Training Report on GSS

8/9/2019 Training Report on GSS

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2.2.2. Brief descriptions of the instruments in the line diagram are-1. Lightening 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 theinstrumentsused in the substation. Even the instruments are very costly, so to prevent any damagelighteningarrestors are used. The lightening arrestors do not let the lightening to fall on the station. Ifsomelightening occurs the arrestors pull the lightening and ground it to the earth. In any substationthemain important is of protection which is firstly done by these lightening arrestors. Thelighteningarrestors are grounded to the earth so that it can pull the lightening to the ground. Thelighteningarrestor works with an angle of 30 to 45 making a cone.

2. C V T

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 operateaprotective 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 the

instrumentation 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 aretypicallysingle-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.

10. Isolators

The use of this isolator is to protect the transformer and the otherinstrument in the line. Theisolator isolates the extra voltage to the ground and thus any extravoltage cannot enter the line.

Thus an isolator is used after the bus also for protection.


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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 require

place.3. The line diagram of the substation:

The Sarusajai 220kv/ 132kv substation, Guwahati, has twostations in it. Firstly the 220kV andnext the 132kV/ 33kV substation.

The 220kV substation has the capacity of 220kv and can step downto 132kv using three input

lines 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 to

the 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 figurebelow

Fig: Line diagram of the Sarusajai substation, Guwahati, Assam.4. Instruments used in the Sarusajai, 220kV grid substation are:

Landscape suited for purpose of explanation: (1) Represents Lord Kelvin's"reduced" area ofthe region, (2) Surface concentric with the Earthsuch that the quantities stored over it and under it are equal; (3)Building on a site of excessive electrostatic charge density; (4)Building on a site of low electrostatic charge density.


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In telegraphy and telephony, a lightning arrester is placed where wires enter astructure, preventing damage to electronic instruments within and ensuringthe safety of individuals near them. Lightning arresters, also called surge

protectors, are devices that are connected betweeneach electrical conductor in a power and communications systems and theEarth. These provide ashort circuit to the ground that is interrupted by a non-conductor, over whichlightning jumps. Itspurpose is to limit the rise in voltage when a communications or power line isstruck bylightning.

The non-conducting material may consist of a semi-conducting material suchas silicon carbide

or zinc oxide, or a spark gap. Primitive varieties of such spark gaps are simplyopen to the air,but more modern varieties are filled with dry gas and have a small amount ofradioactivematerial to encourage the gas to ionize when the voltage across the gap reachesa specified level.Other designs of lightning arresters use a glow-discharge tube (essentially likea neon glowlamp) connected between the protected conductor and ground, or myriadvoltage-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 severalinches indiameter, filled with disks of zinc oxide. A safety port on the side of the devicevents 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 pointsalong the link, orinsulated from the tower structures by small insulators that are easily jumpedby lightningvoltages. The latter allows the pilot wire to be used for communicationspurposes, or to carrycurrent for aircraft clearance lights. Electrical substations may have a web of


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grounded wirescovering the whole plant.

Lightning protection system design

Considerable material is used to make up lightning protection systems, so it isprudent toconsider carefully where a rod structure will have the greatest effect. Historicalunderstanding oflightning, from statements made by Ben Franklin, assumed that each deviceprotected a cone of45 degrees. This has been found to be unsatisfactory for protecting tallerstructures, as it ispossible for lightning to strike the side of a building. A better technique todetermine the effect ofa new arrester is called the "rolling sphere technique" and was developed by Dr

Tibor Horvth.To understand this requires knowledge of how lightning 'moves'. As the stepleader of a lightningbolt jumps toward the ground, it steps toward the grounded objects nearest itspath. Themaximum distance that each step may travel is called the critical distance andis proportional tothe electrical current. Objects are likely to be struck if they are nearer to theleader than thiscritical distance. It is standard practice to approximate the sphere's radius as46 m near the

ground.

Electricity travels mostly along the path of least resistance, so an object outsidethe criticaldistance is unlikely to be struck by the leader if there is a grounded objectsolidly OR within thecritical distance. Noting this, locations that are safe from lightning can bedetermined byimagining a leader's potential paths as a sphere that travels from the cloud tothe 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. Ateach point, weare simulating a potential leader position. Lightning is most likely to strikewhere the spheretouches the ground. Points that the sphere cannot roll across and touch aresafest from lightning.


