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5/18/2015 Selecting,sizingtransformersforcommercialbuildings|ConsultingSpecifyingEngineer
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06/09/2011
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Selecting,sizingtransformersforcommercialbuildingsWhilecommercialbuildingdesignschange,theirelectricalloadsremainfundamentallyunchanged.Properlysizingandselectingtransformersensuresthattheseloadsareaccommodated.ByRalphBaeza,PE,LEEDAP,TLCEngineeringforArchitecture,Miami
Transformers,alongwithotherpowerdistributionapparatus,remainafundamentalcomponentinelectricalsystemsdistributionforcommercialbuildings.Thisarticlepresentsseveralusefuldesignconceptsforselectingandsizingtransformersinthedesignofelectricalsystemsforcommercialbuildings.
Transformerschangevoltagelevelstosupplyelectricalloadswiththevoltagestheyrequire.Theysupplytherequiredincomingelectricalservicetothebuildings.Transformerprimaryandsecondaryvoltagescanbe2,4004,1607,20012,470and13,200for15kVClass,and120,208,240,277,and480for600VClass.
Transformersarelocatedeitheroutdoorsorinsidebuildingsinanelectricalroomorotherareasaspermittedbycode.Theelectricalphasecharacteristicsassociatedwiththetransformersprimarysideis3phase,3wireorDeltaconnected.Thesecondaryis3phase,4wireorWyeconnected.
Constructiontypes
Therearedifferentconstructiontypesfortransformersusedincommercialbuildings.Ourunderstandingoftheirgeneralcharacteristicswillallowthedesignerandendusertomaketheproperselectionfortheelectricalsystemapplication.Followingaresomeofthetransformertypesavailableintheindustryalongwithafewoftheircharacteristics:
Ventilateddrytypetransformersareventilatedbyair,uselargerspaceforclearance,andusedifferentinsulatingmaterialstoaugmentthedielectricstrengthoftheair.Theycontainanenclosuresurroundingthewindingsfortheirmechanicalprotectionandthesafetyof
personnel.Thistypeisthemostcommontobeusedinthebuildingindoorelectricalsystemdistribution.SeeTable1fortypicaldrytypetransformerratings,dimensions,andweights.
Sealeddrytypetransformersaresimilartodrytypeinmostoftheircharacteristics.Thedifferenceistheycontainanenclosedtankwithnitrogenorotherdielectricgastoprotectthewindings.Theycanbeinstalledoutdoorsorindoors.Theyareusefulinareaswithacorrosiveordirtyatmosphere.
Castcoiltransformersareconstructedwiththeprimaryandsecondarywindingsencapsulatedinreinforcedresin.Theycanbeinstalledwheremoistureorairbornecontaminantsexist.
Nonventilateddrytypetransformersaresimilartotheventilatedtypebutaretotallyenclosed.Thistypecanbeinstalledinareaswithcorrosiveordirtyatmosphericconditionswhereitwouldbeimpossibletouseaventilatedtypetransformer.
Oilfilledtransformersareconstructedwiththewindingsencasedinanoiltighttankfilledwithinsulatingmineraloil.Itisgoodpracticetoregularlytestthistypeoftransformerinordertodeterminedielectricbreakdown,whichaffectsitsusefullife.
Applicationtypes
Therearedifferentwaysinwhichtransformersareinstalledandusedaspartofacommercialbuildingelectricalsystem.Theseapplicationtypesinclude:
Indoordistributiontransformersareusedwithpanelboardsandareseparatelymountedtosupplythespecificelectricalloadrequirementsinasystemspecificapplicationwithinthesystemdistribution.Severaltransformertypesratedhigherthan600Vforoilinsulatedtype,higherthan35,000Vfordrytype,andothertransformersratedhigherthan600Varerequiredtobelocatedinvaultrooms,whichmustbebuiltwithfireratedenclosuresdependingonthetransformertypeandapplicablelocalauthorityrequirements,whenindoors.Transformersthatarenotover600Vandarepartoftheindoorbuildingelectricalsystemdistributionhavebothprimaryand
5/18/2015 Selecting,sizingtransformersforcommercialbuildings|ConsultingSpecifyingEngineer
http://www.csemag.com/singlearticle/selectingsizingtransformersforcommercialbuildings/4efa064775c5e26f27bfce4f0a61378e.html 2/4
Transformersthatarenotover600Vandarepartoftheindoorbuildingelectricalsystemdistributionhavebothprimaryandsecondaryvoltagesbelow600Vwiththemostcommonvoltagelevelchangefrom480Vto208Y/120V.
Padmountedtransformersareinstalledoutsideandareconsideredthefirstoptionforsupplyingserviceentrancevoltagetothebuildingelectricalsystembasedontheprojectsizeandrequirements.Theytypicallyhaveprimaryvoltageshigherthan600Vandsecondaryvoltageslowerthan600Vwithcompartmentsfortheassociatedprotectivedevicesassembledinanintegraltamperresistantandweatherproofunit.
