feeders are usua y conventiona overhead ines with u ninsu ated conductors, and fau t rates for these ines are high in comparison wi th cab es or EHV overhead ines. In some countries, ight y insu ated conductors are used, and these reduce the fau t rates experienced. 25.5.1 Circuit Breaker Remote Contro /Monitoring This provides a sma advantage in a erting the operator to a oss of supp y, and a arger one in minimising re storation time. Most OHL fau ts are transient in nature, and therefore circuit b reaker rec osure after a short time interva is ike y to resu t in supp y being restored. The operator may therefore attempt a manua c osure of the circuit br eaker to restore supp y. Use of an auto-rec ose scheme (see Chapter 14) may furt her reduce the disconnection time and re ieve the contro room operator of work oad, especia y in conditions of poor weather when many distribution feeders may suffer transient fau ts. 25.5.2 Automatic Sectiona isers An automatic sectiona iser is a switching device that detects the f ow of current in excess of a set v a ue and opens a switch to disconnect the network downstream of the device. Beca use such devices are usua y po emounted, in ocations remote from a suitab e e ectricity supp y, the sensing and switching mechanism is arranged to be se f-pow ered. The expense of a transformer, etc. to provide such a supp y from the supp y side of the ine is not justified and adds additiona comp ication. By p acing automatic sectiona isers at interva s a ong the ine, it is possib e to disconn ect on y the fau ted section of ine and those beyond it. The number of consumer s affected by a permanent fau t is minimised, and a more precise indication of t he ocation is possib e. For circuits that have more than one feed and a norma y open point (Figure 25.7), oss of supp y unti the fau t is repaired can be i mited to the section in which the fau t ies. The sectiona iser at point B opens automatica y and the operator can take action to open the one at point C. The fau ted section is thus iso ated and (subject to system conditions being satisfa ctory) the sectiona iser at the norma y open point may be c osed. A Sectiona iser operates B C Norma y open point Norma y open point NB: Consumer connections not shown Figure 25.6: Rura distribution network • Response times for ocation and repair of fau ts may be engthy, as the on y ind ication of a fau t having occurred may be customer comp aints of oss of supp y due to the source circuit breaker having tripped. In this case, a consumers fe d by the ine wi suffer oss of supp y, and determining the ocation of the fa u t may take a considerab e time. Many possib e enhancements taking advantage of automation techniques to the basic feeder topo ogy are possib e to improve the situation: a. add remote contro /monitoring to the circuit breaker b. add automa tic sectiona isers Fau t Norma y open point NB: Consumer connections not shown Figure 25.7: Automatic sectiona iser operation
Network Protection & Automation Guide • 449 • Distribution System Automation 25 •
However, there can be drawbacks as we . Grading of the feeder circuit breaker w ith the sectiona isers may be difficu t and resu t in onger fau t c earance tim es for fau ts in the section between the circuit breaker and first sectiona iser . The circuit breaker must be rated for the resu ting fau t duty. Consumers situ ated in hea thy sections of ine may suffer extended vo tage dips, which may giv e rise to prob ems with equipment. An i ustration of the device is given in Fig ure 25.8. A deve opment of the automatic sectiona iser is the automatic rec oser . This device opens when a fau t is sensed, and subsequent y re-c oses according to a preset sequence. It can be thought of as the distribution network equiva e nt to an auto-rec ose scheme app ied to circuit breakers on an EHV transmission ine. It overcomes the disadvantage of a sectiona iser in that transient fau ts do not resu t in oss of supp y to consumers downstream of the device. RTU * * RTU * RTU * RTU RTU * RTU RTU * RTU * RTU * * RTU Norma y open * point * RTU RTU * Norma y open point * RTU * RTU * RTU Distribution System Automation NB: Consumer connections not shown * CB s/rec osers/sectiona isers fitted with t ransducers for vo ts/amps and Fau t Passage Indicator CB s/rec osers fitted with overcurrent/ earth fau t re ay and automatic rec osing device. Figure 25.9: Automated rura distribution network The benefits provided are: a. rapid restoration of supp y to a consumers fo o wing transient fau ts b. disconnection of the minimum number of consumers fo ow ing a permanent fau t c. indication of network performance to the contro centre , inc uding fau t ocation and network oading d. reduced requirement of fie d c rews to carry out manua switching e. reduced fau t ocation time In common with other distribution systems, inte igent devices such as circuit breakers and se ctiona isers fitted with remote contro and current/power sensing devices can be used to gather information on network operating conditions and hence be used as inputs when network enhancement is being considered. With existing equipment, s uch information may not be avai ab e at a un ess a fie d measurement exercise is undertaken. The information can be used not on y to identify constraints in t he network, but a so to determine spare capacity much more accurate y (in terms of permissib e shortterm over oads possib e without excessive temperature rises occurring). Network re-inforcement may then possib y be postponed or even e imin ated, resu ting in Network Protection & Automation Guide Figure 25.8: Modern automatic sectiona iser • 25 •
The first automatic rec osure operates a short time after opening and wi usua y be successfu if the fau t is a transient one. Shou d a fau t sti be detect ed upon the first rec osure, the rec oser de iberate y remains c osed for a sign ificant time to try and c ear the fau t by using the arc energy to burn out the cause of the fau t. The rec oser then opens, and c oses after a pre-set dead tim e. Shou d the fau t sti exist, a further burn time/open/rec ose cyc e is carri ed out, after which a fina open/ ockout operation is performed if the fau t sti exists. The usua remote contro and indication faci ities are provided. Some form of condition monitoring may be used, so that maintenance is requested on y when required, and not on the usua basis of the number of switching operations carried out. Figure 25.9 shows the distribution network of Figure 25.6 after ap p ication of fu automation as described above. • 450 •
reduced capita expenditure requirements. There is a so the potentia for improv ed therma mode ing of p ant, to produce a more accurate therma oss-of ife i ndication. 25.6 COMMUNICATIONS Perhaps the most difficu t task in automating a d istribution network is se ection of the appropriate communications technique for imp ementation of the remote contro /reporting faci ities. Severa techniques a re avai ab e, as fo ows: a. hard-wired b. Pub ic Switched Te ephone Network (PS TN) c. mobi e radio (packet switched data) d. conventiona or ow-powered radio (inc uding Microwave) e. Power Line Carrier Communication (PLCC) Tria s to date appear to indicate that the choice of communications medium is critica . Therefo re, extensive investigations in this area are required. Not a of the possibi i ties are suitab e for a types of distribution system or geographica area, and this needs to be kept in mind. 25.6.1 Hard-Wired Communication Hard-wired commu nication is genera y not a viab e option, as the infrastructure wi not be ava i ab e. The costs of insta ing the required cab ing wi be arge, and it wi norma y be found that there are ess expensive so utions avai ab e. However, in cases where there is infrastructure a ready avai ab e, this so ution wi be at tractive. A cab ing suffers from the possibi ity of fau ts, and therefore an a ternate route, maybe sourced from a te ecomms provider, may be required as back up. 25.6.2 Use of PSTN Network Use of the existing fixed pub ic te ecomms infras tructure wi norma y be feasib e for urban distribution networks. For rura ne tworks, the required infrastructure probab y does not exist. Line qua ity wi b e of critica importance and equipment to ensure detection of errors in transmis sion, and request repeat transmission of data, wi probab y be required. Simi a r y, as substations are areas of high e ectrica interference, appropriate measu res to protect the required hardware in substations wi be required. Technica so utions to these prob ems readi y exist, but appropriate data on the probab e interference eve s, especia y those occurring transient y under fau t conditio ns or due to ightning strikes on overhead ines are required to ensure that equ ipment is Network Protection & Automation Guide proper y specified. Reported experience on schemes in operation suggest that ca set-up times may be s ow, and ine qua ity (even in dense y popu ated urban ar eas) may not be high, eading to s ow data rates and hence restrictions in the a mount of data that can be transmitted in a reasonab e time. 25.6.3 Mobi e Radio Mobi e radio is a quite attractive option. Many companies offer packet-switched data techniques to business users. Fie d experience reported to date indicates t hat this method is we suited to both urban and rura areas. The main prob em i n urban areas appears to be shie ding of the required antennas by other bui ding s or parked vehic es – a prob em shared by a communications techniques invo ving radio. In rura areas, investment may be required to provide the necessary area of cover, and this may take time to achieve, depending on the priorities of the te ecomms provider invo ved. However, mobi e te ecomms service providers are us ua y keen to expand service coverage and sites for the required masts may conve nient y be ocated a ong the right-of-way of the distribution system ines. 25.6 .4 Conventiona Radio Use of radio as a te ecomms medium is we estab ished amo ngst Uti ities. Low powered radio has been used in a number of tria insta atio ns of distribution system automation schemes without significant prob ems. The r equirements for base stations are simi ar to those for mobi e te ecomms, togethe r with the same possib e hazards. One possib e drawback to greater adoption of s uch techniques is that ow-powered radio is not subject to regu ation in some co untries. There is no guarantee that interference from systems operating on the s ame or nearby frequencies wi not occur, nor is there any mechanism avai ab e t o ensure that a frequency, once chosen, is reserved so e y for the user in that area. The regu atory situation cou d be expected to change if wider use of such techniques occurred. 25.6.5 Microwave Transmission Microwave transmission is a p ossibi ity, a though severe y handicapped by the fact that it re ies on ineof–sig ht communications. Numerous repeater stations may therefore be required in hi y terrain. It does not appear to have been used in tria s reported on to date so the practica performance cannot be judged. However, given appropriate terrain,
it sti merits consideration. • • 451 • Distribution System Automation 25 •
25.6.6 PLC Communications Power Line Carrier is a technique that is we known t o Uti ities and makes extensive use of existing Uti ityowned infrastructure. How ever, additiona equipment is needed at each substation to ensure that the signa on y trave s a ong the desired path and is prevented from trave ing a ong oth ers and causing unwanted interference. The additiona equipment required can mak e a new insta ation expensive, and retrofit on existing distribution systems at ower vo tages probab y prohibitive y so. Space provision for the required ine traps and coup ing transformers is required, which may be difficu t to find at many ocations. At higher distribution vo tage eve s (e.g. 66kV/110/132kV), it is more attractive, especia y as it may a ready have been insta ed for other r easons. Data rates may be imited and transmission fai ure may occur under fau t conditions, just at the time when it is most needed. Whichever communications m edium is chosen, care is a so needed in the choice of communications protoco . T he common IEC 60870-5-103 master/s ave protoco used by many protection/measurem ent devices is not who y appropriate for such techniques. It requires po ing b y the master station of the s ave devices on a regu ar basis, whereas initiation by fie d devices is idea y required, in order to imit the communications band width required. Protoco converters may be required in the fie d, making one add itiona source of unre iabi ity. At the Contro Centre, a protoco converter wi a most certain y be required, to interface to the SCADA system in use. Each e ement in the scheme must be re iab e in operation and not be prone to fa se oper ation in any way, otherwise credibi ity is rapid y ost. Not on y wi the schem e fa rapid y into disuse, but a so the experience wi co our the assessment o f future schemes for many years to come. More information on data transmission p rotoco s is to be found in Chapter 24. 25.7 DISTRIBUTION SYSTEM AUTOMATION SOFTW ARE TOOLS To assist the operator of a distribution network, there are a number o f software too s that can be used to assist in making decisions and imp ementing them. They are: a. topo ogy ana ysis b. power system ca cu ations c. power qua ity management d. system configuration management The too s may be avai ab e as on- ine interactive too s, to assist in decision-making, or as off- ine too s to study the impact of decisions (‘what-if’ scenarios). Some of the techno ogy is avai ab e now, especia y in off- ine form, but a features described are under act ive deve opment and can be expected to be avai ab e soon, producing further enhancem ents in distribution network performance. 25.7.1 Topo ogy Ana ysis In its’ simp es t form, topo ogy ana ysis can be simp y an operator disp ay of the distribution network, using co ours to differentiate between the various states in the networ k. The network may be disp ayed in terms of its’ state (energised/non-energised), vo tage eve , or source of supp y. More advanced software too s may invo ve sta te estimation of the network, using historica or assumed data. This is used to fi in gaps in the known network topo ogy, due perhaps to communications fai ur es or use of egacy equipment without communications faci ities on some parts of the network. The resu ts of the ana yses are disp ayed and are used as inputs t o other software too s. 25.7.2 Power System Ca cu ations These invo ve oad f ow and fau t eve ca cu ations to determine network oading, possib e over oads o n equipment and to ensure equipment is operated within fau t eve ratings. Spec ia requirements may exist in imp ementing so ution techniques due to the radia nature of the network. It may a so be necessary to predict network performance in the future by assuming oads, or to assume data where it is acking, by use o f state estimation techniques. The osses in the distribution system, or any par t of it, can be eva uated to determine the efficiency of the network and as an i nput to inte igent configuration too s to assist the operator in se ecting the most appropriate configuration as network conditions change. 25.7.3 Power Qua it y Management Power Qua ity has been covered in Chapter 23. Software can be used for ca cu ating various performance indices re ating to Power Qua ity. The resu ts, whether obtained off- ine or in rea -time, can be used to inf uence the oper ation of the network to minimise either one or severa of the performance indica tors. There may be economic benefits for the Uti ity through more efficient use of the network and avoidance of financia pena ties where performance targets ar
e not met. The too wi use inputs from the Topo ogy Ana ysis and Power System Ca cu ation too s in order for the functions to be carried out. Typica user out puts are tap changer and capacitor switching schedu es, energy osses for the wh o e or se ected parts of the network for defined periods of time, harmonic eve s, data re ating to supp y interruptions Network Protection & Automation Guide • Distribution System Automation 25 • • 452 •
(Customer Minutes Lost, etc), and re iabi ity indices for the network. The data re ating to osses can be sp it into those that are oad re ated and those that are independent of oad. This data can be input into too s re ating to Asset Man agement, as the choice of feeder type/rating and design of transformers, etc. ca n be inf uenced by such factors. 25.7.4 System Configuration Too This too s can be used either off- ine to examine the impact of proposed changes to the networ k, or on- ine to suggest changes to a network to yie d optima resu ts, accordin g to a number of user-specified criteria. The impact of proposed switching seque nces is a so ana ysed, to ensure that the duty imposed is within rating. The use r-specified criteria may inc ude those re ating to Power Qua ity, whi e required inputs are the outputs from the Topo ogy and Power System Ca cu ation too s. A further function of this too is to ca cu ate the optima order of switching in a network to restore supp ies after an incident, whi e maintaining safety. A ter native sequences that can be adopted in the event of fai ure of a device to resp ond to a command are a so avai ab e. • Network Protection & Automation Guide • 453 • Distribution System Automation 25 •
Appendix 1 Ter mino og y
Appendix 1 Termino og y The introduction of computer techno ogy means that the Protection Engineer must now be fami iar with a range of technica terms in this fie d, in addition to th e terms ong associated with Protection and Contro . Be ow is a ist of terms an d their meanings that are now common y encountered in the Protection and Contro fie d. AC A ternating Current ACB Air Circuit Breaker Accuracy The accuracy of a transducer is defined by the imits of intrinsic error and by the imits of va riations. Accuracy c ass A number used to indicate the accuracy range of a measu rement transducer, according to a defined standard. Active power (watt) transduc er A transducer used for the measurement of active e ectrica power ADC Ana ogue to Digita Converter A/D Conversion The process of converting an ana ogue signa into an equiva ent digita one, invo ving the use of an ana ogue to digita co nverter Adjustment The operation intended to bring a transducer into a state of performance suitab e for its use AGC Automatic Gain Contro AI Ana ogue Input AI S Air Insu ated Switchgear Network Protection & Automation Guide • 455 •
A arm An a arm is any event (see be ow) tagged as an a arm during the configurat ion phase A -or-nothing re ay An e ectrica re ay which is intended to be energ ised by a quantity, whose va ue is either higher than that at which it picks up or ower than that at which it drops out Anti-pumping device A feature incorpora ted in a Circuit Breaker or rec osing scheme to prevent repeated operation where the c osing impu se asts onger than the sum of the re ay and CB operating tim es AO Ana ogue Output AR Auto Rec ose: A function associated with CB, imp emente d to carry out rec osure automatica y to try to c ear a transient fau t ARBITER Proprietary protoco for time synchronisation from ARBITER Systems, Inc. Paso R ob es, Ca ifornia USA Bay Set of LV, MV, or HV p ant and devices, usua y contro ed by a bay computer BC Bay Computer. Computer dedicated to the contro of one or severa bays withi n a substation BCD Binary Coded Decima BCP Bay Contro Point. A oca keypad at bay eve to contro the e ements of a sing e bay Biased re ay A re ay in which the characteristics are modified by the introduction of some quantity other tha n the actuating quantity, and which is usua y in opposition to the actuating qu antity Bias current The current used as a bias quantity in a biased re ay BIOS B asic Input/Output System (of a computer or microprocessor) BT Booster Transforme r B o o s t e r Tr a n s f o r m e r A current transformer whose primary winding is in series with the catenary and secondary winding in the return conductor of a c assica y-fed a.c. overhead e ectrified rai way. Used at interva s to ensur e that stray traction return currents, with their potentia to cause interferenc e in nearby communication circuits, are minimised Burden The oading imposed by the circuits of the re ay on the energising power source or sources, expressed a s the product of vo tage and current (vo t-amperes, or watts if d.c.) for a give n condition, which may be either at ‘setting’ or at rated current or vo tage. The ra ted output of measuring transformers, expressed in VA, is a ways at rated curren t or vo tage and it is important, in assessing the burden imposed by a re ay, to ensure that the va ue of burden at rated current is used C Capacitance CAD Comp uter Aided Design Ca ibration The set of operations which estab ish, under speci fied conditions, the re ationship between va ues indicated by a transducer and t he corresponding va ues of a quantity rea ized by a reference standard. (This sh ou d not be confused with ‘adjustment’, q.v.) A p p e n d i x 1 - Te r m i n o o g y • Arcing time The time between instant of separation of the CB contacts and the in stant of arc extinction Auto-transformer A power transformer that does not provi de ga vanic iso ation between primary and secondary windings AUX Auxi iary Auxi iary circuit A circuit which is usua y energised by the auxi iary supp y but is sometimes energised by the measured quantity Auxi iary re ay An a -or-nothing re ay energised via another re ay, for examp e a measuring re ay, for the purpos e of providing higher rated contacts, or introducing a time de ay, or providing mu tip e outputs from a sing e input. Auxi iary supp y An a.c. or d.c. e ectrica supp y other than the measured quantity which is necessary for the correct ope ration of the transducer AV R Automatic Vo tage Regu ator Back-up protection A p rotection system intended to supp ement the main protection in case the atter s hou d be ineffective, or to dea with fau ts in those parts of the power system that are not readi y inc uded in the operating zones of the main protection A1 • • 456 • Network Protection & Automation Guide
CB Circuit Breaker CBC Compact Bay Contro er. Sma capacity bay computer for M edium Vo tage app ications CBCT Core Ba ance Current Transformer CCR Centra Con tro Room CDM Conceptua Data Mode ing is an activity whose aims are: • to define objects and inks and naming conventions for their identifications • to guarantee interoperabi ity between subsystems • to define standard exchange formats between system configurator and subsystem configurators Characteristic ang e The ang e between the vectors representing two of the energising quantities app ied to a r e ay and used for the dec aration of the performance of the re ay Characteristic curve The curve showing the operating va ue of the characteristic quantity corr esponding to various va ues or combinations of the energising quantities Charact eristic Impedance Ratio (C.I.R.) The maximum va ue of the System Impedance Ratio up to which the re ay performance remains within the prescribed imits of accur acy Characteristic quantity A quantity, the va ue of which characterises the ope ration of the re ay, for examp e, current for an overcurrent re ay, vo tage for a vo tage re ay, phase ang e for a directiona re ay, time for an independent ti me de ay re ay, impedance for an impedance re ay Check protection system An auxi iary protection system intended to prevent tripping due to inadvertent operatio n of the main protection system CHP Combined Heat and Power Circuit insu ation v o tage The highest circuit vo tage to earth on which a circuit of a transducer m ay be used and which determines its vo tage test C ass index The number which de signates the accuracy c ass C osing Impu se time The time during which a c osing impu se is given to the CB C osing Time The time for a CB to c ose, from the time of energisation of the c osing circuit to making of the CB contacts Comp iance vo tage (accuracy imiting output vo tage) For current output signa s on y, the output vo tage up to which the transducer meets its accuracy specification Conjunctive test A test of a pr otection system inc uding a re evant components and anci ary equipment approp riate y interconnected. The test may be parametric or specific Conversion coeffi cient The re ationship of the va ue of the measurand to the corresponding va ue of the output C o r e B a a n c e C u r r e n t Tr a n s f o r m e r A ring-ty pe Current Transformer in which a primary conductors are passed through the ap erture of the CBCT. Hence the secondary current is proportiona on y to any imba ance in current. Used for sensitive earth-fau t protection Counting Re ay A re ay that counts the number of times it is energised and actuates an output after a desired count has been reached. CSV Character (or Comma) Separated Va ues form at. A wide y used format for the exchange of data between different software, in which the individua data items a separated by a known character – usua y a comm a CT Current Transformer Current transducer A transducer used for the measuremen t of a.c. current CVT Capacitor Vo tage Transformer. A vo tage transformer that uses capacitors to obtain a vo tage divider effect. Used at EHV vo tages instead of an e ectromagnetic VT for size/cost reasons DAC Digita to Ana ogue Converte r DAR De ayed auto-rec ose D AT Digita Audio Tape DBMS Data Base Management sys tem DCF77 LF transmitter ocated at Mainf ingen, Germany, broadcasting a time si gna on a 77.5kHz frequency • Network Protection & Automation Guide • 457 • A p p e n d i x 1 - Te r m i n o o g y A1 •
