ANALYSISOFADIFFERENTIALANDOVERCURRENTOPERATIONONA345KVHIGHVOLTAGELINEREACTOR

Authors:

Eric Schroeder P.E., Cross Texas Transmission, Amarillo, Texas

Jerry Burton, Cross Texas Transmission, Amarillo, Texas

Luke Hankins, SynchroGrid, College Station, Texas 77845

Joe Perez P.E., SynchroGrid, College Station, Texas 77845

Presented before the

69th Annual Texas A&M Protective Relay Conference

College Station, Texas April 4th – April 7th, 2016

ANALYSISOFADIFFERENTIALANDOVERCURRENTOPERATIONONA345KVHIGHVOLTAGELINEREACTOR

Eric Schroeder P.E., Cross Texas Transmission, Amarillo, Texas eschroeder@crosstexas.com

Jerry Burton, Cross Texas Transmission, Amarillo, Texas jburton@crosstexas.com Joe Perez P.E., SynchroGrid, College Station, Texas, jperez@synchrogrid.com Luke Hankins, SynchroGrid, College Station, Texas, lukeh@synchrogrid.com

I. INTRODUCTION

Highvoltagelinereactorsareusedinlongtransmissionlinestomitigatethehighvoltagelevelscreatedbythelinechargingcapacitance.Theapplicationoflinereactorsshouldbestudiedcarefullysinceanincorrectoperationcanisolatenotonlythereactorbutalsothetransmissionlineitself.Thistypeofapplicationcomeswithchallengesconcerningthesystemandprotectiverelays.Reactorspresentdifficultiestodifferentialalgorithmsduringin-rushsituations.Thisisbecausereactorswillexperiencethesameamountofcurrentsonboththehighandlowsides.Asaresult,thedifferentialcurrentmagnitudecouldbezero,leavingnomagnitudestoextractthesecondharmonicwaveformusedforblockingofthe87function.Theprotectionengineermustunderstandtheprotectionfunctionandoperationalgorithmsoftherelaysbeingappliedinordertoproperlyprotectthereactorandavoidmis-operationsduringin-rushconditions.Thispaperdescribestheanalysisofareactorin-rusheventwherethebackuprelaytrippedondifferentialduringenergizationwhereastheprimaryrelaydidnottripondifferential.Inaddition,thispaperdescribesindetailtheprotectionalgorithmconcepts,waveformbehavior,anddifferentialcharacteristicsoftheprimaryandbackuprelays.Thiswillallowustoseehowthetwoalgorithmsdifferinsecurity,reliability,andsensitivity.II. SYSTEMONELINE

Figure1showsthesystemonelineofthelinereactorsconfiguration.Thenormalreactoroperationisdonethroughtheuseofacircuitswitcher.Thereactorisprotectedbyaredundantsystemusingtwodifferentdifferentialrelaysfromdifferentmanufacturers.ManufacturerAwillbereferredtoastheprimaryandmanufactureBwillbereferredtoasthebackup.ThedifferentialzoneisboundedbyCTswithinthereactor’shighsideandlowside,asshowninfigure2.Ifafaultisdetectedinsidethezoneofprotection,thedifferentialrelayssendatransfertripsignalviaGOOSEtothelinerelaystoopenlocalandremotelinebreakers.Thecircuitswitcheropens30cyclesafterthelinebreakersopen.

BKR_G120

BKR_G130

345 kV Line 1

345 kV 345 kV

Figure1:SystemOneline

BKRG130

Line1toTesla

GY-R3

SWRGR34

BackupPrimary

BKRG120

Primary Backup

345kV

Figure2:ReactorDifferentialProtection

III. SEQUENCEOFEVENTS

Table1showsasummarizedversionofthesequenceofeventsthatoccurredduringthisevent.Asshownbelow,operatorsaretoldtoclosethereactorat09:26:30.887.Fivecyclesafterclosingthecircuitswitcher,thelinerelaysreceiveatransfertripsignalviaGOOSEfromthebackupreactordifferentialrelay.Twocyclesafterthedifferentialtripwasissued,thelocallinebreakersareopened.21cycleslater,theprimaryrelaytripsongroundinstantaneousovercurrent.Finally,34cyclesaftertheinitialdifferentialtrip,thecircuitswitcheropens.

