IEEE Guide for the Protection of Shunt Reactors
IEEE Power Engineering Society Sponsored by the Power System Relaying Committee
IEEE 3 Park Avenue New York, NY 10016-5997, USA
IEEE Std C37.109™-2006 (Revision of IEEE Std C37.109-1988)
20 April 2007
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IEEE Std C37.109™-2006(R2012) (Revision of IEEE Std C37.109-1988)
IEEE Guide for the Protection of Shunt Reactors Sponsor
Power System Relaying Committee of the IEEE Power Engineering Society Reaffirmed 29 March 2012 Approved 6 December 2006
IEEE-SA Standards Board
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Abstract: A comprehensive guide to the methods and configurations for the protection of power system shunt reactors is provided in this guide. The protection of oil-immersed reactors equipped with auxiliary power windings, improved turn-to-turn protection, and use of digital (microprocessorbased) protection for shunt reactors are included. Keywords: air-core, auxiliary power winding, circuit switcher, dry-type reactors, microprocessorbased relays, neutral reactor, oil-immersed, pole disagreement, protection, protective relay, reactance, resonance, series-compensated, shunt reactors, turn-to-turn, voltage-unbalance relaying
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Introduction This introduction is not part of IEEE Std C37.109-2006, IEEE Guide for the Protection of Shunt Reactors.
This guide covers protection of shunt reactors used typically to compensate for capacitive shunt reactance of transmission lines. A survey of shunt reactor protection, conducted in 1979 by the Shunt Reactor Protection Working Group of the IEEE Power System Relaying Committee [B16],a was used as a reference to determine common circuit arrangements and protective relaying schemes for this guide. This revision includes additional equipment arrangements and provides more detail to selected protective schemes. Other arrangements or special applications of reactors such as harmonic filter banks, static var compensation (SVC), high-voltage direct current (HVDC), or current-limiting reactors are not specifically addressed; however, the protective methods described in this guide are usually applicable to this equipment.
Notice to users
Errata Errata, if any, for this and all other standards can be accessed at the following URL: http:// standards.ieee.org/reading/ieee/updates/errata/index.html. Users are encouraged to check this URL for errata periodically.
Interpretations Current interpretations can be accessed at the following URL: http://standards.ieee.org/reading/ieee/interp/ index.html.
Patents Attention is called to the possibility that implementation of this guide may require use of subject matter covered by patent rights. By publication of this guide, no position is taken with respect to the existence or validity of any patent rights in connection therewith. The IEEE shall not be responsible for identifying patents or patent applications for which a license may be required to implement an IEEE standard or for conducting inquiries into the legal validity or scope of those patents that are brought to its attention.
_________________________ a
The numbers in brackets correspond to those of the bibliography in Annex A.
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Participants At the time this guide was completed, the Shunt Reactor Protection Working Group had the following membership: Kevin A. Stephan, Chair Pratap Mysore, Vice Chair John Appleyard Munnu Bajpai Simon Chano
Arvind Chaudhary Roger Hedding Charles Henville
Dean Miller Vittal Rebbapragada Jim Stephens
The following members of the individual balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. William Ackerman Ali Al Awazi Steve Alexanderson Paul Barnhart George Bartok Kenneth Behrendt W. J. Bergman Edward Bertolini Behdad Biglar Wallace Binder Thomas Blackburn Thomas Blair William Bloethe Oscar Bolado Stuart Bouchey Gustavo Brunello Carl Bush Donald Cash Simon R. Chano Tommy Cooper Luis Coronado John Crouse R. Daubert Byron Davenport Paul Drum Fred Elliott Walter Elmore Ahmed Elneweihi Gary Engmann Jorge Fernandez-Daher Anthony Giuliante
Randall Groves Robert Grunert Ajit Gwal N. Kent Haggerty Roger Hedding Charles Henville Jerry Hohn Edward Horgan, Jr. John Horwath James D. Huddleston, III David Jackson Clark Jacobson Lars-Erik Juhlin Gael R. Kennedy Joseph Koepfinger Stephen R. Lambert Gerald Lee Jason Lin Gregory Luri Jesus Martinez Frank Mayle Michael McDonald Nigel McQuin Mike Meisinger Gary Michel Dean Miller Brian Mugalian Anthony Napikoski Jeffrey Nelson Subhash Patel Wes Patterson Carlos Peixoto
Paul Pillitteri Gustay Preininger Madan Rana Radhakrishna Rebbapragada Johannes Rickmann Charles Rogers Dinesh Sankarakurup Devki Sharma Michael Sharp Hong-Ming Shuh Tarlochan Sidhu H. Jin Sim Mark Simon James E. Smith Joshua Smith R. Kirkland Smith Kevin A. Stephan Peter Stevens Ronald Stoner Charles Sufana John C. Sullivan Rick Taylor Demetrios Tziouvaras Eric Udren Charles Wagner Tom Wandeloski Joe Watson James Wilson Philip Winston Larry Yonce Xi Zhu
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When the IEEE-SA Standards Board approved this guide on 6 December 2006, it had the following membership: Steve M. Mills, Chair Richard H. Hulett, Vice Chair Don Wright, Past Chair Judith Gorman, Secretary Mark D. Bowman Dennis B. Brophy William R. Goldbach Arnold M. Greenspan Robert M. Grow Joanna N. Guenin Julian Forster* Mark S. Halpin
Kenneth S. Hanus William B. Hopf Joseph L. Koepfinger* David J. Law Daleep C. Mohla T. W. Olsen Glenn Parsons Ronald C. Petersen Tom A. Prevost
Greg Ratta Robby Robson Anne-Marie Sahazizian Virginia Sulzberger Malcolm V. Thaden Richard L. Townsend Walter Weigel Howard L. Wolfman
*Member Emeritus
Also included are the following nonvoting IEEE-SA Standards Board liaisons: Satish K. Aggarwal, NRC Representative Richard DeBlasio, DOE Representative Alan H. Cookson, NIST Representative Michelle D. Turner IEEE Standards Program Manager, Document Development Matthew J. Ceglia IEEE Standards Program Manager, Technical Program Development
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Contents 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Purpose ................................................................................................................................................ 1 2. Normative references.................................................................................................................................. 1
3. Definitions .................................................................................................................................................. 1
4. Use of reactors............................................................................................................................................ 2
5. Reactor construction and characteristics .................................................................................................... 2 5.1 Dry type ............................................................................................................................................... 2 5.2 Oil-immersed ....................................................................................................................................... 2 6. Typical reactor protection........................................................................................................................... 3
7. Dry-type reactors—application and protection........................................................................................... 3 7.1 Reactor connections............................................................................................................................. 3 7.2 Failure modes and types of faults ........................................................................................................ 5 7.3 System considerations ......................................................................................................................... 5 7.4 Relaying practices................................................................................................................................ 6 8. Oil-immersed reactors—application and protection................................................................................... 9 8.1 Reactor connections............................................................................................................................. 9 8.2 Failure modes and types of faults ........................................................................................................ 9 8.3 System considerations ....................................................................................................................... 12 8.4 Relaying practices.............................................................................................................................. 14 9. Summary of shunt reactor protection ....................................................................................................... 25
Annex A (informative) Bibliography ........................................................................................................... 28
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IEEE Guide for the Protection of Shunt Reactors
1. Overview 1.1 Scope This guide includes description of acceptable protective relay practices applied to power system shunt reactors. The guide covers protection for dry-type (air-core) and oil-immersed type reactors used on power system buses and lines. Also included in this guide are the protection of oil-immersed reactors equipped with auxiliary power windings, improved turn-to-turn fault protection, and use of digital (microprocessorbased) relays for shunt reactor protection.