DCP Device Contro Point: oca keypad on device eve to contro the switchgear , often combined with oca /remote switch DCS Distributed Contro System Dead Ti me (auto-rec ose) The time between the fau t arc being extinguished and the CB c ontacts re-making De-ionisation time (auto-rec ose) The time required for disper sion of ionised air after a fau t is c eared so that the arc wi not re-strike on reenergisation De ayed Auto-Rec ose An auto-rec osing scheme which has a time de ay in excess of the minimum required for successfu operation Dependent time measuring re ay A measuring re ay for which times depend, in a specified manner , on the va ue of the characteristic quantity DFT Discrete Fourier Transform Direct-on-Line A method of motor starting, in which fu ine vo tage is app ied to a stationary motor Drop-out (or drop-off) A re ay drops out when it moves fr om the energised position to the un-energised position Drop-out/pick-up ratio Th e ratio of the imiting va ues of the characteristic quantity at which the re ay resets and operates. This va ue is sometimes ca ed the differentia of the re ay DSP Digita Signa Processing DT Definite time Earth fau t protection system A protection system which is designed to respond on y to fau ts to earth Earthin g transformer A three-phase transformer intended essentia y to provide a neutra point to a power system for the purpose of earthing Effective range The range of va ues of the characteristic quantity or quantities, or of the energising qua ntities to which the re ay wi respond and satisfy the requirements concerning it, in particu ar those concerning precision Effective setting The ‘setting’ of a pr otection system inc uding the effects of current transformers. The effective set ting can be expressed in terms of primary current or secondary current from the current transformers and is so designated as appropriate E ectrica re ay A devi ce designed to produce sudden predetermined changes in one or more e ectrica ci rcuits after the appearance of certain conditions in the e ectrica circuit or c ircuits contro ing it NOTE: The term ‘re ay’ inc udes a the anci ary equipment c a ibrated with the device E ectromechanica re ay An e ectrica re ay in which t he designed response is deve oped by the re ative movement of mechanica e ement s under the action of a current in the input circuit EMC E ectro-Magnetic Compat ibi ity Embedded generation Generation that is connected to a distribution syste m (possib y at LV instead of HV) and hence poses particu ar prob ems in respect of e ectrica protection A p p e n d i x 1 - Te r m i n o o g y • Digita Signa Processor A microprocessor optimised in both hardware architectur e and software instruction set for the processing of ana ogue signa s digita y, through use of the DFT and simi ar techniques Digita Signa Processing A techn ique for the processing of digita signa s by various fi ter a gorithms to obtai n some desired characteristics in the output. The input signa to the processing a gorithm is usua y the digita representation of an ana ogue signa , obtained by A/D conversion Directiona re ay A protection re ay in which the tripping de cision is dependent in part upon the direction in which the measured quantity is f owing Discrimination The abi ity of a protection system to distinguish betwee n power system conditions for which it is intended to operate and those for whic h it is not intended to operate Distortion factor The ratio of the r.m.s. va ue of the harmonic content to the r.m.s. va ue of the non-sinusoida quantity DNP D istributed Network Protoco . A proprietary communication protoco used on second ary networks between HMI, substation computers or Bay Computers and protective d evices DOL Direct-on-Line A1 • • 458 • Network Protection & Automation Guide
e.m.f. E ectro-motive Force ( or vo tage) Energising quantity The e ectrica qua ntity, either current or vo tage, which a ong or in combination with other energ ising quantities, must be app ied to the re ay to cause it to function EPROM E e ctrica y Programmab e Read On y Memory Error (of a transducer) The actua va ue of the output minus the intended va ue of the output, expressed a gebraica y E vent An event is any information acquired or produced by the digita contro sys tem F AT Factory Acceptance Test. Va idation procedures witnessed by the custome r at the factory F a u t Pa s s a g e I n d i c a t o r A sensor that detects the passage of current in excess of a set va ue (i.e. current due to a fau t) at the ocation of the sensor. Hence, it indicates that the fau t ies downstream of the sensor FBD Functiona B ock Diagram: programming anguages One of the IEC 61131-3 G o b a Po s i t i o n i n g S y s t e m A system used for ocating objects on Earth precise y, using a system of sate ites in geostationary orbit in Space . Used by some numerica re ays to obtain accurate time information GMT Greenwic h Mean Time GPS G oba Positioning System G TO Gate Turn-off Thyristor Ha f- dup ex communications A communications system in which data can trave in both dire ctions, but on y in one direction at a time High-speed rec osing A rec osing sch eme where re-c osure is carried out without any time de ay other than that requi red for deionisation, etc. HMI Human Machine Interface. The means by which a hum an inputs data to and receives data from a computer-based system. Usua y takes the form of a Persona Computer (PC) (desktop or portab e) with keyboard, screen and pointing device HRC High Rupturing Capacity (app icab e to fuses) HSR High Speed Rec osing HV High Vo tage HVDC High Vo tage Direct Current I Current ICCP Term used for IEC 60870-6-603 protoco ICT Interposing Current Transformer (soft ware imp emented) I . D . M . T. Inverse Definite Minimum Time I G BT Insu ated Gate Bipo ar Transistor I/O Input/Output IED Inte igent E ectronic Device. Equi pment containing a microprocessor and software used to imp ement one or more fun ctions in re ation to an item of e ectrica • Fiducia va ue A c ear y specified va ue to which reference is made in order to specify the accuracy of a transducer. (For transducers, the fiducia va ue is th e span, except for transducers having a reversib e and symmetrica output when t he fiducia va ue may be either the span or ha f the span as specified by the ma nufacturer. It is sti common practice, however, for statements of accuracy for frequency transducers to refer to ‘percent of centresca e frequency’ and, for phase ang e transducers, to an error in e ectrica degrees.) FPI Fau t Passage Indica tor Frequency transducer A transducer used for the measurement of the frequency of an a.c. e ectrica quantity Fu dup ex communications A communications syste m in which data can trave simu taneous y in both directions Gateway The Gateway is a computer which provides interfaces between the oca computer system and o ne or severa SCADA (or RCC) systems GIS Gas Insu ated Switchgear (usua y SF6) Network Protection & Automation Guide • 459 • A p p e n d i x 1 - Te r m i n o o g y A1 •
equipment (e.g. a bay contro er, remote SCADA interface/protoco converter). A microprocessor-based numerica re ay is a so an IED. IED is a generic term used to describe any microprocessor-based equipment, apart from a computer I G BT Ins u ated Gate Bipo ar Transistor Independent time measuring re ay A measuring re a y, the specified time for which can be considered as being independent, within s pecified imits, of the va ue of the characteristic quantity Inf uence quantity A quantity which is not the subject of the measurement but which inf uences the va ue of the output signa for a constant va ue of the measurand Input quantity The quantity, or one of the quantities, which constitute the signa s received by the transducer from the measured system Instantaneous re ay A re ay that operat es and resets with no intentiona time de ay. Knee-point e.m.f. That sinusoida e.m.f. app ied to the secondary termina s of a current transformer, which, when increased by 10%, causes the exciting current to increase by 50% L Inductance LAN Loca Area Network LCD Liquid Crysta Disp a y LED Light Emitting Diode LD Ladder Diagram. One of the IEC 61131-3 programming anguages LDC Line drop compensator Limiting va ue of the output current The up per imit of output current which cannot, by design, be exceeded under any condi tions Loca Contro Mode When set for a given contro point it means that the co mmands can be issued from this point Lock-out (auto-rec ose) Prevention of a CB rec osing after tripping Long-term stabi ity The stabi ity over a period of one year Low-speed auto-rec ose See De ayed Auto-Rec ose LV Low Vo tage Main protect ion The protection system which is norma y expected to operate in response to a fau t in the protected zone Maximum permissib e va ues of the input current and vo tage Va ues of current and vo tage assigned by the manufacturer which the tr ansducer wi withstand indefinite y without damage MCB Miniature Circuit Breake r MCCB Mou ded Case Circuit Breaker Mean-sensing transducer A transducer which a ctua y measures the mean (average) va ue of the input waveform but which is adj usted to give an output corresponding to the r.m.s. va ue of the input when that input is sinusoida A p p e n d i x 1 - Te r m i n o o g y • NOTE: A re ays require some time to operate; it is possib e, within the above definition, to discuss the operating time characteristics of an instantaneous re ay I n s u a t e d G a t e B i p o a r Tr a n s i s t o r A specia design of transistor that is suitab e for hand ing high vo tages and currents (re ative to an ordinary transistor). Frequent y used in static power contro equipment ( inverters, contro ed rectifiers, etc) due to the f exibi ity of contro of the output Intrinsic error An error determined when the transducer is under referenc e conditions Inverse time de ay re ay A dependent time de ay re ay having an ope rating time which is an inverse function of the e ectrica characteristic quanti ty Inverse time re ay with definite minimum t i m e ( I . D . M . T. ) An invers e time re ay having an operating time that tends towards a minimum va ue with in creasing va ues of the e ectrica characteristic quantity IRIG-B An internationa standard for time synchronisation ISO Internationa Standards Organisation K-b us (K-bus Courier) Term used for the Courier protoco on K-Bus interface for K-R e ay range manufactured by AREVA A1 • • 460 • Network Protection & Automation Guide
Measurand A quantity subjected to measurement Measuring e ement A unit or modu e of a transducer which converts the measurand, or part of the measurand, into a corresponding signa Measuring range That part of the span where the performance comp ies with the accuracy requirements Measuring re ay An e ectrica re ay int ended to switch when its characteristic quantity, under specified conditions and with a specified accuracy attains its operating va ue Metering (non-tariff) Va ues computed depending on the va ues of digita or ana ogue inputs during variab e periods Metering (tariff) Energy va ues computed from digita and/or ana ogue inputs during variab e periods and dedicated to energy measurement for bi ing (tariff) purposes M i d Po i n t S e c t i o n i n g S u b s t a t i o n A subst ation ocated at the e ectrica interface of two sections of e ectrified rai way . It contains provision for the coup ing of the sections e ectrica y in the eve nt of oss of supp y to one section ModBus Proprietary communication protoco us ed on secondary networks between HMI, substation computers or Bay Computers and protective devices MPSS Mid Point Sectioning Substation (e ectrified rai ways) M u ti-e ement transducer A transducer having two or more measuring e ements. The signa s from the individua e ements are combined to produce an output signa co rresponding to the measurand Mu ti-section transducer A transducer having two or more independent measuring circuits for one or more functions Mu ti-shot rec os ing A rec osing scheme that permits more than one rec osing operation of a CB af ter a fau t occurs before ock-out occurs MV Medium Vo tage N/C Norma y C osed N/O Norma y Open Network Protection & Automation Guide Nomina range of use A specified range of va ues which it is intended that an in f uence quantity can assume without the output signa of the transducer changing by amounts in excess of those specified Notching re ay A re ay which switches i n response to a specific number of app ied impu ses NPS Negative Phase Sequence NS Neutra Section (e ectrified rai ways) Numerica re ay A protection re ay whi ch uti ises a Digita Signa Processor to execute the protection a gorithms in s oftware OCB Oi Circuit Breaker O f f - L o a d Ta p C h a n g e r A tap changer that is not designed for operation whi e the transformer is supp ying oad O LTC On Load Tap Changer. O n L o a d Ta p C h a n g e r A tap changer that can be operated whi e the transformer is supp ying oad. Opening time The time betw een energisation of a CB trip coi and the instant of contact parting Operating current (of a re ay) The current at which a re ay wi pick up Operating time (C B) The time between energisation of a CB trip coi and arc extinction Operating time (re ay) With a re ay de-energised and in its initia condition, the time wh ich e apses between the app ication of a characteristic quantity and the instant when the re ay operates Operating time characteristic The curve depicting the r e ationship between different va ues of the characteristic quantity app ied to a re ay and the corresponding va ues of operating time Operating va ue The imiti ng va ue of the characteristic quantity at which the re ay actua y operates • • 461 • A p p e n d i x 1 - Te r m i n o o g y A1 • OHL Overhead ine
OPGW Optica Ground Wire – a ground wire that inc udes optica fibres to provide a communications ink OSI 7- ayer mode The Open Systems Interconnection 7- ayer mode is a mode deve oped by ISO for mode ing of a communications network. Out put common mode interference vo tage An unwanted a ternating vo tage which exist s between each of the output termina s and a reference point Output current (of a transducer) The current produced by the transducer which is an ana ogue functi on of the measurand Output oad The tota effective resistance of the circuits a nd apparatus connected externa y across the output termina s Output power (of a transducer) The power avai ab e at the transducer output termina s Output serie s mode interference vo tage An unwanted a ternating vo tage appearing in series between the output termina s and the oad Output signa An ana ogue or digita r epresentation of the measurand Output span (span) The a gebraic difference betwe en the ower and upper nomina va ues of the output signa Overcurrent re ay A p rotection re ay whose tripping decision is re ated to the degree by which the me asured current exceeds a set va ue. Overshoot time The overshoot time is the dif ference between the operating time of the re ay at a specified va ue of the inpu t energising quantity and the maximum duration of the va ue of input energising quantity which, when sudden y reduced to a specific va ue be ow the operating e ve , is insufficient to cause operation Pa r a m e t r i c c o n j u n c t i v e t e s t A conjunctive test that ascertains the range of va ues of each paramete r for which the test meets specific performance requirements PCB Printed Circuit Board PCC Point of Common Coup ing PED Power E ectronic Device Phase ang e transducer A transducer used for the measurement of the phase ang e between two a.c. e ectrica quantities having the same frequency Pick-up A re ay is said to ‘pick-up’ when it changes from the deenergised position to the energised position Pi ot channe A means of interconnection between re aying points for t he purpose of protection PLC Programmab e Logic Contro er. A specia ised comput er for imp ementing contro sequences using software PLCC Power Line Carrier Com munication Po i n t o f C o m m o n C o u p i n g The interface between an inp ant network containing embedded generation and the uti ity distribution networ k to which the in-p ant network is connected POW Point-on- Wave. Point-on-wave s witching is the process to contro moment of switching to minimise the effects ( inrush currents, overvo tages) Po w e r E e c t r o n i c D e v i c e An e ect ronic device (e.g. thyristor or IGBT) or assemb y of such devices (e.g. inverter ). Typica y used in a power transmission system to provide smooth contro of ou tput of an item of p ant Po w e r f a c t o r The factor by which it is necessar y to mu tip y the product of the vo tage and current to obtain the active power Po w e r L i n e C a r r i e r C o m m u n i c a t i o n A mean of transmitting information over a power transmission ine by using a cariier fraquency superimp ozed on the norma power frequency. PPS Positive Phase Sequence Protected zone T he portion of a power system protected by a given protection system or a part of that protection system Protection equipment The apparatus, inc uding protection re ays, transformers and anci ary equipment, for use in a protection system Pr otection re ay A re ay designed to initiate disconnection of a part of an e ectr ica insta ation or to operate a warning signa , in the case of a fau t or othe r abnorma condition in the insta ation. A protection re ay may inc ude more th an one e ectrica e ement and accessories • A p p e n d i x 1 - Te r m i n o o g y A1 • • 462 • Network Protection & Automation Guide
Protection scheme The co-ordinated arrangements for the protection of one or mor e e ements of a power system. A protection scheme may comprise severa protectio n systems Protection system A combination of protection equipment designed to se cure, under predetermined conditions, usua y abnorma , the disconnection of an e ement of a power system, or to give an a arm signa , or both Protoco A set of ru es that define the method in which a function is carried out – common y used i n respect of communications inks, where it defines the hardware and software fe atures necessary for successfu communication between devices. PSM P ug Setting Mu tip e – a term used in conjunction with e ectromechanica re ays, denoting the ratio of the fau t current to the current setting of the re ay PSTN Pub ic Switc hed Te ephone Network P T 10 0 P atinum resistance temperature probe R Resistanc e R.M.S.-sensing transducer A transducer specifica y designed to respond to the true r.m.s. va ue of the input and which is characterised by the manufacturer f or use on a specified range of waveforms Ratio correction A feature of digita /n umerica re ays that enab es compensation to be carried out for a CT or VT ratio that is not idea Rating The nomina va ue of an energising quantity that appea rs in the designation of a re ay. The nomina va ue usua y corresponds to the C T and VT secondary ratings RCD Residua Current Device. A protection device whic h is actuated by the residua current RCP Remote Contro Point. The Remote Contr o Point is a SCADA interface. Severa RCP’s may be managed with different communi cation protoco s. Physica connections are done at a Gateway or at substation co mputers or at a substation HMI Reactive power (var) transducer A transducer used for the measurement of reactive e ectrica power Rec aim time (auto-rec ose) The time between a successfu c osing operation, mea sured from the time the auto-rec ose re ay c osing contact makes unti a further rec osing sequence is permitted in the event of a further fau t occurring REF R estricted Earth Fau t Reference conditions Conditions of use for a transducer pr escribed for performance testing, or to ensure va id comparison of resu ts of me asurement Reference range A specified range of va ues of an inf uence quantity w ithin which the transducer comp ies with the requirements concerning intrinsic e rrors Reference va ue A specified sing e va ue of an inf uence quantity at which the transducer comp ies with the requirements concerning intrinsic errors Re ay See Protection re ay Residua current The a gebraic sum, in a mu ti-phase system, of a the ine cur rents Residua vo tage The a gebraic sum, in a mu ti-phase system, of a the i ne-to-earth vo tages Response time The time from the instant of app ication of a specified change of the measurand unti the output signa reaches and remains a t its fina steady va ue or within a specified band centred on this va ue Revers ib e output current An output current which reverses po arity in response to a c hange of sign or direction of the measurand Ripp e content of the output With st eady-state input conditions, the peak-to-peak va ue of the f uctuating component of the output r. m . s . Root Mean Square RMU Ring Main Unit ROCOF Rate Of Chan ge Of Frequency (protection re ay) RSVC Re ocatab e Static Var Compensator • Network Protection & Automation Guide • 463 • A p p e n d i x 1 - Te r m i n o o g y A1 • Resetting va ue The imiting va ue of the characteristic quantity at which the r e ay returns to its initia position
RTD Resistance Temperature Detector R TO S Rea Time Operating System RTU Remote Termina Unit. An IED used specifica y for interfacing between a computer and other devices. Sometimes may inc ude contro /monitoring/storage functions S AT S ite Acceptance Test. Va idation procedures for equipment executed with the custo mer on site SCADA Supervisory Contro and Data Acquisition SCL Substation Config uration Language. Norma ised configuration anguage for substation mode ing (as expected by IEC 61850-6) SCP Substation Contro Point. HMI computers at substat ion eve a owing the operators to contro the substation Specific conjunctive test A conjunctive test using specific va ues of each of th e parameters Spring winding time For spring-c osed CB’s, the time for the spring t o be fu y charged after a c osing operation ST Structured Text: One of the IEC 61131-3 programming anguages Stabi ity (of a transducer) The abi ity of a trans ducer to keep its performance characteristics unchanged during a specified time, a conditions remaining constant Stabi ity (of a protection system) The quanti ty whereby a protection system remains inoperative under a conditions other th an those for which it is specifica y designed to operate Stabi ity imits (of a protection system) The r.m.s. va ue of the symmetrica component of the through fau t current up to which the protection system remains stab e Starting re ay A unit re ay which responds to abnorma conditions and initiates the operation of other e ements of the protection system S TATC O M A particu ar type of Static Var Compensator, in which Power E ectronic Devices such as GTO’s are used to gener ate the reactive power required, rather than capacitors and inductors Static re ay An e ectrica re ay in which the designed response is deve oped by e ectronic , magnetic, optica or other components without mechanica motion. Exc udes re a ys using digita /numeric techno ogy S t a t i c Va r C o m p e n s a t o r A dev ice that supp ies or consumes reactive power, comprised so e y of static equipme nt. It is shuntconnected on transmission ines to provide reactive power compens ation S TC Short Time Current (rating of a CT) Storage conditions The conditions , defined by means of ranges of the inf uence quantities, such as temperature, o r any specia conditions, within which the transducer may be stored (non-operati ng) without damage SVC Static Var Compensator A p p e n d i x 1 - Te r m i n o o g y SCS Substation Contro System Setting The imiting va ue of a ‘characteristic’ or ‘ene rgising’ quantity at which the re ay is designed to operate under specified condit ions. Such va ues are usua y marked on the re ay and may be expressed as direct va ues, percentages of rated va ues, or mu tip es SFC Sequentia Function Chart : programming anguages One of the IEC 61131-3 Short-term stabi ity The stabi ity over a period of 24 hours Simp ex communicati ons system A communications system in which data can on y trave in one directio n Sing e-shot rec osing An auto-rec ose sequence that provides on y one rec osin g operation, ock-out of the CB occurring if it subsequent y trips S.I.R. System Impedance Ratio Sing e e ement transducer A transducer having one measuring e e ment SOE Sequence Of Events S OT F Switch on to Fau t (protection) • A1 • • 464 • Network Protection & Automation Guide
