NATIONAL INSTITUTE OF TECHNOLOGY, KURUKSHETRA
SUBMITTED BY: LALIT MEENA 108416(E1)
ELECTRI CAL ENGINEERING
CONTENT S.N o. 1 2 3 4
5 6 7
TOPIC About Indian Railways Acknowledgement Circuit Diagrams Details of Vasai Road(mumbai) substation Bus schemes at Substation Substation equipments Protection equipments
PAGE NO.
ABOUT INDAIN RAILWAYS Indian Railways has 114,500 kilometers (71,147 mi). of total track over a route of 65,000 kilometers (40,389 mi) and 7,500 stations. It has the world's fourth largest railway network after those of the United States, Russia and China. The railways traverse the length and breadth of the country and carry over 30 million passengers and 2.8 million tons of freight daily. It is the world's second largest commercial or utility employer, with more than 1.36 million employees As for rolling stock, IR owns over 240,000 (freight) wagons, 60,000 coaches and 9,000 locomotives. Railways were first introduced to India in 1853. By 1947, the year of India's independence, there were forty-two rail systems. In 1951 the systems were nationalised as one unit, becoming one of the largest networks in the world. IR operates both long distance and suburban rail systems on a multi-gauge network of broad, meter and narrow gauges. It also owns locomotive and coach production facilities.
ACKNOWLEDGEMENT It’s my pleasure to be indebted to various people , who directly or indirectly contributed in the development of this work and who influenced my thinking ,behavior and acts during the course of study. Training at Indian Railways has been a wonderful experience for me. I would like to thank Indian Railways for wholesome development of personality. I express my sincere gratitude to Mr. S.K. PATIL (ADEE/SS/BCT), my mentors that I learnt work ethics besides enhancement of my technical skills. Mr. RATAN PARKHI has been gracious enough to allow me to imbibe educational and professional qualities. I would also like to thank Mr. SINDHEY for his guidance, encouragement and taking pain for my project work. Though tedious at times, the training at Indian Railways was overwhelming. Lastly,I would like to thank almighty and my parents for their moral support and my friends with whom I share my day- today experience and received lots of suggestions that improved my quality of work.
AC POWER TRACTION LEYOUT
DETAILS OF 100/25 KV VASAI ROAD SUB STATION
Incoming feeder from TATA Power Station at 100kv Lightning Arrestor -96 kv Potential transformer 100kv to 110v Double Pole Insulator Bush Coupler Supporting Insulator Current Transformer 800A To 5A Circuit Braker- SF6(Arc Intrruptor ) Zabra Voltage Cable- 28mm2 Lightning Arrestor(Fixed and Moving) Capacitor Bushings Differential Protection –(Zone Protection)
SUBSTATION EQUIPMENT
The main components of a substation are as follows: Autotransformer Circuit breaker Isolator and earth switch Current transformer Capacitive voltage transformer Wave trap Lightning arrester
AUTO TRANSFORMER It is a static device which transfers the A.C. electric power from one circuit to the other at the same frequency. It is used
to step up or step down the voltage. At all the substations except at the generating station the step down transformers are used. There are many types of transformers but in this substation an auto transformer is used.
13.5MVA , 100/25KV AUTO TRANSFORMER There are two auto transformers of 3 phases; 100/25kV auto transformer is installed in Vasai Road Substation. The capacity of each auto transformer is 13.5 MVA. Input of 100kV is coming from TATA POWER PLANT substation and the output is 25 kV . ELECTRICAL SPECIFICATION OF 13.5 MVA, 100/25 KV TRANSFORMER 1 2
Capacity(MVA) Cooling
13.5 ONAN/ONAF/OFAF
3 4 5 6 7
Voltage(KV)HV/LV No. of phases Frequency Connection & symbol Neutral earthing
100/25 3 50Hz Star/ delta-Ynod11 HV solidly earthed
Advantage of Auto Transformer Volume, and hence weight of copper, is proportion to the length and area of cross section of the conductor. The length of the conductor is proportion to number of turns and area of cross section is proportional to the current flowing through it .Hence the weight of copper proportional to the product of current and number of turn . It is used as auto transformer starter to give up to 50 %to 60% of full voltage to the stator of squirrel cage induction motor during starting . It is give a small boost to a distribution cable to correct the voltage drop It is used as a regulating Transformer
Disadvantages:Although Auto transformers have less cost, better regulation and low losses as compared to the ordinary tow winding transformer of same rating. But they are not widely used due to one major disadvantage that the secondary winding is not insulating from primary.
