Distance protection zone 3 misoperation during system wide cascading events: The problem and a survey of solutions

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Electric Power Systems Research

jou rn al h om e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / e p s r

Distance protection zone 3 misoperation during system wide cascading events: The problem and a survey of solutions

A.M. Abdullah

^a,∗

, K. Butler-Purry

^b

aDepartmentofElectricalPowerandMachines,CairoUniversity,Gizah12613,Egypt

bDepartmentofElectricalandComputerEngineering,TexasA&MUniversity,CollegeStation,TX77843,UnitedStates

a r t i c l e i n f o

Articlehistory:

Received10May2017

Receivedinrevisedform20August2017 Accepted21August2017

Keywords:

Distanceprotectionzone3 Blackouts

Cascadingevents Misoperation Security

a b s t r a c t

Distancerelayzone3misoperationhasbeenresponsibleformajorblackoutsaroundtheworld.Zone 3misoperationgenerallyoccursundersystemwidecascadingeventssuchasthe2003Northeastern US–Canadablackoutorunderstressedsystemconditionssuchasthe2015Turkishblackout.Thispaper explainstheproblemofzone3distanceprotectionmisoperation.Thepaperthenproceedstosurveythe literatureforpossiblesolutionstoincreasedistancerelaysecuritytopreventdistanceprotectionmisoper- ation.Threecategoriesofsolutionswereproposedinliteraturetoaddresstheproblemofzone3distance protectionmisoperation.Theﬁrstoneisanticipationandpreventionofmisoperationintheplanning stage.Thesecondoneiscommunicationassistedprotectionschemesthatuseremotemeasurementsto enhancerelaysecurity.Thelastoneuseslocaldatatoenhancedistancerelaysecurity.

1. Introduction

WiththederegulatedmarketstructureintheUnitedStatesand Europe,gridoperatorsareundermorepressuretoreapmoreprof- itsofexistinginfrastructureduetoincreasedcompetition.Thegrid isthusincreasinglyoperatednearthethresholdofstability.Failure ofthegrid,betterknownasblackouts,carriescatastrophic eco- nomicandsocietalsequences.Largeblackoutstendtobedueto eitherextreme naturaleventssuchas hurricanesor a seriesof eventscalledcascadingfailures[1].Inthis paper,weonlyfocus oncascadingevents.Thoseeventscanbeanyofthefollowing:

linetripping,overloadingofotherlines,malfunctionsofprotection systems,poweroscillationsandvoltageinstability[2].Thereason thatisconsideredinthispaperisdistanceprotectionmisoperation whichisacontributingfactorinseventypercentofallcascading events[3].Ifnotdiscoveredandmitigatedinanearlystage,cas- cadingeventsgenerallyleadtoacompleteblackout.Withtoday’s societymuchdependenceonelectricityasaformofenergy,pre- ventingsuchdamageisofhighimportance.

Cascadingfailuresaredeﬁnedas“asequenceofdependentfail- uresofindividualcomponentsthatsuccessivelyweakenthepower system”[4].Sincethe2003US–Canadablackout,cascadingevents

∗Correspondingauthor.

E-mailaddresses:[email protected](A.M.Abdullah), [email protected](K.Butler-Purry).

havedrawnmuchattentionintheindustrialandacademiccommu- nity.Eventhoughtheworldhaswitnessedmanyblackoutspriorto the2003blackout[1],thedramaticcausesandconsequencesofthe 2003blackouthaveleftindustrialandacademiccommunitywith theburdenofexploringthisphenomenoninmoredetail.Tounder- standtheseverityofthe2003blackout[5],itsufﬁcienttosayithad causedthelossof62GWwhichcausedthelightstoturnoffformore than51millionpeopleintheeasterninterconnection.Considering themanycomponentsandthebitsandpiecesinvolved,adomino effectofeventsevolvedslowly(hours)orfast(seconds)according totheregioncausingadegradationoftheintegrityofthesystem leadingultimatelytoacompleteblackout.Themainreasonofthe 2003blackoutwasdistancerelaymisoperation.Dauntingefforts hadtobeexertedtogainmoreknowledgeandunderstandingof theunderlyingphenomenon.

Relaysbydesignactquicklytoremovethefaultfromthesys- tembydisconnectingfaultedlines.However,sometimesrelaysfail toperformsuchfunctionwhichisconsideredaprotectionsystem misoperation.Ofallprotectionsystemmisoperationsthatleadto cascadingevents,thispaperfocusesexclusivelyondistancepro- tectionmisoperation.Aprotectionsystemmisoperationisdeﬁned as“afailuretooperateasintendedforprotectionpurposes”[6].

Variouscategoriesaregivenformisoperationin[6].However,in thispaperthewordmisoperationwillbeusedexclusivelytomean onlyoneofthem,namely,anoperationinwhichaprotectionsys- temtripsahealthylineduetoheavyloadingwhennofaultexists.

Inotherwords,othercausesofdistanceprotectionmisoperation

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Asmentionedin[2],oneoftheeffectivewaystopreventcas- cadingeventsistospecifypotentialundesirablerelayoperations aheadoftime.Inthispaper,weshowthateventhoughdistance protectionmisoperationcanbeanticipatedaheadoftime,preven- tionofthismisoperationisnotpossiblewithdistanceprotection principleonlybecausethedistanceprotectionprincipleisnotable tobeselectiveinsomeregionsofitsoperation.