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Lightning protectors should be placed where they will prevent the sphere fromtouching a

structure. A weak point in most lightning diversion systems is intransporting the captureddischarge from the lightning rod to the ground, though. Lightningrods are typically installedaround the perimeter of flat roofs, or along the peaks of sloped roofsat intervals of 6.1 m or 7.6m, depending on the height of the rod. When a flat roof hasdimensions 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 analysis

A controversy over the assortment of operation theories dates backto the 1700s, when Franklinhimself stated that his lightning protectors protected buildings bydissipating electric charge. Helater retracted the statement, stating that the device's exact mode of

operation was something of amystery at that point. Diversion is a misnomer; no modern systemsare claimed to divertanything, but rather to intercept the charge that terminates on astructure and carry it to theground. The energy in a lightning strike is measured in Joules. Thereason that lightning doesdamage is that this energy is released in a matter of microseconds(typically 30 to 50

microseconds). If the same energy could be released slowly over aperiod of many seconds orminutes, the current flow would be in milliamperes or a fewamperes at most. This is the intentof charge dissipation.


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The dissipation theory states that a lightning strike to a structurecan be prevented by altering theelectrical potential between the structure and the thundercloud.

This is done by transferring

electric charge (such as from the nearby Earth to the sky or viceversa). Transferring electriccharge from the Earth to the sky is done by erecting some sort oftower equipped with one ormore sharply pointed protectors upon the structure. It is noted thatsharply pointed objects willindeed transfer charge to the surrounding atmosphere and that aconsiderable electric currentthrough the tower can be measured when thunderclouds areoverhead.

Lightning strikes to a metallic structure can vary from leaving noevidence 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 anuninstrumented structure must bevisually confirmed, and the random behavior of lightning renderssuch observations difficult.

The research situation is improving somewhat, however. There arealso inventors working on thisproblem, such as through a lightning rocket. While controlledexperiments may be off in thefuture, very good data is being obtained through techniques whichuse radio receivers that watchfor the characteristic electrical 'signature' of lightning strikes usingfixed directional antennas.

Through accurate timing and triangulation techniques, lightning

strikes can be located with greatprecision, so strikes on specific objects often can be confirmed withconfidence.

The introduction of lightning protection systems into standardsallowed various manufactures todevelop protector systems to a multitude of specifications and there


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are various lightning rodstandards. The NFPA's 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 terminalconcept and design from aphysical viewpoint". (Bryan, 1999) The same panel also concludedthat "the recommended[NFPA 780 standard] lightning protection system has never beenscientifically or technicallyvalidated and the Franklin rod air terminals have not beenvalidated in field tests underthunderstorm conditions." In response, the American GeophysicalUnion concluded that "[t]heBryan Panel reviewed essentially none of the studies and literatureon the effectiveness andscientific basis of traditional lightning protection systems and waserroneous in its conclusion

that there was no basis for the Standard." AGU did not attempt toassess the effectiveness of anyproposed modifications to traditional systems in its report.

No major standards body, such as the NFPA or UL, has currentlyendorsed a device that canprevent or reduce lightning strikes. The NFPA Standards Council,following a request for aproject to address Dissipation Array Systems and Charge TransferSystems, denied the request tobegin forming standards on such technology (though the Councildid not foreclose on futurestandards development after reliable sources demonstrating thevalidity of the basic technologyand science were submitted). Members of the Scientific Committeeof the InternationalConference on Lightning Protection has issued a joint statementstating their opposition todissipater technology.


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Various investigators believe the natural downward lightningstrokes to be unpreventable. Sincemost lightning protectors' ground potentials are elevated, the pathdistance from the source to the

elevated ground point will be shorter, creating a stronger field(measured in volts per unitdistance) and that structure will be more prone to ionization andbreakdown. Scientists from theNational Lightning Safety Institute claim that these dissipationdevices are nothing more thanexpensive lightning protectors and that they, unlike traditionalmethods, 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 dissipatersindirectly "eliminate" lightning viathe alteration of a building's 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 thattheir performance iscomparable to conventional terminals and possess no greatenhancement of protection.According to these field studies, these devices have not shown thatthey totally eliminatedlightning strikes.

