Overheadandundergrounddistributionsystemscomponents,part2. Padmountedswitchgear: Generaldescriptionofpadmountedswitchgear: Theseswitchingassembliescanbeclassifiedintoairinsulated,oilsealedinsulated,orSF6/load breakswitchesandvacuumfaultinterrupters.Typically,fortheairinsulatedtypewhenthe separableconnectorsareinplace,theconstructionwillhaveallenergizedpartsenclosedin grounded(shield)enclosure(deadfront).Verificationoftheopenswitch(visiblebreak)is possiblethroughPlexiglasviewingwindows.Thethreephasegangoperatedswitchesinthe assemblyareoperatedwithouthavingtoopenthecablecompartment.The600Adeadbreak bushingsareexternallyreplaceable.Theunitwillhaveparkingstands,replacementfusestorage pockets,groundpadsforgroundingprovisions,doorretainers,latching(3point)arrangement, fuseviewingwindows,liftingprovisions,faultindicatorsandfloorcover(ifrequired).Fig.1.3 showsthemajorcompartmentsandmountingmethodonprecastconcretepad.

Majorcomponentsofoilfilledpadmountedswitchgearunit: Theoilfilledunitswillhaveafillplug,adrainvalve,theoilgauge,thecableentrance,thesteel tank,thespringoperator,themechanicalinterlockoverthefusecompartment,grounding provision,parkingstandsandthehingedcover. MajorcomponentsofSF6/vacuumunits:

Themajorelementsare:theheavygaugeenclosure,sealedswitchcompartment,operating handle(formanualoperation),fillvalve/gaspressuregauge,electroniccontrolpackage(pad lockable),electronicloadtaptripadjustments(knobs,pushbuttons),springassistedswitch operator(withmarkedpositionindicator),provisionsforpadlocking,cableentrancebushings, phaseindicationlabels,parkingstands,deadbreakelbowconnectors,doorlatches,hingeddoors withstoppers,viewingwindows,groundlugs,deepwelllowcurrent(eg.200A)bushings, mechanicaltripandresetlever,provisionsfordoorpadlocking,motoroperator/RTUs.The motor/RTUinstallationwouldprovideremoteoperationoftheswitches(fromacontrolstation) orlocaloperation(motoroperated).Withtheavailabilityofsensors/RTUinthepadmounted switchgearassembly,theremoteindicationoftheloadlevelsandfaults(currents)atthecontrol stationmayassisttheoperatorsinrunningthesystemmoreefficientlywithlessdowntimesand higherlevelsofsupplycontinuity. Productiontestsconductedonpadmountedswitchgearassemblies: Theproductiontestsrunonsuchassembliesare:continuitytest(toassurecorrectinternal connections),hipot(dielectric),pressuretest(toassuretankissealed),theprotective (electronico/c)devicecharacteristicscurveandleakagetests(ifrequired). Remoteoperation&indicationofpadmountedswitchgear: Toremotelyoperateandindicatewithpadmountedswitchgearandundergrounddistribution systems,thefollowingelementsaretobepartoftheswitchgearunit:onesetofthreecurrent sensorsontheloadsideofeachinterrupter(orswitch),sourcesidePTwith120V(forexample) secondaryvoltage,radios/modemsortherequiredcommunicationequipment,connectorfor remoteantenna(ifapplicable),localinterfaceinthecontroller(status,controlswitches, displays),localcommunicationporttoallowsetup/updatingsoftware/troubleshooting/report generationusingalaptop.Shouldductsbe,already,installedtheuseoffiberoptics communicationnetworkwouldbethemostsuitable(overothermeansofcommunications:RF, PLC,.....,etc.).Fiberopticsisimmunetoelectromagneticandradiointerference,thusitcanbe placedinductscontaininghighvoltageCuorAlcables(i.e.15,27.6kV).Theelementsthatare neededtobuildafiberopticnetworkbesidethecablearethetransceiverswiththeserialand opticalports(LEDtransmitterandphotodiodereceiver),theRTUs,opticalsplitters,multiplexes andmodems. Padmountedswitchgearcanbedefined(specified)accordingly:theinsulatingmaterialusedi.e. airinsulated,oilorgas,thenominalvoltageclass,maximumoperatingvoltage,thebasic impulselevel,thecurrentratingsforthedifferentsidesi.e.continuouscurrent,loadinterruption (resistive,inductiveincludingnoloadtransformermagnetizingandcapacitiveincludingcable charging),momentary,faultclose,thedimensionsofthegear,theopeningforthecableentry, propertiesofsteelwork(likethicknessgauge,surfacetreatmentandfinish),theweightandthe assemblyvoltagewithstandabilitytests(A.C.andD.C.).Thespeedofoperation(currenttime curves)forthefusesorprotectivedeviceshadtobespecified. Distributiontransformers:

Methodsofmounting(installing)distributiontransformersonpoles: Thesetransformerscanbefasteneddirectlytothepoles,hungfromcrossarms,mountedon racksorplatformsormountedonbracketsattachedtothepoles.TheKVAratingsforsuch transformersarelowi.e.167or250.Thepolemountedtransformerscanbeinstalledinclusters of3transformersattachedtothesupportingbracketsofwhichthelatterareattachedtothe poles. Mainattachments(accessories)usedtocompletetheinstallationoftheoverhead(pole mounted)distributiontransformer: Thehighvoltagebushingwiththeclamptypeconnectorisconnectedtotheprimary(medium voltagecircuit)andthelowvoltagecablesareconnectedtospadetypeconnectors.Thepole mountedtransformersuseoilastheinsulatingmaterial.Theyareinstalledinmany configurations.Ingeneral,thesetransformersareconnectedtotheprimarycircuitthrougha currentlimitingfuseandafusecutout.Toprotectthetransformersagainstlightningorvoltage surges,theprimaryofthetransformerwillhavealightningarresterconnectedacrossitandthe ground.Thereisanothertypeofpolemountedtransformerswhichisthecompletelyself protectedone(CSP).Primaryfusesandlightningarrestersareincludedwiththetransformer, thusthereisnoneedtoanyexternalprotectivedeviceexceptforacurrentlimitingfuse.

Maincomponents(accessories)thataremountedorincludedwiththepadmountedor vaulttypedistributiontransformerstocompleteitsinstallation: Thedistributiontransformersratingforsinglephasevariesfrom10KVAto1MVA,forthree phase30KVAto2.5MVA.Thepowertransformerscomeinsizesfrom3MVAto150MVAfor 3phaseconstructions.Indistributionsystems,threephasetransformersandthreephasebanks (i.e.3singlephaseconnectedtoprovideadeltaorwye3phaseconfiguration)arequite common.Ingeneral,theprotectionofthepowertransformersisthroughtheuseofprotective relay(o/cordifferential&overcurrentground)andgasrelays.Thedistributiontransformersare protectedbyfuses(currentlimitingandexpulsiontypes).Padmountscanbeclassifiedinto radialfeedandloopfeed.Thepadmountedtransformerswillhaveloadorfaultsensing (expulsion)typefuse.Inserieswiththisfuseacurrentlimitingbackupfuseundertheoil.For vaultmountedtransformers,aseriesofcurrentlimitingandexpulsiontypewithpowerfuseor fuselinkmountedonthepoleorthewallofthevaultaremostprobablyusedasprimary protection.Foratypicalgenerallayoutofavault,refertofig.1.4.Forvaultmountedandpad mounted,theprimaryconnectionismadethroughtheuseofelbows(wherethecablesare

connected)andinsertsinthetransformersconnectedtothedeepwell(cavity)bushings;the secondarywindingsofthetransformersarebroughtoutthroughL.V.bushingsandspade terminals.Otheraccessoriesthatarefoundindistributiontransformersare:pressurerelief devices,fillerplugs,drainplugsand/orsamplingvalves,parkingstandsforelbows,tapchangers (offload),loadbreakswitchesforradialfeedpadmountsandsectionalizingswitchforloop feed. Basicpartsofatransformer? Thegeneralarrangementofanytransformerwillhavethefollowingbasicparts:anironcore consistingoflaminatedsheets,theprimaryandthesecondarywindings.Thereasonofhaving thecoreslaminatedwithinsulationbetweenthelaminationistoreducetheeddycurrents inducedbythealternatingmagneticflux.Theverticalpartsofthecoreareusuallytermedthe limbsandthehorizontalaretheyokes.Thetwodesignsforthecorearethecoretypewherethe ironcoreformingthelimbsaresurroundedbythewindingsandtheshelltypewherethe windingsmorecompletelysurroundedbytheiron(fig.1.5).Thematerialofthecoreiseither thegrainorientedsiliconsteelortheamorphousalloys.Thesiliconsteel(iron)containssilicon

inthe31/2%level.Thethicknessofthelaminatesisintherangeof.014inches(29guage).For highefficiencytransformersormotorsthesteelusedwouldhavesiliconinthe45%range.The steelusedintheseapparatusisdesignatedforexampleasM2(.007"),M3(.009")orM6 (.016").

