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November30.2017

RepurposingtheColstripTransmissionSystemAwhitepaperexplainingthetransmissionquestionsthatarisewhenconsidering

theretirementofcoalfiredgenerationfacilitiesandtheadditionofnewrenewableresourcesontheColstripTransmissionSystem.

ByCharlesA.Stigers–PrincipalPowerSystemsEngineer–USEConsulting

Commissionedby

Forward

ThiswhitepaperexplainsthetransmissionquestionsthatarisewhenconsideringtheretirementofcoalfiredgenerationfacilitiesandtheadditionofnewrenewableresourcesontheColstripTransmissionSystem(CTS).Thewhitepaperalsocontainsadiscussionofseveralpotentialmitigationstoaddressthesequestionsandtherangeofcostsassociatedwithmakingthistransition.Fourscenariosareexamined,eachwithadifferentamountofColstripGeneration(Units1through4)retiringandasimilaramountofwindenergyaddedtothesystem.

Nonewengineeringanalysiswasdonespecificallyforthepreparationofthiswhitepaper.Rather,thepaperisbasedprimarilyonmyknowledgeofhigh-voltagetransmissionsystemsandmyextensiveexperiencewiththeColstripTransmissionSystem(CTS),includingparticipationinnumerousothertransmissionstudiesofthatsystemoverthepast44years.IalsorelyonmydeepinvolvementinthedesignandimplementationoftheexistingRemedialActionScheme(RAS)atColstrip,whichisanimportantcomponentsurroundingmuchofthisdiscussion(seeAccelerationTrendRelay(ATR)innote3attheendofthispaper).

Severalengineeringstudiesthattouchonthistopichavebeencompletedbyvariousgroupsrecently.Whileeachofthesestudies(listedbelowinthispaper)endeavoredtoanswersomebasicquestionsaboutthefeasibilityofretiringthecoal-firedgenerationatColstripandaddingwindgeneration,noneofthemhaveansweredthebasicquestionoftheadequacyorfeasibilityofanyproposedRAS(seenote3)intendedtoproperlyreplacetheATR.ThepurposeoftheATRistoprotecttheMontanatransmissionfromcriticalcontingenciesontheCTS(seenote2).

Mostofthesequestionswillultimatelyhavetobeansweredthroughadditionaltransmissionstudiesandengineeringeffortswellbeyondthosethathavebeendonesofar.Thiswhitepaperidentifiesthisfuturestudyworkandalsosomeoftheotherfinancialandpolicyquestionsthatwillultimatelyhavetobedecidedbythevariouscommercialentitiesinvolved.

CharlesA.StigersPrincipalPowerSystemsEngineerUSEConsulting

NoteAbouttheAuthor:AsanemployeeofMontanaPowerCompany(MPC)andNorthwesternEnergyCorporation(NWE),forover38yearsChuckStigerswasdirectlyinvolvedintheengineeringdesignstudiesandtheplanningandoperationalstudiesoftheColstripTransmissionSystem(CTS).DuringthattimeDr.Stigersperformedavarietyofstudies1ontheCTSandwastaskedwithleadingtheteamthatdesignedtheRemedialActionSchemecurrentlyfunctioningatColstrip.Dr.StigersearnedhisMSandPhDdegreesinphysicsfromTheUniversityofArkansasin1968and1970.HeiscurrentlyemployedbyUtilitySystemEfficiencies,Inc.asaPrincipalPowerSystemsEngineer.Since2011hehascontinuedtoperformtransmissionstudywork.MuchofthisstudyworkinvolvedapplyinghisknowledgeoftheCTS,andworkingwithamodeloftheATRtoproperlyrepresentthatdeviceindynamicsimulations.

ThispaperwascommissionedbyRenewableNorthwest,anon-profitrenewableenergyadvocacyorganizationbasedinPortland,Oregon.

1Powerflowanalysis(steadystatenetworkperformance),Faultdutyanalysis,Dynamicstabilityanalysis(transientstabilityperformance),Sub-synchronousResonanceanalysis,TransientSwitchingAnalysis.,RASdesignanalysisandengineeringdesign.

ExecutiveSummary:RepurposingtheColstripTransmissionSystem

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ExecutiveSummary

ThispaperexplainsthetransmissionquestionsthatarisewhenconsideringtheretirementofcoalfiredgenerationfacilitiesatColstripandtheadditionofnewrenewableresourcesontheColstripTransmissionSystem(CTS).Severalpotentialmitigationstoaddressthesequestions,andtherangeofcostsassociatedwithmakingthistransition,arediscussed.Fourscenariosareexamined,eachwithadifferentamountofColstripGeneration(Units1through4)retiringandasimilaramountofwindenergyaddedtothesystem.

Scenario1:Colstrip1and2retire;610MWofwindenergyisaddedtotheCTS.

Underthisscenariothereshouldbenoimportantpowerflowissues(seeNote1)associatedwiththisfairlymodestchangeintheresourcesatColstrip.However,therewouldbeaneedtodesignaRemedialActionScheme(RAS)2capableofprovidingwindgeneratortrippingforcertaincriticaltransmissionoutagesontheCTStoassuretransientstabilityismaintained(seeNote2).Thedesignstudyforthis“WindRAS”shouldtakeabout3-6months.ItwouldbeessentialforthisRAStocoordinatewellwiththeAccelerationTrendRelay(ATR)—thecurrentRASfortheColstripUnits—andtheotherexistingRASprotectionschemesontheCTS.Toachievethiscoordinationbetweenthe“WindRAS”andtheATR,timingiscritical.The“WindRAS”wouldberequiredtotriptheappropriateamountofwindgenerationaftertheATRhasreacheditstripdecision,butbeforeitistoolatetoproperlyprotectthesystem.

The“WindRAS”wouldbesubjecttoreviewbythevariousWECCreliabilitycommittees(particularlytheRemedialActionSchemeReliabilitySubcommittee,orRASRS)thataretaskedwiththeprotectionofthereliabilityoftheWesternInterconnection.WhiletherearenosignificanttechnicalbarrierstodesigningsuchaRAS,oneshouldanticipatealengthyreviewprocess.ThisshouldbeexpectedtobeaverythoroughanddetailedreviewofthephysicaldesignoftheRAS,andcouldrequire1-2yearsafterthedesignstudyeffortiscomplete.

2RemedialActionScheme(RAS–alsosometimescalledanSpecialProtectionScheme(SPS)isaprotectionschemethatperformsoperationsonasysteminresponsetocertaineventsthatgobeyondtheactionsofsimplefaultprotectionthatisprovidedforeverytransmissionline.Simplefaultprotectiondetectsafaultanywhereonthelineandopensbreakersateitherendofthefaultedlineto“clear”thefault.ARASmaybedesignedtoinitiateactionssuchasgenerationtrippingorotherswitchingactionstopreventasystemfromcollapsingduetoaswitchingevent.EveryRAShasitsownuniquefeatures.


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Scenario2:Colstrip1,2and3retire;1,355MWofwindenergyisaddedtotheCTS.

BecausethisscenariorepresentstheretirementofamuchgreaterfractionofthetotalcapacityoftheColstripgeneration,oneshouldexpecttheneedformoreengineeringanalysisanddesign.TherangeofvariationoftheflowoverPath8(Montana-to-Northwest)wouldbemuchgreaterunderthisscenario.ThisincreasesthedemandforshuntVArresources(seenote1)tokeepthevoltagewellregulatedastheflowvaries.Thisvariancewillsomewhatdependonwhetheranyoftheregulatinggeneration(usedtocompensateforthevariablenatureofthewind)islocalorremote(westoftheCTS).3

AssumingthatthereisnolocalregulatinggenerationtherangeofvariationinflowontheCTSduesimplytothevariabilityofthewindpowerinMontanawouldbegreaterthan1,500MW(partofthiswouldbeduetovariationofotherwindprojectsthatarealreadyinplaceinMontanathatcanbeexpectedtobepartiallycorrelatedwiththe“new”1,355MWofwind.Besidesthisvariation,theloadineasternMontanacanbeexpectedtovaryoverarangeofabout300MW.ThishastheeffectofrequiringalargernumberofvariableVArresourcesthatareflexible(seenote1)tomaintainthevoltagesinMontanawithinanacceptablerange.UnderthisscenarioitisnecessarytostudytheMontanatransmissionsystemunderawiderrangeofoperation,withtheexpectationthatanylevelofwindgenerationcouldlastforasignificantperiodoftime,andmustthereforebetreatedasanormalsystemcondition.

Therequired“WindRAS”forthisscenariowouldtakeonacompletelydifferentcharacterfromthatforScenario1.ItwouldberequiredtooperateindependentoftheATRwhenColstripunit4isoff-lineformaintenance,sincetheATRcanonlyoperatewhenatleastoneoftheColstripunitsispresent.TherearetwooptionsfordesigningaRASforthisscenario:1)ContinuetooperatetheATRforthepurposeofmakingthetrippingdecisionoftheremainingColstripunit(number4);2)RetiretheATRanduseasingleRASdevicetoprovidetrippingbothforColstrip4andforthenewwindgenerators.

Scenario3:AllColstripunitsareretired;2,100MWofwindenergyisaddedtotheCTS.

ThisscenariowouldresultinthemaximumpossiblevariationoftheflowontheCTS.Forthisreasontherecouldbesomesteady-statesystemoperationaldifficulties.WiththeverylimitedamountofconventionalgenerationineasternMontanatherewilllikelybe

3Thispaperwillassumethattheregulatinggenerationisremote(worstcasechoice).AnenergystorageplantsuchasthatproposedfortheHarlowtonareawouldbeapossiblewaytobuildlocalregulatinggeneration.Also,agas-firedcombustionturbine(aero-derivative,notcombinedcycle)locatedintheBillingsareacouldalsobeusedforlocalregulatinggeneration.


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significantvoltageconcernsthatwillhavetobemitigatedwithresourcessuchasVoltageSourceConverters(VSCs,STATCOMS),StaticVArCompensators(SVCs),switchedshuntdevices,orothersuchVARsources.ThenecessaryRASunderthisscenariomaybesimplifiedinsomewaysbecauseitwon’tberequiredtocoordinatewiththeATR.ThisRAScouldbesimplifiedfurtherbyusingacontinuousactingVARsourcesuchasaVSCoranSVCtominimizetransientvoltagedeviationscausedbysomecontingencies.Providingreliableloadserviceunderextremecalmwindconditionswillalsobeaconcern,thoughtheCTScancertainlycarryinenoughpowerfromWestCoastmarkets,assumingthatpowerisavailable.Systemrestorationafteramajoroutagecouldalsobeproblematic.RestartingalargeACpowernetworkwhenmostoftheresourcesarewindpoweredmachineswithinvertersthatrelyonastablesystemwithawell-regulatedfrequencywillrequireastablevoltagesource.ConventionalgeneratorsineasternMontanacanprovidesomestart-up.NorthWesternEnergywillneedtoconducta“blackstartstudy”toconfirmthisisenough.

Scenario4:ThisscenarioconsidersanysystembenefitsfromdifferentlocationsofwindgeneratorswhenallColstripunitsareretiredand2,100MWofwindenergyisaddedtotheCTS.