CVT :

A capacitor voltage transformer (CVT) is a transformer used in powersystems to step-downextra high voltage signals and provide low voltage signals either formeasurement or to operate aprotective relay. In its most basic form the device consists of threeparts: two capacitors across


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which the voltage signal is split, an inductive element used to tunethe device to the supplyfrequency and a transformer used to isolate and further step-downthe voltage for the

instrumentation or protective relay. The device has at least fourterminals, a high-voltageterminal for connection to the high voltage signal, a groundterminal and at least one set ofsecondary terminals for connection to the instrumentation orprotective relay. CVTs are typicallysingle-phase devices used for measuring voltages in excess of onehundred kilovolts where theuse of voltage transformers would be uneconomical. In practice thefirst capacitor, C1, is oftenreplaced by a stack of capacitors connected in series. This results ina large voltage drop acrossthe stack of capacitors that replaced the first capacitor and acomparatively small voltage dropacross the second capacitor, C2, and hence the secondary terminals.

CVT 220 kV ratingType: WP-245 V

Operating voltage: 220/3 kVVoltage factor: 1.5 V for 30 sec.

Test voltage: 460 kV

Test impedance 1050 kv peak

Ellec cap: 440010% PF of 50 Hz

5%

Nominal intermediate voltage 20/3 kv

Spark over voltage: 36 kvVoltage divider ratio 220000/3 /20000/3

Total thermal burden: 1000 VA

Temperature categ: 10 to 55C

Total weight: 900 Kg.


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Wave tape:A device used to exclude unwanted frequency components, such as noise orother interference, of a wave.A device used to exclude unwanted frequency components, such as noise orother interference, of a wave.

Wave trap is an instrument using for tripping of the wave. Thefunction of this trap is that it trapsthe unwanted waves. Its function is of trapping wave. Its shape islike a drum. It is connected tothe main incoming feeder so that it can trap the waves which maybe dangerous to theinstruments here in the substation.

Current transformer:

The instrument current transformer (CT) steps down the current ofa circuit to a lower value and

is 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 theprimary coil, which contains onlya few turns of wire. In this manner, measurements of high values ofcurrent can be obtained. Acurrent transformer should always be short-circuited when not


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connected to an external load.Because the magnetic circuit of a current transformer is designedfor low magnetizing current

when under load, this large increase in magnetizing current willbuild up a large flux in the magnetic circuit and cause thetransformer to act as a step-up transformer, inducing anexcessively high voltage in the secondary when under no load.

These transformers are basically used to get the incoming currenton 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 aCT can be multiplied to determineits absolute maximum measurable primary current. Conversely, theminimum 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 thenominal 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 ambienttemperatures and resultant rating factorswhen CTs are installed inside pad-mounted transformers or poorlyventilated mechanical rooms.Recently, manufacturers have been moving towards lower nominalprimary currents with greater ratingfactors. This is made possible by the development of more efficientferrites and their corresponding

hysteresis curves. This is a distinct advantage over previous CTsbecause it increases their range ofaccuracy, since the CTs are most accurate between their ratedcurrent and rating factorCurrent transformer


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Type 132 kV CTCore 1core 2core 3

Ratio (A/A) 800/1 400/1800/1 400/1800/1400/1Sec. Conn:1S1-1S22S1-2S33S1-3S3Accuracy class:0.25P 10PSBurden (VA):3015

NA

Highest system

Voltage:145 kVinsulation burn275 kV/ 65014 VpIsolator with earth switch (ES):

The instrument current transformer (CT) steps down the current ofa 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 theprimary coil, which contains onlya few turns of wire. In this manner, measurements of high values ofcurrent can be obtained. Acurrent transformer should always be short-circuited when notconnected to an external load.


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Because the magnetic circuit of a current transformer is designedfor low magnetizing current


The main use of using the earth switch (E/S) is to ground the extravoltage which may b

dangerous for any of the instrument in the substation.

