Modeling Metal Oxide Varistors (MOV) in Short Circuit Calcul

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1、SiemensEnergy,Inc.PowerTechnologyIssue111ModelingMetalOxideVaristors(MOV)inShortCircuitCalculationsKrishnatPatilCarlosGrande-MoranStaffSoftwareEngineerPrincipalConsultantkrishnat.patil@siemens.comcarlos.grande@siemens.comIntroductionTheabilitytomodelMetalOxideVaristor(MOV)protectionforseriescompe

2、nsatedlinesinshortcircuit®®calculationsisnowavailableinPSSEVersion33.1.ThisMOVmodelingwillallowPSSEuserstoevaluatetheshortcircuitcapabilityofbreakersusedintheswitchingofseriescompensatedlinesand®seriescapacitorbanksforsymmetricalandasymmetricalfaultcurrents.PSSEfaultcurrentcalculationactivitiesAS

3、CC,IECSandSCMUmodelMOVprotection.Theuseofseriescapacitorsasasolutiontoincreasethepowertransfercapabilityofexistingextrahighvoltage(EHV)transmissionlineshasbeentriedextensivelyinrecentyears.Thegeneralideaistoreducetheinductivereactanceoftheline,therebyincreasingtheelectricallengthandsurgeimpedance

4、loadingofaseriescompensatedtransmissionline.Animportantdesignissueofseriescapacitorsintransmissionlinesistheirovervoltageprotection.Awidelyusedprotectionschemebasedonmetal(zinc)oxidevaristors(MOV)hasproventobeveryeffectiveandreliable.MOVspresentanonlinearresistancecharacteristicthatisapproximated

5、byfundamentalfrequencynonlinearimpedanceduringshortcircuitcalculations.TheseriescapacitorsandMOVprotectionaremodeledtogetherasequivalentbranchimpedance,Zeq=RCeq–jXCeq,controlledby1thebranchcurrentIflowingintotheMOV-protectedseriescapacitorbranch.AtypicalMOV-protectedseriescapacitorarrangementissh

6、owninFigure1.TheMOVprotectionisconnecteddirectlyinparallelwiththeseriescapacitorandperformsitsfunctionbyholdingthemaximumcapacitorvoltagewithinthedesignedprotectivelevel.MOVconductionnormallyoccursonlyduringfaultsbecausethecapacitorprotectivelevelisusuallyspecifiedabovethemaximumvoltagesexpectedd

7、uringoverloadorswingconditions.Innormaloperation,allcurrentflowsthroughtheseriescapacitorandnonethroughtheMOVprotection.1DanielL.Goldsworthy,"ALinearizedModelforMOV-protectedSeriesCapacitors,"IEEETransactionsonPowerSys