外文翻译-单元变电站可靠性设计优化和电气工作场所安全的思考

英文原文CONSIDERATIONSINUNITSUBSTATIONDESIGNTOOPTIMIZERELIABILITYANDELECTRICALWORKPLACESAFETYByDavidB.DurocherSeniorMember,IEEEIndustryManagerEatonCorporationABSTRACTManylegacylowandmedium-voltageunitsubstationsinstalledtodayarebaseduponolderdesignsthattookadvantageofreducedfirstcost“opportunities”allowedbyexistinginstalationcodesandstandards.Fast-forwardtohowthesesubstationdesignsfairinsafetyandreliabilitytoday,particularlyinindustrialprocessapplicationsfoundincement,pulpandpaper,petroleum&chemicalandothers,someoftheexercised“opportunities”appliedinthepastbegintolookmorelikeliabilitiesthanassets.Legacyengineeringdecisionsoncethoughttobeprudenttakeonnewmeaningstoday,particularlywhenthesedecisionsareviewedthroughthelensofemergingnewworkplacesafetystandards.Thecriticalissueofaddressingdestructivearc-flashhazardsassociatedwithpersonsworkingonoraroundenergizedelectricalequipmentmustnowbeconsidered.Becausetraditionalsubstationdesignsoftenappearedtoinvolvesomecompromiseregardingbothsafetyandreliability,adesignteamofamajorprocessindustryusertookafreshlookatunitsubstationdesign.ThedesignreviewtookplaceinconjunctionwithconstructionofaGreenfieldplantbuiltinthespringof2009intheUSA.Thispaperwillreviewthedesignlimitationsoftraditionalunitsubstationconfigurations,offeranoverviewofthealternativesconsideredbytheGreenfieldsiteprojectteam,anddiscusstechnicalandsafetyvalidationofthedesignthatwasultimatelyselectedandinstalled.Economiccomparisonstotraditionaldesigns,changesintheowneroperatingandsafetyproceduresforplantpersonnelasaresultoftheengineeringdesignchanges,andoveralldesignacceptancebyoperationswillalsobereviewedinthispaper.IndexTermsProcessIndustries,PowerDistribution,UnitSubstations,DesignforSafety,ElectricalWorkplaceSafety.INTRODUCTIONLowandmediumvoltageunitsubstationsareapplieduniversalyacrossmosteveryindustry.Atthetree-toplevel,unitsubstationsareusedsimplytotransformmedium-voltage,typically15to25kV,toalowerdistributionvoltage,typically0.48to4.16kV,forapplicationinsupportingahostofvariousmotorandprocessequipmentloads.Fig.1showsatypicallow-voltageunitsubstation.Inthiscase,theprimaryassemblyattheleftisamedium-voltagefusedloadbreakswitch.Forthisexample,wewillassumetheprimaryvoltageis13.8kV.ForassembliesinNorthAmericanindustry,thisassemblyistypicallydesignedtometal-enclosedswitchgearstandardANSI/IEEEStandardC37.20.31.Thisassemblyincludesaload-breakisolationswitchwithratingsof600or1200amperesandamedium-voltagecurrent-limitingfuse,appropriatelysizedtoprotectthetransformer.Theprimaryswitchgearisclose-coupledtoasubstationtransformer,eitherdry-typeorliquidfilled.ThesubstationtransformerisdesignedtoANSI/IEEEStandardC.57.12withwall-mountedprimaryandsecondarybushings.Therearemanydifferentsubstationtransformerdesignalternativestochoosefrom,beyondthescopeofthispaper.Goodinformationonthealternativescanbefoundinothertechnicalpapers,including3.Inthiscase,thetransformerratingisshownat2000kVA.Withasecondarydistributionvoltageat480Y/277volts,thelow-voltagebushingsareshownclose-coupledtometalenclosedlow-voltageswitchgear.InFig.1,thelow-voltageswitchgearconsistsofa3200amperesecondarymainbusandsecondarymetering,withnosecondarymaincircuitbreaker,connectedtofour1200amperefeedercircuitbreakers.Thereareagainvariationsonlow-voltageswitchgeardesigns.Forprocessindustryapplications,mostfrequentlytheseassembliesaremanufacturedtoUL1558Standards4.DESIGNCONSIDERATIONSInanticipationoftheupcomingproject,thedesignteamfortheGreenfieldsitetookonthetaskofinvestigatingexistingunitsubstationconfigurationscarefullytoidentifywheretheremaybesomeinherenthiddenflawsinthedesign.Itisimportanttonotethatprevailingcodesandstandardsregardinginstallationofthisequipmenthadanimpactontheunitsubstationdesign.IntheUS,theprevailinginstallationdocumentthatappliesistheNationalElectricalCode(NEC)5.Letsinvestigatetwoareasofthiscodethatimpactthedesignandinstalationoftheunitsubstationpresentedhere.NECArticle240.21(C)2OvercurrentProtectionArticle240.21(C)oftheNECaddressesrequiredovercurrentprotection,specificalyrelatedtotransformersecondaryconductors.Thearticlestatesthat“asetofconductorsfeedingasingleloadshallbepermittedtobeconnectedtoatransformersecondary,withoutovercurrentprotectionofthesecondary”.Thearticledefinessixconditions,specifiedin240.21(C)(1)through240.21(C)(6),underwhichsecondaryovercurrentprotectionisnotrequired.Sortingthroughthesixoptionsforourclose-coupledunitsubstationexample,pointsustotheconditionoutlinedin240.21(C)(2)whichmostcloselyapplies.Thisconditiongetsfairlyinvolved,withfourdifferentsub-conditions,allwhichmustapplyinordertosatisfytheexceptionofnosecondaryprotection.Relevantlanguageinthesesub-conditionsincludes:“240.21(C)(2):TransformerSecondaryConductorsNotover3m(10ft)Long.(1)Theampacityofthesecondaryconductorsisa).Notlessthanthecombinedcalculatedloadsonthecircuitssuppliedbythesecondaryconductorsb).Notlessthantheratingofthedevicesuppliedbythesecondaryconductorsornotlessthantheratingoftheovercurrent-protectivedeviceattheterminationofthesecondaryconductors.”Thefirstitem(1)a)aboverequiresthattheengineerperformcalculationstodeterminethetotalconductorloadandthenspecifyaconductorsizetosupportthecalculatedload.ReferringbacktotheFig.1example,notethatthesecondaryconductorisspecifiedat3200A.So,althoughthetotalconnectedratedloadofthesecondaryfeederbreakersis4800A(fourbreakersratedat1200Aeach),theNECallowsthedesignertoassumealoaddiversityandsizethesecondarybusassomelowervalue.Theseconditem(1)b.inessencestatesthatthesecondaryconductorampacitybeeithergreaterthantheovercurrentdeviceatwhichtheconductorsterminate(inthisconfiguration,thereisnosuchdevice)orgreaterthanconductororbusratingintheequipmentwheretheconductorsterminate.Fromthislanguage,itseemsclearthatsecondarybusprotectionfortheunitsubstationisnotrequired.Thereisongoingdebateinsomecirclesregardingtheword“device”inthisarticle,assomeseethetermdevicetomeansomethingotherthentheswitchgear.Interestingly,theNECCodeMakingPanelsupportingthisArticleisreviewingthislanguageandconsideringfuturerevisiontoclarifythemeaning.Thisaside,notealsothatArticle240.21(C)includesaFinePrintNotestating“Forovercurrentprotectionrequirementsfortransformers,see450.3.NECArticle450.3EquipmentTransformersArticle450.3oftheNECaddressessecondaryovercurrentprotectionoftransformers.Note2forTable450.3(A)states:“Wheresecondaryovercurrentprotectionisrequired,thesecondaryovercurrentdeviceshallbepermittedtoconsistofnotmorethensixcircuitbreakersorsixsetsoffusesgroupedinonelocation”.Traditionallyreferredtoasthe“sixdisconnect”or“sixhandle”rule,thisprovisionallowstheusertoforegosecondaryovercurrentprotectioninaunitsubstation,providedtherearenomorethansixfeederdevicesintheassembly.FortheexampleshowninFig.1,thisisclearlythecase,sothisassemblycouldbeinstaledwithoutconcernthatthedesignwouldviolatetheapplicableinstallationcode.APPLICATIONWAKE-UPCALLAlthoughthe“sixfeedersnomain”unitsubstationpassesallrequirementsoutlinedintheapplicablestandards,theunitsubstationequipmentmanufacturerandtheprojectteaminvestigatingdesignalternativeswerenotsatisfiedthiswasthebestapproach.Earlierexperiencesinindustrialplantswherearc-flashstudieshavebeenperformedasoutlinedinNFPA-70E6usingcalculationmethodsinIEEE15847yieldedsomeveryrevealinganddisturbingresults.Intheeventofasecondarybusfault,theNFPA-70Estandardrequiresthattheupstreamovercurrentprotectivedevicebeusedindeterminingtheavailablearcingcurrent.Inthiscase,thecurrent-limitingfuseontheprimaryofthesubstationisthedeviceusedinthecalculation.Specifically,Fig.2belowshowscalculationsrevealingarcflashenergiesatthesecondaryswitchgearinexcessof700calories/cm2.TheselevelsaredefinedinIEEE1584asUNAPPROACHABLE,whereeffectivelynoPersonalProtectiveEquipment(PPE)wouldbeadequateinsafeguardingpersonnelshouldabusfaultoccurwhilepersonswereworkingontheenergizedsubstation.Inmanyexistingfacilities,unitsubstationfeederdeviceswereusedasalockout/tagoutpointwhiledownstreamequipmentwasbeingservicedormaintained.Theelevatedarcflashenergieseffectivelymadeitunsafetorack-outasecondaryfeederbreakerwhilethesecondarybuswasenergized.Inprocessindustryapplicationswhereelectricalworkplacesafetyisparamountandenergizedlockout/tagoutiscommon,the“sixfeedersnomain”unitsubstationdesignwassimplynolongerapracticaloption.AnumberofvintageunitsubstationsthatemployedtheconfigurationshowninFig.2,haveeffectivelybeenupgradedtoimprovereliabilityandelectricalsafety.Althoughbeyondthescopeofthispaper,onesuchupgradeispresentedinthecasestudyoutlinedin8.Returningtotheprimarycurrent-limitingfuseintheunitsubstationshowninFig.2,selectingtheratingofthisfusetoaccountfortransformerinrushresultsinameltingtimerequirementupto12Xthetransformerratedprimarycurrentfor0.1seconds.Inthe2000kVAsubstationshowninFig.2,a125Efuseisapplied.Aboltedsecondaryfaultwouldresultinaprimarycurrentoflessthan1000amps,resultinginafuseclearingtimeofover2seconds.Theexamplecalculationassumesanarcingfaultof10,000amperesonthesecondarybus,resultinginafuseclearingtimeof160seconds.Ineitherthecaseofaboltedfaultoranarcingfault,thesecondaryarcflashenergyonthesecondarybusofthisunitsubstationdesignisUNAPPROACHABLE.Inaddition,shouldabusfaultoccurwhilethisassemblywasenergized,thelikelyresultbeyondextremelyhigharcflashenergieswouldbeextensiveequipmentdamagecausedbytheheatenergydevelopedbeforetheprimaryfusewouldclear.Inaprocessindustryenvironment,thistranslatestohoursorperhapsdaysofdowntime.Intheend,theprimaryfuseinthe13.8kVfusedload-breakswitchshowninFig.2,isintendedtoprotectthetransformer,notthesecondarybus.Addingasecondarymaincircuitbreakerwouldresolvethisissueofprotectioninsomeapplications.Thiswouldineffectprotectthesecondarybusdownstreamofthemainbreaker.However,thebusfromthetransformersecondaryterminalsuptothemainisstilnotadequatelyprotected.Inapplicationswheretheprimaryassemblyandtransformerareoutdoorsandcableconnectedtothesecondaryswitchgear,thesecondarybusprotectionissuebecomesmoreproblematic.Clearly,anopportunityexistedfortheprojectdesignteamtoconsiderdesignalternativesthatwouldofferbetterperformance,bothinreliabilityandworkplacesafety.APATHFORWARDVIAPRODUCTTECHNOLOGYRecognizingthelimitationsofthelegacyunitsubstationdesign,theprojectteamworkedwiththepowerdistributionequipmentsuppliertoreviewalternativedesignsthatmightofferimprovedperformance.Becauseoftheextremehazardandpotentialforextendedoutagetime,thegroupquicklydismissedtheage-oldapproachofinstalingunitsubstationsbasedonthe“sixfeeders-nomain”design.Thestrategywastolookatdesignsthatincludedasecondarymainovercurrentprotectivedevice(inthiscase,alow-voltagepowercircuitbreaker)andtheninvestigatedesignalternativesthatmightofferadvantagestothisdesignapproach.Thegrouprecognizedthataddingasecondarymaindevicewouldaddcostandwasinterestedinalternativesthatmightperformaswell,orbetter,thanthesecondarymaindesign.Thegroupconsideredseveralemergingtechnologiesthatmightofferimprovedperformance.Threetechnologieswereconsideredandultimatelyapplied.Thesearediscussedbelow:15kVVacuumPrimaryCircuitBreakerOnetechnologythatappearedpromisingwasintheareaofmedium-voltagevacuumcircuitbreakers.Thegroupbelievedthatapplicationofalow-costcircuitbreakerintheprimaryoftheunitsubstation,providingbothprimaryandsecondarycurrentprotection,wouldbeadesirablealternativetothetraditionalfusedload-breakswitch.Althoughvacuumcircuitbreakershavetraditionallyinvolvedhigherspaceandcostthanafusedswitch,somemanufacturershaddevelopednewervacuumbreakersthatlookedpromising.Fig.3showsandexampleofonesuchdesignavailable.IntheNorthAmericanmarkets,vacuumcircuitbreakersaremanufacturedtoANSIStandardC37.209.Inspiredinpartbyatrendtowardglobaldesignstandards,traditionaldesignshavegivenwaytonewerofferingsthataresmaller,lighter,andhaveimprovedfunctionality.AsisshowninFig.3,althoughthenewerdesignvacuumbreakersareonlyavailableinlimitedratings,mostofferasmallersizewithfewerparts.NotablydifferentfromtraditionalZoneSelectiveInterlockingcyclesona60hertzsystem.Ifhoweverabusfaultshownat(2)onFig.4occurred,themaincircuitbreakerwouldbecalledupontoclearthefault.Withoutzoneselectiveinterlocking,thebreakershort-timedelaytripsettingof0.5secondsor30cycleswoulddictatetheclearingtime.Azoneselectiveinterlocking(ZSI)controlconnectionbetweenallcircuitbreakersaddsintelligencetothissystem.Whenabusfaultoccurs,ZSIallowsthemainbreakertointerrogatethefeederbreakersinthezonetodetermineifthey“see”afaultaswell.Ifallreportbackthatthereisnowdownstreamfault,thenthemainbreakerwilltripwithnointentionaldelay.TheZSIfeatureissimpletoenableandcanoffersignificantadvantagesinreducingpotentialarcflashhazardsdescribedpreviously.Foratypicallow-voltagesystemcapableofdelivering35,000amperessymmetricalfaultcurrent,calculationsinaccordancewithIEEE1584showthataddingZSIcanreducetheincidentenergyfrom43.7calories/cm2to7.0calories/cm2.TheNFPA70EStandardforElectricalSafetyintheworkplacedefinesthefirstconditionaboveasUNAPPROACHABLEandthesecondasHazardRiskCategory2,asignificantdifference.MultipleSettingsGroupsOnefinaltechnologyappliedintodayspowerdistributionsystemsisanewercapabilityofferingmultiplesettingsgroupcapabilityforprotectiverelaysusedwithcircuitbreakers.Althoughthiscapabilityhasbeenafeatureforseveralyearsonafewhigher-endprotectiverelaysusedinmedium-voltagesystems,severaltrippingsystemsappliedinintegraltripunitsoflow-voltagepowercircuitbreakersnowalsoincludethisfeature.InasimilarconceptdescribedaboveinZSIapplications,useofmultiplesettingsgroupsforcircuitbreakertrippingenablesthetrippingsystemtoresponddifferentlyfordifferentsystemconditions.Again,referringtoFig.4,ifadownstreamfaultconditionexisted,thefeedercircuitbreakersettingwoulddictatethatthe0.20secondshort-timedelaysettingtimeoutbeforethebreakertrips.Thepowersystemsengineerdeterminesthissettingtoassurecoordinationwithdownstreamovercurrentprotectivedevicesandsystemloadssothatthedevicenearestthefaulttripsfirst.Insomecasesforinstance,largedownstreammotorsmayhavehighinrushcurrentsorlongaccelerationtimesthatwillaffecttheshort-timesettingofthefeederbreakersintheunitsubstation.Asdiscussedpreviously,addinganintentionaldelaytoabreakerclearingtimecomesatthecostofhigherincidentenergyandarc-flashhazards.Whenpersonnelareworkingindownstreamequipment,suchasalow-voltagemotorcontrolcenter,theopportunityforadroppedtooloraccidentalcontactofatoolorprobebetweenanenergizedconductorandgroundisincreased.Asthiscouldleadtoahigherincidentofshortcircuitsorarc-flashincidents,itisoftenprudenttoreducetripsettingstoenabletheupstreamcircuitbreakertotripfaster.Multiplesettingsgroupseffectivelyallowforthepowersystemsengineertoestablishonegroupofprotectivesettingsduringnormaloperationsandanother“maintenancemode”settingthatcanbeusedwhilepersonnelareworkingindownstreamequipment.Fig.5illustratesapplicationofthemultiplesettinggrouptechnology.AttheleftofFig.5,theintegralLong-time,Short-time,Instantaneous&Ground(LSIG)integraltripunitmountedinthelow-voltagepowercircuitbreakerisequippedwithanon-offswitchthatenablesasecond“group”ofsettings.Inthenormalmode,thepowersystemsengineersettingsarebasedonaselectivelycoordinatedsystem,whileinthemaintenancemode,theLSIGsettingsarereplacedwithaninstantaneousonlysetting,effectivelydisablingthenormalshort-timesettings.Theresultisafasterclearingtimeofthecircuitbreakershouldadownstreamfaultoccur.AttherightofFig.5,notethatthebeforeandaftercoordinationcurvesareshowntodemonstratetheimpactofthemaintenancesetting.Theselectivelycoordinatedcurvessetattheleftshowsthemainandfeedercircuitbreakercurvesandplotsashort-circuitcurrentof5,600amperes.Notethatduetotheshort-timedelaysettingforthefeedercircuitbreaker,thetimetoclearthislowerlevelfaultisextended.Thecurvesetonthefarrightshowsthemaintenancemodeenabled,whicheffectivelyshiftstheinstantaneoussettingofthefeederbreakertotheleft.Theresultinthisexampleisareductioninarc-flashenergyfrom11calories/cm2tolessthan4calories/cm2.Thisdemonstratestheadvantageofthemultiplesettinggroupfeature.Themaintenance(orinstantaneousonly)modeactuallyallowsforfasterclearingtimesthanthenormalinstantaneoussettings,inpartbecausethetrippingsystemrespondstopeakcurrentsasopposedtothenormalRMSorrootmeansquaredcurrents.SincethetrippingsystemisnotburdenedwiththeadditionaRMScalculationbeforesendingasignaltothecircuitbreakertotriponovercurrent,thetimetoactualyopenthebreakercontactsduringafaultisreduced.Typically,instantaneousclearingtimescanoccurin3cyclesratherthanthestandard5-cyletripforthisclassofcircuitbreaker.Althoughclearinginanadditional2cycles(32milliseconds)seemsinsignificant,thisactuallycanmeanadifferenceintheHazardRiskCategory,typicallyreducingthehazardfromHRC2toHRC1.Itisimportanttounderstandthatthemultiplesettingsgroupcapabilitydoesrepresentatrade-offontwodifferentfronts.First,dependingontheinstantaneoussettingselected,selectivecoordinationofthesystemmaybecompromised.IntheFig.5example,notethatthecurvetothefarleftoftheplot(brownincolor)representsanacross-the-linestartofthelargestmotorfedbythissubstationfeederbreaker.Intheselectivelycoordinatedsetting,startingthismotorwouldassurethismotorcouldbestartedwithoutafeederbreakertrip.However,inthemaintenancemode,notefromthecurvesetattherightthatthefeederbreakerwouldindeedtrip.Second,applicationofmultiplesettingsgroupfunctionalitydictatesthatfacilitymaintenancepracticesberevisedandthenadheredto.Maintenancepersonswillneedtoadoptaprocesswherethemaintenancemodecouldbesafelyengagedwhiledownstreamenergizedworkisbeingperformed,andalsobeassuredthattheprotectivesettingswerereturnedtonormalaftermaintenanceiscompleted.Itwouldbetypicalforthemaintenancemodesettingstobeenabledwithalockableswitchanddoor-mountedlightsothisalternativemaintenancesettingcouldbeincludedinthefacilitylockout/tagoutprocedure.Finally,itisimportanttonotethattheOccupationalSafetyandHealthAdministration(OSHA)clearlyprohibitsworkonenergizedequipment.Specificaly,OSHA29CodeofFederalRegulations(CFR)Part1910.333(a)(1)9requiresthatlivepartsbedeenergizedbeforeanemployeeworksonornearthem.Thereissimplynoargumentthatturningthepoweroffresultsinthesafestworkingcondition.However,insomeprocessindustryenvironments,deenergizingthepowersystemis