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FrequencyMonitoringandControlduringPowerSystemRestorationBasedonWideAreaMeasurementSystem

Frequencycontrolduringpowersystemrestorationhasnotbeenstronglyaddressed.Operatorsareoftenconcernedwiththeofflinesizingofloadandgenerationsteps,but,nowadays,theintroductionofWideAreaMeasurementSystem（WAMS）makesitpossibletomonitorthestabilityofpowersystemonline.TheconstraintsofWAMSoperationresultinsomechangesinpowersystemfrequencycontrol.ThispaperproposesanovelmethodologyforfrequencycontrolandmonitoringduringtheearlystepsofpowersystemrestorationbasedonWAMS.Detailedloadmodelingisachievedbasedonthestaticloadmodelingapproach.Powergenerators’modelingisalsoaccomplishedutilizingthesinglemachineequivalentofthepowersystembasedonPMUmeasurements.Simulationresultsofthepresentedmethodologyonthe39busNewEnglandpowersystemclearlyshowtheeffectivenessandapplicabilityoftheproposedmethod.Thesimulationresultsshowthatthepresentedapproachhasacompletelyacceptableprecisionandanoutstandingspeedwithlessthan0.05%error.Theoutstandingspeedofthepresentedapproachalongwiththeresultprecisionwillresultinagreatpromotioninpowersystemrestorationmethodologies.

1.Introduction

Theproblemofrestoringpowersystemsafteracompleteorpartialblackoutisasoldasthepowerindustryitself.Restorationofapowersystemafterasystemblackoutisacomplex,delicate,andtime-consumingproblem[1].

Systemrestorationaftertotalblackoutrequirescoordinationofunits,loadsandtransmissionsystem,andtheassociatedcharacteristics.Furthermore,variousconstraintsimposedingeneratingrestorationplansmustbeconsidered[2].

Nowadays,newtechnologiesprovidepowerfulnewcapabilitiesinareassuchaslarge—scalesystemanalysis,communicationandcontrol,datamanagement,artificialintelligence,andallieddisciplines.Planningofpowersystemrestorationisacombinationalproblem[2].Thisissuealsoinvolvesrestrictionsandconditionsthatmakeitmorecomplicatedforoperatorstojudgerenderingit.Asystemtoprovideanswerstothisissuewithsufficientspeedwhichisappropriateforpracticalapplicationshasnotbeendevelopedwithsuchconventionaltechniquesasoptimizationalgorithms.

Quickrestorationofpowersystemafterablackoutisasignificantpartofsystemoperation.Intheearlystagesofpowersystemrestoration,theblackstartunitsareofthegreatestinterestbecausetheywillproducepowerfortheauxiliariesofthethermalunitswithoutblackstartcapabilities.Theblackstartunitsareusuallythosewithcombustionturbinesorhydroelectricunits[3–6].

Frequencyisanimportantparameterinpowersystems,andaccuratereal-timemeasuredfrequencyishighlydesirabletounderstandthedynamicsofpowersystems.Throughoutthepowersystemrestoration,verylargestepsofpowergenerators’loadingarepronetoresultinfrequencyprotectiontripsandconsequently,prolongthewholeprocessofpowersystemrestoration.

Operatorsareoftenconcernedwiththesizeofloadingstepsofpowergeneratorsbutnowadays,theintroductionofWideAreaMeasurementSystem（WAMS）whichutilizespharosmeasurementunits（PMUs）leadstoonlinepowersystemmonitoringandcontrol.Severalmajorutilitieshaveshownaninterestinthesynchronousphasormeasurementtechnologyapplication.TheseincludeHydro-Qué

bec,AmericanElectricPower,theNewYorkPowerAuthority,´

Electricit´

edeFrance（EDF）,andmanyutilitiesoftheWesternSystemsCoordinatingCouncil（WSCC）suchasBonnevillePowerAdministration（BPA）andSouthernCaliforniaEdisonCompany[7,8].

BasedontheconstraintsofWAMSoperation,powersystemfrequencycontroldiffersfromthoseoftheoldergenerationsofpowersystemoperationandcontrol.ThemaintaskswhichcanbefullyaccomplishedthroughWAMSincludeearlyrecognitionoflargeandsmallsignalinstabilitiesandmaximizationofloadrestorationamount[9].

Itisnecessarytomakethefrequencycontrolsystemduringpowersystemrestorationbemoreeffective,whichrequiresthelocationandthemagnitudeofallgenerationsandloads.Duringpowersystemrestoration,bothgenerationandloadprofilesareconstantlychanginghowevertheconventionalapproachesforfrequencycontrolandprotectionhaveonlyonesetpointforallscenarios.Offlinepowergenerators’loadingoptimizationisacommonpracticeforelectricpowerutilitiesinordertopreventdangerousimbalancebetweenloadandgenerationandstrongfrequencydeviationsduringpowersystemrestorations.Majordrawbackofconventionalapproachesforfrequencycontrolduringpowersystemrestorationisthatlocalprotectiondevicesdonothaveasystemview,and,therefore,theyarenotabletotakeoptimizedandcoordinatedactions.Eveninthecaseoffrequencycontrolandmonitoring,inwhichthefrequencyitselfisasystemindex,theactionsaretakenlocallyonpredefineddesignrules.Carryingoutimproperactionsduringpowersystemrestoration,especiallyintheearlystages,willprolongtheoverallprocess.Generators’loadingisoneofthemostimportantparametersshouldbemanagedconsideringpowersystemoperationalconstraints,loadcharacteristics,andsoforth.Offlineschedulingofgenerators’loadpickupcouldnotguaranteethattheactionswillnotcausefurtherproblems.

