Modeling Current Transformer (CT) Saturation for · PDF file1 Modeling Current Transformer...

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December2011Issue…

1 ModelingCurrentTransformer(CT) SaturationforDetailedProtectionStudies

6 TheWoundRotor(WR)andSquirrel Cage(SQ)InductionMachineModels inthePSCAD®Library

9 NewPTConnections

11 MeettheTeam!

12 PSCAD®2012TrainingSessionsDecember 2011

ThePSCAD®MasterLibrarycontainsanumberofmodelsthatcouldbeusedfordetailedprotectionsystemanalysis. Themostimportantofthesemodelsarethecurrenttransformermodels.Currenttransformersaturationisassociatedwithmanyprotectionproblemsencounteredinpowersystems.CTsaturationisacomplexphenomenonandaccuratemodelinginasimulationenvironmentischallenging.ThemodelsavailableinPSCAD®arebasedontheadvancedmathematicalmodelsproposedbyJilesandAthertonandcommonlyreferredtoasthe

‘Jiles-AthertontheoryofFerromagnetichysteresis.’

ThesedevelopedmodelshavebeenextensivelytestedwithfieldrecordingsincollaborationswithCT,relayandprotectionequipmentvendors.

ThiscaseisusedtoillustratetheeffectsofCTsaturation.ThekeyparametersthatimpactCTsaturationarediscussedinanefforttoguidePSCAD®userswhomayusethismodelfortheirsimulations.

GeneralTheory ThemagneticcharacteristicoftheCTisshowninFigure1(hysteresisnotshown).Inthelinearregion,theCTwillbehavealmostlikeanidealratiochanger.Thatis,theCTsecondarycurrentisanidenticalbutscaleddownreplicaoftheprimarycurrent.However,IftheCTsaturates,morecurrentisrequiredtomagnetizethecoreandasaresultthesecondarycurrent(IS)availableasinputstotherelay

maynotbeanidenticalscaleddownreplicaoftheactualprimarycurrent(IP).Thiscanleadtoprotectionissuesandshouldbegivendueconsideration.

SystemOverview TheACsystemshowninFigure2consistsoftwo230kV,60HzThevaninvoltagesources,a75MVAloadandthree230kVtransmissionlines(125km,75kmand200km).Asinglephase(PhaseA)togroundfaultisappliedbetweenthefirsttwotransmissionlines.

ModelingCurrentTransformer(CT)SaturationforDetailedProtectionStudiesDr. Dharshana Muthumuni, Lisa Ruchkall, and Dr. Rohitha Jayasinghe, Manitoba HVDC Research Centre

Figure1 IM–Fluxcurve

2   P U L S E   T H E M A N I T O B A H V D C R E S E A R C H C E N T R E J O U R N A L

CurrentTransformerSaturation CTsaturationcanbeexplainedusingthesimplifiedequivalentcircuitshowninFigure3.

Inthelinearregionofoperation,magnetizingcurrent(IM1)isverysmallandhenceIP–IMisapproximatelyequaltoIP.ThusISwouldbeascaleddownversion(byafactorofN).IftheCTsaturates,themagnetizingcurrentincreases(IM2).Asaresult,onlyapartofIPisavailablefortransformationtothesecondary.Inthissimulationcase,afaultissimulatedonthetransmissionlineandtheCTisattheendoftheline(whereIabcismeasured).

Note:Figure4showshowtheCTmodelisusedinPSCAD®.Notealsothattheinputprimarycurrent(Iabc)isinkiloampsandthesecondarycurrent(Isabc)isinamps.TheCTsaremodeledas(independent)blocksanddonothavetobeconnectedtotheelectriccircuit.

ThisrepresentationisvalidsincetheCTisessentiallyashortcircuit(inseries)fromtheprimarypowersystemnetworkperspective.

KeyParametersImpactingCTSaturationThefollowingcanhaveasignificantimpactonCTsaturationandshouldbegivendueconsiderationinasimulationstudy:

1) DCoffsetintheprimarysidefaultcurrent.2)RemnantfluxontheCTpriortothefault(ifany).3)Secondarysideimpedanceincludingthoseof therelay,connectingwiresandCTsecondary impedance-thisparameterplaysamajorrolein thelevelofsaturationtheCTwillbesubjectedto.

Illustrativesimulationresultsarepresentedinthefollowingsectionstohighlightsomeofthekeypoints.