Isolator ratings

Voltage rating: 145 kVBasic insulation level: 650 kVp

Current rating: 1250 Amp.

Circuit breaker: using SF6 gas:

Sulphur hexafluoride (SF6) is an inert, heavy gas having gooddielectric and arc extinguishing

properties. The dielectric strength of the gas increases withpressure and is more than ofdielectric strength of oil at 3 kg/cm2. SF6 is now being widely usedin electrical equipment like


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high voltage metal enclosed cables; high voltage metal cladswitchgear, capacitors, circuitbreakers, current transformers, bushings, etc. The gas is liquefiedat certain low temperature,

liquefaction temperature increases with pressure.Sulphur hexafluoride gas is prepared by burning coarsely crushedroll sulphur in the fluorine gas,in a steel box, provided with staggered horizontal shelves, eachbearing about 4 kg of sulphur.

The steel box is made gas tight. The gas thus obtained containsother fluorides such as S2F10,SF4 and must be purified further SF6 gas generally supplier bychemical firms. The cost of gas is

low if manufactured in large scale.


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During the arcing period SF6 gas is blown axially along the arc. Thegas removes the heat fromthe arc by axial convection and radial dissipation. As a result, thearc diameter reduces during the

decreasing mode of the current wave. The diameter becomes smallduring the current zero andthe arc is extinguished. Due to its electronegativity, and low arctime constant, the SF6 gasregains its dielectric strength rapidly after the current zero, the rateof rise of dielectric strength isvery high and the time constant is very small.


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Fig: SF6 circuit breaker.

Gas circuit breaker: high voltage side

Type 220-SFM-20B

Voltage rating: 220kvRated lightening impulse withstand voltage: 1050 kVp

Rated short circuit breaker current: 40 kV

Rated operating pressure: 16.5 kg/ cm2g

First pole to clear factor 1.3

Rated duration of short circuit current is 40 kA for 30 sec.

Rated ling charging breaker breaking current 125 Amp

Rated voltage 245 kVRated frequency 50 Hz

Rated normal current 1600 Amp

Rated closing voltage: 220 V dc

Rated opening voltage 220 V dc

Main parts:

(a) Power circuit

(b) Control circuitGas circuit breaker: low voltage side

Type 120-SFM-32A

Voltage rating: 220kv

Rated lightening impulse withstand voltage: 650 kVp

Rated short circuit breaker current: 31.5 kV

Rated operating pressure: 15.5 kg/ cm2g

First 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 Amp

Rated voltage 245 kV

Rated frequency 50 Hz


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Rated normal current 1250 Amp Rated closing voltage: 220 V dc

Rated opening voltage 200 V dc Main parts:

(a) Power circuit

(b) Control circuit220kv BUS:It is a incoming 220kv feeder BUS from which the line is taken tothe transformer for furtherstep down.

Double main bus & transfer bus systemMerits 1. Most flexible in operation 2. Highly reliable 3. Breakerfailure 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 ofservice at any time formaintenance. 7. Bus fault does not remove any feeder from theservice Demerits 1. High cost dueto three buses Remarks 1. Preferred by some utilities for 400kV and220kV importantsubstations.

Mesh (Ring) busbar systemMerits 1. Busbars gave some operational flexibility Demerits 1. Iffault occurs during busmaintenance, ring gets separated into two sections. 2.Auto-reclosing and protection complex. 3.Requires VTs on all circuits because there is no definite voltagereference point. These VTsmay be required in all cases for synchronizing live line or voltageindication 4. Breaker failureduring fault on one circuit causes loss of additional circuit becauseof breaker failure. Remarks 1.Most widely used for very large power stations having large no. ofincoming and outgoing linesand high power transfer.

Potential transformers: with BUS isolator


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The instrument potential transformer (PT) steps down voltage of acircuit to a low value that can be effectively and safely used foroperation of instruments such as ammeters, voltmeters, wattmeters, and relays used for various protective purposes.

There are two potential transformers used in the bus connectedboth side of the bus. Thepotential transformer uses a bus isolator to protect itself. The mainuse of this transformer is tomeasure the voltage through the bus. This is done so as to get thedetail information of thevoltage passing through the bus to the instrument. There are twomain parts in it (a)measurement; (b) protection.