Thepaperisorganizedasfollows:Section2providesasample distancerelaythatissetaccordingtoNERCstandards.Oncethis relayissetaccordingtoNERCdirectives,itwillbeexplainedin thesamesectionthattherelaymaystillmisoperateundervarious operatingconditions.Anticipationanddetectionofdistancerelay misoperationintheplanningstageisdescribedSection3.Section4 providesanoverviewofthecommunicationassistedschemesthat havebeenproposedtoeliminatethedistanceprotectionmisoper- ation.Lastly,Section5offersasurveyonthemethodsthatwere suggestedinliteraturetoenhancethedistanceprotectionsecurity usinglocaldataonly.

2. Thedistanceprotectionmisoperationproblem

OnAugust14thof2003[5],theUSeasterninterconnectionsuf- feredoneofthelargestblackoutsintherecentUShistory.Three 345kVtransmissionlinessaggedintountrimmedtreesduringthe hotsummerdays.Thetrippingofthoselinescausedanother345kV transmissionlinetocarrysubstantialsystemload.Theheavyload- ingofthis lastlinecoupledwithrelativelylowsystemvoltage, causedthedistancerelaytoconfuseaheavyloadingsituationfor unclearedzone3faultastheimpedanceenteredthethirdzoneof protectionwhichinturnresultedintrippingoftheheavilyloaded line.Thetrippingofthehealthyyetheavilyloadedlineworsened systemconditionsleadingtoachainofeventsthatultimatelyled tosystemcollapse.Also,onMarch31stof2015[9],theTurkish gridsufferedtheworstblackouteverrecordedsince1999whenan earthquakecausedacompleteshutdownofthegrid.Onthecon- trarytothe1999earthquake,the2015blackoutwascausedbya impedanceprotectionmisoperationthattrippedaheavilyloaded lineonthe400kVtransmissionleveleventhoughtherewasno faultonthetrippedlineoranywhereinthetransmissionnetwork.

Ascanbeseenfromtheexamplesin[5,9]inwhichdistancepro- tectionmisoperationhavebeenthemaincauseoftheblackouts orin[10]in whichdistanceprotectionmisoperationhavebeen studiedintheIEEE118bussystem,adistanceprotectionmisop- erationischaracterizedbyadistanceprotectionsystemseeinga heavyloadonalineasafault.Thisconfusionarisesfromthefact thattheimpedancemeasuredbytheimpedanceprotectionsystem coincideswiththatofafault.Thereasonfortheheavyloadcanbe duetoloadshiftingafterafaultasinthe2003US–Canadablackout [5]orduetolinesoutofserviceformaintenancecausingoneline tocarrysubstantialsystempowertransferasinthe2015Turkish blackout[9]orduetoanyunforeseenreasons.

Toillustratethatthisconfusionisnottiedupwithcertainsystem conditionsbutratherinherentinsecurityinthedistanceprotection principle,thesinglelinediagramshowninFig.1isusedtofor- mulatetheproblemingeneralterms.Itwillbeshownbelowthat

theimpedanceofadjacentlineswithoutanyinformationaboutthe systemloaduntilthecoordinationstudyphase.Inthecoordina- tionstudyphase,thetransmissionlineownerchecksallsettings againstapplicablestandards.Thisisexplainedindetailin[11].In thefollowingparagraphs,wewillsetuptherelaysettingsﬁrstthen discusswhathappensinsystemwidecascadingevents.

InFig.1,thedistanceprotectionrelaythatwillbestudiedisthe relayatpointAoflineA–C.LineA–Cisconnectedtothree(3)lines, namelyC–M,C–NandC–P.Thenumberoflinesconnectedtoline A–Cwillnotaffectzone1orzone2settingsbutwillaffectzone3 settings.Aswillbeseenbelow,trippinginzone3becomesmore insecurewithmorelinesconnectedtolineA–Caszone3reach becomeslarger.Tosimplifytheanalysis,alllinesareassumedto havethesameimpedanceaswellastheshortcircuitlevel.How- ever,aswillbeexplainedbelow,thissimpliﬁcationdoesnotaffect thegeneralityoftheproblemformulation.Theimpedanceandthe ratingofthelinesare60and 3000Amp, respectivelyandare takenfrom[12].Thesettingofzone1 isassumedtobe0.85of thelineimpedance.Zone2settingisassumedtobe1.2oftheline impedance.However,someconsiderationisneededtosetthethird zone.Thethirdzonehastobesetsuchthatitcanprotectthelongest adjacentline(assumedtobelineC–Pinthiscase)andtoprotect 20%beyondthatlinetoprovidebackuptotheremotecircuitbreak- ers.IncaseofaboltedthreephasefaultonlineC–Pandassuming thattheshortcircuitcontributionsofallbusesisgiveninFig.1by I_indexwhereindexisthebusname(beingM,N,AorP),thevoltage atdistanceprotectionsystematAcanbewrittenasgiveninEq.(1).

V_A=I_A×Z_A+Z_P×(I_M+I_A+I_N) (1) TheimpedancethatisseenbytherelayAcanthenbewritten asin(2)

ZR=V_A

I_A =ZA+ZP

1+I_M+I_N I_A

(2) Eq.(2)willonlybeapplicabletofaultsonlineC–P,ifweneedto include20%forthelinethatisbeyondbusP,thentheimpedanceZ_P in(2)hasbereplacedby1.2×ZP.Usingthedatain[12]andassum- ingalllinesareidenticalaswellastheirshortcircuitcontribution, thenthesettingofzone3willbeZ_A+3.6×Z_P=4.6×Z_A.Thethree zonesareplottedinFig.2.

Aftersettinguptherelaylocally,applicablestandardsanddirec- tivesneedtobeappliedtothesettingsforcompliancepurposes.

Thisstepinvolvesrunningworstcasepowerﬂowinthesummer peakcase.Themostnotabledirectiveistheloadencroachment.