WiththecontemplationofsuchalargeconcentrationofwindgenerationineasternMontana,utilizingthestrongtransmissionconnectionthattheCTScanprovidewouldbeverybeneficialformovingpoweracrossthearea,supportingtheloadwhenthewindisdown,andexportingthesurpluswhenthewindoutputisatmaximum.Fromatransmissionengineeringperspective,thereisnoobviousadvantagetomovingthewindmachinesortheirpointsofinterconnectionfartherawayfromtheCTS.AtfullcapacityeachoftheabovescenariosrequiresasignificantamountofpowertobeexportedfromtheMontanaareasinceitfarexceedstheindigenousload.TheCTSprovidesthebestcurrentlyavailablemeanstoexportthesurpluspowerfromeasternMontanaintothewesterninterconnection.

TherearelineoutageconditionsalongtheCTSthatcancausethermaloverloadingoncertainelementsoftheCTS(atmaximumflowconditions).ThemostimportantexampleofthisistheoutageofoneColstrip—Broadview500kVline.Undermaximumflowconditions(withtheexistingcoal-firedgenerators)theoutageofoneoftheselinescancausetheflowintheadjacentlinetoexceedthecurrentratingofitsseriescapacitorbank.Aseriescapacitortypicallyhasanemergencyrating,butflowmustbecurtailedbeforeacertaintimehaspassed.Theprotectiverelayingwillautomaticallybypassthecapacitoratthistimelimit.However,bypassingaseriescapacitormaynotbeadesirableoutcome,fromasystemsperspective,sincevoltageswoulddrop.Also,duringsuchanoutage,generationthatisconnectedatColstripisatsomeriskofbeingtripped.IngeneraltheCTSiscapableof


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operatingatornearitsmaximumcapacitywithanysinglelineoutofserviceonatemporarybasis(insteadystateoperation).Placingsomeofthewind-poweredgeneratorssothattheyareconnectedatBroadviewinsteadofColstripshouldmitigatetheissueofasingleColstrip—Broadview500kVlineoutageasdescribedabove.

Ingeneral,noneoftheabovescenariosposeaproblemthatisknowntobeinsurmountable,butallofthemrequiresomeamountofadditionalstudyworkandengineeringtodesignthenecessarysystemreinforcementstoachievecompletelyreliableoperations.Theadditionalstudiesthatneedtobecompletedinclude:1)examiningallsingleanddoublecontingenciesontheCTSwithpowerflow,post-transientpowerflowanddynamicstudies;2)RASdesignandapproval(1-3years)3)PathRatingapprovalthroughthestandardWECCprocess(1-2years).Areasonableexpectationfortheamountoftimeitwilltaketoconductthesestudiesandreceivethenecessaryregulatoryapprovalsis1-3yearsdependingupontheavailablemanpowertocompletethework.

RepurposingtheColstripTransmissionSystem

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Introduction

Thiswhitepaperisanefforttodelineateinbroadtermsthetransmissionissuesthatareinvolvedinretiringexistingcoal-firedgenerationatColstripandaddingnewrenewableresourcessuchaswindandsolartotheColstripTransmissionSystem(CTS).ThespecificquestionofretiringvaryingamountsofgenerationattheColstrippowerplantsineasternMontanaandaddingasimilaramountofwind-poweredgenerationconnectedtotheColstrip500kVsystematColstripandBroadviewisexaminedindetail.AcasualobservermaybetemptedtosuggestthatifsimilargenerationcapacityisconnectedtotheColstripswitchingstation,thetransmissioncapacityshouldsimplyremainthesame.However,thisideaignoresnumeroustransmissionissuesthatarenotapparentifoneisnotfamiliarwiththeelectrictransmissiondesignissuesoftheColstripproject.TheuniquefeaturesoftheCTSpresentcomplicatingfactorsthatmustbeaddressed.

Thispaperdoesnotattempttospecifythedesignofanynecessarysystemchangestoachieveamegawatt-for-megawattreplacementofcoal-firedgeneratorswithwind-poweredgeneration.Thatworkwillneedtobecompletedaspartoftherequiredgeneratorinterconnectionstudiesforspecificwindprojects.InsteaditisageneraldiscussionoftheengineeringissuesthatwouldariseduetotheuniquefeaturesoftheCTSandprovidescontextforthesolutionsandassociatedcosts.Thefocuswillbeonhowthechangefromcoal-firedgenerationtowind-poweredgenerationwouldleadtoadifferentsetofoperatingconcernsfortheColstripTransmissionSystem(CTS)andwhatthesolutionsandrangeofcostsassociatedwiththosesolutionsmaybe.

InthispaperthedesignissuesoftheCTSarediscussedwithoutengaginginalotoftechnicallanguageormathematicalexpressionsthatmightrequirethereadertobeversedinengineeringsubjects.Thegoalisnottocompletelyavoidtechnicalsubjects,butratherpresentthesesubjectsonaverybasicleveltogivetheaveragereaderabetterunderstandingoftheissuesandsolutionsthatexist.TheCTSpresentlyreliesonasophisticatedcombinationoftechnicalfeaturesthathavegreatlyenhancedbothitsreliability,anditscapacity.

Toreplacethecoal-firedgeneratorswithwind-poweredgeneratorsandretainasimilarcapacitywithsimilarreliabilitywillrequirespecificandtargetedengineeringsolutionstomakesurethatthesystemcanmeetperformancerequirements.


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TheColstripTransmissionSystem(CTS)

TheCTSstretchesforroughly500milesacrosstheStateofMontanaandnorthernIdahofromColstripinthesoutheasternquadrantofthestatetointerconnectionpointsintheeasternpartofWashingtonState.TheportionoftheCTSownedbytheColstripConsortium(CPM)extendsapproximately240milesfromColstriptotheownershipchangelocationnearTownsend,Montana.TheBPA-ownedportionofCTSextendsfromTownsendtoaswitchingstationcalled“Taft”whichisapproximately215mileswestneartheMontana–IdahostatelinenorthofI-90.TherearetwoBPAlinesthatinterconnectCTStopointswestofTaft.OneconnectsTafttoaswitchingstationcalled“Bell”nearSpokane,WA(about85mileswest);theotherconnectsTafttoaswitchingstationcalled“Dworshak”(nearthedamwiththesamenameabout75milessouthwestofTaft).


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Thereareeightimportant500kV(EHV)linesegmentsthatrepresentmajorelementsoftheCTSsystem.4EverylinesectionintheCTSisandmustbeseriescompensatedtoachievethepresentratingoftheCTSthroughoutthelengthoftheCTS.(Seriescapacitorbanksaremulti-milliondollarinvestmentswithuniqueinsulationandprotectionissuesthatrepresentsignificantengineeringconsiderationsintheirownright.)

Besidestheseeightlinesthateachrepresentamajorcontingency,thereisanother500kVlineconnectingtheTaftswitchingstationtoastationcalledHotSpringsinNorthwestMontana.ThisconnectsfourNorthwestMontanaHydro(NWMH)largehydro-electricplants(HungryHorse,Libby,CabinetGorgeandNoxon)intoBPA’s500kVsystematTaft.Therearealsoseveral230kVlinesthatknitthesefourplantstogether,andsomethatconnectthemintothegreaterSpokanearea.AthightransferlevelsonthetransmissionpathbetweentheMontanaArea,andtheNorthwestArea(Path8),thereissomeinteractionbetweenthelevelofthetotalgenerationproducedbytheNWMHplantsandtheflowonPath8thatisallowed.Generally,thesefourhydroplantsonlyoperateatpeakoutputduringpeakloadhoursinthespringmonths.Path8usuallyoperatesatmaximum

4Colstrip–Broadview500kVlineA(Broadviewisabout110mileswestofColstrip);Colstrip–Broadview500kVlineB;Broadview–Townsend–Garrison500kVline1(GarrisonisinwesternMontana);Broadview–Townsend–Garrison500kVline2;Garrison–Taft500kVline1(TaftisneartheboundarybetweenMontanaandIdaho);Garrison–Taft500kVline2;Taft–Bell500kVline(BellisnearSpokane,WA);Taft–Dworshak500kVline


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flowduringoff-peakhours.ThethermalplantsatColstripareoftenscheduledformaintenanceduringthespringtoavoidasituationwherethedemandsarehighonPath8duringthisseason.Duringthespringrunoffseason,curtailmentoftheflowovertheCTSmayberequiredoccasionally.Thechangefromcoal-firedgeneration,whichrequiresscheduledmaintenanceoutagesthatcanconvenientlybeusedtoavoidcongestiononPath8,towindpowerthathasnosuchmaintenanceschedulingcharacteristic,maycausetheneedtoworkoutsomeotherarrangementtoavoidtheconflictbetweenthesetwodemandsfortheuseofPath8capacityandtheWestofHatwaipathcapacity.


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ConsiderationsforAddingLargeAmountsofWindGenerationtotheColstripTransmissionSystem

Thenormalconcernsforatransmissionstudytoaccountforwhenexamininganypowersystemarethermaloverloads,voltagecontrolissues,andtransientstability.FortheCTSoneshouldalsoconsiderRASprotectionsimplybecauseofthelongradialcharacterofthesystemwithtwoparallellines.Also,becausetheCTSisseriescompensatedonemuststudySubSynchronousResonation(SSR);however,thespecificchangefromsynchronousmachines(coalfiredgenerators)tonon-synchronousmachines(windgenerators)makesthisissuechangeincharacter.ByfarthebiggestissuetoaddressistheRASprotectionone.IfoneassumesthattheRASprotectionisfunctioningcorrectly,thethermaloverloadissueisrathertrivial,andtheRASaddressesalltransientstabilityissuesautomatically.Voltagecontrolissuesmustbeaddressedinanycase.

Thermaloverloadingoccurswhenthereistoomuchpowerflowingoveraspecifictransmissionlineorsetoflines.Thetransmissionsystemisdesignedtoavoidthermaloverloads,butwhenaportionofthesystemfailsforwhateverreason,thermaloverloadsbecomeapossibilityandmustbemitigated.

TherearelineoutageconditionsalongtheCTSthatcancausethermaloverloadingoncertainelementsoftheCTS(atmaximumflowconditions).ThemostimportantsuchoutageistheoutageofasingleColstrip–Broadviewline(discussedintheexecutivesummaryabove).TherearealsooutageconditionsthatcouldresultinoverloadingoflinesthatoperateinparallelwiththeCTS.


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VoltageControlIssues

WhentheCTSisoperatedatitsmaximumtransfercapabilityitisloadedsomewhatabovetheSurgeImpedanceLoading(seenote1eonquadraturepowerfordetailonthissubject).Atthatflowlevel,voltagesonkeybuseswillbegintodecreasebelowacceptablelevelsunlessadequatereservesofVArsareavailable.ThegeneratorsmustproducemoreVArstocompensateforthetendencyofthevoltagetosag.IfthegeneratorsarenotcapableofproducingtheseVArs(asthecoal-firedgeneratorsnormallyare)thenotherdevicesmaybeneededtoproducetheseVArs.ContinuousVArcontrolisdesirableforqualityperformance.Thistopicisdiscussedfurtherbelow.