Potential transformer ratings:

High voltage side: 245 V

Rated insulation voltage: 395/ 900

Voltage rating: 220/3 kV/ 110/ 3 V

BUS Isolator:

These isolators are used to isolate the incoming high voltage or thehigh incoming current from the incoming feeder which enters thebus. The isolator prevents damage to the instruments by justisolating the line current or the voltage.

Current transformer:

The instrument current transformer (CT) steps down the current ofa 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 theprimary coil, which contains onlya few turns of wire. In this manner, measurements of high values ofcurrent can be obtained. Acurrent transformer should always be short-circuited when notconnected to an external load.


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Because the magnetic circuit of a current transformer is designedfor low magnetizing current


Circuit breaker using SF6 gas:

Sulphur hexafluoride (SF6) is an inert, heavy gas having gooddielectric and arc extinguishingproperties. The dielectric strength of the gas increases withpressure and is more than of

dielectric strength of oil at 3 kg/cm2. SF6 is now being widely usedin electrical equipment likehigh voltage metal enclosed cables; high voltage metal cladswitchgear, capacitors, circuitbreakers, current transformers, bushings, etc. The gas is liquefiedat certain low temperature,liquefaction temperature increases with pressure.

Sulphur hexafluoride gas is prepared by burning coarsely crushedroll sulphur in the fluorine gas,

in a steel box, provided with staggered horizontal shelves, eachbearing about 4 kg of sulphur.

The steel box is made gas tight. The gas thus obtained containsother fluorides such as S2F10,SF4 and must be purified further SF6 gas generally supplier bychemical 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, thearc diameter reduces during thedecreasing mode of the current wave. The diameter becomes smallduring the current zero andthe arc is extinguished. Due to its electronegativity, and low arctime constant, the SF6 gas


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regains its dielectric strength rapidly after the current zero, the rateof rise of dielectric strength isvery high and the time constant is very small.

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 toprevent the high voltage entering the main station. This high voltage is very dangerous to theinstruments used in the substation. Even the instruments are very costly, so to prevent anydamage lightening arrestors are used. The lightening arrestors do not let the lightening to fallon the station. If some lightening occurs the arrestors pull the lightening and ground it to theearth. In any substation the main important is of protection which is firstly done by theselightening arrestors. The lightening arrestors are grounded to the earth so that it can pull thelightening to the ground. The lightening arrestor 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 transformer's 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. Inanideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the

primary 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 makingNS greater thanNP, or "stepped down" by makingNSless thanNP.


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Transformers come in a range of sizes from a thumbnail-sizedcoupling transformer hiddeninside a stage microphone to huge units weighing hundreds of tonsused 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 fortransformers in some electroniccircuits, transformers are still found in nearly all electronic devices


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designed for household("mains") voltage. Transformers are essential for high voltage powertransmission, which makeslong distance transmission economically practical.

Pole-mounted single-phase transformer with center-tappedsecondary. Note use of the groundedconductor as one leg of the primary feeder.An auto transformer 220kv/132kv, in Sub Station, AEGCL,Sarusaji, GuwahatiBasic principles


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The transformer is based on two principles: firstly, that an electriccurrent can produce amagnetic field (electromagnetism) and secondly that a changingmagnetic field within a coil ofwire induces a voltage across the ends of the coil (electromagneticinduction). Changing thecurrent in the primary coil changes the magnetic flux that isdeveloped. The changing magneticflux induces a voltage in the secondary coil.

An ideal transformer.

An ideal transformer is shown in the adjacent figure. Currentpassing through the primary coilcreates a magnetic field. The primary and secondary coils arewrapped around a core of very


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high magnetic permeability, such as iron, so that most of themagnetic flux passes through bothprimary and secondary coils.