1.2 Purpose The purpose of this guide is to provide users of shunt reactors acceptable methods and configurations for the protection of power system shunt reactors.
2. Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. IEEE Std C37.015™-1993, IEEE Application Guide for Shunt Reactor Switching.1, 2 IEEE Std C62.22™, IEEE Guide for the Application of Metal-Oxide Surge Arresters for AlternatingCurrent Systems.
3. Definitions For definitions of terms used in this guide, see The Authoritative Dictionary of IEEE Standards Terms [B8]3 and IEEE Std C37.100™-1992 [B10]. 1 IEEE publications are available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 2 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc. 3 The numbers in brackets correspond to those of the bibliography in Annex A.
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
4. Use of reactors Shunt reactors can be used to provide inductive reactance to compensate for the effects of high charging current of long transmission lines and pipe-type cables. For light load conditions, this charging current can produce more leading reactive power than the system can absorb with the consequent risk of instability or excessive high voltages at the line terminals (Ferranti effect).
5. Reactor construction and characteristics The two general types of construction used for shunt reactors are dry-type and oil-immersed. The construction features of each type, along with variations in design, are discussed in 5.1 and 5.2.
5.1 Dry type Dry-type shunt reactors generally are limited to voltages through 138 kV and can be directly connected to a transmission line or applied on the tertiary of a transformer that is connected to the transmission line being compensated. The reactors are of the air-core (coreless) type, open to the atmosphere, suitable for indoor or outdoor application. Natural convection of ambient air is generally used for cooling the unit by arranging the windings so as to permit free circulation of air between layers and turns. The layers and turns are supported mechanically by bracing members or supports made from materials such as ceramics, glass polyester, and concrete. The reactors are constructed as single-phase units and are mounted on base insulators or insulating pedestals that provide the insulation to ground and the support for the reactor. Since the dry-type shunt reactor has no housing or shielding, a high-intensity external magnetic field is produced when the reactor is energized. Care is thus required in specifying the clearances and arrangement of the reactor units, mounting pad, station structure, and any metal enclosure around the reactor or in the proximity of the reactor. A closed metallic loop in the vicinity of the reactor can produce losses, heating, and arcing at poor joints; therefore, it is important to avoid these loops or to maintain sufficient separation distances. The magnitude of current induced in the loop, which is responsible for extra losses and heating, is dependent on the orientation of the loop with respect to the reactor, impedance of the loop, size of the loop, and distance of the loop from the reactor. Another consideration is the effect of the magnetic fields on the impedance deviation between phases. Methods of minimizing the deviations include adequate separation or arranging the reactors in an equilateral-triangle physical configuration. Deviation from impedance values for reactors will result in a deviation from the actual rating in megavars. The deviation issue as it applies to relaying is discussed in 7.4.3. The reactor manufacturer can provide guidance regarding appropriate clearances or recommendations to minimize stray heating, losses, and impedance deviations. For the same range of applications, the primary advantages of dry-type air-core reactors, compared to oilimmersed types, include lower initial and operating costs, lower weight, lower losses, and the absence of insulating oil and its maintenance. The main limitation for the application of dry-type air-core reactors is that of connection voltage where the reactor size becomes prohibitive for higher transmission system voltages. Since these reactors do not have an iron core, there is no magnetizing inrush current when the reactor is energized.
5.2 Oil-immersed The two design configurations of oil-immersed shunt reactors are coreless type and gapped iron-core type. Both designs are subject to low-frequency long time-constant currents during de-energizing, determined by the parallel combination of the inductance of the reactor and the line capacitance. However, the gapped iron-core design is subject to more severe energizing inrush than the coreless type. Most coreless shunt 2 Copyright © 2007 IEEE. All rights reserved.
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
reactor designs have a magnetic circuit (magnetic shield) that surrounds the coil to contain the flux within the reactor tank. The steel core-leg that normally provides a magnetic flux path through the coil of a power transformer is replaced (when constructing coreless reactors) by insulating support structures. This type of construction results in an inductor that is linear with respect to voltage. The magnetic circuit of a gapped iron-core reactor is constructed in a manner very similar to that used for power transformers with the exception that small gaps are introduced in the iron core to improve the linearity of inductance of the reactor and to reduce residual or remanent flux when compared to a reactor without a gapped core. Oil-immersed shunt reactors can be constructed as single-phase or three-phase units and are very similar in external appearance to that of conventional power transformers. They are designed for either self-cooling or forced-cooling.
6. Typical reactor protection The following two basic shunt reactor configurations are considered: a)
Dry-type, connected ungrounded wye to the impedance-grounded tertiary of a power transformer
b) Oil-immersed, wye-connected, with a solidly-grounded or impedance-grounded neutral, connected to the transmission system Major fault protection for dry-type reactors can be achieved through overcurrent, differential, or negativesequence relaying schemes, or by a combination of these relaying schemes. Protection for low-level turnto-turn faults can be provided by a voltage-unbalance relay scheme connected at the neutral with compensation for inherent unbalance of system voltages and the tolerances of the reactor. Major fault protection for oil-immersed reactors can be achieved through overcurrent relaying, differential relaying, or a combination of both. Protection for low-level turn-to-turn faults can be provided by impedance, negative or zero-sequence overcurrent, thermal, gas-accumulator, sudden-pressure relays, or by a combination of these relays.
7. Dry-type reactors—application and protection
7.1 Reactor connections Dry-type reactor banks are often connected to the delta-connected tertiary of a transformer bank as shown in Figure 1. Each wye-connected, ungrounded reactor bank can be switched on the supply side of the reactor bank, as shown in Figure 1, or on the neutral side, as shown in Figure 2. A grounding transformer having a grounded wye-connected primary and a broken-delta connected secondary, with a grounding resistor, as shown in Figure 1, is often used on the tertiary circuit to provide a limited amount of ground current. It is recommended that the grounding transformer and the grounding resistor be sized for a continuous zero-sequence current at least equal to the zero-sequence current flowing through the tertiary circuit capacitance to ground under ground fault conditions (charging current). In addition, the grounding transformer shall be rated for continuous application of line-to-line voltage in order to withstand a continuous ground fault on the tertiary.
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
Figure 1 —Typical dry-type shunt reactor connection with three-pole supply-side switching and with grounding transformer
NOTE—Grounding transformer not shown.