System disturbance time (auto-rec ose) The time between fau t inception and CB c ontacts making on successfu re-c osure System impedance ratio The ratio of the power system source impedance to the impedance of the protected zone T 101 Term used for IEC 60870-5-101 protoco Ta p c h a n g e r A mechanism, usua y fitted to the primary winding of a transformer, to a ter the turns ratio of the transf ormer by sma discrete amounts over a defined range TC P / I P Transmission Con tro Protoco /Internet Protoco . A common protoco for the transmission of messa ges over the Internet TC S Trip Circuit Supervision TC 5 7 Technica Committee 5 7 working for the IEC and responsib e for producing standards in the fie d of Pr otection (e.g. IEC 61850) TF a) Transfer Function of a device (usua y an e emen t of a contro system b) Transient Factor (of a CT) Through fau t current The cu rrent f owing through a protected zone to a fau t beyond that zone Time de ay A de ay intentiona y introduced into the operation of a re ay system Time de ay r e ay A re ay having an intentiona de aying device TPI Tap Position Indicator (f or transformers) Transducer (e ectrica measuring transducer) A device that prov ides a d.c. output quantity having a definite re ationship to the a.c. measurand Tr a n s d u c e r w i t h o f f s e t z e r o ( i v e z e r o ) A transducer which gives a predetermined output other than zero when the measurand is zero T r a n s d u c e r w i t h s u p p r e s s e d z e r o A transducer whose output is zero when the measurand is ess than a certain va ue Unit e ectrica re ay A sing e re ay that can be used a one or in combinations with others Unit protection A protection system that is designed to operate on y for abnorma conditions within a c ear y defined zone of the power system Unrestricted prot ection A protection system which has no c ear y defined zone of operation and wh ich achieves se ective operation on y by time grading UCA Uti ity Communications Architecture UPS Uninterruptib e Power Supp y U TC Universa Time Coordinates V Vo tage VCB Vacuum Circuit Breaker VDEW Term used for IEC 60870-5-103 protoco . The VDEW protoco is a subset of the IEC 60870-5-103 protoco Ve c t o r g r o u p c o m p e n s a t i o n A feature of digita and numerica re ays that compe nsates for the phase ang e shift that occurs in transformers (inc uding VT’s) due to use of dissimi ar winding connections – e.g. transformers connected de ta/star Vo t a g e t r a n s d u c e r A transducer used for the measurement of a.c. v o tage VT Vo tage Transformer X Reactance Z Impedance • Network Protection & Automation Guide • 465 • A p p e n d i x 1 - Te r m i n o o g y A1 •
Appendix 2 ANSI/IEC Re ay Symbo s
Appendix 2 ANSI/IEC Re ay Symbo s There are two methods for indicating protection re ay functions in common use. O ne is given in ANSI Standard C37-2, and uses a numbering system for various func tions. The functions are supp emented by etters where amp ification of the func tion is required. The other is given in IEC 60617, and uses graphica symbo s. T o assist the Protection Engineer in converting from one system to the other, a s e ect ist of ANSI device numbers and their IEC equiva ents is given in Figure A 2.1. Description Overspeed re ay ANSI 12 IEC 60617 ω> Descrition Inverse time earth fa ultovercurrent relay Definite time earth fault overcurrent relay Voltage restra ine /controlle overcurrent relay Po er factor relay Overvoltage relay Neutral oint is lacement relay Earth fault relay Directional overcurrent relay ANSI 51G IEC 60617 I > Un ersee relay 14 ω< 51N I >
Distance Overtemerature relay Un ervoltage relay Directional overoer re relay lay Un er oer relay Un ercurrent relay 21 26 27 32 37 37 Ζ< θ> U< > 51V 55 59 59N 64 67 U I> cos ϕ > U> Ursd > I > > P> P< I< I> > Negative sequence relay Negative sequence voltage relay Thermal relay Instantane ous overcurrent relay Inverse time overcurrent relay 46 47 49 50 51 I2 > U2 > Directional earth ault relay Phase angle relay Autoreclose relay 67N 78 79 81U 81O 87 I
> ϕ> Ο f< f> Id> Ι
>> > Underfrequency relay Overfrequency relay Differential relay Figure A2.1 – ANS number/ EC symbol comarison Network Protection & Automation Guide • 467 •
Aendix 3 A l i c a t i o n Ta b l e s
Aendix 3 Alication Tables Table A3.1 contains a list of rotection, control and monitoring devices availab le from AREVA. Due to sace limitations, the functionality of some roducts is s ummarised. The list is accurate at the time of comilation, but new roducts are continually being develoed. For a current list of roducts for a articular a lication, availability of older roducts not listed here, or full details of th e functionality of a secific roduct, lease contact your local AREVA reresent ative, or view details on-line at www.areva-td.com. Network Protection & Automation Guide • 469 •
Alication X N N N N N Thermal rotection by PTC. D N rail or flush mounted N N N X Product 21 P111 P120 P121 P122 X X X X X D X X X X X X D D X X X D X X X N X X X N N X X N X X X X X X X P123 P122C P124S P124D P125 Plain Feeder Overcurrent Protection P126 P127 P130C P132 P139 P141 P142 P143 Network Protection & Automation Guide P145 P821 N: non-directional Dis tan ce rot Che ect ck ion syn chr Un oni d erv sin olta g ge Rev er se ow Pha er se und erc Ne urr gat ent ive seq Bro uen ken ce c o ond ver Ne cur uct gat ren or ive t s equ The enc rm eo al v erv CB olta fail ge ure nst ant ane o ns us tan ha tan se e o ou ver Ti sn me cur eut del ren ral aye t ov T d ime erc ha urr del se ent a o ye ver V dn olta cur eut ren ge ral t de o ver Ove end cur rvo ent ren ltag ove t e rcu Res rren idu al o t ver Res vol tric tag te d e /se Wa nsi ttm tive et ear th Pow ric ne fau utr er s al lt win gb Au tor lo c ecl kin ose g Ove rfre que ncy Un der freq uen Cur cy ren td if fere VT su nti a er l visi CT on s ue rvis ion CT vec tor gro VT u /r vec atio tor gro com T r u/ i C en rat ircu sat io c it S ion o C m ue Bc ens ont rvis atio rol/ i on m n M oni eas tor u rem ing ent Eve s nt rec ord Dis ing tur ban ce Pro rec g ra ord mm er abl Set es tin c hem gG rou e lo s gic
• Aendix 3 A3 • 25 27 32P 37P 46 46BC 47 49 50BF 50P 50N 51P 51N 51V 59 59N 64 67W 78 79 81O 81U 87 VTS CTS X X 2 1 X X N N N X X X X X X X X X X X X X X X X X X X X X X X 1 2 2 2 1 X X 2 2 X X 2 X X X X X Single-hase sensing N N N X N N N X N N N X 3P Also includes Switch-on-to-Fault rotection N N N X Comact case version, also includes Switch-on-to-Fault rote ction N N N X Self owered version N N N X 3P Dual owered version D D D X X X S
ingle-hase sensing D N D X X 3P Also includes Switch-on-to-Fault rotection D D D X X X X 3P X X X X X Also includes Switch-on-to-Fault rotection D D X X X X 3P X X X X X X X X X X X X X X X X X 2 4 4 4 4 4 4 4 2 X X X Comact case X X X X X X X X D D D D X X X X 3P X X X X X X X X X X X X X X X X X D D D D X X X X 3P X X X X X X X X X One-box’ solution, including bay control/mo nitoring of u to 6 switching devices, 200 re-rogrammed bay tyes, Switch-on-t o-Fault rotection, Transient Ground Fault Directional, PT100 RTD inut X X X X X X X D D D D X X X X X X X X X X X X X X X X X X X X X D D D D X X X X X 3P X X X X X X X X X X X X X X X X X D D D D X X X X X 3P X X X X X X X X X Additional features: live line working, sequence co-ordination with downstream reclosing e quiment X X X X X X X X D D D D X X X X X 3P X X X X X X X X X As P143 with add itional 10 functions keys + multicolor LEDs, user configurable X N N N N X X X X 1/3 ole CB fail rotection, also includes dead zone and flash over rotections D: directional/non directional C: Control only M: Monitoring only 1P: Single ol e 3P: Three oles
Alication X X X X X X X X X X X X X X X X X X X X X X X X X D X X X N X X X X X X X X N N X X X X X X X X X X X X X Product 21 P521 P541 Network Protection & Automation Guide P542 P543 X P544 X Plain Feeder Differential Protection Fibre-otic to V35 interface for MiCOM P54x series relays Fibre-otic to X21 interface for MiCOM P54x series relays GPS tim e synchronising module for u to 4 MiCOM P545/6 differential relays X X X N N N N P545 P546 P591 P592 P593 P594 P547 MHOA/B/C MBC N: non-directional Dis tan ce rot Che ect ck ion syn chr Un oni d erv sin olta g ge Rev er se ow Pha er se und erc Ne urr gat ent ive seq Bro uen ken ce c o ond ver Ne cur uct gat ren or ive t s equ The enc rm eo al v erv CB olta fail ge ure nst ant ane o ns us tan ha tan se e o ou ver Ti sn me cur eut del ren ral aye t ov T d ime erc ha urr del se ent a o ye ver V dn olta cur eut ren ge ral t de o ver Ove end cur rvo ent ren ltag ove t e rcu Res rren idu al o t ver Res vol tric tag te d e /se Wa nsi ttm tive et ear th Pow ric ne fau utr er s al lt win gb Au tor lo c ecl kin ose g Ove rfre que ncy Un der freq uen Cur cy ren td if fere VT su nti a er l visi CT on s ue rvis ion CT vec tor gro VT u /r vec atio tor gro com T r u/ i C en rat ircu sat io c it S ion o C m ue Bc ens ont rvis atio rol/ i on m n M oni eas tor u rem ing ent Eve s nt rec ord Dis ing tur ban ce Pro rec g ra ord mm er abl Set es tin c hem gG rou e lo s gic 25 27 32P 37P 46 46BC 47
49 50BF 50P 50N 51P 51N 51V 59 59N 64 67W 78 79 81O 81U 87 VTS CTS X X X X X X X X X X 2 4 4 4 4 X X X X Suitable for 2 and 3 terminal lines D D D D X X Suitable for 2 and 3 terminal lines, and two breaker configurations As P543, with increased /O and GPS As P54 4, with increased /O and GPS Fibre-otic to G703 interface for MiCOM P54x serie s relays X N N N X N N N Suitable for 2 and 3 terminal lines, and transformer feeders N N N 3P Suitable for 2 and 3 terminal lines, and transformer feeders D D D X X 1/3P X • 471 • X X X X X X X 4 Phase comarison differential rotection, using Power Line Carrier communication s X Pilot wire differential rotection for 2 or 3 terminal lines. Recommended on ly where comatibility with existing TRANSLAY electromechanical relays is requir ed X Pilot wire differential rotection. Recommended for metallic ilot wire ro tection u to 2.5Ω , isolation up to 15kV D: irectional/non irectional C: Control only M: Monitoring only 1P: Single pol e 3P: Three poles
•
Appen ix 3 A3 •
Application X Compact case X X X X X X X X X X X X X X X X N N N N X X X X Pro uct 21 P630C P631 P632
Transformer Protection an Control P633 P634 • 472 • KVGC 202 X X X X X N X X N X X X P521 P541
Transformer Fee er Overcurrent /Differential P542 MBCI P634 Busbar Protection P740 MCAG34 MFAC34
Network Protection & Automation Gui e N: non- irectional Dis tan ce p rot Che ect ck ion syn chr Un oni erv sin olta gge Rev er se pow Pha er se un erc Ne urr gat ent ive seq Bro uen ken ce c o on ver Ne cur uct gat ren or ive t s equ The enc rm eo al v erv CB olta fail ge ure I nst antane o Ins us tan pha tan se e o ou ver Tisn me cur eut el ren ral aye t ov T ime erc pha urr el se ent a o ye ver V n olta cur eut ren ge ral t ep o ver Ove en cur rvo ent ren ltag ove t e rcu Res rren i u al o t ver Res vol tric tag te e /se Wa nsi ttm tive et ear th Pow ric ne fau utr er s al lt win gb Au tor lo c ecl kin ose g Ove rfre que ncy Un er freq uen Cur cy ren t if fere VT su nti a per l visi CT on s upe rvis ion CT vec tor gro VT u p/r vec atio tor gro com T r up/ ip C pen rat ircu sat io c it S ion o C mp upe Bc ens ont rvis atio rol/ i on m n M oni eas tor u rem ing ent Eve s nt rec or Dis ing tur ban ce Pro rec g ra or mm er abl Set es tin c hem gG rou e lo ps gic •
Appen ix 3 A3 •
25 27 32P 37P 46 46BC 47 49 50BF 50P 50N 51P 51N 51V 59 59N 64 67W 78 79 81O 81U 87 VTS CTS X X X X X X X X X X X X X X X X X X X X X X X 4 4 4 4 4 2 X X
N N N N X X Suitable for 2 win ing transformers. Harmonic/overfluxing/CT saturat ion restraint X X N N N N X X X X X X X X Suitable for 2 win ing transformers. H armonic/overfluxing/CT saturation restraint, a itional PT100, overfluxing prote ction X X N N N N X X X X X X X X Suitable for 3 win ing transformers. Harmonic/ overfluxing/CT saturation restraint, a itional PT100, overfluxing protection X X N N N N X X X X X X X X Suitable for 4 win ing transformers. Harmonic/overflux ing/CT saturation restraint, a itional PT100, overfluxing protection X X Voltage regulating control relay for automatic tap change control N N N N X X X X X X X X X X X X X X X X X X X X X X X X X X 2 4 4 X
X
N N N Suitable fee ers N N N 3P Suit for 2 an 3 terminallines, an transformer able for 2 an 3 terminal lines, an transformer fee ers X X X X C X X X X X X X X 4 4 X
X Translay S pilot wire ifferential protection. Recommen e for metallic pilo t wire protection up to 2.5kΩ, up to 15kV X X X N N N N X X X X X X X X isolation Suitable for mesh busbars, an fixe configuration zones with up to four biase inputsX N N N N XX X High-impe ance ifferential protection relay (current cal ibrate ) High-impe ance ifferential protection relay (voltage calibrate ) D: irectional/non irectional C: Control only M: Monitoring only 1P: Single pol e 3P: Three poles
Application X X X X X X X X X X X Pro uct 21 P430C P432
Network Protection & Automation Gui e P433 P435 X Distance Protection D X X X D D D D X X X 3P X P437 • 473 • P439 P441 P442 P443 D X X D D X X X D D D D X X P444
Mains/Gri Interface P341 P342 Generator Protection P343 MX3IPG2A P941/3 N: non- irectional
Dis tan ce p rot Che ect ck ion syn chr Un oni erv sin olta gge Rev er se pow Pha er se un erc Ne urr gat ent ive seq Bro uen ken ce c o on ver Ne cur uct gat ren or ive t s equ The enc rm eo al v erv CB olta fail ge ure I nst antane o Ins us tan pha tan se e o ou ver Tisn me cur eut el ren ral aye t ov T ime erc pha urr el se ent a o ye ver V n olta cur eut ren ge ral t ep o ver Ove en cur rvo ent ren ltag ove t e rcu Res rren i u al o t ver Res vol tric tag te e /se Wa nsi ttm tive et ear th Pow ric ne fau utr er s al lt win gb Au tor lo c ecl kin ose g Ove rfre que ncy Un er freq uen Cur cy ren t if fere VT su nti a per l visi CT on s upe rvis ion CT vec tor gro VT u p/r vec atio tor gro com T r up/ ip C pen rat ircu sat io c it S ion o C mp upe Bc ens ont rvis atio rol/ i on m n M oni eas tor u rem ing ent Eve s nt rec or Dis ing tur ban ce Pro rec g ra or mm er abl Set es tin c hem gG rou e lo ps gic 25
27 32P 37P 46 46BC 47 49 50BF 50P 50N 51P 51N 51V 59 59N 64 67W 78 79 81O 81U 87 VTS CTS X X X X X X X X X X X X X X X X X X X X X X 4 4 4 4 4 4 4 4 4 X X X X X X
X X X D D D D X X X 3P X X X X C Compact esign case for MV/HV applications wher e 3ph tripping/reclosing is require . Inclu es protective signaling, SOTF an BU protection X X X D D D D X X X 1/3P X X X X X EHV ‘One-box’ solution, inclu ing bay control/monitoring of up to 6 switching evices, 200 pre-programme bay types, SOTF etection X X X X X X X
X X X D D D D X X X 3P X X X X I eal for Peterson groun e or insulate MV netwo rks.Inclu es Directional power protection, stea y state an transient groun fa ult etection X X X D D D D X X X X 1/3P X X X X Same as P433, an in a ition s uitable for HV/EHV applications where PSB, 1/3ph tripping/reclosure an CS are r equire X X D D D D X X X 1/3P X X X
X X X X
Comprehensive 1 pole/3 pole tripping schemes for all applications inclu ing EHV transmission X X X X X X D D D D X X 3P X X X X X X X ‘One-box solution for MV/HV , inclu ing bay control/monitoring of up to 6 switching evices, 3ph tripping/re closing, switch-on-to-Fault etection, PT100 RTD input X X X X
Suitable for MV/HV applications where 3ph tripping/reclosing is require . Inclu esTOR, SOTF, DEF, Emergency Overcurrent on VT failure, an non conventional CT an VT inputs (option) X X X X D X X X D D D D X X X 1/3P X X X X X X X Same as P441 - universal istance relay for single or three pole tripping X X
X X D X X X D D D D X X X 1/3P X X X X X X MiCOMho subcycle istance an irecti onal comparison protection. Full scheme protection with TOR, SOTF, DEF, an 1/3P reclosing. Mho an qua rilateral characteristics X 1/3P X X X X X X X X X X X X X X X X X X X X X X X X 4 4 4 X X X
Same as P442 with increase number of igital I/O D D X X D X X X
X
Inclues ROCOF an Voltage Vector Shift protection functions. 64 functions is ei ther irectional/nonirectional SEF, or REF X X X X X N N N N XX X D X X X X X X M X Also inclu es loss of excitation, overfluxing, stator win ing temperature using PT100 RTD’s, 64 functions is either irectional/non- irectional SEF, or REF X
X XX X X X N N N N X X X D X X X X X X X M X X X X 4 Also inclu es 100% Stator win ing energisation a earth fault, loss of excitation, overfluxing, ina vertent t stan still, stator win ing temperature using PT100 RTD’s, an enhance pole-slip ping protection Rotor earth fault etectionan DC over/un ervoltage X X X X X X X X 4 Inclu es generator abnormal frequency protection, ROCOF an frequency super vision characteristics D: irectional/non irectional C: Control only M: Monitoring only 1P: Single pol e 3P: Three poles •
Appen ix 3 A3 •
Application Pro uct 21 P438 P436
A.C. Electrifie Railway Protection P138 P638 P130C Un er/Over Voltage/ Frequency X X Inclu es ROCOF protection X X X X X X X X X X X P921 P922 • 474 • P923 P941 Loa She ing P942 P943 KAVR 130 Auto-reclose P842
Network Protection & Automation Gui e N: non- irectional Dis tan ce p rot Che ect ck ion syn chr Un oni erv sin olta gge Rev er se pow Pha er se un erc Ne urr gat ent ive seq Bro uen ken ce c o on ver Ne cur uct gat ren or ive t s equ The enc rm eo al v erv CB olta fail ge ure I nst antane o Ins us tan pha tan se e o ou ver Tisn me cur eut el ren ral aye t ov T ime erc pha urr el se ent a o ye ver V n olta cur eut ren ge ral t ep o ver Ove en cur rvo ent ren ltag ove t e rcu Res rren i u al o t ver Res vol tric tag te e /se Wa nsi ttm tive et ear th Pow ric ne fau utr er s al lt win gb Au tor lo c ecl kin ose g Ove rfre que ncy Un er freq uen Cur cy ren t if fere VT su nti a per l visi CT on s upe rvis ion CT vec tor gro VT u p/r vec atio tor gro com T r up/ ip C pen rat ircu sat io c it S ion o C mp upe Bc ens ont rvis atio rol/ i on m n M oni eas tor u rem ing ent Eve s nt rec or Dis ing tur ban ce Pro rec g ra or mm er abl Set es tin c hem gG rou e lo ps gic •
Appen ix 3 A3 • 25 27 32P 37P 46 46BC 47 49 50BF 50P 50N 51P 51N 51V 59 59N 64 67W 78 79 81O 81U 87 VTS CTS X X X X 4 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X 4 4 4 1 2 2 4 X X X D D X -to fault As P438, e for all
X X M Catenary Protection for 25, 50, 60Hz systems. Inclu es switch-on an efrost protection, train start-up restraint, wrong phase coupling for 16 2/3Hz systems X N N N N X M Busbar/fee er protection. Applicabl system frequencies X X N N N N X X M
X X
Transformer protection. Inclu es Buchholz, tank-earth an overfluxing protection . Applicable for all system frequencies X X X Compact case; no CTs optional vers ion of P130C X X X X X
Frequency protection inclu es ROCOF an frequency supervision characteristics. L
oa restoration function also available As P941 plus 7 outputs As P941 plus 7 ou tputs an 8 inputs X X X X X X X 2 4 X X
1/3P High spee 1-Pole/3-Poleauto-reclose/3 Pole elaye auto-reclose relay wit h check synchronism 3P Delaye auto-reclose of 2/3 CB s in mesh-connecte substa tion, inclu ing auto-isolation of circuit isconnectors D: irectional/non irectional C: Control only M: Monitoring only 1P: Single pol e 3P: Three poles
Application Pro uct 21 Check Synch KAVS 100 Motor Protection Tap Change Control Times Delay Relays Control/Tripping relays Intertripping Interposing Relays Auxiliary Relays Test Blocks/Plugs Control mo ule
X X 2 X X N N X 1 P211 LV motors only. Also inclu es prolonge starting/stalle rotor motor, win ing temperature tripping using PTC input. DIN rail or flush mou nte X X X N N NX X X X X X2 P220 Also inclu es protection against excessive s tart time, locke rotor, win ing temperature measurement/trip, spee switch inpu t. Facilities for number of starts limitation an re-acceleration X X X X X N N N N XX X X X X X 2 P225 Also inclu es protection against excessive start time, locke rotor, Facilities for number of starts limi reverse power, anti-backspin. tation, win ing measurement/trip, spee switch input an re-acceleration X X X X X N N N N X X X X X X X 2 P226C Compact case; ual motor protection mo e X X X X X X X N N N N X X D X XX X X X X X 2 P241 Also inclu es protection against ex cessive start time, locke rotor, stalling, reverse power. Facilities for number of win ing temperature measurement/trip using PT100 RTD s sp starts limitation, ee switch input an re-acceleration. Out of step protection for synchronous mot ors P242 X X X X X X X N N N N X X D X X X X X X X X 2 P243 X X X X X X X N N N N X X D X X X X X X X X X 2 KVGC202 Tap change controller, inclu ing line rop c ompensation, circulating current control, reverse current blocking, runaway prot ection, tap changer maintenance monitor an failure etection, tap position in i catorMVTT Digital time elay relay - either elay on pick-up or rop-off MVAJ H eavy uty electromechanical control/CB tripping/intertripping relays, available in a variety of configurations GCMO5 High spee intertripping via pilot wires. I nbuilt pilot isolation MVAW Electromechanical interposing relays for remote cont rol of CB’s, etc. Insensitive to a.c. voltages.Han reset flagavailable PRIMA Co mpact electromechanical auxiliary relays in han , electrical an self reset vers ions, with or without flag MVAA Electromechanical auxiliary relays in han , elec trical an self reset versions, with or without flag. Greater current carrying/b reaking capacity than Prima relays MVUA Time elaye version of the MVAA relay M MLB Test plug for use with MMLG test blocks. Single or multi/finger esign MMLG Test block for use with all varieties of protection relays, particularly MiDOS s eries relays Test block for use with all varieties of protection relays P991 P99 2 Multi-finger test plug for use with P991. Visible automatic shorting of CT cir cuits on insertion into test blocks Single finger test plug for use with P991. F or CT circuit monitoring isolate voltage output P993 It can be combine with 4U protection mo ules, an provi es in epen ent tripping an closing functionali ty as well as CB position in ication P891 N: non- irectional
Dis tan ce p rot Che ect ck ion syn chr Un oni erv sin olta gge Rev er se pow Pha er se un erc Ne urr gat ent ive seq Bro uen ken ce c o on ver Ne cur uct gat ren or ive t s equ The enc rm eo al v erv CB olta fail ge ure I nst antane o Ins us tan pha tan se e o ou ver Tisn me cur eut el ren ral aye t ov T ime erc pha urr el se ent a o ye ver V n olta cur eut ren ge ral t ep o ver Ove en cur rvo ent ren ltag ove t e rcu Res rren i u al o t ver Res vol tric tag te e /se Wa nsi ttm tive et ear th Pow ric ne fau utr er s al lt win gb Au tor lo c ecl kin ose g Ove rfre que ncy Un er freq uen Cur cy ren t if fere VT su nti a per l visi CT on s upe rvis ion CT vec tor gro VT u p/r vec atio tor gro com T r up/ ip C pen rat ircu sat io c it S ion o C mp upe Bc ens ont rvis atio rol/ i on m n M oni eas tor u rem ing ent Eve s nt rec or Dis ing tur ban ce Pro rec g ra or mm er abl Set es tin c hem gG rou e lo ps gic 25 27 32P 37P 46 46BC 47 49 50BF 50P 50N 51P 51N 51V 59 59N 64 67W 78 79 81O 81U 87 VTS CTS X
Network Protection & Automation Gui e X • 475 • D: irectional/non irectional C: Control only M: Monitoring only 1P: Single pol e 3P: Three poles Table A3.1: AREVA Equipment Application List • Appen ix 3 A3 •
In ex
In ex Section Page AREVA equipment application tables . . . . . . . . . . . . . . . . . . . . . . . 20.1-20.8 . . . . 353-369 Section Page A A.C. electrifie railway protection . . . . . . . . Acceleration scheme ( istanc e relays) . . . . . . . . A3 . . . . 469-475 Arc resistance formula . . . . . . . . . 11.7.3 . . . . . . . . 178 . . . . . . . . . . 12.3.3 . . . . . . . . 196 . . . . . . . . 85 Armature reaction, of synchronous machines Arrangement of busbar protection sche mes . . . . . . . . . 5.3 . . . . . . . . 49 Accuracyof transformers A.C. pilot wire protection, unit protection of current multi-en e fee ers . A itional features of numerical relays . . . 6.4.1 . . . . . . . 15.7 . . . . . . . . 239 . . . . . . . 13.3.1 . . . . . . . . 207 . . . . . . . . . . . . . 7.6 . . . . . . . . 107 Arrangement of CT connections: - in high impe ance busbar protection . . . . . . . . . - in low impe ance busbar protection . . . . . . . . . Asymmetry of synch ronous machine . . . . . . . . . . . . Asynchronous running Auto-close circuits Attracte armature relays . . 15.8.5 . . . . . . . . 245 . . 15.9.5 . . . . . . . . 248 Air circuit breakers (ACB’s), use in auto-reclose schemes Algebra, vector Analogue trans ucers Annunciator testing ANSI relay numbers . . . . 14.6.3.2 . . . . . . . . 224 . . . . . . . . 18 . . . . . . . . 5.7
. . . . . . . . 53 . . . . . . . . . . . . . . . . 3.2 . . . . . . . . . 17.16 . . . . . . . . 298 . . . . . . . . 7.2.1 . . . . . . . . 100 . . . . . . . . . . . 22.4 . . . . . . . . 402 . . . . 4.3 . . . . . . . . 33 Analysis, symmetrical component . . . . . . . . . . . . 14.11 . . . . . . . . 229 . . . . . . . . . . . 21.14 . . . . . . . . 396 . . . . . . . . . . . . . A2 . . . . . . . . 467 Anti-pumping evices, auto-reclose schemes . Automatic changeover systems for in ustrial/commercial networks . . . . . . . . . . . . . . . Automatic reclosers Automatic sectionalisers . . . . 18.9 . . . . . . . . 326 . . . . . . . . 14.10.5 . . . . . . . . 228 . . . . . . . . . . . 25.5.2 . . . . . . . . 449 . . . . . . . . 25.5.2 . . . . . . . . 449 Apparent impe ance seen by istance relays, multi-en e fee ers . . . Applicatio n of irectional overcurrent relays . . . . . . . . . . . . 13.4.1 . . . . . . . . 210 . . . . . 14.2 . . . . . . . . 219 Application of auto-reclosing .
