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1、AReviewonOperationofMicroGridsandVirtualPowerPlantsinthePowerMarketsElahehMashhour* S.M., Moghaddas-Tafreshi*K.N.Toosi University of Technology. Faculty of Electrical Engineering, Tehran,IRAN E-mail: mashhouree.kntu.ac.ir of network operation and market transactions in DG operation. These subjects a
2、re described in following.Abstract-TheshareofDistributedGeneration(DG)inthepowersystemgenerationisincreasinglygrownupandfortheeconomicalandtechnicalreasons;theirintegrationisdeeplyconcentratedbytheresearchers.ThispaperreintroducestheintegrationofDistributedEnergyResources(DER),i.e.DGs,controllablelo
3、adsandenergystorages,intomicrogridandVirtualPowerPlant(VPP)andalsoreviewstheiroperationinthepowermarket.Moreover,itdiscussestheperspectiveofmarkettransactionsoffutureVPPsinactivedistributionnetworks.Keywords:VirtualPowerPlant,MicroGridI. IntroductionTheworldisgoingtouseDG,bothfossil-fuel-basedandren
4、ewable-basedone,duetoappearanceofenvironmentalviewpointsandlimitationoffossilfuels.Atpresent,severalsupportiveregulationsinthewholeworldsuchasseparategreenmarketforelectricityproductionofrenewableresources1-31,EuropeanRESDirectivefordevelopmentofrenewableresources4,EuropeanCHPDirectivefbrenergyeffic
5、iencyimprovement5,andsimilarsupportiveregulationsinothercountries.RenewablePortfolioStandard(RPS)inseveralcountriesI6,7,Kyotoprotocolwhichhasaroleinthereductionofgreenhousegasemissions,andsoonarethefactorsthataccelerateDGgrowth.Therefore,thepenetrationofDGinthepowersystemwillbehighlyincreasedinthene
6、wfuture.DuetotheeffectofDGontechnicalparameterofthenetworkandalsoconsideringtherapidextensionofpowermarketinthewholeworld,itisnecessarytocontemplatebothtechnicalaspectsFirstofall,sincetechnicalparametersofthenetworksuchasnodevoltagesandlinecurrentsstronglydependontheDGoperation,thehighpenetrationofD
7、Gmaycreateseveralproblemsinthedistributionnetworksconcerningtostabilityandpowerquality.Furthermore,someconsumerswhoareownersofsomeDGsmayinjecttheirsurpluspowertothegridinsomehoursandcompensatetheirshortagebythegridinothertimes.Therefore,thepowerflowcurrentinthenetworkmaybealteredduringthedayinhighpe
8、netrationofDG.ThispowerexchangebetweentheDGownerandthenetworkisrequiredtobehandled.Inthiscondition,thestandardparadigmofcentralizedcontrol,whichisusedinthecurrentelectricityinfrastructure,willnolongerbesufficientandthedistributedcontrolwillbeconsidered.Secondly,thepowerindustryinthewholeworldisrapid
9、lyrestructuringandthepowersystemoperationshouldbemarket-based.However,ingeneralDGunitscannotbeviablebythemselvesonlyinthepowermarketduetotheirspecialcharacterssuchastheirsmallsizeandstochasticnatureofsomeunits(e.g.windandphotovoltaic).Tosolvetheseissues,DGunitsshouldbecombinedtogetherinoneentity,int
10、egratedentity,andalsocontrolledbyEnergyManagementSystem(EMS),whichisknownasanessentialinfrastructureofthedistributedcontrol.Therefore,amarket-baseddistributedcontrolwillberealizediftheoperatorofintegratedentityisparticipatedinthepowermarketonbehalfofitscomponents.Generallytwointegrationstrategiesare
11、existed,includingintegrationinamicrogridandinaVPP8-27,31.ThispaperreintroducestheconceptsofmicrogridandVPPandalsoreviewstheiroperationinthepowermarket.Moreover,itdiscussestheperspectiveofmarkettransactionoffutureVPPsinactivedistributionnetwork.Theremainingofthepaperisorganizedasfollows.SectionIIdefi
12、nesmicrogridandVPP.SectionIIIandIVreviewtheoperationofmicrogridandinthepowermarketrespectively.Thediscussionsandconclusionsareprovidedinfinalsection.II. DefinitionofIntegratedEntitiesA. Microgrid978-1-4244-3523-4/09/$25.00 c 2009 IEEE273Themicrogridconceptisbasedontheassumptionthatlargenumbersofmicr
