PowerSystemsProtectionsubtransmission,distribution&industriallevels,PartI Introduction: Inthiscoverageofpowersystemsprotection,thesubjectisdividedintothreemajorparts.Thefirstpart willcoveranoverviewofpowersystemsprotectionincludinganintroductiontotheothermajor components,besidetherelays,thatconstituteanoperativeprotectionsystem.Thesecondpartwill coverthehardwareaspectofthesubjectandthethirdwillcoverthecalculations,testing,settingofthe protectiverelays.Also,itwillcoverthesizingofbreakingdevicesandthetypicalprotectionschemesof transformers(poweranddistribution),feeders,busandmotors.Thepresentationcoveragewillendbya fewnumericalexamplestoemphasizethepreviouslygivenprotectiveprinciples. Protectiverelaysaremostlyinstalledinswitchgearassembliesormaybefoundinrelaypanels.The assemblywillbedividedintocompartmentstoaccomodatethedifferentequipmentanddevices.The switchgearassemblywillcontainthecircuitbreakersand/orloadbreakswitchespluspower/current limitingfusesand/ordisconnectswitchesplustheinstrumenttransformers(currentandvoltagefeeding therelaysandmeters)andcontrolpowertransformer.Theseassemblieswillcontainalsothefollowing accessories(tocompletetheoperativesystem):thetestswitches(whichcomeinavarietyofdesignsto suitthedifferentapplications)toisolatetherelaysfromtheirtransformers,assistintestingtherelays andprovidethenecessaryprotection(shortening)ofthecurrenttransformersterminals,terminalblocks thatcomeindifferentdesigns,current/voltageratings,materialsoftheinsulationandscrews/clampsto suitthediversifiedrequirements,thecontrolwires,controlcircuitprotectionfusesormoldedcase(or miniature)circuitbreakersandpowersupply(d.c.batteriesorcontrolpowertransformers)tooperate thebreakingdevices,provideindicationslocally,andsupplytheauxilliary(supply)voltagetosolid state/microprocessorbasedrelays. Circuitbreakers: Ifcircuitbreakersareusedinlowvoltageinstallations,theprotectivedeviceswillbeanintegralpartof it;ifswitchesareused,thentheprotectivedeviceswillbeseparateandprotectionisprovidedbyfuses. Lowvoltagecircuitbreakerscanbedividedbroadlyintomoldedcaseandpower(airmagnetic).In moldedcasebreakers,theprotectivedeviceagainstovercurrentandshortcircuitisprovidedbythermal magneticorelectronic(solidstate)inegralprotectivedevice.Withpowerbreakers,themostcommonly usedconfigurationswillhavetheprotectivedeviceasintegralonewiththebreaker.Somedesignswill havecurrenttransformers(sensors)andrelaysseparatefromthebreakers.Thefunctionsofthe protectivedevicesvaryaccordingtotheuserrequirementsandprotectivephilosophy.Inmedium voltageinstallations,themostcommonconfigurationswillhaveseparateprotectivedevices(relaysor fuses)fromcircuitbreakersorswitches,thoughsomedesignsofcircuitbreakersmayhaveintegral protectivedevices(sensorsandrelays).Forintegralprotectionfromundervoltageconditions,the breakersmaybesuppliedwithundervoltagereleases.Thestandardsthatgovernthedesignandtesting ofcircuitbreakersareeithertheANSIC37ortheIEC56series.TheANSIC37.04and06givesthe ratingsstructureandpreferredratingsforACMVCBs,respectively.StandardC37.09givesthetest procedureforACMVCBs,ratedonasymmetricalcurrentbasis.StandardC37.010givesanoverview oftheapplicationproceduresforACMVCBs.StandardC37.16coversthepreferredratings,related requirementsandapplicationrecommendationsforLVpowerCBs.Ingeneral,theratingsofthecircuit