Theloadencroachmentzoneisanareaoftheprotectionzonein whichtheloadimpedance“encroaches”–intrudes–uponthefault impedance.Loadencroachmentwillobviouslycausemisoperation andshouldberemovedfromthezoneofprotection[12].Toplotthe loadencroachmentzoneaccordingtoNERCdirectives[6,12,13],the loadzoneshouldincludethepointwhichcorrespondsto150%line loadingand0.85perunitvoltage.Thustheloadencroachmentlocus ofthedistancerelayatAwillconsistoftwoparts.Theﬁrstpartwill beanarcofcircleofradiusgiveninEq.(3)whichisgivenasarc RITinFig.1.ThisarcRITcorrespondstotheleastimpedancethat therelayshouldnotissueatripcommandfor.Theothercharacter- isticloadlineswillbetwolinesmakinganangleof±30^◦withthe

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Fig.1. Systemconﬁgurationforformulatingtheproblem.

Fig.2. DistanceprotectioncharacteristicforsysteminFig.1.

x-axis(±30^◦ representsthepowerfactorofthelineunderworst caseloadingcondition)

Z_load= VA

IA =

345kV√ 3 ×0.85

1.5×3000 =57 (3)

TheorangehashedareaRLQTistheloadencroachmentarea.

NERCdirectives[6,12,13]statesthatthisloadencroachmentzone hastoberemovedfromtherelayoperatingzone.Itcanbeseenat oncethatiftheimpedanceseenbytherelaylieswithinthesolid greenareaURIthenarelaymayconfusethisoperatingpointfor afaultsincethepointliesalreadyinzone3.Thisconfusionarises ifthefaultresistanceishighenoughtocausethefaultpointto liewithinthesolidgreenareaURI.InFig.2,thisfaultresistance rangesfrom30to90.Thefaultresistancecanbeobtainedby measuringthedistancebetweenthediameterABofzone3circle topointRandUofthegreenzone.Ifitisknowninadvancethat thefaultresistancecalculatedcannotbeattainedalongtherouteof thetransmissionline,thentheriskofmisoperationisnonexistence andthegreenareacanberemovedfromzone3withoutaffecting thesecurityorreliabilityofthedistanceprotectionsystem.How- ever,faultresistancealongtherouteofthetransmissionlineisnot knowninadvance.Also,oneshouldnotethatsolidstateandelec- tromechanicalrelayscannotbeprogrammed,onlymicroprocessor

relayscan.Thisreallymeansthatolderrelayswillhavetocomply withNERCstandardsbydisablingzone3altogether.Itisshown in[14]thatdisablingzone3insomecaseswillforceprotection engineerstoprovidebackupprotectionsolutionstoremotecircuit breakers.Thismightinvolveaconsiderablecost.Additionally,not allcountriesaroundtheworldhavestandardsasstrictasNERC, sothesolidgreenareaURIexistinthezoneofprotectionwithout regardfromtheprotectionengineer.Lastly,eveniftherelaycom- plieswithNERCdirectives,animpedancecanstillfallanywherein zone3,notonlythesolidgreenareaURI,understressedsystem conditionsasPhadkeandHorowitzpointedoutintheirpaper[14].

Thisshowsclearlytheimpedanceprotectionisinherentlyinsecure understressedsystemconditions.

Attentionisnowgiventotheassumptionsstatedinthebegin- ning.Itwasassumedthatalllineshavethesameimpedanceandall ofthemarecontributingequalcurrentstothefault.Itcanbeseenat oncefromFig.2thatthisassumptionisnotrestrictingthegeneral- ityoftheproblemformulation.Becauseinanycasethegreenarea URIwillexistdue totheloadproﬁleunderstressedconditions.

Additionally,thefaultcontributionofthetransmissionlineswill onlyaffectthediameterBAofzone3whichwillonlyaffectpoint U.PointUcorrespondtothemaxfaultresistance.Stateddiffer- ently,therewillalwaysbeanoverlapbetweenthedynamicrating ofthelineandzone3andtheshortcircuitlevelsfromnearbylines willonlyaffectthesizeoftheoverlap(areaURI)nottheoverlap itself.Lastly,toderiveEqs.(1)and(2),athreephasefaulthasbeen assumed.Thisisduetothefactthatlineoverloadisthreephase phenomenon.However,itshouldnotbehardtobeabletoenvision asinglelinetogroundfaultcausingthesameeffectifoneimpor- tantlineisoperatedwithonlyonephaseduetoalinetolinefault underheavyloadingconditions.

Itshouldbeapparentfromthedescriptionabovethatthemajor issuethatisfacedbytraditionalimpedanceprotectionsystemis that thesteadystateimpedance correspondingtoaheavy load iscoincidenttotheimpedanceunderafaultonaremotelineto whichthedistanceprotectionsystemprovidesbackupprotection.

Itcouldbearguedthatimpedanceprotectionshouldbesupervised byothersteadystateprotectionprinciplestoenhanceitsselec- tivity,i.e.,theabilitytodifferentiatebetweenaloadandafault current.However,otherprotectionprinciples–suchasovercur- rentprotection–thatcanmakethedistinctionbetweenaloadand afaultdependontheanticipatedpowerflowforoperationwhile theprobleminhandisdifferent.Theconfusionthatisseenbythe distanceprotectionisbecausethepowerflowundersystemwide cascadingfailureschangesconsiderablyfromtheplannedpower flow.Inotherwords,theloadencroachmentzonehastobesetfor systemwidecascadingscenariosthatarenotknowninadvance, whichisclosetoimpossibleundertaking.Oneoftheauthors[14]

statesthisfactas:

“TheoverwhelmingthrustoftheNERCrulesandotherinstructions regardingtheapplicationofzone3elementshasbeentoprevent itsoperationduringemergencyconditions.Althoughthisisadesir-

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Fig.3.Impedancemarginandimpedancelocusplotforadistancerelayduringa wideareacascadingscenario.

ablegoal,itshouldberecognizedthatevenwithalltheintelligence availabletomoderncomputerrelays,theproblemofdistinguish- ingafaultfromaheavyloadinarelativelyshorttimeandusing onlythecurrentandvoltagesignalsavailabletotherelaycannotbe solvedineverysingleimaginable(andsomeunimaginable)power systemscenarios”.

