IEEE P1818 / D1 Guide for the Design of Low Voltage Auxiliary Systems for Electric Power Substations

1.0Overview

1.1Scope

This guide will consider the components of both the AC and DC systems and provide guidelines and recommendations for designing the appropriate systems for the substation under consideration. This guide includes the low voltage auxiliary systems from the source(s) to the distribution point(s). Reliability requirements and load characteristics are discussed and distribution methods are recommended.

1.2Purpose

The low voltage AC and DC auxiliary systems comprise very important parts of the substation equipment. The design of the AC and DC auxiliary systems facilitates the safe and reliable operation of the substation. This guide considers various factors that affect the design of the AC and DC auxiliary systems such as reliability, load requirements, system configurations, personal safety and protection of auxiliary systems equipment.

2.0Normative References

3.0Definitions

4.0Design of Substation AC Systems

Theobjectiveofthissectionistoprovidetherequiredinformationfor thesubstationengineerto designsafeandeconomicalAC stationauxiliarysystemas applicablefor eachsubstation.The stationpower for mostsubstationscanberepresentedby theblockdiagramshown infigure4-1 below.Detailedinformationisgiveninthesectiongivennexttoeachpart.

Figure4-1:BlockDiagramfor TypicalSubstationAC StationPower[DML1]

Theabovefigurerepresentsanultimatestationpower configurationthatcanbeappliedtoany substationdependingon substationsizereliabilityandloadrequirements.One sourceis designatedas thenormalfeed, andasecondor thirdsourceisused as abackup.A loss of thenormal or preferred sourcewillresultintransferringtheloadtothebackupsource.In substations with multiple sources, thesources arenormally connectedtoatransferringscheme.One or moreAC panelsareused toservethesubstationload as required.

As afirststeptothedesignprocess, thedesignengineermustreviewthedesigncriterionfor the stationpower;thenumberof sources, sourcetype:threephaseor singlephase,required transformerrating,connectionandotherfactorsthatmayaffectthefinalconfigurationof thestationpower for theapplicablesubstation.Thedesigncriterionisdiscussed insection4.1 below.

4.1 Design Criteria

In general,thedesigncriteriaoftheAC auxiliarysystemaredeterminedby thedemandloadof theconnectedKVA ofthesubstationloads,as wellas thevoltageratingsandnumberof phases of thesubstationequipmenttobesupplied. Auxiliary transformers and other station power components should consider substation expansion and/or anticipated growth rate. Timing of any proposed expansion may dictate initial installation or deferral of station power components. Someloadsmaybeidentifiedas critical,which requiresAC servicetobe maintainedatalltimes.Dependingupon such criticalloads,the substationmayrequiretwo or three AC stationservicesourceswiththeabilitytotransferloadsbetween sources.

Due totheimportanceof thestationpower totheoperationandreliabilityof thesubstation,the followingfactorsmustbeconsideredinordertodeterminetherequiredstationpower configurations:

4.1.1 System Stability[DML2]

Systemstabilityconsiderationsareimportantforthereliabilityrequirementsofthestation power. If theloss of acertainsubstationwillresultinsystemdisturbancenotonlywithinthe owner utilitysystembutmightalsohaveacascadingeffecton neighboringutilitieswhich mayresultinablackoutconditioninthearea.[DML3]

4.1.2 Customer Service and Loss of Revenue

Somesubstationsserve verycriticalloadssuch as hospitals,manufacturingcomplexes, governmentaloffices,schools andotherimportantloads, or serve large blocks of load wherethesubstationreliability requirementsarehigh.Somesubstationsareconnectedtopower plants wheretheloss of the substationequipment mayresultintrippingtheplantwhichresultsinlossofrevenuefor theutility.More than one station power source may be warranted for these types of substations. Other less criticalsubstationsmayhavelimitedeffecton thecustomers’service andonesourcefor the stationpower maybejustified.

4.1.3 Equipment Protection

Substationequipmentprotectionconsiderationsmustbegiventoallsubstationsregardlessof thesize,howeverhighandextrahighvoltagesubstationscontainhighcostequipmentsuchas transformerswherethecoolingsystemisconsideredveryimportanttotheoperationof this equipmentandthereforeabackupsourceis generallyrequired.Protectiverelaysor other electronic control equipment locatedinhightemperatureareasmay requireacontinuouscoolingsystemandthereforethesecondsourceis generally required.

4.1.4 Design Considerations

ReviewComment–

ThedesignermayconsiderthefollowinglistwhendesigninganAC systemfor Substation:

  • Locationof AC equipmentindoor/outdoor
  • Numberof AC Panels
  • Essentialloadsconnections
  • Non-essentialloadsconnections
  • Conductortypeandsize
  • Voltagedrop calculations
  • NEC requirements

4.1.5 Selection of auxiliary system voltage

ReviewComment–

Severalsecondaryvoltagelevelsareavailablefor acauxiliarysystems.Whendeterminingthe voltagelevelneeded,thedesignermayuse astandardvoltageleveldeterminedby thedesigner’s power systemor use avariationtomaintainthevoltagelevelsoftheequipmentbeingsupplied. Eitherway, thedesignerneedstorememberthefactorsinsection4.2:ConnectedUltimateLoad.

4.2 Load Requirements

AC auxiliarypower systemsarevitaltothesuccessfuloperationof asubstation.TheAC auxiliarysystemsuppliesauxiliaryACpower toallAC loadslocatedwithinthesubstation. Becauseof thecriticalnatureofmuchof theseloads,itisnecessarytocalculateallof the requiredAC loadswithinthesubstationtoadequatelydesigntheAC auxiliarypower system.