Description TimeStampValue(hrs:min:sec:ms)CircuitSwitcherGR34closes 09:26:30.887

(5cycleslater)backuprelayoperatesonBPhaseDifferential 09:26:30.972

1/4ofacycleafterdifftrip,linerelaysreceivetransfertripfromthereactorbackupdifferentialrelay

09:26:30.974

2cyclesafterdifftrip,breakersG13&G12open 09:26:31.005

20cyclesafterdifftrip,theprimaryreactordifferentialissuesatripongroundinstantaneousovercurrent

09:26:31.308

21cyclesafterdifftrip,thebackupreactordifferentialissuesatripongroundinstantaneousovercurrent

09:26:31.324

34cyclesafterdifftrip,R34opens 09:26:31.537

Table1.SequenceofEvents

Thesummaryofthesequenceofeventstellsusthattherearequiteafeweventshappeningthatneedfurtherinvestigation.Wecanseethatonlythebackuprelaydetectsadifferentialfault.Thisalready

raisesseveralquestionsabouttheseoperations.Isthereaninternalfaultinthereactor?Whydidn’ttheprimaryrelayseeaninternalfault?Whydobothrelaysoperateoninstantaneousgroundovercurrent?

Thefirststepinanalyzinganeventistochecktherelaysettingsthatareinservice.Thisprocedurecanbelaborioussinceitrequiresthatallsettingsberecalculated.Thisallowsustouncovererrorsthatmighthaveslippedduringinitialcommissioning.Thenextsectiondigsdeeperintheanalysisoftherelaysettingsanddescriptionofitsuse.

IV. DIFFERENTIALRELAYSETTINGS

Thisreactorapplication,asshowninthesystemoneline,usestworedundantcurrentdifferentialrelaysforprimaryandbackupprotection.Inaddition,twodifferentrelaymanufacturersareutilizedwheretheprotectionalgorithmsarecalculateddifferently.Asummaryofthesettingsfromtheprimaryrelayusedtoprotectthereactorisshownintable2.

Description Setting ValueDiff.ElementOperatingCurrentPickup(p.u.) O87P 0.5Slope1Setting(%) SLP1 35Slope2Setting(%) SLP2 75UnrestrainedElementCurrentPickup(p.u.) U87P 1.00IncrementalOperateCurrentPickup(p.u.) DIOPR 1.2IncrementalRestraintCurrentPickup(p.u.) DIRTR 1.2EnableHarmonicBlockingDifferentialElement E87HB YEnableHarmonicRestraintDifferentialElement E87HR NSecond-HarmonicPercentage(%) PCT2 10

Table2:PrimaryRelayDifferentialSettings

Thesettingsarecomposedofaminimumpickupdifferential087PalongwithahighinstantaneousU87Psetting.The087Psettingoffersaverysensitivethresholdthatallowstherelaytoisolateinternalfaultsveryquickly.TheU87Pisaninstantaneoussettingwheretherestraintcurrentisnottakenintoconsideration,makingitsuitableforhighmagnitudeinternalfaults.Itoperatesdirectlyasthesummationofthefiltereddifferentialcurrents.Wenoticedthatharmonicblockingandharmonicrestraintsettingsarealsoavailablewithharmonicrestraintnotbeingused.Thesecondharmonicsettingissetto10%ofthefundamental.Thismeansthatiftherelaydetectsasecondharmoniccontentabove10%,therelaywillblockthedifferentialelementfromoperating.Harmonicblockingandrestraintareusedinordertoincreasethesecurityanddependabilityofthealgorithmduringin-rushorexternalfaultevents.

Theslopecharacteristicforthisrelayisshowninfigure3.Thisrelayusesoneoftwoslopesaspartofthedifferentialcharacteristic.Therelay’sinternalalgorithmsdecidewhichslopetousebasedonthebehaviorsofthecurrents.Slope1,setat35%,istypicallyusedtoincreasetherestrainoftherelayinordertoavoidoperationsduetoCTerrors,transformerlosses,highloadconditions,etc.Slope2,setat75%,givestherelaymoresecuritybyincreasingtherestrainregionofthedifferentialplaneandisusedforhigh-throughfaults,highclose-inexternalfaults,CTsaturation,etc.Increasedrestrainisnecessarywhendealingwitherrorsofahighermagnitude.

Figure3:PrimaryRelayDifferentialPlane

Thebackuprelayusessimilarsettingstotheprimaryrelay.Asummaryofthesettingsfromthebackuprelayusedtoprotectthereactorisshownintable2.