Figure2 ACsystemmodel

Figure3 SchematicrepresentationofCT(left)andthesimplifiedequivalentcircuit(right)

Figure4 CTPSCAD®implementation

D E C E M B E R 2 0 1 1 3

Case1–Impact of DC offset in the primary fault current:ThepointonthevoltagewaveformattheinstantofthefaultdeterminestheleveloftheDCoffsetintheprimarycurrent.ThemaximumDCoffsetwilloccurwhenthefaultisappliedatavoltageminimum(t=0.49167s).TheresultsinFigure5aoccurwhentheDCoffsetissignificant.

Ascanbeseen,theDCoffsetcausestheflux(Figure5b)tobedrivendownandintosaturation.TheCThassaturatedafterabouttwocycles.ThereductionofthesecondarycurrentisevidentfromFigure5a.

ThesimulationresultsinFigure5bdemonstrateasituationwhenthereisnoDCoffset(faultisappliedatt=0.4876s,voltagemaximum).Ascanbeseen,theCTdoesnotgointosaturationandonlyasmallamountofmagnetizingcurrentisrequiredtomagnetizethecore.Therefore,thesecondarycurrentisanexactbutscaleddownreplicaoftheprimarycurrent.

InFigure5aalsonotetheremnantfluxintheCTcoreoncethefaultiscleared.EffectsofremnantfluxwillbediscussedinCase2.

Figure5a DCoffsetpresent Figure5b NoDCoffsetpresent

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Impact of DC offset in the primary fault current...

4   P U L S E   T H E M A N I T O B A H V D C R E S E A R C H C E N T R E J O U R N A L

Note:theB-HlooptrajectoryoftheCTduringthefaultisshowninFigure6.TheformationoftheminorB-HloopsandhysteresisareaccuratelymodeledbasedontheJiles-Athertontheoryofferromagnetichysteresis.SuchdetailedrepresentationsoftheCTbehaviorarenecessaryfordetailedprotectionsystemanalysis,suchas:

•CTresponseduringautoreclose•ProtectionschemeswithCTsoperatinginparallel –6CTsinparallelinathree-phasetransformer differentialscheme –3CTsinparallelinanearthfaultrelayscheme – CTconnectionsingeneratorprotections

Case2–Reclosing the line while the fault is still present (auto reclose):Inthefirstfaultevent,saturationhadtakenapproximatelytwocycles.Ifthelineisreclosedwhilethefaultisstillpresent,theCTmaysaturatemuchfasterduetothepresenceoftheremnantflux.ThisisdemonstratedinFigure7.Ascanbeseen,thesaturationoccursinapproximatelyhalfacycle,possiblybeforetherelayhadtimetorespond.

Figure7 Case2results

Figure6 B-Hlooptrajectory

D E C E M B E R 2 0 1 1 5

Figure8 Case3results

Case3–Effect of the secondary impedance:TheCTsecondarysideburdenimpedancehasasignificantimpactonCTsaturation.InCase1,theburdenwassetto2.5Ω.TheresultsshowninFigure8wereobtainedbyreducingtheburdento0.5Ωinthesimulation.Ascanbeseenfromtheresults,thefluxisnotdrivendownasfar.ItalsotakesalongertimefortheCTtobecomesaturated(approximately6cycles).

Note:TheCTmodelusedintheexamplecaseisthatofasingleCToperatingindependentlyofanyotherCTsintheprotectionscheme.Ingeneral,aprotectionschememayhaveanumberofCTsoperatinginparallel.TheresponseofoneCTmayimpactthebehaviouroftheotherandthus,shouldbeappropriatelymodeled.ContactthePSCAD®TechnicalSupportTeamformoreinformationonspecificmodels.

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The response of one CT may impact the behaviour of the other and should be appropriately modeled…

6   P U L S E   T H E M A N I T O B A H V D C R E S E A R C H C E N T R E J O U R N A L6   P U L S E   T H E M A N I T O B A H V D C R E S E A R C H C E N T R E J O U R N A L

Dr. Dharshana Muthumuni, Manitoba HVDC Research Centre

TheinductionmachinemodelsaretwoofthemostwidelyusedcomponentsfromthePSCAD®MasterLibrary. ThePSCAD®libraryhastwoinductionmotormodels:

1)Squirrelcageinductionmachinemodel representingadoublecagedesign–(SQ).2)Woundrotorinductionmachinemodel–(WR).

TheTechnicalSupportTeamreceivesquestionsfromourusersastowhichmodeltheyshouldusetorepresentasetofspecificdataprovidedbytheequipmentvendor.ThegoalofthistechnicalnoteistoprovidethenecessaryinformationtohelpPSCAD®userswhenfacedwithsuchquestions.

