18047364 project-on-substation-220kv-132kv

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18047364 project-on-substation-220kv-132kv

  1. 1. Hi this is a project on a 220 kV Grid substation in Sarusajai,Guwahati, AssamMost of the documents are taken from different search engines andI am not responsible for any wrong information here. All the dataare from the substation itself. -Preetom Parasar1. Introduction: 1.1. About the substation: 1
  2. 2. The substation in Sarusajai, Guwahati-781034, Assam was completed by the year 1986 underAEGCL; it is one of the largest power grids in the state of Assam and the north eastern India.This substation has the capacity of 220kv and can step down to 132kv using three input linesthrough the incoming feeders. The input feeders are namely: AGIA- i; AGIA-ii; LONGPI- i; LONGPI- ii; SAMUGURI-i; SAMUGURI- ii.All these feeders come into the substation with 220kv. The substation has another substation under it. The capacity of this is 132kv/33kv. Thissubstation was completed by the year 1997 under AEGCL. The purpose of this station was tostep down the 132kv to direct distribution to the 33kv/11kv substations in six different areas ofthe state. The substation of 132kv/33kv has six outgoing feeders, namely: JWAHAR NAGAR, GARBHANGA, MIRZA, PALTAN BAZAR, and Kahilipara STATION.These out going feeders are of 33kv line. The most important of any substation is the grounding of the instruments, transformers etc. usedin the substation. For grounding of the substation a metallic square or some poly shaped metalboxes are placed in the ground. These ground the extra high voltage to the ground. As it isdangerous to us to go near the instrument without proper earth. If the instruments are not groundproperly they may give a huge shock to anyone who would stay near it and also it is dangerousfor the costly instrument as they may get damaged by this high voltage.1.2. Construction – Site Selection & LayoutE HV S U B STATIONEHV Sub-Station forms an important link between Transmission network and Distributionnetwork. It has a vital influence of reliability of service. Apart from ensuring efficienttransmission and Distribution of power, the sub-station configuration should be such that itenables easy maintenance of equipment and minimum interruptions in power supply. Flexibilityfor future expansion in terms of number of circuits and transformer MVA Capacity also needs tobe considered while choosing the actual configuration of the sub-station. EHV Sub-Station isconstructed as near as possible to the load center. The voltage level of power transmission isdecided on the quantum of power to be transmitted to the load center.Generally, the relation between EHV Voltage level and the power to be transmitted is as follows:S.NO. POWER TO BE TRANSMITTED VOLTAGE LEVEL1) Upto 150 MVA 132 KV.2) From 150 MVA to 300 MVA 220 K.V.3) 300 MVA to 1000 MVA 400 K.V.1.3. SELECTION OF SITE 2
  3. 3. Main points to be considered while selecting the site for EHV Sub-Station are as follows:i) The site chosen should be as near to the load center as possible.ii) It should be easily approachable by road or rail for transportation of equipments.iii) Land should be fairly leveled to minimize development cost.iv) Source of water should be as near to the site as possible. This is because water is required for various construction activities; (especially civil works,), earthing and for drinking purposes etc.v) The sub-station site should be as near to the town / city but should be clear of public places, aerodromes, and Military / police installations.vi) The land should be have sufficient ground area to accommodate substation equipments, buildings, staff quarters, space for storage of material, such as store yards and store sheds etc. with roads and space for future expansion.vii) Set back distances from various roads such as National Highways, State Highways should be observed as per the regulations in force.viii) While selecting the land for the substation preference to be given to the Govt. land over private land.ix) The land should not have water logging problem.x) The site should permit easy and safe approach to outlets for EHV lines.1.4. Process of Land Acquisition After the selection of site of the proposed EHV Substation and finalization of the area required, proceedings for acquisition of land have to be initiated. The acquisition of land generally takes quite a long time. Forecasting and planning of substation and selection of substation site needs to be done much in advance taking into account the normal period of acquisition of land. The acquision of land should not in any way disturb the commissioning of programme of sub-staion. In MSETCL a land acquisition is carried out by the concerned Civil wing. The proposal for acquisition of land is submitted to the District Collector in case of Govt. land and to the land 3
  4. 4. acquisition officer in case of private land through the PWD Authorities accompanied by following documents: (i) 7/12 abstracts along with the resolution of local authority. (ii) Village map with suitable land duly marked on it. (iii) Sales statistics around the area. (iv) No Objection Certificate from the Forest Deptt. if applicable. (v) The certificate in case of private agricultural land that the owner of the land does not become landless if his land is acquired. (vi) These papers should be submitted after their due scrutiny. The land selected should be marked on the village map by taking joint measurements with Revenue Authorities. (vii) Once the proposal is submitted to land acquisition officer further legal proceedings are completed by him, and the award is given followed by allocation of land to the utility.Possessions of land are taken by taking joint measurements.1.5. STORAGE OF EQUIPMENTS FOR THE SUB STATION: All the substation equipments / materials received on site should be stored properly, either in theoutdoor yard or in the stores shade depending on the storage requirement of that particularequipment. The material received should be properly counted and checked for any damages / breakages etc.The storage procedure for main equipment is as follows:i) EHV C.T.s and P.T.s Normally, 132 KV C.Ts. and P.Ts are packed and transported in wooden crates vertically while those of 220 KV and 400 KV are packed in iron structures for extra supports with cross beams to avoid lateral movement. 132 KV C.Ts. and P.Ts. should be stored vertically and those of 220 KV and 400 KV should be stored in horizontal position. C.Ts and P.Ts. packed in wooden crates should not be stored for longer period as the packing would may deteriorate. The wooden packages should be stored on a cement platform or on M.S. Channels to avoid faster deterioration of the wooden crates. C.Ts and P.Ts packed in iron cases stored in horizontal position should be placed on stable ground. No C.Ts and P.Ts. should be unpacked in horizontal position.ii) L.A. s. and B.P.I. These are packed in sturdy wooden case as the porcelain portion is very fragile. Care should be taken while unpacking, handling and storage due to this reason.iii) Batteries, Acid, Battery charger C & R panel, A.C.D.Bs copper piping, clamp connectors, hardwares etc. should be stored indoor. 4
  5. 5. iv) Circuit breakers: The mechanism boxes of 33 KV – V.C.Bs should be stored on raised ground and properly covered with tarpaulins or should be stored in door. The interrupter chambers should be stored on raised ground to avoid rain water in storage area.v) E.H.V. C.B. Now-a-days SF6 circuit breaker are used at EHV rottages. The control and operating cabinets are covered in polythene bags and are packed in wooden and iron crates. These should be stored on raised ground and should be covered with tarpaulins. The arcing chambers and support insulators are packed in iron crates and transported horizontally. The +ve pressure of SF6 gas is maintained in these arcing chambers to avoid the ingress of moisture. It should be ensured that this pressure is maintained during the storage. Other accessories like pr. Switches, density monitor, Air Piping, control cables, wiring materials, SF6 gas pipes; SF6 cylinder should be stored in store shed.vi) Power transformers: The main Tank - The transformer is transported on trailor to substation site and as far as possible directly unloaded on the plinth. Transformer tanks up to 25 MVA capacity are generally oil filled, and those of higher capacity are transported with N2 gas filled in them +ve pressure of N2 is maintained in transformer tank to avoid the ingress of moisture. This pressure should be maintained during storage; if necessary by filling N2 Bushings - generally transported in wooden cases in horizontal position and should be stored in that position. There being more of Fragile material, care should be taken while handling them. Rediators – These should be stored with ends duly blanked with gaskets and end plates to avoid ingross of moisture, dust, and any foreign materials inside. The care should be taken to protect the fins