Figure 2 —Dry-type shunt reactor connection with two-pole or three-pole neutral-side switching
The grounding scheme for the tertiary is a high-resistance method utilizing the broken-delta secondary of the grounding transformer to insert the resistance, as well as provide indication of a ground fault on the tertiary circuits. This method offers the following advantages: a)
The zero-sequence resistance helps stabilize the neutral.
b) The probability of ferroresonance is reduced. c)
The voltages to ground on the tertiary circuits due to switching are minimized.
d) Currents due to line-to-ground faults are minimized; a few amperes are typical.
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
e)
Excellent ground fault protection is afforded by the voltage relay (59N), across the resistor.
f)
Any number of banks can be switched without sacrificing the foregoing advantages.
The multiple advantages of this method have been demonstrated for some time. However, other tertiary grounding arrangements such as a zig-zag transformer with a grounding resistor can also be applied. Surge arrester selection, coordination, and application for protection of shunt reactors are covered in IEEE Std C62.2-1987 [B12] and IEEE Std C62.22.4
7.2 Failure modes and types of faults The faults encountered in dry-type reactor installations can be categorized as follows: a)
Phase-to-phase faults on the tertiary bus, resulting in a high-magnitude phase current
b) Phase-to-ground faults on the tertiary bus, resulting in a low-magnitude ground current, dependent upon the size of the grounding transformer and resistor as well as the total capacitance in the circuit c)
Turn-to-turn faults within the reactor bank, resulting in a very small change in phase current
Phase-to-phase faults are not likely to occur in dry-type reactors when the configuration is that of singlephase units arranged with adequate separation between phases. However, instances have been reported where arcing from a faulted reactor contacted the tertiary bus to initiate a phase-to-phase fault. Since dry-type reactors are mounted on insulators or supports that provide standard clearances to ground, direct winding-to-ground faults are not likely to occur without unusual circumstances, such as when an animal bridges the insulation to ground. The damage that occurs for a winding-to-ground fault depends on how much ground current is permitted by the grounding transformer. Winding-insulation failures in dry-type reactors can begin as tracking due to surface contamination, insulation deterioration, or as turn-to-turn faults. Once an arc is initiated, these failures, if not detected promptly, can flashover the entire winding due to the strong interaction of the arc with the magnetic field of the reactor, channeling of the arc byproducts along the cooling ducts of the reactor, and excessive heat due to high circulating current in the first shorted turn. The result is a phase-to-neutral fault that increases the current in the unfaulted phases to a maximum of the square root of three times normal phase current. This increase in phase current, if not detected, can cause thermal damage of the unfaulted phases of the reactor bank.
7.3 System considerations Reactors are often connected in ungrounded wye on a transformer delta ungrounded tertiary. This connection affords a lower voltage and a higher current rated reactor than if the reactors were connected directly on the high-voltage side. The effective reactance referred to the high-voltage side is proportional to the square of the turns ratio of the transformer multiplied by the low-voltage reactance value. Faults on only one phase of the ungrounded wye reactor have little effect on the associated transmission supply system, unless the fault is allowed to evolve and include two phases. Failure of an entire reactor leg, when all windings of one phase are shorted to the neutral connection point, has only minor impact on normal system load currents but results in the reactor current in the unfaulted phases being increased by square root of three times normal phase reactor current. If there is a strong possibility, due to physical arrangement, for example, of a phase-to-neutral fault evolving to a phase-to-phase fault, this fault should be detected as quickly as possible and the reactor isolated by tripping its associated switching device. If the reactor 4
For information on references, see Clause 2.
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
switching device does not have fault interrupting capability, the transformer bank should be tripped to clear the fault. Subclause 8.4.8 contains additional information on utilizing less-than-maximum fault interrupting devices. Subclause 4.3 of IEEE Std C37.015-1993 can be consulted for guidance. When a faulted reactor is isolated from the tertiary circuit, the voltage on the transmission line tends to increase. Studies of the system should be made to be sure that the loss of the reactor does not cause a significant overvoltage condition on the system.
7.4 Relaying practices 7.4.1 Protection for phase-to-phase faults Relaying protection for phase-to-phase faults generally consists of overcurrent (50/51), differential (87), or negative-sequence current (46), relaying schemes, or a combination of these relaying schemes. Common schemes are illustrated in Figure 3. Unbalance currents in the reactor can be detected using negative-sequence overcurrent relays. Relays should be set above the levels of unbalance seen in normal service either due to voltage unbalance or due to manufacturing tolerance of the reactor. Tripping should also be delayed to coordinate with reclosing times during single-phase tripping and reclosing and also with other protection devices that operate during faults external to the reactor. The use of negative-sequence relays can also detect an open circuit on the neutral side of an ungrounded wye-connected shunt reactor bank. 7.4.2 Protection for phase-to-ground faults Typical ground fault protection is shown in Figure 1. The broken-delta output of the grounding transformer is monitored by an overvoltage relay (59N), equipped with a harmonic filter to reject any third harmonic voltage that can be present. Depending on the voltage at the grounding resistor, the primary of an auxiliary voltage transformer can be connected across the grounding resistor with the secondary of the auxiliary voltage transformer supplying the overvoltage relay. An accepted practice is to alarm but not trip for this condition. This relay cannot differentiate between a reactor ground and a ground on other portions of the tertiary system. 7.4.3 Protection for turn-to-turn faults Turn-to-turn faults in dry-type reactors present a formidable challenge to the protection engineer. The current and voltage changes encountered during a turn-to-turn fault can be of the same order of magnitude as variations expected in normal service, and therefore, sensitive, reliable protection using the common relaying schemes described in 7.4.1 and 7.4.2 is not possible. The voltage-unbalance relaying scheme has been applied for this protection. Such a scheme is illustrated in Figure 4(a) and described in RD-3221 Operating Description [B18]. The voltage appearing between the neutral connection of the reactor bank and ground can be the result of the following: a)
Reactor bank unbalance due to a faulted reactor
b) Reactor bank unbalance due to manufacturing and configuration tolerances5 c)
Tertiary bus voltage unbalance with respect to ground
5 Per IEEE Std C57.21-1990 [B11], in the case of a three-phase shunt reactor or a bank made up of three single-phase reactors, the maximum deviation of impedance in any one phase shall be within 2% of the average impedance ohms of the three phases. For drytype shunt reactors without magnetic-field shielding, this tolerance applies only when units are arranged in an equilateral-triangle configuration and isolated from any external magnetic influences.
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
The manufacturing tolerance produces a fixed-error voltage that can be negated by an equal and opposite voltage generated by means of a phase-shifting network. System voltage unbalance can be variable; however, a given percent change in system unbalance affects both the reactor bank neutral-to-ground voltage and the grounding transformer broken-delta voltage to the same degree, and therefore, these two voltages can be used to cancel each other. The summing-amplifier output of Figure 4(a), or the output of a similar comparator, is thus representative of the degree of unbalance due only to the faulted reactor, and hence, this scheme can discriminate between a turn-to-turn fault and other sources of unbalance.