Induction law

The voltage induced across the secondary coil may be calculatedfrom Faraday's law ofinduction, which states that:

whereVS is the instantaneous voltage,NS is the number of turns inthe secondary coil andequals the magnetic flux through one turn of the coil. If the turns ofthe coil are orientedperpendicular to the magnetic field lines, the flux is the product of

the magnetic field strengthBand the areaA through which it cuts. The area is constant, beingequal to the cross-sectional areaof the transformer core, whereas the magnetic field varies with timeaccording to the excitation


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The transformer is based on two principles: firstly, that an electriccurrent can produce amagnetic field (electromagnetism) and secondly that a changingmagnetic field within a coil ofwire induces a voltage across the ends of the coil (electromagneticinduction). Changing thecurrent in the primary coil changes the magnetic flux that isdeveloped. The changing magneticflux induces a voltage in the secondary coil.

An ideal transformer.

An ideal transformer is shown in the adjacent figure. Currentpassing through the primary coilcreates a magnetic field. The primary and secondary coils arewrapped around a core of very


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high magnetic permeability, such as iron, so that most of themagnetic flux passes through bothprimary and secondary coils.

Induction law

The voltage induced across the secondary coil may be calculatedfrom Faraday's law ofinduction, which states that:

whereVS is the instantaneous voltage,NS is the number of turns inthe secondary coil andequals the magnetic flux through one turn of the coil. If the turns ofthe coil are orientedperpendicular to the magnetic field lines, the flux is the product of

the magnetic field strengthBand the areaA through which it cuts. The area is constant, beingequal to the cross-sectional areaof the transformer core, whereas the magnetic field varies with timeaccording to the excitation

attached across the terminals of the secondary coil, it appears tothe primary circuit to have animpedance of. This relationship is reciprocal, so that the impedanceZP of theprimary circuit appears to the secondary to be.Detailed operation

The simplified description above neglects several practical factors,

in particular the primarycurrent required to establish a magnetic field in the core, and thecontribution to the field due tocurrent in the secondary circuit.

Models of an ideal transformer typically assume a core of negligiblereluctance with two


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windings of zero resistance. When a voltage is applied to theprimary winding, a small currentflows, driving flux around the magnetic circuit of the core. Thecurrent required to create the flux

is termed the magnetizing current; since the ideal core has been assumedto have near-zeroreluctance, the magnetizing current is negligible, although stillrequired 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 associatedvoltage drop, and so the voltages

VP and VS measured at the terminals of the transformer, are equalto the corresponding EMFs.

The primary EMF, acting as it does in opposition to the primaryvoltage, is sometimes termed the"back EMF".This is due to Lenz's law which states that theinduction of EMF would always besuch that it will oppose development of any such change inmagnetic field.

Transformer ratingType of cooling:capacity:OFAFONAFONAN100mV80mV60mVNo load voltage: 220kv/ 132kvLine current: HV 262.43 Amp.209.94 Amp.157.46 Amp.LV437.38 Amps349.91 Amp.


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262.43 Amp.Impedence voltage: 12.5V1u2u

3u3w2w2v1v1w3vVector symbol: YNad1Lightening arrestors:

Lightening arrestors are the instrument that are used in theincoming feeders so that to preventthe high voltage entering the main station. This high voltage is verydangerous to the instrumentsused in the substation. Even the instruments are very costly, so toprevent any damage lighteningarrestors are used. The lightening arrestors do not let the lighteningto 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 theselightening arrestors. The lighteningarrestors are grounded to the earth so that it can pull the lighteningto the ground. The lighteningarrestor works with an angle of 30 to 45 making a cone

Current transformers

Current transformers are basically used to take the readings of the

currents entering thesubstation. This transformer steps down the current from 800amps to 1 amp. This is donebecause we have no instrument for measuring of such a largecurrent. The main use of thistransformer is (a) distance protection; (b) backup protection; (c)measurement.


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Current transformer LV sideType 132 kV CTCore 1core 2

core 3Ratio (A/A) 800/1 400/1800/1 400/1800/1400/1Sec. Conn:1S1-1S22S1-2S33S1-3S3Accuracy class:0.25P 10PSBurden (VA):3015

NA

Highest system

Voltage:

145 kVinsulation burn275 kV/ 65014 VpIsolator:

The line isolators are used to isolate the high voltage from flowthrough the line into the bus.

This isolator prevents the instruments to get damaged. It alsoallows the only needed voltage andrest is earthed by itself.