of radiators while unloading and storage to avoid further oil leakages. The radiators should be stored on raised ground keeping the fins intact. Oil Piping. The Oil piping should also be blanked at the ends with gasket and blanking plates to avoid ingross of moisture, dust, and foreign All other accessories like temperature meters, oil flow indicators, PRVs, buchholtz relay; oilsurge relays; gasket ‘ O ‘ rings etc. should be properly packed and stored indoor in store shed.Oil is received in sealed oil barrels . The oil barrels should be stored in horizontal position withthe lids on either side in horizontal position to maintain oil pressure on them from inside andsubsequently avoiding moisture and water ingress into oil. The transformers are received on sitewith loose accessories hence the materials should be checked as per bills of materials.1.6. SUBSTATION STRUCTURES: It should be properly checked as per bills of materials of stacked and stored outside.1.7. THE FIRE PROTECTION: The fire protection device should be kept in store yard for safety of equipments during storage.1.8. INSURANCE – Transport of equipments from one place to other is done, where they are actually erected. During transport, erection and commissioning the equipments may get damaged resulting into loss, hence the equipments should be insured against storage, transportations, erections, testing and commissioning as the paying authorities feels suitable and can be insured with ( Govt. Insurance Fund / Insurance Authority ) the cost of equipments and the period of insurance should be mentioned.1.9. Earthing design 5
  6. 6. Guiding standards – IEEE 80, IS: 3043, CBIP-223. 400kV & 220kV system are designed for 40kA. Basic Objectives: Step potential within tolerable Touch Potential limit Ground Resistance Adequacy of Ground conductor for fault current (considering corrosion)1.10. Various outdoor equipments used are as follows:1.10.1.L.A. - To discharge the switching and lightening voltage surges to earth. Coupling capacitor with line matching units – These are high pass Filters ( carrier frequency 50 KHZ to 500 KHZ ) pass carrier. Frequency to carrier panels and power frequency parameters to switch yard.1.10.2.Wave Traps. - Low pass filter when power frequency currents are passed to switch yard and high frequency signals are blocked. Line Isolator with E.B. – To isolate the line from Sub Station and earth, it under shut down.1.10.3.Current transformers. – These are used for i) Measurement of current ii) Protection current circuits when currents are passed through protective relays.1.10.4.Potential transformers – A) Measurement of voltage. B) Provide secondary voltage for protection purposes and measurements.1.10.5.Isolators ( w.o. EB ) without earth blade. – To isolate the bay from the Bus.1.10.6.Circuit Breakers - These are used to operate on the Fault either on line or transformer, depending upon where it is connected. This isolates the Faulty line or equipments from the live portion of the Sub Station by opening automatically through protective relays; control cables etc. in a definite time.1.10.7.Battery sets – to provide adequate D.C. supply voltage for operation of protection system and circuit breakers. When A.C. supply fails as an emergency stand by. Battery Charges to provide appropriate D.C. Voltage for operation of protective systems and circuit brakes. To keep the battery set continuously is charged condition as it gets discharged to certain extent due to internal resistance of batteries.1.11. CONTROL AND RELAY PARTS - These are used to control the operations of breakers, isolates, through protective relays installed on these panels various protection schemes for transformers, lines etc, are provided on these panels. AC & DC DB’S – These are used for extending A.C. & D.C. supplies whenever required through various circuits. There are two main Buses in this arrangement connected by each diameter. i) Through either of line breakers the line side Main Bus can be charged normally (Bus- I). 6
  7. 7. ii) The line breaker, tie breaker and IInd Bus breaker if closed in series will charge the IInd Main Bus. iii) Outage on anyone Bus can be availed without interruption on any Bus. The second Bus can feed all the loads. iv) Breaker from any bay can be taken out for maintenance without interrupting the supply. v) For efficient working two diameters are required having source in each diameter preferably connected diagonally opposite to two different buses. vi) If both the sources are connected to same Bus (i.e. from one side only one tie breaker can be attended at a time). vii) If all the four breakers connected to Bus are out the transformer can be charged through the breaker from remote substation source. viii) Changing over as in case of 2 Bus or 3 Bus systems is not necessary as supply is not interrupted, in any case as said above. ix) All the breakers in the diameters are in energized position including tie breakers to keep the system in tact in case of any fault. x) On line or transformer fault the tie breaker with respective line or transformer breaker will trip. xi) On Bus fault on any Bus only the two breakers (of two diameters) connected Bus will Trip. xii) The Teed-point remains unprotected in any of line or transformer or bus faults hence the Teed point protection is given by differential relay. In case of this protection the breakers (2 Nos.) connected to Teed point (tie breaker + Bus breaker) will Trip.1.12. CLEARANCES Distinction should make between electrical clearances; necessary to ensure satisfactoryperformance in service and safety clearances which are required for safety of personnel ininspection; operation and maintenance work. Electrical clearances - This is minimum distancerequired between live parts and earth materials (earth clearance) or between live parts ofdifferent potentials (phase clearances) in order to prevent flashovers. Safety clearances to theconductor - (Live Part) – Minimum distance required but live conductor and the limits of worksection for safety to personnel working.1.13. Ground Clearances – 7
  8. 8. The minimum distance required between any exposed insulator which supports or contains liveconductor, and limits of work section where safety of personnel is ensured. Work Section - Thespace where the person may work safely provided he remains with in that space. 132 KV Single– Bus system1) Minimum clearance to earth on air for : 132 KV level – 1070 mm 22 KV level – 280 mm 33KV level – 380 mm 220 KV level – 1780 mm 400 KV level – 3500 mm2) Minimum clearance between phases in air : 22 KV level – 330 mm 33 KV level – 430 mm132 KV level – 1220 mm 220 KV level – 2060 mm 400 KV level – 4000 mm Sectionalclearances : 22 KV level – 2745 mm 33 KV level - 2770 mm 132 KV level – 3505 mm 220 KVlevel – 4280 mm 400 KV level – 6500 mm3) This is Minimum clearance from any point where a man may be required to stand to thenearest live conductor in air.4) Ground clearance. 33 KV – 3.7 meters. 132 KV – 4.6 meters. 220 KV – 5.5 meters. 400 KV -8.0 meters. 132 KV SYSTEM: Height of Bus from ground – 8 meters, Bay width - Single Bus –11 meters. Double Bus – 12 meters. Distance of Earth wire from ground – 10.5 meters. Ph to PhDistance – 3 meters. Between equipments At right angle to Bus – 3 metersFor 220 Kv System – (Single Bus) i) Height and Bus forms ground – 12.5 meter ii) Width to Bay – 18 Meters. iii) Distance bet formulations – 4.5 meters. iv) Distance bet equipments – 4.5 meters. ( meter) ( Right angle to Bus) v) Height of Earth wire from ground 15.5 meters 220 KV/ Bus two / three. vi) Height of 1st Steels Bus above ground – 18.5 meters. vii) Height of IInd Bus – 25 meters. From ground. viii) Height of Earth wire – above ground – 28.5 meters. 400 KV Height of Main Bus from ground –15.6 meters. Height of stab Bus from ground – 22 meters. Height of Earth conductor – 30 meters above ground Bay width – 27 meters. Distance bet Equipments (Right angle to Bus) - > 6 Meters. Distance bet. Phases 7 meters.2. Single line diagram (SLD) A Single Line Diagram (SLD) of an Electrical System is the Line Diagram of the concernedElectrical System which includes all the required ELECTRICAL EQUIPMENT connection 8
  9. 9. sequence wise from the point of entrance of Power up to the end of the scope of the mentionedWork. As in the case of 132KV Substation, the SLD shall show Lightening Arrestor, State ElectricityBoards C.T/P.T Unit, Isolators, Protection and Metering P.T & C.T. Circuit Breakers, againIsolators and circuit Breakers, Main Power Transformer, all protective devices/relays and otherspecial equipment like NGR, CVT, GUARD RINGS, SDR etc as per design criteria.2.1. Fig: Single line diagram of substation. As these feeders enter the station they are to pass through various instruments. The instrumentshave their usual functioning. They are as follows in the single line diagram. 1. Lightening arrestors, 2. C V T 3. Wave trap 4. Current transformer 5. Isolators with earth switch 6. Circuit breaker 7. Line isolator 8. BUS 9. Potential transformer in the bus with a bus isolator 10. Isolator 11. Current transformer 12. Circuit breaker 13. Lightening arrestors 14. Transformer 15. Lightening arrestors with earth switch 9