Figure 3 —Common protective relaying schemes for dry-type reactors
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
(a)
(b) Figure 4 —Voltage-unbalance relay protection for (a) dry-type reactors and (b) dry-type reactors (alternate method)
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
When the voltage-unbalance relaying scheme is applied, consideration should be given to the effect of a tertiary bus fault to ground on the operation of the reactor protection relays. If the tertiary bus ground relay (59N) is connected to trip the tertiary bus source, the reactors will be de-energized and the response of the reactor neutral voltage relays is immaterial. However, if the bus ground relay only provides an alarm, it is generally considered desirable to keep the reactors in service during the ground fault and the following points about the voltage unbalance scheme should be reviewed: a)
Under ground fault conditions, the neutral voltage and the grounding transformer broken-delta voltage will have high levels. These voltages should cancel in the comparator circuit; therefore, the comparator should be linear up to the maximum voltages obtained during a ground fault. Failure of these two voltages to cancel results in an erroneous output from the comparator and possibly causes the overvoltage function (59) to trip falsely. An alternate scheme shown in Figure 4(b) keeps the comparator from seeing the large neutral and grounding transformer voltages. The connection used provides a summation of the neutral and the grounding transformer output so that the comparator circuit is only presented with the differential voltage during a reactor fault.
b) If the voltage used to supply the phase-shifting network is affected by a tertiary bus ground fault, then the compensation for reactor unbalance can be changed in magnitude or phase angle, possibly resulting in a false trip. This can be avoided by using a phase-to-phase, rather than phase-to-ground voltage as the source for the phase-shifting network, as illustrated in Figure 4(b). When dry-type reactors are constructed using multiple parallel circuits per coil, the voltage unbalance scheme might not have sufficient sensitivity to detect a single-turn fault in one of the parallel windings. Some manufacturers, Shunt Reactor Bulletin [B20] and Recommendations for Protective Relays [B1], of such reactors propose a split-phase protection system, similar to that used on hydrogenerators for turn-to-turn fault protection, as shown in Figure 5(a) and Figure 5(b). Neutral switching is possible with the scheme shown in Figure 5(b), while it is not with the scheme in Figure 5(a).
8. Oil-immersed reactors—application and protection
8.1 Reactor connections Oil-immersed reactors are often connected to one or both ends of a long transmission line, as shown in Figure 6(a), and are usually wye-connected with a solidly-grounded neutral. These reactor banks can be switched or permanently connected to the line. Another reactor bank arrangement for single-phase tripping and reclosing of circuit breakers is the fourreactor scheme (Edwards et al. [B5]) shown in Figure 6(b). In this application, a fourth reactor is connected between the reactor bank neutral and ground to suppress the secondary arc current in a faulted and disconnected phase conductor during single-phase fault interruption. Oil-immersed reactors can also be connected to the substation bus, and as with line-connected reactors, are generally solidly grounded and can be either switched or permanently connected. Relaying protection for bus-connected reactors and for four-reactor configured banks is basically the same as that used for lineconnected, solidly-grounded, oil-immersed reactors (Kimbark [B13]).
8.2 Failure modes and types of faults The failures encountered with oil-immersed reactor installations can be categorized as follows:
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a)
Faults resulting in large changes in the magnitude of phase current, such as bushing failures, insulation failures, etc.
b) Turn-to-turn faults within the reactor winding, resulting in small changes in the magnitude of phase current c)
Miscellaneous failures such as auxiliary power winding faults, overvoltage, low oil, loss of forced-cooling, and pole disagreement
(a)
(b) Figure 5 —Split-phase protection: (a) three-phase sensing and (b) single-phase sensing
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(a)
(b) Figure 6 —One-line diagram of (a) line-connected, switched shunt reactors and (b) line-connected shunt reactor with neutral reactor
Due to the proximity of the winding with the core and tank, winding-to-ground failures can occur. The magnitude of current resulting from this type of fault is dependent upon the location of the winding-toground fault with respect to the reactor bushing. The farther the fault is away from the bushing, the lower the fault current. Bushing failures within or external to the tank, as well as faults on the connection between the transmission line and the reactor bank, can result in large increases in the magnitude of phase current. Low-level faults within an oil-immersed reactor will result in a change in the reactor impedance, and can increase the operating temperature, internal pressure, and accumulation of gas. If not detected, the turn-toturn fault is likely to evolve into a major fault. 11 Copyright © 2007 IEEE. All rights reserved.
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8.3 System considerations 8.3.1 Clearing of faults A typical relaying practice for line-connected reactors is to trip the local line breaker and transfer trip the remote line breaker. A dual channel is recommended for extra security (The Art of Protective RelayingEHV Systems [B19]). For a reactor fault in a direct-connected line reactor, both line breakers are usually locked out so as to block reclosing of the line. For a fault in a switched line reactor, on a line where rapid reclosing is desired, both line breakers can be tripped, the reactor bank switching device opened, and then the line breakers can be automatically reclosed if system conditions permit. When a circuit switcher is utilized as the reactor bank switching device, a blocking or coordinated tripping scheme can be applied. In this scheme, the circuit switcher interrupts reactor faults within its rating, and the terminal breaker operates only on higher level faults beyond the rating of the circuit switcher. Other users, with concern for reliability of trip blocking, may choose to operate the higher capacity terminal breaker directly for faults beyond the rating of the circuit switcher without blocking trip of the circuit switcher and depend on the faster circuit breaker to clear the fault. However, use of a full-rated circuit breaker for reactor switching eliminates the need for a coordinated tripping scheme. 8.3.2 Resonance phenomenon The distributed shunt capacitance of the transmission line can form a parallel-resonant circuit with the shunt reactor(s) having a natural frequency close to 60 Hz. This resonant circuit can be troublesome and should be taken into account by the system planner and the relay protection engineer. When a deenergized transmission line with directly-connected reactor(s) is physically close enough to another energized line for the two lines to be electrically coupled, it is possible for higher-than-rated system voltage to develop across the “deenergized” reactor. This problem can be prevented by isolating the reactor by means of a dedicated reactor switching device at the same time as, or immediately following, the deenergizing of the line (Pickett [B15]). Another phenomenon of concern to the relay protection engineer occurs when a series-compensated transmission line is de-energized. The parallel-resonant circuit can produce a damped sinusoidal voltage at a frequency generally less than 60 Hz, which can last several seconds, with an initial voltage that can approach rated voltage. This substantial voltage, at a reduced frequency, can cause misoperation of impedance relays used to protect shunt reactors, unless the impedance relays are specifically designed for the application. Figure 7 represents a one-line diagram of shunt reactor application on a series capacitors compensated transmission line. If the main shunt reactor winding is protected with a differential relay (87) scheme, then phase and ground faults on the series-compensated line should not have any undesirable impact on the shunt reactor protection. Figure 7 illustrates a fault applied on the series-compensated line. It is noted that before opening the line circuit breaker, the voltages are severely depressed on the line side and bus side of the shunt reactor installation. As shown from Figure 8, the current in the shunt reactor is small as a result of the depressed fault voltage. Upon opening of the line circuit breaker(s), the series capacitors will transiently discharge (IOSC) through the line and shunt reactor (assuming the capacitors are not bypassed for a line fault). The use of overcurrent protection (50), as shown in Figure 7, is covered in 8.4.1.