Automation: - application tables . . . . . . . . . . . . . A3 - istribution sys tems . . . . . . . 25.1-25.7 - substations . . . . . . . . . . . . 24.1-24.7 . . . . 469-475 . . . . 443-453 . . . . 423-441 . . . . . . . 9.14.3 . . . . . . . . 134 Application tables for AREVA equipment . . . . . . . . . . . . . . . . . . . A3 . . . . 469-475 Application of unit protection systems to mesh corner substations . . . . . . . . . . . . . . . . 10.8.2 . . . . . . . . 159
Application of unit protection to breaker an a half substations . . 10.8.2 . . . . . . . . 159
Auto-reclosing: . . . . . . . . . 14.1-14.12 . . . . 219-231 - anti-pumping evi ces . . . . . . . 14.10.5 . . . . . . . . 228 - application of . . . . . . . . . . . . . . 14.2 . . . . . . . . 219 - circuit breaker characteristics . . . . . . . 14.4.1.3, 14.6.3 . . . . 221-224 - ea time . . . . . . . 14.4.1, 14.6.4, 1
4.9 221-225-226 - e-ionisation of fault path . . . . . . . . . . 14.4.1.4, 14.6 .2 . . . . 222-224 - elaye . . . . . . . . . . . . . . . . . . 14.9 . . . . . . . . 226 - high-spee . . . . . . . . . . . . . . . . 14.8 . . . . . . . . 226 • 477 • Network Protection & Automation Gui e
Section Page Balance voltage unit protection scheme for Tee’ fee ers . . . . . Bar primary cu rrent transformers . . . . . Section Page - initiation of . . . . . . . . . . . . . . 14.10.1 . . . . . . . . 228 - lock-o ut . . . . . . . . . . . . . . . . 14.10.7 . . . . . . . . 228 - multi-shot . . . . . . . . . . . . . . 14.10.9 . . . . . . . . 229 - number of shots (attempts) . . . . . . . . . . 14.4.3, 14.6.6 . . . .223-225 - on EHV lines . . . . . . . . . . . 14.5-14.9 . . . . 223-226 - on HV istribution networks . 14.3-14.4 . . . . 220-221 - operating features . . . . . . . . . . 14.10 . . . . . . . . 227 - reclaim time . . . . . . . . 14.4.2, 14.10.6 . . . . 222-228 - reclosing impul se . . . . . . . . . . 14.10.4 . . . . . . . . 228 - reset time . . . . . . . . . . . . . . 14.4.1.5 . . . . . . . . 222 - single-phase . . . . . . . . . . . . . . . 14.7 . . . . . . . . 225 - single-shot . . . . . . . . . . . . . . . 14.6. 6 . . . . . . . . 225 - system stability an synchronism . . . . . . . . . . 14. 4.1.1 . . . . . . . . 221 - three-phase . . . . . . . . 14.3, 14.6, 14.9 . 220-2 23-226 - type of loa . . . . . . . . . . . . . 14.4.1.2 . . . . . . . . 221 - u se with blocking schemes . . . . 14.8.1 . . . . . . . . 226 - use with transfer trip protection schemes . . . . . . . . . 14.8.1 . . . . . . . . 226 - use with zone 1 extension scheme . . . . . . . . . . . . . . . . . 14.8.2 . . . . . . . . 226 Auto-synchroniser . . . . . . . . . . . . 22.8.2 . . . . . . . . 407 . . 13.3.2 . . . . . . . . 207 . . . . . . . . . . 6.4.5.2 . . . . . . . . 87
Basic circuit laws,theorems an network re uction . . . . Bearing failures on m otors Behaviour of istance relays with earth faults . . . . . . . . Biase iff erential relays . . . . . . . 3.6 . . . . . . . . 24 . . . . . . . 19.10 . . . . . . . . 348 . . . . 13.2.2.3 . . . . . . . . 204 . . . 10.4.2 . . . . . . . . 155 Bias in unit protection systems . . . . . . . . 10.4.2 . . . . . . . . 155 schemes, istance protection: . . . . . . . . . . . 12.4 . - multi-en e Blocking fee ers . . . . . . . . . 13.5.3 . - using zone 1 element . . .. . . . 12.4.1 .2 . - using zone 2 element . . . . . . . 12.4.1.1 . - weak infee con itions . . . . . . . 12.4.2 . Boun ary characteristic of istance relay . . . . . . . . B ranch law Breaker fail protection . . . . . . . 198 . . . . . . . 199 . . . . . . . 198 . . . . . . . 199 . . . . . . . 213
. . . . . . . . 11.2 . . . . . . . . 172 . . . . . . . . 24 . . . . . . . . . . . . . . . . 3.6.1.1 •
Auto-transformer: - equivalent circuits . . . . . . . . . . . 5.16 - Fee ing of a.c. electrifie railways: . . . . . . . . . . . . . . . . . 20.6 . backup prote ction . . . . . . . . . 20.6.6 . escription . . . . . . . . . . . . . . 20.6.1 . istance protection zonereaches . . . . . . . . . . . . . . . . 20.6.3 . ist ance protection zone time elay settings . . . . . . . . . . . . 20.6.4 . implic ation of two-pole switching an auto-reclosure . . . . . . . . 20.6.5 . protecti on philosophy . . . . . . . 20.6.2 . - positive sequence equivalent circuit . . . . . . . . . . . . . . .. . . 5.16.1 - protection . . . . . . . . . . . . . . . . 16.12 . - special con itions of neutral earthing, zero sequence reactance . . . . . . . . . . . . . . . . 5.16.3 - zero sequence equivalent circuit . . . . . . . . . . . . . . . . . . 5.16.2 . . . . . . . . 15.9.6.6 . . . . . . . . 250 . . . . . . . 14.4.1.3 . . . . . . . . 221 . . . . . . . . 59 . . . . . . . 361 . . . . . . . 365 . . . . . . . 361 . . . . . . . 363 . . . . . . . 364 Breaker operating times Broken elta connection of voltage transformers . . . . Buchholz protection Buch holz relay . . . . . . . 6.2.6 . . . . . . . . . 81 . . . . . . . . . . 16.15.3 . . . . . . . . 269 In ex . . . . . . . . . . . . . 16.15.3 . . . . . . . . 269 . . . . . . . 364 . . . . . . . 363 . . . . . . . . 59 . . . . . . . 267 . . . . . . . . 60 . . . . . . . . 60 Busbar: - faults . . . . . . . . . . . . . . . . . . . . 15.2 . . . . . . . . 23 4 - principles . . . . . . . . . . . . . . . . . 15.7 . . . . . . . . 239 - Prot ection: . . . . . . . . . . . 15.3-15.10 . . . . 234-251 - Differential . . . . . . . . . . . . 15.7-15.10 . . . . 239-251 high impe ance . . . . . . . . . . . . 15.8 . . . . . . . . 241 low impe ance . . . . . . . . . . . . 15.9 . . . . . . . . 246 numerical . . . . . . . . . . . . . . . 15.10 . . . . . . . . 251 - fr ame-earth (Howar ) . . . . . . . . 15.6 . . . . . . . . 236 - mesh corner . . . . . . . . . . . . . 15.7.2.1 . . . . . . . . 240 - schemes . . . . . . . . . . . . . . . . . . 15.5 . . . . . . . . 236 - spee . . . . . . . . . . . . . . . . . . . 15.3.1 . . . . . . . . 234 - stability . . . . . . . . . . . . . . . . . 1 5.3.2 . . . . . . . . 234 - types of protection system . . . . . . . .15.4 . . . . . . . . 235 Bushing current transformers . . . . 6.4.5.2 . . . . . . . . 87 B Back-up protection . . . . . . . . . 2.9, 20.5
. . . . . 11-361 C Cable circuits . . . . . . . . . . . . . . . . 5.18 . . . . . . . . 62 . . . . . . . . 69 Balance voltage system, unit protection . . . . . . . . . . . . . . . 10.5 . . . . . . . . 156 Cable ata: • 478 • . . . . . . . . . . . . . . . . . 5.24
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Section Cable glan insulation in frame-earth protection schemes . . . . . . . . . . . C alculation of overcurrent relay settings . . . . . . . . Page Choice of harmonic (transformer protection) Section . . . . . . . Page . . . . . . . 264 16.9.2 . . . . . . . . 15.6.1 . . . . . . . . 236 Choice of reclaim time . . . . . . . . . 14.6.5 . . . . . . . . 225 . . . . . . . . 9.13 . . . . . . . . 132 Calculationof series impe lines an cables) . . . Calculation of ance (overhea shunt impe ance (overhea lines an cables) . . . Calculations, fault Calculati ons, examples of – see example relay setting calculations Capacitor control . . . 5.19 5.20 . . . . . . . . 62 . . . . . . . . . . . 63 . . . 31-35-41 . . . . . . . . 4.2, 4.4-4.6 . . . . . . . . . . . . 18.11.1 . . . . . . . . 328 . . . . . . . . . . . 18.11 . . . . . . . . 328 . . . . Capacitor protection Capacitor voltage transformers: - transient behaviour . . . . . . - voltage prot ection of . . . . . Carrier channels: Carrier unit protection systems . Casca e voltage transformers Catenary thermal protection Catenary back-up protection 6.3 . . . . . . . . 83 . . . . . . . . 84 . . . . . . . . 84 . . . . . . . . 117 . . . . 6.3.2 . . . . 6.3.1 Circuit breakers: - Characteristics: .3 . . . . . . . . 221 ea time . . . . 225 for auto-reclose schemes . . 24 interrupting time . . . . 9.11.1,
closing time . . . . . . . . . . . . 14.4.1 . . . . . . . . . . . . 14.6.4 . . . . . . . . . . . . . 14.4.1.3, 14.6.3, . . . 221-2 14.4.1.3 . . . . 131-221 monitoring in nume
rical relays . . . . . . . . . . . 7.6.3 . . . . . . . . 108 opening time . . . . . . . . . . . 14.4.1.3 . . . . . . . . 221 - Types: air . . . . . . . . . . . . . . . . . . . 18.5.3 . . ... . . . 322 miniature (MCB) . . . . . . . . . 18. 5.1 . . . . . . . . 320 moul e case (MCCB) . . . . . . 18.5.2 . . . . . . . . 3 21 oil . . . . . . . . . . . . . . . . . . . . 18.5.4 . . . . . . . . 322 SF6 . . . . . . . . . . . . . . . . . . . 18.5.6 . . . . . . . . 322 vacuum . . . . . . . . . . . . . . . . 18.5.5 . . . . . . . . 322 Circuit laws . . . . . . . . . . . . . . . . . 3.6.1 . . . . . . . . . . . . . . 3.4 . . . . . . . . . . . . . . 3.6.2 . . . . . . . . . . . . . . 3.4.1 . . . . . . . . 24 . . . . . . . . 20 . . . . . . . . 24 . . . . . . . . 20 . . . . . . . . . . . . . 8.6.3 . . . . 10.9 . . . . . . . . 160 . . . . . . . . 82 Circuit quantities Circuit theorems Circuit variables . . . . . 6.2.8 . . . . . . 20.4 . . . . . . . . 360 . . . . . . 20.5 . . . . . . . . 361 CBCT – see Core balance current transformers Characteristic angle of a relay Chara cteristic time/current curves of IDMT relays . . . . . . . . 9.14.2 . . . . . . . . 133 Circulating current system, unit protection . . . . . . . . . . . . . . . 10.4 . . . . . . . . 154 . . . . . . . . 9.4 . . . . . . . . 126 Characteristics: - of circuit breakers . . . . . . . . . 14.4.1.3 . . . . . . . . 221 - of istance relays . . . . . . . . . . . . 11.7 . . . . . . . . 175 - of generators . . . . . . . . . . . . . 5.2-5.6 . . . . . . 47-51 - of motors . . . . . . . . . . . . . 19.2-19.7 . . . . 337-346 - of overcurrent relays . . . . 9.4-9.8, 9.14, 126-129-133 . . . . . . . . . . . . . . . . . . . . . . . . 9.16 . . . . . . . . 136 Check feature for frame-earth busbar protection . . . . . . . . Check synchroniser . . . . 15.6.4 . . . . . . . . 238 Classical single-phase fee ing of a.c. electrifie railways: . . . . . . . 20.3 . - istance protection zone reaches . . . . . . . . . . . . . . 20.3.3 . - fee ing iagram . . . . . . . . . . . 20.3.1 . - impact of regenerative braking . . 20.3.6 . - loa avoi ance . . . . . . . . . . . . 20.3.4 . - mo ern relay charac teristics . . . 20.3.5 . - protection philosophy . . . . . . . . 20.3.2 . Classi fication of power system isturbances . . . . . . . . . . . . Class PX (IEC 6004 4) current transformers . . . . . . . . . . Class X (BS3938) current transformer s . . . . . . . . Closing impulse time . . . . . . . 356 . . . . . . . 354 . . . . . . . 359 . . . . . . . 359 . . . . . . . 359 . . . . . . . 355 . . . . . 23.2 . . . . . . . . 411 . . . . . . 6.4.4 . . . . . . . . 86
. . . . . . 14.9.2, 22.8.1 . . . . 227-406
Check system: - for frame-earth protection . . - for high impe ance busbar prote ction . . . . . . . . . - for low impe ance protection Choice of ea time . . . . . . . . 6.4.4 . . . . . . . . 86 . . 15.6.4 . . . . . . . . 238 . . 15.8.3 . . . . . . . . 244 . . 15.9.3 . . . . . . . . 248 . . . . . . . . . 14.10.4 . . . . . . . . 228 . . 14.4.1.3 . . . . . . . . 221 Closing time of circuit breakers Co-or ination problems in in ustrial an commer cial networks . . . . . . . . . . . . . • 479 • . . . . . . . . . . 14.6.4 . . . . . . . . 225 . . . . . . 18.7 . . . . . . . . 324 Network Protection & Automation Gui e In ex • . . . . . . . 354
Section Combine ifferential an restricte earth fault protection of transformers Page Cross country fault analysis Section . . . . . . 4.4.6 Page . . . . . . . . 37 . . . . . . . 16.10 . . . . . . . . 264 Current ifferential protection scheme . . . . . . . . . . . . . . . . 10.8, 10. 10 Current istribution ue to a fault . . . . . . . . . . . . . . . . . . . . 4.5 . . . . . . . 4.5.1 . . . . 158-160 Combine I.D.M.T. an high set instantaneous overcurrent relays Commissioning te sts Communication links . . . . 8.1, . . . . 9.5 . . . . . . . . 127 . . . . . . . . 38 . . . . . . . . 39 . . . . . . . . . . . 21.9 . . . . . . . . 387 Current istribution factors 24.4, 25.6 . 113-427-451 Communication protocols . . . . . . . . 24.4 . . . . . . . . 427 Current polarisation of irectional earth fault relays . . . . . . . . . . . 9.1 7.1.2 . Current reversal on ouble circuit lines, istance protection . . . . . . . 139
Communications for istribution networks . . . . . . . . . . . . . . . . Compari son of transfer trip an blocking schemes . . . Complex numbers Complex variable s . . . 25.6 . . . . . . . . 451 . . . . . . . . . . 13.2.2.1 . . . . . . . . 204 . . . . . . . . 9.10 . . . . . . . . 129 . . . . . . . 12.6 . . . . . . . . 200 . . . . . . . . 19 . . . . . . . . 19 . . . . . . . . 18 Current setting of a relay . . . . . . . . . . . . . 3.3.2 . . . . . . . . . . . . . 3.3.1 . . . . . . . . . . . . . 3.3
Complex quantities Complex transmission circuits, protection of . . . . . . . . . . . . . 13.1-13.7 Composite error of current transformers . . . . . . . . . . . . . . . . 6.4.2 . . . . 203-215 . . . . . . . . 86 Con ition monitoring: - of circuit breakers . . . . . . . . . . 24.5.1 - of gene rators . . . . . . . . . . . . . 24.5.1 - of switchgear . . . . . . . . . . . . . 24.5.1 - of transformers . . . . . . . 16.18, 24.5.1 Con itions for irection comparison, unit protection . . . . . . . . 431 . . . . . . . . 431 . . . . 273-431 . . . . . . . . 431 . . . . . . . 10.3 . . . . . . . . 154 Connections for irectional relays . . . . . . . . . . . . . . Contact systems f or relays . . 9.14, 9.17.1 . . . . 133-139 . . . . . . . . 12 . . . . . . 2.10.1 • Control equipment application tables . . . . . . . . . . . . . . . . Convention of irection of current flow, unit protection Co-or ination of earth fault relay s in three-phase four wire systems . . . . . . . . . . . . . A3 . . . . 469-475 . . . . . . 10.2 . . . . . . . . 154 . . . . . 18.7.2 . . . . . . . . 325 Co-or ination proce ure for overcurrent protection relays Core-balance current t ransformers . . . . . . . . . . 9.2 . . . . . . . . 123 . . . . . . . 132 Co-or ination of relays with fuses . 9.12.3 . . . . . . . . . . 6.4.5.3 Current transformers: . . . . . . . . 6.4-6.5 . . . . . . 85-96 - accuracy . . . . . . . . . . . . . . . . . 6.4.1 . . . . . . . . 85 - accuracy class . . . . . . . . . . . . . . 6.4.2 . . . . . . . . 86 - accuracy limit current . . . . . . . . 6.4.3 . . . . . . . . 86 - bar primary . . . . . . . . . . . . . . 6.4.5.2 . . . . . . . . 87 - bushing . . . . . . . . . . . . . . . . . 6.4.5.2 . . . . . . . . 87 - class X (BS 3938) . . . . . . . . . . . 6.4.4 . . . . . . . . 86 - c lass PX (IEC 60044-1) . . . . . . . 6.4.4 . . . . . . . . 86 - construction . . . . . . . . . . . . . . . 6.4.5 . . . . . . . . 87 - equivalent circuit . . . . . . . . . . . . . 6.4 . . . . . . . . 85 - errors . . . . . . . . . . . . . . . . . . . 6.4.1 . . . . . . . . 85 - hall-effect . . . . . . . . . . . . . . . 6.5
.2.1 . . . . . . . .96 - harmonics in . . . . . . . . . . . . . . 6.4.11 . . . . . . . . 92 - hybri . . . . . . . . . . . . . . . . . . 6.5.1.2 . . . . . . . . 93 - knee-point voltage . . . . . . . . . . . 6.4.4 . . . . . . . . 86 - linea r . . . . . . . . . . . . . . . . . . . 6.4.63 . . . . . . . . 88 - line current . . . . . . . . . . . . . . . 6.4.6 . . . . . . . . 88 - magnetisation curve . . . . . . . . . . 6.4.4 . . . . . . . . 86 - novel . . .. . . . . . . . . . . . . . . . . . 6.5 . . . . . . . . 92 - open circuit secon ary voltage . . . . 6.4 . . . . . . . . 85 - optical . . . . . . . . . . . . . . . . . . . 6.5.1 . . . . . . . .92 - polarity check . . . . . . . . . . . . 21.9.3.1 . . . . . . . . 3 88 - rate short-time current . . . . . . . 6.4.9 . . . . . . . . 89 - ratio che ck . . . . . . . . . . . . . . 21.12.2 . . . . . . . . 395 - saturation . . . . . . . . . . . . . . . . 6.4.10 . . . . . . . . 89 -secon ary current rating . . . . . . . 6.4.8 . . . . . . . . 88 - secon ary win ing impe ance . . . 6.4.7 . . . . . . . . 88 - summation . . . . . . . . . . . . . . 6.4.5.4 . . . . . . . . 87 - supervision in numerical relays . . . . . . . . . . . . . . . . . . . 7.6. 2 . . . . . . . . 108 - tests . . . . . . . . . . . . . . . 21.9.3, 21.12 . . . . 388-394 - test win ings . . . . . . . . . . . . . 6.4.12 . . . . . . . . 92 transient response . . . . . . . . . . 6.4.10 . . . . . . . . 89 In ex . . . . . . . . 87 Core faults: - in generators . . . . . . - in power transformers D D.C. ramp tests . . . . . . . . . . . . . . 21.3.3 . . . . . . . . 375 . . . . . . . . . . 8.7.1 . . . . . . . . 120 . . . . . . . . . 17.3 . . . . . . . . 283 . . . . . . . 16.2.6 . . . . . . . . 257 D.C. voltage signalling • 480 •
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Section Dea line charging . . . . . . 14.9.2, Page . . . . 227-406 Section Directional comparison blocking schemes . . . Directional comparison unblocking scheme . . . . . . . 12.4, Page . . . . 198-213 22.8.1 Dea time in auto-reclose schemes 13.5.5 . . 14.4.1, 14.6.4, 14.9 . .221-225-226 Definite time overcurrent relay . . . . . . . . . . . . . . Definition of: - pro tection equipment - protection scheme . . - protection system . . . . . . . . . . . . 12.5 . . . . . . . . 199 . . . . . . . . . 9.9 . . . . . . . . 129 Directional control of impe ance type istance relays . . . 11.7.2 . . . . . . . . 176 . . . . . . . . . . 2.2 . . . . . . . . . . 2.2 . . . . . . . . . . 2.2 . . . . . . . . .7 . . . . . . . . .7 . . . . . . . . .7 Definitions of terms use in protection, control an automation . . . . . . De-i onisation of fault path . . . . Directional relays: - connections . . . . . . . . . . . . . . 9.17.1 . . . . . . . . 139 - earth fault . . . . . . . . . . . . . 9.17-9.19 . . . . 138-141 - pha se fault . . . . . . . . . . . . . . . . 9.14 . . . . . . . . 133 - overcurrent . . . . . . . . . . . . . . . . 9.14 . . . . . . . . 133 Discrimination: - by cu rrent . . . . - by time . . . . . . - time an current . . . . . . . . . . . . 9.3.2 . . . . . . . . 124 . . . . . . . . 9.3.3, . . . . . . . . . A1 . . . . 455-465 . . . . . . . 14.4.1.4, 14.6.2 . . . . 222-224 . . . . . . . . . . . . 9.3.1 . . . . . . . . 124
Delaye auto-reclose scheme . Delta-connecte win ing of a transformer . . . . . . Delta/star transformer overcurrent protection . . . . . 14.9 . . . . . . . . 226 9.11 . . . . 125-130 . . . 16.2.3, 16.7 . . . . 256-260 . . . . . . . . . . 16.6 . . . . . . . . 260 Design for maintenance of protection schemes . Determination of sequence current s . . . . . . . . 21.16 . . . . . . . . 397 . . . . . . . . . . . . . . 4.4 . . . . . . . . 35 Device numbers, list of ANSI . . . . . . . A2 . . . . . . . . 467 Differential protection: - igital systems . . . . . . . . . . . . . . 10.8 . . . . . . . . 158 - electromechanical systems . . . . . . 10.7 . . . . . . . . 156 - for sectionalise an uplicate busbars . . . . . . . . . . . 15.7.1 . . . . . . . . 239 - numerical systems . . . . . . . . . . . 10.8 . . . . .. . . 158 ofbusbars . . . . . . . . . . . . 15.7-15.10 . . . . 239-251 - of irect conne cte generators . . . . . . . . . . . . . . . . 17.5 . . . . . . . . 284 - of ge nerator-transformers . . . . . . 17.6 . . . . . . . . 285 - of multi-en e fee e rs . . . . . . . . 13.3 . . . . . . . . 207 - of parallel fee ers . . . . . . . . . . 13.2.1 . . . . . . . . 204 - of transformer fee ers . . . . . . . 10.12.2 . . . . . . . . 168 - static systems . . . . . . . . . . . . . . 10.7 . . . . . . . . 156 -using high impe ance relays . . . . . 10.5 . . . . . . . . 156 - usi ng low impe ance relays . . . . . 10.4 . . . . . . . . 154 - using optical fibre signalling . . . 10.8.1 . . . . . . . . 158 Digital relays . . . . . . . . . . . . . . . . . 7.4 . . . . . . . . 102 Digital current ifferential protection systems . . . . . . . . . . . . . 10.8 . . . . . . . . 158 . . . . . 22.3 . . . . . . . . 401 Digital trans ucer technology Direct an qua rature axis values of machines . . . . . . . . . . . . . . 5.8, 5 .11 . . . . . . 53-55
Distance relay: - apparent impe ance seen by . . . 13.4.1 . . . . . . . . 210 application example . . . . . . . . . 11.12 . . . . . . . . 189 - application pr oblems . 11.10, 13.4, 14.8 186-210-226 - Application to: a.c. electrifie railwa ys . . . 20.2, 20.6 . . . . 354-361 multi-en e fee ers . . . . . . . . . 13.5 .