  10. 10. 16. Circuit breaker 17. Current transformer 18. Isolator 19. Bus 20. Potential transformer with a bus isolator 21. A capacitor bank attached to the bus.2.2. Brief descriptions of the instruments in the line diagram are-1. Lightening arrestorsLightening arrestors are the instrument that are used in the incoming feeders so that to preventthe high voltage entering the main station. This high voltage is very dangerous to the instrumentsused in the substation. Even the instruments are very costly, so to prevent any damage lighteningarrestors are used. The lightening arrestors do not let the lightening to fall on the station. If somelightening occurs the arrestors pull the lightening and ground it to the earth. In any substation themain important is of protection which is firstly done by these lightening arrestors. The lighteningarrestors are grounded to the earth so that it can pull the lightening to the ground. The lighteningarrestor works with an angle of 30° to 45° making a cone.2. C V TA capacitor voltage transformer (CVT) is a transformer used in power systems to step-downextra high voltage signals and provide low voltage signals either for measurement or to operate aprotective relay. In its most basic form the device consists of three parts: two capacitors acrosswhich the voltage signal is split, an inductive element used to tune the device to the supplyfrequency and a transformer used to isolate and further step-down the voltage for theinstrumentation or protective relay. The device has at least four terminals, a high-voltageterminal for connection to the high voltage signal, a ground terminal and at least one set ofsecondary terminals for connection to the instrumentation or protective relay. CVTs are typicallysingle-phase devices used for measuring voltages in excess of one hundred kilovolts where theuse of voltage transformers would be uneconomical. In practice the first capacitor, C1, is oftenreplaced by a stack of capacitors connected in series. This results in a large voltage drop acrossthe stack of capacitors that replaced the first capacitor and a comparatively small voltage dropacross the second capacitor, C2, and hence the secondary terminals.3. Wave trapWave trap is an instrument using for tripping of the wave. The function of this trap is that it trapsthe unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is connected tothe main incoming feeder so that it can trap the waves which may be dangerous to theinstruments here in the substation. 10
  11. 11. 4. Current transformerCurrent transformers are basically used to take the readings of the currents entering thesubstation. This transformer steps down the current from 800 amps to 1 amp. This is donebecause we have no instrument for measuring of such a large current. The main use of thistransformer is (a) distance protection; (b) backup protection; (c) measurement.5. Lightening arrestors with earth switchLightening arrestors after the current transformer are used so as to protect it from lightening i.e.from high voltage entering into it. This lightening arrestor has an earth switch, which can directlyearth the lightening. The arrestor works at 30° to 45° angel of the lightening making a cone. Theearth switch can be operated manually, by pulling the switch towards ground. This also helps inbreaking the line entering the station. By doing so maintenance and repair of any instrument canb performed.6. Circuit breakerThe circuit breakers are used to break the circuit if any fault occurs in any of the instrument.These circuit breaker breaks for a fault which can damage other instrument in the station. Forany unwanted fault over the station we need to break the line current. This is only doneautomatically by the circuit breaker. There are mainly two types of circuit breakers used for anysubstations. They are (a) SF6 circuit breakers; (b) spring circuit breakers.The use of SF6 circuit breaker is mainly in the substations which are having high input kv input,say above 220kv and more. The gas is put inside the circuit breaker by force ie under highpressure. When if the gas gets decreases there is a motor connected to the circuit breaker. Themotor starts operating if the gas went lower than 20.8 bar. There is a meter connected to thebreaker so that it can be manually seen if the gas goes low. The circuit breaker uses the SF6 gasto reduce the torque produce in it due to any fault in the line. The circuit breaker has a direct linkwith the instruments in the station, when any fault occur alarm bell rings.The spring type of circuit breakers is used for small kv stations. The spring here reduces thetorque produced so that the breaker can function again. The spring type is used for step downside of 132kv to 33kv also in 33kv to 11kv and so on. They are only used in low distributionside.7. Line isolatorThe line isolators are used to isolate the high voltage from flow through the line into the bus.This isolator prevents the instruments to get damaged. It also allows the only needed voltage andrest is earthed by itself.8. BUS 11
  12. 12. The bus is a line in which the incoming feeders come into and get into the instruments for furtherstep up or step down. The first bus is used for putting the incoming feeders in la single line.There may be double line in the bus so that if any fault occurs in the one the other can still havethe current and the supply will not stop. The two lines in the bus are separated by a little distanceby a conductor having a connector between them. This is so that one can work at a time and theother works only if the first is having any fault.9. Potential transformers with bus isolatorsThere are two potential transformers used in the bus connected both side of the bus. The potentialtransformer uses a bus isolator to protect itself. The main use of this transformer is to measurethe voltage through the bus. This is done so as to get the detail information of the voltage passingthrough the bus to the instrument. There are two main parts in it (a) measurement; (b) protection.10. IsolatorsThe use of this isolator is to protect the transformer and the other instrument in the line. Theisolator isolates the extra voltage to the ground and thus any extra voltage cannot enter the line.Thus an isolator is used after the bus also for protection.11. Current transformerCurrent transformers are used after the bus for measurement of the current going out through thefeeder and also for protection of the instruments.12. Circuit breakerThe circuit breakers are used to break the circuit if any fault occurs in the circuit of the anyfeeders.13. Lightening arrestorsThe use of lightening arrestors after the bus is to protect the instrument in the station so thatlightening would not affect the instruments in the station.14. TransformerThere are three transformers in the incoming feeders so that the three lines are step down at thesame time. In case of a 220kv or more kv line station auto transformers are used. While in caseof lower kv line such as less than 132kv line double winding transformers are used.15. Lightening arrestors with earth switchThe lightening arrestors are used with earth switch so that lightening would not pass through theinstruments in the station.16. Circuit breaker 12
  13. 13. The circuit breakers are used to break the circuit for any fault.17. Current transformerCurrent transformers are used to measure the current passing through the transformer. Its mainuse is of protection and measurement.18. IsolatorThese are used to ground the extra voltage to the ground.19. BusThis bus is to carry the output stepped down voltage to the required place.20. Potential transformer with a bus isolatorTwo PT are always connected across the bus so that the voltage across the bus could bemeasured.21. Capacitor bank attached to the bus.The capacitor banks are used across the bus so that the voltage does not gets down till the requireplace.3. The line diagram of the substation:The Sarusajai 220kv/ 132kv substation, Guwahati, has two stations in it. Firstly the 220kV andnext the 132kV/ 33kV substation.The 220kV substation has the capacity of 220kv and can step down to 132kv using three inputlines through the incoming feeders. The input feeders are namely: AGIA- i; AGIA-ii; LONGPI- i; LONGPI- ii; SAMUGURI-i; SAMUGURI- ii.All these feeders come into the substation with 220kv.The purpose of the 132kV/ 33kV substation was to step down the 132kv to direct distribution tothe 33kv/11kv substations in six different areas of the state. The substation of 132kv/33kv has six outgoing feeders, namely: JWAHAR NAGAR, GARBHANGA, MIRZA, PALTAN BAZAR, and Kahilipara STATION.These out going feeders are of 33kv line.The complete line diagram of the station are shown in the figure below: 13
  14. 14. Fig: Line diagram of the Sarusajai substation, Guwahati, Assam.4. Instruments used in the Sarusajai, 220kV grid substation are:Lightening arrestors: 14