Colstripunits1and2areeachcapableofapproximately115MVArofreactivepoweroutputatfulloutput(continuouslyforlongperiodsoftime).Colstripunits3and4areeachcapableofapproximately270MVArofcontinuousreactivepoweroutput(thesemachineshavearatedpowerfactorcapabilityof95percent).Togetherthefourunitscanproduceanapproximatetotalof770MVAr.ThisVArcapacityisusedtotightlycontrolthevoltageontheColstrip500kVbus.TheycontinuouslysupplyVArsasnecessarytomaintainaconstantvoltageonthatbusoverawiderangeofflowconditions.ThisistheprincipalvoltagecontrolontheCTS.

Ifthe2,100MWofcapacityofthecoal-firedunitsisreplacedwithwind-poweredgeneration,itwouldrequireapproximately1,400largewind-turbinegenerators(thesefiguresarebasedona1.5MWwindunitsize).Thetotalreactivepowercapacityofthe1,400wind-poweredgeneratorswouldbeabout480MVAr(basedonanassumedpowerfactorcapabilityof97.5percent).UnderpartiallycalmedconditionstheVArcapacityofthewindfleetwouldbediminishedduetothegeneratorsthatarestopped(oroff-line).Thismeansthatthewindvariation(atlowspeeds)maycontributetovariabilityofsystemvoltageaswindmachinesdropoutatthecriticallowwindspeed.ItwillbemorechallengingtomaintainaconstantvoltageontheCTSusingafleetofwindgeneratorsthatisconstantlyinastateofflux.ItmaybewisetoinstallsomeVArgeneratingdevices(e.g.synchronouscondensers,VSCs,orSVCs)toassureacceptablevoltagelevelsaremaintainedsmoothlywithsomereserveVArsavailable.

Apowerflowstudyshouldbedonetoevaluatetheminimum(andmaximum)VArcapacityrequiredofthewindfleetunderpartially(andtotally)calmedconditionswheresomeunitsarenotavailableduetocalmconditions.UsingwindmachinesthatarecapableofproducingsomeVAroutputevenwhentheyarestoppedwouldhelptomitigatethisissue.Thesestudiesshouldbedoneduringheavyloadconditions,andalsoduringlightloadconditions(calmwindsmayoccurduringeithercondition).Voltagecontrolissuesmaydiffersignificantlyforthesetwoconditions.


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TheauthorhasproducedasinglepowerflowcasewithnoColstripgeneration,noauxiliaryloadsatColstrip,andnowindgenerationineasternMontana.Thepurposeofthiscasewasonlyto“calibrate”theremarksinthiswhitepaper.Thecasesolved,andvoltageswereinacceptableranges(somebuseswereafewpercentbelowthevoltagetheyareoperatedattoday).ThiscasesuggeststhattheexistingswitchedshuntdevicesontheCTSbusesarecapableofachievingacceptablevoltagesontheCTSforheavyloadconditionsandnocontinuousVArsupplyfromeitherColstripgeneratorsorthewindgeneratorsineasternMontanatoday.Thiswasonlyacursorylook,loadswerenotvaried(heavysummerloads2025wereused),andnodynamicstudieswereperformed.Underlightloadconditions,thevoltagemightbetoohigh.Thissinglecasedoesnotchangetheneedforacomprehensivestudy.

Also,atmaximumwindconditionsastudywouldbeneededtoverifythatsufficientVArsareavailabletomaintainadequatevoltageontheCTSusingarealisticfeedersystemtoconnectthewindmachinestotheCTS(completewithtransformersandintermediatetransmissionthatisreasonablyrepresentativeofactualgeneratorinterconnections).Thisshouldbeperformedatbothheavyloadandlightloadconditionsbecausemaximumwindconditionsmayoccurduringeitherloadingcondition.OneshouldexpectthemaximumflowontheCTStooccurwhenwindgenerationisatamaximumandtheloadinMontanaisataminimum.

Reactivepower(VArs)providedbyancillarydevices(capacitors,reactors,“synchronouscondensers”andstaticVArdevices,SVCsandVSCs)canbeusedtoprovidevoltageregulationonanACsystem.Thesebecomemorenecessaryifthe(real)powerflowlevelonthesystemvariesoverawiderange,andiftheflowlevelcyclesmoreoften.Problemscausedbyhighflowlevels(lowvoltage)cangenerallybemitigatedbyreducinggenerationtoreduceflow,butproblemscausedbylowflowlevels(highvoltage)wouldhavetobemitigatedbytheuseofshuntreactors(orcomparabledevices)thatabsorbtheexcessVArsthatthelinesaregenerating.

TheCTSpresentlyhasanassortmentofswitchableshuntreactorsthatareusedtoabsorbVArsunderlightpowerflowconditions(duringgeneratoroutagesprimarily).WhentherealpowerflowontheCTSislow,thevoltagetendstogohighduetotheVArsthatareproducedbythecapacitiveshuntreactanceofthe500kVlines.Asthewindspeed(averagedoverthewholecollectionofwindturbinesinMontana)decreases,therealpowerflowontheCTSwoulddecrease.Ifcalmconditionsprevailgenerally,therewouldbeaneedtotakemeasurestocontrolhighvoltageasthe500kVlinesincreasetheirVArproduction.Theseswitcheddevicesallowstep-wisecontrolofthesystemvoltage,butdonotprovidecontinuouscontrol.TheprimarysourceofcontinuouslyvariableVArsupplytodayistheset


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ofcoal-firedgeneratorsatColstrip.ItmaybenecessarytosupplementthecontinuousVArsupplyofmodernwindmachineswithaSVCorVSC,especiallyundercalmwindconditions.Thecostofsuchdevicesdoesnotrepresentasignificantportionoftheprojectcosts.

Therewouldbeaneedforpowerflowstudiesthatspecificallyaddresshowtomitigatethisoveravarietyofpossibleconditions.Includinglightandheavyloadconditionsforbothhighwind(fullgeneration)andcalmwind(windgeneratorsoffduetoverylightwinds).TheCTShasmanyfeaturesthatifappropriatelyusedwouldlikelybehelpfulformanagingtheCTSsystemvoltages.Someoftheseoptionsshouldonlybeusedwhenallothermethodshaveproventobeinadequate.Thesefeaturesinclude:

1. SwitchedshuntreactorsthatarealreadyinplaceatColstrip,Broadview,Garrison,andTaft.

2. SeriescapacitorsthatcanbeswitchedouttoincreaseVArlossesinthelineswhenthepowerflowinginalineislow.Unfortunatelythesehavelittleeffectwhentheseriespowerflowingislow.

3. OpeningonesegmentofalinetodeliberatelyincreaseVArlossesintheadjacentsegmentcanhelpcontrolhighvoltage.(Thisshouldnotbecommonpractice.)

Itmaybenecessarytoaugmentthevoltagecontrolfeaturesinthelistabovetoprovidemorevoltagecontrolatverylowflowconditions(partiallycalmed).Thiscanonlybeassessedbyperformingtherangeofstudiesdescribedabove.Totalsystemloadcyclesdailyandbothhighandlowwindconditionsmayoccuratanytimeofday.Thepossibilitythatitmaybenecessarytoaugmentthevoltagecontrolfeaturesaboveisgreatestwhenallofthecoal-firedgenerationisoff-line.Thenumberofdutycyclesofthethreetypesofdeviceslistedabove(peryear)maywellincreaseduetothevariabilityofthewindresource(totaloutput).Thismaycausetheneedtoreplaceswitchesthatareusedforvoltagecorrectionmoreoftenthanitisrequiredforthesystemwithcoal-firedgeneration.OnewaytomitigatethiswouldbetoaddsomecontinuouslyactingVArdevicessuchassynchronouscondensers,staticVArcompensators(SVCs),VoltageSourceConvertersetc.tothesystem.


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DynamicStudyIssues(“TransientStability”)

Transientstabilityreferstoatransmissionsystem’sabilitytowithstandamajorfaultandquicklyreturntoastableconditionbeforeadditionalproblemsoccurorspreadtootherpartsofthesystem.TheCTSiscurrentlyconfiguredwithaRAS(theATR)thatisdesignedtotripgenerationatColstripintheeventofthelossofatransmissionline.ThistrippingmechanismprotectsthetransmissionsystemandallowsforanincreasedtransfercapabilityonPath8.ThereisalsoaRASownedbyBPAthatswitchesreactorsoffatGarrisonforcertaincontingencies.

EarlyinthedesignphaseoftheColstripproject(inthe‘70s)itwasestablishedthatroughly2,200MWofgenerationwouldbeconstructed,andthatthiswouldrequiretwo500kVtransmissionlinesoperatinginparallel.IntermediatestationsattwolocationsbetweenColstripandthewesternterminusweredesignedtoallowforinterconnectionwiththeMontanasystem,andtoimprovetransientstabilitybyinsuringfaulteventsonlyresultedinapartiallossofeitherline.Theideawastosectionalizethesystemintothreeroughlyequalsegments.Historicaleventsforcedachangethatmadethecentralsectionmuchlongerthantheothertwosegments.ThisactuallyreducedthenativecapacityoftheCTS.ThishadtobecompensatedforbysupplyingamoreeffectiveRAS.

IntheearlystagesofplanningfortheCTSitwasdecidedthatthistransmissionsystemwouldbe“justenough”toservetheseplants.Itwaswellunderstoodthatmaintainingtransientstabilitywouldbeachallenge.Everyknowndeviceforenhancingtransientstabilitywasatleastconsidered.Muchwasdoneintheearlystudyphasetoreviewalloptionsforwaystoimprovestabilityandkeeptheoverallcostdown.Thisidealeddirectlytotheunderstandingthatgeneratortrippingwouldbeneededforcertaincontingencies(especiallythe“side-by-side”lineoutages).

Thus,theCTSreliedimplicitlyuponaRemedialActionScheme(RAS)fromitsinception.TheoriginalpresumptionwasthatthisRASwouldbea“DirectTransferTrip”(DTT)scheme.TherearenumeroussuchRASschemesinthewesterninterconnection.ThebasicideaofsuchaRASisthateachsystemoutage(lineortransformer)thatwouldrequiregeneratortrippingwouldbeidentifiedthroughourstudyeffort.Thisisnottoodifficultifthereareveryfewcriticalcontingenciesforwhichtrippingisneeded.Thencommunicationsline(s)betweentheswitchingstation(s)wherethelineterminalsforanyoutageoccurswoulddeliveratrippingsignaltotheappropriategenerator(s)tocausethemtobetrippedfortheoutage.Thisseemssimpleenough,butonemustrecognizethatthereareliterallydozensofoutagestostudyontheCTStobesurethattheRASwillcoverallnecessarycontingencies.SystemperformancewithoutaRASvariesgreatlywiththetotalflowontheCTS.Somelineterminalsarehundredsofmilesfromthegenerators.Communication


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circuitsthatarehighlyreliablearesometimeshardtofind(ortoputitanotherway,veryexpensive)inMontana.Thesecommunicationlineswouldneedtoberedundant(includingrouteredundancy)andalsosecuretopreventexcessivefalsetripping.ForeachoutagetheRASmustpromptlyinitiateanacceptableamountofgeneratortripping,enoughtoassuresystemstabilityispreservedoverarangeofflowlevels.Also,thepreciseamountofrequiredgeneratortrippinggenerallydependsontheflowlevelsonthecriticalelementsoftheCTS.Trippinglargecoal-firedgeneratorsisquiteexpensive.Itrequiressubstantialquantitiesofauxiliaryfueltorestartaplant,andthereisacertainriskofdamagetocriticalequipmenteverytimeaunitistripped.Forthisreasonitwasalwaysconsideredveryimportanttominimizefalsetripping.SavingonthecostofcommunicationsisoneofthegreatadvantagesoftheATR.