. . . . . . . 212plain fee ers . . . . . . . . . . . . . . 11.6 . . . . . . . . 174 parallel fee ers . . . . . . . . . . . 13.2.2 . . . . . . . . 204 - Charac teristics: . . . . . . . . . . . . . 11.7 . . . . . . . . 175 amplitu e an phas e comparison . . . . . . . . . . . . . . 11.7.1 . . . . . . . . 176 fully cross polarise mho . . . . 11.7.5 . . . . . . . . 179 lenticular . . . . . . . . . . . . . . 11.7.4.3 . . . . . . . . 179 offset mho . . . . . . . . . . . . . . 11.7 .4 . . . . . . . . 178 other . . . .. . . . . . . . . . . . . . 11.7.9 . . . . . .. . 182 partially cross-polarise mho . . 11.7.6 . . . . . . . . 180 plain i mpe ance . . . . . . . . . . 11.7.2 . . . . . . . . 176 qua rilateral . . . . . . . . . . . . . 11.7.7 . . . . . . . . 181 self polarise mho . . . . . . . . . 11.7.3 . . . . . . . . 177 - current reversal on ouble circuitlines . . . . . . . . . . . . . 13.2.2.1 . . . . . . . . 204 - earth fault impe ance measurement . . . . . . . . . . . . . 11.9.2 . . . . . . . . 185 - effect of source impe an ce . . . . . 11.9 . . . . . . . . 184 - effect of earthing metho . . . . . . 11 .9 . . . . . . . . 184 - example setting calculation . . . . 11.12 . . . . . . . . 189 - features of . . . . . . . . . . . . 11.7, 11.11 . . . . 175-188 - forwa r reach limitations . . . . . 11.10.5 . . . . . . . . 187 - implementation . . . . . . . . . . . . . 11.8 . . . . . . . . 182 - minimum length of line . . . . . . 11.10.2 . . . . . . . . 186 - minimum voltage at relayterminals . . . . . . . . . . . . 11.10.1 . . . . . . . . 186 - multi-en e fee ers . . . . . . . . . . 13.4 . . . . . . . . 210 - over-reach . . . . . . . . . . . . . . . 11.10.4 . . . . . . . . 187 - parallel fee ers . . . . . . . . . . . . 13.2.2 . . . . . . . . 204 - percentage over-reach . . . . . . . 11.10.4 . . . . . . . . 187 • 481 • Network Protection & Automation Gui e In ex •
Section - percentage un er-reach . . - Performance of: igital types . . . . . . . . ele ctromechanical types numerical types . . . . . . static types . . . . . . . . . Page Section Page . . . . 11.10.3 . . . . . . . . 187 . . . . . 11.3.2 . . . . . . . . 173 . . . . . 11.3.2 . . . . . . . . 173 . . . . . 11.3.1 . . . . . . . . 172 . . . . . 11.3.1 . . . . . . . . 172 - in rural areas . . . . . . . . . . . . . mary istribution systems . . . 25.3 . . . tion systems . . . . . . . . . . . . . . . tools . . . . . . . . . . . . . . 25.7 . . mer earthing of generators . . Disturbance . . 17.2,
. . 25.5 . . . . . . . .449 - of pri . . . . . 445 - of secon ary istribu 25.4-25.5 . . . . 447-449 - software . . .. . . 452 Distribution transfor recor er
•
- phase fault impe ance measurement . . . . . . . . . . . . . 11.9.1 . . . . . . . . 184 - principles of . . . . . . . . . . . . . . . . 11.2 . . . . . . . . 17 2 - Schemes: . . . . . . . . . . . . . 12.1-12.6 . . . . 193-200 acceleration . . . . . . . . . . . . . 12.3.3 . . . . . . . . 196 Blocking over-reaching scheme s: . . . . . . . . . . . . . . . . 12.4 . . . . . . . . 198 using zone 1 element . . . . . 12.4.1.2 . . . . . . . . 199 using zone2 element . . . . . 12.4.1.1 . . . . . . . . 198 comparisonof transfer trip an blocking schemes . . . . . . . . 12.6 . . . . . . . . 200 irectional unblocking . . . . . . . . comparison . . . . . . . 12.5 . . . . . . . . 199 irect un er-reach transfer tripping sche me . . . . . . . . . . 12.3.1 . . . . . . . . 195 lines using high-spee auto-re close . . . . . . . . . . . . . . 14.8 . . . . . . . . 226 multi-en e fee ers . . . . . . . . . 13.5 . . . . . . . . 212 permissive over-reach transfer trippin g . . . . . . . . . . 12.3.4 . . . . . . . . 196 permissive un er-reachingaccel eration . . . . . . . . . . . . . 12.3.3 . . . . . . . . 196 permissive un er-re ach transfer tripping . . . . . . . . . 12.3.2 . . . . . . . . 195 trip transfer . . . . . . . . . . . . . . 12.3 . . . . . . . . 195 weak infee con itions . 1 2.3.5, 12.4.2 . . . . 197-199 zone 1 extension . . . . . . . . . . . 12.2 . . . . . . . . 194 - Settings: . . . . . . . . . . . . . . . . . 11.6 . . . . . . . . 174 other zones . . . . . . . . . . . . . . 11.6.4 . . . . . . . . 175 reverse zones . . . . . . . . . . . . 11.6.4 . . . . . . . . 175 zone 1 . . . . . . . . . . . . . . . . . 11.6.1 . . . . . . . . 174 zone 2 . . . . . . . . . . . . . . . . . 11.6.2 . . . . . . . . 175 zone 3 . ... . . . . . . . . . . . . . 11.6.3 . . . . . . . . 175 - starters for switche istanceprotection . . . . . . . . . . . . . . . . 11.8.1 . . . . . . . . 183 - switche istance protection relay s . . . . . . . . . . . . . . . . . . . 11.8.1 . . . . . . . . 183 - un er-reach . . . . . . . . . . . . . . 11.10.3 . . . . . . . . 187 - un er-reach on parall el lines . . 13.2.2.2 . . . . . . . . 204 - voltage supervision in . . . . . . . 11.10.7 . . . . . . . . 188 - zones of protection . . . . . . . . . . . 11.6 . . . . . . . . 174 Distance protection – see istance relay Distribution system aut omation . . . . . . . . . . . . . . 25.1-25.7 . . . . 443-452 - communications f or use in . . . . . . 25.6 . . . . . . . . 451 - in urban areas . . . . . . . . . . . . . . 25.4 . . . . . . . . 447 . . . . . . . . . . . . . . . . . . . . . . 17.8.2.2, 17.8.2.3 .
. . . 282-290 . . . . . . . 290 . . . . . . . . . . . 22.9 . . . . . . . . 407 Disturbance recor er function in numerical relays . . . . . . . Double busbar su bstation, application of auto-reclose to Double circuit lines, current reversal on . . . . . . . . . . . Double frequency or broa ban trap . . . . . . . . 7.6.4 . . . . . . . . 108 . . . 14.12.1 . . . . . . . . 229 . . . . 13.2.2.1 . . . . . . . . 204 . . . . . . . . . . . 8.6.3 . . . . . . . . 117 Dual fe substations, earth fault protection in . . . . . . . . 18.7.2 . . . . . . . . 325 . . . . 21.7 . . . . . . . . 383 Dynamic vali ation type testing E Earth fault impe ance measurement using istance relays . . . . . . . . . . . . . . 11.9.2 . . . . . . . . 185 In ex
Earth fault protection for entire generator stator win ing . . . . . . . 17.8.4 . . . . . . . . 291 Earth fault protection: - of in uction motors . . . . . . . . . . 19.6 . . . . . . . . 342 - of generators . . . . . . . . . . . 17.5, 17.8 . . . . 284-288 - of transformers . . . . . 16.6-16.8, 16.10 . 260-261-264 - on insulate networks . . . . . . . . . 9.18 . . . . . . . . 139 - on Petersen Coil earthe networks . . . . . . . . . . . . . . . . . 9.19 . . . . . . . . 141 - sensitive . . . . . . . . . . . . . 9.16.3, 9.17 . . . . 138-138 - time gra ing of . . . . . . . . . . . 9.16.2 . . . . . . . . 137 - usingovercurrent relays . . . . 9.16-9.19 . . . . 136-141 - with resi ually-connecte CT’s . . . . . . . . . . . . . . . . . . . 18.7.1.1 . . . . . . . . 324 Earthing arrangements for frame-earth protection . . Earthing, system, effect of on zero sequence quantities . . . . . . 15.6.1 . . . . . . . . 236 . . . . . . . 4.6 . . . . . . . . . 41 Earthing of istribution transformers use for generator earthing . . . . 17.8.2 .2, 17.8.2.3 • 482 • . . . . 290-290
Network Protection & Automation Gui e
Section Earthing transformer protection . . . . . . . . Effective setting of earth fault relays . . Page Section Page . . . . . . . . . . 16.11 . . . . . . . . 267 . . . . . . . . . . . 9.16.1 . . . . . . . . 136 Effective setting of electromechanical earth fault relays . . . . . . . . . . . . . . . . - in uction motor . . . . . . . . . . . . . 19.7 . . . . . . . . 346 - overhea lines . . . . . . . . . . . . . . 5.22 . . . . . . . . 67 - transformer . . . . . . . . . . . . 5.14-5.15 . . . . . . 56-57 - synchronous generator . . . . . . 5.2-5.10 . . . . . . 47-55 - voltage transformer . . . . . . . . . . . 6.2 . . . . . . . . 80 Errors: - in current transformers . . . . . . . . 6.4.1 - in relay s . . . . . . . . . . . . . . . . . 9.11.2 - in voltage transformers . . . . . . . . 6.2.1 Event recor er . . . . . . . . 85 . . . . . . . . 80 . . . 9.16.1.2 . . . . . . . . 137 Effect of: - earthing metho s on istancerelays . . . . . . . . ... . . . 11. 9 . . . . . . . . 184 - fault current on istance multi-en e fee ers . relays, . . . 13.4.3 . . . . . . . . 211 pre-fault loa on istance relays, multi-en e fee ers . . . . 13.4.2 . . . . . . . . 211 - saturation on machine reactances . . . . . . . . . . . . . . . . 5.12 . . . . . . . . 55 - source impe ance on i stance relays . . . . . . . . . . . . . . 11.9 . . . . . . . . 184 - system eart hing on zero sequence quantities . . . . . . . . . . . 4.6 . . . . . . . . . 41 Electrical faults in stator win ings . . . . . . . . . . Electrical type tests . . . . . 17.3, . . . . . . . . 131 . . . . . . . . . . . . . . . 22.9 . . . . . . . . 407 19.6 . . . . 283-342 . . . . . . . . . . . . 21.2 . . . . . . . . 372 Electromagnetic compatibility type tests . . . . . . . . . . . . . . . . . . . 21.3 . . . . . . . . 373
Example relay setting calculations: - a.c. electrifie railways . . . . . . . . 20.8 . - earth fault, overcurrent . . . . . . . 9.20.2 . - complex transmission circuits . . . . 13.7 . - istance . . . . . . . . . . . . . . . . . 11.12 . - g enerator . . . . . . . . . . . . . . . . 17.22 . - in uction motor . . . . . . . . . . . . 19.14 . - in ustrial systems . . . . . . . . .. . 18.12 . - overcurr ent . . . . . . . . . . . . . . . . 9.20 . - parallel fee ers – istance . . . . 1 3.7.1 . - parallel fee ers - overcurrent . . . 9.20.3 . - phase fault . . . . . . . . . . . . . . . 9.20.1 . - ring main . . . . . . . . . . . . . . . . 9.20.4
. - transformer . . . . . . . . . . . . . . . 16.19 . - unit protection . . . . . . . . . . . . . 10.12 . Examples of auto-reclosing schemes . . . . . . . . . . . . . . . 3 .
. . . .
. . . .
. . . . 365 . . . . . . . 148 . . . . . . . 189 . . . . . . . 308 . . . . 349 . . . . . . . 330 . . . . . . . 143 . . . . . . . 149 . . . . . . . 14 . . . . . 150 . . . . . . . 276 . . . . . . . 167 . . . . . . . 215 . . . . 215
Electromagnetic voltage transformers: . . . . . . . . . . . . . . . . . 6.2 - ca sca e type . . . . . . . . . . . . . . . 6.2.8 - construction . . . . . . . . . . . . . . . 6.2.5 . - errors in . . . . . . . . . . . . . . . . . . 6.2.1 - prot ection of . . . . . . . . . . . . . . 6.2.4 . - resi ual connection of . . . . . . . . 6.2.6 . - transient performance . . . . . . . . 6.2.7 - voltage factor . . . . . . . . . . . . . . 6.2.2. Electromechanical relays Electromechanical uni t protection systems . . . Embe e generation . . . . . . . . . 7.2 . . . . . . . . 80 . . . . . . . . 82 . . . . . . . . 80 . . . . . . . . 81 . . . . . . . . 81 . . . . . . . . 81 . . . . . . 14.12 . . . . . . . . 229 Examples of electromechanical unit protection systems . . . . . Examples of stat ic unit protection systems . . . . 10.7 . . . . . . . . 156 . . . . . . . . 82 . . . . . . . . . 10.7 . . . . . . . . 156 . . . . . . . . 81 . . . . . . . . 99 Extremely inverse overcurrent relay . . . . . . . . . . . . . . . . . . . . . . . 9.7 . . . . . . . . 128 . . . . . . . . . . 10.7 . . . . . . . . 156 F Factors influencing HV auto-reclose schemes . . . . . . . . . . . 14.4 . . . . . . . . 221 . . . . . . . . . . 17.21 . . . . . . . . 306 En zones of a protecte fee er ( istance protection schemes) . . Environmental type tests Equations an network connections for various types of faults . . . . . . Equivalent circuits: - auto-transformer . . - cables . . . . . . . . . - cu rrent transformer . . . 12.1 . . . . . . . . 193 . . . . . . . . . 21.5 . . . . . . . . 380
Factory (pro uction) tests on relays . . . . . . . . . . . Failure of the prime mover Fast transient test Fault calculations . . . . . . . . 21.8 . . . . . . . . 386 . . . . . 17.19.1 . . . . . . . . 303 . . . . . . . . . . 4.4 . . . . . . . . 35 . . . . . . . . . . . . 21.3.5 . . . . . . . . 375 . . . . . . . . . . . 4.2-4.6 . . . . . . 31-41 . . . . . . . . . . 5.16 . . . . . . . . . . 5.23 . . . . . . . . . . . 6.4 . . . . . . . . 59 . . . . . . . . 68 . . . . . . . . 85 Fault current contribution from in uction motors . . . . . . . . . . 18.8 . . . . . . . . 326 . . . . . . Fault Passage In icator (FPI) • 483 • 25.4 . . . . . . . . 447
Network Protection & Automation Gui e In ex • Examples of Power Quality issues . . . 23.6 . . . . . . . . 419
Section Feeer substation protection for a.c. electrifie railways . . Ferro-resonance F iel suppression of synchronous machine . Flags Page Section Page . . . . . . 20.7 . . . . . . . . 365 . . . . . . . . 84 . . . . . . . . . . . . . . 6.3.3 . . . . . . . . 17.15.5 . . . . . . . . 298 . . . . . . . . 13 . . . . . . . . . . . . . . . . . . . . . 2.10.2 Forwar reach limitations of istance relays . . . . . . . . . . . . 11.10.5 . . . . . . . . 187
Four-switch mesh substation, auto-reclosing applie to . . . Four-wire ual-fe substations . . . 14.12.3 . . . . . . . . 230 . . . 18.7.2 . . . . . . . . 325 FPI – see Fault Passage In icator Frame-earth protection (Howar protection): . . . . . . . . . . . 15.6 . - check system . . . . . . . . . . . . . 15.6.4 . - sch eme for ouble bus station .. 15.6.3 . - single busbar . . . . . . . . . . . . .. 15.6.1 . - with sectione busbars . . . . . . . 15.6.2 . Frequency shift key e signals, protection signalling using . . Frequency variations ) Power quality ) . . . . . . . . . . . . 236 . . . . . . . 238 . . . . . . . 238 . . . . . . . 236 . . . . . . . 237 . . . . . 8.7.3 . . . . . . . . 121 . . . . . . . . . 23.3.5 . . . . . . . . 414
Fully cross-polarise mho relay Fun amentals of protection practice . . . . . . . . . . . . . Fun amental theory . . . 11.7.5 . . . . . . . . 179 . . . . 2.1-2.12 . . . . . . . 5-14 . . . . . . . . . . 3.1-3.7 . . . . . . 17-28 Fuses for use with istribution transformers . . . . . . . . . . . Fusing factor . . . . 16.6.1 . . . . . . . . 260 . . . . . . . . . . . . . . . 18.4.3 . . . . . . . . 320 •
G Gas an oil surge relay (Buchholz relay) for transformers . . . . . . . . . . . . 16.15.3 . . . . . . . . 269 Gas evices for protection of transformers . . . . . . . . . . . . . . 16.15 . . . . . . . . 268 Generator: . . . . . . . . . . . . . 17.1-17.22 . . . . 281-308 - Complete schem es: for irect-connecte generators . . . . . . . . . . . . . 17.20.1 . . . . . . . . 304 generator-transformer units . 17.20.2 . . . . . . . . 305 - earthing . . . . . . . . . . . . . . . . . . 17.2 . . . . . . . . 282 - examples of protec tion calculations . . . . . . . . . . . . . . . 17.22 . . . . . . . . 308 - Impe ances: irect axis . . . . . . . . . . . . . 5.8, 5.11 . . . . . . 53-55 effect of satu ration . . . . . . . . . 5.12 . . . . . . . . 55 negative sequence . . . . . . . . . . . 5.9 . . .. . . . . 54 positive sequence . . . . . . . . . . . 5.8 . . . . . . . . 53 qua rature axis . . . . . . 5.5, 5.8, 5.11 . . . 50-53-55 zero se quence . . . . . .. . . . . . . 5.10 . . . . . . . . 55 - Protection: ifferent ial protection of irect-connecte generators . . 17.5 . . . . . . . . 284 iffe rential protection of generator-transformer units . . . . . . . . . . . . . . . . . . . 17.6 . . . . . . . . 285 io e failure . . . . . . . . . . . . 17.15.4 . . . . . . . . 297 earth fault for entire stator win ing . . . . . 17 protection .8.4 . . . . . . . . 291 embe e generation . . . . . . . 17.21 . . . . . . . . 306 failure of prime mover . . . . . 17.19.1 . . . . . . . . 303 ina vertent en ergisation . . . . . 17.13 . . . . . . . . 295 loss of excitation . . . . . . . . . . 17.16 . . . . . . . . 298 loss of Utility supply . . . . . . . 17.21.1 . . . . . . .. 306 loss of vacuum . . . . . . . . . . 17.19.3 . . . . . . . . 304 low forwar power . . . . . . . . 17.11.1 . . . . . . . . 293 mechanical faults . . . . . . . . . . 17.19 . . . . . . . . 303 negative phase sequence . . . . . 17.12 . . . . . . . . 293 neutral voltage isplacement . . . . . 17.8.1.3, 17.8. 2.4 . . . . 289-290 overcurrent protection . . . . . . . 17.7 . . . . . . . . 28 6 overfluxing . . . . . . . . . . . . . 17.14.1 . . . . . . . . 295 overfrequenc y . . . . . . . . . . . 17.14.2 . . . . . . . . 295 overheating . . . . . . . . . . . . . . 17.18 . . . . . . . . 303 overspee . . . . . . . . . . . . . . 17.1 9.2 . . . . . . . . 303 overvoltage . . . . . . . . . . . . . . . 17.9 . . . . . . . . 292 pole slipping protection . . . . . . 17.17 . . . . . . . . 301 revers e power . . . . . . . . . . . 17.11.2 . . . . . . . . 293 rotor earth faults . . . . . . . . . 17.15.1 . . . . . . . . 296rotor faults . . . . . . . . . . . . . . 17.15 . . . . . . . . 296 rotor shorte turn protection . . . . . . . . . . . . . 17.15.3 . . . . . . . .297 stator earth fault . . . . . . . . . . . 17.8 . .. . . . . . 288 unbalance loa ing . . . . . . . . 17.12 . . .. . . . . 293 un erfrequency . . . . . . . . . . 17.14.2 . . . . . . . . 295 unervoltage . . . . . . . . . . . . . 17.10 . . . . . . . . 292 voltage controlle overcurrent . . . . . . . . . . . . 17.7.2.1 . . . . . . . . 287 voltagerestraine overcurr ent . . . . . . . . . . . . 17.7.2.2 . . . . . . . . 287 win ing protection .. . . . . . . . . 17.4 . . . . . . . . 284 Generator-transformer protection Gra in g margins for overcurrent relays: - earth fault relays . . - fuse to fuse . . . . . - fuse to relay . . . . . • 484 • . . . . . . . . . 17.6 . . . . . . . . 285 In ex 9.11-9.12 . . . . . . . . . 9.12.2 . . . . . . . . 132 . . . . . . . . . 9.12.3 . . . . . . . . 132
. . . . . . . . . 9.16.2 . . . . . . . . 137 . . . . 130-131
Network Protection & Automation Gui e
Section - relay to relay Gra ing of ring mains . . . . . . 9.15, Page Impe ance notation . . . . . 135-150 Section . . . . . . . . . . . . 3.5 Page . . . . . . . . 23 . . . . . . . . . . . . . . 9.12.1 . . . . . . . . 131 9.20.4 H Har ware architecture of numerical relays . . . . . . . . . . . . 7.5.1 . . . . . . . . 104 Harmonic content of inrush waveform, transformer . . . . Harmonics (Power Qualit y) . Harmonics in current transformers . . . . . . . . . . . 16.3.1 . . . . . . . . 259 . . . . . 23.3.4 . . . . . . . . 414 . . . . . . . . . . 6.4.11 . . . . . . . . 92 Impe ances: - Generator: irect-axis . . . . . . . . . . . . . . . 5.11 . . . . . . . . 55 negative sequence . . . . . . . . . . . 5.9 . . .. . . . . 54 positi ve sequence . . . . . . . . . . . 5.8 . . . . . . . . 53 qua rature axis . . . . . . . . . 5.5, 5.11 . . . . . . 50-55 zero sequence . . . . . . . . . . . . . 5 .10 . . . . . . . . 55 - In uction motor: negative sequence . . . . . . . . . . 19.7 . . . . . . . . 346 positive sequence . . . . . . . . . . . 19.7 . . . . . . . . 346 - Transformer: positive sequence . . . . . . . 5.14, 5.17 . . . . . . 56-60zero sequence . . . . . . . . . 5.15, 5.17 . . . . . . 57-60 Implementatio n of istance relays . . . . . . . . . . . . . . . Impulse type test for a relay Incorrect relay operation, reasons for . . . . . . . . . . . . . . . . 11.8 . . . . . . . . 182 . . . . . 21.5.5 . . . . . . . . 381 Heating of in uction motor win ings . . . . . . . . . . . . High frequency istu rbance test . . . 19.3, 19.7 . . . . 338-346 . . . . . . . . . . . High impe ance busbar feature . . . . . . .
. . 21.3.4 . . . . . . . . 375 ifferential protection: . . . . . . . . . 15.8 . - check . . . . . . 15.8.3 . - CT supervision . . . . . . . . . .
. . . 15.8.4 . - effective etails of . . . . . . . . .8.2 . - stability . . . . ays in busbar protection .
setting . . . . . . . . . . . . 15.8.2 . - practical . . 15.8.6 . - primary operating current . .. . . 15 . . . . . . . . . . . . . 15.8.1 . High impe ance rel .