Thegoverningdesignrequirementsof thesubstationAC auxiliarysystemshouldbesafe,reliable operationandallowancefor futureloadgrowth.Someapplicationsmayrequireadditional uninterruptablepower supplies.

In bothtransmissionanddistributionsubstations,thesubstationAC auxiliarysystemsare typicallyused tosupplyloadssuch as, butarenotlimitedto:

  • EssentialLoad
  • Non EssentialLoad
  • MaintenanceLoad
  • ConstructionLoad

4.2.1 Essential Load

These loads are related to equipment operation and are necessary to the proper function of the substation.

a)Transformercooling,oilpumps,andloadtapchangers

b) Substationbatterychargingsystems

c) Circuitbreakeraircompressorsandchargingmotors

d) Power circuitbreakercontrolcircuits

e)Power equipmentheatingcircuits

f) Communicationsequipment

g) Relaying,supervisory, alarm,andcontrolequipment

h) AC/DC converter– uninterruptablepower supplies

i)AC poweredmotoroperateddisconnectswitches

4.2.2 Non-Essential Load

Providedescription–

a)Outdoor lighting,securitysystemsandreceptacles

b) Controlbuildingor switchgearbuildinglighting,HVAC, andreceptacles

4.2.3 Maintenance Load

Providedescription–

4.2.4 Construction Load

Providedescription–

4.3 One-Line Diagram

Thedesignermayconsiderthefollowinglistwhen designingone-linediagram.

  • Transformerconnectionrequirementswhenthree-phasetransformerisused
  • TransformerKVA andvoltagerating
  • Possibleavailablesources
  • Determinepreferredsources
  • Numberof AC Panelsrequired
  • Methodof connectingtheACpanelstothetransformer

4.3.1 Distribution Substation

A distributionsubstationAC stationserviceauxiliarysystemmaybeas simpleas astep down transformerwithsecondaryvoltagetoanAC panelboardto distributeserviceloads. Dependingupon thesizeandcriticalityof thedistributionsubstation,analternatesourceand manualor automatictransferswitchmaybeincorporated.

ProvideSamples-

ConverttoBlockDiagram–

4.3.2 Transmission Substation

TheAC stationserviceauxiliarysystemnormallyrequiresaprimaryandabackupstation servicesource.Typically,theprimarysourceisfedwithinthesubstationfromasubstation bus or transformertertiary.Thebackupsourcemayalsocomefromasubstationbus or transformertertiary,or fromanexternalsourceoutsidethesubstationor by abackup generator.Thedesignshouldincludethemostreliableandefficientmeansfor two auxiliary systemsources. Theserviceloadsshouldbefedthroughanautomatictransferswitchto provideredundancyandreliabilitytoservethesubstationAC loads.

Many AC auxiliarysystemsintransmissionsubstationwillincludeAC panelsinthe substationyardtoserveoutdooryardequipmentandlightingcircuits.Dependingupon the voltage,yardequipmentmaybeservedfromACpanelswithinthecontrolbuilding.Large threephaseequipmentloadsmayrequireyardpanelsrated480 VAC, whichwould require voltagestepdown for othersubstationloads.Circuitswithinthecontrolbuildingshouldbe servedfromAC panelswithinthebuilding.

ProvideSamples-

ConverttoBlockDiagram–

4.4 Station power Source

4.4.1 Single-Phase or Three-Phase

AC auxiliarysystemsarecriticalinasubstation.AC power isused toprovideload requirementsfor all essential and non-essential loads within the substation.Thedesignershouldperformaload analysison alltheAC loadsinthesubstationinordertodecidewhetherasingle-phaseor three-phasesourceisneeded.Economicanalysis and determination of essentialandnon-essentialloadsshould beconsideredbeforemakingafinaldecision.

4.4.1.1 Single-Phase AC Source

Single-phasetransformersaretypicallyused insmallersubstations where the load demand does not warrant a three phase source.If the substationhas apotentialexpansion or serves critical loads,abackup station power circuit may be considered. Thesetwo circuitsshallbefromindependentsources. A tertiarywindingof thepower transformermay alsobe used for anAC source if available.Giventhesignificanceof thesubstation,abackupgenerator shall alsobeconsideredfor providing an auxiliarysourceduringanoutage.Theloadanalysisshall determinethesizeof thesingle-phasetransformers.Allcriticalor essential loadsshallbeservedatanytime.Thesingle-phasesourceisused when allthesubstationloadscanoperateat120 or 240 volts,andtheKVA demandedby theloadsisnotlargeenoughtorequirehigh ampacityconductorsandbreakers.Itisveryimportantfor thedesignertospecifytheratedAC voltageon theequipmentspecification.

4.4.1.2 Three-Phase AC Source

Substationswiththree-phasevoltageequipmentandhighstation power load requirements should considera three-phaseAC source.LargebulksubstationswithhighMVA autotransformersare commonexamplesof substationswitha three-phaseAC source.Thefans on highMVA autotransformersandsomeoilpumpscan beratedat480 Volts.The240/120 voltDeltaand 240/120 volt Wye connections arecommonthree-phaseAC station power connections since they canalsosupply120 voltswhen itisafour wireconnection.Standard three-phaseAC sourcescan beobtainedwithathree-phasetransformer,athree-phasebankingof single-phase transformers,or a backupgeneratorfor auxiliarysource.