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Figure 7 —Fault on series-compensated line with shunt reactors
Figure 8 —TNA study results of series-compensation and reactor oscillations
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8.4 Relaying practices 8.4.1 Protection for large-magnitude faults Relaying protection for faults producing large increases in the magnitude of phase current is generally a combination of overcurrent (50/51), differential (87), or distance (21), relaying. Common schemes are illustrated in Figure 9. One of the principal difficulties with shunt reactor protection is false relay operation during iron-core reactor energizing and de-energizing. During these periods, dc offset with long time-constants and lowfrequency components of the reactor energization current cause many of the problems. High-impedance differential relays are generally recommended over summation-connected ordinary low-impedance overcurrent relays for this reason (Englehardt [B6]). Percentage-restrained (biased) low-impedance differential relays that perform as well as high-impedance relays can also be applied. Where ordinary lowimpedance differential relays are used, it is generally recommended that the relay be sufficiently desensitized to prevent misoperation. Phase overcurrent protection can be useful for protection for large magnitude internal faults. Phase instantaneous overcurrent protection should be set higher than the maximum current that can flow in the reactor during transient conditions. During energization, transient offset in the current can result in peak currents approaching twice rated levels. Since the reactor impedance is proportional to the frequency of the voltage applied to it, higher than normal currents will also flow during conditions of lower than normal frequency (such as during the de-energization of the line/reactor combination as noted in 8.3.2). Therefore, immunity to the transient offset of current and low-frequency current is an important attribute of a reactor instantaneous phase overcurrent relay. That is, the relay should be tuned to respond to fundamental frequency only. If the relay is immune to off-nominal frequency current, the likely other source of high phase current is temporary high voltage on unfaulted phases during single line-to-ground faults on the power system. Highvoltage (HV) and extra-high-voltage (EHV) systems are usually effectively grounded, and the rise in voltage on an unfaulted phase rarely exceeds 1.3 per unit during a single line-to-ground fault (see the standard definition of “ground fault factor” in IEEE 100 [B8]). Therefore, a setting of 150% of reactor current at rated voltage is normally sufficient to override temporary overvoltages. However, the maximum temporary overvoltage should be determined for each application of an instantaneous phase overcurrent relay. Differential schemes have been applied as primary protection for the detection of winding-to-core or winding-to-tank faults. The ground differential (87G) or restricted ground fault schemes are useful when the shunt reactors are a single phase per tank design. Figure 10 shows a high-impedance ground differential (87G) scheme; however, a low-impedance scheme can also be implemented. Where a reactor differential relaying scheme is used that is sensitive to mismatch in current transformer performance during a fault, it is recommended that the current transformers on both sides of the reactor have similar excitation characteristics. Ground fault back-up protection can be provided by a neutral overcurrent relay. 8.4.2 Protection for turn-to-turn faults Phase overcurrent relay schemes might not be sufficiently sensitive to provide adequate protection for turnto-turn faults and differential relay schemes normally cannot detect such faults. Distance relays or ground overcurrent relays offer some improvement in protection, but the sudden-pressure relay or gas-accumulator relay or both generally provide a sensitive means of detecting turn-to-turn faults within oil-immersed reactors.
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Figure 9 —Common protective relaying schemes for oil-immersed reactors
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Figure 10 —Ground differential scheme for oil-immersed reactors
Nondirectional ground overcurrent relays provide some protection, but there should be time-delay coordination provided for external faults and current transformer saturation. The coordination is for external unbalanced faults, because such faults unbalance the voltages supplied to the reactor, and also result in unbalanced currents flowing in the three phases of the reactor. Directional ground overcurrent relays can provide faster protection provided special precautions are observed to provide sufficient polarizing quantities for the directional element and to overcome current transformer saturation during reactor energization and de-energization. For grounded neutral reactors, a ground overcurrent relay controlled by a directional relay can be used to discriminate between unbalanced currents due to external faults and unbalanced currents due to internal faults, such as shorted turns. A problem arises in the application of directional relays to detect shorted turns in a reactor if the unbalanced current flow is not sufficient to unbalance the phase voltages sufficiently for a zero-sequence voltage or negative-sequence voltage polarized directional relay to sense the fault. This problem can be overcome by using a directional relay that has the polarizing voltage reinforced by voltage developed from using some relay operating current through an impedance. As long as this impedance is less than the reactor impedance, the additional polarizing reinforcement provided by the operating current should not be sufficient to cause incorrect directional indication for an unbalanced external fault. A short time delay (few hundred milliseconds) on the directionally-controlled ground overcurrent tripping function is helpful to increase the security of this protection. Figure 11(a) is a one-line diagram showing the connection of a ground overcurrent relay (50N) on a reactor bank grounded neutral. Starting of the ground overcurrent relay should be controlled by a negativesequence polarized directional relay (67Q). Figure 11(b) shows an alternative scheme using a zerosequence polarized directional relay (67N). An arrow indicates the forward direction for which the directional relay is connected (i.e., operating direction). It is apparent that with the operating direction indicated, the directional relay might not operate for winding to ground faults in the reactor. This is not of 16 Copyright © 2007 IEEE. All rights reserved.
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concern since other reactor protection (for high fault current protection) should detect such faults. The directional ground relay is applied for turn-to-turn fault protection only. Note that the ground overcurrent relay and the directional ground overcurrent relay are both connected to a current transformer on the ground connection, and not to the residual connection of the three phase current transformers. By using the connection shown, the risk of undesirable operation of the overcurrent detector or directional element due to unequal saturation of the three phase current transformers during reactor energization is eliminated. Unequal saturation of the phase current transformers can easily result during the period after energization of the reactor due to the long time-constant of decay of any dc component present in the initial phase current. In the case of the negative-sequence directional relay, there is also a possibility of apparent high levels of negative-sequence current due to unequal saturation of the phase current transformers immediately after energization or de-energization. Unlike the case of the ground directional overcurrent function, there is no alternative connection to avoid undesirable operation. It is therefore necessary to block the turn-to-turn protection for some time after energization until after the transient dc component of the phase current has completely decayed.
