Download - Selecting, sizing transformers for commercial buildings _ Consulting-Specifying Engineer.pdf

5/18/2015 Selecting,sizingtransformersforcommercialbuildings|ConsultingSpecifyingEngineer

http://www.csemag.com/singlearticle/selectingsizingtransformersforcommercialbuildings/4efa064775c5e26f27bfce4f0a61378e.html 1/4

Trending NewProducts Power Fire,LifeSafety Electrical,Lighting BuildingTypes CodesandStandards HVAC

Weeklymerger,acquisition,dealupdate:May15

Achievingrealworldbuildingenergyperformance

Transferswitches

Arcflashrelay

Yourquestionsanswered:ElectricalSystems:ArcFlashReductionandNEC240.87Requirements

Energyefficiencywebcast:Variablefrequencydrives(VFDs),variablespeeddrives(VSDs),andmotors

YorkLXseriespackagedunits

Graduatingfromsmarttointelligentbuildings

Casestudy:Designinganintelligentbuildingsystem

Video:RetroCxoftheSanDiegoConventionCenter:CaseStudy

Forgotpassword?

Username

*******SubscribeManageAccount

MagazineNewslettersWebcastsResearch

Sponsoredby:

06/09/2011

Share

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



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).



ArticlesCareerSmartCaseStudiesandWhitePapersCodes&StandardsMagazine

NewProductsNewProductsInnovationsfromtheIndustryCSEProduct&MediaShowcase

TopicsAutomation,ControlsCodes&StandardsElectricalSystemsFire&LifeSafetyHVAC/R

Training,ToolsAppsforEngineersCareerSmartCaseStudyDatabaseVideos

EventsandAwards40Under40MEPGiantsProductoftheYearUpcomingEvents

News,Views,BlogsCareerSmartCollaboratingagainstcounterfeitelectricalproductsCuttheCopper

NewslettersCodes&StandardsElectricalSolutionsFire&LifeSafetyHVACNewsWatch:Data

AdvertisingCaseStudyDatabaseListingContactUsInnovationsfromtheIndustry

AboutUsTermsofUseAppsforEngineersControlEngineeringAboutCFEMediaLLC.AdvisoryBoard



IndustryRoundupMagazineArchivesProjectsinProgress

ProductoftheYear LightingPlumbing,Piping

EngineeringChangeIEEEPowerPerspectivesIndustryNewsInsightsonPowerResearchAnalystBlogsRSSFeedsSocialMediaUnderstandingSpecifications,Codes,andStandards

CentersNewsWatch:EducationalFacilitiesNewsWatch:HospitalsPurePower

InternetProfilesMagazineCSEProduct&MediaShowcase

HowtoContributeContactUsMagazineContentStreamMagazineArchivesEGSAOnPeakPerformancePlantEngineeringPrivacyPolicyPurePowerSubscribe