  15. 15. Firstly we can see lightening arresters. These lightening arrestors can resist or ground thelightening if falls on the incoming feeders. The lightening arrestors can work in a angle of 30degrees around them. They are mostly used for protection of the instruments used in thesubstation. As the cost of the instrument in the station are very high to protect them from highvoltage from lightening these lightening arrestors are used.Fig. lightening arrestor. It is a device used on electrical power systems to protect the insulation on the system from thedamaging effect of lightning. Metal oxide varistors (MOVs) have been used for power systemprotection since the mid 1970s. The typical lightning arrester also known as surge arrester has ahigh voltage terminal and a ground terminal. When a lightning surge or switching surge travelsdown the power system to the arrester, the current from the surge is diverted around the protectedinsulation in most cases to earth. Landscape suited for purpose of explanation: (1) Represents Lord Kelvins "reduced" area ofthe region, (2) Surface concentric with the Earth such that the quantities stored over it and underit are equal; (3) Building on a site of excessive electrostatic charge density; (4) Building on a siteof low electrostatic charge density. 15
  16. 16. In telegraphy and telephony, a lightning arrester is placed where wires enter a structure,preventing damage to electronic instruments within and ensuring the safety of individuals nearthem. Lightning arresters, also called surge protectors, are devices that are connected betweeneach electrical conductor in a power and communications systems and the Earth. These provide ashort circuit to the ground that is interrupted by a non-conductor, over which lightning jumps. Itspurpose is to limit the rise in voltage when a communications or power line is struck bylightning. The non-conducting material may consist of a semi-conducting material such as silicon carbideor zinc oxide, or a spark gap. Primitive varieties of such spark gaps are simply open to the air,but more modern varieties are filled with dry gas and have a small amount of radioactivematerial to encourage the gas to ionize when the voltage across the gap reaches a specified level.Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glowlamp) connected between the protected conductor and ground, or myriad voltage-activated solid-state switches called varistors or MOVs. Lightning arresters built for substation use areimpressive devices, consisting of a porcelain tube several feet long and several inches indiameter, filled with disks of zinc oxide. A safety port on the side of the device vents theoccasional internal explosion without shattering the porcelain cylinder.Electric power system lightning protection High-tension power lines carry a lighter conductor (sometimes called a pilot or shield) wireover the main power conductors. This conductor is grounded at various points along the link, orinsulated from the tower structures by small insulators that are easily jumped by lightningvoltages. The latter allows the pilot wire to be used for communications purposes, or to carrycurrent for aircraft clearance lights. Electrical substations may have a web of grounded wirescovering the whole plant.Lightning protection system design Considerable material is used to make up lightning protection systems, so it is prudent toconsider carefully where a rod structure will have the greatest effect. Historical understanding oflightning, from statements made by Ben Franklin, assumed that each device protected a cone of45 degrees. This has been found to be unsatisfactory for protecting taller structures, as it ispossible for lightning to strike the side of a building. A better technique to determine the effect ofa new arrester is called the "rolling sphere technique" and was developed by Dr Tibor Horváth.To understand this requires knowledge of how lightning moves. As the step leader of a lightningbolt jumps toward the ground, it steps toward the grounded objects nearest its path. Themaximum distance that each step may travel is called the critical distance and is proportional tothe electrical current. Objects are likely to be struck if they are nearer to the leader than thiscritical distance. It is standard practice to approximate the spheres radius as 46 m near theground.Electricity travels mostly along the path of least resistance, so an object outside the criticaldistance is unlikely to be struck by the leader if there is a grounded object solidly OR within thecritical distance. Noting this, locations that are safe from lightning can be determined byimagining a leaders potential paths as a sphere that travels from the cloud to the ground. Forlightning protection, it suffices to consider all possible spheres as they touch potential strikepoints. To determine strike points, consider a sphere rolling over the terrain. At each point, weare simulating a potential leader position. Lightning is most likely to strike where the spheretouches the ground. Points that the sphere cannot roll across and touch are safest from lightning.Lightning protectors should be placed where they will prevent the sphere from touching a 16
  17. 17. structure. A weak point in most lightning diversion systems is in transporting the captureddischarge from the lightning rod to the ground, though. Lightning rods are typically installedaround the perimeter of flat roofs, or along the peaks of sloped roofs at intervals of 6.1 m or 7.6m, depending on the height of the rod. When a flat roof has dimensions greater than 15 m by 15m, additional air terminals will be installed in the middle of the roof at intervals of 15 m or lessin a rectangular grid pattern.Evaluations and analysisA controversy over the assortment of operation theories dates back to the 1700s, when Franklinhimself stated that his lightning protectors protected buildings by dissipating electric charge. Helater retracted the statement, stating that the devices exact mode of operation was something of amystery at that point. Diversion is a misnomer; no modern systems are claimed to divertanything, but rather to intercept the charge that terminates on a structure and carry it to theground. The energy in a lightning strike is measured in Joules. The reason that lightning doesdamage is that this energy is released in a matter of microseconds (typically 30 to 50microseconds). If the same energy could be released slowly over a period of many seconds orminutes, the current flow would be in milliamperes or a few amperes at most. This is the intentof charge dissipation.The dissipation theory states that a lightning strike to a structure can be prevented by altering theelectrical potential between the structure and the thundercloud. This is done by transferringelectric charge (such as from the nearby Earth to the sky or vice versa). Transferring electriccharge from the Earth to the sky is done by erecting some sort of tower equipped with one ormore sharply pointed protectors upon the structure. It is noted that sharply pointed objects willindeed transfer charge to the surrounding atmosphere and that a considerable electric currentthrough the tower can be measured when thunderclouds are overhead.Lightning strikes to a metallic structure can vary from leaving no evidence excepting perhaps asmall pit in the metal to the complete destruction of the structure. When there is no evidence,analyzing the strikes is difficult. This means that a strike on an uninstrumented structure must bevisually confirmed, and the random behavior of lightning renders such observations difficult.The research situation is improving somewhat, however. There are also inventors working on thisproblem, such as through a lightning rocket. While controlled experiments may be off in thefuture, very good data is being obtained through techniques which use radio receivers that watchfor the characteristic electrical signature of lightning strikes using fixed directional antennas.Through accurate timing and triangulation techniques, lightning strikes can be located with greatprecision, so strikes on specific objects often can be confirmed with confidence.The introduction of lightning protection systems into standards allowed various manufactures todevelop protector systems to a multitude of specifications and there are various lightning rodstandards. The NFPAs independent third party panel found that "the [Early Streamer Emission]lightning protection technology appears to be technically sound" and that there was an "adequatetheoretical basis for the [Early Streamer Emission] air terminal concept and design from aphysical viewpoint". (Bryan, 1999) The same panel also concluded that "the recommended[NFPA 780 standard] lightning protection system has never been scientifically or technicallyvalidated and the Franklin rod air terminals have not been validated in field tests underthunderstorm conditions." In response, the American Geophysical Union concluded that "[t]heBryan Panel reviewed essentially none of the studies and literature on the effectiveness andscientific basis of traditional lightning protection systems and was erroneous in its conclusion 17