Table3:BackupRelayDifferentialSettings

Thisrelayalsohasminimumandhighoperatingpickupsettingsthataresettothesamepickupastheprimaryrelay.Eventhoughthesamepickupcriteriaisusedforbothrelays,thesettingsthemselvesappeartobedifferentbecauseeachrelaycalculatedtheperunitvaluedifferently.Thisrelayusesharmonicblockingandaninhibitin-rushsettingcalledadaptivesecondharmonic.Similartothepreviousrelay,thesecondharmonicissetto10%ofthefundamental.Inaddition,thesecondharmonicvalueascomparedtothefundamentalmustbehigherthanthesettingsinatleast2outof3phasesinorderforthedifferentialtobeblocked.Thiswillbeveryimportantinformationthatwillbevisuallyexplainedinthewaveformanalysissection.

Description Setting ValuePERCENTDIFFERENTIAL Function EnabledPERCENTDIFFERENTIAL Pickup 0.100pu(0.5A)PERCENTDIFFERENTIAL Slope1 25%PERCENTDIFFERENTIAL Break1 1.570puPERCENTDIFFERENTIAL Break2 7.840puPERCENTDIFFERENTIAL Slope2 98%PERCENTDIFFERENTIAL Inrush

InhibitFunction

Adapt.2nd

PERCENTDIFFERENTIAL InrushInhibitMode

2-out-of-3

PERCENTDIFFERENTIAL InrushInhibitLevel

10.0%fo

PERCENTDIFFERENTIAL Function EnabledPERCENTDIFFERENTIAL Block OFF

Theslopecharacteristicofthebackuprelayisshowninfigure4.Thisrelayalsousesthedualslopecharacteristictoimprovethesecurityoftherelays.However,unlikethepreviousrelay,theslopesarefixedforbothinternalandexternalfaults.

Figure4:BackupRelayDifferentialPlane

V. PROTECTIONALGORITHMS

Inordertounderstandthereasonsbehindthereactordifferentialrelayoperation,itisnecessarytounderstandhoweachindividualrelayalgorithmworks.Eachmanufacturerhasitsownpreferredmethodofprotectionalgorithmthateithermakesthemmoresecureandlesssensitiveorlesssecureandmoresensitive.BelowisareviewonhowbothrelaysestablishtheoperateIOPandrestrainIRTcurrentsalongwiththeharmonicrestraintandblockingthatareneededindifferentialapplications.

OperateandRestraintCurrents:

Theprimarycalculatestheoperateandrestraintcurrentsonaperphasebasis.TheformulasforAPhasedifferentialforthehigh(IAT)andlowside(IAW)ofthereactorphasorcurrentsareshowninequations1and2below.

IOPA = |IAT + IAW|(1)

IRTA = |IAT| + |IAW|(2)

ThebackupcalculatestheoperateandrestraintcurrentsforAPhaseasshownbelow:

IOPA = IAT + IAW(3)

IRTA = MAX |𝐼𝐴𝑇| , (|𝐼𝐴𝑊|) (4)

Noticethatinbothrelays,thedifferentialcurrentsIOPAiscalculatedbyaddingthephasorcurrentsthatareprotectingthereactor.Theabsolutevalueofaphasorcalculationresultsintakingmagnitudesonly

andnottheangles.Themajornoticeabledifferenceishowtherestraintcurrentiscalculated.ThiswillbeshownvisuallyinsectionVII.Ineithercase,bothsetsofcurrentdifferentialmethodshavetoovercometherestraincurrentalongwiththeslopesettinginorderfortherelaytooperateasshowninequation5.

IOP > IRT ∗ SLP(5)

HarmonicRestraintandBlocking:

Bothrelaysofferextrasecurityalgorithmsforin-rushconditions.Theprimaryrelayhastheabilitytoapplyharmonicrestraintandblockingbeforethedifferentialfunctionissuesatrip.