Curvesthatdefinetheelectricsteelpropertiesofatransformers: Thereareafewcurvesthatdefinetheimportantpropertiesoftheelectricsteelasusedin transformers,theyare: BHloop:themagneticinduction(inweber/m2,forexample)vs.themagneticfieldstrength, alsotermedmagnetizingforce(inampereturns/cmorperm)itisknownasthehysteresisloop. thedcmagnetizationcurve:whichisthemagneticinduction(B)vs.themagnetizingforce(H). Coreloss:themagneticinductions(eg.inweber/sq.m)vs.thecorelossinwatts/LB. theVAlosscurve:theexcitingvoltampererms/LBvs.excitinglossesinVA/LB(Pa). theanglefromrollingdirection:itistheanglefromtherollingdirectionindeg.vs.Pa. Propertiesoftheelectricsteel,structuresensitive&nonstructuresensitive: Thepropertiesofthemagneticmaterialsdependon:thechemicalcomposition,fabrication processandheattreatment.Saturation(magnetization)changesslowlybyvariationinchemical compositionbutisunaffectedbyfabrication(includingimpurities)orheattreatment. Permeability()whichequalsB/H,coerceiveforce(itisthedcmagnetizingforceatwhichthe magneticinductioniszerowhenthemayerialisinasymmetricallycyclicallymagnetized condition)andhysteresislossarestructuresensitivei.e.affectedbycomposition,impurities, strain,temperature,crystalstructureandorientation.Atconstantmagneticfield,thecoreloss increaseswithincreasedsheetthickness.Itistheeddycurrentcomponentinthecorelossesthat increaseswiththeincreaseinthethickness. Amorphousmetals: Amorphousmetalsarealloyswithnoncrystallineatomicstructure.Theatomsarearranged

randomlyinrelationtoeachother.Itiseasiertomagnetizethistypeofalloysthancrystalline ones.Ifthistypeofmaterialisusedtobuilddistributiontransformers,lowercorelosseswill produced.Thecoolingrateoftheliquidalloytoobtaintheamorphousmetalstructureisinthe orderof1milliondegreespersecond.Thereareafewmethodsofquenchingtoproducethis material.Theprocessthatisusedinpracticeistheplanarflowcasting.Thistechnique,ina simplifiedmannerproducesthesolidifiedmetalthroughthefollowingsteps: liquidalloyismeltedanddeliveredtoaholdingreservoir. thealloyisdeliveredthroughatapinthebottomofthereservoirtothecastingnozzleandthen thequenchingbelt. thequenchingbelthasthecoolingboxandleadsthequenchedribbontothemeasuringstand andwindingmachinery. Coresofdistributiontransformers: Thecoreconstructioncanbeanyofthefollowing,functionoftheratingandthedesign:wound, butt,ormittered(fig.1.5a).Thewound(spiral)coremayhavethesteelsheetscuttopre determinedlengths(commonlyused)orsheetswithnocuts,thisisacommondesignwithoil filleddistributiontransformers.Thebutt(andlap)designwillhavetwodifferentsizesofsheets, thefirstmakeupthelegsandtheothertomaketheyokes.Inthisdesign,thegapsbetweenthe differentsteelparts(inthefluxpath)maybethereasonforthenoiseandtheincreaseinthe requiredampereturnstoachievethedesired(rated)fluxdensity.Themitteredcorewillhavethe sheetsforthelegsandyokescutat45inordertohavethefluxpathalwaysinthedirectionthe steelwasrolled(grainoriented).Hotrolledorcoldrolledsteelsareusedintransformerswith regulargrainorhighpermeabilitygrainorientedproperties.

Arrangementsofcoilsofdistributiontransformers:? Themajorwindingtypesaretheconcentric(thel.v.isclosertothecoreandtheh.v.iswound ontopofthel.v.)andthesandwiched(wherethesecondarywindingsandwichisontopand bottomoftheprimaryone),fig.1.6.Thewiresusedinformingthecoilsareinsulatedcopperor aluminum.Thecoilsareoftheprewound(formed)constructionandcanbeofthecylindricalor disctype.Cylindricalcoilsarewoundinhelicallayers,withlayersinsulatedfromeachother. Insulatingcylindersareplacedbetweenthecoreandthefirstcoil.Theyare,also,placedbetween thecylindricalwindings.Disccoilsmaybeoneormultiturnsperlayer.Multilayershavean insulatingmaterialbetweenthem.Acompletewindingconsistsofstackeddiscsofcoilswith intercoilinsulation.Thewindingconfigurationwillhaveaneffectonthetransientresponseof thetransformer.Indrytypetransformers,therearethreemajortypesofwindings/windings insulationcombination.Theyare:opencoil,castcoilandcoatedcoil.Thewirescanhaveanyof thefollowingforms:circular,rectangular(strip)oroval.