PARTS OF AN AUTOTRANSFORMER Conservator tank Conservator tank shall have adequate capacity with highest and lowest visible level to meet the requirements of total cold
oil volume in transformer and cooling equipment from minimum ambient temp. To 100 deg C. Conservator should notobstruct any electrical connection to transformer. Dehydrating Filter Breather Conservator shall be fitted with dehydration filter breather. It shall be so designed that:a). Passage of air is through silica gel. b). Silica gel is isolated from atmosphere by oil seal. c). Moisture absorption indicated by change in colour. Silica gel is replaced when half to two third of the silica gel as become saturated and turned pink in colour. For its reuse, silica gel should be heated in a well ventilated oven at a temp. Of 130 to 138 deg Celsius until the entire mass has achieved the bright colour.
Silic a gel breather of ICT1 and ICT2 CORE Core is manufactured from lamination of cold rolled grain oriented silicon steel which gives very low specific loss at operating flux densities. Joints of the laminations are designed
such that the electromagnetic flux is always in the direction of grain orientation. The core is of 3 limbs, 3phases. WINDINGS Windings are made from paper insulated copper conducors. Parallel conductors are transposed at regular intervals throughout the winding for ensuring equal flux linkage and current distribution between strands. Insulation spacers in the windings are arranged such that oil is directed through the weights for ensures proper cooling. COOLING For ONAN cooling, oil flows through the winding and external radiators units attached to the tank by thermo phonic effect. Radiator cooler units consist of pressed steel sheet radiators mounted directly on the tank. ONAF cooling is achieved by providing adequate number of fans. OFAF cooling is achieved by providing adequate number of in line oil pumps.
WINDING TEMPERTAURE INDICATORS The winding temperature relay indicates the winding of the temperature of the transformer and operates the alarm, trip and cooler control contacts.Windingtemp.thermometer bulb is in a separate thermometer well near the top of the tank. Wire heater oil is either inserted into or wrapped around the thermometer well which surround the temp.sensitive bulb. In some transformers a current transformer is around is of the three winding leads & provides current directly to the heater coil in proportion to winding current. In other transformer the current transformer supplies current to an auto transformer that supplies currently to the heater coil. The heater warms s the bulb & the dial indicates heat temp. But it is not the true hottest spot temp. the increase in temp. of the resistance is proportionate to that of the winding. The sensor bulb of the
instrument is located in the hottest oil of the transformer plus the winding temp. rise above hot oil i.e. the hotspot temperature.
WINDINGTEMPRATURE INDICATOR
TAP CHANGER Transformer is provided with on load tap changer. The tap changing can be designed for changing the taps in the following modes: a). manually from local motor drive unit. b). manually from remote. c). automatically from the supervisory system. The on load tap changing will be self contained unit housed in the main transformer tank. This consists of diverter unit & selector unit. In selector unit termination will be made. Since some amount arcing take place during switching operation from one tap to other. The oil inside the diverter unit will
deteriorate faster. Hence this oil in diverter chamber cannot be allowed to mix with the oil in the main transformer. Oil load tap changer is provided with a separate conservator and oil surge relay.
GAS AND OIL ACTUATED (BUCHHOLZ) RELAY Gas and oil actuated relay is fitted in the feed pipes from the expansion vessel to the tank for collection of gas generated in the oil. A pre-set volume of gas collection in the relay, or an oil surge toward the expansion vessel will cause the alarm or trip contacts to operate respectively. In the event of serious oil loss from the transformer, both alarm and trip elements operate. In the pipes connection between On Load Tap changer and its oil expansion vessel an oil surge relay fitted. This relay operate on principle of oil surge impinging on a flag causing operation of the mercury switch connect to the trip circuit. Gas actuated alarm switch is eliminated because the gas generated during normal tap changing operation will give unnecessary alarm.