TheexistinggeneratorsatColstriparelargesteamturbine-generators.Thereareonlyfourlargesteamturbine-generatorsintheColstripproject.TheRASforthesemachinesonlyneedstomakefourtrip/notripdecisions,oneforeachofthefourunits.Thetotalgenerationoneachunitistrackedinreal-time,andeachofthe15possiblecombinationsofgeneratorsistotaledandsortedsothatwhentheATRproducesatrippingdecision,itcanquicklyselecttheappropriatecombinationofgeneratorstotriptogetjusttherightamountofgenerationtripping(withina“step-size”thatisaboutthesizeofonesmallunit).


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Designinga“WindRAS”fortheCTS

ToretiretheexistinggeneratorsatColstripandaddacomparableamountofwindgeneration,a“WindRAS”willhavetobedevelopedfortheCTSinordertomaintainacomparabletransfercapability.AlloftherecentCTSstudiesdiscussedinthispaperassumed(withoutproof)thatasufficientRASwasindeedinplaceandsomeassumedthatitwasaseffectiveastheexistingATRRASatColstripandcompatiblewithit(underallscenarioswhereanyofthefourColstripunitsarestilloperating).No“WindRAS”designhasactuallybeenproposedorchosen.Obviously,whena“WindRAS”designisactuallyproposed,itwillhavetodemonstrateitseffectivenessthroughtesting.Thiscanbeaccomplishedthroughadditionalstudies.TheinterconnectionstudiesforlargewindplantsconnectingtotheCTSconductedbyNorthwesternEnergysofarsuggestthatthecostofsuchaRASwouldbe$1millionto$4million.AlargepartofthecostofaRASisintheengineeringeffort.Thecostofthemajorequipmentwoulddependverymuchonthechosendesign.Longcommunicationpathsthatinvolvestringingfiberopticsovergreatdistancescouldgreatlyincreasethecost.Thelogicdevicesthataretypicallyusedareeitherlargeprogrammablelogiccontrollersorhardenedcomputers(rackmountedwithspecialfeaturesforhandlingmanydigitalinputs/outputs).Cleverengineeringmightsavesomemoney,butitisnotwiseto“gocheap”onthistypeofequipment.

Totriptheequivalentofonesmallunitofcoal-firedgenerationwouldrequireapproximately200wind-turbinestobetripped.Theequivalentofonelargeunitwouldbeabout500wind-turbines.Trippingsuchmassivenumbersofwindgeneratorsmightinvolvesubstantialamountsofswitchingequipment,dependinguponhowthewindgeneratorsareorganizedandconnectedtotheCTS.ItisnotdesirabletoseparatethewindgeneratorsintoverymanysmallparcelsbecausethecostofaRASwouldbehigherifitwererequiredtooperatemoreswitches.ThebestplanforsuchaRASwouldbetobringallsignalsfromremotelinesintoasinglelocationwherethe“brains”oftheRASislocated(redundantofcourse).Inorderto“manage”theamountofgenerationtotrip,thereshouldsomesimplewaytototalizethepowerfromeachgroup(thathasindividualtrippingavailable)ofwindmachinessothatonecantripthedesiredamountofwindpowerquickly,andavoidtrippingallofitforeverycontingency.GivingtheRASthecapabilitytotrippartialsetswouldbehighlydesirableinordertomaximizethecollective“plantfactor”forthefullsetofwindplantsbyavoidingexcessivetripping.Sinceeverygroupwouldhavevariableoutput,thiscouldrequireacarefullyengineeredRASdesign.

Ifoneiscontemplatingreplacementofthecoal-firedgeneratorswithwind-poweredmachinesontheCTS,thereislittlechoicebuttodevelopaDTTRAS.Thebestwaytocontrolthecostsistotakethemaximumadvantageofcommunicationpathsthatarealreadyin


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placeforthecurrentrelayingschemesthatprovideprotectionfortheCTS(faultdetectionandbreakeroperations).AdditionalcommunicationlinesmaystillberequiredtoprovidetheredundancyneededtomeetpresentdayRASdesignrequirements.ThiswillrequiresomeresearchtodeterminehowmanyoftheneededcommunicationlinesforanyproposeddesignareavailableintheexistingCTScommunicationslines.

ThemostimportantdesignfeaturenecessarytominimizetheadditionalrequirementsfortrippingthewindmachinesinvolvesthetopologyoftheconnectinglinesthatfeedpowerfromthewindplantstotheCTSsystem.Thepowerfromsizablegroups(200–500MW)ofwindcapacityeachshouldbefedthroughintermediatevoltage“trunk”linestotheBroadviewandColstripswitchingstations.Trippingshouldbeaccomplishedbyopeningthese“trunk”lines(mostly230kV).Eachtrippingactionwouldthustripasizeableamountofgeneration.Thecostofthehigh-reliabilityredundantcommunicationsrequiredtogetsignalstothese“trunk”linescouldthusbeminimized.Allofthese“trunk”linesshouldterminateclosebytheexistingCTSswitchingstations.

Thecontrollerforthe“WindRAS”probablyshouldbeatBroadview(itshouldberedundanttoo).TheotherchoiceisColstrip.AnothersetofredundantcommunicationslinesfromthesetwocontrollersshouldbesetupbetweenBroadviewandColstrip.ThisshouldcarrytrippingsignalstoColstrip(orBroadview)fromthecontrollerforeach“group”ofgeneratorsconnectedthere(all“feeders”shouldconvergeonthesetwostations).Costscouldbekeptdownbyminimizingthenumberof“groups”ofgeneratorsconnectedatColstrip.Also,trippinginformation(formatteddata)shouldbetransmittedtotheATRforthosescenariosthatthe“WindRAS”needstocoordinate.

Onepossible“simpleidea”forscenario1(whereColstripUnits3and4arestilloperating)wouldbetohavetheATRprovideacommunicationssignaltothe“WindRAS”controllerthatwouldtripthewind-poweredgeneratorsthataredesignatedtobe“replacements”forColstripunits1and2wheneveritwouldbecallingfortheirtrippingatpresent.Therewouldlikelybeaneedto“recalibrate”theATRtomakesuretrippingisreasonablyaccurate.

Thereareotherwaysthatmayworkjustaswell.ThemainrequirementforanyproposedRASdesignwouldbethatathoroughstudyhasbeendoneinadvanceoftheinstallationtoverifythesystemperformanceisassured.Thestudywouldbefairlyeasytoperform,butifthefirstideadidnotwork,onewouldneedtobepreparedwithotherideas.

Strongevidenceneedstobeprovidedtoshowthatanysuch“WindRAS”isreliabletothestandardsoftheregulatorsthatmustapproveanysuchschemeintheWECC.


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TheRoleofInertiainDynamicEvents

TheeffectofhavingalargeamountofwindgenerationconcentratedineasternMontanaduringaserioustransmissionoutageisanareathatrequiresfocusedstudy.Aseriousefforttomodelthedynamicsofthesystemwouldberequiredtoanswercriticalquestionsabouthowsuchachangewouldaffectperformance(long-termdynamicsstudiescouldbeusedtogainmoreunderstandingofthisissue).Theratioofthetotalwind-generationtothetotalgenerationprovidedbyrotatingsynchronousmachines(includinghydro-electric)isanimportantparameterinevaluatingthisconcern.MostoftherotatinginertiaineasternMontana(roughly90percentofit)isinthecoal-firedmachinesatColstrip.Therearejustafewsmallcoal-firedgeneratorsintheareabesidestheColstripunits.Therearegas-firedcombustionturbinegeneratorsinAnaconda,andnumeroussmallhydro-electricplantsinGreatFallsandpointsfurtherwest.So,forscenario2,3and4discussedinthispaper,therearesituationswherethetotalrotatinginertiaineasternMontanawouldbereducedbyafactoroften.Ifthispartofthetransmissionnetworkwereislandedforanyreason,thefrequencywouldbetentimesmoresensitivetoloadandgenerationchanges.Whilethissuggestsaninvestigationisinorder,thereisnotproofthatthiswouldleadtounacceptableoperationoftheremainingsystemaslongasitisconnectedtotheWECCthroughtielines.

Windgeneratorswouldactuallyeliminatesomeofthedynamicissuesdiscussedabove.Sincethewindgeneratorshavenoeffectiveinertia,ittakeslesstransmissionstrengthtostoptheover-speedeventthatcanoccurwhenthereisatransienteventcausedbyalinefault.However,steadystatevoltageperformancewouldneedtobemaintainedintheaftermathofacontingencythatrequirestrippingofwindgeneratorsasnecessary.AwelldonestudycoulddetermineifaproperlysizedstaticVArdeviceorvoltagesourceconverter(orseveralthatarestrategicallyplaced)couldmitigatethelowvoltagedipthatoccursduringadynamiceventprecipitatedbyafaultedlineandtheaftermath.Suchadevicemightallowtheamountofgeneratortrippingtobereduced(comparedtotheATRtrippingamount)forsomeofthelesscriticalcontingencyevents.


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OtherDynamicsIssues

Aproperdynamicstudymustmodeleachswitchingsequenceingreatdetailusingpreciseknowledgeaboutthespeedofthecircuitbreakers,protectionrelays,andanyautomaticswitchingthatmayexist(suchasaRAS).Theremaybeaneedforatrippingsignaltotravelovergreatdistancesandrelaysmust“pick-up”uponthearrivalofthissignalsothereareadditionaltimerequirementsherethatmustbeproperlymodeled.Itisveryimportantthatany“WindRAS”thatneedsto“coordinate”withtheATR(tobecompatible)mustactaftertheATRtrippingalgorithmshavehadtimetoact.Ifthisisnotdone,theveryactionoftrippingthewindgenerationearlywilltendtohavetheeffectof“blinding”theATRtotheevent.ThismayresultinthefailureoftheATRtoactforacriticaloutage.

Theexistingcoal-firedgeneratorsrequireauxiliaryfueltorestartthem.Thismakesthecostoftrippingthemsignificantevenwithoutregardtothecostofreplacementenergy.Thereisnosuchrequirementforfuelforawindmachine.Therealsoisnoneedforalengthydelaybeforeawindmachinecanberestarted(assumingishasnotbeendamagedbytheshutdown).Theauthorisnotawareofaseriousriskofdamagethatwouldresultfromfrequenttrippingofa“type-4”windmachine;however,thisisnotatrivialmatter,sincemanyunitscouldbeexposedtotrippingeachtimetheRASmustact.Forthispaperitisassumedthisisnotaproblem.Thus,forthisconcernitwouldappearthatwindgeneratorshaveacertainadvantageovercoal-firedgenerators.