  18. 18. that there was no basis for the Standard." AGU did not attempt to assess the effectiveness of anyproposed modifications to traditional systems in its report.No major standards body, such as the NFPA or UL, has currently endorsed a device that canprevent or reduce lightning strikes. The NFPA Standards Council, following a request for aproject to address Dissipation Array Systems and Charge Transfer Systems, denied the request tobegin forming standards on such technology (though the Council did not foreclose on futurestandards development after reliable sources demonstrating the validity of the basic technologyand science were submitted). Members of the Scientific Committee of the InternationalConference on Lightning Protection has issued a joint statement stating their opposition todissipater technology.Various investigators believe the natural downward lightning strokes to be unpreventable. Sincemost lightning protectors ground potentials are elevated, the path distance from the source to theelevated ground point will be shorter, creating a stronger field (measured in volts per unitdistance) and that structure will be more prone to ionization and breakdown. Scientists from theNational Lightning Safety Institute claim that these dissipation devices are nothing more thanexpensive lightning protectors and that they, unlike traditional methods, are not based on"scientifically proven and indisputable technical arguments". William Rison states that in hisopinion the underlying theory of dissipation is "scientific nonsense". According to these sources,there is no proof that the dissipation arrangement is at all effective. According to opponents ofthe dissipation technology, the various designs of dissipaters indirectly "eliminate" lightning viathe alteration of a buildings shape and only have a small effect (either intended or not) becausethere is no significant reduction to the susceptibility of a structure to the generation of upwardlightning strokes. Some field investigations of dissipaters show that their performance iscomparable to conventional terminals and possess no great enhancement of protection.According to these field studies, these devices have not shown that they totally eliminatedlightning strikes.CVT :A capacitor voltage transformer (CVT) is a transformer used in power systems to step-downextra high voltage signals and provide low voltage signals either for measurement or to operate aprotective relay. In its most basic form the device consists of three parts: two capacitors acrosswhich the voltage signal is split, an inductive element used to tune the device to the supplyfrequency and a transformer used to isolate and further step-down the voltage for theinstrumentation or protective relay. The device has at least four terminals, a high-voltageterminal for connection to the high voltage signal, a ground terminal and at least one set ofsecondary terminals for connection to the instrumentation or protective relay. CVTs are typicallysingle-phase devices used for measuring voltages in excess of one hundred kilovolts where theuse of voltage transformers would be uneconomical. In practice the first capacitor, C1, is oftenreplaced by a stack of capacitors connected in series. This results in a large voltage drop acrossthe stack of capacitors that replaced the first capacitor and a comparatively small voltage dropacross the second capacitor, C2, and hence the secondary terminals. 18
  19. 19. CVT 220 kV ratingType: WP-245 VOperating voltage: 220/√3 kVVoltage factor: 1.5 V for 30 sec.Test voltage: 460 kVTest impedance 1050 kv peakEllec cap: 4400±10% PF of 50 Hz ± 5%Nominal intermediate voltage 20/√3 kvSpark over voltage: 36 kvVoltage divider ratio 220000/√3 /20000/√3Total thermal burden: 1000 VATemperature categ: 10 to 55°CTotal weight: 900 Kg.Wave tape:A device used to exclude unwanted frequency components, such as noise or other interference, of a wave.A device used to exclude unwanted frequency components, such as noise or other interference, of a wave.Wave trap is an instrument using for tripping of the wave. The function of this trap is that it trapsthe unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is connected tothe main incoming feeder so that it can trap the waves which may be dangerous to theinstruments here in the substation. 19
  20. 20. Current transformer:The instrument current transformer (CT) steps down the current of a circuit to a lower value andis used in the same types of equipment as a potential transformer. This is done by constructingthe secondary coil consisting of many turns of wire, around the primary coil, which contains onlya few turns of wire. In this manner, measurements of high values of current can be obtained. Acurrent transformer should always be short-circuited when not connected to an external load.Because the magnetic circuit of a current transformer is designed for low magnetizing currentwhen under load, this large increase in magnetizing current will build up a large flux in the magnetic circuit and cause the transformer to act as a step-up transformer, inducing an excessively high voltage in the secondary when under no load.These transformers are basically used to get the incoming current on the incoming feeders. Itsteps down the incoming 800 amps to 1 amps.Rating factor:Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determineits absolute maximum measurable primary current. Conversely, the minimum primary current a CT canaccurately measure is "light load," or 10% of the nominal current (there are, however, special CTsdesigned to measure accurately currents as small as 2% of the nominal current). The rating factor of aCT is largely dependent upon ambient temperature. Most CTs have rating factors for 35 degrees Celsiusand 55 degrees Celsius. It is important to be mindful of ambient temperatures and resultant rating factorswhen CTs are installed inside pad-mounted transformers or poorly ventilated mechanical rooms.Recently, manufacturers have been moving towards lower nominal primary currents with greater ratingfactors. This is made possible by the development of more efficient ferrites and their corresponding 20
  21. 21. hysteresis curves. This is a distinct advantage over previous CTs because it increases their range ofaccuracy, since the CTs are most accurate between their rated current and rating factorCurrent transformerType 132 kV CT Core 1 core 2 core 3Ratio (A/A) 800/1 400/1 800/1 400/1 800/1 400/1Sec. Conn: 1S1-1S2 2S1-2S3 3S1-3S3Accuracy class: 0.2 5P 10 PSBurden (VA): 30 15 NAHighest systemVoltage: 145 kV insulation burn 275 kV/ 65014 VpIsolator with earth switch (ES):The instrument current transformer (CT) steps down the current of a circuit to a lower value andis used in the same types of equipment as a potential transformer. This is done by constructingthe secondary coil consisting of many turns of wire, around the primary coil, which contains onlya few turns of wire. In this manner, measurements of high values of current can be obtained. Acurrent transformer should always be short-circuited when not connected to an external load.Because the magnetic circuit of a current transformer is designed for low magnetizing currentwhen under load, this large increase in magnetizing current will build up a large flux in the magnetic circuit and cause the transformer to act as a step-up transformer, inducing an excessively high voltage in the secondary when under no load.The main use of using the earth switch (E/S) is to ground the extra voltage which may bdangerous for any of the instrument in the substation.Isolator ratingsVoltage rating: 145 kVBasic insulation level: 650 kVpCurrent rating: 1250 Amp.Circuit breaker: using SF6 gas:Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc extinguishing 21
  22. 22. properties. The dielectric strength of the gas increases with pressure and is more than ofdielectric strength of oil at 3 kg/cm2. SF6 is now being widely used in electrical equipment likehigh voltage metal enclosed cables; high voltage metal clad switchgear, capacitors, circuitbreakers, current transformers, bushings, etc. The gas is liquefied at certain low temperature,liquefaction temperature increases with pressure.Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas,in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur.The steel box is made gas tight. The gas thus obtained contains other fluorides such as S2F10,SF4 and must be purified further SF6 gas generally supplier by chemical firms. The cost of gas islow if manufactured in large scale. 22
  23. 23. During the arcing period SF6 gas is blown axially along the arc. The gas removes the heat fromthe arc by axial convection and radial dissipation. As a result, the arc diameter reduces during thedecreasing mode of the current wave. The diameter becomes small during the current zero andthe arc is extinguished. Due to its electronegativity, and low arc time constant, the SF6 gasregains its dielectric strength rapidly after the current zero, the rate of rise of dielectric strength isvery high and the time constant is very small. 23