BUCHHOLZ RELAY BUSHING
Transformers are connected to HV lines, and therefore, care is to be taken to prevent flash-over from the high voltage connections to earthed tank. Connections from cables are made in cable boxes, but overhead connections are made to be brought through bushings specially designed for different classes of voltages. The winding connection passes from the windings to terminal bushings. Terminal bushings upto 36 KV class, 3150 Amps, are normally of plain and porcelain and oil in contact type. Bushings of 52 KV class and above are of oil impregnated condenser type. The oil inside the condenser bushings will not be communicating with the oil inside the transformer. Oil level gauge is provided on the expansion chambers of the condenser bushings.
INSULATING OIL In the Auto Transformer, we use mineral oil .It acts as an insulating fluid & heat transfer medium to carry off excess heat generated by the losses of power transformers. To ascertain the requirements: 1 2 3 4 5 6
properties
of
oil
for
meeting
certain
Kinematic viscosity Pour point Carbon type composition Oxidative ageing Density Oxidation stability It should be free from moisture
For testing of insulating oil it stand for 60 KV for up to flash time.
CIRCUIT BREAKER It is a device which makes & breaks the circuit under no load, full load & fault condition. It can be operated manually under normal conditions & automatically under abnormal conditions. WORKING PRINCIPLE It consists of fixed & moving contacts called as electrodes under normal operating conditions. These contacts remain closed & will not open automatically until the system become faulty. When fault occurs the tripping coil of the circuit breaker gets energized & the moving contacts are pulled apart. An arc is produced between them. The production of arc delays the current production & generates heat which may damage the system. Sulphur-Hexafluoride Circuit Breaker In this circuit breaker, sulphur hexafluoride gas is used for the extinguishing the arc.
SULPHURHEXAFLOURIDE CIRCUIT BREAKER
PRINCIPLE OF ARC EXTINCTION IN SF6 In SF6 circuit breaker, the SF6 gas is blown axially convection & dissipation. This reduces the arc diameter during the decrease mode of current wave & an arc in extinguished. In sf6 circuit breaker, the gas is made to flow from high pressure to low pressure through a convergent divergent nozzle. When moving contact is struck from the fixed contact, an arc is produced in between them. Gas nozzle is located such that the flow of gas covers the arc reduce its diameter. Finally, arc reduces to zero at current zero. Under normal conditions, the moving contact is connected with the fixed contact. The outer cylinder is coupled with the moving contact whereas piston is fixed. When fault occurs the moving contacts separated from the fixed contact. ADVATANGES: There are some advantages of SF6 circuit breaker over the conventional breakers given below: (1) Due top outstanding arc quenching property of SF6, the arcing time is very small. This reduces contact erosion. (2) Using SF6 gas at low pressure & low velocity, the current chopping can be minimized. (3) During arcing of SF6 circuit breaker, no carbon dioxide is formed and hence no reduction of dialectical strength. (4) SF6 circuit breaker is salient in operation and moisture ingression into the gas cycle is almost nil. (5) SF6 breaker is performance is not affected due to variation in atmospheric condition. (6) SF6 breaker is compact in size and electrical clearance is directly reduced. DISADVANTAGES: The only disadvantage is that SF6 is suffocating. In case of leakage of breaker tank, this gas is heavier than air settled in the surrounding and may lead to
suffocation of the operating personals .However, It is non poisonous. It also destroys our ozone layer.