Ontheotherhand,wind-poweredgeneratorshavesomefeaturesthatmayleadtodifficultiesnotfoundwithlargecoal-firedmachines:

1. Windturbinegenerators(including“type-4”machinesdonothavealargeshorttermemergencycapability(abovetheirnameplatecapacity)togenerateVArsthatimprovesystemperformanceduringemergencyconditionssuchasafaultedline.ThegeneratorsatColstripareequippedwithhighspeedexcitationsystems.TheserespondtoafaultbyquicklycausingthemachinestogenerateVArsatveryhighlevels(wellabovetheirsteady-staterating)forashortperiodoftime.WithlargeamountsofnewwindgenerationaddedtotheCTS,VoltageSourceConvertersstrategicallyplacednearloadcenterscouldhelpwiththisconcern,especiallyatBillings.

2. Windturbinegeneratorscannotbecountedontobeavailabletoservelocalloadduringoutages.Dispatchablereservegenerationwillbeneededtoservelocalloadduringsuchevents.InthesediscussionsthisisassumedtoexistwestoftheCTS.

3. Windturbinegeneratorsdonotgenerallyhavethecapabilitytoprovidefrequencyregulationinawaythatadvantagesthereliabilityofthepowersystemduringlargegenerationoutages.


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4. Whenadynamiceventoccursthewindmachinesdonotrespondtothefrequencyincreasebyameasurableincreaseinitsrotorspeedinthewaythatasynchronousmachinedoes.ThisimpliesthatadeviceliketheATRcannotworkonawindmachine(becauseitsprinciplesofoperationarenotcompatible).


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Sub-SynchronousResonance(SSR)Concerns

TheCTStransmissionlinesareseriescompensated(seriescapacitorsareconnectedtooneendofeachlinesection).Thisimprovespowerflowandstabilityperformance.Italsoreduceslossesinthelines,andhasotherbenefits(withoutseriescompensationonecouldarguethatathirdlinewouldhavebeenneeded,oratleastsomeotherremedywouldhavebeenneeded).However,seriescompensationalsocomplicatestheoperationandprotectionoftheCTS.Aseriescompensatedlinehasanaturalresonantfrequency.Typically,thecapacitiveimpedanceissmallerinmagnitudethanthereactiveimpedanceoftheline.Thisassuresthattheresonantfrequencyislessthanthesystemoperatingfrequency(hence“sub-synchronous”).Networksofsuchlinesnormallyhavemultipleresonantfrequenciesdependingonthenetworktopology.Ifavoltageisappliedat(ornear)aresonantfrequencyofthenetworkextremelyhighvoltagesmayoccur.Thismustbeavoidedtopreventapossiblecatastrophicfailureorseveredamagetotheline.Lineequipment(especiallyseriescapacitors)couldbedestroyed.Also,generatorsfeedingpowerintothesystemcouldbeseverelydamaged.Themostseriousnegativeeffectsofthisphenomenonarecollectivelyreferredtoas“Sub-SynchronousResonance”(SSR).

TherearethreetypesofSSR,5allofwhichhavesevereeffectsthatmustbeevaluatedforaseriescompensatedtransmissionsystemthatiswellconnectedtolargesteamturbinegenerators.AllwerestudiedfortheCTSduringthedesigneffort.

WhenweperformedtheoriginalSSRstudyworkforColstrip,wehadtostudyeachofthepossiblecombinationsofgeneratorsonlinebecausethesystemresonantfrequencieschangeforeachconfiguration.OfthefourColstripunits,theSSRriskisgreatestforColstrip

5ThethreetypesofSSRare:

1. Inductiongeneratoreffect:Becauseofthenaturalresonance,ageneratormayspontaneously“feed”thesystemresonancegeneratingcurrentatthatfrequency).Thiscanseverelydamagethewindingsinthegenerator.

2. TorsionalResonance:Whenasystemresonanceoccursasub-synchronoustorqueisdevelopedontherotorofeachconnectedgeneratorduetothecurrentflowinginthemachinewindings.Thistorquecanstimulatearesonanceintheturbine-generatormechanicalsystemthatcancauseseriousdamagetotheshaftofthemachine.

3. TransientTorqueAmplification:Whenswitchingoccursonthetransmissionsystemeachswitchoperationcausesatransienttorqueoneachconnectedgeneratorshaft.Thefirstswitchingeventstimulatesatorsionalvibrationthatmaythenbeenhancedbythenextswitchingevent(timingiscritical).Thiscanleadtoexcessivestressonthegeneratorshaftasitrings.Shaftfailureispossible.


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1and2.ThustheSSRrisktotheColstripunitswouldbegreatlyreducedforallofthescenariostreatedinthispapersincetheyarepresumedtoberetiredforallscenarios.

The“InductionGeneratorEffect”istheonlyoneofthethreetypesofSSRdescribedabovethatcouldbeconsideredanissueforwindgeneratorsingeneral.Type4windgeneratorsshouldnothavethisproblematallsincetheirinductiongeneratorisisolatedfromthegridbytherectifierinverterinterface.However,therehavebeencaseswherewindturbinegeneratorshadinteractionswithseriescompensatedlinesincloseproximitytotheirterminals.

Ibelievethepotentialissueformodernwindmachinesisthatthevoltageregulatorsintheseunitshavecontrollersthatarecapableofrespondingtovoltagechangesatfrequenciesthatarehighenoughtostimulatethesub-synchronousresonancesoftheseriescompensatedsystem.Insomecasesthetimedelayinthecontrolloopbetweenthevoltagesensorandtheamplifiercontrollingthevoltageregulatormayleadtoapositivefeedbackloopwhichcouldbedangeroustothegenerator.

Thelengthofthewindfeederlines,andthelocationofthePOI,wouldchangetheresonantfrequencyofthesystemforeachinstallation.Thisissueshouldbeexaminedthroughafrequency-domainstudyforeachgeneratorlocation,andifthereareissuesidentifiedforaparticularlocationthenthevoltageregulatorsonthewindmachinesatthatlocationshouldbemodifiedwithafilter(notchorlowpass)thatisdesignedtoavoidthisproblem.

Special“frequencyscan”programsarewidelyavailablethatcanbeusedtocalculatethefrequencyresponseofthepowersystematanybusinthesystemtodeterminethesystemresonantfrequenciesatspecificbuses.Thistypeofprogramshouldbeappliedbeforethegeneratorsareconnected.

Afterthefrequencyscanstudyforeachsiteiscompleted,afrequencyresponsetestmustbeappliedtotheinvertersofthespecificwindmachineschosenforthatsitetodetermineiftheyarecapableofexcitingthesystemresonance(s).Filtersshouldbeappliedtoeachmachineatthesitetoblockthegeneratorfromproducingpowerattheundesirablefrequencies.


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ExistingStudiesReview

a. 2014NorthernTierTransmissionGroup(“NTTG”)PublicPolicyStudy:6ThisstudybyNTTGexaminedtheretirementofColstripUnits1and2andtheadditionof610MWofnewwindinthearea.Thiswasonlyapowerflowstudyandalthoughitnaturallydidnotfindanymajorissues,itisoflimitedusebecauseitdidnotlookatthemoreimportanttransientstabilityquestions.ThepowerflowcaseswerecreatedundertheassumptionthataRASwasinplacetotriptheappropriateamountofgeneration(withoutproofoftheefficacyofanyspecificRASdesign).

b. NorthWesternEnergyStudies:7

NorthWesternhaspubliclyreleasedatleasttwostudiesexaminingcomponentsofscenarioswherealloforsomenumberoftheColstripUnitsareretiredandwindisaddedtotheCTS.OneofthestudieslooksatascenariowhereColstripUnits1and2areretiredandreplacedwithamixofdifferentresources,buttheremainingtwoColstripunitscontinuetorun.Thisstudyconductedadynamicstabilityanalysisandfoundnomajorissues,butitdoesidentifytheneedforadditionalstudyworkrelatedtotheRAS,frequency,voltagesupport,andpathrating.TheotherNorthWesternEnergyStudylookedatdifferentscenarioswhereallfourunitsatColstripareretiredandasmuchas2,520MWofnewwindisaddedtothesystem.Thisstudydidnotexaminethetransientstabilityquestions.

c. WesternWindandSolarIntegrationStudy,Phase3,Section4.2.1:8ThisstudyconductedbyNRELlookedatvariousscenariosofhighlevelsofnewrenewablesacrosstheWestandalsoconductedsomefocusedanalysisontheColstripareabecauseofthehistoryofstabilityissuesthere.ItexaminedlargeadditionsofwindandsolarinMontanawhileeitherallorthreeofthefourColstripUnitswerestilloperating.OnlyoneswitchingeventinMontanawasstudiedfordynamicperformance.ThiseventrepresentsthemostcriticalsinglecontingencyeventfortheCTS.Forthiseventthesystem(today)wouldgenerallybestablewithnoRASaction,butmaynotmeetvoltageperformancerequirements.ThecasewasrunfortheBaseCaseandforthe“Hi-mix”case.ThegreaterstartingangleindicatesthatMontanawasexporting

6http://nttg.biz/site/index.php?option=com_docman&view=download&alias=2405-nttg-report-for-the-2014-2015-public-policy-consideration-scenario-final-05-13-2015&category_slug=ppc-draft-report&Itemid=317http://www.mtaffordableelectricity.org/wp-content/uploads/2015/11/Attach-B-4-15-NWE-Study-Colstrip-111d-Shutdown-Impact-on-Transmission-5-8-15-Final.pdfhttp://www.northwesternenergy.com/docs/default-source/cpp/epa-cpp-transmission-impact-11-17-15-final-pdf8http://www.nrel.gov/docs/fy15osti/62906.pdf


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morepowerforthe“Hi-mix”case.ThefactthatthechangeinangleatthemaximumpointintheswingwasgreaterfortheBaseisademonstrationofthefactthatwindmachinescanbeexpectednottoaccelerateasmuchassynchronousmachineswouldduringswingevents.Interestingly,thisstudyfoundthatthestabilityofthesystemactuallyslightly“improved”withtheadditionsofnewrenewablestothesystem(becausetheswinganglewasreduced).Thisfacthaslittlebearingontheover-allperformanceofthesystemwithahighpenetrationofwindgenerationasareplacementforthecoal-firedplants.Thecaseisnotacriticalcase.Theover-allperformancewillneedtobeevaluatedforallcriticalcases.Itisquitelikely,thatthemostcriticalcasewill“setthelimit”fortransfercapability.NoRAS(noteventheexistingone)wasappliedforthisstudy.IftheATRhadbeenapplied,theremighthavebeengeneratortrippingforeitheroneorbothofthesecases.

d. NorthWesternEnergyGenerationInterconnectionSystemImpactStudies,ProjectNumbers:31,99,101,and115.9Theseinterconnectionstudiesexaminethelocalsystemrequirementsfornewgenerationinterconnectionsforvariouswindprojects.Thesestudiesdonottypicallyexaminethetransmissionquestionsweareconsideringhere,butareusefulinthattheyidentifytheneedforthesegeneratorstohaveanewRASandestimatethecosttobeintherangeof$1-$4million.

e. 2016NTTGPublicPolicyStudy(draft):10

ThisstudylooksatretiringColstripUnits1,2,and3andadding1,494MWofnewwindtotheCTS.ThestudysuggeststhatreplacingcoalwithwindmaybefeasibleontheCTS.Thisstudydidconductatransientstabilityanalysisandfoundnoviolationsundertheirscenariosandassumptions.However,theassumptionsforthisstudyweresimplytoooptimistic.ThisstudyalsoassumedthataneffectiveRASwouldbeinplacebutdidnotspecifythedesignofsuchaRAS.Instead,allwindgenerationwastrippedinstantly(toofasttoberealistic)foreverycriticaloutage.ThisisnotevenpossibleforarealisticRASdesign.Thismayhaveeffectively‘blinded’theATRoratleastdelayeditsresponse(seediscussionabove).Thisstudydidnotconsidersub-synchronousresonanceissuesnordoesitconstituteapathstudy.