  24. 24. 24
  25. 25. Fig: SF6 circuit breaker. 25
  26. 26. Gas circuit breaker: high voltage sideType 220-SFM-20BVoltage rating: 220kvRated lightening impulse withstand voltage: 1050 kVpRated short circuit breaker current: 40 kVRated operating pressure: 16.5 kg/ cm2gFirst pole to clear factor 1.3Rated duration of short circuit current is 40 kA for 30 sec.Rated ling charging breaker breaking current 125 AmpRated voltage 245 kVRated frequency 50 HzRated normal current 1600 AmpRated closing voltage: 220 V dcRated opening voltage 220 V dcMain parts: (a) Power circuit (b) Control circuitGas circuit breaker: low voltage sideType 120-SFM-32AVoltage rating: 220kvRated lightening impulse withstand voltage: 650 kVpRated short circuit breaker current: 31.5 kVRated operating pressure: 15.5 kg/ cm2gFirst pole to clear factor 1.5Rated duration of short circuit current is 31.5 kA for 30 sec.Rated ling charging breaker breaking current 50 AmpRated voltage 245 kVRated frequency 50 Hz 26
  27. 27. Rated normal current 1250 AmpRated closing voltage: 220 V dcRated opening voltage 200 V dcMain parts: (a) Power circuit (b) Control circuit220kv BUS:It is a incoming 220kv feeder BUS from which the line is taken to the transformer for furtherstep down.Double main bus & transfer bus systemMerits 1. Most flexible in operation 2. Highly reliable 3. Breaker failure on bus side breakerremoves only one ckt. From service 4. All switching done with breakers 5. Simple operation, noisolator switching required 6. Either main bus can be taken out of service at any time formaintenance. 7. Bus fault does not remove any feeder from the service Demerits 1. High cost dueto three buses Remarks 1. Preferred by some utilities for 400kV and 220kV importantsubstations.Mesh (Ring) busbar systemMerits 1. Busbars gave some operational flexibility Demerits 1. If fault occurs during busmaintenance, ring gets separated into two sections. 2.Auto-reclosing and protection complex. 3.Requires VT’s on all circuits because there is no definite voltage reference point. These VT’smay be required in all cases for synchronizing live line or voltage indication 4. Breaker failureduring fault on one circuit causes loss of additional circuit because of breaker failure. Remarks 1.Most widely used for very large power stations having large no. of incoming and outgoing linesand high power transfer.Potential transformers: with BUS isolator The instrument potential transformer (PT) steps down voltage of a circuit to a low value thatcan be effectively and safely used for operation of instruments such as ammeters, voltmeters,watt meters, and relays used for various protective purposes. There are two potential transformers used in the bus connected both side of the bus. Thepotential transformer uses a bus isolator to protect itself. The main use of this transformer is tomeasure the voltage through the bus. This is done so as to get the detail information of thevoltage passing through the bus to the instrument. There are two main parts in it (a)measurement; (b) protection.Potential transformer ratings: 27
  28. 28. High voltage side: 245 VRated insulation voltage: 395/ 900Voltage rating: 220/√3 kV/ 110/ √3 VBUS Isolator:These isolators are used to isolate the incoming high voltage or the high incoming current fromthe incoming feeder which enters the bus. The isolator prevents damage to the instruments byjust isolating the line current or the voltage.Current transformer:The instrument current transformer (CT) steps down the current of a circuit to a lower value andis used in the same types of equipment as a potential transformer. This is done by constructingthe secondary coil consisting of many turns of wire, around the primary coil, which contains onlya few turns of wire. In this manner, measurements of high values of current can be obtained. Acurrent transformer should always be short-circuited when not connected to an external load.Because the magnetic circuit of a current transformer is designed for low magnetizing currentwhen under load, this large increase in magnetizing current will build up a large flux in the magnetic circuit and cause the transformer to act as a step-up transformer, inducing an excessively high voltage in the secondary when under no load.Circuit breaker using SF6 gas:Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc extinguishingproperties. The dielectric strength of the gas increases with pressure and is more than ofdielectric strength of oil at 3 kg/cm2. SF6 is now being widely used in electrical equipment likehigh voltage metal enclosed cables; high voltage metal clad switchgear, capacitors, circuitbreakers, current transformers, bushings, etc. The gas is liquefied at certain low temperature,liquefaction temperature increases with pressure.Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas,in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur.The steel box is made gas tight. The gas thus obtained contains other fluorides such as S2F10,SF4 and must be purified further SF6 gas generally supplier by chemical firms. The cost of gas islow if manufactured in large scale.During the arcing period SF6 gas is blown axially along the arc. The gas removes the heat fromthe arc by axial convection and radial dissipation. As a result, the arc diameter reduces during thedecreasing mode of the current wave. The diameter becomes small during the current zero andthe arc is extinguished. Due to its electronegativity, and low arc time constant, the SF6 gasregains its dielectric strength rapidly after the current zero, the rate of rise of dielectric strength isvery high and the time constant is very small. 28
  29. 29. Lightening arrestor:These lightening arrestors are used to prevent the lightening from damaging the instruments inthe substation. Lightening arrestors are the instrument that are used in the incoming feeders so that to preventthe high voltage entering the main station. This high voltage is very dangerous to the instrumentsused in the substation. Even the instruments are very costly, so to prevent any damage lighteningarrestors are used. The lightening arrestors do not let the lightening to fall on the station. If somelightening occurs the arrestors pull the lightening and ground it to the earth. In any substation themain important is of protection which is firstly done by these lightening arrestors. The lighteningarrestors are grounded to the earth so that it can pull the lightening to the ground. The lighteningarrestor works with an angle of 30° to 45° making a cone.Auto transformer: Transformer is static equipment which converts electrical energy from one voltage to another.As the system voltage goes up, the techniques to be used for the Design, Construction,Installation, Operation and Maintenance also become more and more critical. If proper care is exercised in the installation, maintenance and condition monitoring of thetransformer, it can give the user trouble free service throughout the expected life of equipmentwhich of the order of 25-35 years. Hence, it is very essential that the personnel associated withthe installation, operation or maintenance of the transformer is through with the instructionsprovided by the manufacture. It is a device that transfers electrical energy from one circuit to another through inductivelycoupled conductors — the transformers coils. Except for air-core transformers, the conductorsare commonly wound around a single iron-rich core, or around separate but magnetically-coupled cores. A varying current in the first or "primary" winding creates a varying magneticfield in the core (or cores) of the transformer. This varying magnetic field induces a varyingelectromotive force (EMF) or "voltage" in the "secondary" winding. This effect is called mutualinduction. If a load is connected to the secondary, an electric current will flow in the secondary windingand electrical energy will flow from the primary circuit through the transformer to the load. In anideal transformer, the induced voltage in the secondary winding (VS) is in proportion to theprimary voltage (VP), and is given by the ratio of the number of turns in the secondary to thenumber of turns in the primary as follows: By appropriate selection of the ratio of turns, a transformer thus allows an alternating current(AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NSless than NP. 29
