IMPACT OF MAINTENANCE STRATEGY ON RELIABILITY

IMPACTOFMAINTENANCESTRATEGYONRELIABILITYFinalreportbytheIEEE/PESTaskForceonImpactofMaintenanceStrategyonReliabilityoftheReliability,RiskandProbabilityApplicationsSubcommitteeJuly19992TaskForceChair:J.EndrenyiMembers:S.AboreshaidJ.McCalleyR.N.AllanS.MeliopoulosG.J.AndersT.C.MielnikS.AsgarpoorP.NituR.BillintonN.RauN.ChowdhuryN.D.ReppenE.N.DialynasL.SalvaderiM.FipperA.SchneiderR.H.FletcherCh.SinghC.Grigg3IMPACTOFMAINTENANCESTRATEGYONRELIABILITY1MandateandScopATaskForceontheeffectofvariousmaintenancestrategiesonreliabilityhasbeenestablishedbythIEEESubcommitteeonApplicationofProbabilityMethods(presentlyReliability,Risk&ProbabilitApplicationsSC).ThemandateoftheTaskForcewassetattheTF'smeetingduringthePESWinterMeetinginNewYork,January1995.Itcontainsthefollowingagenda.1.Definetheareaofinvestigation.Proposeterminologywhereambiguityexists.2.Reviewliteratureonmaintenancemethodsandrankthemforapplicabilityinpowersystems.Includeanassessmentoftherecentlydevelopedmethodscommonlycalledreliability-centeredmaintenance(RCM)whichappearmostlyheuristicbutmaycontainusefulobservationsormodels.3.Reviewthemajormaintenancepoliciespresentlypracticedinelectricutilities.4.Reviewthedataneedsofmaintenanceprogramsanddiscussdatacollectionmethods.5.Discusstheinterpretationofdiagnostictests,andthewaysofdeterminingthestageofageinginwhichadeviceisfoundduringinspection.6.Describeapproachestodevelopingamaintenanceprogramwhichcouldincreasethecostandreliabilitybenefitsfromthosegainedwhenusingtraditionalpolicies.Discussthecoststobeincluded.7.Surveypresenteffortsofdevelopingsuchapproaches.8.Produceadocumentwhichsummarizesthefindingsandoffersrecommendations.Assesstheneedforfurtherrefinementssuchasconsideringtheeffectsofloadlevelsorcostoptimization.Thestructureofthepresentreportcloselyfollowsthemandate.Thefocusoftheworkisonproviding4clearconcepts,identifyingneedsandcriteria(particularlyaboutcosts)formaintenanceintheelectricpowerindustry,anddiscussingapproacheswhichcanbestsatisfytheneeds.Thereportisnotaimedatdevelopingacomprehensivemaintenanceprogram;rather,itisrestrictedtotheevaluationoffactorstobeconsideredinsuchadevelopment.Thegoalofthereportistoincreasetheindustry'sawarenessothistoolforimprovingsystemreliability,atoolwhichhasnotbeenfullyexploitedinthepast.2TerminologyInthefollowing,definitionswillbegivenofafewfundamantalconceptsdiscussedinthisreport.Whilethetermsbelowwillbeusedthroughoutthereport,itmustbeunderstoodthatthereisnostandardnomenclatureinthisfield,andtheliteraturehasbeenemployingavarietyoftermsforsomeoftheconcepts.Inthenotestothedefinitions,severalalternativetermsarelistedwhicharefrequentlyusedinpowersystemreliabilitystudiesorinthecomponentandcircuitreliabilityliterature;inproposingthedefinitionsbelow,anattemptismadetoofferaconsistentsetwhichmaybeacceptabletomostusers.Failure-Theterminationoftheabilityofadevicetoperformarequiredfunction.Randomfailure-Afailurewhoserateofoccurrence(intensity)isconstant,andindependentofthedevice'scondition.Note1:Inotherwords,thechancesofafailureoccurringinanyshorttimeinterval,assumingthatthedevicehasbeenworkinguptothattime,isalwaysthesame.Inthemorepreciseterminologyofreliabilitytheory,afailureisrandomifthedensityoftheconditionalprobabilitythatitoccursintheinterval(t,t+Δt),giventhatthedevicewasinaworkingconditionatt,isconstant(independentoft).Ingeneral,thisdensityiscalledthehazardfunction,andifitisconstant,thehazardratorfailurerate.Note2:Inabroadersense,failureswhoseoriginsarenotwellunderstoodandthereforeareperceivedasbeingabletooccuratanytimeareoftensaidtoberandom.Ifforeasiermathematicalmodelingitisassumedthatsuchfailurescanoccuratanytimewithequalprobability,thenthebroaderconceptisreducedtotheabovedefinition.Note3:Therateofrandomfailuremaydependonexternalconditions.Forexample,theratesoflightningoricestorms,andtheratesofresultingrandomfailures,wouldbdifferentineachseason.Deterioration(wearorwear-out)-Aprocessbywhichtherateoffailureincreasesduetolossostrength,theeffectsofusage,environmentalexposureorpassageoftime.5Note:Thetermisalsousedtodescribetheaccumulatedresultsoftheprocess.Deteriorationfailure-Afailureresultingfromthedeteriorationofadevice.Restoration-Anactivitywhichimprovestheconditionofadevice.Ifthedeviceisinafailedcondition,theintentofrestorationisthere-establishmentofaworkingstate.Replacement-Restorationwhereinadeviceisremovedandoneinbetterconditionisputinitsplace;ifthedeviceisfailed,itisreplacedbyaworkingone.Itisoftenassumedthatthedevicesoinstalledisnew.Repair-Restorationwhereinafaileddeviceisreturnedtooperablecondition.Note:Itiscommontousethetermcorrectivemaintenanforbothreplacementandrepair.Minimalrepair-Repairoflimitedeffortwhereinthedeviceisreturnedtotheoperablestateitwasinjustbeforefailure.Maintenance-Restorationwhereinanunfaileddevicehas,fromtimetotime,itsdeteriorationarrested,reducedoreliminated.Note:Itiscommontocallthisconceptplannedmaintenanorpreventivemaintenance.Thesetermsaremeanttocontrastwitcorrectivemaintenan(seeRepair).Theyarredundantbytheabovedefinition,andarenotusedinthisreport.Schedu