外文翻译原文-继电保护装置的电力系统监测和控制

IEE2ndInternationalConferenceonAdvancesinPowerSystemControl,OperationandManagement,December1993,HongKongPowerSystemMonitoringandControlFacilitiesonProtectiveRelaysSMHadenandRSquiresGECALSTHOMProtection&ControlLtd,Stafford,UKAbstractItisnowpossibletoconsiderintegratingthefunctionsofthepowersystemprotectionsystemswiththoseofthelocalandremotedatacollectionandcontrolsystems.Astructuredapproachtothisintegrationisnecessaryhowever,ifthefullbenefitsaretoberealized.Asolutionwhichwillsolvemanyoftheproblemspreviouslyassociatedwiththisintegrationispresentedtogetherwithanexampleofhowitmightinfuturebeappliedinatypicalsubstation.Keywords:Digitalcommunications,Integration1.IntroductionThecurrentpracticeinpowersystemtransmissionanddistributionenvironmentsistoseparatethefunctionsofthelocalcontrol,protectionandsupervisorycontrolanddataacquisition(SCADA)systems.Onereasonforthishasbeenthetechnicalconstraintthathaslimitedtheamountofintegrationwhichcanbercliablyachieved.Localcontrolfacilitieshaveconsistedofhardwiredpanelstakingupmuchcontrolroomspace.Controllogichasbeenprovidedbyhardwiredcontactsorprogrammablelogiccontrollers.Untilrecentlymuchoftheprotectionequipmenthasconsistedofanaloguedevices,againtakingupmuchspace.Mostmodernprotectiondevicesusingelectronicandmicroprocessortechnologieshavesofarconcentratedonreducingthespacetakentoimplementtraditionalprotectionfunctions.Generally,SCADAsystemshavebeenaddedmorerecentlyandhavesuppliedtheirowntransducers,interfaceunitsandwiring.Theseharegrownupinparallelwiththelocalcontrolandprotectionsystemsdespitethefactthatthisoftenresultedinmuchfunctionalduplication.WhereinformationconcerningtheprotectionoperationhasbeenrequiredbytheSCADAsystemthishasbeenderivedinasecondaryfashion,forexample,feedingtheprotectionoutputsbackintoSCADAdigitalinputunits.Recenttechnologyadvanceshaveledtotherealizationthatthisdegreeofduplicationisbecominglessandlessnecessary.Giveninfinitecomputingpoweritcouldbearguedthattheinformationnecessaryto91performprotectionfunctionsisallavailableorcanbemadeavailableontheSCADAnetwork.ItisconceivablethenthattheSCADAsystemcouldperformitsownprotectionalgorithmsandissueitsowntripsignalsthroughitscontrolnetwork.Inpracticereliabilityrequirementsandtheneedforrapidfaultclearanceshavelimitedthisapproachtoafewspecializedinstancessuchaslongtimethermaloverloadprotection.AfarmoreviableapproachistomaketheinformationandcontrolfacilitieswithintherelayavailabletotheSCADAnetwork.IfthisisdonemanyofthecostsassociatedwiththeSCADAanalogueanddigitalI/Osystemscanbereduced.Additionally.ifcontrolfacilitiesprovidedwithintheprotectionequipmentareutilized,acentralsubstationcomputercanalsobeusedtoreplacemuchofthelocalcontrolsystem.Onereasonforthefailureofsystemstointegrateprotectionfunctionswithinanoverallcontrolpackageisthesheeramountofprocessingrequired.Moderndigitalprotectionrelaysusestateoftheartmicroprocessorstoprovidecomplexprotectionfunctions.Whenmanyofthesearespreadaroundasubstationitisclearthattheprocessingpowerrequiredtoabsorbtheirfunctionsatacentralpointisformidable.OntheotherhandtheanalogueanddigitaltransducersusedbytheSCADAsystemarcrelativelysimpledevicesasarethedigitaloutputunits.TheirequivalentsalreadyexistwithintheprotectionandmakingthemavailabletotheSCADAsystemoftenrequireslittlemorethantheadditionofcommunicationsfacilities.TheabilityoftheprotectionequipmenttoreplacemuchofthelocalcontrolandSCADAI/Osystemshingesontheabilityoftheprotectionequipmenttocommunicateinastructuredanddeterministicway.ItisessentialthattheprotectionperformanceisnotcompromisedwhilstatthesametimetherequirementsofthelocalcontrolandtheSCADAsystemsarestillmet.FromthelocalcontrolandSCADApointofviewtheprincipalrequirementsareforanalogueinputsformeasurementanddatalogging,digitalinputdataforannunciatorsandalarms,anddigitaloutputsforcontrollingplant.Mostmeasurementdataisusedforgeneralindicationpurposeswhereanaccuracyoff5%issufficient.Analogucanddigitaldatausedforvisualindicationsanddataloggingrequirescanratesofaroundonceasecond.Wheresequenceofeventrecordingisrequiredrelativeaccuracyacrossthesystemismoreimportantthanabsoluteaccuracy.Generallyaresolutionof+lmsshouldbeaimedfor.Controlresponsetimesshouldbearound200ms.2.ProtectiveRelayCommunications2.1CommunicationsPhilosophyTheprotectiverelaysprimefunctionremainstheprotectionofthepowersystem.Itisessentialthereforethattherelaysprotectionperformanceisnotcompromisedbytherequirementsofdatamonitoringandcontrol.Forthisreasonitisconsiderednecessarytoprovidemonitoringandcontrolcommunicationsseparatefromanycommunicationsrequirementsoftheprotection.Thusinablockingschemeforexample,blockingsignalswouldbetransmittedovertheirownprotectionsignallinglinke.g.pilotwires,andnotoverthemonitoringandcontrolcommunicationslink.Inthiswaythedeterministicbehaviouroftheprotectionismaintained.Also,thereremainsthoseuserswhodonotyetneedsomeorallofthefeaturesavailableItisimportantfortheseusersthattheoperationoftherelaydoesnotdependonthemonitoringandcontrolcommunicationslinkandthatthefullprotectioncapabilitiescanstillberealizedwhensuchlinkshaenotbeeninstalledThefullbenefitsofrelaycommunicationswillonlybeachievediftheycanbeinstalledatalltherelevantpointsonautilityspowersystem.Thiswillnothappenovernightanditisthereforeveryimportantthatanychosensystemcanbeinstalledonapiecemealbasisacrossasystemasitbecomesrequired.Oneofthemajorfactorsirffluencingthetakeupofrelaycommunicationswillbethecosttotheuser.Thiscostconsistsnotjustoftheadditionalcostofthehardwareontherelaybutalsowiringcosts,set-upandconfigurationcostsandon-goingoperationalcosts.Itisimportantthereforethatstepsaretakentocontrolallofthesecostareas.SetagainstthesecostsshouldbethesavingsontheSCADAsystemandtheoperationalsavingswhichresultfromtheincreaseinsystemdataavailable.2.2CommunicationsTopologyItispossibletoconnecttheSCADAsystemandtheprotectiverelaysusinganumberofdifferentcommunicationstopologiesThechoiceoftopologyisimportantasithasadirectbearingonthecommunicationsefficiencyofthesystem1SCADASYSTEM1,/ILIFigure1:SimpleProtectiodSCADATopologyAsimpleformofconnectionistoconnecteachrelayseparatelytoremoteterminalunits(RTUs)fittedwithdigitalcommunicationsfacilities.TheseRTUsinturnconnecttotheSCADAnetwork-seeFigure1.TheseRTUsactasnetworkswitches,themainSCADAsystembeingresponsiblefortheactualpollingofinformation.Inthistopologytheprotectiverelayshaveeffectivelybecomeintelligenttransducers.ThereisasavingfortheSCADAsystemintermsofthetransducersthathavebeenreplacedbutthismaybeoffsetbythemorecomplexRTUs.Evenatthissimplelevelhowever,therearebenefitstotheSCADAsystemintheamountofadditionaldatathatisavailablefromtherelays.Unfortunatelyitisthissameincreasedamountofdatawhichultimatelylimitstheperformanceofsuchsystems.1SCADASYSTEM1IFigure2:UseofMultidropConnectionsAnimprovedcommunicationstopologyisillustratedinFigure2.SeveralrelaysareconnectedtoasingleRTUonasinglecommunicationsspur.Thisrelieson92theprotectiverelaysbeingfittedwithacommunicationslinkcapableofmultidropconnection.InthisschemetheRTUisnowresponsibleforthepollingofallunitsattached.Inthiswayinformationcanbepre-processedandoveralldataratescanbereduced.ThisrequiresamorecomplexRTU,howeverasingleRTUcanhandlemorerelayssofewermayberequiredandwiringcomplexityreduced.Intheorythisprinciplecouldbeextendedtotheentiresubstation,usingjustoneRTUtocommunicatewithallprotectiverelays.Inpracticethisisnotpossibleduetodatarateconsiderationsandisalsoundesirablefromareliabilitypointofview.Thenumberofdeviceswhichcanbecontinuouslymonitoredcontrolledonasinglechannelisdependentonboththebaudrateusedandtheamountofdatatobetransferred.igure3:UseofSubstationCentralComputerAmoresophisticatedtopologyisshowninFigure3.ThistopologyutilizesanIBMPCcompatiblecomputerasasubstationcomputer.Wherereliabilityisthoughttobeaproblemasecondslavecomputerisaddedinparallelwiththefirst.ThesubstationcomputerreplacestheRTUsdescribedaboveandgivesanumberofadvantagestotheuser.FirstlythereisnowalocalcontrolpointwithinthesubstationinadditiontotheremotecontrolfacilitiesoftheSCADAnetwork.Thiscantaketheformofamimicdiagramprogram,completewithpanelmetering,annunciators,etc.,freeingmuchofthelocalcontrolsystempanelspaceifrequired.Thesubstationcomputerisresponsibleforthecontinuousmonitoringofalltheconnecteddevicesandcarriesoutsubstantialdatapre-processingforthemainSCADAsystem.InparticularthePCperformslocaldataloggingtoitsowndisk,relievingtheSCADAsystemofasubstantialprocessingburden,especiallyduringandimmediatelyfollowingfaultsituations.ThisdataissubsequentlyavailablebothlocallyatthesubstationPCandremotelyontheSCADAsystem,asrequired.Thesubstationcomputercanalsobeusedasasingleaccesspointtoallrelaysatcommissioningtime.Separatecommunicationsspursarelikelytobetakentoeachsubstationsection,eachcapableofsupporting32relays.UptoeightspurscanbeprovidedbyasinglePCgivingatheoreticalcapacityof256relays.OnsuchasystemitisstillpossibleforamodernPCtopollandextractdatafromeachrelayatarategreaterthanonceasecond.Intheunlikelyeventthatthisnumberofrelaysisinsufficientfurthersubstationcomputersmaybeadded.ThesemaybeindependentlyconnectedintotheSCADAsystem.Alternatively,anoptionaladditionallevelofsubstationcomputerwiththesamecontrolfacilities,maybeadded,asinFigure4.Notenowthateachsubstationcomputermaybephysicallyremote.ItisalsoworthnotingthatthisfinaltopologyhasineffectbecomeaminiSCADAsysteminitsownright.Formanysmallerutilitiesthissolutionmayofferalltheremotefacilitiesrequired.Ga,-iIIISCADASYSTEMigure4:MultiLevelTopology2.3CommunicationsHardwareHardwarefordigitalcommunicationscantakemanyforms,mostofwhicharenotsuitableforuseinpowersystemenvironments.Thefirstchoicetobemadeisbetweenparallelandserialsystems.Parallelsystemsinvolvethetransmissionofseveralbitsofinformationconcurrentlyoverseveralseparatewires(typicallyeightorsixteen).Suchsystemsofferfasterdatatransferratesthanserialsystemsbutinvolvefarhigherwiringcosts.Forthisreasontheyarenotsuitableasauniversalsolutionforpowersystemmonitoringandcontrol.Serialcommunicationsinvolvethetransmissionofstreamsofdataonebitatatimeoverasinglepairof93wires.Clearlywiringcostsarereducedattheexpenseofoveralldatatransmissionrateswhichareproportionallylower.Formonitoringandcontrolapplicationstheslowerdataratesremainacceptableandserialtypecommunicationsareusedalmostexclusively.ThecommunicationshardwaremostcommonlyusedbyprotectiverelaysatpresentconformstotheEIAsRS232standard.Thistakestheformofthefamiliar25or9wayDconnector.Thishasusuallybeenusedtoconnecttherelaytoapersonalcomputer(sometimesindirectly,viaamodem)allowingtherelaytobeset-upandallowingpostfaultinformationtobeextracted.RS232connectionsareconvenientbecauseoftheiralmostuniversalavailability.RS232connectionsdohaveanumberoflimitationswhichmakethemlesssuitableinmonitoringandcontrolapplications.ThemostseriousoftheseisthatRS232isdesignedforpointtopointsystems.Asingledevicecanonlycommunicatewithoneotherdeviceoveragivenlink.Ifcommunicationswithmoredevicesarerequired,astheyarefordatamonitoringandcontrolwithinsubstations.separatelinksmustbeprovided.Alternatively,multiplexersorcodeswitchescouldbeaddedthoughthiswouldgenerallyimposesomesortoflimitationonhowthelinksareoperated.RS232alsoimposesalimitonthephysicallengthofthecommunicationslinkofjustover15,andamaximumdatarateof19.2kbaud.Thiscanalsobeovercomebutagainrequiresadditionalequipment.FinallyRS232doesnotofferanysignificantlevelofisolation.OpticallyisolatedRS232portscanbecreatedbuttheseareexpensive.AmoresuitablecommunicationsstandardisRS485.Thisallowsforamultidropsystemwithupto32nodesonasinglespur,sufficienttoconnectatleastasinglebayofrelays.RS485specifiesamaximumtransmissiondistanceof1200metresandamaximumdatarateoverthisdistanceoflookbaud,significantlyfurtherandfasterthanRS232.ItusesabalanceddriveranddifferentialsignallingwhichislesssusceptibletointerferencethantheunbalanceddriverreferencedtogroundasusedinRS232systems.RS485requiresasingleshieldedtwistedpaircablewhichislowcostandeasytoterminate.Withintheelectricalindustryingeneralthishastypicallybeenterminatedineither25or9wayDconnectorssimilartothoseusedbyRS232.Inasubstationenvironmenttheseconnectorsarenotreallysuitableandapairofconventionalterminalsispreferred.Theuseofopticalfibrestoconnectdirectlybetweenrelaysremainsexpensiveformostusers,especiallyatdistributionvoltagelevels.Fibresarehoweversuitableforconnectingthelocalnetworkofrelaystoremotemasterstationswheredistancesexceed1200metresorwheretheriskofinterferenceishigh.Insuchcasesmodemsareusedtointerfaceagroupofrelaystoanopticalfibre.Aswithelectricalbasedcommunications,anumberofdifferentsolutionsareavailable.Fordistancesofuptofourorfivekilometres,850nmmultimodefibresarequiteadequateandofferrelativelylowcomplexity.Forgreaterdistancesuptoaround25kilometres,1300nmsinglemodefibrescanbeused.Theserequiremorecomplextransmittersandreceivers,howevertheirwidespreaduseinthetelecommunicationsindustrymeansthattheymaywellbecomemoreeconomicthan850nmfibresevenovershortdistances.Serialdatacommunicationsmaybeclassifiedaseitherasynchronousorsynchronous.RS232communicationsportsonprotectivcrelaysareinvariablyasynchronous.Inasynchronoussystemstimingorsynchronizationinformationistransmittedtogetherwitheachcharacter.Insynchronoussystemseitheraseparateclockistransmittedorthereceiverderivestheclockinformationfromthedataitself.Synchronoussystemsaremorecomplexthanasynchronoussystemsbutroughly20%moreefficient.Moreimportantlyinter-charactergapsonsynchronoussystemsarefixed.Thisallowsfrequencymodulated(FM)codingmethodstobeusedwhichresultinnoDCcomponentinthesignal.HencetheproblemsofisolationinRS232systemscaneasilybeovercomeusingsimpletransformerisolation.AsecondbenefitofFMcodingisthatthetwosignalwiresarenon-polarizedandmaybefreelyinterchangedthroughoutasystem.Modernserialcontrolchipsallowtheextracomplexityofsynchronoustransmissiontobeabsorbedatnoextracost.Insummary,ofthecommoncommunicationsinterfaces,multidropsynchronousRS485transmissionusingsomeformofFMencodingiscurrentlythemostsuitableforuseinpowersystemdatameasurementandcontrolapplications.Thiscanprovidefasteconomiccommunicationswithelectricalinterferenceimmunitysufficientforpowersystemenvironments.2.4CommunicationsLanguageSuccessfuldigitalcommunicationsdependsnotjustoncompatiblecommunicationshardwarebutalsoonthecommunicationslanguageandprotocolthatareused.Traditionallyrelaymanufacturers(incommonwiththoseinotherfields)havedevelopedtheirownlanguages.Thishasbeenlessimportantinthepastwhentherehasbeennoneedtointegratetherelaysintocontrolsystems