Withthistypeof AC source,there areseveraltypesof differentconnectionsinordertofittheloadrequirements.The standardIEEEC57.105providesaguidefor applicationoftransformerconnectionsinthree-phasedistributionsystems.Someutilitiesutilizethethree-phaseAC sourceas a standardACpower supplytothesubstations.Aftertheloadanalysis,thedesignershall determinetheratingof three-phasevoltage,thetypeof connectionandweatherathree- phasetransformeror transformerbankisneeded.

4.4.2 Station Power Available Sources

Availablesourcesarerepresentedin figure4-1. Thesourcedeemedthemostreliablesourceistypically designatedandused as primaryor normalsource. Thesecondsourceisdesignatedas a backupsourceandused onlywhen theprimarysourcehas beenlost.Athirdsource,ifused, willbeused as asecondbackup andwillbeconnectedonlyifboththeprimaryand secondsources havebeenlost.

Thedesignermustdeterminethepower sourcetoservethesubstationAC loadasdescribedinsection4.2. In mostcases,thedesignermaywanttoconsideraprimaryanda secondarysourcebasedon thedesigncriteria. Four sources thatarecommonly used as substationAC auxiliarypower sources include:

1. Power TransformerTertiary

2. SubstationBus

3. DistributionLine

4. StandbyGenerators

Eachsourcehas advantagesanddisadvantages.Substationlocation,substationequipment, andbus configurationsmaydictatewhichsourceispreferred.In substationswhereAC auxiliarypower reliabilityiscritical,multiplesourcesof AC auxiliarypower maybeutilized withatransferschemetoswitchbetweenanormalAC auxiliarypower sourceandan emergencyAC auxiliarypower source.

4.4.2.1 Power Transformer Tertiary

Whenused ina substation,thetertiaryof athreewindingor autotransformercanprovidea good sourcefor stationpower applications.Whentheprimaryandsecondarywindings are connected“wye”,athird or tertiarywindingconnectedindeltamustbeused for stabilizing purposes. A tertiarywindingpresentsalow impedancepathtozerosequencecurrents and harmonics,therebyreducingthezerosequenceimpedancepresentedtotheoutsideworld, whileavoidingtheproblemof tankheating.Thetertiarywindingtypicallyhas a volt-ampereratingbetween20-30% of thevolt-ampereratingoftheprimarywinding, andtypicallyhas amediumvoltageratingupto40kV. If thereareplanstouse the transformertertiaryfor a sub-transmission or distribution source and/or for stationpower purposes, thetertiarywindingisbroughtout to external bushings for user connection. Otherwise,thetertiarywindingisburied inside the transformer.Seefigure4-2 for transformerconnection.

Thevolt-ampereratingofthetertiarywindingtypicallyexceedsthemaximumvolt- ampererequirementofasubstation’sAC auxiliarypower loadandis,therefore,an extremelyadequateAC auxiliarypower source.

Considerationshouldbegiventotheavailablefaultcurrentatthetertiarybus for phase-to-phase and three-phase faults. In caseof thecurrentmagnitudeexceedingtheinterruptingratingof theprotectiveequipmentsuch as thefuse or thecircuitbreakers,severaloptionscanbeemployedtoreducethefault current.Theseoptionsincludeinstallingcurrentlimitingfuses, resistors, reactorsor increasingthetransformertertiaryimpedance.

Anotherconsiderationshouldbegiventothedetectionoftheground faulton thetertiary bus. Thetertiarybuses on threephasepowertransformersaregenerallyshort and may notrequireanyground faultprotection.However, when singlephasepower transformersareused toconstructathreephasebank,bus runs thatconnectandformthe tertiarybus becomemuchlongerandaremorelikelytobesubjecttoaphasetoground fault.A singlephasetoground faulton thetertiarywillnot generatefaultcurrentinthe tertiary deltaandwillnottripthedistributiontransformerhighsideprotection,nor willthefault tripatypicalpower transformerbankdifferentialscheme.Someutilitieschoosetoinstall a ground detectionnetworkon thetertiarydeltatodetectthissinglelinetoground fault, signalanalarm,butnottripthepower transformerbank.A commonmethodfor constructingthisground detectionnetworkistoinstallthreesmalldeltaconnected transformerswithagroundingresistor across the open delta.Therewillbezerovoltagedroppedacross the groundingresistorundernormaloperatingconditions.Therewillbeavoltagedropacross thegroundingresistor due to circulating current in the broken delta duringaphasetoground faultandthatindicatesthata phasetoground faulton thedeltatertiaryhas occurred.

Figure4-2[DML4]:TertiaryDeltaBus withGroundingResistor(NormalSystemConditions[DML5])

Figure4-3:TertiaryDeltaBus withGroundingResistor(A Phase Grounded)

One disadvantagetousingapower transformertertiaryas anAC auxiliarypower sourceis thatwhen thepower transformerisremovedfromserviceduetoeithermaintenanceor failure,theAC auxiliarypower sourceisalsoremoved.Thispossibilitygenerallyleads utilitiestoinstallanemergencyback-upACauxiliaryservicefor such cases.This emergencyserviceisgenerallyprovidedfromeitherasecondpower transformerlocated withinthesubstation,anunaffectedhighvoltagebus, anoffsitedistributionfeeder,or an emergencygenerator.