3. Detectionandmitigationofzone3misoperationinthe planningstage

MostIndependentSystemOperators(ISOs)[15,16]todayuseN- 1criteriontojudgewhetherthesystemissecureaftertheremoval ofonelineinplanningstage.However,giventhatzone3distance protectionmisoperationisnottriggereduntiltwoorthreelinesgo outofservice[5],thedistributionofpowerﬂowishardtobetaken intoaccountintheplanningstageasitwouldmeanperformingN-2 andN-3contingencyanalysiswhichisexpensivecomputationally.

Thus,itisincreasinglyhardtoassessprotectionsystemcapability underthemoststressfulsystemconditions.Duetothat,theauthors in[17,18]providedanefficientmethodtostudythesensitivityof zone3undervariousoperatingconditionswithouttheneedfor performinglargenumberof powerflow studies.In[17,18],the authorsdefinealinearizedimpedancemarginofadistancerelay usingsystemvoltages,injectedpowerandshuntsusceptance.The impedancemarginisdefinedasthedistancebetweenthemeasured impedancelocusintheR–Xplanetotheboundaryoftheoperat- ingcharacteristicoftherelayandisshowninFig.3.Bydoingso, therelaysthatmaymisoperatecanbeanticipatedaheadoftime inanefficientmannerintheplanningstageusingsimpleformulas.

However,itisshowninthepaperthatifthechangesinthevoltages orpowerinjectionsareelectricallyfarawayfromtherelayunder study,asituationthatalwaysexistsinwideareacascadingscenar- ios,theerrorintheanalysisbecomesunacceptableespeciallyfor longlines.

In[19],theauthorsproposeblockingzone3ofcertaindistance relaysaheadoftimebasedonofﬂinesimulations.Theauthorspro- posethatthesystemoperatorperformcontingencyanalysisusing crediblehistoricalcontingenciesinplanningstagetoobservethe impedancetrajectory ateach distance relayin thesystem.The contingencyscenariostobeperformedhavetoincludemorethan onecontingencytocreateanimpedancetrajectory.Therelaysthat misoperateduringeachcontingencyscenarioduetotheimpedance trajectoryenteringzone3whennofaultconditionsexistshould beshortlisted.Ofthoserelaysthatareshortlisted,certainrelays

cadesinrealtime.Anindexthatisusedtomonitordistancerelays isalsoproposed.Iftheindicesassociatedwiththedistancerelays exceedtheirthreshold,thendistanceprotectionmisoperationis abouttooccurand thesystemoperator hastheoptionto stop distancerelaysfromoperation.However,inpracticeonlyN-1con- tingenciesarestudiedandthustheseverityindexischosenbased onthesecases.Nevertheless,ifthesystementersN-2orN-3con- tingencyconditions,tuningofthe“severity”thresholdwillbea difﬁculttaskcomputationally.

Lastly,distanceprotectionco-ordinationhasbeenexploredin [21,22].An SVM for each relayis trained using theimpedance trajectorythat isseen bytherelayduringfault conditions.The impedancetrajectoryisobtainedbyusingatransientstabilitypro- gram.SVMisthentrainedforvariousscenariostodistinguishzone 1,zone2andzone3faults.Trainingscenariosincludecasesfordif- ferentfaulttypesatdifferentdistancesawayfromtherelaythathas theSVMandatdifferentfaultresistances.TestingscenariosforSVM includefaultsthathavenotusedintraining.Byensuringthatthe relaysarewellco-coordinatedundervariouscontingencycondi- tions,unnecessarytripcouldbeavoided.However,theimpedance trajectorywilldependonthesystemtopologyatthetimeoffault occurrenceandthishasnotbeentakenintoconsiderationin[21,22]

whichcouldleadtolargeofﬂinetrainingset.

Duetothefactthatdistanceprotectionco-ordinationrequires large amount of ofﬂine simulations by performing many contingency conditions, theauthors in[23] introduced theidea of

“distance of impact” to automate the distance protection co- ordination. However, the authors use the super computer to performtheircomputationsattheﬁrststage.Oncethedistance ofimpactofeachrelayhasbeencalculated,co-ordinationofthe distancerelaysbecomesstraightforward.Ashortcomingofthedis- tanceco-ordinationapproachisthateventhoughitcanensurethat distancerelaysneveroverreachforfaultsbeyondtheirreach,little researchhasbeenperformedtostudywhethertheco-ordination canensurethatrelayswillnotmisoperateunder heavyloading conditions.

Ascanbeseenabove,anticipationdetectionofdistancepro- tectionmisoperationintheplanningstageisa hardtask. Tobe able tofully prevent distanceprotection misoperation, an N-x, wherexisgreaterthan1,contingencyanalysisneedtobedone.

Eventhough this couldbedone for small benchmarksystems, contingencyanalysis,underuncertainloadandgeneration,iscom- putationallyintensiveforlargesystemconsistingofthousandsof buses.ManyISOsmaynothaveaccesstosupercomputerstorun suchintensivecomputations.