. . . . . . . 241 . . . . . . . 244 . . . . . . . 245 . . . . . . . 242 . . . . . . . 245 . . . . . . . 242 . . . . . . . 241 . . . . . . . . . 2.4 . . . . . . . . .9 In icating lamps in protection schemes . . . . . . . . . . . . . . In ication or flags In uction motor characteristics . . . . . 2.10.2 . . . . . . . . 13 . . . . . . . . 13 . . . . . . . . . . . . 2.10.2 . . . . . . . . . . . 19.3-19.7 . . . . 338-346 . . . . . . . . . . 15.8 . . . . . . . . 241 In uction motor equivalent circuit . . . . . . . . . . . . . . . . . . . . . 19.7 . . . . . . . . 346 High spee auto-reclosing: - on EHV systems . . . . . . . . . 14.6-14.8 . . . . 223-226 - on lines using istance relays . . . . 14.8 . . . . . . . . 226 High v oltage capacitor of line coupling equipment Howar protection (Frame-earth prote ction) HRC fuse applications . HRC fuses Hysteresis effect . . . . . . . 8.6.3 . . . . . . . . 117 . . . . . . . . 15.6 . 18.4 . . . . . . . 236 . . . . 260-319 . . . . . 16.6.1, . . . . . . . . . . . . . . . . . . 18.4 . . . . . . . . 319 . . . . . . . . . . . . 6.4.10.2 . . . . . . . . . 91 I IDMT overcurrent relay IEC relay symbols . . . . . . . . . . . 9.4 . . . . . . . . 126 . . . . . . . . . . . . . . . A2 . . . . . . . . 467 Impact of Power Quality problems . . . . . . . . . . . . . . . . . . . 23.3 . . . . . . . . 413 In ustrial an commercial power system protection: - automatic changeover system s . . . . . . . . . . - busbar arrangement . . - capacitor control . . . . - coor ination problems
• 485 • . . . 18.1-18.13 . . . . 317-335 . . . . . . . . 18.9 . . . . . . . . 326 . . . . . . . . 18.2 . . . . . . . . 31 8 . . . . . . 18.11.1 . . . . . . . . 328 . . . . . . . . 18.7 . . . . . . . . 324 Network Protection & Automation Gui e In ex • High resistance earthing of generators . . . . . . . . . . . . . . . . 17.2 . . . . . . . . 282 In uction motor protection: . . . . . . . . 19.2-19.12, 19.14 337-348-349 - bear ing failures . . . . . . . . . . . . 19.10 . . . . . . . . 348 - earth fault . . . . . . . . . . . . . . . . 19.6 . . . . . . . . 342 - examples of . . . . . . . . . . . . . . . 19.14 . . . . . . . . 349 - fault current contribution from . . . 18.8 . . . . . . . .326 - faults in rotor win ings . . . . . . . . 19.8 . . . . . . . . 347 - locke rotor . . . . . . . . . . . . . . . . 19.4 . . . . . . . .339 - lossof loa . . . . . . . . . . . . . . . 19.12 . . . . . . . . 348 - mo ern relay esign . . . . . . . . . . 19.2 . . . . . . . . 337 - negative ph ase sequence . . . . . . . 19.7 . . . . . . . . 346 - overcurrent . . . . . . . . . . . . . . . . 19.5 . . . . . . . . 342 - stalling . . . . . . . . . . . . . . . . . 19.4.2 . . . . . . . . 340 - starting . . . . . . . . . . . . . . . . . 19.4.1 . . . . . . . .339 - thermal . . . . . . . . . . . . . . . . . . 19.3 . . . . . . . . 338 - un ervoltage . . . . . . . . . . . . . . 19.11 . . . . . . . . 348
Section - iscrimination in . . . . . . . . . . . 18.3 . - examples of protection . . . . . . . 18.12 . - fault current contribution from in uction motors . . . . . . . . 18.8 . - HRC fuses: . . . . . . . . . . . . . . . . 18.4 . - characteristics . . . . . . . . . . . . . 18.4.1 . - iscrimination . . . . . . . . . . . . . 18 .4.2 . - effect of ambient temperature . . 18.4.4 . - fusing factor . . . . . . . . . . . . . . 18.4.3 . - protection of cables by . . . . . . . 18.4.3 . - prot ection of motors by . . . . . . 18.4.5 . - motor power factor correction . 18.11 .2 . - power factor correction . . . . . . . 18.11 . - protection against overvo ltage . . 18.10 . - protection against reverse phase sequence . . . . . . . . . . . . 18.10 . - protection against un ervoltage . . . .. . . . . . . . . . 18.1 0 .- protection of capacitors . . . . . . 18.11.3 . In ustrial circuit breakers In ustrial consumers, auto-reclosing requirements Initial commissioning tests I njection test equipment, secon ary . . . . . . . . . . Injection tests, primary Injection tests, secon ary Page . . . . . . . 330 . . . . . . . 326 . . . . . . . 319 . . . . . . . 319 . . . . . . . 319 Section - for multi-terminal lines . . . . . . . . 13.5 . - for transformers . . . . . . . . . .. . 16.17 . - intertripping, performance requirements . . . . . 8.5.1 . - metho s . . . . . . . . . . . . . . . . . . . 8.4 . - permissive tripping, pe rformance requirements . . . . . 8.5.2 . - permissive tripping, signalling . . . 8.4.2 . - signalling for . . . . . . .. . . . . . . . . 8.4 . Interturn fault protection for the generator stator win ing Interturn faults in power transforme rs . . . . . . . . Page . . . . . . . 212 . . . . . . . 273 . . . . . . . 115 . . . . . . . 114 . . . . . . . 115 . . . . . . . 115 . . . . . . . 114 . . . . . . . 320 . . . . . . . 320 . . . . . . . 320 . . . . . . . 320 . . . . . . . 320 . . . . . . . 329 . . . . . . . 328 . . . . . . . 327 . . . . . . . 32 7 . . . . . . . 327 . . . . . . . 330 . . 17.3.3 . . . . . . . . 283 . . . . . . . 16.2.5 . . . . . . . . 257 Inverse overcurrent relays: - extremely inverse . . . . . - IDMT . . . . . . . . . . . . . - very inverse . . . . . . . . Issues with numerical relays . . . . . . . . 9.7 . . . . . . . . 128 . . . . . . . . 9.4 . . . . . . . . 126 . . . . . . . . 9.6 . . . . . . . . 128 . . . . . . . 7.7 . . . . . . . . 109 . . . . . . . . 18.5 . . . . . . . . 320 . . . . 14.4.1.2 . . . . . . . . 221 . . . . . . . 21.9 . . . . . . . . 387 J Junction law . . . . . . . . . . . . . . . 3.6.1.2 . . . . . . . . 24 . . . . . . . . 21.10 . . . . . . . . 389 . . . . . . . . . 21.12 . . . . . . . . 394 . . . . . . . . 21.11 . . . . . . . . 392 . . . 16.2 . . . . . . . . 256
K Kennelly s star/ elta theorem Knee-point voltage of current transformers . . . . . 3.6.2.3 . . . . . . . . 25 Inrush waveform of transformers In ex Instantaneous overcurrent relays, characteristics of . . . . . . . . . . . . . . . . . . . . . 6.4.4 . . . . . . . . 86 . . . . 9.5 . . . . . . . . 127 . . . . . . 79-92 • Instrument transformers: . . . . . . 6.1-6.5 - capacitor voltage . . . . . 6.3 - current . . . . . . . . . . . . . . . . . . . . gnetic voltage . . . . . . . . 6.2 - novel . . . . . . . . . . . . . 6.5 Insulate networks, earth fault protection of . . . . . nsulation tests on protection schemes uring commissioning
. . 6.4 . . . .
. . .
. . . . . electroma . . . . . . . . . I
. . . . . . . . 83 . . . . . . . . 85 . . . . . . . . 80 . . . . . . . . 92 L Lenticular characteristic of istance relay . . . . . . . . . . . . 11.7.4.3 . . . . . . . . 179 Line attenuation, power line carrier communications . . . Line charging current Line trap . . . . . . 8.6.3 . . . . . . . . 117 . . . . 9.18 . . . . . . . . 139 . . . 9.18.2, . . . . . . . . . . . . . . . . . . . . . . . . 21.9.1 . . . . . . . . 387 9.19, 10.8, . 140-141-158 16.16.1.4 . . . . . . . . 272 . . . . . . . . . . . . . . . . . . . 8.6.3 . . . . . . . . 117 . . . . . . . A2 . . . . . . . . 467 . . . . . . A2 . . . . . . . . 467 Interference an noise, effect on protection signalling . . . . . . . . Intertri pping:- blocking scheme, performance requirements - blocking scheme,signalling - by .c. signal on separate pilots . . . . . . . . . . . . . . . - irect trip ping, signalling . . . . . . 8.6 . . . . . . . . 116 List ofANSI evice numbers List of IEC protection symbols Live line reclosing L oa she ing . . . . . . . . . . . . . 14.9.1 . . . . . . . . 227 . . . . . 8.5.3 . . . . . . . . 116 . . . . . 8.4.3 . . . . . . . . 115
. . . . . . . . . . . . . . . 24.5 . . . . . . . . 430 . . . . . 8.7.1 . . . . . . . . 120 . . . . . 8.4.1 . . . . . . . . 115 Lock-out in auto-reclosing schemes . . . . . . . . . . . . . 14.6.6, 14.10.7 Log ic systems • 486 • . . . . 225-228 . . . . . . . . . . . . . . . . 7.6.6 . . . . . . . . 109 Network Protection & Automation Gui e
Section Loss of excitation protection for generators . . . . . . . . . . Loss of excitat ion protection for synchronous motors . . . Loss of vacuum in turbines Page Mathematical operators Maximum torque angle of relay – see relay characteristic an gle Section . . . . . . . . . 3.3.3 Page . . . . . . . . 19 . . . . . 17.16 . . . . . . . . 298 . . . . 19.13.1 . . . . . . . . 348 . . . . . 17.19.3 . . . . . . . . 304 Low forwar power protection of generators . . . . . . . . . . . Low impe ance b iase ifferential protection type MBCZ . . . . . . . . . . . . . . . . . 17.11.1 . . . . . . . . 293 Measuring transformers: . . . . . . 6.1-6.5 - capacitor voltage . . . . . . . . . . . . . 6.3 - current . . . . . . . . . . . . . . . . . . . . 6.4 - electromag netic voltage . . . . . . . . 6.2 - novel . . . . . . . . . . . . . . . . . . . . . 6.5 Measurement centres . . . . . . 79-92 . . . . . . . . 83 . . . . . . . . 85 . . . . . . . . 80 . . . . . . . . 92 . . . . . . . . . . . 22.6 . . . . . . . . 404 . . . . . . 15.9.6 . . . . . . . . 248 Low impe ance ifferential protection (biase ): - Busbars: . . . . . . . . . . . . . . . . . 15.9 . - check feature . . . . . . . . . . . . . 15.9.3 . - CT supe rvision . . . . . . . . . . . . . 15.9.4 . - effective setting . . . . . . . . . . . . 15.9.2 . - primary operating current . . . . . 15.9.2 . - stability . . . . . . . . . . . . . . . . . 15.9.1 . - for generators . . . . . . . .. . . . . 17.5.1 . - for transformers . . . . . . . . . . . . . 16.8 . Low spee ( elaye ) auto-reclosing . . . . . . . . . . . . . . . . . . . . . 246 . . . . . . . 248 . . . . . . . 248 . . . . . . . 247 . . . . . . . 247 . . . . . . . 247 . . . . . . . 284 . . . . . . . 261 Measurements: . . . . . . . . . . . 22.1-22.9 . . . . 399-408 - isturbance reco r ers . . . . . . . . . 22.9 . . . . . . . . 407 - general characteristics . . . . . . . . 22.2 . . . . . . . . 399 - measurement centres . . . . . . . . . 22.6 . . . . . . . . 404 - synchronisers . . . . . . . . . . . . . . . 22.8 . . . . . . . . 406 - tariff metering . . . . . . . . . . . . . . 22.7 . . . . . . . . 4 05 - trans ucer selection . . . . . . . . . . 22.5 . . . . . . . . 402 - using a nalogue trans ucers . . . . . 22.4 . . . . . . . . 402 - using igital trans uce rs . . . . . . . 22.3 . . . . . . . . 401 Merz-Priceprotection systems Mesh law Mesh substation, four-switch, auto-reclosing applie to . . . Metering, tariff . . . . . 10.1 . . . . . . . . 153 . . . . . . . . 24 . . . . . . . . . . . . . . . . . 3.6.1.3
14.9 . . . . . . . . 226 . . 14.12.3 . . . . . . . . 230 M Machine reactances (synchronous machines) Machine reactances, effect of saturati on . Magnetically polarise armature relays . . . . . . . . . . . . . . . . . . . . . . . . . 22.7 . . . . . . . . 405 . . . . 8.7 . . . . . . . . 120 Metho s of protection signalling 5.4-5.12 . . . . . . 49-55 . . . . . . . 11.7.6 . . . . . . . . 180 . . . . . . . 11.7.3 . . . . . . . . 17 7 . . . . . . . . . . 7.2.1 . . . . . . . . 100 Magnetisation curve of a current transformer . Magnetising inrush in power trans formers Maintenance of protection equipment . . . . . . . . . 6.4.4 . . . . . . . . 86
Microprocessor-base portable test sets . . . . . . . . . . . . . . . . Micropro cessor-base simulation equipment . . 21.10.2 . . . . . . . . 390 . . . . . . . . . . 16.3 . . . . . . . . 258 . . . . . . . . . 21.7.2 . . . . . . . . 383 . . . . . . . . . . 21.15 . . . . . . . . 396 Microwave communications for rural istribution systems Miniature circuit breake rs (MCB’s) . . . . . . . . . . . . . . . . . 25.6.5 . . . . . . . . 451 Maloperation with reverse faults, istance relays, multi-en e fee ers . . . . . . . . . . 13.4.4 Manipulation of complex quantities . . . . . . . . . . . . . . . . . . . 3.3 . . . . . . 18.5.1 . . . . . . . . 320
. . . . . . . . 212 Minimum length of line for y terminals .
istance protection . . . . . Minimum voltage at rela
. . . . . 11.10.2 . . . . . . . . 186 . . . . . . . . 18 Manual closing of circuit breakers, auto-reclosing schemes . . . . . . . 14.10.8 . Margins, gra ing for overcurrent relays . . . . . . . . . . . . . . . . . . 9 .11-9.12 . . . . . . . . . . . 11.10.1 . . . . . . . . 186 . . . . . . . 228 Mobile ra io communications for rural istribution systems . Monitoring of Power Quality • 487 • . . . . 25.6.3 . . . . . . . . 451 . . . . 130-131 . . . . . . 23.4 . . . . . . . . 415 Network Protection & Automation Gui e In ex • . . . . . . . . . . . 5.12 . . . . . . . . 55
Mho relay: - fully cross-polarise . . - lenticular . . . . . . . . . - offset m ho . . . . . . . . - partially cross-polarise - self-polarise . . . . . . . . . . . . . . 11.7.5 . . . . . . . . 179 . . . . . . . 11.7.4 . . . . . . . . 17 8 . . . . . . . 11.7.4 . . . . . . . . 178
Section Motorcurrents uring stall con itions . . .19.4 . Motor currents uring startin g con itions . . . Motor protection Page . . . . . . . 339 Section Numerical busbar protection Numerical busbar protection, reliability consi erati ons . . . Page . . . . . 15.10 . . . . . . . . 251 . . . . . . . . . . 19.4 . . . . . . . . 339 . . . 18.11.2 . . . . . . . . 329 . . . . 15.10.1 . . . . . . . . 252
Motor power factor correction Motor win ings, heating of . . . . . . . . . 19.1-19.14 . . . . 337-351 . . . . . . . 19.3 . . . . . . . . 338 Numerical ifferential protection system . . . . . . . . . . . . . . . . . . Num erical relay issues Numerical relays . . . 10.8 . . . . . . . . 158 . . . . . . . . . . . 7.7 . . . . . . . . 109 Moul e case circuit breakers (MCCB’s) . . . . . . . . . . . . . . Multi-en e fee ers protection of . . . . . . . . . . . . . . . . . 7.5-7.7 . . . . 102-109 . . . . . . . . 7.5.2 . . . . . . . . 106 . . . 18.5.2 . . . . . . . . 321 Numerical relay software . . . . . . . . 13.3–13.5 . . . . 207-212 . . . . . . 14.10.9 . . . . . . . . 229 NVD – see neutral voltage isplacement relay Multi-shot auto-reclosing Mutual compensation . . . . . . . . . 13.2.2 . . . . . . . . 204 O Offset mho istance relay Ohm istance relay Oil an gas evices for transformer protection . . . . . . . 11.7.4 . . . . . . . . 178 . . . . . . . . . . . 11.7.2 . . . . . . . . 176 Mutual coupling, effect of on unit an istance protection schemes .
. . 13.2 . . . . . . . . 203 N Nature an effect of transformer faults . . . . . . . . . . . . . 16.1 . . . . . . . . 255 . . . . . . . . . 16.15 . . . . . . . . 268 . . . . . . Oil circuit breakers (OCB’s) Operating features of auto-reclose schemes Operation in icators . . . . . . . . 19.7 . . . . . . . . 346 . . . . . . 4.3.2 . . . . . . . . 34 18.5.4 . . . . . . . . 322 Opening time of circuit breakers . 14.4.1.3 . . . . . . . . 221 Negative phase sequence protection of generators . Negative phase sequence prote ction of motors . . . . . . . . . . 17.12 . . . . . . . . 293 . . . . . . . . . . 14.10 . . . . . . . . 227 . . . . . . . . . . 2.10.2 . . . . . . . . 13 Negative sequence network Operation of in uction motors with unbalance voltages . . . Operators in vector algebra Optical fibre channels Out-of-step tripping for generators . . . . . . . . . 19.7 . . . . . . . . 346 . . . . . . . . 19 In ex Negative sequence reactance: - in uction motor . . . . . . . . - synchronousgen erator . . . Network connections for various types of fault . . . Network re uct ion . . . . . . . 3.3.3 . . . . . 19.7 . . . . . . . . 346 . . . . . . 5.9 . . . . . . . . 54 . . . . . . . . . . 8.6.5 . . . . . . . . 119 . . . . . . . . . 4.4 . . . . . . . . 35 . . . . . . . . 25 . . . . . . 11.7.8, 17.17 . . . . 181-301 . . . . . . . . 12 •
. . . . . . . . . . . . 3.6.3 Output evices, of relays Overcurrent protection . . . . . . . . . 2.10 Neutral voltage isplacement relay . . . . . . . . . . . . 17.8.1.3, 17.8.2.4 No ise an interference in protection signalling systems Non-linear resistor, use i n busbarprotection schemes Non-unit transformerfee er protection schemes Notati on, impe ance . . . . . . . 9.1-9.21 . . . . 123-151 . . . . 289-290 . . . . . 8.6 . . . . . . . . 116 . . . 15.8.6.7 . . . . . . . . 246 . . . . . 16.16.1 . . . . . . . . 270 . . . . . . . . 23 . . . . . . . . 92 . . . . . . . . . . . . 3.5 . . . . . 6.5 Novel instrument transformers Number of shots, auto-reclosing schemes . . . 14.4.3, 14.6.6, . . . 223-225 . . . . . . . . . . . . . . . . . . . . . . . 14.10.9 . . . . . . . . 229
Overcurrent relay: - 90° qua rature connection . . . . 9.14.2 . .. . . . . . 133 - calculation of settings . . . . . 9.13, 9.16 . . . . 132-136 - efinite time . . . . . . . . . . . . . . . . 9.9 . . . . . .. . 129 - irectional earth fault . . . . . . . . . 9.17 . . . . . . . . 138 - irectional phase fault . . . . . . . . 9.14 . . . . . . . . 133 - earth fault protection . . . . . . . . . 9.16 . . . . . . . .136 - extremely inverse . . . . . . . . . . . . . 9.7 . . . . . . . . 128 - gra ing margin . . . . . . . . . . 9.11-9.12 . . . . 130-131 - instan taneous . . . . . . . . . . . . . . . 9.5 . . . . . . . . 127 - inverse efinite minimum time . . . . 9.4 . . . . . . . . 126 - other characteristics . . . . . . . . . . . 9.8 . . . . . . . . 129 - timing error . . . . . . . . . . . . . . . 9.11.1 . . . . . . . . 131 - transient over-reach . . . . . . . . . . 9.5.1 . . . . . . . . 128 - very inverse . . . . . . . . . . . . . . . . 9.6 . . . . . . . . 128 • 488 • Network Protection & Automation Gui e
Section Overcurrent starters for istance relays . . . . . . Overfluxing in power transf ormers Page Permissive intertrip over-reaching scheme Permissive intertrip un er-reaching sc heme Section Page . . . . . . . . 11.8.1 . . . . . . . . 183 . . . . . . . . . 12.3.4 . . . . . . . . 196 . . . 16.2.8.3, . . . . . . . . . . . . . . . . . . . . . . 16.2.8.4, . . 16.13 . . . . . . . 258-258 . . . . 268 . . . . . . . . 12.3.2 . . . . . . . . 195 Overhea lines: . . . . . . . . . . . 5.18-5.22 - calculation of impe ances . . 5.19-5.20 - ata . . . . . . . . . . . . . . . . . . . . . 5.24 - equivalent cir cuits . . . . . . . . 5.21-5.22 - with or without earth wires . . . . . 5.21 Ove rheating of power transformers . . . . . . 62-67 . . . . . . 62-63 . . . . . . . . 69 . . . . . . 66-67 . . . . . . . . 66 Petersen Coil, protection of networks earthe using: . . . . . . . . 9.19 . . . . . . . . 141 Phase comparison protection schemes . . .. . . . . . . . . . 10.10.1, 10.11 Pha se comparison protection schemes for Tee’ fee ers . . . Phase fault impe ance mea surement using istance relays . . . . . . . . . 160-162 . . . . 13.3.3 . . . . . . . . 208 . . . . . . . . . . . . 16.4 . . . . . . . . 259 . . . . . . . 17.18 . . . . . . . . 303 Overheating of generators Overloa protection: - of generators . . . . . . . . . . . . . . 17.18 . . . . . . . . 303 - of motors . . . . . . . . . . . . . . . . . 19.3 . . . . . . . . 33 8 - of transformers . . . . . . . 16.2.8.1, 16.4 . . . . 257-259 Over-reach, tra
nsient, of a relay . . . . . . . . . . . . . . . . . . . 9.5.1 . . . . . . . . 128 . . . 11.10.4 . . . . . . . . 187 . . . 9.11.3 . . . . . . . . 131 . . . . . . . . . 11.9.1 . . . . . . . . 184 Phase fault overcurrent relay settings . . . . . . . . . . . . . . Phase-phase f ault analysis Phase-phase-earthfault analysis . . . . . . . . . . . . Phase rev ersal protection . Phase shift ue to system capacitance . . . . . . . . . Phase unbalance relays Plain impe ance relay . . . . . . 9.13 . . . . . . . . 132 . . . . . . . . 36 . . . . . . . 4.4.2 Over-reach of a istance relay Overshoot of overcurrent relays Overspee of gene rators Overvoltage protection of generators . . . . . . . . . . . . . . 4.4.3 . . . . . . . . 36 . . . . . . . 18.10 . . . . . . . . 327 . . . . . . . 17.14.2 . . . . . . . . 295 . . . . . . 10.11.1 . . . . . . . . 162 . . . . . . . . . . 17.9 . . . . . . . . 292 . . . . . . . . . . . . . . 19.7 . . . . . . . . 346 . . . . . . . . . 11.7.2 . . . . . . . . 176 Overvoltaqes (Power Quality) 23.3.3 . . . . . . . . 414 Pole slipping of generators . . . . . . . . . . 13.2.2 . . . . . . . . 204 Parallel fee ers, istance protection of Parallel fee ers, unit protection of . . . . . . 17.17 . . . . . . . . 301 . . . . . . . . 22 Polyphase systems . . . . . . . . . . . . . 3.4.4 . . . . . . . . . . . . 13.2.1 . . . . . . . . 204 Parallel fee ers, overcurrent protection of . . . . . . . . . . Parallel operati on with Utility network . . . . . . . . . . 9.14.3 . . . . . . . . 134 . . . . . . . . . 17.21 . . . . . . . . 306 Positive sequence equivalent circuits: - auto-transformer . . . . . . . . . . .