HarmonicRestraint:Whentheprimaryrelayissettoharmonicrestraint,theoperatingcurrentiscalculatedasfollows:

IOPA > IAT + IAW ∗ 𝑆𝑙𝑜𝑝𝑒 + 𝐾2 ∗ 𝐼𝑂𝑃𝐴𝑝ℎ2 + 𝐾4 ∗ (𝐼𝑂𝑃𝐴𝑝ℎ4)(6)

K2andK4arethe2ndand4thharmonicsettingsandIOPAph2andIOPAph4arethe2ndand4thharmonicvaluesfoundintheoperatecurrentorcurrentdifferentialsummation.Equation6showsthattheoperatingcurrentIOPA“mustovercomethecombinedeffectsoftherestrainingcurrent,IRTA,andtheharmonicsoftheoperatingcurrentfortheelementtoassertatripoutput.Anymeasurableharmoniccontentprovidessomebenefittowardthegoalofpreventingdifferentialrelayoperationduringin-rushconditions”[1].Harmonicrestraintisgenerallyslower,buthasimproveddependabilitywhenenergizingafaultedtransformerorreactor.Also,becausetheharmonicsaresummed,harmonic restraintismoresecureduringin-rushconditions.Therestraintmethodisshowninfigure5below.

•10

•K4th

+

Σ

Σ

•f(SLP1,SLP2)IRTA

I2nd

I4th

IOPA

• • •

TRIP

Figure5:PrimaryRelayHarmonicRestraintSupervision

HarmonicBlocking:Whentheprimaryrelayisintheharmonicblockmode,theIOPAoperatingcurrentis“independentlycomparedwiththerestraintcurrentandtheselectedharmonics.”[3]Theharmoniclogicisshowninfigure6.Weseethattheoperatecurrentstillhastoovercometherestraintandslopesettingasshowninequation7.

IOPA > IAT + IAW ∗ 𝑆𝑙𝑜𝑝𝑒(7)Beforeadifferentialtripisdeclared,theamountofsecondharmoniccontentischeckedwithintheoperatecurrent.Ifthe2ndharmonicmeasuredvalueisgreaterthanthepercentsetting,thedifferentialtripisblocked.Forexample,fora2ndharmonicsettingof10%,whenthefundamentaloperatecurrent

hasavalueof10Aandthe2ndharmonicfoundintheunfilteredoperatecurrenthasavalueof1A,therelaydifferentialelementwillbeblocked.“Whentheharmoniccontentisbelowthespecifiedthreshold,theharmonicblockinghasnoeffect.”[3]

+

+

+

+

IAM4

IOPA•f(SLP1,SLP2)IRTA

IPU

AND AND

OR

2nd-HarmonicBlocking

87BL

87R

xth-HarmonicBlocking

TRIP

ph2

ph4

IAM2

Figure6:PrimaryRelayHarmonicBlocking

Inaddition,theprimaryrelayusescommoncrossblockingwhichblocksthedifferentialofany2ndharmonicphasethatisabovethegivensetting.Thisisshowninfigure7below.

OR87BL1

TRIPAND

OR

87BL287BL3

87R187R287R3

Figure7:PrimaryRelayHarmonicCrossBlocking

Thebackuprelaydoesnotofferarestraintharmonicblockingfeature.However,itdoesoffertwotypesofharmonicblockingtechniques:traditionalandadaptive2ndharmonicblocking.Thetraditional2ndharmonicrestraintrespondstotheratioofmagnitudesofthe2ndharmonicandfundamentalfrequencycomponents.Ifthe2ndharmoniccontentfoundinthedifferentialcurrentishigherthanthegivensettings,therelayblocksthedifferentialsetting.Thisissimilarorthesameastheharmonicblockingtechniqueoftheprimaryrelay.Theadaptive2ndharmonicrestraintrespondstomagnitudesandphaseanglesofthe2ndharmonicandthefundamentalfrequencycomponent.Thebackuprelaymanufacturerclaimsthattheadaptiveharmonicrestraintalgorithmsuccessfullyrestrainstrippingwhenfacedwithlowlevelsofsecondharmoniccurrentduringanin-rushevent[3].Theharmonicblockinglogicofferedbythebackuprelayisshowninfigure8.

ANDSlope

Functionladlar

DisabledAdapt.2nd

Trad.2nd

=0

=1

Disabled5th

=0=1

lad2>=LEVEL

lad5>=LEVEL

1outof32outof3Average

lad2

lad5

2ndHarmonicBlock

5thHarmonicBlock

ORA-PhaseDiffOpB-PhaseDiffOpC-PhaseDiffOp

DiffOp

Figure8:BackupRelayHarmonicandCrossBlocking

Sincethesecondharmoniciscalculatedonaperphasebases,therelayoffers4differentmodesofharmonicblocking.