Circuit breaker:


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The circuit breakers are used to break the circuit if any fault occursin any of the instrument.

These circuit breaker breaks for a fault which can damage otherinstrument in the station. For

any unwanted fault over the station we need to break the linecurrent. This is only doneautomatically by the circuit breaker.

132kv BUS:

This bus is to carry the output stepped down voltage to the requiredplace.Potential transformer: 2 with bus isolator

Two PT are always connected across the bus so that the voltageacross the bus could be

measuredCurrent transformers are basically used to take the readings of thecurrents entering thesubstation. This transformer steps down the current from 800amps to 1 amp. This is donebecause we have no instrument for measuring of such a largecurrent. The main use of thistransformer is (a) distance protection; (b) backup protection; (c)measurement.

Current transformer LV sideType 132 kV CTCore 1core 2core 3Ratio (A/A) 800/1 400/1800/1 400/1800/1

400/1Sec. Conn:1S1-1S22S1-2S33S1-3S3Accuracy class:0.2


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5P 10PSBurden (VA):30

15NA

Highest system

Voltage:

145 kVinsulation burn275 kV/ 65014 VpIsolator:

The line isolators are used to isolate the high voltage from flowthrough the line into the bus.

This isolator prevents the instruments to get damaged. It alsoallows the only needed voltage andrest is earthed by itself.

Circuit breaker:

The circuit breakers are used to break the circuit if any fault occursin any of the instrument.

These circuit breaker breaks for a fault which can damage otherinstrument in the station. Forany unwanted fault over the station we need to break the linecurrent. 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 voltageacross the bus could bemeasured

This bus is used for the 33kV line. This bus carries 33kV voltage.Potential transformer on the bus:


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The potential transformer is used in the bus only. This is becauseto measure the voltage in thebus. The use of the potential transformer is to measure and toprotect the instruments.BUS isolator:

The bus isolator is used to isolate the extra high voltage throughthe bus.Current transformer:

The use of the CT here is to protect the instrument and formeasurement purpose.Circuit breaker:

The circuit breaker breaks the circuit whenever there is any fault inthe line.Line isolator with earth switch (E/S):

An isolator with switch is part of an electrical circuit and is oftenfound in industrial applications,however they are commonly fitted to domestic extractor fans whenused in bathrooms in the UK.Isolator switches may be fitted with the ability for the switch topadlock such that inadvertentoperation is not possible . In some designs the isolator switch hasthe additional ability to earththe isolated circuit thereby providing additional safety. Such an

arrangement would apply tocircuits which inter-connect power distribution systems where bothend of the circuit need to beisolated. Major difference between isolator and circuit breaker isthat isolator is an off-loaddevice, whereas circuit breaker is an on-load device.

Lightening arrestors:

Lightening arrestors are used to protect the instruments fromlightening.

Capacitor bank:A capacitor bank is used in the outgoing bus so that it canmaintain the voltage level same in theoutgoing feeder.Capacitor Control


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is usually done to achieve as many as possible of the followinggoals: Reduce losses due toreactive load current, reduce kVA demand, decrease customerenergy consumption, improve

voltage profile, and increase revenue. Indirectly capacitor controlalso results in longerequipment lifetimes because of reduced equipment stresses.Experience shows that switchedfeeder capacitors produce some of the fastest returns on equipmentinvestment.

Sources of Energy Loss

Energy losses in transmission lines and transformers are of two

kinds: resistive and reactive. The former are caused by resistivecomponent 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

apacitance (negative), and circuit inductance (positive). When a heavy inductiveload is connected to the power grid, a large positive reactive power componentis added, thereby increasing observed power load (Fig. 1). This increases lossesdue to reactive load current, increases kVA demand, increases customer energyconsumption, usually degrades voltage profiles, and reduces revenue.

Reactive Compensation

When capacitors of appropriate size are added to the grid at appropriatelocations, the abovementioned losses can be minimized by reducing the reactive power componentin Fig. 1, therebyreducing the observed power demand. There are many aspects to thiscompensation and itseffects, depending on where capacitors get to be located, their sizes, and detailsof the

distribution circuit. Some are discussed below.

