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ThehistoryofEnergyManagementSystemsforthepast10yearshasbeenacontinualprogresstowarddistributedsystems.AnEMSiseasilydividedalongfunctionallines,apropertythatmanysuppliersandutilitieshavetakenadvantageofindistributingsystemsamongseveralcomputers.ThetechniqueofusingmultipleidenticalcomputerstohandleanEMSwasacceptableaslongastheloadwasevenlybalancedbetweenthedistributedparts.Theexpansionthattookplacefrom1980to1985,however,hasbeenpredominatelyinthenetworkanalysisarea.Thissegmentofthesystemsofaroutweighstheothersthatabalancedsystemapproachisnolongerfeasible.Thedualor“quad”minicomputersystemscannotsupportamodernEMSbutremainanexcellentapproachforevenverylargesupervisorycontrolanddata-acquisition(SCADA)systemsandSCADAsystemswithgenerationcontrolfunctions.
Certainproblemsinherentindistributedsystemsareamplifiedinsystemsthatcombinediversetypesofcomputers.Thetaskofmaintainingthesystemcanbeverydifficultifeachcomputerhasaseparatedatabase.Programminglanguagesandtechniquesthataredifferentalsocomplicatethemaintenanceproblem.Themethodsusedbydifferentsupplierstoaddresstheneedforadistributedarchitectureandtheproblemsassociatedwithsuchanarchitecturearepresentedinthefollowingsections.
ConceptualModelofanEMS
TheinformationusedtodevelopthemodelofanEMSpresentedherewastakenfromastudyperformedbyFerrantiInternationalControlsCorporationin1983anddiscussedintheAugust1985issueofIEEESpectrum.Conceptually,anEMSconsistsoffourparts:
1.Networkanalysis:
stateestimation,contingencyselectionandanalysis,optimalloadflow,voltagecontrol,dispatcher'
sloadflow,anddispatchertrainingsimulator.
2.Generationschedulingandcontrol:
automaticgenerationcontrol,economicdispatch,andinterchangescheduling.
3.Database:
datastorageandretrievalservicesforallareas.
4.SCADA:
dataacquisition,alarmprocessing,andman/machine.
Figure1showstherelationshipbetweenthevariouspartsoftheconceptualmodel.Themostnotablecharacteristicsofthismodelare:
1.Otherthannetworkanalysis,floating-pointprocessingisminimal.
2.Thetotalprocessingrequirementismuchhigherfornetworkanalysisthanforallotherpartscombined.
3.Allareasrequireaveryhightransactionratewiththedatabase(whereatransactionisdefinedasacompleteaccessofthedatabase).
4.TheSCADAfunctionmustrespondtoahighinterruptload.
Itisclearthatthediverserequirementsofthesedifferentareascannotbesatisfiedbyasinglecomputerorbymultiplecopiesofthesamecomputer.Acomputationalloadofover9MIPScannotbesustainedinconjunctionwith800interruptspersecond.Computersthatareverygoodfloating-pointprocessorsdonothandleinterruptswell.Ingeneral,everythingthatcanbedonetoimprovetheperformanceofacomputer(pipelining,caching,etc.)makesitlesseffectiveatprocessingcontextswitchesandinterrupts.Forexample,apipelinedmachinegetsitsspeedbyprocessingseveralinstructionsatonce,eachoneclockcyclebehindtheother.Ifaparticularinstructiontakeseightcycles,bythetimeitmakesitswaythroughthepipeline,sevenotherinstructionsarepartwayintothepipeline.Aninterruptorcontextswitchattheendofthisinstructioncausesthecontentsofthepipelinetobelost.Inthisexample,partsofseveninstructionsarelost.Memorycachingpresentsadifferentproblem.Thespeedofthecomputerdependsonthecachehitrate(thatis,thepercentageofthetotalmemoryaccessesthatcanbesatisfiedoutofcache).Thedifferenceinperformancewithandwithoutcachecanbeasmuchas4to1,butisusually2to1.Theeffectofaninterruptoncacheisthattheinterruptroutineanditsdatareplacethecontentsofcache.Inareal-timesystem,aninterruptusuallycausesataskswitch.Whenthetaskisinvoked,itanditsdatawillagainreplacethecontentsofcache.Whentheinterruptandthetaskitcausestoexecutearecompleteandthesystemreturnstotheinterruptedtask,thecachemustbereloadedagain.Sincetheinterruptroutineandthesubsequenttaskareeachexecutedonlyonce,cachedoesnothelpthemmuch.Evenworse,theinterruptedtask(thatprobablywashelpedbycache)mustreestablishthecachecontentsaftereveryinterrupt.
InanEMS,theSCADAloadischaracteristicallyalargenumberofsmall,quicktasks.Thistypeofloaddefeatsapipelinedorcachedcomputerbycausingthepipelinetobelostandthecachetobereloadedoften.Ontheotherhand,networkanalysisischaracteristicallyasmallnumberoflarge,long-runningtasks.Thesetypesofloadsaregreatlyaidedbypipeliningandcaching.So,clearly,differenttypesofcomputersareneededfornetworkanalysisandSCADA.