4. Communicationassistedschemes

The2003blackoutspointedouttheimportanceofhavingevents alongwiththetimetheyoccurred.Investigatorsspentmuchoftheir timetryingtomatchupthewaveformstoreconstructthesequence ofeventsthatledtotheblackout. Itwasthen apparentthatto facilitatethetransferandcomparisonofwaveforms,allsamples needtohaveatimestampforthispurpose.PhasorMeasurement Unit(PMU)orsynchrophasortechnologywasthenrecommended

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Fig.4. Protectionlogicofthepaperin[28].

toaddressthisshortcoming[5].Bythetimeof2003blackout,state estimationwasaverymatureﬁeld,butPMUsopenednewareasfor theapplicationofstateestimationbyreducingthetimeneededto dostateestimationduetowidespreaddeploymentinthetransmis- sionnetwork.APMUmeasuresthepositivesequencevoltageand current(boththemagnitudeandangle),whichopensnewareasfor adaptiverelayingandwideareacontrolandprotection.Intypical distanceprotectionschemes,therelayisonlyappliedtoasingle transmissionline.However,inawideareaprotectionscheme,a completearea(severaltransmissionlines)canbeprotectedusing selectedPMUdeviceswithouttheneedtoapplyPMUstoeachsin- glebusinthesystem.BytransferringinformationinbetweenPMUs inthepowersystem,accuratedecisionregardingthenatureofthe impedancefallinginzone3canbemadeandmisoperationcanbe avoided.

WiththeintroductionofPMUs,severalfaultdetectionandloca- tionsmethodshave beenproposed. Thesalient featureof PMU detectionschemes[24]isthatusingthecommunicationlinksto transferdatabetweenthetwoendsofthelines,severalconclusions canbedrawnregardingwhetherthelineisundergoingabnormal conditions.AsalientfeatureofthePMUalgorithmistheabilityto monitorthestatusofthelineandcomputetheparametersofthe lineonlineinaveryaccurateway.

Duetothefactthatzone3ofdistancerelaysmaynotbeable differentiatebetweenheavylineloadandanactualfaultonthe system,theauthorsin[25]proposedthatatoolbeinstalledatthe controlcentre,alsoknownasIndependentSystemOperator(ISO), tocontinuouslysupervisetheoperationofzone3elements.The toolwillconsistoftwomodules,acentralcontrolunit(CCU)and severalregionalcontrolunits(RCUs).TheCCUwillbeinstalledin theISOwhiletheRCUswillbeinstalledatselectindividualsub- stations.The CCUmeasuresthe lineoutagedistributionfactors andgenerationshiftfactorsfortheentirepowergridandsends allfactorstotheindividualRCUs.RCUsuselocalmeasurementsat thesubstationandcommunicatewithoneanother.Theindividual RCUswilldifferentiatebetweenfaultsandtransferofpowerﬂow andsupervisedistancerelaysbasedontheinformationreceived fromtheCCUandotherRCUs.Eventhoughthissolutionmightbe abletopreventallzone-3misoperations,itneedsconsiderablecost toconstructthecommunicationinfrastructurethatisneededto transmitthedatabetweentheindividualsubstationandtheISO.

Theauthorsin[26]usessynchronizedsamplesfrombothends ofthelinetocheckwhetherthetransmissionlinehasbeentripped successfullyornot.Theinstantaneouspowerfrombothlineends arecalculated.Itisshowninthepaperthatafterfaultinception,the

instantaneouspoweratbothendsbecomespositive.Thisholdtrue evenundersystemswithweekinfeed.Themethodisapplicableto allsystemsotherthanradialsystems.However,duetotheneedto transferdatabetweenbothends,signiﬁcantcostmaybeincurredto establishsuchalgorithmacrosseachtransmissionlineinthegrid.

Theapproachthathasbeenproposedin[26]hasbeenvalidated usingﬁelddatain[27].

In[28],distributedPMUsareusedtoreachadeﬁnitivedecision abouttheexistenceofthefaultinthesystemusingasynchropha- sorstateestimator(SynSE).TheSynSEisusedtodetectnetwork topologychangesaswellasdeterminethefaultlocationsasshown in Fig. 4. However, the grid has be completely observable by PMUs.Thus, sitingPMUshastobedonecarefullytonotcreate unobservableislandsunderstressedsystemconditionsandcertain contingencyconditions.

In[29]synchrophasorsareproposedtosupervisezone3oper- ation.Speciﬁcindicesareproposedtoassertthefaultusingthe currentswhichresultinaveryrobustzone3distanceprotection system.Thealgorithmformsasupernodefromthecertaingroups ofPMUs. Bysummingthecurrent goingintothesupernode, a decision canbereachedwhetheradisturbanceexists ornot.If adisturbanceisdetected,anotherlogicisinvokedtodetermine whetheritisafaultforthegroupofPMUswiththehigherdeviation.

Thelogicthatisinvokedafterdisturbancedetectionisimpedance based.Aweightedfaultdetectionindexisdeﬁnedforthispurpose.

Theweightsusedtodeﬁnetheindexdependonthereachofthe respectivezone 3distanceprotectionrelay.If afterdisturbance detection,theweightedfaultindexisexceeded,afaultinzone3 isdeclaredandtherelayisrestrainedfromoperation.However,to fullytakeadvantageofthemethod,strategicallylocatedPMUshave toexistinthesystemwhichmakestheapproachhighlydependent onthetopologyandanytransmissionsystemupgrades.Addition- ally,certaincontingencyconditionscancausesomefaultsnottobe detectedduetothelocationofthePMUs.