Anotherdisadvantagetousingapower transformertertiaryas anAC auxiliarypower sourceisthatspecialLoadTapChanger(LTC)designs arerequired[DML6]tostabilizethetertiarywindingvoltage(IEEEC57.12.10AppendixA.1.2). Adding theseLTC’stothe transformer’stertiarywindingwilladdadditionalcosttothepower transformer.

4.4.2.2 Substation Bus

Anotheravailablesourcefor stationpower istouse ahighvoltagebus as asource.This possiblesourceisnormallyused when othersourcesarenotavailableduetoitsrelatively highcost.APower VoltageTransformer(PVT), sometimesknown as aStationService VoltageTransformer(SSVT), isthedevicethatmaybeused totransformthebus voltage totheAC auxiliaryvoltage.Thesedevicesareavailablefor voltagesbetween34.5 to230 kV. One or moreSSVT’s mightberequiredas requiredby thestationpower load.See figures4-4, 4-5, 4-6, 4-7, 4-8 &4-9 for possibleconnections.

TheSSVT isnormallylocatedwithinthelineor bus relayszonesof protection.A faulton theSSVT willbeclearedby theprotectiverelayfasterthananyhighsidefuse. Also the sizeof requiredfuse maynotbeavailablefor certainvoltagelevel.Theprotection engineermustbeconsultedfor thefinalwhen determiningtherequiredSSVT protection. Low sideovercurrentprotectionof thesecondaryconductorsused for auxiliarystation servicearetypicallyappliedas closetothesecondaryterminalsas possible.Surge protectionistypicallyneededon thehighsideconnectionof theSSVT. If arresters protectingotherequipmentinthestationarecloseenoughtoprotecttheSSVT, a dedicatedarresterfor thePVT maynotberequired.Guidanceon surge protectionand separationeffectscanbefound inIEEEC62.22,Guidefor theApplicationof Metal- OxideSurge Arresters for Alternating-CurrentSystems.Dependingon thebus configurationandtheimportanceof theswitchingstation,multiplePVT’s maybe required toprovideadequateauxiliaryAC service.Thefiguresbelowshow somebus arrangementsandpossibleSSVTplacements.

For lower voltage substation busses up to 34.5 kV, standard distribution transformers can be used as the source, whether single phase or banked for three phase service.

(There were figures here, but they did not copy or move well when I shuffled the document - not sure they are necessary)

Wheretwo stationservicefeedsareprovidedwithintheswitchingstation,eitherbothby bus, or oneby bus andtheotherby someothersource,thefeedstypicallyareswitchedon thesecondarysidewitheitheramanualor automatictransferswitch.A typical arrangementisshown inFigure4-10.

4.4.2.3 Distribution Line / Feeder

Distributionfeedersmaybeused as anormalor emergencyAC auxiliarypower source. Utilizingthedistributionfeederas thenormalsourcecanbebeneficialifthatfeederisnot directlysuppliedby theutilitysubstation.In thatcase,theACauxiliarypower source would stillbeenergizedintheeventofade-energizedutilitysubstation.Thedisadvantageofusingadistributorfeederisthattheutilitysubstationisdependingupon thereliabilityofthatfeedertosupplyAC auxiliarypower. Thereistherisk oflosingAC auxiliarypower fromthedistributorfeedereventhoughtheutilitysubstationisstillenergized.

Someutilitiesuse adistributorfeederas a back-uptoapower transformeror bus normal AC auxiliarysupply.Thereareengineeringaspectstoconsiderwhen usinganoutside distributionfeederas anemergencyback-upsource.Whenusingthedistributionfeederas aback-upsource,considerationhastobetakenas how toswitchbetweenthetwo supplies. Thisisdonethroughamanualor automatictransferswitch.Considerationalso mustbegivenas towhetheror nottoswitchtheneutralconnectionswhenswitchingthelivelines.IfthedistributionfeederhasalreadybeentransformedtoanAC auxiliary voltage(i.e.120/240volt),theneutralhas beensinglepointgrounded,mostlikely,atthe transformersecondaryconnectionpoint.Conversely,thenormalAC auxiliarysourceis typicallysinglepointgroundedtothesubstationground grid.In thatcase,itisdesirable toswitchtheneutralfromthenormalsourcetotheneutralof theemergencysource.If bothneutralsaresinglepointgroundedontothesameground, thenitisnotnecessaryto switchtheneutralsof theAC auxiliarysupplies.

4.4.2.4 Standby Generators

Generatorsmayalsobeused as an ACauxiliarypower source.In substations,generators aretypicallyused as anemergencypower sourceinsteadof apermanentpower source. Thisisduetothedisadvantagesof usinggeneratorsas apermanentAC auxiliarysource. Choosingtouse generatorsas apermanentAC auxiliarysourcewillrequireadditional designconsiderations.Fireprotectionsystemswillneedtobedesignedtoprotectthe substationequipmentfromageneratorfile.Fuelstoragesystemswillneedtobeinstalled tohouse thefuelneededtorun thegenerators.Thegeneratorsmayalsobehoused ina separatebuildingstructure,whichrequirestheinstallationofaventilationsystem.

Generatorsused as anemergencyAC auxiliarypower sourcehavemoremeritthanasa permanentsource.As anemergencysource,thereisnotthesameneedfor fireprotection installation,fuelstoragesystem,or buildingventilation(ifthegeneratorsarelocated outdoors intheswitchyard).