GrowthinSCADAtakestheformofhorizontalgrowth[replicatingcomponentstoaddmoreCRTsandremoteterminalunits(RTUs)]andgrowthinnetworkanalysistakestheformofverticalgrowth(expandingthesizeofthecomputerusedfornetworkanalysis).Toaccommodatethistypeofgrowth,theSCADAportionshouldbehorizontallyexpandableandthenetworkanalysisportionshouldbeverticallyexpandable.Thatis,theSCADAcomputer(s)shouldbeincrementallyexpandableasnewenddevicesareadded,andthenetworkanalysiscomputer(s)mustbeupgradeableorreplaceabletoaccommodateadditionalfunctionsandbiggerpowersystemmodels.Ifthegrowthratefrom1980to1985ismaintaineduntil1990,networkanalysiswillrequirealmost160MIPSofprocessingpower!
Generationschedulingandcontrolissmallenoughtobeplacedwhereveritisconvenientandisnotsignificantintermsofexpansion.
Thefollowingsectionswillconsiderthethreemajorarchitecturesinusetodayandwillcontrastthemtotheconceptualmodel.Thefollowingdiscussionshouldbetreatedasatheoreticaldiscussion,notadescriptionofanyexistingsystems.Certainly,manyexamplesofperfectlyviablesystemsthataresimilartooneofthefollowingarchitecturescanbefound.Inreality,mostsystemsaresomewherebetweentwoofthesearchitectures.Forexample,theOntarioHydrosystemdescribedearlierhasattributesofboththecentralizedanddistributedarchitectures.
CentralizedArchitecture
ThecentralizedarchitectureshowninFig.2istypicalofseveralofferingsavailabletoday.Thissystemconsistsofapairofmainframecomputersandsomenumberofmicroprocessorsconnectedtothemainframe.
Themicroprocessorsperformlow-leveldata-acquisitionanddisplaysupportfunctions,leavingthebulkofSCADAinthemainframesalongwithnetworkanalysisandgenerationschedulingandcontrol.
TherelationshipbetweenthissystemandtheconceptualmodelisshowninFig.3.Thisapproachpartiallyaddressestheproblemofhandlingalargenumberofinterrupts;
themicroprocessorsabsorbsomeofthem.ItprovidessomelevelofhorizontalexpansionwithinthemicroprocessorsbutdoesnotallowforexpansionofSCADAprocessingcapabilityindependentofnetworkanalysis.Thissystemprovidesexcellentverticalexpansion;
typicalmainframefamilieshaveprocessorsfromsmalltoverylarge,butthelackofhorizontalexpansioncapabilitycouldresultinbeingforcedintoamainframemodelupdatetoaddRTUsorCRTs.
Themaintenanceofthissystemisextremelysimple.Infact,somesuppliersappeartohaveabandonedthedistributedarchitecturebecauseoftheirexperienceintryingtosolvethemaintenanceproblemsthatarepresentinearliersystemsthatlackedacentralizeddatabasemanagerandcommonprogrammingtools.
Theresponsivenessofthistypeofsystemwillvarygreatlywithsystemconditions.Duringupsetconditions,averyhighnumberofalarmsmaybereceived.Processingthesealarmswilltakeresourcesawayfromnetworkanalysisanddisplaysupport.Settingtheprioritiesofthesystemsuchthatprocessingalarmsissecondarytonetworkanalysiscancausealargealarmsbacklog,hidingserioussystemconditionsfromtheoperator.Whileitmaybereasonabletoallownetworkanalysisfunctionstobesloweddownbyalarmprocessingwiththesetoffunctionsavailabletoday,asclosed-loopfunctionslikevoltagecontrolbecomemoreprevalent,thismethodofresolvingresourcecontentionwillbecomelessacceptable.
Inaddition,whatisreasonabletoasystemdesigner(slowingdownnetworkanalysis)maybeunreasonabletothesystemoperator.Inthefuture,utilitiesmayhaveengineersonduty,studyingthesystemasitgoesdown.Ifthesystemisnotresponsive,includingnetworkanalysis,duringupsetconditions,theengineerandtheoperatorwillbeunhappywithit.
ClusteredArchitectures
TheuseoftheVAXCluster[fromDigitalEquipmentCorporation(DEC)]toimplementadistributedEMSisarecentdevelopmentthathasbeenadoptedinvaryingdegreesbyseveralsuppliers.ThesystemdepictedinFig.4istypicalofthesesystems.ThistypeofsystemusuallyconsistsofsomenumberofVAX8000seriescomputers,performingnetworkanalysisandgenerationcontrolandscheduling,somenumberofsmallerVAXcomputersperformingSCADA,andtwoHSC(hierarchicalstoragecontroller,anintelligentstoragedevice)50sand70sactingasacommondatabase.MostsuppliersusemicroprocessorstohelphandletheSCADAload,particularlyfordataacquisition.Eachcomputerhasadedicatedbackupinsomesystemswhereasothersprovidedegradedoperationwhenoneofthenetworkanalysiscomputersfails.
TherelationshipbetweenthistypeofsystemandtheconceptualmodelisshowninFig.5.Thisarchitecturedoesnothandletheproblemsassociatedwithinterruptsverywell.TheVAXitselfdoesno