In[30],agentsareusedtoaidzone3relayelementswithout enforcing adecision. Inthis scheme,eachimpedance relaywill havethecapabilitytocommunicatewithotheragentsinthenet- workthatprotectthesametransmissionline. Ifthemajorityof theagentsinformsthelocaldistancerelaythattheimpedancein itszone3reachisactuallyduetoafault,thenthelocaldistance relayshouldbeenergizedandatripcommandhastobeissued.

Anoptimizationapproachissetupsuchthatthecommunication delaybetweenthevariousagentsarelessthan1second,whichis thetimeofoperationofzone3.Asimilarapproachhasbeenpro-

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willbeﬂaggedashavingafault.Nextcomesthetaskofidentify- ingthefaultedline.Fortheareathatisﬂaggedashavingafault,a distancequantitywillbecalculatedforeachlinewithinthearea, i.e.,eachlinewillbeassumedfaultedandadistancequantitywill becalculated.Ifthedistancequantityfallsbetween0to1then thefaultedlinewillbedetermined.Ifmorethanonelineisfound tobefaulted,anestimationofrelativeresidualerrorismadeand thelinewiththeminimumresidualerrorwillbeselectedasthe faultedline.Itisclearfromthisdescriptionthattwofaultswithin anyprotectedzoneorcrosscountryfaultswillbedetectedasone faultonly.Andeventhoughoneofthetwofaultsmayhavenot beencleared,thealgorithmmaynotbeabletodetectsuchsitua- tion.

In[33],abackupwideareaprotectionschemeisproposed.Short windowDFTisusedtoextractthephasorinformationfromthe threephasevoltagesandcurrents.Inthisscheme,theabsolutedif- ferenceofthebusanglesandcurrentsareusedtodetectthefault.

Theminimumvoltagemagnitudeestablishestheclosestbustothe faultandthemaximumcurrentangledifferencebetweenthebuses establishesthefaultedlines.However,forthemethodtowork,the minimumvoltagethresholdhastobeestablished.Itisshownin thepaperthatavoltagethresholdlessthan0.95meansthatthere isafaultonasystem.Thisimmediatelypointsoutthatincaseof voltageinstabilityconditions,themethodcanoperateerroneously.

In[34],anotherwideareaprotectionschemeisproposed.The solutionconsistsoftwocomponents:afaultelementidentiﬁcation (FEI)andafaultareadetection(FAD).TheFEIisusedtoidentifythe faultedelementinthezonebeingprotected.Afterthat,thefault isolationisrealizedbycoordinationamongareacircuitbreakers breakers.AspartoftheFEI,themeasuredvoltageandcurrentofone terminaloftheprotectedareaareusedtoestimatethevoltagesat theotherend.Ifaninternalfaultoccurswithinthezonebeingmon- itored,thentheestimatedvaluewillbedifferentfromthemeasured valueatthatbuscausingafaulttobedetected.Ontheotherhand, thefaultedareaisdetectedthroughFAD.Thesubstationsthatare withinthefaultedareaneedtosendtheinformationtothecentral controlroom.Afterwards,thecentralcontrolroomwillsearchthe suspectedfaultylinesandidentifyactualfaultylinesquickly.The useofFADconceptreducesthecommunicationoverheadrequired bythescheme.

Theauthorsin[35]usePMUs,thatareinplaceaspartofawide areaprotectionscheme,todetectthepowerflowtransferduetothe removaloffaultedlinefromservice.In[35],theloadflowtransfer toalinecanbecalculatedusingthenetworktopologyviathedis- tributionfactors.Ifthemeasuredpowerflowtransfersignificantly mismatchesthecalculatedpowerflowtransfer,thenafaultwillbe declared.Basedonthatdetection,zone3isadaptivelyadjustedto preventmisoperationwhicheliminatedistanceprotectionzone3 misoperation.

EventhoughPMUbasedschemesandwide-areabasedschemes canofferattractivesolutionstoeliminatetheproblemofdistance protectionmisoperation,thesesolutionsrequiresigniﬁcantcom- municationinfrastructurecost.Additionally,thepowergridis a criticalinfrastructureandthecybersecurityriskmaybeeminentif thegridisbroughtonlineforcommunicationspurposes.

usedtodeterminewhetherafaulthasoccurredwithinthereach oflocaldistancerelay.TheDCdecayingfaultcomponentisrecon- structedfromthemeasuredcurrentsinallthreephases.Sincethe faultisthreephase,aDCdecayingcomponentmustexistinone ofthephases.Atransientmonitoringfunctionwillthenbedefined tobethemaximumofallthreephasesDCdecayingcomponentsin onecycle.Also,forthethreephasetobeasserted,thelineloadangle hastobegreaterthan50degrees.Boththemonitoringfunctionas wellasthelineloadanglehastobetrueforathreephasefault tobedeclared.Thedrawbackofthismethodisthatthefaultmust havesignificantdecayingDCcomponentwhichmakesitchalleng- ingforcertainfaultincipientanglesandtransmissionlinelengths asasignificantdecayingDCcomponentcanonlyoccurwhenthe faultoccursatcertainincipientanglesandundercertainX/Rratios.

Additionally,thepaperassumestheanglebetweenthecurrentand thevoltageattherelaytobemorethan50degreesasafaultindi- cator.However,ithasbeenpointedoutin[14]thatunderstressed systemconditionstheanglemayindeedexceedthatthresholdwith nofaultonthesystem.Also,iftheswingfrequencyinthesystemis anticipatedtobelargerthan5Hz,thresholdselectionfortransient monitoringbecomesahardtaskwhichcouldmisleadtheschemeto confusepowersystemswingforthreephasefaults.Lastly,theeffect offaultresistancehasnotbeenstudiedinthepaper.Faultresistance canpotentiallycausetroublesettingthethresholdforthetransient monitoringfunctionasitaffectstheDCdecayingcomponent.