Mathematically,theSQcagemachinecanberepresentedbytheWRmachine.TheWRmodelcouldalsobeusedtorepresentadoublecageSQmachine.Thetwoexamplesbelowwilldescriberelevantdataentryconsiderationsandalsocompareresultsforvalidationpurposes.

Thesimplesimulationcaseshownbelowisusedtohighlightthekeypoints.ASQmachineandaWRmachinewithcomparableparameters(identicalratingsanddata)areconnectedtothesamesupplysource(60Hz,0.69kV).

Example1:Modelingasinglecageinductionmachine TheSQcagemachinemodelortheWRmachinemodelmaybeusedtorepresentasinglecage(SQ)machine(IM_study_01.pscx).

Theequivalentcircuitofadoublecagedesign(asimplementedinPSCAD®)squirrelcagemachineisshownbelowinFigure2.

Theequivalentcircuitofawoundrotormachinemodel(singlerotorwinding)isshowninFigure3.

TousetheSQcagemachinemodeltorepresentasinglecagemachine:

–Makethe‘secondcageresistance’(RC2)and the‘secondcageunsaturatedreactance’(XLC2) relativelylarge(comparedtotheotherleakage inductances/resistances).Inthiscase,thevalues usedareRC2 =5PUandXLC2 =5PU,whichismuch largerthanRC1 =0.0507PUandXMR =0.091PU.–GivetheSQcage‘rotorunsaturatedmutual reactance’(XMR)thesamevalueastheWR ‘rotorleakagereactance’(XLR).

TheWoundRotor(WR)andSquirrelCage(SQ)InductionMachineModelsinthePSCAD®Library

Figure1 Circuitdiagram(WR–top,SQcage–bottom)

Figure2 SQcage(doublecage)equivalentcircuit

Figure3 WRIM(singlewinding)equivalentcircuit

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Figure4showsthedataentryfortheSQcage(left)andWRmodels(right).Byusingequivalentvalues,bothmodelsshowcomparablebehaviour(Figure5)andrepresentasinglecagemachinedesign.

ThesimulationresultsshowninFigure5showthatthespeed(W–WR,W2–SQcage)andtorque(T–WR,T2–SQcage)ofbothmachinesareidentical.Thus,anyoneoftheinductionmachinemodelsmaybeusedtorepresentasinglecageinductionmachine.

Example2:Modelingadoublecageinductionmachine. TheWRmachinemodelcanbesetuptorepresentadoublecageSQcagemachine(IM_study_02.pscx).

IntheWRmodel,selectthe“No.ofRotorSquirrelCages=1”,asshowninFigure6.TheequivalentcircuitrepresentationisasinFigure7.

Figure4 SQcageandWRsetupconfiguration

Figure5 Simulationresults(IM_study_06_a.pscx)

Figure6 WRconfiguration

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8   P U L S E   T H E M A N I T O B A H V D C R E S E A R C H C E N T R E J O U R N A L

Figure8showsthedataentryfortheWRmodel.Withtheappropriatedata,theSQcageandWRmachinemodelswillgiveidenticalresults(Figure9).

Conclusions Ascanbeseenfromtheresults,aSQcageandWRmachinemodeldeliverequivalentresultswhenconfiguredproperly.Hence,aSQcagemachinemodelcanbeaccuratelyrepresentedusingaWRmachinemodel.PSCAD® users are encouraged to use the WRIM model due to its more conventional and straight forward parameter configuration.

PSCAD®Cases:IM_study_01.pscxandIM_study_02.pscx

Figure8 WRsetupconfiguration

Figure9 Simulationresults(IM_study_06_b.pscx)

A SQ cage and WR machine model deliver equivalent results when configured properly…

D E C E M B E R 2 0 1 1 9

NewPTConnectionsDr. Namal Fernando, Manitoba Hydro

Background Generator and transformer protection upgrade at Seven Sisters generation station.Therearesixunitsandtheexistingelectromechanicalrelaysarebeingreplacedwithmulti-functionaldigitalrelays.Theprojectincludesreplacementoflineendpotentialtransformers;theexistingtwopotentialtransformers(i.e.opendeltawithphaseBgrounded)arebeingreplacedwiththreepotentialtransformers(groundedwye).Thereisonlyonesynchronizingsystemforallsixunits.Incomingvoltagereferenceislinevoltage;phaseABwithBatgroundpotential.

Problem Sincethenewdesignincludesthreepoten-tialtransformerswithneutralgrounded,wecouldnotconnectdirectlytotheexistingsynchronizersystem.TheexistingsystemisbasedonphaseBatgroundpotentialandcommontoalltheunits,whereastheunitswiththenewprotectiondesignwillhaveneutralpointgrounded(i.e.,forunitswithnewprotectiondesign,thephaseBisnolongeratgroundpotential).