13、ogeneratorsareconnectedtonetworkandthatthesecanbeusedtoreducetherequirementfortransmissionandhighvoltagedistributionassets.Accordingtotherelevantliterature,differentdefinitionsareprovidedformicrogrid.In8,microgridisaclusterofloadsandmicrosourcesoperatingasasinglecontrollablesystemthatprovidesbothpow
14、erandheattoitslocalarea.Microsourcesaresmall(lessthan100kW)unitswithpowerelectronicinterfaces,placedatcustomersites,lowcost,lowvoltageandhighlyreliablewithfewemissions.AccordingtothedefinitionproposedbyConsortiumforelectricalReliabilityTechnologySolutions(CCERTS_),microgridisanintegratedpowerdeliver
15、ysystemconsistingofinterconnectedloadsanddistributedenergyresourceswhich,asanintegratedsystem,canoperateinparallelwiththegridorinaninternationalislandmode9).MicrogridisdefinedinEuropeanmicrogridsprojectasfollows:Interconnectionofsmall,modulargenerationtolowvoltagedistributionsystemsformsanewtypeofpo
16、wersystem,themicrogridIO.Microgridscanbeconnectedtomainpowernetwork(non-autonomousmode)orbeoperatedislanded(autonomousmode)inacoordinatedandcontrolledway10.Intheautonomousmode,themicrogridservestheelectricalloadwithoutpowerfromtheutility;basicallythemicrogridisitsownstand-alonegrid.Inthenon-autonomo
17、usmode,themicrogridproducespowerwhileinterconnectedtotheutilitysystem11J.Maintainingofvoltagesandload-frequencycontrolinanautonomousmicrogridissocomplicatedthannon-autonomousone.Innon-autonomousmode,ifproblemswiththeDGresourcescomeup,thecustomercanstillhavepower,whilemaintenanceisperformedontheDGres
18、ources.Inthismode,thecustomerscansigncontractswiththeutilitytosellexcesspowerintotheutilitygrid.Also,iftheutilityhasapoweroutage,theDGresourcesofthecustomerwilldisconnectfromtheutilitygridandserveonlythecustomersconnectedload.Theautonomousmicrogridsareusedforplacesthatarefarawayfromthenetworkandthen
19、etworkextensionisexpensiveHLB. Virtualpowerplant(VPP)Similartomicrogrid,VPPisacombinationofDGs,controllableloadsandenergystorages;however,itisawiderconceptthanmicrogrid.Ingeneral,VPPisaconcepttoaggregateanumberofDERofvarioustechnologieswithvariousoperatingpatternandavailabilitythatconnectedtovarious
20、pointsindistributionnetworkforthepurposeoftradingelectricalenergyortoprovidesystemsupportservices.Accordingtotherelevantliterature,differentdefinitionsareprovidedforVPP.In12,13,VPPisanaggregationofmanycombinedheatandpowermicrounitswhichareconnectedtolowvoltagedistributionnetwork.In141,VPPisdefinedas
21、anaggregationofdifferenttypesofdistributedresourceswhichmaybedispersedindifferentpointsofmediumvoltagedistributionnetwork.In15,16VPPdefinesasamulti-technologyandmulti-siteheterogeneousentity.AccordingtoEuropeanfenixproject,VPPiscomposedofanumberofvarioustechnologieswithvariousoperatingpatternsandava
22、ilabilitywhichtheycanconnecttodifferentpointofdistributionnetwork17.18.Accordingtothisdefinition,VPPisaflexiblerepresentationofaportfolioofDERthatcanbeusedtomakecontractsinwholesalemarketandtoofferservicestothesystemoperator.TwotypesofVPParedefinedinfenixproject,i.e.CommercialVPP(CVPP)andTechnicalVP
23、P(TVPP).ACVPPhasanaggregatedprofileandoutputwhichrepresentsthecostandoperatingcharacteristicsfortheDERportfolio.TheimpactofdistributionnetworkisnotconsideredintheaggregatedCVPPprofile.TheTVPPconsistsofDERfromthesamegeographiclocation.TheTVPPincludestherealtimeinfluenceofthelocalnetworkonDERaggregate
24、dprofileaswellasrepresentingthecostandoperatingcharacteristicsoftheportfolio18.III. OperationofMicroGridsIncontrolstrategiesofmicrogrid,thereareseverallevelsofdecentralizationthatcanbepossiblyappliedrangingfromthefullydecentralizedapproachtoahieraticalcontrol19.Accordingtothefullydecentralizedapproa
25、ch,themainresponsibilityisgiventothecontrollersofthemicrogeneratorsthatcompetetomaximizetheirproductioninordertosatisfythedemandandprobablyprovidethemaximumpossibleexporttothegridtakingintoaccountmarketprices19,20,21.Thisapproachisbasedonthemulti-agenttechnologyandprovideseffectivesolutionsforanumbe
