Selecting, sizing transformers for commercial buildings _ Consulting-Specifying Engineer.pdf
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Forgot password? Username ******* Subscribe Manage Account Magazine Newsletters Webcasts Research Sponsored b 06/09/2011 Share Selecting, sizing transformers for commercial buildings While commercial building designs change, their electrical loads remain fundamentally unchanged. Properly sizing and selecting transformers ensures that these loads are accommodated. By Ralph Baeza, PE, LEED AP, TLC Engineering for Architecture, Miami Transformers, along with other power distribution apparatus, remain a fundamental component in electrical systems distribution for commercial buildings. This article presents several useful design concepts for selecting and sizing transformers in the design of electrical systems for commercial buildings. Transformers change voltage levels to supply electrical loads with the voltages they require. They supply the required incoming electrical service to the buildings. Transformer primary and secon dary voltages can be 2,400; 4,160; 7,200; 12,470; and 13,200 for 15-kV Class, and 120, 208, 240, 277, and 480 for 600-V Class. Transformers are located either outdoors or inside buildings in an electrical room or other areas as permitted by code. The electrical phase characteristics associated with the transf ormer’s primary side is 3-phase, 3-wire or Delta connected. The secondary is 3-phase, 4-wire or Wye connected. Construction types There are different construction types for transformers used in commercial buildings. Our understanding of their general characteristics will allow the designer and end user to make the proper selection for the electrical system application. Following are some of the transformer types available in the industry along with a few of their characteristics: Ventilated dry-type transformers are ventilated by air, use larger space for clearance, and use different insulating materials to augment the dielectric strength of the air. They contain an enclosure surrounding the windings for their mechanical protection and the safety of personnel. This type is the most common to be used in the building indoor electrical system distribution. See Table 1 for typical dry-type transformer ratings, dimensions, and weights. Sealed dry-type transformers are similar to dry type in most of their characteristics. The difference is they contain an enclosed tank with nitrogen or other dielectric gas to protect the windings. They can be installed outdoors or indoors. They are useful in areas with a corrosive or dirty atmosphere. Cast-coil transformers are constructed with the primary and secondary windings encapsulated in reinforced resin. They can be installed where moisture or airborne contaminants exist. Nonventilated dry-type transformers are similar to the ventilated type but are totally enclosed. This type can be installed in areas with corrosive or dirty atmospheric conditions where it would be impossible to use a ventilated-type transformer. Oil-filled transformers are constructed with the windings encased in an oil-tight tank filled with insulating mineral oil. It is good practice to regularly test this type of transformer in order to determine dielectric breakdown, which affects its useful life. Application types There are different ways in which transformers are installed and used as part of a commercial building electrical system. These application types include: Indoor distribution transformers are used with panelboards and are separately mounted to supply the specific electrical load requirements in a system-specific application within the system distribution. Several transformer types rated higher than 600 V for oil insulated type, higher than 35,000 V for dry type, and other transformers rated higher than 600 V are required to be located in vault rooms, which must be built with fire-rated enclosures depending on the transformer type and applicable local authority requirements, when indoors.
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Transcript of Selecting, sizing transformers for commercial buildings _ Consulting-Specifying Engineer.pdf
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5/18/2015 Selecting,sizingtransformersforcommercialbuildings|ConsultingSpecifyingEngineer
http://www.csemag.com/singlearticle/selectingsizingtransformersforcommercialbuildings/4efa064775c5e26f27bfce4f0a61378e.html 1/4
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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
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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).
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5/18/2015 Selecting,sizingtransformersforcommercialbuildings|ConsultingSpecifyingEngineer
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5/18/2015 Selecting,sizingtransformersforcommercialbuildings|ConsultingSpecifyingEngineer
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