4.5 Conductor Selection

Thissectioncoversboththeprimaryandsecondaryconductors.Theprimaryconductorcanbe eitherabareconductoror insulatedcabledependinginthelocationof thetransformer.Whenthe transformerislocatednearthesource,abareconductoreitherAA or coppercanbeused. When thetransformerislocatedawayfromthesource,InsulatedcableCopperor AllAluminumcanbe used. Thecableisnormallyinstalledinsideaconduitandburiedbelowgrade.

Insulatedcableisnormallyusedfor thesecondaryconductor.Thefollowingmustbeconsidered when selectingeithertheprimaryor thesecondaryconductor.Therearethreemainrequirements for theconductorused for theauxiliarypower system.IEEE525 providesinformationon the selectionandapplicationofcablesandconductorfor ACauxiliarypower systems.

4.5.1 Cable InsulationVoltage Rating

Thecableinsulationvoltageratingisselectedbasedon thephasetophaseoperatingvoltage andtheexpectedclearingfaultclearingtime.In general,for aclearingtimeequalto1 minute,100% insulationisselected,for faultclearingtimegreaterthanoneminutebutless thanonehour, 133% insulationisselected.In eithercase,theprotectiverelaysmustclearthe faultbeforetheinsulationcanbedamagedas calculatedinthesectionsbelow.Theprimary cablestandardvoltagesare5, 8,15,25,28 and35kV. Thesecondarycableinsulatingis normallygivenintwo ratings;600V or 1000V.

4.5.2 Cable InsulationType

Theinsulationtypemustbealsoselectedto meetthelocationconditionsuch as dry, wetor both.Table4-2 shows someof theinsulationtypesthathaveused insubstationapplication. ReferNEC for moreinformation.

Table4-2 SelectedCableInsulationType

THW‐2 / ThermoplasticInsulation(usuallyPVC), HeatResistant(90°Crating),
suitablefor Wetlocations
THWN‐2 / Sameas THWexceptNylon jacketoverreducedinsulationthickness.
Also ratedTHHN.
THHN / ThermoplasticInsulation(usuallyPVC), High HeatResistant(90°C
rating),dry locationsonly,Nylon jacket.Also ratedTHWN.
XHHW‐2 / Cross-linkedPolyethyleneInsulation(X) High HeatResistant(90°C
rating)for Wetanddry locations.
RHH / RubberInsulation(we actuallyuse cross-linkedpolyethylenebecause
itqualifiesfor rubber),High HeatResistant(90°Crating)for dry locationsonly.
RHW‐2 / RubberInsulation(again,cross-linkedpolyethyleneisused by most manufacturers),HeatResistant(90°C),Suitablefor Wetlocations.
USE‐2 / Underground ServiceEntrance.Mostutilizecross-linkedpolyethylene
insulationratedfor 90°Cindirectburialapplications.Productis usuallytripleratedRHH—RHW—2-USE-2.

4.5.3 Conductor Size Calculations

Thefollowingfactorsmustbeconsideredwhen selectingtheconductorsize:

  • RequiredAmpacity
  • TemperatureCorrection
  • AllowableConnectorTemperature
  • VoltageDrop limitations

4.5.3.1 Required Ampacity

Theconductorsizemustbeselectedwithapropersizetocarrythemaximumstation power load.Thisisimportanttopreventconductoroverheatingandinsulationdamage andthereforefire.As afirststeptheampacityrequirementsof theconductormustbe determined.Stationpowerloadconsistsof acontinuousloadwhichisexpectedtocontinuefor 3 hours or more.Thenon-continuousloadisexpectedcontinuefor short timeonly.Thefollowingsubstationloadcanbeclassifiedas continuousload:

  • ControlBuildingAir conditionload
  • Transformercoolingloads
  • Lightingload
  • BatteryChargerload

Loadthatisnon-continuouscanbeidentifiedas follows:

  • Breakermotorload
  • Testequipmentload
  • Motor operatorload.

Branchconductorsshouldbesizedtomeetequipmentloadrequirements.An overload factorof 1.25 shouldbeapplied.Equipmentratingdatawillprovidetheload requirements.Someloadsareof short duration,such as breakerspring or compressor motors.Others arecontinuousduration,suchas heatersandtransformerfans. As loads areaggregatedatpanelstheconductorssupplyingthepanelcanbesizedusingaload factorbasedon theanticipatedcoincidencetheofbranchloads.

Oncethecontinuousandnon-continuousloadisdetermined,theprimaryandsecondary conductorsrequiredsizecanbedeterminedas discussed below:

4.5.3.1.1 Primary Conductor

AccordingtheNEC 215.15 (B1) theratingoftheconductorshallbeselectedto equaltoatleasttheKVA ratingof thetransformer.Sincethestationpower primarycurrentisnormallylow duetotransformerlow kVA ratingandthehigh primaryvoltage,theconductorsizeisnormallyselectedtoexceedtheNEC requirements.Thereforeno overheatingwilloccurandthevoltagedrop is acceptable.Whenaninsulatedcableisused,considerationsmustbegiventothe conductorstrengthinordertopreventbreakageanddamageduringpulling.

4.5.3.1.2 Secondary Conductor

For secondaryconductorsrated600Vor less, theNEC requirestheconductorbe ratedequaltothenon-continuousloadplus125% of thecontinuousloadas definedinsection4.3.4.1 above.Using theNEC appropriateampacitytables,a preliminaryconductorsizecanbeselected.