26、rofspecificoperationalproblemsincontrollingmicrogrids19,20.Inordertoachievethefullbenefitsfromtheoperationofmicrogrids,itisimportantthattheintegrationofthemicrosourcesintolowvoltagegridsandtheirrelationwiththemediumvoltagegridwillcontributetooptimizethegeneraloperationofthesystem22.Toachievethisgoal
27、,thehierarchical(centralized)controlofmicrogridisproposedforwhichthreecontrollevelsaredistinguished19,23,24:1-DistributionManagementSystem(DMS).2- MicroGridCentralController(MGCC).3- LocalControllers(LC),whichcouldbeeithermicrosourcecontrollersorloadcontrollers.DMSisresponsibletomanageandcontrolthed
28、istributionareacomprisingseveralfeedersincludingseveralmicrogrids.ThemaininterfacebetweentheDMSandthemicrogridistheMGCC.Itisthemainresponsiblefortheoptimizationofthemicrogridoperation,oralternatively,itsimplycoordinatesthelocalcontrollers,whichassumethemainresponsibilityforthisoptimization.Thelowerc
29、ontrollevelconsistsoftheLCthatcontrolDER.A.MarketpoliciesofamicrogridwithcentralizedcontrolInamicrogridwithcentralizedcontrol,MGCCisresponsibleforthemaximizationofthemicrogridvalueandtheoptimizationofitsoperation.ItusesthemarketpricesofelectricityandprobablyDemandSideManagement(DSM)requeststodetermi
30、netheamountofpowerthatthemicrogridshouldexchangewiththedistributionsystem,optimizingthelocalproductioncapabilities.Itmightuseloadforecasts(electricalandpossiblyheat)andforecastsofpowerproductioncapabilities.Thedefinedoptimizedoperatingscenarioisachievedbycontrollingthemicrosourcesandcontrollableload
31、sinthemicrogridbysendingcontrolsignalstothefield.Inthisframework,non-criticalcontrollableloadscanbeshed,whenitisnecessary.Furthermore,itisnecessarytomonitortheactualactiveandreactivepowerofthecomponents.Thesetechniquescanbeconsideredequivalenttothesecondarycontroloftheinterconnectedgrid23.Twomarketp
32、oliciesarcassumedforMGCCthatcontainofbehaviorlikeagoodcitizenandbehaviorlikeanidealcitizen23,25.Inthefirstpolicy,theMGCCaimstosatisfythelocalenergydemandusingitslocalproduction,whenitisfinanciallybeneficial,withoutexportingpowertotheupstreamdistributiongrid.Thisisequivalenttothegoodcitizenbehavior.F
33、ortheoveralldistributionsystemoperation,suchbehaviorisbeneficial,becauseatthetimeofpeakdemandleadingtohighelectricityprices,themicrogridrelievespossiblenetworkcongestionbypartlyorfullysupplyingitsenergyneeds.Fromtheend-userpointofview,theMGCCminimizestheoperationalcostofthemicrogrid,takingintoaccoun
34、tmarketprices,demandsandDGbids.End-usersofthemicrogridsharethebenefitsofreducedoperationalcosts.InthiscaseMGCCcollectsthemarketpricesforpower,theloaddemands(probablyforecastedbyshorttermloadforecastingprograms)andthebidsofmicrogridstominimizetheenergycostsforthewholemicrogridforeachofdefinedinterval
35、,e.g.eachof15minutesinterval,subjectedtotheconstraintssuchasactiveandreactivepowerbalance,P-Qcurveforeachoneofthegeneratorunitsandthegridtechnicallimitsofeachunit,etc23.Inthesecondpolicy,themicrogridparticipatesintheenergymarketofthedistributionarea,buyingandsellingactiveandreactivepowerfrom/tothegr
36、id,probablyviaanaggregatororsimilarenergyserviceprovider.Accordingtothispolicy,theMGCCtriestomaximizethevalueofthemicrogrid,i.e.maximizethecorrespondingrevenuesoftheaggregator,byexchangingpowerwiththegrid.Theend-usersarechargedfortheiractiveandreactivepowerconsumptionatmarketprices.Themicrogridbehav
37、esasasinglegeneratorcapabilitytorelivethepossiblenetworkcongestion,notonlyinthemicrogriditself,butalsoviaexportingenergytonearbyfeedersofthedistributionnetworklikeanidealcitizen.Inthiscase,MGCCconsidersthemarketpriceforbuyingandsellingenergytothegrid,(hepowerdemand(probablyfromshorttermloadforecasti
