The art of designing the auxiliary system of a power plant | EEP

Power plant auxiliary system is usually pretty complicated and consists of dozen of motors, transformers, capacitor banks, ... Searchfor:CyberMondayOffer❱❱Get20%OFFonProPremiummembershipand30%OFFoncourses&bundleswithdiscountcodeCYBERMONDAY21(expiresonDec3rd).MoreInformation»ElectricMotor/EnergyGeneration/ProtectionTheartofdesigningtheauxiliarysystemofapowerplantByEdvard|February,24th2020|4comments|Translate|SavetoPDF♛Subscribe|Share!Home/TechnicalArticles/TheartofdesigningtheauxiliarysystemofapowerplantAuxiliarysystemdesignPowerplantauxiliarysystemisusuallyprettycomplicatedandconsistsofdozenofmotors,transformers,capacitorbanks,variablefrequencydrives,PLCs,andotherelectricaldevices.Therefore,protectionrelayingcannotbesimple,anditrequiresspecialattentionforeachcomponentofapowerandcontrolsystemaswellascoordinationbetweenthem.TheartofdesigningtheauxiliarysystemofapowerplantThistechnicalarticlewillshedsomelightonhowanauxiliarysystemshouldbedesignedinordertosustainthemainproductionfacilitiesinpowerplants.Specialattentionisgiventotheprotectioncoordinationofmotors,transformers,andotherdevicesfromthepointofviewofthenormalandemergencyoperationoftheentireplant.FaultedelectricalequipmentMUSTberemovedfromserviceasfastaspossible.Formanyelectricalfaultsorabnormaleventswithintheplantthismayrequirethatthegeneratorberemovedfromthesystem,theexcitationsystemtripped,theturbinevalvesclosedandtheboilerfiresextinguished.Oftenthisisnotacceptable.However,itisnecessarythatvitalservicessuchasbearingoilpumps,instrumentaircompressors,exhaustandpurgingfans,etc.bemaintainedeventhoughtheunithasbeentrippedandisintheprocessofbeingshutdown.Inaddition,theauxiliarysystemmustbeconfiguredtoallowtheunittoreturntoserviceassoonaspossible.Tableofcontents:Auxiliarysystemofaunit-connectedgeneratorWhichtypeofcircuitbreaker?Phasefaultprotection(51)NeverunderestimategroundfaultprotectionBustransferschemes(alternativesource)Generatorbreaker(neededornot?)Auxiliarysystemofaunit-connectedgeneratorAportionofatypicalauxiliarysystemofaunit-connectedgeneratorisshowninFigure1below.The4kVauxiliarybusisfeddirectlyfromthe20kVgeneratorleadsorfromthestartuptransformerandisthesourceforthemajormotors.Asunitsizesincrease,theauxiliaryloadincreasesproportionately,requiringhigherratedtransformersandhigherrated,highervoltagemotors.Thishasresultedinhigherbusvoltages,suchas6.9kVand13kV.Phasefaultcurrentsalsoincreased,requiringswitchgearwithhigherinterruptingcapacity.Insizingtheswitchgeartherearetwocontradictoryfactorsthatmustbeconsidered.Theimpedanceofstandardtransformersincreasesastheirratingsincrease.Sincethenormalandshort-circuitcurrentsarealsoincreasing,thereisagreatervoltagedropbetweentheauxiliarybusandthemotor.Figure1–Singlelinediagramofatypicalpowerplantauxiliarysystemwithunit-connectedgeneratorNormaldesignpracticeistomaintainatleast85%voltageatthemotorterminalsduringmotorstarting.Ifthestandardtransformerimpedanceisspecifiedtobeatalowervaluetoreducethevoltagedropandmaintainthe85%voltagecriterion,theinterruptingcurrentwillincreaserequiringlargerratedswitchgearIfthetransformerimpedanceisraisedtoreducethefaultcurrent,andhencetheinterruptingcapacityrequirementoftheswitchgear,thevoltagedropwillbetoohigh.Theartofdesigningtheauxiliarysystemmusttakeallofthesefactorsintoaccount.Transformerscanbespecifiedwithspecialimpedancesatagreatercost.Theauxiliarysystemcanbedesignedwithseveralbussectionsthusreducingthetransformerratingforeachsection.Current-limitingreactorscanbeusedeitherasseparatedevicesorincorporatedintheswitchgear.Inadditiontothe4kV(orhigher)bus,alowervoltageauxiliarybussystemisusedtofeedthedozensorhundredsofsmallermotors,heatingandlightingloadsthatarepresentintheplant.Thenominalvoltageratingofthislowervoltagebussystemcanbe600or240V.ThelowervoltagebusesareenergizedfromthehighervoltagebusasshowninFigure1.Automaticthrow-overschemesbetweentheseveralbussectionsorbetweenthegeneratorstep-up(GSU)andstartuptransformerareusedintheeventofa4kVbusfaultorfailureofa20kV/4kVor4kV/600Vtransformer.Inaddition,manualthrow-overprovidesflexibilityformaintenancewithoutremovingthegeneratorfromservice.Thecircuitbreakersusedonthelowervoltagebusesareincludedinthemetal-enclosedswitchgearandarecoveredinANSIstandardsC37.20-1andC37.20-3.Theymaynotbedraw-outtypenecessarily,don’thaveCTsandmaybemountedinmotorcontrolcenters.Circuitbreakersmaybeairtypeormoldedcasebreakerswithlimitedinterruptingcapacity.Protectionisprovidedbyseriestripcoilsorthermalelements.GobacktoContentTable↑Whichtypeofcircuitbreaker?Therearemanycircuitbreakerdesignsdependingupontheparticularapplication:OilcircuitbreakersAircircuitbreakersVacuumcircuitbreakersSulfurhexafluoride(SF6)circuitbreakersVacuumcircuitbreakersextinguishthearcinagapoflessthan13mm(0.5in)becausetherearenoconstituentsinthevacuumthatcanbeionizedtosupportthearc.Sulfurhexafluoride(SF6)circuitbreakersextinguishthearcusingoneoftwomethods:thepufferdesignblowsthearcoutwithasmallamountofgasblastedinarestrictedarcspace;therotatingarcdesignusestheelectromagneticeffecttorotatethearcthroughSF6thatcoolsandextinguishesit.Nowadays,vacuumandSF6circuitbreakersaremorecommonlyused.Moreaboutwhichoneandwheretoapply,youcanlearnhere.Thesecircuitbreakersaremainlydrawouttype,allowingthebreakertoberemovedformaintenance.Figure2–ABBUniGearZS1vacuumcircuitbreakers(VCB)–photocredit:slaters-electricals.comFigure3–Mediumvoltagedraw-outvacuumcircuitbreaker,typeVD4–photocredit:slaters-electricals.comBusesratedabove2400Vusemetal-cladswitchgearasdefinedinANSIstandardC37.20-2.TheswitchgearcompartmentcontainstheCTs,auxiliarycontactsand,usually,therelaysandmeters.GobacktoContentTable↑Phasefaultprotection(51)Thephaseovercurrentrelays(51Aand51B)onthesecondaryoftheunitauxiliaryandstartuptransformersprovidebusprotectionandbackuprelayingforindividualmotorprotectionandswitchgear.Figure1indicatesthegeneralarrangementofthebusesandloadsandshowstheprotectionofthe2000hpmotorandthe7500hpmotor.Ideally,thebackupovercurrentrelays51Aand51Bshouldhavepickupsettingsgreaterthanthehighestmotorprotectionrelay,andtimedelayslongerthanthelongeststartingtime.Thesesettingsmaybesohigh,orthetimessolong,thattheprotectionisnotacceptableandmodificationsorcompromisesarerequiredasdiscussedbelow.Iftherelaysarealsotheprimarybusprotectiverelays,thesettingsmaybesohighthattheremaynotbeenoughbusfaultcurrenttoprovidesufficientmargintoensurepickupfortheminimumbusfault.Figure4–Phaseovercurrentrelays(51Aand51B)onthesecondaryoftheunitauxiliaryandstartuptransformersasabusprotectionandbackuprelayingforindividualmotorprotectionandswitchgearEvenifcoordinationistheoreticallypossible,therequiredtimedelaymaybetoolongtobeacceptable.Somecompromisesarepossible.Sincethelargestmotorswillprobablyhavedifferentialprotection,thebackupfunctioncouldconsidercoordinatingwiththeovercurrentrelaysofthesmallermotorswithanassociatedreductioninpickup.Assumingthatthedifferentialrelaysarealwaysoperative,coordinationwiththelargermotorsisnotaproblemsincethedifferentialprotectionisinstantaneous.Coordinationwouldbelostifthedifferentialrelaysfailtoclearafaultandthetime-delayovercurrentrelaysmustdoit.Thisisusuallyanacceptablerisk.Abusdifferentialrelaycouldbeusedtoprovideprimaryprotectionandtheovercurrentrelaysprovidebackupprotectionformotorrelayorswitchgearfailures.Thetimedelaymaythenbeacceptable.Thepickupsettingmuststillrecognizethemagnitudeofstartingcurrentofthelargestmotor.Ifitcannotbesetabovethisvalue,aninterlockmustbeprovidedwhichwillblockthebackuprelay.Figure5–OvercurrentbackupandbusdifferentialprotectionschemeTheovercurrentbackuprelaywillsee thetotalcurrentsuppliedtothe