ReliabilityStandardsfortheNorthwestPowerSystem.docx

ReliabilityStandardsfortheNorthwestPowerSystem.docx

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ReliabilityStandardsfortheNorthwestPowerSystem

ReliabilityStandardsfortheNorthwestPowerSystem

Abstract

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