Insummary,noneofthesestudiesidentifiedanyfatalflawsforconvertingtheCTStotransportwindenergy,atleastnotforthescenariosandassumptionsthatweremadefor

9Availableat:http://www.oatioasis.com/NWMT/NWMTdocs/GenConnect7.html10http://www.mtaffordableelectricity.org/wp-content/uploads/2015/11/Attach-B-4-15-NWE-Study-Colstrip-111d-Shutdown-Impact-on-Transmission-5-8-15-Final.pdf


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eachstudy.Thisismostlybecauseallthesestudieswerelimitedinsomefashionandthusnotcapableofrevealingsuchflaws.Allofthestudiesrecognizedtheneedformorecomprehensivestudyworkand,ultimately,thedesignandengineeringofspecificmitigations.Thetoolsexisttomakethesescenariosrunreliably,butthetimeitwilltaketodothestudyworkanddesignthechangestothesystemissignificant.Thispaperdescribestheadditionalstudyandengineeringworkthatneedstobeaccomplishedandtimingconsiderationsforcompletingthiswork.


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ExaminationofFourColstripRetirementandNewWindScenarios

Scenario1:

ThisscenariopostulatesthatColstrip1and2havebeenretiredand610MWofwind-poweredgenerationisaddedtotheColstripswitchingstation(thetotalnetgenerationfromunitsoneandtwoisapproximately610MWmaximum).

Forthisscenariopowerflowissueswouldbeminimalandthisispartiallysupportedbythepreviousstudyworkdiscussedabove.(Although,therewasnodemonstrationoftheeffectivenessofanyparticularRASdesign.)

Underthisscenario,thevariationofthewindpowerfromzerotomaximumdoesnotrepresentaseriouschallengetotheabilityoftheexistingCTSequipmenttomaintainvoltageswellwithinacceptablelimits.ThisisbecauseColstrip1and2together,amounttoonlyapproximately29percentofthetotalColstripnetcapacityandthepercentageoftheavailableVArcapacityforthesetwounitsislessthan29percentofthepresenttotal.TheCTStotalgenerationvariesbythisamount(610MW)fairlyoftenforvariousgeneratoroutages,ordispatchingchoices.TheCTSworksverywelloverthisrangeonasteady-statepowerflowbasis.Colstrip3and4haveplentyofreserveVArcapacityformaintainingvoltagesineasternMontanaatfullloadwhentheyareon-line.

Ifonlyoneofthelargeunitswereinserviceforthisscenario,VArsupplyshouldstillbeadequate;however,toproperlyevaluatethisscenario,oneshouldstudythesystemwitheitherColstripunit3orunit4off-line(andunavailable).Thisisnecessarybecauseallcoal-firedgenerationplantsrequireannual(ornearlyannual)maintenanceoutagesthatlastforweeks.Duringthemaintenanceoutagetheplantwouldnotbeavailableatall.

Ifthewindpowerisoff(orverylow)underthisscenariothereareshuntreactorsavailableatBroadviewandColstrip.Thesewouldhelpkeepvoltagesundercontrol.Plannedgeneratoroutagesforthecoal-firedunits(3and4)couldbemanagedtoavoidsimultaneousoutages(aforcedoutageofthelastColstripunitshouldbemanageable,butrare).

Forthisscenarioa“WindRAS”isneededtotakesomeorallofthewind-poweredgenerationoutofservicerapidlyforcertaincontingenciesatmaximumflow.ThisRASwouldhavetocoordinatewellwiththeATRwhichprovidesforthetrippingoftheColstripcoal-firedgenerators.ThismeansitwouldhavetotripinatimeframethatisafterthedecisiontimeoftheATR,butfastenoughtobeeffective.Iwouldnotexpectthistobeaprohibitivelydifficultthingtodo,buttherewouldbeaseriousneedtoadequatelystudythis,andprovethatonehasaschemethatismutuallyacceptabletothewind-power


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ownersandtheColstripowners.QualifyingaRASinthepresentdayregulatoryenvironmenttakessignificanttime.ThesoonerthisworkcanbeginthelessdelaywilloccurbetweenthetimethatColstripUnits1and2retireandthetimethatnewgenerationcantakeservice.TheWECCRASRS(RemedialActionSchemeReliabilitySubcommittee)conductsthetechnicalreviewoftheproposedRASdesign.

Withawell-designed“Wind-RAS”thereislittlereasonforconcernaboutasignificantreductioninthepathratingofPath8.Anyincrease(ordecrease)intheratingwouldhavetobedemonstratedthroughtheresultsofthestudyeffortneededtoqualifytheRAS.Thisstudywouldalsobethedefinitivetestofthesignificanceofconcernsaboutinertia.AproperRASdesignneedstomakesurethattrippingofthewind-poweredmachinesdoesnotgetinitiatedbeforetheATRhasmadeitstrippingdecision,butbeforethereisasignificantdeteriorationofthepowersystemconditions(droppingbusvoltages)hasoccurredinanysimulatedevent.Theauthorhasbeeninvolvedinotherstudiesthatshowthisisinfactfeasible,howeverthesestudieswerenotspecificallyfocusedonsubstitutingwindforcoal.

Scenario2:

ThisscenariopostulatesthatColstrip3hasbeenretired(inadditiontounits1and2)andthat1,355MW(thisisthetotalnetcapacityofthethreeunits)ofwind-poweredgenerationisaddedtotheCTS.Thisrepresentsapproximately64.5percentofthetotalcoal-firedgenerationcapacityatColstrip.

Thisscenariowouldleadtoamuchgreatervariability(morethandoublethatofScenario1above)ofthegeneration(bothrealpowersupplyandVArsupply)availableatthesourceendoftheCTS.MeaningfulstudiesshouldincludeexaminingthissystemwithColstrip4out-of-service(asaplanned,long-termoutage).Thisisneededbecausecoal-firedgeneratorsgenerallyrequireannualmaintenanceoutages.Theseoutagesoftenlastforweeks.Forsuchanextendedperiodoftimeitisreasonabletoexpectthewindtobecalmedforapartofthattime.ThismeanstherewouldbeverylittlegenerationavailableineasternMontana.TheflowontheCTSwouldbereversed.WithnoVArresourcesexceptforswitchedshuntdevicesandthreemuchsmallercoal-firedmachines(onenearColstrip,oneinHardenandtheotherinBillings)voltagecontrolwouldbesomewhatdifficult.Itmayrequireextra-ordinarymeasurestooperatetheCTSundertheseconditions.Somewind-poweredgeneratorsarecapableofsupplyingVArswithnowindavailable—thiscouldbehelpfulforprovidingsmoothervoltageregulationundercalmconditions.


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Forthisscenarioa“WindRAS”isneededtotakesomeorallofthewind-poweredgenerationoutofservicerapidlyforcertaincontingencies.ItwouldhavetoeithercoordinatewellwiththeATRwhichprovidesforthetrippingoftheoneremainingColstripcoal-firedgenerator(unit4),orsimplyreplaceit.Inanycasethis“WindRAS”wouldhavetobeabletofunctionwhenunit4isoutofservice(thisoutageimpliesthatnoATRisavailable).TherearetwobasicoptionsforsuchaRASdesign:

Option1:

WhenColstripunit4ison-line,the“WindRAS”wouldhavetocoordinatewiththeATR.Thismeansthatthe“WindRAS”wouldhavetoexecuteitstrippingactionaftertheATRhashadenoughtimetoevaluatetheeventforthetrip/notripdecisionforColstripunit4.And,whenColstripunit4isoff-line,the“WindRAS”wouldhavetobeabletoapplyanynecessarytrippingforthewindgenerationthatisonlinewhenacriticaleventoccurs.

Option2:

WhenColstripunit4ison-linethe“WindRAS”wouldneedtoincludethatunitinitstrippinglogic.WhenColstripunit4isoff-line,the“WindRAS”wouldhavethesamefunctionasforoption1above.

Thestudyefforttodesigna“WindRAS”forthisscenariowouldthushavetobeflexibleenoughtocopewithtwoquitedifferentsituations.Effectively,the“Wind-RAS”mustfunctionfortwoquitedifferentversionsofthepowersystem(withColstrip4on,andwithitoff)andwouldlikelyrequiremoretimetoproduce(roughly6-12months).

Oneshouldexpectthereviewprocesstobelongertoo(possiblyupto3years).TheWECCRASRS(RemedialActionSchemeReliabilitySubcommittee)conductsthetechnicalreviewoftheproposedRASdesign.

Themainrequirementfordevelopingthe“WindRAS”wouldbetotestthedesignadequatelyinadvanceofitsinstallationthroughanextensivestudyeffort.Thiswouldrequireexaminationofeverysingleandeverydoublecontingency.Becausethisscenarioinvolvesamuchlargerfraction(butnotall)oftheexistingColstripcapacity,thisstudyworkwouldneedtobemorecomprehensiveandwouldtakemoretimethantheonerequiredforscenario1above.AllcaseswouldhavetobeperformedwithandwithoutColstrip4on-line.

Withawell-designed“WindRAS”thereislittlereasonforconcernthattherewouldbeasignificantreductionintheratingofPath8.Anyincrease(ordecrease)instabilityconcernswouldhavetobeansweredthroughthestudyefforttoqualifythe“WindRAS”.Thiswould


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alsobethedefinitivetestofwhethersignificanceofconcernsaboutinertia(atleastfortransientswitchingeventsontheCTS)isjustified.Aproper“WindRAS”designneedstomakesurethattrippingofthewind-poweredmachinesdoesnotactuallyoccurbeforetheATRhasmadeitstrippingdecision,butdoesactintimetoassuretheproperrecoveryofthepowersystemaftertheswitchingevent.Therehavebeenstudiesthatshowthisisinfactfeasible.

Scenario3:

Thisscenariopostulatesthatallfourofthecoal-firedgeneratorsatColstriphavebeenretiredandthat2,100MWofwind-poweredgeneratorsareaddedtotheCTS.

ThisscenariowouldimplythatnoVArresourcesareavailablefromthecoal-firedgeneratorsregardlessoftheamountofwind-poweredgenerationavailable.Somewind-poweredgeneratorsarecapableofsupplyingVArswithnowindavailable—thiswouldbehelpfulforprovidingbettervoltageregulationundercalmconditionsthoughitmaynotbenecessarygiventheexistinggenerationandswitchableshuntdevicesthatareavailableineasternMontana.