(a)
(b) Figure 11 —Turn-to-turn fault protection for grounded reactor using (a) negative-sequence and directional control and (b) zero-sequence directional control
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There is also the possibility of some ground current and negative-sequence current flowing in a reactor directly connected to a line terminal during the discharge of line charging current immediately after deenergization of a transmission line. To avoid the possibility of undesirable tripping by sensitive negativesequence or zero-sequence overcurrent turn-to-turn fault protection, this protection should be blocked by an undervoltage relay and timer combination. The undervoltage relay immediately blocks the overcurrent tripping, and the time delay on reset retains the blocking function for some seconds after energization to ensure no misoperation due to false directional or overcurrent operation due to unequal saturation of the phase current transformers during energization. To avoid the possibility of a severe internal turn-to-turn fault depressing the terminal voltage low enough to block the turn-to-turn fault protection, instantaneous phase overcurrent protection should also be applied. The undervoltage detector should be set to drop out below the phase to neutral voltage expected due to winding faults that result in enough current to operate the instantaneous phase overcurrent protection. The drop in voltage on the reactor terminals at the current level at which instantaneous phase overcurrent protection operates can be simply calculated by multiplying the setting of the instantaneous phase overcurrent element by the maximum positive-sequence source impedance at the terminal of the reactor. For example, consider a 135 Mvar, EHV reactor applied at a location where the minimum fault current is 20 per unit on a 100 MVA base. The rated current of the reactor is about 1.35 per unit on a 100 MVA base. Assume the instantaneous phase overcurrent protection is set at 150% of rated reactor current or 2.0 per unit. The positive-sequence source impedance is 1/20 = 0.05 per unit on a 100 MVA base. The voltage drop at a current of 2.0 per unit will be 2 × 0.05 = 0.1 per unit, or 10%. Therefore, if the undervoltage function supervising the sensitive turn-to-turn fault protection is set at 80% of rated voltage, and if there is an internal turn-to-turn fault of sufficient severity to depress the terminal voltage to a level low enough to block the turn-to-turn protection, the instantaneous phase overcurrent protection should operate. Distance relays have been applied to detect shorted turns in iron-core shunt reactors. The use of distance relays for this type of protection is possible due to the significant reduction in the 60 Hz impedance of a shunt reactor under turn-to-turn fault conditions. The turn-to-turn fault sensitivity that can be achieved is limited by the apparent impedance seen by the relay during the inrush period when the reactor is energized. The relay reach should be set below the reduced impedance seen during this inrush period and should be selected so that the relay does not operate incorrectly on the natural frequency oscillation that occurs when a compensated transmission line is de-energized. Split-phase protection is an option for reactors in the EHV range and is shown in Figure 5(a) and Figure 5(b). For such applications, the disc-type reactor windings are split into two parallel groups with separate neutral connections brought out for each group. Two alternatives using three-phase and singlephase sensing are shown in Figure 5(a) and Figure 5(b), respectively. In Figure 5(b), the two neutral end leads are brought together in opposition through a current transformer, which picks up the current difference. A turn-to-turn fault in the winding creates an imbalance between the impedances of the two halves and creates a corresponding current imbalance. The relay used is a three-phase overcurrent relay (51), typically set at approximately 2.5% of reactor rated current. The gas-accumulator relay is applicable on reactors that are equipped with conservator tanks and have no gas space inside the reactor tank. This relay is inserted in the pipe between the reactor and the expansion chamber (conservator). Low-energy partial discharges, creepage, and overheating caused by turn-to-turn faults, or by high contact or joint resistance cause the insulation at these points to slowly decompose while evolving gas. The gas rises through the oil and is accumulated in the relay. The relay should also operate for severe internal arcing or heavy-current flashovers, which force oil through the relay at a high velocity before the gases rise through the system to the device. This device is commonly known as a Buchholz relay. The sudden-gas-pressure relay, also known as a fault-pressure relay, is applicable to gas-cushioned oilimmersed reactors. The relay is mounted on the reactor tank in the region of the gas space at the top of the reactor, and consists of a pressure-sensing bellows, a pressure-actuated switch, and a pressure-equalizing orifice. The relay operates on the difference between the pressure in the gas space of the reactor and the pressure inside the relay. During slow pressure variations associated with reactor temperature changes, the 18 Copyright © 2007 IEEE. All rights reserved.
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pressure-equalizing orifice should equalize the pressure between the relay and the reactor, and thus prevent operation. For internal arcing that produces large amounts of gas and a sudden rise in gas pressure, the bellows should expand, causing the relay to operate. The sudden-oil-pressure relay, another type of fault-pressure relay, is applicable to all oil-immersed reactors. The relay is mounted on the reactor tank below the minimum deenergized liquid level. Oil fills the lower chamber of the relay housing, within which a spring-backed bellows is located. The bellows is completely filled with silicone oil. There is also silicone oil in the upper chamber, which is connected to the bellows via an equalizer hole. Should an internal fault develop, the resulting rapid rise in oil pressure, or pressure pulse, is transmitted to the bellows, and the relay should operate. In the event of gradual increases in oil pressure, due to temperature variations in the reactors, the equalizing hole stabilizes the pressure in the bellows and should keep the relay from operating. 8.4.3 Neutral or “fourth leg” reactor protection Neutral reactors, used for secondary arc extinction on single-phase line tripping, are connected between the neutrals of the line phase reactors and the ground mat of the substation. The impedance and voltage rating of the neutral reactors should be based on the shunt capacitance of the transmission line and can be determined by electromagnetic transients program or other transient switching program studies. When the reactor bank is energized, there is normally very little voltage being applied across the neutral reactor. The transmission line phase-to-ground voltages are normally balanced. This normal lack of voltage across the neutral reactor causes the significant difference in the fault conditions encountered by the neutral reactor as compared to phase reactors. This difference affects the protection for the reactor. Voltage is developed across the neutral reactor during a line ground fault and the open phase period for the singlephase tripping. This period can range from 500 ms to 2 s depending on the system. It is preferred to detect problems with the neutral reactor before it is called upon to function for a single-phase trip operation. With the normal lack of applied voltage, prior detection of failures is difficult to impossible with electrical fault detection devices. If the neutral reactors are oil immersed, they have failure modes similar to the oil-immersed phase reactors. A limited way of accomplishing early detection of failure is to apply physical trouble detectors: low oil level, sudden-pressure, and pressure-relief device. For the sudden-pressure or the pressure-relief device, some unbalance in the system is needed to supply the energy to generate the gas. The sudden-pressure relay is critical for detecting turn-to-turn faults in the reactor. A single-phase differential relay can be applied to detect winding-to-core or winding-to-tank faults. Since the magnitude of the fault currents are usually low and the duration of the fault condition is limited to the time of the open-phase condition, the differential relay is possibly the only electrical device that is fast and sensitive enough to detect this type of fault. Due to the application, neutral reactors are normally not sized for continuous operation. Typically, they are selected with a 10 s rating for the condition of one phase open. According to IEEE Std 32-1972 [B9], a neutral reactor has an inherent continuous current capability of 3% of the 10 s rating. A time-delay overcurrent relay is applied to protect the reactor from abnormal operation of the system. The neutral reactor has a unique application that makes it possible to remove the reactor from service by closing a bypassing device as shown in Figure 12. The bypassing device shorts out the reactor, removing the voltage from it. This device can be triggered to operation by the protective relays. 8.4.4 Auxiliary power winding protection In some cases, station service can be provided from a shunt reactor auxiliary winding. Figure 13 illustrates a one-line diagram showing a protection scheme for a solidly-grounded neutral auxiliary shunt reactor winding. This arrangement provides protection for all types of phase faults and ground faults on the auxiliary supply side of the installation.