  30. 30. Transformers come in a range of sizes from a thumbnail-sized coupling transformer hiddeninside a stage microphone to huge units weighing hundreds of tons used to interconnect portionsof national power grids. All operate with the same basic principles, although the range of designsis wide. While new technologies have eliminated the need for transformers in some electroniccircuits, transformers are still found in nearly all electronic devices designed for household("mains") voltage. Transformers are essential for high voltage power transmission, which makeslong distance transmission economically practical.Pole-mounted single-phase transformer with center-tapped secondary. Note use of the groundedconductor as one leg of the primary feeder.An auto transformer 220kv/132kv, in Sub Station, AEGCL, Sarusaji, GuwahatiBasic principles 30
  31. 31. The transformer is based on two principles: firstly, that an electric current can produce amagnetic field (electromagnetism) and secondly that a changing magnetic field within a coil ofwire induces a voltage across the ends of the coil (electromagnetic induction). Changing thecurrent in the primary coil changes the magnetic flux that is developed. The changing magneticflux induces a voltage in the secondary coil.An ideal transformer.An ideal transformer is shown in the adjacent figure. Current passing through the primary coilcreates a magnetic field. The primary and secondary coils are wrapped around a core of veryhigh magnetic permeability, such as iron, so that most of the magnetic flux passes through bothprimary and secondary coils.Induction lawThe voltage induced across the secondary coil may be calculated from Faradays law ofinduction, which states that:where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φequals the magnetic flux through one turn of the coil. If the turns of the coil are orientedperpendicular to the magnetic field lines, the flux is the product of the magnetic field strength Band the area A through which it cuts. The area is constant, being equal to the cross-sectional areaof the transformer core, whereas the magnetic field varies with time according to the excitation 31
  32. 32. of the primary. Since the same magnetic flux passes through both the primary and secondarycoils in an ideal transformer, the instantaneous voltage across the primary winding equalsTaking the ratio of the two equations for VS and VP gives the basic equation for stepping up orstepping down the voltageIdeal power equationThe ideal transformer as a circuit elementIf the secondary coil is attached to a load that allows current to flow, electrical power istransmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectlyefficient; all the incoming energy is transformed from the primary circuit to the magnetic fieldand into the secondary circuit. If this condition is met, the incoming electric power must equalthe outgoing power. Pincoming = IPVP = Poutgoing = ISVSgiving the ideal transformer equationTransformers are efficient so this formula is a reasonable approximation.If the voltage is increased, then the current is decreased by the same factor. The impedance inone circuit is transformed by the square of the turns ratio. For example, if an impedance ZS is 32
  33. 33. attached across the terminals of the secondary coil, it appears to the primary circuit to have animpedance of . This relationship is reciprocal, so that the impedance ZP of theprimary circuit appears to the secondary to be .Detailed operationThe simplified description above neglects several practical factors, in particular the primarycurrent required to establish a magnetic field in the core, and the contribution to the field due tocurrent in the secondary circuit.Models of an ideal transformer typically assume a core of negligible reluctance with twowindings of zero resistance. When a voltage is applied to the primary winding, a small currentflows, driving flux around the magnetic circuit of the core. The current required to create the fluxis termed the magnetizing current; since the ideal core has been assumed to have near-zeroreluctance, the magnetizing current is negligible, although still required to create the magneticfield.The changing magnetic field induces an electromotive force (EMF) across each winding. Sincethe ideal windings have no impedance, they have no associated voltage drop, and so the voltagesVP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs.The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the"back EMF".This is due to Lenzs law which states that the induction of EMF would always besuch that it will oppose development of any such change in magnetic field.Transformer ratingType of cooling: capacity: OFAF ONAF ONAN 100mV 80mV 60mVNo load voltage: 220kv/ 132kvLine current: HV 262.43 Amp. 209.94 Amp. 157.46 Amp. LV 437.38 Amps 349.91 Amp. 262.43 Amp.Impedence voltage: 12.5V 33
  34. 34. 1u 2u 3u 3w 2w 2v 1v 1w 3v Vector symbol: YNad1Lightening arrestors:Lightening arrestors are the instrument that are used in the incoming feeders so that to preventthe high voltage entering the main station. This high voltage is very dangerous to the instrumentsused in the substation. Even the instruments are very costly, so to prevent any damage lighteningarrestors are used. The lightening arrestors do not let the lightening to fall on the station. If somelightening occurs the arrestors pull the lightening and ground it to the earth. In any substation themain important is of protection which is firstly done by these lightening arrestors. The lighteningarrestors are grounded to the earth so that it can pull the lightening to the ground. The lighteningarrestor works with an angle of 30° to 45° making a coneCurrent transformers: 34
  35. 35. Current transformers are basically used to take the readings of the currents entering thesubstation. This transformer steps down the current from 800 amps to 1 amp. This is donebecause we have no instrument for measuring of such a large current. The main use of thistransformer is (a) distance protection; (b) backup protection; (c) measurement.Current transformer LV sideType 132 kV CT Core 1 core 2 core 3Ratio (A/A) 800/1 400/1 800/1 400/1 800/1 400/1Sec. Conn: 1S1-1S2 2S1-2S3 3S1-3S3Accuracy class: 0.2 5P 10 PSBurden (VA): 30 15 NAHighest systemVoltage: 145 kV insulation burn 275 kV/ 65014 VpIsolator:The line isolators are used to isolate the high voltage from flow through the line into the bus.This isolator prevents the instruments to get damaged. It also allows the only needed voltage andrest is earthed by itself.Circuit breaker:The circuit breakers are used to break the circuit if any fault occurs in any of the instrument.These circuit breaker breaks for a fault which can damage other instrument in the station. Forany unwanted fault over the station we need to break the line current. This is only doneautomatically by the circuit breaker.132kv BUS:This bus is to carry the output stepped down voltage to the required place.Potential transformer: 2 with bus isolatorTwo PT are always connected across the bus so that the voltage across the bus could bemeasured. 35
  36. 36. BUS isolator:This are used for the protection of the instruments.Current transformer:Current transformers are used to measure the current passing through the transformer. Its mainuse is of protection and measurement.Lightening arrestors:The use of lightening arrestors after the bus is to protect the instrument in the station so thatlightening would not affect the instruments in the station.Transformer: with two windingsThe simplified description above neglects several practical factors, in particular the primarycurrent required to establish a magnetic field in the core, and the contribution to the field due tocurrent in the secondary circuit.Models of an ideal transformer typically assume a core of negligible reluctance with twowindings of zero resistance. When a voltage is applied to the primary winding, a small currentflows, driving flux around the magnetic circuit of the core. The current required to create the fluxis termed the magnetizing current; since the ideal core has been assumed to have near-zeroreluctance, the magnetizing current is negligible, although still required to create the magneticfield.The changing magnetic field induces an electromotive force (EMF) across each winding. Sincethe ideal windings have no impedance, they have no associated voltage drop, and so the voltagesVP and VS measured at the terminals of the transformer, are equal to the corresponding EMFs.The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the"back EMF".This is due to Lenzs law which states that the induction of EMF would always besuch that it will oppose development of any such change in magnetic field.Lightening arrestors:The use of lightening arrestors after the bus is to protect the instrument in the station so thatlightening would not affect the instruments in the station.Current transformer:These transformers are used for measurements and protections.Circuit breaker:The use of the circuit breaker again is to break the circuit if there is any fault in the line.Isolator:Isolator ground the extra voltage to the earth.33kv BUS: 36