In[37]therateofchangeofvoltageisusedasatriprestraintto supervisedistanceprotectionmisoperation.Theideaisthatfault occurrenceorfaultclearingcausesthevoltageseenbytherelay tochangedrastically.Theauthorsproposeifthelocalrelaysenses thatthesystemvoltageisstressedaccordingtotheconditionslisted in[38],therateofvoltagechange(V/t)willbeusedtoassert whetherafaulthasoccurredandclearedwithinzone3protection usingtwo thresholds for both fault occurrenceand fault clear- ing.Additionally,theauthorsproposedtouseathermal loading monitoringfunctiontodecidewhetherthemaximumconductor temperaturehasbeenreached.Thetemperaturemonitoringfunc- tionwillstartoperationafterthefaulthasbeendetected.Ifthe temperaturemonitoringfunctiondeclaresthatthelinetempera- turehasreachedmaximumlimit,thelinewillbetrippedwhether thefaulthasbeenclearedornot.However,forthetripcommand tobesecure,largeamountofofﬂinesimulationsneedtobecarried outtoknowtheworstcaserateofchangeofvoltage.Themajor disadvantageofthemethodisthatundervoltageinstability,the (V/t)criterionisnotexclusivepropertyforfaultsaspointedin [39]sinceundervoltageinstability,asuddengeneratortripping couldcausethesame(V/t).

Sinceitishardtodistinguishbetweenevolvingpre-blackout event and short circuit conditions using magnitude of voltage, anothertrip restraint quantity, namely (I/V)has beenused alongwith(V/t)criteriontotripzone3securelyin[39].The (I/V)thresholdvaluehasalsobeenobtainedusingofﬂinesim- ulations.Thedisadvantageofthemethodin[39]isthat neither contingencyanalysisnorsystemloadinglevelshavebeentaken intoconsideration.

Theuseoffaultgeneratedhighfrequencycomponentshasalso beeninvestigatedinliterature[40].Suchusagecangiveamore

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Fig.5.Creationofadaptiveloadblinderin[45].

preciseanswerwhetherafaulthasoccurredthusmakingtripping inzone3moresecure.In[41],theenergyofthefirstthreecurrent levelsandthethirdapproximationisusedtotrainaprobabilis- ticclassifier.Theenergyiscalculatedusingcertainlevelsofthe discretewavelettransformofthethreephasecurrents.Usingthis energyfeatures,adecisionismadewhetheratransientsignalifdue toafaultornon-faultconditiononaline.Asimpleronlinetransient classifierhasbeenproposedin[42],wherethetransientevents occurringonthetransmissionlinesnearbyanydistancerelayare identified.Modaltransformationis usedtotransformthephase currentsintomodalcurrents.Thediscretewavelettransformcoef- ficientsoftheaerialmodes[43]arecombinedinacertainmanner totrainafeedforwardneuralnetworktoclassifythosetransients.

However,themajordrawbacksofboth[41,42]isthattheycan- notbeusedtotellwhetherthefaultiswithintherelayprotection reacheventhoughtheycantellwhetherafaulthasoccurredinthe network.Thesepaperscanonlybeusefulifamethodisfoundto usethetransientclassiﬁcationasanentrancepointtodetermining whetherthefaultiswithintheprotectionreachoftherelaybutthis hasnotbeendoneinliteraturetothebestofourknowledge.

Variousadaptivezone3settingswerealsoinvestigatedinliter- aturewherezone3reachisadjustedbasedonlocalinformation.In [44,45],ANNhasbeenusedtopredictthecorrectloadblinderunder varioussystemconditions.Theloadblinderisthencombinedwith thezone3settingstoblockanyundesirabletripping.Theloadblin- derisonlyactivatedunderbalancedconditionstomakesurethat theheavyloadingisnotconfusedforunbalancedfaultconditions.

Theloadblinderisdeterminedbasedonofﬂinesimulations.The simulationstakeintoaccounttheloadinglevelofthepowersys- tem,faultincipientangle,sourceimpedanceratio,faultresistance andfaultlocation.ThefeaturesusedtotrainANNistheactiveand reactivepower,voltageaswellastherateofchangeofboththe voltagesandcurrentsseenattherelayterminalasshowninFig.5.

Asimplelineconnectedtoanotherlinewithaloadin between isusedtotestthemethodwhichisadisadvantageofthemethod.

Also,theeffectofthesystemtopologywasn’tstudied.Itisalsofore- seenthattheinclusionforcontingencyanalysisinANNtrainingwill requirelargeofﬂinesimulationsandwillmakeloadblinderselec- tiondifﬁculttask.Additionally,onlythreephasefaultcurrentshas beenusedtotraintheANNandincaseasinglelinetogroundfault occursandcausethefeaturestobedifferentthantheonesusedin training,themethodisexpectedtofailbecauseANNisknownto havebadgeneralizationcapabilities[46].

In[47,48],theauthorsproposechangingzone3reachduring emergencyconditionstopreventdistanceprotectionmisoperation ifcertainsystemconditionsaremet.Ifthosesystemconditionsare met,zone3protectionreachwillbeshrunktozone2whichin turngivesmaximumsecurity.Duringstressedsystemconditions, theimpedanceseenbythelocalrelayapproachestheoriginalzone

Fig.6. TZMAandTZPAofthemethodin[47].