.Whentherelayshavebeenintegratedabespokesolutionhasbeennecessarywithcustomprogrammingforeachdifferentrelay.The94costsofthishavebeenuneconomicformostusers.Untilnownolanguagesuitableforusebyallprotectiverelayshasbeenproposed.Themajordrawbackwithmostlanguagesisthattheyassumethemasterstationmusthaveanintimateknowledgeoftherelay.Ifaparticularpieceofdatahasbeenrequiredithasbeenaskedforusingitsmemorylocationintherelayorsomedevicespecificcode.Thisaddressmustbeexplicitlycodedintothemasterstationsoftware.Moreovertherelayhastypicallyrespondedwithrawunformatteddata.Themasterstationmustassumeascaleforthereplyandconvertitaccordingly.Wherealargeamountofdatamustbeextractedfromarangeofrelaystheproblemsarefurthercompounded.Evenwhencommunicationsareonaonetoonebasiserrorscanstilloccurifthemasterstationsoftwareversiondoesntmatchthatoftherelay.Thelanguagepresentedhereovercomestheseproblems.ItissuitableforusebyallrelaysandotherI/Odevicesanditdoesnotrequirethemasterstationtohaveanintimateknowledgeofeachrelaytype.Thelanguageisdesignedaroundadatabasestoredineachindividualrelay.Therelayusesthisdatabasetostorealldataandsettings.Thecontentsofthedatabasearethenmadeaccessibleoverthecommunicationslink.Thestructureofthedatabaseisverysimilartothatofaspreadsheet,consistingofindividualcellsorganizedintorowsandcolumns.Asinglecellisthesmallestunitofaccesswithinthedatabaseandcanthereforecontainonlyoneitemofdata.Thesystemdiffersfrompreviousonesinthattheoverallsystemdatabaseisnowdistributedamongsttheindividualrelaysandnotconcentratedinthemasterstation.Theadvantageofthisisthatrelayspecificinformationisembeddedwithintherelayandnotsomespecializedmasterstationsoftware.Moreoverthecommunicationslanguagecanbereducedtoasetofgenericcommandswhichperformspecificoperationsonthedistributeddatabase.HeadmgCeUi+3SemneIContrulCellValueCeUwyigure5:DatabaseLayout&CellTypesThedatabaseconsistsofthreedifferenttypesofcell,eachonebeingasupersetoftheprevious.seeFigure5above.ThethreecelltypesareHeadingCells;ValueCellsandSetting/ControlCells.HeadingCellscontainasimplepieceoftext.Theseareusedthroughoutthedatabaseasplace-markerstosplitthedatabaseintodifferentareas.ThemostcommonHeadingCellsarethedatabasecolumnheadingcells.ValueCellscontainapieceoftexttodescribetheircontentsandavaluewhichmayberead.Intrinsicinthisvalueisadatatypewhichinstructsthemasterstationhowthedataistobeconverted.TypicalValueCellsaremeasuredvaluessuchasphasecurrents,deviceinformationsuchasmodelnumber,waveformrecords,etc.Setting/ControlCellsaresimilartoValueCellsbuttheircontentscanbechanged.Thesecellsadditionallycontaininformationabouttheminimumandmaximumvaluesforthecellandthevalidsteps.TypicalSettingKontrolCellsarerelayprotectionsettingssuchascurrentthresholds,systemcontrolcellssuchascircuitbreakercontrol.etc.Individualcellsaregroupedtogetherintocolumnsofrelatedinformation.Thecellinthefirstrowofeachcolumnisaheadingcellwhichdescribesthecontentsofthecolumn.Thisorganizationisinvariantacrossallrelays.Thusthecontentsofanyrelaycanbereadinthesameway.Firstthecolumnheadingsareextractedandpresentedtotheuserasamenu.Fromthismenutheuserselectsaparticularcolumn.Thetextandvaluesforeachcellintheselectedcolumnarethenextractedandagainpresentedtotheuserasamenu.Individualcellsmaythenbeselectedforfurtheroperation.Typicallythiscouldbechangeofsetting,assignmenttoameasurementvalueonamimicdisplay,logtodiscorrealtimegraphing.Inpracticeitisfoundthatallrelaytypescontainacertainamountofcommoninformation.Thisincludestherelaytype,niodelnumberandserialnumber,municationsaddress.etc.Thisinformationisgenerallyrequiredbythemasterstationwhentherelayisfirstconnecte