transmission line protections 1

8/3/2019 Transmission Line Protections 1

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----ByH.RaghavendraRao,SE(RETD)/KPCL

BusBarprotections

Busbarsarelargesectionconductors,towhichallcircuitsandequipmentsareconnected

Busbarprotectionisintendedtodetectfaultswithinthebusandtrip

allconnectedcircuits Whyseparatebusbarprotectionisrequired

Insubstationallequipmentslikegenerators,transformersandfeedershavetheirownprotections.Fastactingprotectionsoftheseequipmentswillnotoperateforbusfault.Abusfaultisexternaltotheequipmentprotectionsandareclearedonbackup.

Thetimedelayedclearanceonbusfaultshavinghighestfaultcurrentscancauselargesystemdisturbances

Equipmentsliketransformersandgeneratorsarestressedduetosustainedshortcircuits

Ontrippingofequipmentbackupprotectionsallcircuitsofthe bustrip

outandhealthysectionofthebusalsoislost Busfaultsmaycauseseriousfireanddamageotherequipmentsif not

clearedquickly


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BusBarprotections

Whybusbarprotectionnotalwaysprovided Probabilityoffaultoccurrenceinbusbarsareless Nocostlyandlargeequipmentsarepartofthebus-PT,CT,LAetc Anywaythebackupprotectionsoftheequipmentsconnectedtothebuswill

isolatethefault Falsetrippingofbusprotectionishighlyundesirablewhichcausesdisconnection

oflargeno.ofgenerators,feedersetc.fromthesystemandpartialblockouts CostofbusbarprotectionandassociatedadditionalCTcores

HencebusbarprotectionisprovidedselectivelyforEHVandlargersubstations

Busbarprotectionalsoprovidesdiscriminativetrippingoffaultedbussectiononlyretaininghealthysectioninservice.Lessdisruption

Busfaultscanoccurdueto Wrongonloadoperationofisolators FailureofCT/PT/LAetc

Flashoverofbusinsulators Snappingofjumpers

Unknowinglyleavingsafetyearths


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BusBarprotections

Requirementsofbusbarprotection Veryhighspeedofoperationtojustifytheirapplication

Stabilityisofparamountimportance.Falsetrippinghighlyundesirable.Shallnotoperateheavythroughfaults

Selectivetrippingwheremultiplebussectionsarethere

Typeofbusconfigurations Singlebus Doublebus

Singleordoublebuswithsectionalising Mainandtransferbus

Oneandhalfbreakersystem

Busprotectionzonetooperateforfaultsinbusuptotheequipmentbreakerterminals(DefinedbyCTlocations)

Typesofbusprotections Frameearthprotection/Phasecomparison/Currentdifferential OnlyCurrentdifferentialprotectionisused


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BusBarprotections

Busdifferentialprotectionprinciple Sumofallcurrentsintothebusisequaltosumofallcurrents goingoutofthe

businhealthyandexternalfaultconditionthatisthealgebraicsumofallcurrentsiszero

Duringafaultwithinthebusthesumisnotzerosincethefaultcurrentfromthebusisflowingintothefault.Thecurrentsaddupandthedifferentialcurrentispresent

Thepresenceofdifferentialcurrentisusedtooperatetheprotection ToobtainalgebraicsumCTsofallcircuitsareconnectedinparallelwith

correctpolarityandtherelayisconnectedacross

AllCTstohaveidenticalratiosandhighkneepointvoltages

Busdifferentialprotectioncanbeprovidedonlyforearthfaultssincethesefaultsaremorelikely.thiscaseall3phaseCTsofallcircuitsare inparallelandrelayacross.Butmostlyall3phasesareprovidedwithseparateprotectionelements.Separatesetofelementsforeachseparablesectionofthebus.

Ctconnectionstooverlapthezonestoavoidblindzones.

ForoneandhalfbreakerbusseparateCtandrelaysforeachbus.FordoublebusCTswitchingbyisolatorcontactstoconnecttheCTtorespectivezone

Thetriplogictotriponlythosebreakersconnectedtothefaultybus.Thisismetthroughisolatorcontacts


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BusBarprotections

BusbarprotectiontohaveIN/OUTswitchingfacilty sinceduringoperationslikebuschangeoveretc.thebusbarprotectionistobeout.