1. Per-phase:Inper-phasemode,therelayperformsin-rushrestraintindividuallyineachphase.

2. 2-out-of-3:In2-out-of-3mode,therelaychecksthesecondharmoniclevelinallthreephasesindividually.Ifanytwophasesestablishablockingcondition,theremainingphaseisrestrainedautomatically.

3. Averaging:Inaveragingmode,therelayfirstcalculatestheaveragesecondharmonicratioand

thenappliestheinrushthresholdtothecalculatedaverage.

4. 1-out-of-3:In1-out-of-3mode,allthreephasesarerestrainedwhenablockingconditionexistsonanyonephase.1-out-of-3modetypicallyrevertsbacktoper-phasemodeafterashorttimedelaytoallowtrippingincaseaninternalfaultoccursduringenergization.

VI. WAVEFORMANALYSIS

Thissectionwillusetheinformationthatwasexplainedabovetodeterminethebehaviorofthetwodifferentialrelaysduringthereactorin-rush.

Figure9showsaCOMTRADErecordofthereactorin-rushthatwasobservedduringenergizationcapturedbythebackuprelay.Equalphaseshavebeensuperimposedwitheachotherinordertoshowtheirangleseparation.

Figure9:ReactorIn-RushCurrentsfromBackupRelay

Ascanbeseen,thewaveformsignatureandphasevectorsindicatethateachequalphaseis180degreesfromeachother.Forexample,thehighsideandlowsideofphaseAare179.7degreesapart,indicatedbychannels1and5respectively.TheBphasesare135degreesapartandtheCphasesare179degreesapart.Aninternalfaultisdeclaredwhentheanglebetweenthecommonphasesarelessthan90degrees.Itisclearthatthereisnointernalfaultbasedonthewaveformanalysis.However,thebackuprelaydeclaresadifferentialoperationontheBPhase.Thisresultsintrippingthelinebreakersand345kVlineoutofservice.Theprimaryrelaydoesnotseeadifferentialoperationduringtheenergization.

Let’sevaluatetheperformanceofeachrelaybylookingintotheoperate,restraint,and2ndharmonicblockingfunctionsbyusingtheformulasoutlineintheprotectionalgorithmssection.

Inordertocalculatetheoperateorcurrentdifferentialofthisrelay,onemustfirstfilterthecurrentsusingaFastFourierorCosineFilter.Thisisdoneinordertoextractallharmoniccomponentsfromthewaveformexceptthe60Hzsignal.Relaysoperateonlyonthefundamentalsignalforallprotectionfunctions.UsingWavewin,wecaneasilyfilterthefundamentalsignalforouranalysis.

1. Takethefundamentalofeachphasecurrent.2. Calculatetheoperatecurrentforeachphasedifferential.

IOPA = |IAT + IAW|IOPB = |IBT + IBW|IOPC = |ICT + ICW|

3. TaketheRMSofeachoperatecurrent.

SincetheBphaseisthecurrentthatoperated,theIOPBwascalculatedfirst.Figures10and11showtheIOPBoriginallygivenbytheprimaryandbackuprelayrecordsalongwiththeIOPBcalculatedbyWavewin.NoticethattheoriginalIOPBwaveformsareslightlydifferentfromtheonescalculatedbyWavewin.Thisisbecauseofthesamplingrateofeachrelay.Inaddition,thebackuprelaycutsoffitsmeasurementwhenthecurrentsvaluefallsbelow0.1A.Anyvaluebelow0.1Aisnottakenintoaccountinprotectionfunctions.Nevertheless,theWavewin-calculatedvaluessimulatetheoriginalrelaysignalsverywell.

Figure10:IOPBCurrentDifferentialforPrimaryRelay

Figure11:IOPBCurrentDifferentialforBackupRelay

Theminimumoperatecurrentofbothrelaysaresettotripata0.5Asecondary.BasedontheinformationseeninFigures10and11,bothrelaysIOPBreachedbeyondthesettingpointof0.5Asecondary.Theprimaryrelayshowsavalueof0.51ampsandthebackuprelayshowsavalueof0.45ampsor0.095p.u.Thesesmalldiscrepanciesaremostlikelyduetothesamplingrateoftherelaydonefortheprotectionfunctions.