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Figure 12 —Neutral reactor with bypass device
Figure 13 —Shunt reactor with grounded wye auxiliary power winding 20 Copyright © 2007 IEEE. All rights reserved.
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Phase overcurrent relays and a residually-connected ground overcurrent relay are shown in this application. Instantaneous phase (50) and ground (50N) overcurrent relays should be slightly time-delayed (62), to assure proper coordination with the downstream protection provided for the auxiliary supply feeders. Even though slightly delayed, both instantaneous phase and ground overcurrent elements initiate fast tripping of the low-voltage circuit breaker (LVCB). The time-delay elements of both phase (51) and ground (51N) overcurrent relays should also be coordinated with the downstream feeder protections and provide an additional important back-up protection function by tripping the high-voltage circuit breaker (HVCB) of the main shunt reactor in case the fault is not eliminated by the feeder protection or the instantaneous protections on the auxiliary reactor side. Another auxiliary power winding configuration6 includes the use of an intermediate transformer before distribution to the load. The extension of the auxiliary winding to the intermediate transformer is still part of the shunt reactor and, for any fault, thereon requires tripping of the entire shunt reactor bank. Therefore, some users choose to operate this circuit ungrounded. Consequently, for a phase-to-ground fault between the auxiliary winding and intermediate transformer, there can be negligible fault current, allowing the continued operation of the auxiliary winding to provide substation power supply. As shown in Figure 14, a ground fault sensing scheme should provide an alarm. This scheme consists of three voltage transformers, a resistor, and an overvoltage relay, 59N, connected similar to the grounding transformer discussed in 7.1 and 7.4.2. Protection of the distribution system supplied by the auxiliary winding should be coordinated with the protection of the shunt reactor itself. Typical station service arrangements are normally provided with emergency supplies (not shown in Figure 14). Feeder bus tie and feeder breakers should also be provided with overcurrent protections. It is therefore recommended to account for the various possible means of auxiliary power distribution and provide adequate coordinated protection schemes taking into consideration both the thermal capability of the auxiliary reactor winding and the short-circuit capability of the circuit breakers. 8.4.5 Loss of cooling Oil-immersed reactors are sometimes built with forced-cooling to reduce size and cost. For such reactors, the cooling is usually critical and should be operational any time the reactor is energized. The loss of cooling can be detected by monitoring the oil flow with flow indicators, monitoring the ac supply voltage to the cooling fans and oil pumps, and by monitoring the temperature with temperature relays. The oil-flow and ac supply voltage indicators are usually connected for alarm only. The temperature relays are generally connected to trip and remove the reactor from service. To adequately protect the reactor, a combination of all the above indicators is usually recommended. 8.4.6 Overvoltage Overvoltage relays can be used to disconnect reactors under extreme high-voltage conditions, but in this case, the associated transmission line should be de-energized at the same time; otherwise disconnection of the reactors tends to further aggravate the overvoltage condition on the system. Reactor overvoltage relays should be set to accommodate planned contingencies.
6 Some shunt reactor designs with auxiliary power windings can be adversely affected by open-circuiting the auxiliary winding while energized. As a precaution, inadvertent opening of a controlled circuit interrupting device can be avoided by using noncontrolled devices such as fuses.
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Figure 14 —Shunt reactor with ungrounded wye auxiliary power winding
8.4.7 Pole disagreement protection In the application of shunt reactors at the terminals of EHV lines and buses, it is generally desirable to provide a means of switching the reactor bank for protection of the reactors and/or for system operating requirements. Due to the voltage level involved, the switching equipment often consists of single-pole devices that are not mechanically linked, with each pole having an independent operator. With such an arrangement, the possibility exists that one pole might not operate coincidentally with the other poles, thus creating an undesirable imbalance in system voltages or, in case the switching equipment were called upon to isolate the reactor bank to clear a fault, might fail to remove the faulted reactor from service. To ensure that all poles of the switching equipment function in unison, two common methods of detecting pole disagreement are presented, and can be used either together or separately. The first method utilizes auxiliary contacts on the various pole operators of the switching equipment, interconnecting “a” and “b” contacts of the devices, so that if all poles are not open or closed at the same time, a trip circuit should be provided to trip all poles of the switching equipment or, additionally, to trip back-up circuit breakers to isolate the switching equipment. Such a scheme is shown in Figure 15, which illustrates the application for a three-pole reactor switching arrangement. A second method of detecting pole disagreement uses a pole disagreement relay designed to compare the currents in each reactor connected to the transmission system. One way that this comparison can be made is illustrated in Figure 16, in which a spare reactor is provided that can be switched to replace any of the normal phase reactors. The scheme shown in Figure 16 provides for two trip outputs with separately adjustable time delays. The shorter delay is used to trip the reactor switch(es) in the event of a current disagreement between phases. The longer delay trips local and remote line circuit breakers if the first trip fails to clear the pole disagreement condition. As shown, the scheme relies on a multiphase comparison of logic derived from the line current inputs, including that from the spare reactor, and coordinating timers.
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Figure 15 —Typical auxiliary contact disagreement circuit in circuit breaker control wiring
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Figure 16 —Pole disagreement protection for three-phase reactor installation with switchable spare reactor
8.4.8 Microprocessor-based relays Several benefits have been demonstrated for using microprocessor-based relays. The first is self-testing. A microprocessor relay can continually check itself to see if it is working properly. Should something go awry, a self-check failure alarm occurs and the relay takes itself out of service. It is no longer necessary to wait for a fault to find a relay has failed. A microprocessor-based relay can also display metering information, which can be used to determine the integrity of current transformers and potential transformers. Another benefit from using a microprocessor-based relay is data storage. With the ability to
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store data, microprocessor-based relays can store numerous fault records and oscillographic data to aid in the post-fault analysis. Microprocessor relays have the ability to store several setting groups and switch between them using internal programmable logic with or without auxiliary inputs. Depending on system conditions external to the relay such as inrush current or dissipating trapped charge during line de-energization, it might be desirable to change a relay setting for the condition to make it more or less sensitive. The flexibility of a microprocessor relay to be set to respond to either rms or fundamental gives the user the ability to desensitize the relay to all harmonics, or respond to all harmonics below the Nyquist limit dictated by the sampling rate. Digital filtering is used to remove the dc component from the sampled waveform. The programmability of microprocessor relays can contribute to the ease of implementing different control options when special situations occur. One example is when a circuit switcher is used to protect a shunt reactor as discussed in 8.3.1. Should the fault current be too high as to exceed the rating of the circuit switcher, logic can be employed through the microprocessor-based relay, which blocks the circuit switcher from operating and trips a back-up breaker with minimal additional wiring. Custom logic can also be used to open a load-break switch during pole disagreement while normal fault clearing can be through a circuit breaker. Many microprocessor relays contain multiple protective functions within one device. The user should consider redundancy when using multifunction devices.
9. Summary of shunt reactor protection This clause includes Table 1 through Table 6 that represent a summary of shunt reactor protection.