Busbarcurrentdifferentialisof2types HighImpedancedifferentialprotection

LowImpedancediffrential protections

HighimpedanceisconnectingallCTsinparallelandacrosstherelay.Nobiascircuit.Highimpedanceoftherelaycircuitmade to

providethestabilitygivesthisname Lowimpedanceprotectionisprovidingabiaswindinginrelayfor

stabilityandlowerimpedancepresentedtoCT

AdvantagesofhighImpedance

Simplerobustandreliable AllCTconnectionsneednotbebroughttorelaypanelandcanbe

paralleledattheyard.Reducescabling

Lessexpensive


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BusBarprotections

DisadvantagesofhighImpedance

CTstohavehigherkneepointvoltagessincetheimpedance

presentedtoCTishigh

AllCTstoidenticalinratio.Noslightdeviationispermitted AddingofnewcircuitsinexpansionisdifficultSimplerobustand

reliable

HighImpedancecircuitandstabilising resistance

DuringacloseinexternalfaulttheCTinthecircuitinwhichfaultoccurredwillbecarryingmaximumcurrentwithallotherCTscarryinglesscurrent.ProbabilityofthisCTgettingsaturatedismore.During

suchCTsaturationtoprovidestabilityastabilising resistorisaddedtorelaysensingcircuitsothatcurrnet throughrelayislessthanpickup

value-----Id=If(Rct+Rl)/(Rstab=Rrelay) Relayisvirtuallysetasvoltagepickup

CTstohavekneepointvoltage>2(voltageatCTreqd.forrelaypickup)


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BusBarprotections

Atypicalhighimpedanceschemefordoublebus

CTsofcircuitsswitchedbyisolatorcontactstotherespective zone

BuscouplerhasCTsonbothsidesortwocoresandconnectedtwo

zonesseparately AllcircuitshaveseparateCts forcheckzone

EachzonehasCTsupervisionrelayforsensinganyctopenand

shortingthetrippingthezones

Trippingthroughrespectiveisolatorcontacts Discriminationcanalsobeprovidedbyovercurrentrelaysinstead

ofisolatorCTswitching.Butthisrequirestimedelayhencenotpreferred


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Breakerfailprotection Allprotectionsoftheequipmentsfinallygivecommandtothebreakerto

tripandisolate. Ifthebreakerfailstotripprotectionswillbeineffectiveand equipmentsget

damaged

EHVbreakersarereliablebuttheyfailtotripdueto Defectivemechanism Air/hydraulicpressureforoperationinadequate Gaspressureduetoleak

Tripcoilsburnt BreakerDCfailing Lowimpedancescheme

Duringsuchbreakerfailurestheequipmentcanbeisolatedonlybytrippingallbreakersconnectedtothebusandmakingbusdead

TotakecareofsuchbreakerfailuresLocalBreakerbackupprotectionisprovided

Schemelogicdetectscontinuedpresenceofcurrentflowafterasmalldelayfromtheenergisation oftriprelayswhenthebreakerhasnottrippedandtripsallbreakersconnectedtothebusthroughthebusbartripcircuit

LBBandbusbarprotectionoperatethroughsametripcircuits


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TransmissionLineProtections Transmissionlinesofdifferentvoltagelevelsofdifferentlengthsand

differentcapacitiesareinservice Theyformthegridnetworkcarryingpowerfromsourcetoload

centers Faultsintransmissionlinesaremorefrequentcomparedtofaultsin

otherpowersystemcomponents.Thelineslieinforests,hillyregionsandaresubjecttoweathering,bird/animalfaultsetc

Thefaultsthatoccurcouldbe

Ph-Groundfaults Ph-phfaults Ph-Ph-Groundfaults

3Phasefaults Brokenconductor

Allfaultsintransmissionlinearetobeclearedwithouttimedelayforsecuredoperationofthesystem


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TransmissionLineProtections Faultsresultinhighmagnitudeofcurrentsandsimple

overcurrentandearthfaultrelayswereusedinearlier

andsmallerradialfeeders.Thesecannotprovidefasttrippingbecause Timegradingistobedonefordiscriminatedtrippingthustime

delay

Thecurrentgradingcannotbedonesincethefaultcurrentisalwaysnotthesameanddependonsystemconditions

Trippingtakesplaceinbothdirectionflow

Directionalovercurrentprotectionprovideddirectionaldiscrimination.Directionalrelaysareoperationalevennowfor

lowervoltageoneendfedfeeders areTransmissionlinesofdifferentvoltagelevelsofdifferent

lengthsanddifferentcapacitiesareinservice


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TransmissionLineProtections Adventofimpedancerelayssuitedmostforlineprotections Impedancerelaysareenergised by2quantitiesvoltageandcurrentand

measuretheV/Iratioandtripwhenthemeasuredimpedanceislessthanthesetvalue

Sincethedistancealongthelineuptofaultpointisproportionaltotheimpedance,theimpedancemeasuringrelayisalsocalledasdistancerelay