4.5.3.2 Temperature Corrections

If theambienttemperatureatthesubstationlocationisdifferentthantheambient temperatureused for calculatingtheNECampacitytables,theselectedconductor ampacityshouldbecorrectedfor thenew ambienttemperature.Thefollowingequation canbeused tocalculatetheampacityratingof theconductorbasedon thenew ambient temperature:

4.1

Where:

I'=ampacitycorrectedfor ambienttemperature

I =ampacityshown inthetables

Tc=temperatureratingof conductor(°C) Ta'=new ambienttemperature(°C)

Ta=ambienttemperatureused inthetable(°C)

4.5.3.3 Correction for number of conductors

Whenmorethanthreecurrentcarryingconductorseithersingleor multiconductorcables areused for stationpower, theconductorratingshouldbede-ratedaccordingtoNEC table310.15(B)(3)(a)isshown belowas table4-2

Table4-2 (NEC Table310.15(B)(3)(1) AdjustmentFactorsfor More ThanThreeCurrent-CarryingConductorsinaRacewayor Cable

Numberof
Conductors1 / PercentofValuesin
Table310.15(B)(16)through Table310.15(B)(19) as Adjustedfor Ambient TemperatureifNecessary
4-6 / .80
7-9 / .70
10-20 / .50
21-30 / .45
31-40 / .40
41 andAbove / .35

4.5.3.4 Voltage Drop Verifications

Thevoltagefor boththeprimaryandsecondaryconductorsmustbe maintainedatlow values.For stationpower applications,thevoltagedrop isnormallylow for theprimary conductorsandisnotanissue. For thesecondaryconductors,dependingon thedistance betweenthetransformerandAC panelmustbe maintained.Voltagedrop limitsmaybe giveneitheras atargetpercentdrop (typically3% for abranchcircuitor 5% includingthefeeder)or as theequipmentvoltagelimitations.In substationswithlongcablelengths, thevoltagedrop considerationsmayrequirealargerconductorsizethantheampacity requirements.

4.6Station Power Transformer

4.6.1 Single-Phase

Single-phasedistributiontransformersaremanufacturedwithoneor two primarybushings. Thesingle-primary-bushingtransformerscanbeused onlyon groundedwye systemsifthey areproperlyconnected.An example,single-phasetransformerconnectedtoathree-phase2,400-voltL-Ltoobtainl20-voltsingle-phaseisshown. Theconnectionsarethesameforthefollowingvoltagelevels:4,800 voltL-L,7,200 voltL-L,13,200 voltL-L,and34,400 voltL

A single-phasetransformerconnectedtoathree-phase2,400-voltL-Lsystemisshown below toobtain120/240-voltsingle-phase,three-wireservice.Normallythewireconnectedtothe centerlow-voltagebushingwillbeconnectedtoground.Groundingthewiretothecenter bushinglimitsthesecondaryvoltageaboveground to120 volts,eventhoughthewiresconnectedtotheoutsidesecondarybushings have240 voltsbetweenthem.

Thesingle-phasedistributiontransformerconnectedto4,160Y/2,400 voltsisshown belowto obtain120/240-voltsingle-phasesecondaryservice.Otherstandardthree-phasesystemvoltagesare12,470Y/7,620V, 13,200Y/7,620V and13,800Y/7,970 V.

4.6.2 Three-Phase Connection

Single-phasetransformerscanbeconnectedtoobtainthree-phasesecondaryvoltages.The four commonconnectionsareshown below.

4.6.2.1 Open Delta Connections

Theopendeltabankisoftenthemosteconomicalchoicefor servingsmallthreephase loads,particularlywhen commonlyavailabledistributiontransformerscanbeused. The costoftheadditionalKVA capacityinthetwo transformerswillnormallybe muchless thanthecostof anadditionaltransformer,fuse, andinstallationlabor.

Open deltabankswillcarry57.7% of theequivalentthreephasecapacity.

Example:

3- 25 KVA transformers3 phasecapacity=75 KVA.

2- 25 KVA transformers3 phasecapacity=57.7% of 75 KVA or 43.275 KVA

4.4.2.2 Common Three Phase Connections

Delta-Delta

Wye-Wye

Delta-Wyeor Wye-Delta

4.6.2.3 Personnel Safety

Allengineering,construction,andmaintenanceshalladheretospecificcodesand standardsas wellas theOwner’s operatingpolicytoensurepersonnelsafety.

4.6.2.4 Ferroresonance

Ferroresonanceisaconditionthatcreatesahighvoltagebetweenthetransformerprimary windingandground. Thehighvoltagecanbeas muchas fivetimesof theprimary voltage.In such casesthetransformer,cablesinsulation,or otherequipmentcanbe damaged.Whenpresent,thetransformerwill makesounds thatarenotof itsnormalhum.

Ferroresonanceoccursunderthefollowingconditions

  • Threephase
  • Ungrounded primaryandtransformergrounded
  • Longprimarycable,producingahighcapacitance
  • No loadon thebankor lightlyloaded

4.6.3 Station Power Transformer Types and Ratings

In thissectionthefollowingtransformerrequirementswillbeconsidered:

  • Quantity of station power transformers required
  • Station power transformer ratings
  • Transformer impedance
  • Transformer connections

4.6.3.1 Quantity of station power transformers required

Stationloaddictatethenumberof transformersthatmustbeused for stationservice transformer.For substationswithsingle-phaseandlightto mediumloads,single-phase transformerisused. For substationswiththree-phasehighloads,athreetransformeris normallyselected.Otherloadssuch as maintenanceequipmentmaydictatethenumberof transformerstobeused for stationpower.