AnecessityexiststodevelopafrequencycontrolandmonitoringapproachduringpowersystemrestorationbasedonWAMSthatcancustomizefrequencycontrolalgorithmsdynamicallyinresponsetoanysystemcondition.Propercoordinationofpowersystemoperationcharacteristics,especiallyWAMScharacteristicsencounteringpowersystemfrequencycontrolandmonitoring,andpowersystemrestorationplanningisthemainobjectiveofthepaper.

ThispaperpresentsasystematicmethodforpowersystemfrequencymonitoringandcontrolduringpowersystemrestorationbasedonWAMS.Thepresentedapproachwhichconsistsofdetailedloadmodelingandpowergenerators’loadingoptimizationistopreventdangerousimbalancebetweentheloadandgenerationandstrongfrequencydeviationsthroughpowersystemrestorationswithapracticallyacceptablespeedandaccuracy.Comparedwiththeconventionalpowersystemrestorationapproacheswhichutilizeofflinepowergenerators’loadingoptimization,thepresentedapproachoptimizesthegeneratorsloadingjustbasedonthecurrentstateofpowersystemwhichresultsinasafe,smooth,andquickrestoration.

UtilizationofWAMSprovidestheoperationsystempredictionofapracticallypreciseloadandgenerationmodeling;

inordertoachievedetailedloadmodel,staticloadmodelingapproachisutilized.Powergenerators’modelingisalsoaccomplishedutilizingthesinglemachineequivalentofthepowersystembasedonPMUmeasurements.SuchahighdegreeofprecisioninloadandpowergenerationmodelingcouldnotbeachievedwithoutusingWAMS.

Propercoordinationofloadandpowergenerationmodelsleadstoareasonableestimationoftheactivepowerimbalanceandthesteady-statefrequency.Usingthesamemodel,theamountofloadpickuporgenerationincreaserequiredtokeepthefrequencywithintheallowablerangescanbecalculated.Powergeneratorsarestudiedbasedontheassociatedclassicalmodels.Thegenerators’internalvoltage,reactanceandrotorphaseangleareestimatedusingPMUmeasurements.Themechanicalpower,inertiaconstant,anddampingconstantcanbeobtainedbyaleastsquareerrorfittingonswingequationsofthemultimachinesystem.

TheproposedapproachforfrequencycontrolandmonitoringduringtheearlystagesofpowersystemrestorationconsideringWAMSapproachconsistsofacoupleofsteps:

preparationofthesinglemachineequivalentmodelbasedonPMUmeasurementsandestimationoftheactivepowerimbalanceandpredictingthesteady-statefrequency.Thesameapproachcanbesuccessfullyutilizedtodeterminetheamountofloadpickuporgenerationincreaserequiredtomaintainthefrequency.Themodelishighlyapplicabletoassessfrequencydeviationsduringpowersystemrestorationplanningwhenthereisnotenoughtimetousedetailedpowersystemfrequencycontrolmodeling.

Theproposedapproachforpowersystemfrequencymonitoringandcontrolduringpowersystemrestorationassistspowersystemoperatorduringtherestorationprocess.Sincetherestorationshouldbesafe,smooth,andquick,suchanapproachshouldbeabletoquicklyandpreciselypredictfrequencyoscillationsandadjustgenerators’loadingsothattheoperationalrisksbeasminimumaspossible.Withintheearlystagesofpowersystemrestoration,inwhichthereareafewonlinepowergenerators,subsystemsarepronetobeunstable.Improperpowersystemfrequencymonitoringandcontrol,especiallywithinsuchstages,willprolongtheoverallprocessofpowersystemrestoration.

Simulationresultsofthepresentedmethodologyonthe39busNewEnglandpowersystem[10]clearlyshowtheeffectivenessandapplicabilityoftheproposedmethod.Itisnoteworthythattheresultsshowthatthepresentedapproachhasacompletelyacceptedprecisionandanoutstandingspeed.Comparedwiththewell-knownpowersystemmodelingsoftwarepackages,powersystemfrequencypredictionisaccomplishedwithlessthan0.05%error.Theoutstandingspeedofthepresentedapproachalongwiththeresultprecisionwillresultinagreatpromotioninpowersystemrestorationmethodologies.

Therestofthepaperisorganizedasfollows.Section2describestheconceptsandbasicformulationoffrequencymonitoringandcontrol.Section3presentstheresultofpowersystemparallelrestoration