Excitingcurrentcomponents: Areminder,theexcitingcurrentcanbebrokendownintoafundamentalandafamilyofodd harmonics(usingFourieranalysis).Thefundamentalcomponentcanfurtherberesolvedinto twocomponents,oneinphasewiththecountere.m.f.andanotheroutofphaseby90.Thecore lossabsorbedbythehystersisandeddycurrentlossesinthecore,accountforthatcomponentin phasewiththee.m.f.Themagnetizingcurrentequalstheexcitingcurrentminusthecoreloss componentwhichmeansitisequaltoalltheharmonicsplustheoutofphasefundamental component.Atconstantmagneticinductionandmagneticstrengthfield,thetotallosses decreasewiththeincreaseinthepermeability.Forthesamepermeability,thelossesincrease withtheincreaseinthesheetthickness.Theincreaseofthegrainsizenumberreducesthe losses,aswellastheincreaseinpercentsilicon(increaseintheresistivity)reducesthelosses. Theincreaseinthetensilestrengthwillreducethelosses.

Methodsofinsulationofcoilsoftransformers: Theopencoilusesamethodwherebythetransformercoilsafterbeingwoundareimmersedin aninsulatingvarnishlikesilicone.Thevarnishfillstheairvoidsandcoatsthecoilsurfacewith about2mils(.002")protectivecoating.Thedifferenttypesofwoundcoilsusedinthismethod canbeanyofthefollowing:barrel(cylindrical)ordiscorsectional.Thecastcoilisusedwith thebarrelorsectionalwindings.Thecoilsareplacedinacastingmould.Themouldandcoils arethenplacedinavacuumchamberandevacuated.Anepoxyresin(oflowviscosity)isput (injected)insidethemould(undervacuum).Themouldwithitscontentsisthenplacedinan oventosolidify(ovenbake)theresin.Theresultinginsulationcoatingis250mils, approximately.Thecoatedcoilisusedwithsimilarwindingsasusedwiththecastcoil insulatingtype.Afterthewindingswereprepared,thecoilsareplacedinavacuumchamberand evacuated,thenfloodedundervacuumwithalowviscosityepoxyresin.Thecoilsaredrained

andbaked(tosettheresin).Thisprocessisrepeated,butthistimewithahighviscosityresin. thecoatingoverthewindingswillbeabout100mils. Transformerlossesunderload&noloadconditions: Lossesindistributiontransformerscanbeclassifiedintoloadandnoloadlosses.Theload

lossesequalI2Randascanbeseenvarieswiththesquareoftheloadcurrent.Itis,also,referred toaswindinglosses.Thenoloadlossesaretheresultoftheelectriccurrentsandmagnetic fieldsnecessarytomagnetizethetransformerscore.Thenoloadlossesarepresentaslongas thetransformeriskeptenergized(itisindependentofthetransformerloading).Auxiliarylosses (likefansenergyconsumptionthatischargedtothedemand)arenotavailablewiththe distributiontransformersbutarepresentinthesubstationtransformersabove5MVAratings. Thetotalowningcostofatransformerconstituteoftheinitialcost(purchasingprice),costof noloadlosses(overtheexpectedlifeofthetransformer)andloadlosses(overtheexpectedlife ofthetransformer)andmaybethemaintenancecost(overtheexpectedlifeofthetransformer). Thelossescostandmaintenance(ifincluded)arepresentedasfirstcost(presentvalue)inorder tomaketheevaluationofthetotalowningcostofthedifferentavailabletransformersforeach sizeorrating(oftransformer)possible. Causesofgeneratedgasesinoilfilledtransformer: Thepropertiesofthenewoiltobeusedintransformers,thetestsperformedontheoil,the acceptablevaluesfromthedifferenttests,theinterpretationoftheusedoiltestresultsandthe instrumentsusedtodetectgasesinoilarecoveredfullyinASTMandIEEErelatedstandards. Gasesduetocompositionofoilandsolidinsulationresultfromconductortemperature(dueto loadlosses)andexposuretoarctemperature.Gasesunderlowenergydischargesandpartial discharge(corona)conditionsareformed,mainlybyionicbombardment.Theproductsthat resultfromthermaldecompositionofoilimpregnatedcellulosematerialarecarbonoxides(CO &CO2)andhydrogen(H2)ormethane(CH4). Factorsaffectingthegenerationofgasinoilimmersedtransformers: Thevolumeofthegeneratedgasanditsratedependonthetemperatureandthevolumeofthe heatedmaterial.Thebreakingofcarbonhydrogenandcarboncarbonbondsresultfromthe thermalorelectricalfaultsunderoil.Thearcunderoilwillhaveahighpressuregasbubblewith thefollowingfluids,fromtheoutsidetowardthearc,inwardly(ofthebubble):oil,wetoil vapour,superheatedoilvapour,hydrocarbons(C2H2acetylene)andhydrogen.Thearcrunsin amixtureofhydrogenions,metalvapour,electrons.Thus,tousethepresenceofthegasinthe transformeroilasanindicationofthepresenceofafault,threedistinctivetypesoffaultshaveto bedefined.Thethreetypesare:thermal,electrical(lowintensitydischarges)andhighintensity electricalarcs.Thegasesthatmaybefoundintransformeroileitherundernormalorfaulty conditionsare:methane(CH4),ethylene(C2H4),ethane(C2H6),acetylene(C2H2),hydrogen (H2),carbonmonoxide(CO)andcarbondioxide(CO2).Thethermalfaultsthatwillcausethe oiltemperaturetorisetoupto500deg.CwillproduceH2,CH4andtracequantitiesofC2H4& C2H6.TemperaturesinthemiddlezonewillgeneratesignificantquantitiesofC2H6&C2H4. Attheupperendofthethermalfaultsandhighintensityarcing(electrical)faults,the