Thisweresuitedforlineprotectionsincerelayoperationwasnotdependentonfaultcurrent,nocurrentgradingandcouldbesettooperate fortherequiredlengthoftheline

Typeofdistancerelays Plainimpedancerelay,reactancerelay,MHOrelay,

ThecharacteristicsoftherelaysdepictingtheirtrippingandnontrippingzonescouldbeconvenientlyshownonanR-Xdiagram.Sincelinealsohasitscharacteristicimpedancetherelaycharacteristicadoptabilitycouldbeanalysed inanR-Xdiagram

Moderndistancerelaysinadditiontostandardcircularcharacteristicscanalsoprovidequadrilateral/Lenticular etc.whichcanbemadepreciselyfittherequirement

Staticandnownumericalrelayscanbemadetoprovideanyoperatingcharacteristic


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TransmissionLineProtections Distancerelayschemesappliedtolineprotectionsbysettingtheir

impedancepickuptocovertha transmissionlinelengthasrequired.Distancerelaysformthemainprotectionforlines

Thebackuptodistanceprotectionisbydirectionalovercurrentrelaysforlinevoltagesupto220Kv andfor400KVthebackupisalsobyonemoredistancescheme.Nowadaysfor220KVlinesalsobackupisbydistance

DistancemeasuringelementisrequiredforallPh-PhfaultsandPh-gnd.Faults.Thus6elementsreqd.Furtherforbackupforremotefaultsdifferentimpedancesetrelaysreqd.3suchzonesaregenerallyused.Thus18elementsreqd.

Onedistanceelementforallmeasurementwhichisswitchedbystarterelaysareusedfor66KVandbelowvoltagelines.Switchedschemescouldbewithbothphaseswitchingandzoneswitching

For220KVlinesseparaterelaysforphasesbutsamerelayswitchedforzonesareused.Nowwithnumericalrelaysprovidingnumberofmeasuring

elementsisnotadditionallyexpensiveandhenceallschemesare nonswitched

Inadditiontomainandbackupasaboveovervoltageprotection isprovidedfor400KVlines


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TransmissionLineProtections

3Zonedistanceprotection Allfaultswithinprotectedsectionistobeclearedwithoutanytimedelay Ideallythedistancerelayistobesettoreachuptothenextremoteend.

Thismaycauseundesiredtrippingevenforfaultsinthenextsection Relayoverreaches(operatesformorethanthesetImpedance)dueto

inaccuraciesofrelay(10%erroe) CTPTerrors Lineparametersanddistanceconsideredforsettingmaynotbeexactdueto

routeetc.

Hencetheinst.Trippingdistancerelay(Zone1)istobesetforlessthanline

length,80to85%adopted Forprovidingprotectionforbalance20%anotherdistanceelement(Zone2)provided.Tocoveranyunderreachingrelayissetformorethan thelinelengthbutlessthanzone2reachofnextsection.Upto50%ofnextshortestsectionadopted.Thisistimedelayedtoallowthezone1elementattheotherendtotrip

Backupprotectionforfaultsinthenextsectiontocoverfailureoftrippingbyotherendrelaysprovidedby Zone3element.Thebackupisrequiredtoreachuptotheendofthelongestlineattheotherend.Timedelaymorethanzone2timerequired.next faults


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Standard3zonedistancescheme

A B C

Z1A

Z2A

Z3A

Z1B

Z2B

Z3B

D

Z2timeZ3time

Z1time

Zone1A=80%ofprotectedzone=80%ofA-B,Time=Instantaneous

Zone2A=Upto50%ofnextshortestsection=(A-B)+50%of(B-D),Time=0.3to0.5sec

Zone3A=Uptoslightlymorethan100%ofnextlongestsection=120%of(A-B)+100%of

(B-C),Time=06to1.0sec.


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Carrieraidedprotections

Blockingscheme

Remoteendequipmentsendssignalifthefaultisintheothersection.Reverselookingdistancerelayisusedtosendthesignal.