. 5.16 - cables . . . . . . . . . . . .. . . . . . . 5.23 - in uction motor . . . . . . . . . . . . . 19.7 . - overhea transmission lines . . . . . 5.22 - syn chronous machine . . . . . . . . . . 5.8 - transformer . . . . . . . . . . . . . . . . 5.14 Positive sequence reactance of synchronous machine . . . . . . . . . 5.4, 5.8 Positive sequence network . . . . . . . . . .4.3.1 Power/angle curve . . . . . . . . . . . . . . . . .14.5 . Power factor correction . . . . . . . . . . . .18.11 . Power Line Carrier Communications (PLCC) technique . . . . . . . . . . . . . . . . . . . . . . . .8.6.3 • 489 • . . . . . . . . 59 . . . . . . . . 68 . . . . . . . 346 . . . . . . . . 67 . . . . . . . . 53 . . . . . . . . 56 Partially cross-polarise mho relay . . . . . . . . . . . . . . . . Peak voltage evelope by current transformers . . . . . 11.7.6 . . . . . . . . 180 . . . . . . 49-53 . . . . . . . . 34 . . . . . . . 223 . . . . . . . 328 . . . . 6.4, 15.8.6.7 . . . . . 85-246 Percentage un er-reach of istance relays . . . . . . . . Perio ic maintenance t ests . . . . . 11.10.3 . . . . . . . . 187 . . . . . 11.3 . . . . . . . . 172 Performance of istance relays . . . . . . 21.15 . . . . . . . . 396 . . . . . . . . 117
Network Protection & Automation Gui e In ex • P Plain tone signals, protection signalling using . . . . . . . 8.7.2 . . . . . . . . 120
Section Power Line Carrier Communications in rural istribution networks . . . Power Lin e Carrier unit protection phase comparison schemes Page Section Page . . . . . . . 25.6.6 . . . . . . . . 452 . . . . . . . . . . 13.3.3 . . . . . . . . 208 Power Quality: . . . . . . . . . . . 23.1-23.7 . . . . 411-420 - causes . . . . . . . . . . . . . . . . . . . 23.3 . . . . . . . . 413 - classification . . . . . . . . . . . . . . . 23.2 . . . . . . . . 411 - examples of . . . . . . . . . . . . . . . 23.6 . . . . . . . . 419 - impact of . . . . . . . . . . . . . . . . . 23.3 . . . . . . . . 413 - management in istribution systems . . . . . . . . . . . . . . . . . 25.7.3 . . . . . . . . 452 - monitoring . . . . . . . . . . . . . . . . 23.4 . . . . . . . . 415 - reme ial measures . . . . . . . . . . . 23. 5 . . . . . . . . 417 Power quantities . . . . . . . . . . . . . . 3.4.3 . . . . . . . . 22 - busbars . . . . . . . . . . . . . . 15.1-15.10 . . . . 233-251 - capacitors . . . . . . . . . . . . . . . 18.11.3 . . . . . . . . 330 - complex transmission c ircuits . . . . . . . . . . . . . . . 13.1-13.8 . . . . 203-217 - generators . . . . . . . . . . . . 17.1-17.21 . . . . 281-314 -motors. . . . . . . . . . . . . . 19.1-19.13 . . . . 337-351 - multi-en e fee ers – istance protection . . . . . . . 13.4, 13.5 . . . . 210-212 - multi-en e fee ers – unit protection . . . . . . . . . . . . . . 13.3 . . . . . . . . 207 - parallel fee ers . . . . . . . . . . . . . 13.2 . . . . . . . . 203 - phase reversal . . . . . . . . . . . . . 1 8.10 . . . . . . . . 327 - series compensate lines . . . . . . . 13.6 . . . . . . . . 214 - synchronous motors . . . . . . . . . 19.13 . . . . . . . . 348 -tr ansformers . . . . . . . . . . . 16.1-16.10 . . . . 255-279 - transformer-fee er s . . . . . . . . . 16.16 . . . . . . . . 270 - voltage transformers . . . . . . . . . 6.2.4 . . . . . . . . . 81 Protection performance Protection requirements for busbars .. . . . . . . . Protection reset time, auto-reclose schemes Prote ction scheme, efinition of . . . . . . . . . . . . . 2.4.6 . . . . . . . . .9 Power swing blocking Power system plant . . . . . . . . . 11.10.6 . . . . . . . . 188 . . . . . . . . 47 . . . . . . . . . . . . . 5.1 Protection relay – see Relay . . . . . . . . . 15.3 . . . . . . . . 234
Power transformer – see Transformer Primary istribution system automation . . . . . . . . . . Primary protection . Primary injection testing Prime mover, failure of . . . . . . . 25.3 . . . . . . . . 445 . . . . . . . . . . . . . 2.9 . . . . . . . . . 11 . . . . . . . . 21.12 . . . . . . . . 394 . . . . . . . . 14.4.1.5 . . . . . . . . 222
. . . . . . . . . . . . . 2.2 . . . . . . . . .7 . . . . . . . . 17.19.1 . . . . . . . . 303 . . . . . . . 11.2 . . . . . . . . 1 72 . . . 9.3 . . . . . . . . 124 Principles of istance relays Principles of time/current gra ing In ex Principles of unit protection systems . . . . . . . . . . . . . . . . . . . . 10.1 . . . . . . . . 153 Private generation – see embe e generation Private pilot wires an channels . . . 8.6.1 . . . . . . . . 116 Protection signalling: . . . . . . . . . 8.1-8.7 . . . . 113-120 - effect of int erference . . . . . . . . . . 8.6 . . . . . . . . 116 - effect of noise on . . . . . . . . . . . . . 8.6 . . . . . . . . 116 - for unit protection schemes . . . . . . 8.2 . . . . . . . . 113- intertripping . . . . . . . . . . . . . . . . 8 .4 . . . . . . . . 114 - metho s . . . . . . . . . . . . . . . . . . . 8.7 . . . . . . . . 120 - performance requirements . . . . . . . 8.5 . . . . . . . . 115 - signalling metho s . . . . . . . . . . . . 8.7 . . . . . . . . 120 - transmiss ion me ia . . . . . . . . . . . 8.6 . . . . . . . . 116 Protection system, efin ition of Pull-out protection for synchronous motors . . . . . 2.2 . . . . . . . . .7 •
Pro uction testing of relays . . . . . . . 21.8 . . . . . . . . 386 Programmable scheme logic in numerical relays . . . . . . . . . . . . . . 7.6.6 . . . . . . . . 109 . . . . . . . 19.13.1 . . . . . . . . 348 Protection against: - asynchronous operation for generators . . . . . . . . . . . . . 17.16 . - loss of Utility supply . . . . .. . . 17.21.1 . - pole slipping for generators . . . . 17.17 . - power swings – istance relays . . 11.7.8 . - su en restoration of supply . . 19.13.2 . Protection equipment, efinition of . . . . . . . . . . . . . . . . . . 2.2 . . . . . . . 298 . . . . . . . 306 . . . . . . . 181 . . . . . . . 301 Q Qua rature axis machine impe ances . . . . . . . . . Qua rilateral relay . . . . . . 5.5, 5.11
. . . . . . 50-55 . . . . . . . 349 Qua rature connecte relays . . . . . 9.14.2 . . . . . . . . 133 . . . . . . . . . . . . 11.7.7 . . . . . . . . 181 . . . . . . . . .7 . . . . 371-397
Protection equipment testing . 21.1-21.15 Protection of: - a.c. electrifie rail ways R Ra io channels, protection signalling • 490 • . . . . . . . . . . . 8.6.4 . . . . . . . . 119 . . . . . 20.1-20.8 . . . . 353-369 Network Protection & Automation Gui e
Section Ra io communications for istribution systems . . . 25.6.3, Page . . . . 451-451 Section Page 25.6.4 Railways: . . . . . . . . . . . . . . . 20.1-20.8 . . . . 353-369 - autotransfor mer fee ing . . . . . . . 20.6 . . . . . . . . 361 - catenary backup protection . . . . . 20.5 . . . . . . . . 361 - catenary thermal protection . . . . . 20.4 . . . . . . . . 360 - classical single-phase fee ing . . . . 20.3 . . . . . . . . 354 - example of protection . . . . . . . . . 20.8 . . . . . . . . 365 - fee e r substation protection . . . . . 20.7 . . . . . . . . 365 - protection of a.c. electrifie . 20.1-20.8 . . . . 353-369 - protection philosophy . . . . . . . . . 20.2 . . . . . . . . 354 RCA - see Relay characteristic angle Reactances: - au to-transformer - cables . . . . . . . . 5.17 . . . . . . 59-60 . . . . 5.18-5.20 , 5.23, . . . 62-63-68 . . . . . . . . . . . . . . . . . . . . . . . . 5.24 . . . . . . . . 69 in uction motor . . . . . . . . . . . . . 19.7 . . . . . . . . 346 - overhea transmission lines . . . . . . . . . . . . . 5.18-5.22, 5.24 . . . 62-67-69 - synchronous machine . . . 5.4, 5.8-5.12 . . . 49-53-55 - transforme r . . . . . . . . 5.14-5.15, 5.17 . . . 56-57-60 . . . . . . . . 5.16, . . . . . . . . . 5.3 . . . . . . . . 49 Reaction, armature, of synchronous machines Reclaim time in auto-reclosing . . . . . . . . . 14.4.2, . . . . . . . . . . . . . . . . . . . . . . . 14.6.5, 14.10.6 . . . . . . . 222-225 . . . . 228 Reclosers, automatic . . . . . . . . . . 25.5.2 . . . . . . . . 449 Reclosing: - elaye . . . . . . . . . . . . . . . . . . 14.9 . . . . . . . . 22 6 - high-spee . . . . . . . . . . . . 14.6-14.8 . . . . 223-226 - of EHV networ ks . . . . . . . . . . . . 14.5 . . . . . . . . 223 - of HV networks . . . . . . . . . . . . . 14.3 . . . . . . . . 220 -of live lines . . . . . . . . . . . . . . . 14.9.1 . . . . . . . . 227 Recommen e gra ing margins . . . . 9.12 . . . . . . . . 131 . . . 22.9 . . . . . . . . 407 Recor ers, event an isturbance Relay: - application of irectional overcurrent . . . . . . . . . . . . . . 9.14 .3 . . . . . . . . 134 - application tables . . . . . . . . . . . . . A3 . . . . 469-475 - bias (of ifferential relays) . . . . 10.4.2 . . . . . . . . 155 - ch aracteristic angle (RCA) . . . . . . . . . . . . . 9.14.2, 11.7.3 . . . . 133-17 7 - Characteristics: ANSI . . . . . . . . . . . . . . . . . . . . 9.4 . . . .. . . . 126 custom . . . . . . . . . . . . . . . . . . 9.8 . . . . . . . . 129 ef
inite time . . . . . . . . . . . . . . . 9.9 . . . . . . . . 129 istance . . . . . . . . . . . . . . . . . 11.7 . . . . . . . . 175 IDMT . . . . . . . . . . . . . . . . . . . . 9.4 . . . . . . . . 126 extremely inverse . . . . . . . . . . . . 9.7 . . . . . . . . 128 • 491 • IEC . . . . . . . . . . . . . . . . . . . . . 9.4 . . . . . . . . 126 instantane ous . . . . . . . . . . . . . . 9.5 . . . . . . . . 127 neutral voltage isplace ment . . . . . 17.8.1.3, 17.8.2.4 . . . . 289-290 very inverse . . . . . . . . . . . . . . . 9.6 . . . . . . . . 128 - connections for irectional elements . . . . . . . . . . . . . 9.14, 9.17.1 . . . . 133-139 - contact systems . . . . . . . . . . . . 2.10.1 . . . . . . . . 12 - current setting . . . . . . . . . . . . . . 9.10 . . . . . . . . 129 - ata management in numerical relays . . . . . . . . . . . . 7.7.2 . . . . . . . . 110 - eterioration in service . . . . . . . . 2.4.5 . . . . . . . . . 9 - Example setting calculations: istance . . . . . . . . . . . . . . . . 11.12 . . . . . . . . 189 generator . . . . . . . . . . . . . . . 17.22 . . . . . . . . 308 in uction motor . . . . . . . . . . . 19.14 . . . . . . . . 349 overcurrent . . . . . . . . . . . . . 9.20.1 . . . . . . . . 143 ring main . . . . . . . . . . . . . . . 9.20.4 . . . . . . . . 150 transformer . . . . . . . . . . . . . . 16.19 . . . . . . . . 276 unit protection . . . . . . . . . . . . 10.12 . . . . . . . . 167 - features of numerical relays . . . 7.5 -7.6 . . . . 102-107 - flags . . . . . . . . . . . . . . . . . . . 2.10.2 . . . . . . . . 13 - gra ing margins . . . . . . . . . 9.11-9.12 . . . . 130-131 - ins tallation of . . . . . . . . . 2.4.3, 21.16 . . . . . . 9-397 - issues with nume rical relays . . . . . . 7.7 . . . . . . . . 109 - maximum torque angle – see rela y characteristic angle - operation in icators . . . . . . . . . 2.10.2 . . . . . . . . 13 - output evices . . . . . . . . . . . . . . 2.10 . . . . . . . . 12 overcurrent . . . . . . . . . . . . . . . . . 9.4 . . . . . . . . 126 - oversho ot . . . . .. . . . . . . . . . . 9.11.3 . . . . . . . . 131 - performance, ef inition for istance relays . . . . . . . . . . . . 11.3 . . . . . . . . 172 - p ro uction testing . . . . . . . . . . . 21.8 . . . . . . . . 386 - relay charact eristic angle . . . . . . 9.14.2 . . . . . . . . 133 - routine maintenance of . . . . . . . 21.15 . . . . . . . . 396 - selectivity . . . . . . . . . . . . . . . . . . 2.5 . . . . . . . . . 10 - selftesting . . . . . . . . . . . . . . . . . 7.5 . . . . . . . . 102 - setting stu y, basic requirements . . . . . . . . . . . . 9.2 . . . . . . . . 123 - spee of operation . . . . . . . . . . . . 2.7 . . . . . . . . . 10 - stability . . . . . . . . . . . . . . . . . . . 2.6 . . . . . . . . . 10 - targets . . . . . . . . . . . . . . . . . . 2.10.2 . . . . . . . . 13 - testing an commissioning . . . . . 7.7.2, 21.9-21.12 . 110-387-394 - t ime gra ing margin . . . . . . . . . . 9.11 . . . . . . . . 130 - timing error . . . . . . . . . . . . . . . 9.11.1 . . . . . . . . 131 - transient over-reach . . . . . . . . . . 9.5.1 . . . . . . . . 128 - trip circuit supervision . . . . . . . . . 2.12 . . . . . . . . 14 - tripping circuits . . . . . . . . . . . . . 2.11 . . . . . . . . 13 - type testing of . . . . . . . . . . 21.2-21.7 . . . . 372-383 - voltage setting in busbar protection . . . . . . . . 15.8.6.3 . . . . . . . . 245 Network Protection & Automation Gui e In ex •
Section Page Rotor earth faultprotection for synchronous generator . Rotor protection (gener ator) Rural secon ary istribution system automation . . . . . . Section Page Relay types: . . . . . . . . . . . . . . . 7.1-7.5 . . . . . 99-102 - electromec hanical . . . . . . . . . . . . 7.2 . . . .. . . . 99 - ifferential . . . . . . . . . . . . 10.4-10.11 . . . . 154-162 - igital . . . . . . . . . . . . . . . . . . . . 7.4 . . . . .. . . 102 - irectional . . . . . . . . . . . . . 9.14, 9.17 . . . . 133-138 - istance . . . . . . . . . . . . . . . . . . 11.7 . . . . . . . . 175 - earth fault . . . . . . . . . . . . . . . . 9.16 . . . . . . . . 136 - numerical . . . . . . . . . . . . . . . 7.5, 7.6 . . . . 102-107 - overcu rrent . . . . . . . . . . . . . . . . . 9.4 . . . . . . . . 126 - ROCOF . . . . . . . . . . . . . . . . . 17.21.2 . . . . . . . . 307 - static . . . . . . . . . . . . . . . . . . . . . 7.3 . . . . . . . . 101 - voltage vector shift . . . . . . . . . 17.21.3 . . . . . . . . 307 Reliability of protection equipment . . . . . . . . . Reliability of numerical busbar protection . . . . . . . . . . . . . 2.4 . . . . . . . . .9 . . . . 17.15.1 . . . . . . . . 296 . . . . . 17.15 . . . . . . . . 296 . . . . . . 25.5 . . . . . . . . 449 S Salient pole rotor, synchronous machine Saturation of current transformers . . . . . . . . . . . 5.5 . . . . . . . . 50 . . . . . . . . . . 6.4.10 . . . . . . . . 89 . . . . . . . . 15.10.1 . . . . . . . . 252 Secon ary istribution system automation . . . . . . . . . 25.4-25.5 . . . . 447-449
Remanence flux, effect ofin a current transformer . . Reme ial measures for Pow er Quality problems Rente pilot circuits an channels . . . . . Requirementsof the signalling channel Resi ual compensation in istance relays . . . . Resi ua l current . . . . . 6.4.10.2 . . . . . . . . . 91 Secon ary injection test equipment . . . . . . . . . Sectionalisers, automatic S electivity of protection equipment . . . . . . . . . . . . . . . . . 21.10 . . . . . . . . 389 . . . . . . 21.11 . . . . . . . . 392 Secon ary injection testing
. . . . . . . . . 23.5 . . . . . . . . 417 . . . . . . . 25.5.2 . . . . . . . . 449 . . . . . . . . . . . 8.6.2 . . . . . . . . 116 . . . . . . . . . . 2.5 . . . . . . . . . 10 . . . Self-testing (of numerical relays) . . . . . . . . . . . 8.5 . . . . . . . . 115 7.5 . . . . . . . . 102 . . . . . . . . . 11.9.2 . . . . . . . . 185 . . . . . . . . . . . . . . 4.6.1 . . . . . . . . . 41 Sensitive earth fault protection: - of generators . . . . . . . . . . . . 17.8.1 .2 . . . . . . . . 288 - of motors . . . . . . . . . . . . . . . . . 19.6 . . . . . . . . 342 - overcurrent . . . . . . . . 9.16.3, 9.17-9.19 . 138-138-141 Sens itivity of protection equipment . . . . . . . . . Sequence networks . . . . . . . . . . 2.8 . . . . . . . . . 11 . . . . . . . . 33 In ex Resi ual flux, effect of in a current transformer . . . . . . . 6.4.10.2 . . . . . . . . . 91 Resi ually connecte voltage transformers . . . . . . . . . . . Resi ual voltage . . . . . . . . . . . . . 4.3 . . . . . 6.2.6 . . . . . . . . . 81 . . . . . . . . . . . . . . 4.6.1 . . . . . . . . . 41 •
Restricte earth fault protection: - of generators . . . . . . . . . . . - of tr ansformers . . . . . . . . . . Reverse faults, maloperation of istance protecti on schemes on Reverse looking relay setting – . . 17.8.3 . . . . . . . . 291 . . . 16.7 . . . . . . . . 260 Sequence reactances: - auto-transformer . . . . . . . . . . . . 5.16 . . . . . . . . 59 - cables . . . . . . . . . . . . . . . . . . . 5.23 . . . . . . . . 68 in uction motor . . . . . . . . . . . . . 19.7 . . . . . . . . 346 - overhea t ransmission lines . 5.18-5.22 . . . . . . 62-67 - synchronous generator . . . . . . 5.8-5.10 . . . . . . 53-55 - transformer . . . . . . . . . . . . 5.14-5.15 . . . . . . 56-57 Series compensate lines, protection of . . . . . . . . Series sealing, relay tripping circuits . . . . . . . . . . 13.4.4 . . . . . . . . 212 13.6
. . . . . . . . 214 istance relay blocking schemes . 12.4.1.1 . Reverse power protection of generat ors . . . . . . . . Ring main unit Ring mains: - example of gra ing - gra ing of . . . . . . . ROCOF relay . . . . . . . 198 . . . . . . . . . . 2.11.1 . . . . . . . . 14 . . . . . . 17.11.2 . . . . . . . . 293 Setting of overcurrent relay SF6 circuit breakers, use in auto-reclose schemes S hunt fault equations Shunt reinforcing, relay tripping circuits • 492 • . . . . . . . 9.10 . . . . . . . . 129 . . . . . . . . . . . . . . . 25.4 . . . . . . . . 447 . . . . 14.6.3.3 . . . . . . . . 225 . . . . . . . . 35 . . . . . . . . . 9.20.4 . . . . . . . . 150 . . . . . . . . . . 9.15 . . . . . . . . 135 . . . . . . . . . . . 4.4 . . . . . . . . . . . . . . . 17.21.2 . . . . . . . . 307 . . . . . . . . . . 2.11.2 . . . . . . . . 14
Network Protection & Automation Gui e
Section Shunt reinforcing with series sealing, relay tripping circuits Signalling, prote ction Signalling performance requirements . . . . . . . Sign conventions . . . . 2.11.3 Page in uction motors . . . . . . . . 14 Section . . . . . . . . . 19.3, Page . . . . 338-342 19.6 . . . . . . . . . 8.1-8.7 . . . . 113-120 Stator win ings, electrical faults in: - generators . . . . . . . . . . . . . . . . 17.3 . . . . . . . . 283 - in uction motors . . . . . . . . 19.3, 19.6 . . . . 338-342 Stea y-state theory of synchronous machines . . . . . . . . . . . 5.4 . . . . . . . . 49 . . . . . . . . . . 8.5 . . . . . . . . 115 . . . . . . . . . . . . . 3.4.2 . . . . . . . . . 21 . . . . . . . . 8.6.3 . . . . . . . . 117 . . . . . . 14.7 . . . . . . . . 225 Single frequency line trap Single phase open circuit fault analysis . . . . . . . . Single phase to earth fault analysis Single phase systems Single-phase auto-reclosing . . . . . . . . 4.4.5 . . . . . . . . 37 . . . . . . . . . . 4.4.1 . . . . . . . . . . . 3.4.4 . 14.4.3, . . . . . . . . 36 . . . . . . . . 22 . . . . 223-225 Substation Control an Automation . . . . . . . . . . . . . . 24.1-24.7 . . . . 423-441 - communication protocols . . . . . . 24.4 . . . . . . . . 427 - example s of . . . . . . . . . . . . . . . 24.7 . . . . . . . . 433 - functionality . . . . . . . . . . . . . . . 24.2 . . . . . . . . 423 - har ware . . . . . . . . . . . . . . . . . 24.3 . . . . . . . . 425 - system configuration . . . . . . . . . 24.6 . . . . . . . . 432 - system testing . . . . . . . . . . . . . . 24.6 . . . . . . . . 432 - topology . . . . . . . . . . . . . . 24.2-24.3 . . . . 423-42 5 Sub-transient reactance of generators . . . . . . . . . . . . . . 5.8, Single shot auto-reclosing Single switch substation, auto-reclose applie to . . 14.6.6 5.11 . . . . . . 53,
55 . . . . . . 14.12.2 . . . . . . . . 230 Software version control in numerical relays . . . . . . . Su en restoration of supply, synchronous motor protection Summation arrangement s, unit protection . . . . . . . . Superposition theorem Supply interruptions (P ower Quality) . . . Supervision of trip circuits . . . 19.13.2 . . . . . . . . 349 . . . . . . . 7.7.1 . . . . . . . . 109 Softwaretools for istribution system automation . . . . . . . Software type te sts Spee of protection equipment Stabilising resistance . . . . . . . 10.6 . . . . . . . . 156 . . . . . . . . 25 . . . . . . . . 14 . . . . . 25.7 . . . . . . . . 452 . . . . . . . . 3.6.2.1 . . . . . . . . 2.12 . . . . . . . . . . . . 21.6 . . . . . . . . 381 . . . . . 2.7 . . . . . . . . . 10 . . . . 2.6 . . . . . . . . . 10 Stability of protection equipment Stalling of in uction motors Stan by transform er, auto-close schemes . . . . . . . . . . . 23.3.8 . . . . . . . . 415 . . . . . . . . . . 10.4.1 . . . . . . . . 155 . . . . . 19.4.2 . . . . . . . . 340 . . . 9.4 . . . . . . . . 126 Switch-on-to-Fault feature Synchronisers Synchronising . . . . . . 11.6.4 . . . . . . . . 175 . . . . . 4.3 . . . . . . . . 33 Symmetrical component theory Synchronism check relay Stan ar IDMT overcurrent relays . . . . . 14.9.2, 22.8 . . . . 227-406 . . . . . . . . . 14.11.1 . . . . . . . . 229 . . . . . . . . . . . . . . . . 22.8 . . . . . . . . 406 Star-connecte win ing of a transformer, impeance earthe . 16.2.1 . Star-conne cte win ing of a transformer, soli ly earthe Star/ elta theorem, Kennelly s St arting protection for in uction motors
. . . . . . . 256 . . . . 16.2.2 . . . . . . . . 256 . . . . 3.6.2.3 . . . . . . . . 25 . . . . . . . . . . 19.4.1 . . . . . . . . 339 Starting relays for switche istance protection . . . . . Static relays . . . . . . 11.8.1 . . . . . . . . 183 . . . . . . . . . . . . . . . . . . 7.3 . . . . . . . . 101 Static ifferential protection systems . . . . . . . . . . . . . . Stator protec tion: - for generators Synchronous machines: . . . . . . . . . . 5.2 . . . . . . . . 47 - armature reac tion . . . . . . . . . . . . 5.3 . . . . . . . .49 - asymmetry . . . . . . . . . . . . . . . . . 5.7 . . . . . . . . 53 - cylin rical rotor . . . . . . . . . . . . . . 5.4 . . . . . . . . 49 - irect axis reactances . . . . . . 5.4-5.11 . . . . . . 49-55 - negative sequence reactance . . . . . 5.9 . . . . . . . . 54 qua rature axis reactances . . . 5.5, 511 . . . . . . 50, 55 - reactances . . . . . . . . . . . . . . 5.4-5.12 . . . . . . 49-55 - salient pole rotor . . . . . . . . . . . . . 5.5 . . . . . . . . 50 - saturation, effect on reactances . . 5 .12 . . . . . . . . 55 - stea y-state theory of . . . . . . . . . 5.4 . . . . . . . . 49 - transient analysis . . . . . . . . . . . . . 5.6 . . . . . . . . . 51 - zero sequence reactance . . . . . . . 5.10 . . . . . . . . 55 Synchronous mot or protection: . . . 19.13 . - low forwar power protection . . . . . . . . . . . . . 19.13.2.2 . - out-of step protection . . . . . . . 19.13.1 . • 493 • . . . . . . . 348 . . . . . . . 349 . . . . . . . 348 . . . . . . 10.7 . . . . . . . . 156 . . . . . . . . . . 17.4-17.5 . . 284-28for
Network Protection & Automation Gui e In ex • . . . . . . . . . . . . . . . . 22.8 . . . . . . . . 406
Section - protection against su en restoration of supply . . . . - un erfrequency prote ction Page - Type: . . . . . . . . . . ynamic vali ation electrical . . . . . . electromag netic compatibility . . . . environmental . . . pro uct safety . . . software . . . . . . . Theorems, circuit . . . . . . Section 21.2-21.7 Page . . . . 372-383 . . . . 19.13.2 . . . . . . . . 349 . . 19.13.2.1 . . . . . . . . 349 . . . . . . . . . . 21.2 . . . . . . . . 372 . . . . . . . . . . 21.3 . . . . . . . . 373 . . . . . . . . . . 21.5 . . . . . . . . 380 . . . . . . . . . . 21.4 . . . . . . . . 379 . . . . . . . . . . 21.7 . . . . . . . . 383
System configuration, substation automation an control . . . . . . System confi guration software tools for istribution systems . . System earthing, effect of on zero sequence quantities System fault stu ies . . . 24.6 . . . . . . . . 432 . . . 25.7.4 . . . . . . . . 453 . . . . . . . . . . 21.6 . . . . . . . . 381 . . . . . . . 4.6 . . . . . . . . . 41 . . . . . . . . 38 . . . . . . . . . . . . . 3.6.2 . . . . . . . . 24 . . . . . . . . . . . . 4.5
System testing, substation control an automation . . . . . . . . . . . . System tripping angles, unit protection schemes System Zo/Z1 ratio . . 24.6 . . . . . . . . 432 Theory: - current transformer . . . . . . . . . . . 6.4 - synchronous machine . . . . . . 5.2-5.12 - transformer . . . . . . . . . . . . 5.14-5.16 - voltage tra nsformer . . . . . . . . . . . 6.2 . . . . . . 47-55 . . . . . . 56-59 . . . . . . . . 85 . . . . . . . . 80 . . . . . . . 10.11.2 . . . . . . . . 164 . . . . . . . . . . . . . 4.6.2 . . . . . . . . . 41
System ZS/ZL ratio, istance relay performance . . . . . . . 11.4 . . . . . . . . 173
Thermal protection: - of a.c. electrifie . . . . . . . 20.4 . . . . . . . . 360 . 17.18 . . . . . . . . 303 - of motor . . . . . . . . . 338 - of transformer . . 273 Thévenin s theorem . . . . . . . . . . . 3.6.2.2 . . . . . .