temperaturerangewillbe700to1800deg.Candthegasesyieldedare:H2,C2H4andtracesto significantamountsofC2H2.Thermaldecompositionofcelluloseandothersolidinsulating materialswillproduceCO,CO2andwatervapour.TheratioofCO2/COortheppm(quantity) maybeusedasanindicationoftheinsulationdisintegration.Forelectricallowintensity dischargesunderoil,H2,CH4andtracesofC2H2willbeproduced. Differenttypesoftransformers: Thedifferenttypesoftransformersfoundindistributionsystemsare:Power(upto10MVA) liquidfilled(oil),power(over10upto100MVA)oilfilledwithradiators/fans(oneor2sets), singlephasedistributiontransformers/oilfilled(withorwithoutradiators/fans)upto500KVA, threephasedistributiontransformers/oilfilled(withorwithoutradiators/fans)upto1.5MVA, drytypepowertransformers/3phase300KVAto2MVAorsiliconefilledorepoxyresin insulatedforindoorinstallations.Alloilfilledtransformersareinstalledoutdoorunlessa speciallayoutwithfireproof(resisting)materialandappropriatebarriersareused,thenindoor installationispossible.Distributiontransformerscanbeofthepolemounted,vaultor padmountedtype.Theprimaryvoltageofpowertransformerscanbeashighas750KV,though themostcommonare345KV,220KV,115KV,fordistributiontransformersashighas72KV thoughthemostcommonare34.5,25KV(27.6KV),15KV. Standards: Thestandardsthatgoverndistributiontransformersare:CSA"Singlephase&threephase distributiontransformers"Std.C2,CSA"Drytypetransformers"C9,CSA"Guideforloading Drytypedistributionandpowertransformers"C9.1andCSA"Insulatingoil"C50.The distributiontransformersaredefinedasfollows:thevoltageratings(insulationclasslevelof primaryh.v.winding,theprimaryandsecondarywindingsratedvoltage),shortcircuit capabilityforafaultonthebushingsofthetransformer(currentvalueanditscorresponding duration),dielectrictestvalues(appliedvoltagefor1minute,fullwaveandchoppedBILand timetoflashoverforthechopped),outdoortransformerbushingsratings(definedbytheir insulationclass,60HZ1minute/dry,10second/wetdielectricwithstandability,thefullwaveand choppedBIL),audiblesoundlevelsandinducedvoltagetests. Lightningarrestersparameters: TheimportantparametersbywhichL.A.aredefinedare:dutycyclevoltage,impulsetestcrest voltage,powerfrequencyvoltage(dryandwetforoutdoorinstallations),impulsecurrent rating,maximumcontinuousoperatingvoltage,switchingsurgescapability,highcurrent/short timeandlowcurrent/longdurationrating,materialofhousing,designofinternalsie.gappedor gaplesselements(nonlinearresistancematerial).