ThetotalcustomerloadineasternMontana(connectedthroughtheBroadviewandColstripswitchingstations)variesfromabout300MWto600MW.Underextremelycalmconditions(lowavailabilityofwindgeneration)theflowwouldbewesttoeastontheCTS.ThepowerflowneededtosupplyloadsinMontanawouldcomemostlyfromthewestandenterthroughtheGarrisonandBroadviewswitchingstations.Astheloadvaries,switchedshuntreactorsatGarrisonandBroadviewwouldneedtobeswitchedon(oroff)inordertokeepthevoltagesonthe500kVbuses(andtheunderlyingMontanasystem)withintheirdesiredlimits.ThevoltagechangeasdevicesareswitchedwouldbemorenoticeablesincetherewouldnotbecontinuouscontroloftheCTSvoltagebytheColstripgenerators.ContinuouscontrollerVArresourcescanbeappliedinordertoevenoutthevoltageundertheseconditions.Devicesthatcanprovidethiscapabilityare:1)Synchronouscondensers,2)VoltageSourceConverters(VSCs),3)STATCOMsand4)StaticVArCompensators.

Forthisscenarioa“WindRAS”isneededtotakesomeorallofthewind-poweredgenerationoutofservicerapidlyforcertaincontingencies.This“windRAS”wouldhavetobeabletofunctionadequatelyforeverycritical500kVcontingencyontheCTS.Ideally,itwouldissuemeasuredamountsoftripping,insteadofsimplytrippingallofthewind-poweredgeneratorsforeverycontingencyknowntobeanissue.Atreducedamountsofwindgeneration,itmaynotneedtotripatall.


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Themainrequirementfordevelopingthe“WindRAS”wouldbetotestthedesignadequatelyinadvanceoftheinstallationthroughanextensivestudyeffort.Thiswouldrequireathoroughstudyeffortexaminingeveryplausiblecontingency.

Withawell-designed“Wind-RAS”itshouldbepossibletoavoidasignificantreductionintheratingofPath8.Becauseoftheintermittentnatureofthewindpowertheamountoftimethatthesystemwouldbeloadedhighenoughtorequirethemaximumamountsofgeneratordroppingshouldbelower.Anyincrease(ordecrease)instabilityconcernswouldhavetobeansweredthroughthestudyefforttoqualifythe“WindRAS”.Thiswouldalsobethedefinitivetestforconcernsabouttransientstabilityperformance.Aproper“WindRAS”designneedstomakesurethattrippingasufficientfractionofthewind-poweredmachinestakesplaceintimetoassurethatthetransientstabilityperformancestandardsoftheWECCaremet.TheRASRS(RemedialActionSchemeReliabilitySubcommittee)conductsthetechnicalreviewoftheproposedRASdesign.

Scenario4:

Thisscenariopostulatesthatallfourofthecoal-firedgeneratorsatColstriphavebeenretiredandaskstheopenquestionwhethertheremightbe“systembenefitsassociatedwithmovingsomeofthewindtoothersubstationsinMontana,”ascomparedtosimplyconnectingthemtotheColstripandBroadviewswitchingstationsasproposed.

Inshort,therearenoobviouselectricalbenefitstobegainedbyconnectingthewindtootherstations.TheremaybesomesmallbenefittoconnectingmoretoBroadview,andlesstoColstrip,butthisisnotcritical.(ItwouldreducetheprobabilitythataColstrip–Broadview500kVoutagewouldcauseanoverload.)TheCTSiscertainlythemostsubstantialtransmissiontoconnectsuchalargeamountofgenerationtotheMontanatransmissionsystem.

Itwouldbeawisechoicetodividethewind-poweredgeneratorsintosubstantiallyequalsizedgroups.Eachgroupshouldbeaboutthesamesize(say300–500MW).Forexamplefourequalgroupswitheachgrouphavingapproximately525MW.Thiswouldsimplifytheefforttoproperlystudyanddesignthe“Wind-RAS”.TheRASRS(RemedialActionSchemeReliabilitySubcommittee)conductsthetechnicalreviewoftheproposedRASdesign.


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NOTES:

1. Powerflowstudies(steady-state):Issuesthatmustbeaddressedinpowerflowstudiesare:

a. Voltageregulationastheflowlevelvariesfromminimumflowconditionstomaximumflowconditionsbothforthefullsystem,andforoutagesituations.(Voltageatintermediatestationsthatarenotregulatedbygeneratorsmaysagduringheavyflowconditionsonthelines,andfloattoohighwhenthelinesarelightlyloaded.)Voltageregulationisgenerallyachievedbyusingdevicesthatproducequadraturepowertomanagethevoltagewhenthesystemisunderstress.Voltageregulationisthemostimportantsteady-stateissueforthescenariosdiscussedinthispaper.

b. Thermaloverloading(flowmayexceedratingofaline,orseriesconnectedlineequipmentsuchasseriescapacitors,switchesetc.).Thermaloverloadingisgenerallycausedwhenexcessrealpowerisflowingthroughcriticalelementsinthenetwork.

c. RealPowerIssues:ThefirstthingIshouldsayaboutrealpowerissuesisthatthereshouldbeminimalproblemsprovidedtheamountofwind-poweredgenerationisnogreaterthanthatproducedbythecoal-firedgeneratorsinplacetoday.Realpowerisrealpowerwhetheritisproducedbywindturbinegeneratorsorsteamturbinegenerators.Thisistheunderlyingfactthatpromotestheideathatthecoal-firedgeneratorscansimplybereplacedwithwindgenerators.Thisiswhyoneshouldnotexpectnewrealpowerflowproblemstocomeupbecauseofthesubstitutionofwind-powerforcoal-firedpower.Thisimplies,however,thatstudiesthatonlyexaminesteady-staterealpowerflowissueswillnotrevealanyproblemsandreallydoesnotaddresstheprimaryconcernsinvolvedwiththescenariosexaminedinthispaper.


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d. ReactivePowerIssues:11Inthepowertransmissionstudybusinesswegenerallyonlyuse“reactive”unitstoavoidconfusionasmuchaspossibleandsimplytreattheVArsasasignedquantity.Inthisterminology,ashuntcapacitorinjects“reactive”VArsintothepowersystem,andashuntreactorabsorbs“reactive”VArsfromthesystem.Generatorscan(anddo)producequadraturepowerofeithersign.Theydothiswhiletheysimultaneouslyproducerealpower.Typically,coal-firedgeneratorshavelargercapacitytoproduceVArs(ofeithersign)thandowindmachines.ShuntcapacitorscanbeusedtogeneratesupplementaryVArsasneeded,butthesetypicallyswitchinlargesteps.Large“synchronouscondensers”canproduceVArswithcontinuouscontrolsimilartoacoal-firedgenerator.Youcanthinkofa“synchronouscondenser”asageneratorthathasnoprimemover.Becausethereisnoprimemoveritcannotproducerealpower,butithasanexcitationsystemjustlikeageneratordoes,andcanproducereactivepowerofeithersign.Thesecanbeusedtoregulatethevoltageonabussmoothly.

e. Reactivepowerissuesintransmissionlines:Atransmissionlineproduces(orconsumes)VArs.Infact,alinecannotbestoppedfromdoingthis.ThenetVAroutputofatransmissionlinedependsuponhowmuchrealpowerisflowingthroughit.Whenatransmissionlineisidle(connectedbutwithzerorealpowerflowing),itgeneratesthemaximumamountofVArspossible.Itdoesthisasanaturaloutcomeofthephysicalcharacteristicsoftheline(largeparallelconductorsstretchingformilesoverthesurfaceoftheearthwhichisconductive).Atransmissionlineactslikeaverylongcapacitor.

Thelongertheline,themorecapacitanceithas.(Whenphasewiresare“bundled”thisaddscapacitivereactancebecausetherearemorewires.)AllofthelinesintheCTSare“bundled”.Thisisnecessarytomitigatecoronalossesandtoreducetheelectricfieldsthatthelinesproduceatgroundlevel.

11Definitionsoftwocomponentsofpower:WhenACcurrentflowsinanydevice(e.g.powerline);theappliedvoltageisnotnecessarilyinphasewiththecurrent.Foranyphaserelationship,onemaythinkofthecurrentashavingtwocomponents:1)Thecomponentthatisexactlyinphasewiththevoltage:Thiscomponentiscalled“Realpower”(measuredinWatts,Kilowatts(kW)andMegawatts(MW).Realpoweractuallyiscarryingenergythroughthedevicethatiscapableofdoingworkovertime.Normally,wejustshortenthenameto“power”,sinceitisthepartoftheACcurrentthatcarriesenergyfromthesourcetotheload.2)Thecomponentthatisexactly90degreesoutofphasewiththevoltage.Thiscomponentiscalled“Quadraturepower”(or“VArs”).Quadraturepowerdoesnotcarryenergythroughadevicethatiscapableofdoingwork.Itcanhowever,haveaveryimportanteffectontheperformanceofatransmissionsystem.


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Astherealpowerflowonatransmissionlineincreases,thereareVArlossesinthelineduetoitsseriesinductance.Atacertainpredictablerealpowerflowlevel,theVArlossesinthelinebecomelargeenoughtomatchtheVArsbeingproducedbytheshuntcapacitanceoftheline.Thisflowlevelisreferredtoas“surge-impedanceloading”(SIL).(SeriescapacitorshavetheeffectofincreasingtheeffectiveSILofatransmissionline.)WhentheflowisaboutequaltotheSILthelineneitherconsumesnorgeneratedVArs(VArlossesmatchtheVArsthatarebeinggenerated).AtflowsgreaterthantheSIL,theVArlossesgrowlarger,andthereisnetlossofVArsintheline.AtflowshigherthantheSILasystemwillbegintohavelowvoltageproblemsastheVArlossesinthelinecontinuetoclimb.

IfnothingisdonetocounterbalancetheeffectoftheVArlossesthataregeneratedinthelinesasdescribedabove,thevoltageatthebusesalongthelinemaybetoohighatlowflowlevels,andtoolowathighflowlevels.Neitherconditionisacceptable.

AtlowflowlevelsvoltagesonkeybusescouldgotoohighunlesstherearedevicesavailabletoabsorbtheexcessVArsbeinggeneratedbythetransmissionlines.Again,ifthegeneratorsarenotcapableofabsorbingthesurplusVArs,otherdeviceswillhavetobeputintothesystemtodothis.Suchdevicesincludesynchronouscondensers,largestaticVArcompensators,switchedcapacitorbanks,andswitchedshuntreactors.

2. Transientstabilitystudies:Transientstabilityissuesareaboutthedynamicperformanceofthetransmissionsystemduringthefirstfewsecondsafterafaulthasoccurred.ForanyACpowersystemtheremaybecertaincriticalcontingencieswherethesystemmaybedynamicallyunstablebecauseacriticalline(orsetoflines)mustbeopenedtoclearthefault.Suchaneventmayleadtoa“blackout”ofasignificantpartofthetransmissionsystem.Ataminimum,aproperevaluationofaproposalsuchasthereplacementofthecoal-firedplantsatColstripwithwindgeneratorsofequalnetcapacityrequiresathoroughstabilitystudy.ThisisbecausetheCTSalreadyreliesuponaRASfortransientstabilityperformancefornumerouscontingencies.