4.6.3.2 Station Power Transformer Ratings

Thecapacityof atransformerisdeterminedby theamountof currentitcancarry continuouslyatratedvoltagewithoutexceedingthedesigntemperature.Transformerrating isgiveninkilovolt-amperes(kVA) sincethecapacityislimitedby theloadcurrentwhichis proportionaltothekVA regardlessof thepower factor.ThestandardkVA ratingsaregiven inTable4-1 below.

Table4-1:StandardRatingsofDistributionTransformerkVA

OverheadType / Pad MountedType
Single
Phase / Three
Phase / SinglePhase / Three
Phase
5 / 15 / 25 / 75
10 / 30 / 37.5 / 112.5
15 / 45 / 50 / 150
25 / 75 / 75 / 225
37.5 / 112.5 / 100 / 300
50 / 150 / 167 / 500
75 / 225 / 750
100 / 300 / 1000
167 / 500 / 1500
250 / 2000
333 / 2500
500

4.6.3.3 Transformer Impedance

4.6.3.4 Transformer Connections

Dependingon numberof transformerselectedfor stationpower applicationstwo typesof connectionsareemployed:

4.6.3.4 .1 Single Phase Transformer Application

Single-phasedistributiontransformersaremanufacturedwithoneor two primary bushings. Thesingle-primary-bushingtransformerscanbeused onlyon groundedwye systems.For thisconnectiontheH1 bushingisconnectedtooneof theavailablephases whiletheotherbushingisconnectedtoground as shown infigure4-11 below.

Figure4-11:SinglePhase toGround Connection

Whenadeltasystemisavailable,a phasetophasevoltageisappliedbetweenthetwo bushings H1 andH2 as shown infigure4-12 below.

Figure4-12:SinglePhaseTransformerwithPhasetoPhase Connections

Theprimaryvoltagecanbeanyof thefollowing;2400,4800,13,200 and34500 Volts. TheSecondaryvoltagecanbeanyofthefollowing120/240and480/120Volts.

4.6.3.4.2 Three Phase Transformer Connections

Three-phasetransformerconnectioncanbeachievedby usingtwo or threesingle-phase transformersandconnectedas required.Theuser canalsospecifythree-phasetransformerconnectedas specifiedby theuser. Whenthree-phasetransformerisrequired a padmountedthree-phasetransformerisnormallyused for thestationpowerapplications.A padmountedthree-phasetransformerisapplicabletobelowgrade connectionfromboththeprimaryandthesecondary’ssides.

Whenselectingathree-phasetransformerthefollowingmustbeconsideredbefore selectingthetransformerconnection:

  • Therequiredsecondaryvoltage
  • Safety
  • Ferro resonancecondition

4.6.3.4.3 Secondary Voltage

ThefollowingsecondaryTransformerconnectionmustbeselectedinordertoobtainthe secondaryvoltagethatwillberequiredfor theapplication.

a)Three-Phase Secondary Connections–Delta

Three-phase transformers or banks with delta secondaries will have simple nameplatedesignations such as 240 or 480. If one winding has a mid-tap, say for lighting, then thenameplate will say 240/120 or 480/240, similar to a single-phase transformer with acenter tap. Delta secondaries can be grounded at the mid-tap or any corner.

b)Three-Phase Secondary Connections–Wye

Popular voltages for wye secondaries are 208Y/120, 480Y/277, and 600Y/347.

4.7Transfer Switch

4.7.1 General

Theneedfor anauxiliarypower systemtransferswitchisrelatedtothecriticalityofthe substation.If onlyonestationservicepower sourceisavailable,atransferswitchmaynotbe required.If thereareno criticalAC systemrequirements,theDC batterysystemmaybe sufficienttooperatethecriticalDC systemsuntiltheAC stationservicepower isrestored.

Most substationsareprovidedwithtwo sourcesof stationserviceAC power. Thetwo sources of stationservicepower aregenerallydesignatedas theprimarysourceandthe alternate(or backupor secondary)source.Bothsources shouldbeof equalreliability.

Tosimplifytheoperationof thetransferbetweensources, a“breakbeforemake”operationis suggested.Thiswillensurethatsources thatareoutof phasewithoneanotherdo notoperate inparallel.In thecaseof manualoperationof thetransferswitch,itmaybedesirableto disable or lockoutonesourcewhiletheothersourceisbeingused. In bothcases,sufficient trainingshouldbeprovidedtooperatorstoensurethatsourcesarenotparalleled.

Sincetheauxiliarypower sourcescanbesuppliedatdifferentvoltagesthantheutilization voltageinthesubstation,thetransferswitchcanbeappliedateithertheprimaryor secondary voltage.Thehighervoltageapplicationresultsinlowercurrentratedequipment.13.8kV,12.47kV, 4.16kV, 480V and240/120Varecommonauxiliarypower voltagesandthetransferswitchcanbeappliedatanyof thesevoltages.Theauxiliarypower sourcecanbeeitherthree-phaseor single-phasedependingon thestationservicerequirements.Transferswitches typicallycanbepurchasedwithtwo, threeorfour poles.A four poleswitchhas theabilityto switchtheneutralandisnecessaryon asystemthathas separatelyderivedsystem.

Smallerratedtransferswitchescanbewallmounted.Floormountedswitchesarecommon. Transferswitchescanbepurchasedfor indooror outdoormounting.