CURRENT TRANSFORMER These are the instrument transformers which are basically step down transformers.Current transformer is used to step down the a. c. from higher to lower value. The primary winding of a having one or a few turns of thick wire is connected in series with the line whose current is to be measured. The secondary having large number of carries the instrument an ammeter directly connected across it. The current transformer works under short circuit condition. The secondary of transformer is connected to the instrument placed on panel boards. The current transformer burden values are very low i.e.15 VA for 1A.current transformer ratios are so chosen such that they supply 1a or 5A at nominal primary current. For measurement CT’s may utilize low inductance cores or special core materials for high accuracy measurement& protection. CTs may use air gapped cores to meet special requirement for asymmetrical fault currents. CT’s are also known as “SERIES TRANSFORMER”.Current transformers are very accurate and long lasting reliable equipment.
CURRENT TRANSFORMER
TYPES OF CT 1). DEAD TANK CT In the dead tank C.T, the primary winding is on the top where as the secondary winding is placed on bottom as shown in figure.
1. Dome
2. Nitrogen filling valve
3. Primary terminal
4. Collar
5. Porcelain insulator with insulation
6.
7. Adaptor cylinder
8. Secondary cores
9. Base
Primary
10. Oil drain plug
conductor
2). LIVE DESIGN TYPE CT In the live tank C.T., both primary &secondary windings are on the top. The fig. is as shown below:-
ISOLATOR It is a device which opens and closes the contacts only under no load conditions. Its main purpose is to isolate a portion of the circuit from the other. These are generally placed on both sides of the circuit breaker in order to make repair and maintain the circuit breaker without any danger. These are never opened until the circuit breaker in the same circuit is opened and always closed before the circuit breaker is closed.
(Horizontal Double break type)
CAPACITIVE VOLTAGE TRANSFORMER CVT is an instrument transformer which is used for voltage measurement and protection system and their high frequency point is used for PLCC system. In EHV system CVT’s are used instead of VT’s due to economical reason.
1. Oil level indicator units
2. Expansion device3. Capacitor
4. Insulating oil5. Porcelain insulator6. Sealing 7. Electromagnetic unit8. Low voltage terminals box 9. Series inductance10. Medium voltage transformer
11. Damping circuit against ferro-resonance effects
There are 280 – 300 elements in C1 & C2 C1 will be about 260 to 280 elements C2 will be 15 to 20 elements Ratio of C1/ C2 is about 20 400/ 20 = 20kV (Tap Voltage)
LIGHTNING ARRESTORS LA is used for protection of substation equipments(eg transformer), from Lightning surges and over voltage. It behaves like a insulator in normal operating voltage and gives a continuous path in case of high voltage surges. It is first equipment of the substation which is placed at the line entrance to the substation to protect bay equipments and is also placed on both sides of the transformer. Concept of Leakage Current The leakage current through the arrester is broadly classified as… 1. Capacitive components current originated from permittivity of ZnO elements, stray capacitance, grading capacitors. 2. Resistance component current originated from ZnO elements, Porcelain surface current (Pollution). MEASUREMENT PRINCIPLE 1. Harmonic are created in leakage current on application of fundamental frequency, due to non linear voltagecurrent characteristics of surge arresters. 2. 3rd Harmonic is the largest Harmonic component of the resistive current. EFFECT OF 3RD HARMONICS IN SYSTEM VOLTAGE 1. 3rd Harmonic in system voltage creates capacitive harmonics currents which affect the measured value. 2. Error in the measured values may be considerable. 3. As reported, 1% 3rd Harmonic in system voltage may introduce error upto 100% in the measured value.
WAVE TRAP It is PLCC equipment. It is highly inductive device which is used for PLCC purpose, it block the high frequency PLCC signal to enter in the switchyard of the substation and it allows the power frequency 50 Hz to pass through it and enter into switchyard. It is connected in series with the EHV line.
( Wave Trap)
CONTROL ROOM Control room consists of : relay panel control panel SCADA
RELAY PANEL It is used for protection purposes ,the relays get supply from CT or CVT. The relays used are numeric relays .They send tripping signal to the circuit breaker during fault condition.
(A view of Relay panel in a Relay Room)
CONTROL PANEL
It displays the current,voltage,power reading for all load centers,line feeders etc. .at the time of fault ,the alarm sounds or any red alert signal turns on. Generally, red and white flags are present. Red flags give indication for a major fault and white flag give indication for a minor fault. The panel has several transducers i.e. voltage, current, power, frequency transducers. These transducers receive the input from the relay unit. The quantities are then converted to DC signals, which are displayed on digital meters.