Inaddition,thesizeofthecommercialfacilitywilldeterminetheappropriateapproachfordesigningtheelectricaldistributionsystemforthespecificapplication.Inthiselectricalsystemdesign,thetransformercanbeusedaspartofasubstation,primaryunitsubstation,secondaryunitsubstation,ornetworkconfiguration.
Sizing
TheelectricalsizeofthetransformerloadisratedinkVA.Thisratingprovidestheassociatedpoweroutputdeliveredforaspecificperiodbytheloadsconnectedtothetransformeronthesecondarysideoftheequipment.Theloads,whicharecalculatedaspartofthebuildingelectricalsystemdesignphase,areshownintheconstructiondocumentsrespectiveequipmentschedulesinVAorkVA.
Ageneralapproachtodeterminingtransformercapacityandselectingtheproperratingforthedesignapplicationistoobtainthecalculateddesignloadfromtherespectiveelectricalscheduleandadd20%sparecapacityforfutureloadgrowthtobeshownintheequipmentschedule,unlessotherwisedirectedbythefacilitybasedondesignparameters.Forexample,thecodebaseddemandloadofa208Y/120V,3phase,4wirepanelboardis42kVA,whichdoesdonotincludesparecapacityforfuturegrowth.Therefore,thetransformersizerequiredforconvertingthesystemvoltagefrom480V,3phase,3wireto208Y/120V,3phase,4wireis:
TransformersizeinkVA=42kVAx1.25=52.5kVA
Therefore,a75kVAtransformerwouldbeselectedforthisapplicationoutoftheavailablestandardratingsfora480Vprimaryto208Y/120Vsecondary.Themostcommonbuildingindustrystandardratingsare3,6,9,15,30,37.5,45,75,112.5,150,225,300,500,750,and1,000kVA.
Theabovesimplecalculationmeetstheintenttoachievethenormallifeexpectancyofatransformer,whichisbasedonthefollowingbasicconditions:
ThetransformerisequaltoorlessthanitsratedkVAandratedvoltage.Theaveragetemperatureofthecoolingairduringa24hourperiodis86F.Thetemperatureofthecoolingairatnotimeexceeds104F.
Selecting
TransformerselectionstartswiththekVAratingrequiredtosupplytheloadsconnectedintheelectricalsystem.Anotherconsiderationforindoordistributiontransformersisthetypeofload:linearornonlinear.Linearloadsincluderesistiveheatingandinductionmotorsnonlinearloadsareproducedbyelectronicequipmentthatcontributestothedistortionoftheelectricalpowersignalsbygeneratingharmonics.Theharmonicsresultingfromnonsinusoidalcurrentsgenerateadditionallossesandheatingofthetransformercoils,whichreducethetransformerlifeexpectancy.
IndoortransformersfornonlinearloadscanbeselectedwithaKrating,whichallowsthetransformertowithstandnonlinearconditionsintheelectricalsystem.Kratedtransformersdonotmitigateoreliminateharmonics.However,theydoprotectthetransformeritselffromdamagecausedbyharmonics.Forharmonicmitigation,Kratedtransformerscanbecombinedwithharmonicfiltersorchokes.Forlinearloadapplications,transformersshouldbeselectedwithlowercorelosses.Otherfactorsthatshouldbeconsideredinselectingtransformersarevoltageratingsforbothprimaryandsecondary,voltagetaps,efficiency,impedancevalue,typeofcoolingandtemperaturerise,voltageinsulationclass,basicimpulselevel,andsoundlevel.
Practicalapplications
Inthepasttwoyears,twolargeprojectsinMiamiDadeCountyhavebeenbuilt:theFloridaInternationalUniversityfootballstadiumandMiamiInternationalAirportSouthTerminal.Bothprojectsincludeddrytype480V,3phaseto208Y/120VVstepdowntransformers(inNEMA2enclosures),rangingfrom15kVAto112.5kVAintheelectricalsystemdistributiondesign.
The18,688seatFIUfootballstadiumwasdesignedwithabout12transformersaspartoftheelectricalsystemdistributioninordertosupplygeneralusereceptacles,smallmotors,andotherloadsinthestadiumbuildingstructureandtheattachedfieldhousebuilding.TheMIASouthTerminalexpansionwasdesignedwithabout50transformerswithsimilarintentasthestadiumsbutamorediversegroupofloadsforthe208Y/120V3phase,4wiresystem,whichalsoincludedlightingloads,signage,telecommunication,securitysystems,andotherloadspartofthisbuildingproject(Figure1).
5/18/2015 Selecting,sizingtransformersforcommercialbuildings|ConsultingSpecifyingEngineer
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5/18/2015 Selecting,sizingtransformersforcommercialbuildings|ConsultingSpecifyingEngineer
http://www.csemag.com/singlearticle/selectingsizingtransformersforcommercialbuildings/4efa064775c5e26f27bfce4f0a61378e.html 4/4
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