Solution InstallanisolatingtransformerbetweenthelinevoltageABandtheincomingvoltageinputtothesynchronizersystem.

PSCAD®Application ItiswellknownthatPTsshouldbeconnectedincorrectphasesequence.Whilethecorrectconnectionscanbedeterminedthroughsimplehandcalculationsbyconstructingrelevantphasordiagrams(Figure1),thisisnotalwaysstraightforward.

Therearemanyinstanceswhereincorrectconnec-tionshavecausedunittrippingatthecommissioningandtestingstage.Thiscouldcreateaheadacheforallinvolvedandtheconsequencescanbesignificant.PSCAD®providesasimpleandefficientwaytoverifythenewconnections.WeusedasimplePSCAD®simulationcircuittoverifythatthePTconnectionscorrespondingtotherunningandincomingvoltagestothesynchronizerareconnectedproperly.

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Figure1 Phasordiagram

1 0   P U L S E   T H E M A N I T O B A H V D C R E S E A R C H C E N T R E J O U R N A L1 0   P U L S E   T H E M A N I T O B A H V D C R E S E A R C H C E N T R E J O U R N A L

ThePSCAD®caseusedforthestudyisshowninthefigurebelow.Thesimulationisnotcomplex.However,wefindthistobeausefulengineeringapplication.TheschematicdiagramsareshowninFigures2and3.

TheimportantdataforthissimulationisjustthePTvoltage(turns)ratios.Otherdetails,suchassaturation,etc.arenotimportantforthisbasicanalysis.

Figure2 NewPTconnection

Figure3 OldPTconnection

PUBLICATIONAGREEMENT#41197007RETURNUNDELIVERABLECANADIANADDRESSESTO

MANITOBAHVDCRESEARCHCENTRE211COMMERCEDRIVE

WINNIPEGMBR3P1A3CANADA

T+12049891240F+12049891277info@pscad.com

D E C E M B E R 2 0 1 1 1 1

JuanCarlosGarciaAlonso,P.Eng.Power System Simulation & Studies Engineer

JuanCarlos(JC)receivedhisElectricalEngineeringdegreefromtheNationalUniversityofColombia,Bogota,Colombiain1996andsubsequentlyreceivedhisM.Sc.degreefromtheUniversityofManitoba,Canada.HedesignedpowertransformerswithPauwelsTransformersinWinnipegandin2006joinedtheMHRC.JCparticipatesinvariousengineeringserviceprojectsintheareasofinsulationcoordination,VSCs,protectionandsuperconductivemagneticdevices,amongothers.HismainfocusduringthisperiodhasbeenwritingnewmodelsformagneticsimulationwithPSCAD®.

JCcanbefoundenjoyinghisfavouriteoutdooractivities,suchasrockclimbing,canoeingandhiking.

ArashDarbandiEngineering Application Specialist

ArashreceivedhisElectricalEngineeringdegreefromtheUniversityofManitoba,Canadain2010andiscurrentlypursuinghisM.Sc.atthesameuniversity.Inhisshorttimeherewithus,ArashhasimprovedthelinefeaturesoftwareofourIceVisionsystemanddevelopedanewalgorithmtoimprovetheexistingsoftware.ArashassistedwiththeLineFaultLocator(LFL)productbydevelopingnewfactoryacceptancetestsfortheLFL,improvingsoftwarequal-ityandperformance.Arashiscurrentlyinvolvedwiththebatteryre-purposingresearchprojectdevelopingapowerelectronicgridinterconnectionandareactivepowerstudy.Heisalsoperformingreactivepowerstudiesindifferentplants,aswellasprovidingrecommendationstoclientstoimprovetheirsystemperformance.

Arashenjoysahealthyandactivelifestyle,volunteeringwiththeUniversityofManitobaSatelliteteamastheirTechnicalCoordinator,andtravellinginternationallytoexperiencecultureandcuisine.

Manitoba HVDC Research Centre

MeettheTeam!

TheManitobaHVDCResearchCentrepridesitselfonitsexcellentcustomersupportandservice.Oursuccessisadirectresultofourclientfocusedefforts.Wearecommittedtoprovidingourclientswiththebestpossiblesupporttoensureoptimumsuccesswithourproductsandservices.“MeettheTeam”willbearegularlypublishedadditiontothePulseNewslettertointroduceourexperiencedteammembers.ThispublicationfeaturesJuanCarlosGarciaAlonsoandArashDarbandi;justafewofthedynamicstaffmemberswearefortunatetohaveattheManitobaHVDCResearchCentre(MHRC).