SCADA
SCADA is an acronym that stands for Supervisory Control and Data Acquisition. SCADA refers to a system that collects data from various sensors at a factory, plant or in other remote locations and then sends this data to a central computer which then manages and controls the data. A SCADA system usually includes signal hardware (input and output), controllers, networks, user interface (HMI), communications equipment and software. All together, the term SCADA refers to the entire central system. The central system usually monitors data from various sensors that are either in close proximity or off site (sometimes miles away). (Programmable Logic Controller) is usually used to control a site automatically. The SCADA system also provides a host control functions for the supervisor to control and define settings. For example, in a SCADA system a PLC can be used to control the flow of cooling water as part of an industrial process. At the same time the supervisor can use the Host control function to set the temperature for the flow of water. It can also have alarms and can record the flow of water temperature and report back to the SCADA system. The RTUs and PLCs are responsible for data collection such as meter readings, equipment status etc and communicate back to the SCADA system. This data can be stored in a database for later analysis or monitored by a supervisor to take appropriate actions if required. SCADA systems typically implement a distributed database, which contains data elements called tags or points. A point represents a single input or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point is representative of an actual input or output connected to the system, while a soft point represents the result of logic and math operations applied to other hard and soft points.. Point values are normally stored as valuetimestamp combinations; the value and the timestamp when the value was recorded or calculated. A series of valuetimestamp combinations is the history of that point. It's also common to store additional data with tags such as: path to field device and PLC register, design time comments, and event alarming information.
PROTECTION
RELAYS Protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective section from the rest of the system. We have seen that whenever the fault occur on the power system, the relay detects that fault and closes the trip coil circuit. This result in the opening of the circuit breaker which disconnects the faulty section. Thus, the relay ensures the safety of the circuit equipment from damage which may be caused by the faulty current. ESSENTIAL ELEMENTS OF RELAY All relays have the following three essential fundamental elements in the block diagram. i.
ii.
iii.
Sensing Element: - Sensing or measuring element is the element which responds to the change in the magnitude or phase of the actuating quantity e.g. current in the over current relay. Comparing Element: - It is the element which compares the action of the actuation quantity of the relay with pre-designed relay setting. The relay only pick up if the actuating quantity is more than the relay setting. Control Element:- when a relay pick up, it accomplishes a sudden change in the controlled quantity such as closing of trip coil circuit.
FUNDAMENTAL REQUIREMENT OF RELAY
The main function of the protective relay is to disconnect the faulty section of the power system through circuit breaker abruptly before damaging the costly equipment. In order to perform this function satisfactory, it should have the following important features:i. ii. iii. iv. v. vi.
Selectivity Sensitivity Reliability Speed or quickness Simplicity Economical
All protective switchgears have the above features as above mentioned.
TYPES OF RELAYS There are many kinds of relays applied in the power system. The relays can be designed and constructed to operate in response to one or more electrical quantities such as voltage, current, phase angle, etc. The relays are classified in different ways.
1) ACCORDING TO CONSTRUCTION AND PRINCIPLE OF OPERATION Thermal relays:- the heating effect of electric current is used for the operation of these relays. Electromagnetic attraction type relay: - The operation of these relays depends upon the movement of an armature
under influence of attractive forces due to the magnetic field set up by current flowing through the relay coil. Induction type: - Electromagnetic induction phenomenon is used for the operation of these relays. By induction eddy current are induced in the aluminum disc, free to rotate, which exerts torque on it. 2)
ACCORDING TO APPLICATION
Over current, over voltage relays:-These relays operate when the current, voltage or power rises beyond a specific value. Under voltage, under current relays: - These relays operate when the current, voltage or power falls below a specific value. Directional or reverse current relays:-These relays operate when the applied current assumes a specified phase displacement with respect to the applied voltage and the relay is compensated for fall in voltage. Directional or reverse voltage relay: - These relays operate when the applied voltage and current assumes a specified phase displacement and no compensation is allowed for fall in voltage. Distance relays:-The operation of these relays depends upon the ratio of the voltage to the current. Differential relays:- the operation of these relays takes place at some specific phase difference or magnitude difference b/w two or more electrical quantities.