  37. 37. This bus is used for the 33kV line. This bus carries 33kV voltage.Potential transformer on the bus:The potential transformer is used in the bus only. This is because to measure the voltage in thebus. The use of the potential transformer is to measure and to protect the instruments.BUS isolator:The bus isolator is used to isolate the extra high voltage through the bus.Current transformer:The use of the CT here is to protect the instrument and for measurement purpose.Circuit breaker:The circuit breaker breaks the circuit whenever there is any fault in the line.Line isolator with earth switch (E/S):An isolator with switch is part of an electrical circuit and is often found in industrial applications,however they are commonly fitted to domestic extractor fans when used in bathrooms in the UK.Isolator switches may be fitted with the ability for the switch to padlock such that inadvertentoperation is not possible . In some designs the isolator switch has the additional ability to earththe isolated circuit thereby providing additional safety. Such an arrangement would apply tocircuits which inter-connect power distribution systems where both end of the circuit need to beisolated. Major difference between isolator and circuit breaker is that isolator is an off-loaddevice, whereas circuit breaker is an on-load device.Lightening arrestors:Lightening arrestors are used to protect the instruments from lightening.Capacitor bank:A capacitor bank is used in the outgoing bus so that it can maintain the voltage level same in theoutgoing feeder.Capacitor Controlis usually done to achieve as many as possible of the following goals: Reduce losses due toreactive load current, reduce kVA demand, decrease customer energy consumption, improvevoltage profile, and increase revenue. Indirectly capacitor control also results in longerequipment lifetimes because of reduced equipment stresses. Experience shows that switchedfeeder capacitors produce some of the fastest returns on equipment investment.Sources of Energy LossEnergy losses in transmission lines and transformers are of two kinds: resistive and reactive. Theformer are caused by resistive component of the load and cannot be avoided. The latter, comingfrom reactive component of the load, can be avoided (Fig. 1). Reactive losses come from circuit 37
  38. 38. capacitance (negative), and circuit inductance (positive). When a heavy inductive load isconnected to the power grid, a large positive reactive power component is added, therebyincreasing observed power load (Fig. 1). This increases losses due to reactive load current,increases kVA demand, increases customer energy consumption, usually degrades voltageprofiles, and reduces revenue.Reactive CompensationWhen capacitors of appropriate size are added to the grid at appropriate locations, the abovementioned losses can be minimized by reducing the reactive power component in Fig. 1, therebyreducing the observed power demand. There are many aspects to this compensation and itseffects, depending on where capacitors get to be located, their sizes, and details of thedistribution circuit. Some are discussed below.Energy Loss ReductionMore than one half of system energy loss is caused by the resistance of the feeders. To minimizeenergy losses it is, therefore, important to locate feeder capacitors as close to the loads aspossible. Substation capacitors cannot do the job - the reactive load current has already heatedfeeder conductors downstream from the substation. Reducing reactive current at the substationcan not recover energy losses in the feeders. Another way to minimize energy losses is to usecapacitor banks that are not too large. This makes it possible to put the banks on-line early in theload cycle. Since energy saved is the product of power reduction and the time the banks are on-line, the overall energy reduction is usually greater than when using large banks which are turnedon for shorter amounts of time (Fig. 2). 38
  39. 39. Demand ReductionWhen capacitors are on-line reactive current and, therefore, total line current is reduced. Duringheavy load periods this has several advantages: The peak load is increased when it is mostneeded (essentially releasing demand), the effective line current capacity is increased, and theoperating line and transformer temperatures are reduced – prolonging equipment lifetimes. Thelatter makes it possible to upgrade lines and transformers less frequently. All of these contributeto reduced costs and higher revenues.Voltage ProfileDistribution feeder demand capacity is usually limited by voltage drop along the line. Thecustomer service entrance voltage must be stable, usually ±5% to ±10%. The feeder voltageprofile can be ‘flattened’ by connecting large capacity banks to the grid. Several benefits becomeavailable: The kVA demand can be increased to arrive at the original voltage drop (this isequivalent to releasing feeder demand), the substation voltage can be lowered to reduce peakdemand and save energy, or the service entrance voltage can be allowed to increase therebyincreasing revenue (at the expense of less than optimum kVA demand).System ConsiderationsObviously properly switched capacitors located at appropriate locations along distributionfeeders provide great financial benefits to the utility.If there is to be only one capacitor bank on a uniformly loaded feeder, the usual two-thirds, two-thirds rule gives optimum loss and demand reduction. This means that the bank kVAr size shouldbe two-thirds of the heavy load kVAr as measured at the substation, and the bank should belocated two-thirds the length of the feeder from the substation. If the objective is voltage controlthe bank should be farther from the substation.With several banks on a uniformly loaded feeder, the total capacitor kVAr can more closelymatch the total load kVAr. Depending on the type of the switching control used, multiple bankson a feeder can lead to ‘pumping’ as the controls affect the operating points of each other.Usually no more than three or four banks are used per feeder. 39
  40. 40. In the case of concentrated industrial loads, there should be a bank, sized to almost equal thereactive load current, located as close to each load as possible (Fig. 3).Types of ControlVAr control is the natural means to control capacitors because the latter adds a fixed amount ofleading VArs to the line regardless of other conditions, and loss reduction depends only onreactive current. Since reactive current at any point along a feeder is affected by downstreamcapacitor banks, this kind of control is susceptible to interaction with downstream banks.Consequently, in multiple capacitor feeders, the furthest downstream banks should go on-linefirst, and off-line last. VAr controls require current sensors.Current control is not as efficient as VAr control because it responds to total line current, andassumptions must be made about the load power factor. Current controls require current sensors.Voltage control is used to regulate voltage profiles, however it may actually increase losses andcause instability from highly leading currents. Voltage control requires no current sensors.Temperature control is based on assumptions about load characteristics. Control effectivenessdepends on how well load characteristics are know. Not useful in cases where thosecharacteristics change often. Temperature control does not require any current sensors.Time control is based on assumptions about load characteristics. Control effectiveness dependson how well load characteristics are know. Not useful in cases where those characteristics changeoften. Time control does not require any current sensors.Power factor control is not the best way to control capacitor banks because power factor by itselfis not a measure of reactive current. Current sensors are needed.Combination control using various above methods is usually the best choice. If enough current,and/or other sensors are available, a centrally managed computerized capacitor control systemtaking into account the variety of available input parameters can be most effective, thoughexpensive to implement.BUS isolator: 40
  41. 41. The bus isolators are used to isolate high voltage entering the bus or entering the substation.Line Current transformer:The current transformer used in the line is known as the line current transformer. The main use ofthis current transformer is to measure and to protect the instruments.Line circuit breaker:The circuit breaker used in the line is known as the line circuit breaker. The use of the circuitbreaker in the outgoing feeder is to break the circuit when the any fault occurs in the line i.e, anyfault on the outgoing feeder.Line isolators with earth switch:The line isolator with earth switch is to isolate the extra high voltage through the feeders goingout of the station. The isolator used in the line is known as the line isolator.To output feeder:The outgoing feeders are used to give the step down voltage to the required area. This feederssupply the voltage to the required place for further step down or its use in the place.The Sarusajai substation has six output feeders it six different places namely: JWAHAR NAGAR, GARBHANGA, MIRZA, PALTAN BAZAR, and Kahilipara STATION. 41

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