railway catenary . . . . . . . . . . . of generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.3 . . . . . . . 16.4, 16.18 . . . . 259. . 25
T Tables, equipment application Tank-earth protection (transformers) . . . . . Tar iff metering Targets . . . . . . A3 . . . . 469-475 Three-phase auto-reclosing . . . . . . . . . 14.3, 14.6, 14.9 . 220-223-226 . . . . . . . . 37 . . . . . . . . . 16.14 . . . . . . . . 268 Three-phase fault calculations Three-phase fault analysis Three-phase fault calc ulations Time/current characteristics of overcurrent relays . . . . . Time gra i ng of earth fault relays . . . . . . . . . . . . . . Time synchronisation in num erical relays . . . . . . . 4.2 . . . . . . . . . 31 . . . . . . . . . . . . . . . 22.7 . . . . . . . . 405 . . . . . . . . 13 . . . . . . . 4.4.4 . . . . . . . . . . . . . . . . . . . 2.10.2 . . . . . . 4.2 . . . . . . . . . 31 . . 13.3 . . . . . . . . 207 In ex Tee fee er unit protection schemes . . . . . . . . . . . . . 13.3.2-13.3.4 Tel ephone type pilots Teleprotection signalling systems Terminology Test block Test plug . . . . . . . 8.6.1, Three terminal lines, protection of . . . . 207-209 . . . . 116-116 8.6.2 . . . . . . . 9.4 . . . . . . . . 126 . . . . . . . . . . . . . . 8.3 . . . . . . . . 114
. . . . . . 9.16.2 . . . . . . . . 137 . . . . . . • . . . . . . . . . . . . . . . . . . A1 . . . . 455-465 . . . . . . . . . . . . . . . . 21.10.1 . . . . . . . . 390 Time multiplier setting (TMS) 9.4 . . . . . . . . 126 . . . . . . . . . . . . . . . . . 21.10.1 . . . . . . . . 390 . . . . . . 7.6.5, 10.8.1 . . . . 108-158 Tests: - commissioning . . . . . . . . . . injection tests . . . . . . . . . . . . . . . . . . . 21.15 . - primary injection . ion . . . . . . . . . . . . . . . . 21.8 . . 21.13 . - secon ary injection . . . . . l an automation systems . . . . . . . . . ation . . . . . . . . . . . . . . 21.14 .
. . . . 21.9 . - equipment, secon ary 21.10 . - maintenance . . . . . .. . . . . . . . . . . . 21.12 . - pro uct - protection scheme logic . . . . . . . . . . . 21.11 . - substation contro . 24.6 . - tripping an alarm annunci
. . . . . . . 387 . . . . . . . 389 . . . . . . . 396 . . . . . . . 394 . . . . . . . 386 . . . . . . . 395 . . . . . . . 392 . . . . . . . 432 . . . . . . . 39 6 Topology of substation control an automation systems . . . . Transactor Trans u cers: . . . . . . . . - analogue . . . . . . . . - igital . . . . . . . . . . measurement centres - selection of . . . . . . . . . . . . 24.2 . . . . . . . . 423 . . . . . . . . . . . . . . . . . . 10.5 . . . . . . . . 156 . . . . . 22.3-22.6 . . . . . . . . . 22.3 . . . . . . . . 401 . . . . . . . . . 22.5 . . . . . . . . 402 . . . . . . . . . 22.6 . . . . . . . . 404 . . . . . . . . . 22.4 . . . . . . . . 402 . . . . 401-404 Transfertrip istance protection schemes: . . . . . . . . . . . . . . . . . . 1 2.3 . - irect un er-reach scheme . . . . 12.3.1 . - permissive over-reach schem e . . 12.3.4 . • 494 • . . . . . . . 195 . . . . . . . 195 . . . . . . . 196 Network Protection & Automation Gui e
Section - permissive un er-reaching acceleration scheme .. . . - permissive un er-reach scheme . . . . . . . . . . . . - weak infee con itions . . Page Transient over-reach of a relay Section Page . . . . . 12.3.3 . . . . . . . . 196 . . . . . 12.3.2 . . . . . . . . 195 . . . . . 12.3.5 . . . . . . . . 197 . . . . 9.5.1 . . . . . . . . 128 Transient reactance of generator . . . . . . . . . . . . . . 5.6, 5.8.2 . . . . . . 51-54
Transformer: - con ition monitoring . . . . . . . . 16.18 . . . . . . . . 273 core faults . . . . . . . . . . . . . . . 16.2.6 . . . . . . . . 257 - equivalen t circuits . . . . . . . . 5.14-5.15 . . . . . .56-57 - faults, nature an effe ct . . . . . . . 16.2 . . . . . . . . 256 - impe ances . . . . . . . . . . . . . . . . 5.17 . . . . . . . . 60 - Instrument: . . . . . . . . . . . . . 6.1-6.5 . . . . . . 79-92 capacitor voltage . . . . . . . . . . . . 6.3 . . . . . . . . 8 3 current . . . . . . . . . . . . . . . . . . 6.4 . . . . . . . . 85 electromagn etic voltage . . . . . . . 6.2 . . . . . . . . 80 magnetising inrush . . . . . . . . . . 16.3 . . . . . . . . 258 novel . . . . . . . . . . . . . . . . . . . . 6.5 . . . . . .. . 92 overheating . . . . . . . . . . . . . . 16.4 . . . . . . . . 259 overloa con itions . . . . . . . 16.2.8.1 . . . . . . . . 257 overvolta ge con itions . . . . . 16.2.8.3 . . . . . . . . 258 positive sequence equivalen t circuit . . . . . . . . . . . . . . . . . . 5.14 . . . . . . . . 56 - Protecti on . . . . . . . . . . . . 16.5-16.15 . . . . 260-268 Buchholz relay . . . . . . . . . . 16.15.3 . . . . . . . . 269 combine ifferential an earth–fault . . . . . . . . . . . . . . 16.10 . . . . . . . . 264 elta-connecte win ings . . . . . 16.7 . . . . . . . . 260 elta/star connecte . . . . . . . . . 16.8 . . . . . . . . 261 ifferential . . . . . . . . . . . 16.8-16.10 . . . . 261-264 example s of . . . . . . . . . . . . . 16.19 . . . . . . . . 276 intertripping . . . . . . . . . . . . . 16.17 . . . . . . . . 273 oil an gas evices . . . . . . . . . 16.15 . . . . . . . . 268 overcurrent . . . . . . . . . . . . . . . 16.6 . . . . . . .. 260 overfluxing . . . . . . . . . . . . . . 16.13 . . . . . . . . 268 overloa protection . . . . . . . 16.2.8.1 . . . . . . . . 257 restricte earth fault . . . . . . . . 16.7 . . . . . . . . 260 tank-earth . . . . . . . . . . . . . . 16.14 . . . . . . . . 268 thermal . . . . . . . . . . . . . 16.4, 16.18 . . . . 259-273 - zero sequence equivalent circuit . . 5.15 . . . . . . . . 57 - R eactances: positive . . . . . . . . . . . . . . 5.14, 5.17 . . . . . . 56-60 zer o . . . . . . . . . . . . . . . . 5.15, 5.17 . . . . . . 57-60 Transformer-fee e r protection of . . . . . . . . . . 10.12.2, Transient response: - of a capacitor voltage transformer . . . . . . . . - of a current transformer - of a voltage transformer Transients (Power Quality) Transl ay balance voltage electromechanical unit protection system . . . . . . . . . 6.3.2 . . . . . . 6.4.10 . . . . . . . 6.2.7 . . . . .
. . . . . . . . 84 . . . . . . . . 89 . . . . . . . . 82 . . . . . . . . 415 23.3.10 . . . . . . . 10.7.1 . . . . . . . . 156 Translay S static circulating current unit protection . . . . Transmission lin e with or without earth wires . . . .Transmission lines: - ata . . . . . . . . . - equivalent circuit - series impe ance - shunt impe ance . . . . . 10.7.2 . . . . . . . . 157 . . . . . . . . 5.21 . . . . . . . . 66 . . . . . . . . . . . . 5.24 . . . . . . . . 5.19, . . . . . . . . . . . . . . . 69 . . . . . . 62, . . . . . . . . . . . . . 5.18-5.22 . . . . . . 62-67 5.22 . 5.20, 5.22 67 . 63, 67
Transmission me ia interference an noise: . . . . . . . . . . 8.6 - optical fib re channels . . . . . . . . . 8.6.5 - power line carrier communications techniqu es . . . .8.6.3 - private pilot wires an channels . . . . . . . . . . . . . . 8.6.1 - ra io channels . . . . . . . . . . . . . 8.6.4 - rente pilot wires an channels . . . . . . . . . . . . . . 8.6.2 Trip circuit supervision Tripping cir cuits Tripping an alarm annunciation tests . . . . . . . . . . 2.12 . . . . . . . . . . . . . . . 2.11 . . . . . . . . 116 . . . . . . . . 119 . . . . . . . . 117 . . . . . . . . 116 . . . . . . . . 119 . . . . . . . . 116 . . . . . . . . 14 . . . . . . . . 13 . . . . . . . . . . . . 21.14 . . . . . . . . 396 Turns compensation of a current transformer . Turns compensation of a voltage tr ansformer . . . . . . . . 6.4.1.2 . . . . . . . . 85 16.16 . . . . 168-260 Transient analysis of synchronous machines Transient factor of a current transfo rmer Transient fault Transient instability in unit protection systems . . . . . . . . . . 6.2.1 . . . . . . . . 80 . . . . . . . . . . . 5.6 . . . . . . . . . 51
. . . . . . . . . . 6.4.8 . . . . . . . . 88 . . . . . . . . . . . . . . . 14.1 . . . . . . . . 219 . . . . . . . . 10.4.1 . . . . . . . . 155
Type testing of relays: . . . . . . 21.2-21.7 . . . . 372-383 - ynamic vali ati on . . . . . . . . . . . 21.7 . . . . . . . . 383 - electrical . . . . . . . . . . . . . . . . . . 21.2 . . . . . . . . 372 - electromagnetic compatibility . . . . 21.3 . . . . . . . . 373 - environmental . . . . . . . . . . . . . . 21.5 . . . . . . . . 380 - pro uct safety . . . . . . . . . . . . . . 21.4 . . . . . . . . 379 - software . . . . . . . . . . . . . . . . . . 21.6 . . . . . . . . 381 • 495 • Network Protection & Automation Gui e In ex •
Section Types of busbar protection systems Typical examples of time an current gra ing, overcurrent relays . . . . Page Variation of resi ual quantities Vector algebra Very inverse overcurrent relay V ibration type test Voltage an phase reversal protection . . . . . . . . . . . Section . . . . 4.6.3 Page . . . . . . . . 42 . . . . . . . . 18 . . 15.4 . . . . . . . . 235 . . . . . . . . . . . . . . . . 3.2 . . . . . . . . . 9.20 . . . . . . . . 143 . . . . . . 9.6 . . . . . . . . 128 . . . . . . . . . . . . 21.5.5 . . . . . . . . 381 U Unbalance loa ing (negative sequence protection): - of generators . . . . . . . . . . . . . . 17.12 . . . . . . .. 293 - of motors . . . . . . . . . . . . . . . . . 19.7 . . . . . . . . 346 Un erfrequency protection of generators . . . . . . . . . Un er-power protection of generators . . . . . . . . . . . . 17.14.2 . . . . . . . . 295 . . . . . . 18.10 . . . . . . . . 327 Voltage cont control using substation automation equipment . . . . . . . Voltage rolle overcurrent protection . . . . . . . . . . . . .. Voltage epen ent over current protection . . . . . . . . . . . . . . Voltage ips (Power Quality) Volt age istribution ue to a fault . . . . . . . . . . . . . Voltage factors for vo ltage transformers . . . . . . . . . . . . 24.5 . . . . . . . . 430 . . 17.7.2.1 . . . . . . . . 287 . . . . . . . 17.11.1 . . . . . . . . 293 . . 11.10.3 . . . . . . . . 187 . . . 17.7.2 . . . . . . . . 287 . . . . Un er-reach of a istance relay Un er-reach of istance relay on parallel lines . . . . . . . . . Un ervoltage (Power Quality) 23.3.1 . . . . . . . . 413 . . . 13.2.2.2 . . . . . . . . 204 . . . . . . . . . . . 4.5.2
. . . . . . . . 40 23.3.9 . . . . . . . . 415 . . . . . . . 6.2.2 . . . . . . . . . 81 . . . . . . . . 415
Unit protection: . . . . . . . . . . 10.1-10.13 . . . . 153-169 - balance volta ge system . . . . . . . 10.5 . . . . . . . . 156 - circulating current system . . . . . . 10.4 . . . . . . . . 154 - igital protection systems . . . . . . 10.8 . . . . . . . . 158 - electromechanical protection systems . . . . . . . . . . . . . . . . . . 10.7 . . . . . . . . 156 - numerical protection systems . . . . 10.8 . . . . . . . . 158 - static protection systems . . . . . . . 10.7 . . . . . . . . 156 - summation arrangements . . . . . . 10.6 . . . . . . . . 156 Unit p rotection schemes: - current ifferential . . . . . . 10.4, 10.10 . . . . 154-16 0 - examples of . . . . . . . . . . . . . . . 10.12 . . . . . . . . 167 -multi en e fee ers . . . . . . . . . . 13.3 . . . . . . . . 207 - parallel fee ers . . . . . . . . . . . . 13.2.1 . . . . . . . . 204 - phase comparison . . . . . . . . . . . 10.11 . . . . . . . . 162 in . . . . . . . . . . . . . . - signalling . . 8.2 . . . . . . . . 113 - Tee’ fee ers . . . . . . . . . 13.3.2-13.3.4 . . . . 207-209 - Translay . . . . . . . . . . . . 10.7.1, 10.7.2 . . . . 156-157 - us ing carrier techniques . . . . . . . . 10.9 . . . . . . . . 160 Unit transformer protection (for generator unit transformers) . . . . . . . . . Urban secon ary istribution system automation . . . . . . Voltage fluctuations (Power Quality) . 23.3.6 Voltage limit for accurate reach p oint measurement . . . . . . . . Voltage restraine overcurrent protection . . . . . . . . . . . . . . Voltage spikes (Power Quality) Voltage surges (Power Qual ity) . . . . 11.5 . . . . . . . . 174 . . 17.7.2.2 . . . . . . . . 287 . . . . . 23.3.2 23.3.2 . . . . . . . . 413 . . . . . . . . 413 • . . . . . 17.6.2 . . . . . . . . 286 Voltage transformer: . . . . . . . . . 6.2-6.3 . . . . . . 80-84 - capacitor . . . . . . . . . . . . . . . . . . 6.3 . . . . . . . . 83 - casca e . . . . . . . . . . . . . . . . . . 6.2.8 . . . . . . . . 82 - construction . . . . . . . . . . . . . . . 6.2.5 . . . . . . . . . 81 - errors . . . . . . . . . . . . . . . . . . . 6.2.1 . . . . . . . . 80 - phasing check . . . . . . . . . . . . 21.9.4.3 . . . . . . . . 389 - polarity check . . . . . . . . . . . . 21.9.4.1 . . . . . . . . 388 - ratio check . . . . . . . . . . . . . . 21.9.4.2 . . . . . . . .389 - resi ually-connecte . . . . . . . . . 6.2.6 . . . . . . . . . 81 - secon ary lea s . . . . . . . . . . . . . 6.2.3 . . . . . . . . . 81 - supervision in is tance relays . . 11.10.7 . . . . . . . . 188 - supervision in numerical relays . . . 7.6.2 . . . . . . . . 108 - transient performance . . . . . . . . 6.2.7 . . . . . . . . 82 - voltage factors . . . . . . . . . . . . . 6.2.2 . . . . . . . . . 81 Voltage unbalance (Power Quality) . .
. . . . . . . . . . . In ex . . . . . . 25.4 . . . . . . . . 447 23.3.7 . . . . . . . . 415 Voltage vector shift relay . . . . . . 17.21.3 . . . . . . . . 307 V Vacuum circuit breakers (VCB’s) Van Warrington formula for arc resistance . . . . . . . 18.5.5 . . . . . . . . 322 W Warrington, van, formula for arc resistance . . . . . . • 496 • . . . . . . . 11.7.3 . . . . . . . . 177 . . . . . . . . 11.7.3 . . . . . . . . 177 Network Protection & Automation Gui e
Section Wattmetric protection, sensitive Woun primary current transformer . . . . . . . . Page Section Page . . 9.19.2 . . . . . . . . 142 . . . . . . . . 6.4.5.1 . . . . . . . . 87 Z Zero sequence equivalent circuits: - auto-transformer . . . . . . . . . . . 5.16 .2 - synchronous generator . . . . . . . . 5.10 - transformer . . . . . . . . . . . . . . . . 5.15 Zero sequence network . . . . . . . . . 4.3.3 . . . . . . . . 60 . . . . . . . . 55 . . . . . . . . 57 . . . . . . . . 35 Zero sequence quantities, effect of system earthing on . . . . . . . 4.6 . . . . . . . . . 41 Zero sequence reactance: - of cables . . . . . . . . . . . .. . . . . . 5.24 of generator . . . . . . . . . . . . . . . 5.10 - of overhea lines . . . . . . . . 5.21, 5.24 - of transformer . . . . . . . . . . 5.15, 5.17 Zone 1 extension scheme ( istance protection) . . . . . . . . . . . . . . . . . . . 69 . . . . . . 66-69 . . . . . . 57-60 . . . . . . . . 55 12.2 . . . . . . . . 194 Zone 1 extension scheme in auto-reclose applications . . Zones of protection Zon es of protection, istance relay . . . . . . . . . 14.8.2 . . . . . . . . 226 . . . . . . . . .8 . . . . . . . . . . . . . 2.3 . . . . . . . . . . . . 11.6 . . . . . . . . 174 Network Protection & Automation Gui e • 497 • In ex •
