ReliabilityStandardsfortheNorthwestPowerSystem文档格式.docx
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Forlackofabettermeasure,theelectricutilityindustrygenerallyreferences95-percentreliabilityasa“standard”fortheNorthwestpowersystem,butitisagoalandnotanofficialregulation,requirementorcode.Asthenation’selectricityindustrybecomesincreasinglycompetitive,andasournationbecomesincreasinglydependentontechnologythatrequireshighlyreliableelectricpower,theissueofaregionalornationalreliabilitystandardloomslarge.Whatisanacceptablestandard?
Howwoulditbecreated,andwhatwoulditaddress?
Shouldtherebeonestandardorseveral?
Occasionalpoweroutagesareaninconvenienceforsomeandadisasterforothers.Perhapsthereisnoneedfora“standard,”asconsumerswhoneedhighlyreliablepowercanpayforthatprivilegebypurchasingtheirownbackupgenerators,andthoseforwhomreliabilityislesscriticalmightbewillingtoacceptwhateverreliabilitytheirpowersupplierprovides--andatalowercost.Thispaperexplorespasteffortstodefineandassessreliability,andrecommendsnextstepsfordevelopingastandardorstandards.Separatestandardsforgenerationplanning,operationandtransmissionprobablyarewarranted.Foroperations,adeterministicstandardmightbecraftedaroundminimumreserverequirements.Forgenerationplanningpurposes,aprobabilisticstandard(onethatincorporatesthefrequency,durationandmagnitudeofcustomerinterruptions)couldserve.Fortransmission,itseemsappropriatethatanindependentRegionalTransmissionOperator(RTO)wouldestablishastandard,withattentiontoreducingcongestionandrewardingdistributedgenerationwhenitrelievesstressonthesystem.
DefiningthePowerSystem
Apowersystemcanbeverycomplex,integratingmanydifferenttypesofgeneratingresourcestoprovideelectricitytoanumberofcustomerswithvaryingrequirements.Eachindividualcomponenthasitsowncharacteristics,includingitsdependabilityorreliability.Onecanimaginethedifficultyintryingtoevaluatethereliabilityofsuchacomplexsystemasawhole.Tosimplifythetasksomewhat,powersystemsaregenerallybrokendownintothreemajorcomponents:
thegenerationsystem,thebulktransmissionsystemandthedistributionnetwork.Thegenerationsystemincludesallresourcesthatproduceelectricity,includingoilandgas-firedturbines,coalandnuclearplants,hydroelectricdams,windmillsandallothergenerators.Thebulktransmissionsystemincludeshighvoltagetransmissionlinesthatconnectgeneratorstodistributionnetworks.Thedistributionnetworks,madeupofallthelowervoltagelines,deliverelectricitytoindividualcustomers.
Toourknowledge,noonehasattemptedtoevaluatethereliabilityofapowersystemasawhole.Generally,individualutilitiesassessthereliabilityoftheirowndistributionnetworks.Thispaperfocusesonmethodstoassessthereliabilityofthegenerationcomponentofthepowersystem.
DefiningReliability
Ingeneralterms,reliabilityisameasureofhowwellasystemperformsitsexpectedfunction.Anothersystemcharacteristicthatiscloselyassociatedwithreliabilityisadequacy.Asystemisadequateifithassufficientresourcestoperformitsfunction.Asystemcanbeadequatebutunreliable.However,ifasystemisinadequate,thenbymostdefinitionsitisalsounreliable.
Forelectricalpowersystems,thesetwotermscanbedefinedmorespecifically.TheNorthAmericanElectricReliabilityCouncildefinespowersystemreliabilitytobe:
thedegreeofperformanceoftheelementsofthebulkelectricsystemthatresultsinelectricitybeingdeliveredtocustomerswithinacceptedstandardsandintheamountdesired.Reliabilitymaybemeasuredbythefrequency,durationandmagnitudeofadverseeffectsontheelectricsupply.Electricsystemreliabilitycanbeaddressedbyconsideringtwobasicandfunctionalaspectsoftheelectricsystem--adequacyandsecurity.
Adequacy-Theabilityoftheelectricsystemtosupplytheaggregateelectricaldemandandenergyrequirementsofthecustomersatalltimes,takingintoaccountscheduledandreasonablyexpectedunscheduledoutagesofsystemelements.
Security-Theabilityoftheelectricsystemtowithstandsuddendisturbancessuchaselectricshortcircuitsorunanticipatedlossofsystemelements.
TheNERCdefinitionsofadequacyandsecurityapplytoboththegenerationandtransmissionsystems.Bothsystemsmustbeabletocontinueoperationafterasuddendisturbance--eitheralossofamajortransmissionlineoramajorgenerator.Apowersystemisunreliableifeitheritstransmissionorgenerationsystemsareinadequate.Unfortunately,analyzingthecompositereliabilityofbothsystemsisverydifficult.Becauseofthisdifficultyandbecauseinpracticethereliabilityofeachsystemisusuallymeasuredseparately,thefocusofthispaperwillbeonthegenerationsystem.However,theNorthwestPowerPlanningCouncilisalsoparticipatingindiscussionsregardingthebulktransmissionsystemandisinfavorofestablishingaRegionalTransmissionOperatorthatwillhavesomecontrolovertransmissionreliability.