3)
ACCORDING TO TIME OF OPERATION
Instantaneous operation takes place
relays:- in these instantaneously
relays, i.e.
complete the
operation is complete in a negligible small interval of time from the incidence of the actuating quantity. Definite time lag relays:- in these the operation takes place after a definite time lag which is independent of the magnitude of actuating quantity. Inverse time lag relays:- In these relays the time of operation is inversely proportional to magnitude of actuating quantity. Inverse definite minimum time lag relays (IDMT):In these relays the time of operation is approx. inversely proportional to magnitude of actuating quantity, but it is never less than a definite minimum time for which relay is set.
DIFFERENTIAL PROTECTION RELAY Features
Processors system with powerful 16 bit microprocessor. Insensitive against CT errors, transient phenomenon & interfaces. Simple setting & operation using the integrated panel. Storage of fault data &a instantaneous value during a fault for fault recording. Continuous monitoring of the hardware & software of the relay. Continuous calibration of operational measured value & indication on front display. OPERATION Differential protection system operate acc. To the principle of current comparison. It is known as current balance system. They utilize the fact that the current leaving a healthy protected object is as that which entered it. The sec. winding of CT1& CT2, which have the same transformation ratio, may be
so connected that current I flows through it. A measuring element M is connected at electrical balance pt. under healthy condition, no current flow in the measuring element M. when fault occurs in b/w the two CT’s, the currents at the ends of the section are unequal, the measuring element is subjected to a current I1+I2 prop. To I1+I2 the sum of two inflowing faults currents. When an external fault causes a heavy current to flow through the protected zone diff. in the MAJ. Characteristic of the CT under condition of saturation may cause a current flow through M. if the magnitude of current lies above the response threshold, the system would issue a trip signal.
RESTRICTED EARTH FAULT PROTECTION Detects earth fault in power transformers, shunt reactors, neutral grounding trans. Or rotating machines, the star point of which is led to earth. A precondition is that a CT is installed in the star point lead. i.e. b/w the star point & earth. The star point CT & the three phases CT’s define the limits of the protection zone exactly.
DISTANCE PROTECTION RELAY Basic Principle of Distance protection A distance relay, as its name implies, has the ability to detect a fault within a pre-set distance along a transmission line or power cable from its location. Every power line has a resistance and reactive per kilo meter related to its design and construction so its total impedance will be a function of its length or distance. A distance relay therefore looks at current and voltage and compares these two quantities on the basis of Ohm’s law.
Basic principle of operation The voltage is fed onto one coil to provide restraining torque; the current is fed to the other coil to provide the operating torque. Under healthy conditions, the voltage will be high (i.e. at full-rated level), whilst the current will be low (at normal load value), thereby balancing the beam, and restraining it so that the contacts remain open. Under fault conditions, the voltage collapses and the current increase dramatically, causing the beam to unbalance and close the contacts.
Distance Protection Relay Application onto a power line Correct coordination of the distance relays is achieved by having an instantaneous directional zone 1 protection and one or two more time-delayed zones. A transmission line has a resistance and reactance proportional to its length, which also defines its own characteristic angle.
Zone 1 The relay characteristic has also been added, from which it will be noted that the reach of the measuring element has been set at approximately 80% of the line length. This ‘under-reach’ setting has been purposely chosen to avoid. These measuring elements in known as zone 1 of the distance relay and are instantaneous in operation. Zone2 To cover the remaining 20% of the line length, a second measuring element can be fitted, set to over-reach the line, but it must be time delayed by 0.5 s to provide the necessary coordination with the downstream relay. This measuring element is known as zone 2. It not only covers the remaining 20% of the line, but also provides backup for the next line section should this fail to trip for whatever reason. Zone 3 A third zone is invariably added as a starter element and this takes the form of an offset mho characteristic. This offset provides a closing-onto-fault feature, as the mho elements may not operate for this condition due to the complete collapse of voltage for the nearby fault. The short backward reach also provides local backup for a bus-bar fault. This element can also be used for starting a carrier signal to the other end of the line. The zone 3 element also has another very useful function. As a starter it can be used to switch the zone 1 element to zone 2, reach after say 0.5 s, thereby saving the installation of a second independent zone 2 measuring element so reducing its cost.