Energy Loss Reduction

More than one half of system energy loss is caused by the resistance of thefeeders. To minimizeenergy losses it is, therefore, important to locate feeder capacitors as close tothe loads as


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possible. Substation capacitors cannot do the job - the reactive load currenthas already heatedfeeder conductors downstream from the substation. Reducing reactive currentat 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 bankson-line early in theload cycle. Since energy saved is the product of power reduction and the timethe banks are on-line, the overall energy reduction is usually greater than when using largebanks which are turnedon for shorter amounts of time (Fig. 2).

Demand Reduction

When capacitors are on-line reactive current and, therefore, totalline current is reduced. Duringheavy load periods this has several advantages: The peak load isincreased when it is most

needed (essentially releasing demand), the effective line currentcapacity is increased, and theoperating line and transformer temperatures are reduced prolonging equipment lifetimes. Thelatter makes it possible to upgrade lines and transformers lessfrequently. All of these contributeto reduced costs and higher revenues.


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Voltage Profile

Distribution feeder demand capacity is usually limited by voltagedrop along the line. Thecustomer service entrance voltage must be stable, usually 5% to10%. The feeder voltageprofile can be flattened by connecting large capacity banks to thegrid. Several benefits becomeavailable: The kVA demand can be increased to arrive at the originalvoltage drop (this isequivalent to releasing feeder demand), the substation voltage canbe lowered to reduce peakdemand and save energy, or the service entrance voltage can beallowed to increase thereby

increasing revenue (at the expense of less than optimum kVAdemand).

System ConsiderationsObviously properly switched capacitors located at appropriatelocations 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 meansthat 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. Ifthe 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 switchingcontrol used, multiple bankson a feeder can lead to pumping as the controls affect theoperating points of each other.Usually no more than three or four banks are used per feeder.


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In the case of concentrated industrial loads, there should be abank, sized to almost equal thereactive load current, located as close to each load as possible (Fig.3).Types of Control

VAr control is the natural means to control capacitors because thelatter adds a fixed amount ofleading VArs to the line regardless of other conditions, and lossreduction depends only onreactive current. Since reactive current at any point along a feederis affected by downstreamcapacitor banks, this kind of control is susceptible to interactionwith downstream banks.Consequently, in multiple capacitor feeders, the furthestdownstream banks should go on-linefirst, and off-line last. VAr controls require current sensors.

Current control is not as efficient as VAr control because itresponds to total line current, andassumptions must be made about the load power factor. Currentcontrols require current sensors.Voltage control is used to regulate voltage profiles, however it mayactually increase losses and


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cause instability from highly leading currents. Voltage controlrequires no current sensors.

Temperature control is based on assumptions about loadcharacteristics. Control effectivenessdepends on how well load characteristics are know. Not useful incases where thosecharacteristics change often. Temperature control does not requireany current sensors.

Time control is based on assumptions about load characteristics.Control effectiveness dependson how well load characteristics are know. Not useful in caseswhere those characteristics change

often. Time control does not require any current sensors.

Power factor control is not the best way to control capacitor banksbecause power factor by itselfis not a measure of reactive current. Current sensors are needed.

Combination control using various above methods is usually thebest choice. If enough current,and/or other sensors are available, a centrally managedcomputerized capacitor control systemtaking into account the variety of available input parameters can bemost effective, thoughexpensive to implement.

BUS isolator

The bus isolators are used to isolate high voltage entering the busor entering the substation.Line Current transformer:

The current transformer used in the line is known as the linecurrent transformer. The main use ofthis current transformer is to measure and to protect theinstruments.Line circuit breaker:

The circuit breaker used in the line is known as the line circuitbreaker. The use of the circuitbreaker in the outgoing feeder is to break the circuit when the any


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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 lineisolator.To output feeder:

The outgoing feeders are used to give the step down voltage to therequired area. This feederssupply the voltage to the required place for further step down or itsuse in the place.

The Sarusajai substation has six output feeders it six different

places namely:JWAHAR NAGAR, GARBHANGA, MIRZA, PALTAN BAZAR, andKahilipara STATION

Training Report on GSS

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