3. RASissues:ThepurposeofaRASistoprotectthereliabilityofapowersystemforeventsthatcannotbeallowedtoproceedwithoutinterventionbeyondthatprovidedby“simpleprotection”.RASissuesareaboututilizingspecialrelayprotectionschemestoperformnon-traditionalswitching(suchasgeneratordropping,orswitchingofother


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systemcomponents)topromotetheabilityofthesystemtorecoverfromacriticalcontingency.ARAScanexpandtheoperatingrangeofaspecifictransmissionsystemtransferpath(perhapsonethatisotherwise“weak”)beyondwhatcanbeachievedthrough“simple”protectionschemes.(Simpleprotectionschemescanbeconsideredtobethosethatareaccomplishedbyusingtraditionalline-relayprotectiontoopenafaultedlineatbothendsinordertoremoveitfromserviceand“clear”thefault.)ARAScanbeusedtoremedya“powerflow”problemsuchasanoverloadorlowvoltagecondition,oratransientstabilityproblem.TherearenumerouscriticalcontingenciesintheCTSforwhichoperationatfullcapacitywithnoRASwouldresultinunacceptableperformance.Insomecasesthesystemwouldbecompletelyunstable.ForthisreasonitismandatorythattheCTSbeequippedwithafunctioningRASthatiscapableofprotectingagainstthelossofsystemstabilityorinsomecasesunacceptablevoltageperformance(regardlessofthetypeofgeneratorsusedtosupplythepower).Withtheexistingsystem,theRASthathandlesthebulkoftheseissuesistheATR.Thisdeviceemploysanunusualtechnology.ItmonitorstherotorspeedofthesynchronousmachinesatColstrip,andmakesalltrippingdecisionsbasedontherelativespeed,acceleration,andangleoftherotorsduringapotentiallyunstableevent.WiththereplacementoftheColstripsynchronousmachineswithwindgenerators,theATRcouldnotfunctionbecauseoftheprinciplesofoperationofthedevice.Instead,itwillbenecessarytobuildanewRASschemebasedonsomeothertechnology.ThemostobviouschoiceisaconventionalDirectTransferTrippingscheme(DTT).Inthispapertheterm“WindRAS”hasbeenusedconsistentlytorefertothisscheme.Regardlessofthescenario,someformof“WindRAS”willbenecessarytoprovidefortrippingthewindgenerators.ThisrepresentsthesinglemostcriticalengineeringdesignissuethatmustbemetinordertoprovidecomparablereliabilityandcapacityfortheCTSunderthescenariosconsideredinthispaper.

4. SeriesCompensation:Sometimes,“seriescompensation”isusedtoeffectivelyreducetheseriesimpedanceofaline(capacitorsareplacedinserieswithsometransmissionlines).Thishasthebeneficialeffectofimprovingpowerflowcapacityontheline,andvoltageperformance.Itcanbeasolutiontoproblemsdescribedin(Note1)aboveanditcanalsohelpwithtransientstabilityissues,(Note2)above.Youcanthinkof


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seriescompensationasawaytomakeaweaklinestronger.Whenpoweristransferredoververylongdistances(hundredsofmiles)itrequiresacarefuldesigntoachievehighpowertransfers.Ingeneralthelongerthedistance,themoreimportantitistousehighertransmissionvoltages(toreducetherequiredamountofcurrent),themoreimportantitbecomestouseseriescompensation(toeffectivelyreducetheimpedance),andthemorelikelyitbecomesthattransientstabilitywillalsobeaproblem.AlloftheseissueswereimportanttothedesignoftheCTS.

5. Inertiaissues:

a. Inaswitchingeventcausedbyafault,higherinertiainthesourcecan“buytime”fortheresponseoftheprotectionsystemallowingforalongerfaultclearingtime.

b. Thedownsideofhighinertiaisthatifahighinertiageneratorgainsenoughspeedduringafault,itwillbehardertoslowitdownwithouthavingitlosesynchronism.(Thereisnoperfectvalueforinertiafromatransientstabilityperspective.)Duringaswitchingevent(causedbyafault)theaccelerationoftheunitsnearthefaultedlinecontinuesatleastuntilthefaultiscleared(normallythisis3-4electricalcyclesorabout0.050-0.067seconds).Asthecircuitbreakersopenforthefaultedline(removingitfromthesystem)theremainder(weakened)systemmustbecapableofslowingthemachinesthatwereacceleratedbythefaultandrestoringequilibrium.Allofthekineticenergythathasbuiltupintherotorsofthegeneratorsmustbeabsorbedandtheymustberestoredtosynchronousspeed.

c. Forlongtermfrequencyperformanceofacompletepowersystemthemoretotalconnectedinertiathebetter.Thehighinertiamakesiteasiertokeepthesystemfrequencyconstant.Highinertiasmoothstheresponseofthesystemtochangingloadsandchangingtotalgeneration.Reducingtheinertiainalargesystemwouldeventuallyresultindifficultiesmaintainingaconstantfrequency.EasternMontanaiswellconnected(throughtheCTSandotherweakerties)totherestofthewesterninterconnection.ReducingthetotalinertiaineasternMontana,asexaminedthroughthescenariosinthepaper,willmakeeasternMontanamoredependentupontherestofthewesterninterconnectiontomaintainaconstantfrequency.Iftheentirewesterninterconnectionissimilarlyreducinginertia,eventuallythewholesystemwillhavedifficultiesmaintainingaconstantfrequency.

d. Foraswitchingeventsuchastheonedescribedin(b)above,windturbines(type4)wouldnotaccelerateasmuch(theinverteronlytrackssystemfrequency—theprimemovermaybegintodumpwindtoavoidover-speedingtheturbine)duringthefault,


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buttheywouldgoonsupplyingconstantelectricpowertoatransmissionsystemaslongastheirlocalvoltagepermitsthat.Theinvertercontrollerstrivesforconstantpower.Duringanoutage,thesystemmightneedrelieffromthisflow(dependinguponwhichlinemustbeopened).Anysynchronousmachinethatmightbeoccupyingthesamesystemwouldbeacceleratingfromitsownprime-mover—andfromthepresenceofthewindmachinestoo.So,itwouldaccelerateevenfaster.Asthesynchronousmachinesnearbyaccelerate,thewindmachineswillmatchfrequencywiththem.Wind-poweredgeneratorsactuallydohavearotatinginertia(onthewindturbinesideoftherectifier),butthisinertiaiscompletelyde-coupledfromthesynchronouspowersystem.The“type-4”wind-poweredgenerator(thetypeproposed)actuallyproducesACpowerwithinductiongenerators(onthewindturbinesideoftherectifier,thispoweristhenrectifiedtoproduceDCpower,andfinally,theDCpowerisagainconvertedtoACpower(onthesystemsideoftheinverter)atthesystemlocalfrequency(regardlessofthesystemconditions).Withthisdesign,thewind-poweredmachinesshouldbeconsideredtohavenoeffectiveinertia(onthesystemsideoftheinverter).

6. EnergyManagementissues:Powerflowproblemsshouldnotbeconfusedwithenergymanagementproblems.Ofcoursethepowergeneratedbycoal-firedgeneratorsiscapableofremainingconstantforverylongperiodsoftime(days).Thepowerproducedbywind-poweredgeneratorsmaystayfairlyconstantduringwindyperiodsbutthendropsoffinasomewhatunpredictablewayasthewindspeeddrops.Thisvariabilityraisesuniqueconcernsthatmustbeexamined.Fromatransmissionowner’sperspectivethismerelymeansthatthetransmissionwillbeidlemuchofthetimeiftheregulatinggeneration(generationthatistherespecificallytomakeupfortheabsenceofthewindpowerduringcalmedperiods)isnotlocatednearthelocationofthewindgeneration.Whentheiroutputdropsregulatinggenerationisneededtomeetload.Thetreatmentofenergymanagementissueswillultimatelydependonthebusinessstructuresunderlyingthebuildoutofthescenariosdescribedinthispaper.Althoughtheseissuesareimportant,theyarenotthesubjectofthispaper.

7. Sub-SynchronousResonance:Aseriescompensatedlinehasanaturalresonantfrequencythatislowerthanthesystempowerfrequency(hencethetermsub-synchronousresonance).Networksofsuchlineshavemultipleresonantfrequencies.Everytransmissionoutagechanges


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theoverallsetofresonantfrequenciesofthesystem.Ifavoltageisappliedat(ornear)aresonantfrequencyofthenetworkextremelyhighvoltagesmayoccuracrossacapacitorbankoronanybusasverylargesub-synchronousfrequencyvoltagesoccuracrosssystemelements.Thismustbeavoidedtopreventapossiblecatastrophicfailure.Lineequipment(especiallyseriescapacitors)couldbedamaged,andalsogeneratorsfeedingpowerintothesystemcouldbedamaged.ThemostseriousnegativeeffectsphenomenonofthisarecollectivelyreferredtoasSub-SynchronousResonance(SSR).12

8. PathRatingProcess:TheWECChasapathratingprocessthatisdesignedtoassurethatwhenanewpathisproposedormodified(newtransmissiontielineoroldpathimprovement)thestudyworkneededtojustifytheproposedratingisproperlyperformedwithfullattentionpaidtothepotentialforconflictswithexistingpaths.Thisprocessisapartofthenormaltransmissionplanningprocess.Peerreviewisthekeystoneofthisprocess.ThepathownermustannouncetheirplantobuildthenewpathormodifyanexistingoneandinviteinterestedWECCmemberstoattendaseriesofmeetingswherethepathownerpresentstherequiredinformationconcerningtheplanofservice,expectedratingandotherbasicinformationincludingthetargetdateforcompletionoftheproject.SincePath8isanexistingpath,thisprocesscouldbeabbreviated.Thedocumentcontainingtherulesforthisprocessis:“Project_Coordination_Path_Rating_and_Progress_Report_Processes_proposed_changes_2015-09-11.pdf”(orasuccessordocumentavailablethroughWECC).

AsPathoperator,NWEwouldhavetotaketheleadinconductingthisseriesofmeetings.

12TherearethreetypesofSSR:1)Inductiongeneratoreffect(Becauseofthenaturalresonance,ageneratormayspontaneously“feed”thesystemresonancegeneratingcurrentatthatfrequency).2)TorsionalResonance(Whenasystemresonanceoccursasub-synchronoustorqueisdevelopedontherotorofeachconnectedgeneratorduetothecurrentflowinginthemachinewindings.Thistorquecanstimulatearesonanceintheturbine-generatormechanicalsystemthatcancauseseriousdamagetotheshaftofthemachine.)3)TransientTorqueAmplification.(Whenswitchingoccursonthetransmissionsystemeachswitchoperationcausesatransienttorqueoneachconnectedgeneratorshaft.Thefirstswitchingeventstimulatesatorsionalvibrationthatisthenenhancedbythenextswitchingevent(timingiscritical).Thiscanleadtoexcessivestressonthegeneratorshaftasitrings.Shaftfailureispossible.

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