bus,whereasthedifferentialrelayonlyseesthedifferencebetween supplycurrentandloadcurrentasdiscussedpreviously.Therelay settingsforovercurrentbackupprotectionsaresomewhatdifficultandusuallyemployaninstantaneousandtimedcomponent.Afterall,the objectiveisthatbreakers X and Y shouldclearthefaultbeforethe backupprotectionoperatestotripthemainsupplybreaker S.GobacktoContentTable↑NeverunderestimategroundfaultprotectionTheimportanceofgroundfaultprotectioncannotbeoveremphasized.Groundisconsideredtobeinvolvedin75–85%ofallfaults.Inaddition,phaseovercurrentmayoftenreflectatemporaryprocessoverloading,whilegroundcurrentisalmostinvariablyanindicationofafault.Auxiliarysystemsmaybeeitherdelta-orwye-connected.Adeltasystemisnormallyoperatedungroundedandisallowedtoremaininservicewhenthefirstgroundindicationappears.Itisgenerallyassumedthatthefirstgroundcanbeisolatedandcorrectedbeforeasecondgroundoccurs.Itisnotuncommonforsystemsof600Vandlesstobedelta-connected.Medium-voltagesystems(5kVto15kV)aregenerallyoperatedinwye,withaneutralresistortolimitthegroundcurrenttosomedefinitevalue.Theresistorhasatime-relatedcapability,e.g.10s,atthemaximumgroundcurrentanditisafunctionofthegroundprotectivesystemtoremoveallfaultswithinthistimeconstraint.Figure6–2.0ΩneutralresistorsintheauxiliaryandstartuptransformersInFigure6,groundfaultsonthe4kVsystemarelimitedbythe2.0Ωneutralresistorsintheauxiliaryandstartuptransformers.Themagnitudeofthemaximumfaultcurrentisthelineto-groundvoltagedividedbythe2.0Ωresistor.Thenominalvoltageofthebusis4kVbutitsnormaloperatingvoltageis4160V.Therefore,themaximumgroundcurrentis4160/(√3×2)or1200A.Coordinationmust,ofcourse,beginattheload.IfthemotorgroundovercurrentprotectionisprovidedbythetoroidalCTshowninFigure7thereisnocoordinationproblem.Figure7–Currenttransformersinrelayprotectionapplications(photocredit:merko.ee)Thesecanhavearatioof50:5resultinginarelaycurrentof120A.Setaninstantaneousrelayat5.0A.IfaresidualgroundrelayisusedasshowninFigure8,themaximumgroundfaultthroughtheCTsonbreakersAandBis1200/600=2.0A.Setthetime-delaygroundovercurrentrelaysat0.5Aand15–30cycles.Themotorrelaystriptheassociatedfeederbreaker,51Aand51Btripthe4kVmainbreakersandtheneutralrelays51Ntriptheirassociatedprimarybreakers.Figure8–ResiduallyconnectedgroundrelayGobacktoContentTable↑Bustransferschemes(alternativesource)Itiscommonpracticetoprovideabustransferschemetotransfertheauxiliarybustoanalternativesourceintheeventofthelossoftheprimarysource.Inpowerplants,thepurposeofthisalternativesourceisnottomaintainnormaloperationbuttoprovideastartupsource,toactasaspareintheeventanauxiliarytransformerfailsandtoprovidefororderlyandsafeshutdown.Inindustrialplants,thealternativesourcemighthaveadifferentpurpose,suchastoprovideflexibilityinproductionorsupplysomefacilitiesfromtheutilityandothersfromalocalgenerator.Thetransferschememustconsiderseveralfactors.Amanual,livetransferisperformedbytheoperatorwhileboththenormalandstartupsourcesarestillenergized.Ifthetwosourcescanbeoutofsynchronism,itwillbenecessarytoincludesynchronizingequipment.Someschemesmonitorthisresidualvoltageandallowclosingtothealternativesourceonlyafterthisvoltagehasbeensignificantlyreduced.Figure9–AtypicalunittostationswitchboardbustransferschemeGobacktoContentTable↑Generatorbreaker(neededornot?)Figure10showsageneratorbreakerasanalternativefacility.Thisiscommonforgeneratorsthatareconnectedtoacommonbus,suchasinahydroplant.Withtheadventoftheunitsystem,however,thisconfigurationhasnotbeenusedasoften.Theunitsystemrequiresthattheboiler,turbine,generatorandGSUtransformerbeoperatedasasingleentityandthelossofanyoneelementrequiresthatallofthemberemovedfromservice.Thegeneratorbreakeristhenunnecessary.Inaddition,astheunitsizesincreased,theinterruptingcapabilityofageneratorbreakerbecametechnicallydifficult.A1300MWgeneratorcancontributeasmuchas100000Atoafaultatthegeneratorvoltagelevel,e.g.onthebusfeedingtheauxiliarytransformers.Notonlyissuchabreakerextremelycostly,itmustbeplacedbetweenthegeneratorandthestep-uptransformer,whichaddsconsiderablelengthtothebuilding.Thisintroducescoststoeverysegmentoftheconstructionandinstallation.Figure10–GeneratorbreakerasanalternativefacilityNevertheless,thegeneratorbreakerhascanbeextremelyuseful.Itsmostimportantadvantageisthefactthat,forafaultonthegeneratororauxiliarybuses,withoutageneratorbreakertoremovethegeneratorcontributionfromthefault,thegeneratorwillcontinuetofeedthefaultuntilthegeneratorfielddecays.Thiscantakeasmuchas7–10s.Duringthistimetheenergyinthefaultwillresultinextensivephysicaldamagetoalloftheconnectedequipmentandgreatlyincreasesthepossibilityoffire.ReferringtoFigure1,withoutageneratorbreaker,startupisaccomplishedbyenergizingtheauxiliarybusesthroughthe800kVbreakerF,thestartuptransformerand4kVbreakerB.Synchronizingisdonethrough800kVbreakerE. Intheeventofaunittrip,theunitisremovedfromthesystembyopeningbreakerEandtheauxiliarybusistransferredtothestartuptransformerbyopening4kVbreakerAandclosingbreakerB.BreakerFisoperatednormallyclosed.Ifthestartuptransformerisconnectedtosomeothersystem,thenbreakerBmustbeclosedwithsynchronizingrelays.Ifageneratorbreakerisprovided,atstartupthegeneratorbreakerisopenandtheauxiliarybusesarefedthroughtheGSUtransformerand4kVbreakerA.GobacktoContentTable↑Sources:PowerSystemRelayingbyStanleyH.Horowitz,ArunG.Phadke(purchasethehardcoverfromAmazon)ScienceandReactorFundamentals–Electrical|CNSCTechnicalTrainingGroupSwitchgearsbookbyBHEL–BharatHeavyElectricalsLimitedRelatedEEP'scontentwithsponsoredlinksPremiumMembershipGetaccesstopremiumHV/MV/LVtechnicalarticles,electricalengineeringguides,researchstudiesandmuchmore!Ithelpsyoutoshapeupyourtechnicalskillsinyoureverydaylifeasanelectricalengineer.MoreInformationEdvardCsanyiElectricalengineer,programmerandfounderofEEP.HighlyspecializedfordesignofLV/MVswitchgearsandLVhighpowerbusbartrunking(<6300A)inpowersubstations,commercialbuildingsandindustryfacilities.ProfessionalinAutoCADprogramming.4CommentsJessieCorbitoJul17,2021Iwouldliketoknowallofthistopic..ReplyGodwillMugabeFeb25,2020I’mconfidenttoalwaysrefertoyourportalinfutureforconsultativepurposesReplyGodwillMugabeFeb25,2020I’mextremelyinterestedinyourportal,mindrefreshingfortechnicians,likeI’mcomingoutofaseminarReplyKhaledabdoFeb24,2020Ineedunderstandingdirectionalovercurrent&directionalearthfaultReplyLeaveaCommentCancelReplyTelluswhatyou'rethinking...wecareaboutyouropinion!Pleasekeepinmindthatcommentsaremoderatedandrel="nofollow"isinuse.So,pleasedonotuseaspammykeywordoradomainasyourname,oritwillbedeleted.Let'shaveaprofessionalandmeaningfulconversationinstead.Thanksfordroppingby!Yourrealname*Emailaddress*ProvethatyouarenotR2D2.Solvesomemath:  −  2  =  fourΔCyberMondayOffer🎀Get20%OFFonEEPProPremiummembershipand30%OFFoncourses&bundleswithcodeCYBERMONDAY21(expiresonDecember3rd).MoreInfoSubscribetoNewsletterSubscribetoourWeeklyDigestnewsletterandreceivefreeupdatesonnewtechnicalarticles,videocoursesandelectricalguides.Subscribe©2021EEP-ElectricalEngineeringPortal.AllRightsReserved|PrivacyPolicy|TermsandConditionsThecontentiscopyrightedtoEEPandmaynotbereproducedonotherwebsites.TechnicalArticlesTechnicalArticlesCategories(42)ElectricalGuidesElectricalEngineeringPowerSubstationsControl&ProtectionGenerationT&DABBProtectionBooksSiemensEEBasicsSchneiderElectricCTsIndustrialAutomationDataCenterGuidesABBDrivesGuidesAlternativeEnergyElectricalDesignDocsElectricalToolsExcelSpreadsheetsSoftwareReviewsVideoTrainingsElectricalEngineeringPLCProgrammingTesting&Commiss.EEResourcesHomeSitemapEEPAcademyCoursesContact/AdvertisementContactAdvertisementFreeNewsletterEEWeeklyDigestFacebookLinkedinLinkedinTwitterPremi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