Table 1 —Properties of shunt reactors Dry air-core type
Oil-immersed (self-cooled or forced-cooled)
Typical connections: Connected to the tertiary winding of a transformer or directly connected to system
Typical connection: Solidly-grounded or impedancegrounded neutral connected directly from the system
Voltage range: 138 kV or below
Voltage range: Above 34.5 kV
Magnetizing inrush: No magnetizing inrush upon energization (no iron core)
Coreless type: Less severe energizing inrush
Peak current during energization up to: 2 2 × Inominal due to transient offset
Gapped iron-core type: Severe energizing inrush
Table 2 —Failure modes in dry air-core reactors Dry air-core type
Cause of failure
Phase-to-phase faults on tertiary bus bar resulting in high magnitude phase current
Arcing from a failed reactor not detected fast enough and causes phase-to-phase fault on the tertiary bus bar due to fault ionization
Direct winding-to-ground faults
Usually caused by bridging the insulation to ground (animal)
Turn-to-turn faults
Insulation failure can result in arc faults. If not detected fast enough, the fault can cascade the entire winding. It can thermally damage the healthy phases if the reactor is ungrounded. Ihealthy phase = 3 × Inominal
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
Table 3 —Failure modes in oil-immersed reactors Oil-immersed reactors
Cause of failure
Phase-to-phase faults and phase-to-ground faults resulting in high magnitude phase and ground current
Arcing from a failed reactor not detected fast enough
Direct winding-to-ground faults
Usually caused due to the proximity of the winding with the core and tank. The magnitude of the fault decreases as the fault is located closer to the neutral side.
Turn-to-turn faults
Insulation failure can result in increased temperature, internal pressure build-up and gas accumulation. This type of fault can result in a major fault if not detected quick enough.
Miscellaneous faults
Loss of cooling—low oil level
Table 4 —Typical protection requirements for shunt reactors Faults
Time clearing requirements
High magnitude phase faults
Fast clearing time determined by the fault withstand time of the primary equipment supplying the reactor or the critical clearing time of the transmission line
Low magnitude phase-to-ground faults
Minimum clearing time required without inducing false trips
Low magnitude turn-to-turn faults
Minimum clearing time required without inducing false trips
Miscellaneous faults
Alarm: sufficient time required for operators to respond Trip: Required time below damaging levels
Table 5 —Typical protection functions for dry air-core and oil-immersed reactors Type
Protection performance
87
Fast, secure for medium to high magnitude faults. Not efficient for turn-to-turn faults or for partial internal winding faults.
21
Fast for high magnitude faults. Turn-to-turn faults with long time delay might require differential set below inrush apparent impedance but above turn-to-turn shorted impedance.
50/51PH
Reactor/equipment fault withstand
50/51N,50/51G,87G,67N,67Q
Detects winding faults to ground. Partial turn-to-turn protection
46
Turn-to-turn and winding-to-ground protection
59
Overexcitation of the reactor iron core/system protection
Gas, oil pressure
Turn-to-turn/internal tank faults/tank rupture
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Table 6 —Typical primary, secondary, and back-up protection schemes Type
Protection performance
Dry-type reactors 50/51,87,46
Phase-to-phase faults
59∠ (broken delta)
Phase-to-ground faults
Voltage unbalance scheme with compensation
Turn-to-turn faults
Oil-immersed reactors 50/51,87(HI),21
Large magnitude faults
50/51N
Ground faults
50N,67N,67Q
Turn-to-turn faults
Sudden-pressure/gas-accumulator relay/21
Internal tank faults/tank rupture
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Annex A (informative) Bibliography [B1] ASEA Electric Recommendations for Protective Relays, Pamphlet ZF27-004E Reg. 4771. ASEA Brown Boveri, Protective Relay Division, Allentown, PA, 1985. [B2] Blackburn, J. L., “Protection of Shunt Reactors,” Silent Sentinels RPL 77-1, Westinghouse Electric Corporation Publication, Nov. 1977. [B3] Carlson, L., et al., “Single-pole reclosing on EHV lines,” International Conference on Large HighVoltage Electrical Systems, CIGRE, Paris, France, paper no. 3103, 1974. [B4] Copper, J. W., and Eilts, L. W., “Relay for ungrounded shunt reactors,” IEEE Transactions on Power Apparatus and Systems, vol PAS-92, pp 116–121, Jan/Feb. 1973. [B5] Edwards, L., Chadwick, Jr., J. W., Riesch, H. A., and Smith, L. E., “Single-pole switching on TVA’s Paradise-Davidson 500-kV line design concepts and staged fault test results,” IEEE Transactions on Power Apparatus and Systems, vol PAS-90, pp. 2436–2450, Nov/Dec. 1971. [B6] Engelhardt, K. H., “EHV line-connected shunt reactor protection application and experience,” International Conference on Large High-Voltage Electric Systems, CIGRE, Paris, France, paper no. 34-09, 1984. [B7] Feldman, J. M., and Wilson, D. D., “Shunt reactor compensation on present and future transmission systems,” American Power Conference, Apr. 1969. [B8] IEEE 100™, The Authoritative Dictionary of IEEE Standards Terms, Seventh Edition. [B9] IEEE Std 32™-1972, IEEE Standard Requirements, Terminology, and Test Procedures for Neutral Grounding Devices.7, 8 [B10] IEEE Std C37.100™-1992, IEEE Standard Definitions for Power Switchgear. [B11] IEEE Std C57.21™-1990, IEEE Standard Requirements, Terminology, and Test Code for Shunt Reactors Rated Over 500 kVA. [B12] IEEE Std C62.2™-1987 (withdrawn), IEEE Guide for Application of Gapped Silicon-Carbide Surge Arresters for Alternating-Current Systems. [B13] Kimbark, E. W., “Suppression of ground-fault arcs on single-pole switched EHV lines by shunt reactors,” IEEE Transactions on Power Apparatus and Systems, vol. 83, no. 3, pp 285–290, Mar. 1964. [B14] LaForest, J. J., et al., “Resonant voltages on reactor compensated extra-high-voltage lines,” IEEE Transactions on Power Apparatus and Systems, vol PAS-91, pp 2528–2536, Nov/Dec. 1972.
7
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IEEE Std C37.109-2006 IEEE Guide for the Protection of Shunt Reactors
[B15] Pickett, M. J., et al., “Near resonance coupling on EHV circuits: I-Field investigations,” IEEE Transactions on Power Apparatus and Systems, vol PAS-87, pp 322–325, Feb 1968. [B16] Power System Relaying Committee Report, “Shunt reactor protection practices,” IEEE Transactions on Power Apparatus and Systems, vol PAS-103, pp. 1970–1976, Aug. 1984. [B17] Reactors, International Electrotechnical Commission Publication 289, 1968. [B18] S&C Electric Company, Chicago, IL, RD-3221 Operating Description, Aug. 1985. [B19] “The Art of Protective Relaying—Power Systems Protection for EHV Systems,” General Electric Company Publication, GET-7207, Jan. 1965. [B20] Trench Electric, Toronto Ontario, Canada. Shunt Reactor Bulletin T100-35-02l, May 1984.
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