Thisprovesthatthecurrentdifferentialleveldidgoabovetheoperatesetting.However,therelayshavetocheckthe2ndharmoniccontentofthewaveformbeforeitdeclaresaninternaltrip.Thiswasexplainedintheprotectionalgorithmsection.Inasimilarmanner,wearegoingtouseWavewintochecktheharmoniccontentofeachdifferentialcurrentforeachrelay.

1. Takethefundamentalofeachphasecurrent.2. Calculatetheoperatecurrentforeachphasedifferential.

IOPA = |IAT + IAW|IOPB = |IBT + IBW|IOPC = |ICT + ICW|

3. Extractthe2ndharmonicsignalofeachunfilteredoperatephasedifferential.

Figures12and13belowshowtheratioofthesecondharmoniccontentascomparedtothefundamentalIOPBsignal.Figure12showstheratiocalculatedbytherelayandWavewinshownasIHB2

andthe2ndHarmonicratiofortheprimaryrelayshownasIB.Figure13showstheharmonicspectrumshowingthesameresult.Itcanbeseenthattheharmonicratioshowninbothsignalsisaround50%whichisabovetherelaysettingof10%.Sincetheharmonicratioisabove10%,thedifferentialelementwillbeblockedfortheprimaryrelay.Inaddition,sincethisrelayusesharmoniccrossblocking,allthreephasedifferentialfunctionswillbeblocked.

Figure12:PrimaryRelayRatioof2ndHarmonicandFundamentalforIOPB

Figure13:PrimaryRelayHarmonicContentofUnfilteredIOPB

Figures14showsthe2ndharmonicratioascomparedtothefundamentalcalculatedbythebackuprelayandWavewin.Basedonthewaveform,wecanobservethattheratioisabout50%whichissimilartowhattheprimaryrelaycalculated.Figure15showstheharmonicspectrumshowingthesameresult.

Figure14:BackupRelayRatioof2ndHarmonicandFundamentalforIOPB

Figure15:PrimaryRelayHarmonicContentofUnfilteredIOPB

Basedonthisanalysis,theBphasedifferentialproducedenoughharmoniccontenttoblockthedifferentialfunctionfromoperating.Sowhydidthedifferentialfunctionstilloperate?Let’slookattheothertwophases’2ndharmoniccontentandseewhattherelaycalculated.

Figures16and17showtheharmoniccontentoftheAphasedifferential.Noticethatthe2ndharmoniccontentcalculatedbytherelayiszero.Ourcalculationshowsacontentof29%.ThechallengewiththissignalisthatthehighandlowsidesofphaseAarealmostidentical.Sowhenyouaddthetwosignalstogetthedifferentialmagnitude,theresultproducesalmostnocurrent.Asaresult,thereisno2ndharmonicsignaltoextract.Inaddition,theactualcurrentvaluesaresosmallthattheygobelowthethresholdcutoffoftherelay.Atthatpoint,therelayinterpretsthatthereisnocurrenttobemeasured.

Figure16:PrimaryRelayRatioof2ndHarmonicandFundamentalforIOPA

Figure17:BackupRelayRatioof2ndHarmonicandFundamentalforIOPB

Figures18and19Error!Referencesourcenotfound.showthe2ndharmoniccontentoftheCPhase.Themeasuredvaluecontinuestobeachallenge,butthesignaturereflectsamuchcleanershapethantheBphase.The2ndharmonicvaluecalculatedbytherelayiszeropercentsincethemeasuredvalueswerebelowthe0.1Athreshold.Ourcalculatedvaluegivesa2ndharmoniccontentofover100%.

Figure18:PrimaryRelayRatioof2ndHarmonicandFundamentalforIOPC

Figure19:BackupRelayRatioof2ndHarmonicandFundamentalforIOPC

Thesecondharmonicanalysisofallthreephasesrevealedthattherewasenoughsecondharmoniccontentinallthreephases.However,theAandCphases2ndharmoniccontentsdefaultedtozeroduetotheminimummeasurementcutoffoftherelays.ThatlefttheBphaseastheonlycurrentwithenough2ndharmoniccontenttoblockthedifferentialelementonbothrelays.Sincetheprimaryrelayusesharmoniccrossblocking,therelayonlyneedstoseeonephasetoblockthedifferential.However,thebackuprelayneedsatleasttwophasestoblockthedifferential.Asaresult,thebackuprelaytrippedondifferential.