38、ng),thebidsofmicrosourcesforactivepowerandthemaximumcapacityallowedtobeexchangedwiththegrid(withregardtosomecontractualagreementoftheaggregatororthephysicallimitsoftheinterconnectionlinetogrid).Thenitismaximizedthemicrogridvaluethatisthedifferencebetweenincomesandexpenses,subjecttorelatedconstraints
39、.Theconstraintsareincludethepreviouspolicyconstrainsplustotheconstraintsrelatedtocapacityoftheinterconnectionandtheactivepowerpolicycontractedbytheaggregator23.Inthebothpolicies,demandsidebidsandadequacyorpowerqualityissuescanbeincludedintheoptimizationroutines.IV. OperationofVPPIn26VPPdispatchesits
40、unitsonlybasedontheminimizationofshorttermvariablecostsforserviceproviding.Forthispurpose,VPPsolveanoptimizationprogram,inwhichtheobjectivefunctionisthesumofheat,electricaswellasCHPplantsfuelcostsaddtocost/revenueoftheelectricpowerexchangedwiththegridineachhour.Theconstraintfunctionisthebalanceofele
41、ctricaswellasthermalenergybetweenVPPgeneratorsandVPPloads.Theauthorsof26developtheirmodelin27byproposingthecoordinatedoptimizationofVPPandDSOtomaintainthesecurityofdistributionnetwork.ThisisbecauseDSOisresponsibletomaintainthesecurityofthedistributionnetwork.Itshouldhandlethenetworkinsuchawaythatthe
42、voltagetoleranceineachbustobemetandthecongestiontobeavoided.Infact,ifVPPoptimizationisperformedwithoutDSOregulation,i.e.VPPoperatorindependentlyminimizesitstotalshorttermcostsincludingproductioncostsofVPPunitsandthecostofpowerexchangewiththegrid,andtherebysuppliesitsheatingandelectricaldemand,thisma
43、yleadtoviolationofvoltagesandcurrentslimitsindistributionnetwork.Aninferiorsolutionisthat(heDSOperformstheregulatingactionsonthenetworkonlybyactingonthemainsubstationOnLoadTapChangerTransformer(OLTCT)andontheavailableshuntcompensators.ButthecoordinatedoptimizationofVPPandDSOisbetterandsuitablesoluti
44、on.Theproposedalgorithmin27isbasedonthesolutionofafurtheroptimizationproblem,whichisaimedtoharmonizetheoperationsofbothDSOandVPPinordertoachievethedesiredregulationwithminimumwheelingcosts,anextracostaccountingforthedistributionnetworklosses.Here,theDSOwouldperformasortofvirtualactiveandreactivepowe
45、rdispatchingof(heDGsbysolvinganoptimizationproblemtominimizethenetworklosseswhichcorrespondtominimizethecostofactiveandreactivepowerimportedfromtheHVgrid,sincethedistributedpowergenetionanddemandarefixedboundaryconditions.Inordertodoso,theDSOneedscompleteknowledgeofthesystemstate(generatedanddemande
46、dactiveandreactivepoweratanybusbar)andcostfunctionsoftheVPPgenerators.Theconstraintsareincludedpowerflowbalanceequationsateachbusofthenetworkforbothactiveandreactivepower,voltagesboundariesand(hecurrentlimitationsofthefeeders.Itistobenoted(hatactiveandreactivepoweroftheDGunitswhicharebelongtotheVPP,
47、areconsideredasfixedvaluesinpowerbalanceconstraintofeachbus,whiletheyareoptimizationvariablesinVPPoptimizationalgorithm.Asaresultoftheoptimization,thisalgorithmprovidesanestimationofthenodalpricesforactiveandreactivegridexchangepowerrequiredbytheVPPoptimizationalgorithm.VPPhasthechoicetoadequateitsp
48、roductiononthebasisofsuchsignals,inwhichcasethewheelingcostswouldbeminimized.ThustheoveralloptimizationprocedureisaccomplishedbytheiterativeapplicationofbothDSOandVPPoptimizationalgorithms,wherethecommonvariablesarealternativelytreatedasoptimizationparametersinonealgorithmandfixedboundaryconditionsi
49、ntheother.Theterminationcriterionistheminimizationofthepricevaluesvariationofenergyinthenetwork.In26,27,althoughVPPexchangeselectricpowerwiththegridinregardtothepricesofwholesalemarket,ithasapassiveroleinmarkettransactions.Itisduetothefactthatitdoesnotbidtothepowermarketasaparticipant.In15,16,ageneralframeworkforoperationoftheVPPinthepowermarketisproposed.Inthisscheme,theDGunitsoffertheirproposalsforsaleofenergytotheVPPoperator,whoformulatestheenergyprice