TheNorthwestPowerPlanningCouncilhasdefinedtheterms“reliable”and“adequate”inaslightlydifferentwayforthegenerationsystem.WhentheCouncildiscussesreliability,itmeanstheshort-termabilityoftheexistingandplannedgenerationsystemtodeliverelectricitywhenneeded,includingunusuallycoldperiodsortimeswhengeneratorsareoutofservice.IntheCouncil’sMarch2000analysis,itconcludedthatthegenerationsystemwas“inadequate”meaningthatmoreresourceswereneededinordertolowertheriskofblackoutstoanacceptablelevel.Inthatsense,theCouncilconcludedthatthepowersystemwasalsounreliable.
ReserveRequirements
Tohelpensureahigherlikelihoodofuninterruptedservice,allutilitiesarerequiredtomaintainacertainlevelofreservemargin.Areservemarginissimplyresourcecapabilityinexcessoftheexpectedpeakdemand.Reserveresourcesarecalledintoservicewhenunexpectedoutagesoccur.Ifreserveresourcesareusedmoreoftenthanexpected,itimpliesthatthepowersystemisinadequateandthereforeunreliable.
TheWSCCrequiresreservesequalto5percentofloadsuppliedbyhydroelectricresourcesplus7percentofloadsuppliedbythermalgeneration,withspinningreservesnotlessthanonehalfofthetotaloperatingreserve.WSCCiscurrentlyreviewingthispolicyandmaychangetheserequirements.Increasingthereservemarginprovidesamorereliablepowersystembutatahighercost.Decreasingthereservemarginsavesmoneybutcouldmeanhigherlikelihoodofcurtailment.
CapacityandEnergy
Forapowersystem,plannersmustalsodistinguishbetweencapacityandenergyshortfalls.Thecapacityofthesystemisitsabilitytodeliverpowertomeetinstantaneousdemandrequirements.Thepowersystemmustalsobeabletodeliver,onasustainedbasis,theaverageenergyrequirementsoftheregion.Anotherwaytolookatthedifferencebetweencapacityandenergy(especiallyforapredominantlyhydrosystem)istoconsiderfuelasopposedtomachines.Doesthesystemhaveenoughmachinestomeetthepeakdemand?
Doesthesystemhaveenoughwaterandfueltorunthemachineswhenneeded?
Curtailmentofservicecanoccurifthesystemhasinsufficientmachinesorfuelorboth.
Mostpowersystemsintheworldplanforcapacityneeds--ensuringthatenoughresourcesareinplacetoservethepeakdemand.Theirabilitytomeetloadislimitedbytheirpeakgeneratingcapacity,ratherthanthefuelavailablefortheirgeneratingfacilities.Forthesesystems,thetotalgeneratingcapabilityusuallyfarexceedstheaverageenergyrequirements.
Forsystemsthatincorporatealargeshareofhydroelectricgeneration,however,planningisusuallydoneonanenergybasis.Thisisbecausereservoirsbehindthedamsgenerallycannotholdenoughwatertoruneachprojectatfullcapacityallyearlong.Thesesystemsareconsideredenergylimitedandcapacitysurplus.FortheNorthwestregion,thepeakingcapacityisabout42,000megawattsandtheaverageannualenergyproductionisabout21,000averagemegawatts.
Inacapacity-limitedsystem,noadditionalamountoffuelwillincreaseservicetocustomers.Inanenergy-limitedhydroelectricsystem,noadditionalnumberofturbineswillhelp.(Althoughaddingothernon-hydrogeneratorsobviouslywouldimprovethesituation).Tomakemattersmorecomplicated,quiteoftentheoperationofresourcesislimitedbyconstraintsdesignedtoprotectairorwaterqualityortoenhancefishsurvival.
Howdocapacityandenergyrelatetoreliabilityandadequacy?
Asystemisadequateifithasenoughresourcesandfueltomeetboththepeakdemandandtheannualaverageenergyrequirement.Asystemisreliableifitcandeliverpowerwhencalledupontodosowithinacceptablestandards,takingintoaccountscheduledandreasonablyexpectedunscheduledoutagesofsystemelements.(Systemsaregenerallydesignedtowithstandthelossofamajortransmissionlineorthelargestgeneratingresource).
Uptothispoint,wehavedefinedreliabilityinqualitativeterms.Inordertomeasurereliability,aquantitativedefinitionmustbedeveloped.Inthefollowingsection,severalapproachesareexplored.
MeasuringReliability
Thereisnosingleindexthatisuniversallyusedtoexpressthereliabilityofapowersystem.Reliabilityindicescanbebroadlycategorizedaseitherdeterministicorprobabilistic.
Deterministicindicesarecalculatedwithknownsystemparametersandprovideastaticlookatthesystem.Theirad