CTTconductedin-rushtestingoftheoriginalrecordbychangingthefollowingsettings:• AdaptiveBlockingModefor2outof3• AdaptiveBlockingModeperphase• TraditionalblockingMode2outof3• TraditionalblockingModeAverage

Table4showstheresultsforthein-rushperformedbyCTT.Eachtimethetestwassettotwooutofthreephasesforharmonicblocking,therelaytrippedduringthein-rush.

TestDescription TripAdaptiveBlockingModefor2outof3 YAdaptiveBlockingModeperphase NTraditionalblockingMode2outof3 YTraditionalblockingModeAverage N

Table4:In-rushTestingResults

Basedonthetestsaboveandharmonicanalysisperformedforthisevent,CTTdecidedtoimplementtheperphasemethodusingthetraditionalsecondharmonicsetting.

VII. PRIMARYANDBACKUPRELAYDIFFERENTIALPLANES

Figures20and21belowshowthedifferentialbehaviorduringthein-rusheventfortheprimaryandbackuprelaysrespectively.Eachmanufacturerhasitsownwayofdeterminingitsoperateandrestraintcurrents.Referringbacktoequations2and4,wecanseethatthereisamajordifferenceinhowtherelayscalculateeachphase’srestraintcurrentvalues.Theprimaryrelayaddsthehighandlowsidereactorphasorcurrentstogetherwhereasthebackuprelayuseswhichevervalueishigherinmagnitude.

Thisresultsintheprimaryrelayrestrainingitsdifferentialcurrentsonamuchlargerscaleascomparedtothebackuprelay.Theincreasedrestraintpresentintheprimaryrelayallowsittooperatecorrectlyduringanin-rusheventsuchasreactorenergization.Thisisthefundamentaldifferencebetweenthetwoalgorithmsthatmakestheprimaryrelaymoresecureandreliable.Ontheotherhand,thebackuprelayisfasterandmoresensitiveduringinternalfaults,butisalsopronetomisoperationsduetothelowerrestraintquantities.Itisveryclearthatthebackuprelayoperatedduringtheenergization.

Figure20:2ndHarmonicDifferentialPlaneforPrimaryRelay

Figure21:2ndHarmonicDifferentialPlaneforBackupRelay

VIII. RESONANCEEFFECT

Analyzingfigure22belowshowsthatthetransmissionlinecurrentsextinguishedapproximately5cyclesafterthereactordifferentialrelaytripped.However,thereactorcontinuedmeasuringcurrentsonthehighandlowsides.BasedontheSER,thecircuitswitcherremainedclosedforapproximately30cycles

afterthereactordifferentialtrip.Thesourceofthecurrentsbeingmeasuredbythereactorrelaysisfromthedischargeenergyfromthe345kVline.Thisenergyisbuiltintothelinecapacitancepropertiesforlonglines.Basedonthewaveformanalysisinfigure23,thereseemstobearesonanceeffectbetweenthelinecapacitanceandthereactorreactanceoscillatingaround47Hz.Thevoltagesandcurrentsstarttooscillateat47Hz,producingnon-sinusoidalsignalswhichmakethisshort-termsystemhighlyunbalanced.ThevoltagelevelsonCPhaseforalinetogroundvaluereachashighas326KV.Thisisalmostashighasthephasetophasevalueof345kV.

Figure22:RecordShowingT-LineOpened5cyclesafterReactorTrip

Figure23:ResonanceEffectduetoLineDischarge

Theresonanceeffectalsoaffectedtheinstantaneousandtimeovercurrentsettingsassertingatripsignal.Thegroundovercurrentwaveformisshowninfigure24.Theinstantaneousground50Gelement

inboththeprimaryandthebackuppickedup20cyclesaftertheinitialdifferentialtrip.Asaresult,bothrelayssentsignalstoopenbothlinebreakers,lockingthelineout.However,thelineswerealreadyopenduetothereactordifferentiallockout.Thispresentsachallengeonreclosingforregularlinetogroundfaulteventssincethecircuitswitcherwillnotopenthecurrentsafter30cycles.Thelinewillbelockedoutbeforethereclosingattemptisperformed.Itisimportantthatthegroundovercurrentelementsdonotpickupforthelinedischargecurrentandresonanceeffectwhenthelineistrippedduringnormallinetogroundfaults.