Thetransferswitchmaybeas simpleas two inputsources withswitchingdevicesandone outputtotheload.Thetransfersystemmaybeas elaborateasaunitswitchgearconsisting of two inputswitchingdevices,two transformers,two maincircuitbreakers,onetiecircuit breakerandmultiplebranchcircuitbreakers.

Figure4.9-1 [DML7]SimpleTransfer switch

Figure4.9-2 ComplexTransferSystem

Anotherconsiderationwhendesigningthetransfersystemisthereliabilityof thetransfer switch.Itmaybeprudentto makeprovisionstobypass theswitchintheeventof theswitch’s failure,maintenanceor replacement.Thismaybeaccomplishedby havingathirdsource routedtothesubstationAC loadcenterthatisleftnormallyopenandlockedoutuntilitis needed.Itmaybe morecosteffectivetorouteanothersetof conductorsfromeitheror bothof theprimaryandalternatesourcetothesubstationAC loadcenter.Similartothetransfer operation,trainingandprocedureshouldbeprovidedtotheoperatorso thatitwillbe unlikelytoparallelsourcesfor abypass option.

4.7.2 Manual Transfer Switch

For less criticalsubstationsamanualtransferswitchwillprovidethecapabilityof transferring fromtheprimarytothealternatesource.Themanualtransferswitchwillbeamuchsimpler andlowercostswitchthananautomatictransferswitch.However,theuse of themanual transferswitchwillrequirestationalarmstoalertoperationspersonnelof theloss of the primarysourceanddispatchingpersonneltothesubstationtooperatethemanualtransferswitch.Proper designof theDC batterysystemisrequiredtoprovidecontinuous operationof criticalsystems(systemprotectionfunctions,controlandbreakertripping)while personnelresponds andmanuallyoperatesthetransferswitch.

Ifthesubstationhas onlyonesourceofAC power, amanualtransferswitchmaystillbe desirableas aconnectionpointfor atemporaryAC alternatesource,such as aportable generator.

Themanualtransferswitchconsistsof two (2) manuallyoperatedswitchingdevices(usually circuitbreakers)capableof interruptingtheloadcurrentofthetransferswitch.Thetwo switchingdevicesaretypicallymechanicallyinterlockedtopreventbothACsources from beingconnectedinparallel.Faultcurrentinterruptioncapabilityisnotrequiredinthetransfer switch,butcouldbeincludedorprovidedseparately.Indicationof sourcestatus(hotordead)isnottypicallyprovided.Somepointofalarmisnecessarytodetecttheloss ofthe primaryACsource.

4.7.3 Automatic Transfer Switch

Criticalsubstationsor substationswithcriticalAC loadswillrequireatransferswitchthat willautomaticallytransferfromtheprimarysourcetothealternatesourcewhen theprimary sourceislost.

Transfershouldoccurunderthefollowingconditions:

1) Thereshouldbeatimedelayon loss of theprimarysource.Thisistopreventtransfer for momentaryproblemswiththeprimarysource.

2) Thealternatesourceisavailable.Thisistopreventtransfertoadeadsource.

Automaticreturntotheprimarysourceshouldoccuronlyaftertheprimarysourcehas been restoredfor aspecifiedtimeperiodtopreventreturntoanunstablesource.

Theautomatictransferswitchconsistsoftwo (2) electricallyoperatedswitchingdevices (usuallycircuitbreakers)capableof interruptingtheloadcurrentof thetransferswitch.The two switchingdevicescanbeelectricallyand/ormechanicallyinterlockedtopreventbothAC sources frombeingconnectedinparallel.Faultcurrentinterruptioncapabilityisnotrequired in thetransferswitch,butcouldbeprovidedoraddedseparately.Detectionandindicationofsourcestatus(hotor dead)isrequired.Timedelaysandcontrolsequencingisnecessaryto preventtransferringtoadeadalternatesourceor topreventnuisancetransferringtounstable sources. Indicatinglightsandrelaysareusuallyprovided.Alarmindicationof transfershould beprovided.Closeandlatchcapabilitymustalsobeconsiderinequipmentrating.

4.7.4 Alternate Methods

If bothsources aredesignatedas primarysources,theAC loadcanbedividedbetweenthe two sources withthetransferswitchsystemconsistingof thetwo normallyclosedprimary circuitbreakersandanormallyopentransfercircuitbreaker.

4.8AC Panel Bus rating[DML8]

4.9 AC Panels

ReviewandComment-

4.9.1 Present and Future Load Accommodation

Thenumberofbranchcircuitsisdependenton theultimatestationbuildout.Thestation arrangementdrawingshouldprovidethenumberof eachtypeof equipment(transformers, circuitbreakers,etc,).Thevoltageandcurrentrequirementsofeachpieceofequipmentwill determinethebranchcircuitbreakerratingrequirements.For futureequipmentinstallations, therequirementsshouldbeaworst caseestimation.Allowancefor future“unknowns” or sparebranchcircuitbreakersshouldbeprovided.

4.9.2 Load Classification (or Segregation)

ItmaybedesirabletoseparatecriticalloadstodifferentAC panels.Non-criticalloads canbeconnectedtodifferentpanelsfromthecriticalloads.

4.9.3 Number of Panels required

Thenumberof panelswilldependonseveralitems;

1) Voltageandphaserequirements- Generally,thehigherthevoltageof theauxiliary power system,thefewercircuitbreakersthatcanfitinthepanel.Two andthreepole circuitbreakerswillrequiremorespacewithinthepanel