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ExpandingKnowledgeThefollowingcoursesareavailable,aswellascustom training courses–pleasecontacttraining@pscad.comformoreinformation.

FundamentalsofPSCAD®andApplicationsIncludesdiscussionofACtransients,faultandprotection,transformersaturation,windenergy,FACTS,distributedgeneration,andpowerqualitywithpracticalexamples. Duration: 3 Days

AdvancedTopicsinPSCAD®SimulationIncludescustomcomponentdesign,analysisofspecificsimulationmodels,HVDC/FACTS,distributedgeneration,machines,powerquality,etc. Duration: 2–4 Days

HVDCTheory&ControlsFundamentalsofHVDCTechnologyandapplicationsincludingcontrols,modelingandadvancedtopics. Duration: 4–5 Days

ACSwitchingStudyApplicationsinPSCAD®Fundamentalsofswitchingtransients,modelingissuesofpowersystemequipment,straycapacitances/inductances,surgearresterenergyrequirements,batchmodeprocessingandrelevantstandards,directconversionofPSS/EfilestoPSCAD®. Duration: 2–3 Days

DistributedGeneration&PowerQualityIncludeswindenergysystemmodeling,integrationtothegrid,powerqualityissues,andotherDGmethodssuchassolarPV,smalldieselplants,fuelcells. Duration: 3 Days

LightningCoordination&FastFrontStudiesSubstationmodelingforafastfrontstudy,representingstationequipment,straycapacitances,relevantstandards,transmissiontowermodelforflash-overstudies,surgearresterrepresentationanddata. Duration: 2 Days

MachineModelingincludingSRRInvestigationandApplicationsTopicsincludemachineequations,exciters,governors,etc.,initializationofthemachineanditscontrolstoaspecificloadflow.AlsodiscussedaretypicalapplicationsandSSRstudieswithseriescompensatedlinesasthebasecase. Duration: 2 Days

ModelingandApplicationofFACTSDevicesFundamentalsofsolid-stateFACTSsystems.Systemmodeling,controlsystemmodeling,convertermodeling,andsystemimpactstudies. Duration: 2–3 Days

TransmissionLines&ApplicationsinPSCAD®

Modelingoftransmissionlinesintypicalpowersystemstudies.Historyandfundamentalsoftransmissionlinemodeling,discussiononmodels,suchasPhase,Modal,BergeronandPIintermsofaccuracy,typicalapplications,limitations,etc.,examplecasesanddiscussionontranspo-sition,standardconductors,treatmentsofgroundwire,cross-bondingofcables,etc. Duration: 3 Days

WindPowerModelingandSimulationusingPSCAD®

Includeswindmodels,aero-dynamicmodels,machines,softstartinganddoublyfedconnections,crowbarprotection,lowvoltageridethroughcapability. Duration: 3 Days

ConnectwithUs!January11–13,20122012PSCAD®andRTDSAsiaConferencewww.nayakpower.comBangalore,India

January18–22,2012Elecrama–2012www.elecrama.com/elecrama2012/index.aspxMumbai,India

January24–26,2012DistribuTECHConference&Exhibitionwww.distributech.com/index.htmlSanAntonio,Texas,USA|Booth#4541

March27–30,20122012PSCADEuropeanUserGroupMeetingtraining@pscad.comCastelldefels,Spain

June3–6,2012AWEAWindpower2012Conference&Exhibitionwww.windpowerexpo.orgAtlanta,Georgia,USA|Booth#7513

August27–31,2012CIGRESession44andTechnicalExhibitionwww.cigre.org/gb/events/sessions.aspParis,France

Moreeventsareplanned!Pleaseseewww.pscad.comformoreinformation.

PSCAD® Training Sessions

Hereareafewofthetrainingcoursescurrentlyscheduled.Additionalopportunitieswillbeaddedperiodically,sopleaseseewww.pscad.comformoreinformationaboutcourseavailability.

April17–19,2012FundamentalsofPSCAD®andApplications

May15–17,2012HVDCTheoryandControls

September11–13,2012FundamentalsofPSCAD®andApplications

September18–20,2012WindPowerModeling&SimulationusingPSCAD®

November27–29,2012HVDCTheoryandControls

AlltrainingcoursesmentionedaboveareheldattheManitobaHVDCResearchCentreInc.Winnipeg,Manitoba,Canadatraining@pscad.com www.pscad.com

PleasevisitNayakCorporation'swebsitewww.nayakcorp.comforcoursesintheUSA.

For more information on dates, contact training@pscad.com today!