Figure24:ResonanceEffectonGroundOvercurrentElements

IX. CONCLUSION

Sincedifferentialrelaysoffersolutionsformultipleapplications,onecanconcludethattherelayengineermustdeeplyunderstandnotonlytheelementbehavior,butalsohoweachrelaycalculatesitsprotectionfunctionsforthegivenapplication.Thispaperhasdescribedthebehaviorofareactorenergizationandtheresponseoftwodifferentdifferentialrelays.Inaddition,thispaperhasprovidedinformationthatequipsthecustomerandsettingsengineerwiththenecessaryinformationtoproperlyavoidoperationsduringin-rushconditions.

X. REFERENCES

[1] Behrendt,K.;Fischer,N.;Labuschagne,C.,“ConsiderationsforUsingHarmonicBlockingandHarmonicRestraintTechniquesonTransformerRelays,”SchweitzerEngineeringLaboratoriesInc.,2006.

[2] GEURSeriesInstructionManualGEK-113628A,“T60TransformerProtectionSystem,”Markham,

Ontario,Canada,2015.[3] Hunt,R.;Schaefer,J.;Bentert,B.,"PracticalExperienceinSettingTransformerDifferentialInrush

Restraint,"61stAnnualConferenceforProtectiveRelayEngineers,pp.118-141,1-3April2008.

[4] Nashawati,E;Fischer,N.;Le,B.;Taylor,D.,“ImpactsofShuntTractorsonTransmissionLine

Protection,”38thAnnualWesternProtectiveRelayConference,October2011.[5] SEL-487E-3,-4RelayInstructionManual20150626,“CurrentDifferentialandVoltageProtection,”

SchweitzerEngineeringLaboratoriesInc.,2011-2015.

XI. BIOGRAPHY

EricSchroederjoinedCrossTexasTransmissioninJanuary2013andhasmorethan20yearsofexperiencemanagingelectrictransmissionutilitiesandconsultinginthepowerdeliveryindustry.AtCrossTexasTransmission,Ericisresponsibleformanagingbothfieldoperationsandcontrolcenteroperations.PriortojoiningCrossTexasTransmission,EricwasanexecutivetransmissionmanageratTexasMunicipalPowerAgency,overseeingtheelectrictransmissionbusiness.Priortothat,hewasaprojectengineeratPOWEREngineers,aglobalconsultingengineeringfirm.Ericalsohasownedhisownbusinessintheenergyindustryandisaregularspeakeratenergyandutilityconferences.EricholdsaBachelorofScienceinelectricalengineeringfromtheUniversityofTulsa.

JerryBurtonjoinedCrossTexasTransmissioninNovember2013andhasover18yearsofexperienceintheelectricalfieldonprojectsintheresidential,commercial,oilandgas,processandgeneration/transmissionindustries.Mr.Burtonhasfilledseveralpositionsfromapprenticetogeneralforeman,seniorrelaytechnicianandmostrecentlySubstationSuperintendent.Mr.Burtonhasawidevarietyofknowledgeasitpertainstorelaytesting,commissioning,preventivemaintenanceandsubstationconstruction.Mr.BurtoncurrentlyholdsaTexasDepartmentofLicensingandRegulationJourneymanElectricianlicenseandaSubstationJourneymanElectriciancertificatethroughtheUSDepartmentofLabor.

JoePerezreceivedhisB.S.degreeinElectricalEngineeringfromTexasA&MUniversityin2003.JoeistheauthorofmanyrelayapplicationnotesandhaspresentedtechnicalpapersatWPRC,TexasA&MandGeorgiaTechRelayConferences.JoeistheownerofSynchroGrid,aregisteredprofessionalengineerinthestateofTexasandamemberofPSRC,IEEE,andPES.JoeresidesintheBryan/CollegeStationarea.Hecanbecontactedatjperez@synchrogrid.com

LukeHankinsisfromCleveland,Texas.HegraduatedfromTexasA&MUniversitywithaBachelor’sofSciencedegreeinElectricalEngineering.HeiscurrentlyanE.I.T.andworkingforSynchroGridasaDesignEngineer.Inadditiontosubstationdesign,Lukeisinchargeofrelaysettingsverificationandmis-operationanalysis.HealsowritescodeinC++andVBAthataidincompanyoperation,automatingtasksandimprovingefficiency.

XII. ACKNOWLEDGEMENT

TheauthorswouldliketothankHaleyTriburforhercontributionsinmakingthispaperpossible.