3border,whetherthisborder isMHOcharacteristicsorpolygo- nalone.Theauthorsproposetodefineafourthprotectionzone, calledthirdzoneproximityarea(TZPA),aswellasafifthprotection zone,calledthirdzonemodificationarea(TZMA),tochangezone 3protectionreachbasedoncertaincriteria.TheTZMAisaregion withinTZPAbutclosertozone3protectionzone.Theauthorspro- videdrulesonhowtosetbothTZMAandTZPA.Oncetheimpedance entersthisTZPAandcrosseszone3fastenough,thefaultwillbe declared.However,iftheimpedanceentersTZPAandstayswithin TZPA,theTZMAlogic willbeputintoaction. Zone3protection willbeshrunktozone2iftheimpedancecrossesTZMAwithina minute.Thisoneminuteisusedtoadjustforrestorativecorrective forcesofthegirdsuchasgeneratorexcitationcontrolsandtrans- formerloadtapchanger.Insummary,zone3protectionwillonlybe changediftheimpedancecrossesTZMAwithacertainratewithina specifiedtimedelayonceitstartschanginginTZPA.Theseideasare illustratedinFig.6.Although,theapproachisverypromising,afast evolvingsysteminstabilitycouldmisleadthescheme.Additionally, itcanbeseenthattheapproachproposedsacrificesdependability forsecurityasevidentbyshrinkingzone3;ifafaultoccurswithin theoriginalzone3protectionzonebeyondtheoriginalzone2reach afterzone3protectionzoneisshrunk,theapproachproposedby theauthorswillrelyheavilyontheclosestrelayandcircuitbreaker toclearthefault.However,ifthisrelayorcircuitbreakerfailsto clearthefault,thefaultwillgounclearedworseningthealready stressedsystemconditions.Lastly,theproposedalgorithmwillfail definitelyincasetheimpedancestayswithinTZPA,suddenlyjumps tooriginalzone3duetozone3faultbeforethetimedelayexpires thenstaysinzone3afterfaultclearingduetolineheavyloading conditions.

Lastly,theauthorsin[49]proposedtoidentifypowerﬂowover- loadbasedonanewlyproposedconcept.Thisconceptisbasedon thephasorrelationshipsinthecomplexphasorplane.Thepaper onlydifferentiatesbetweenthreephase faultsand overloadsas overloadsarethreephasebalancedphenomenon.Sincethepaper onlyaimstodistinguishthreephasefaultsfromoverload,anythree phase faultona transmissionlinecan beanalyzedwithoutthe needofinformationfromtheotherendofline.Thisisduetothe factthatthearcvoltageisnotaffectedbytheinfeedfaultcurrent makingthefaultpointgroundedthroughthearcresistance.This propertyisusedtoderivethecriteriontodifferentiatebetween

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Insummary,theuseofrateofchangeofvoltagesandcurrents canpreventtheproblemofdistanceprotectionmisoperation in mostoccasionsbutdoesnotfullypreventit.Othermethodsuse previousoperationalexperiencewithdistancerelaystopropose solutionstotheproblemofdistanceprotectionmisoperation.How- ever,systemconditionsand scenarios thatwerenot previously encounteredcouldmisleadthoseproposedsolutions.Theuseof highfrequencyfaultgeneratedtransientsseemstobeapromising areabutitisscarcelyresearchedinliterature.

6. Conclusion

The problem of distance protection misoperation has been presented.Variousapproachestosolvetheproblemhavebeensur- veyedandorganizedintothreemaincategories.Themethodsin eachcategoryhavebeenexplained.Additionally,theadvantages anddisadvantagesofeachmethodhavebeenpointedout.

Theﬁrstcategoryisanticipationofdistanceprotectionmisop- erationintheplanningstage.Inthiscategory,distanceprotection misoperationcouldbeanticipateddayaheadbasedonthefore- casted load and generation as well as contingency conditions.

However,duetothelargesizeofmoderndaypowersystems,such anticipationiscomputationallyintensive.

The second category is communication assisted protection schemes.Themainideainthiscategoryisthatusinginformation frombothendsofthelineorinformationfromvarioussubstations inthenetwork,ablockingcommandcanbeissuedtotheaffected distancerelay.Nevertheless,thesecommunicationassistedpro- tectionschemeshavenotfoundwideindustryacceptancedueto cybersecurityrisksaswellastheeconomiccostthatisrequiredto buildsuchsystems.

Thethirdcategoryismodiﬁcationofthelocaldistanceprotec- tionfunctionusinglocalinformation.Themainideainthisfamilyof solutionsisthatusingoperationalsystemexperienceaswellasthe localdata,onecantellwithcertaintythatadistancerelayisaboutto misoperate.Bydetectingsuchconditions,ablockingcommandcan beissuedtoguardtherelayagainstmisoperation.Nevertheless,the methodsinthislastcategorymayfailundersystemconditionsthat werenottakenintoconsiderationwhiledevelopingthesemethods.

Themost appropriate methodtomitigate zone 3misopera- tionisdependentonwhatisthemostimportantfactor forthe utilitycompany.Forexample,oneutilitycompanymayprefera communication-assistedschemeusingremote measurementsto eliminatethepossibilityfordistanceprotectionmisoperationwhile accepting the cybersecurity vulnerabilities that are introduced.

Anotherutilitymaynotbewillingtoacceptthecybersecurityrisks orthecostofconstructingsuchschemeandrequiresasolution usinglocalmeasurementsorminimumremotemeasurements.The authorsthinkthatmethodswhichuselocalrelaydataareworthy ofresearchattentionascybersecuritythreatsarebecomingamajor concern.

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