Thezone2elementwilloperateasperzone2delayifsignalisreceivedsincereceiptofsignalimpliesthatthefaultisnotinthesection

Ifthefaultisintheprotecteddirectionthenthereverselookingdistancerelayatfarendwillnotpickupandsendsignalimpliesthat

thefaultisinthesection.ThenZonetimedelayisshortedandzonetripalsowillbeinstant

Inblockingschemenonavailabilityofsignalingequipmentwillnotpreventinstanttripping.ThismaybedisadvantagealsosinceforfarendfaultsalsoZone2maytripinstantlyifsignalingisfaulty

Ifsignalingchannelisfaultyschemeisrevertedtooriginalscheme Asmalltimedelayreqd forzone2extendedtriptoavoidinstanttrippingduetodelayinsignaltransmission


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Carrieraidedprotections

Permissiveunderreachtransfertripscheme

Signalistransmittedwhenzone1elementpicks

upandonreceiptofsignalzone2tripactsinstantly.Ifthefaultisoutsidenosignalistransmittedandschemeoperateswiththezonetimedelaysset

Permissiveoverreachtransfertripscheme

Itissimilartopermissiveunderreachexceptthatthesignalistransmittedbytheoverreachingzone2elements


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Powerswingdetection Powerswingsinthesystemoccurduetodelayedfaultclearances,sudden

changeinloadflows,generatorsrunningwithevacuationthrough longlinesetc

Duringpowerswingsthetwoendvoltagevectorsstartslippingrelativetoeachotherwithcurrentalsoswingingfrommaximumtominimum

Theimpedancemeasuredduringpowerswingswillbevaryingwiththelocusenteringtherelaycharacteristic

Therelayses itsimilartoa3phasefaultandmaytrip Forrecoverableswingssuchtrippingisnotdesirable Henceduringpowerswingstrippingistobeblocked.Forsomespecialcases

trippingonpowerswingisrequired.Howeverdetectionofpowerswingis

requiredsothatselectivelytrippingorblockingcanbeadopted Powerswingdetectionisbasedontheprinciplethattheimpedancevectorduring

swingsmovesslowerfromloadpointtoinsidewhereasduringfaultstheimpedancevectorsuddenlymovesintotherelaycharacteristic.Discriminationofpowerswingandfaultbasedonthis

Detectorhasacharacteristicwithreachhigherthanthezone3distancerelayreachandtheannularareaistheswingband.Ifthelocusentersswingdetectorcharacteristicandremainsinthebandformorethansetdelayitisdetectedaspowerswing.Ifthelocusleavesthebandwithinthattimethan itisnotapowerswing.Timetakentotravelfromoutercharacteristictoinnercharacteristicismorethan30MSitisdetectedasswing


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AutoReclosing

Autoreclosingschemeperformsautomaticclosingofthelinebreakerafterthelinehastrippedonfaultandlinehasbeendeenergised frombothends

Mainfeaturesofautoreclosingschemeare Selectabletoreclose on1ph.Faultsor3phasefaultsorboth

Adeadtimeisallowedfromfaulttrippingtoreclosing command.Thisistoallowarcextinguishinganddeionising timeofthefaultarcforsuccessfulreclosing

Areclaimtimeisallowedbeforegoingforsubsequentautorecloseafterbreakerisclosedeitherbyautorecloserorbymanually. Thisistotakecareofbreakerdutycycle.Afteroneclose-openoperationthebreakermechanismtobereadyforreclosing Airpressure/springcharging,Reclaimtimeforbreakerarcchamber

For3phasereclosingcheckfordeadlineconditionorsynchronismofbusandlinesuppliesbeforereclosing

Lockoutofreclosingafteroneshotclosingandretrippingonpermanent/semipermanentfault


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Autoreclosing

Requirementsinlineprotectionsystemforapplicationofautoreclosing

Thebreakershallhaveseparatetrippingarrangementforeachpole

forsinglepoletrippingandreclosing

Thedistancerelayshallhaveindividualphasetrippingasperfaultedphase

Thebreakershallbesuitableforautorecloseduty

Thelineprotectionsystemshallhaveinstantaneoustrippingonbothsidesforallfaultsinprotectedsectionwhichisprovidedbyeithercarrieraideddistanceprotectionorunitprotectionlikephasecomparisonrelayingwithphasesegregation

Autoreclosingisenabledonlyontrippingbydistancerelayinzone1orzone2extendedandisblockedfortrippingby

Zone2andzone3distance/Overvoltage/busbarprotection/intertripfromotherend/manualtrip

transmission line protections 1

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