OVERCURRENT RELAY- TYPE MCA 4B/5B3 relay are used for restriction earth fault protection. 4B3 relay are available in two voltage setting (15 -170V and 115270 V) and 5B3 relay has single voltage setting 15- 270 V.
AUXILIARY RELAY Auxiliary relay is the range of attracted armature, auxiliary/protection relays operated by DC or AC voltage. A product designation number following the ‘AR’ identifies the particular features of the model.Product designations: First designation elements.
:
Second digit Third digit
: :
Number of identical Type of Flag. Type of contact reset.
AUXILLARY RELAY
LIGHTNING MAST: Lightning phenomenon can be the cause of severe failures in substations such asinsulation flashover and damage of substation devices .the tipof mast is sufficiently small then the mast attracts lightningflashes more easily than the shielding wire. Masts arethus preferred to shielding wires for lightning protection for substations. In a substation, a height of a device can range betweena height of a mast and zero. As the goal of the substationprotection, the heights of the masts, as well as the locationsof the masts, should be determined to protect properly for alldevices within this range. This is, the protected zone of themasts should cover the devices as much as possible, the protected zone of one mast is based on: themast height of h and the object protected, which has a heightof y and is located at a distance of x from the foot of the mast. Limited Conditions:
In case of more than two masts, for a certain nominal systemvoltage, three conditions are considered as the follows: • Each mast of this voltage level is contained by at leastone three-mast group. • For outside masts, two consecutive outside masts are twoends of a side of a three-mast group. They are neighbormasts. • Each outside mast has only two neighbor masts.
LIGHTNING ARRESTOR The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against travelling waves, which may reach the terminal apparatus. The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device, which conducts the high voltage surges on the power system to the ground. The action of the lightning arrester is as under: (i) Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting (ii) On the occurrence of over voltage, the air insulation across the gap breaks down and an arc is formed providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.
(iii) As the gap sparks over due to over voltage, the arc would be a short-circuit on the power system and may cause powerfollow current in the arrester. Since the characteristic of the resistor is to offer low resistance to high voltage (or current), it gives the effect of short-circuit. After the surge is over, the resistor offers high resistance to make the gap nonconducting. TYPES OF LIGHTNING ARRESTERS: There are several types of lightning arresters in general use. They differ only in constructional details but operate on the same principle viz, providing low resistance path for the surges to the round. Following are the different types of lightning relays: 1. Rod arrester 2.Horn gap arrester 3. Multigap arrester 4. Expulsion type lightning arrester 5. Valve type lightning arrester
SURGE ABSORBER Surge Absorber acts as a load that utilizes the high current until it exhausts. Surge absorber degrades faster than surge diverter. It is also called surge modifier, surge capacitor and surge reactor. A surge modifier is a small shunt capacitor connected between line and earth or a series air cored inductor. It means a capacitor connected in parallel with an equipment provide some degree of protection against surges
before the surge can impress high voltage on the equipment.
GROUND WIRES The shielding of station and incoming lines is done by ground wires which limit the severity of surges. If supports are available,we may run it over the station in such a way that all equipments lie in protected zone. For a small substation, it sometimes is sufficient to run one or two ground wires across the station from adjacent line towers. If it is not possible to run ground wires to run over the substation then it may be possible to erect masts or rods at corners or over vertical columns so that the buses and equipments will fall within the cone of protection of masts and rods. Breakage of shieldingwires (due to lightning current or poor maintenance) can causecatastrophic faults in substations.