breakersarethelimitsoftheoperatingconditionsofthefollowingparameters,forwhichthebreakers aredesigned(withoutcausingdamagetoitselforthesurroundingsfortheprojectedlifetimeofsuch device): ratedoperatingvoltagerange(eg.minimum,nominalandmaximum). ratedfrequency(eg.400c/sor50/60c/s). ratedmaximumcontinuouscurrent(eg.1200,2000,...). ratedinterruptingcurrentsunderthedifferentoperatingvoltagesofthebreakers. rateddielectricstrengthlimitormaximumvoltagewithstandability(eg.forpowerfrequencyvoltages, impulsevoltagewaveshapesandswitchingvoltageimpulses). dutycycletowhichthebreakersaredesignedandtested. ratedoperatingtimes. equipmenttowhichthebreakerswillbesuitabletoapplyandusesafely. mechanicaldurabilityundernoload,fullloadandshortcircuitcurrents. controlcircuitsupply(operating)requirements. Loadbreakswitches:Mediumvoltageloadbreakswitchesinserieswithpowerorcurrentlimiting fusesinmediumvoltageinstallationsisacommonconfiguration.Thoughthisdesignismuchsimpler indesignandmaintenancethancircuitbreakers,ithasitsdisadvantages.Theconstructionisofthe fixedtype(vs.drawoutwithcircuitbreakers).Ifthefuseisblown,thecelldoorhastobeopenedand thefusesreplaced(vs.reclosingofbreakersafterfaultremoval)arelativesaferoperation.Thebreaker constructionismorecompactthantheswitch(asclearanceshavetobemetinair).Lowvoltage switchescanbeofthemoldedcasetypeorthesafetyswitchconstruction.Bothtypeswilrequirefuses toprovidetheprotectiontothesystemandconnectedequipment. Currenttransformers:InstrumentstransformersarecoveredinANSIstandardC57.13andCSACAN3 C.13.Currenttransformerscomeinafewformstoprovideforthespacelimitationsincertaindesigns andtherequiredhighaccuracyforothers.Currenttransformerscanbeclassifiedbroadlyintobar, window,bushingandwound(forhigheraccuracy).Currenttransformersforprotectionapplication (ratherthanmetering)donotrequireahighaccuracyatratedcurrentasmuchasrequiretheiroperation withoutsaturationathighfaultcurrents.ForeaxampleaC.Trated2.5L200willhavea2.5%errorat 20timesitsratedsecondarycurrenttimestheburdenconnectedtothesecondary.Iftheratedsecondary currentis5thenthemaximumburdentobeconnectedis2ohm,ifratedsecondaryis1thentheburden is10.The200isthekneevoltageonthesecondaryexcitingvoltagevs.excitatoncurrentcurveofC.T. TheotherimportantpiecesofinformationthatarerequiredabouttheC.T.are:thesecondaryresistance, crosssectionareaofthecoreandthesaturationfluxdensityforthesiliconeirongradeusedforthecore sothattheC.T.canbecheckedforitsproperoperationunderthegivenconditions.Current transformersaresubjectedtoheavyprimarycurrrentforshortperiodsoftimeduringfaults.Mechanical damagemaybecausedbymagneticforcesinthewindingsduetothefirstcycle(andsubsequentones) peakfaultcurrentandisproprtionaltosquarethepeakcurrent.Thermaldamagetothetransformer insulation(andmaybeitscopper)mayoccurduetoheatingofthewindingduetotheintegrated effectivevalueofthefaultcurrentoverthetimeperioduntilthefaultremoval.(Note:thesame discussionisapplicablewhensizingcablesandcablesupports).Partialdichargeininstrument

transformersareobtainedbytestingatthefactoryandtheacceptablelevelvariesdependingonthe requirementoftheenduser,thoughthelimit(amaximumvalue)isgivenas50picocoulomb.A measureoftheconditionoftheisulatingmaterialisthedielectriclosswhichisgivenbythedissipation orpowerfactor.Anidealinsulationmaterialshouldhaveaverylowpowerfactorcloseto0(=cos= tan ).Polarityofinstrumenttransformersisimportantwithdifferentialprotectionschemes.Polarity markingsdesignatetherelativeinstantaneousdirectionsofcurrentinthetransformerleads.Ahighand alowvoltage(orcurrent)leadshavethesamepolarityatagiveninstantifthecurrententersthehigh voltageleadandleavesthelowvoltageleadatthisinstant(orviceversa),givingtheeffectasthough thetwoleadsbelongtothesamecontinuouscircuit.Thepolarityoftransformerscanbeadditive(H1& X1arediagonallyopposite)orsubtractive(H1&X1areadjacent).A.C.orD.C.polaritytestscanbe performedatsitetodeterminethepolarityofunmarkedtransformers. Potential(voltage)transformers:Thistypeofinstrumenttransformersprovideanisolationbetweenthe highvoltagecircuitandthecontrol/meteringcircuit.Italsoprovidesastandardlowvoltagesignal irrelevantofthevoltageclassofthehighvoltagesystem.Inrelaycircuits,P.T.isusedinvoltage restrained,voltageanddistancerelays.TheburdenoftheloadstothesecondaryoftheP.T.winding shouldnotexceedthemaximumdesignatedtotheP.T.EachP.T.isratedforthermalwithstandabilityat differentambienttemperatures.Accordingtothestandardstherearethreegroupsofpotential transformers,theydefinetheratedvoltageoftheP.T.andwhatshouldbethesystemvoltagetowhich thisP.T.shoudbeappliedto.They,also,definetheratedovervoltagefactorandthedurationofthe overvoltageforeachgroup.DefinitionofPTgroups. Testswitches:Theycomeinmanyconfigurationsanddesigns,someareoftheopenstyleandothers covered(opaqueortransparent).Theycanbefrontorbackconnectedtotheotherdevices.Theswitch cancomewithdifferentnumberofpolesuptotenwithanycombinationofcurrenttypeorpotential typepoles.Broadly,thepoletypecanbeclassifiedaccordingly:blankspace,singlepolepotential, gangedtwoormorepotentialpoles,throughbar,fusemountingclip(max.30amp.),singlepoleshort circuitingassembly,singlepoletestjack/shortcircuitingcurrentassemblyandthreepoleshort circuitingcurrentassembly. Terminalblocks:Terminalblockscomeinmanyshapesandformstosuitthespecificneedsoftheuser, standardfeedthroughterminals,C.T.doubleclamp,slidinglink,knifedisconnectterminals,ground, neutral/disconnect,miniaturefeedthroughterminals,justtolistafewtypes.Theyareratedupto750 volt,uptoabout130amp.Certaindesignscomeupto3tierfordensewiringapplications.Thematerial oftheinsulatingmaterialcanbeblackphenolic,melamineorpolyamide. Controlwires:ThethreemostcommonlyusedtypeshaveasinsulatingmaterialTBS,SISorteflon. ThesizevariestoprovidetheminimumpossibleburdenontheC.T.orP.T.thatitisconnectedto.For C.T.,siesAWG8,10and12arecommon.ForP.T.sizes,AWG12and14arwnotunusual.Theratings forTBSvs.SISareasfollows:temperaturerating:90Cvs.90C,voltagerating600vvs.600V,solid orstrandedforbothfrom#14to.2,insulaqtionthicknessforbothisthesamethoughthebasematerial ofTBSisPVC(withcottonbraid)andforSISisXLPE(filled).Wireswithtefloninsulationhave highertemperatureratings. Stationarybatteries:Thestationarybatteryisdesignedtoserveasanauxilliary/standbysourceof

powertoalldevicesconnectedtoit.Thebatteryisnormallymountedonracksandiscontinously chargedexceptforintermittentdischargingperiodsofvaryingtimesandpower.Batteryvoltage graduallydeclinesduringdischargeandshouldnotbepermittedtodropbelowtheminimumtolerated bytheloadplusthelinedrop.Toprotectthebatteryagainstoverdischarge,alowvoltagerelay(d.c.) canbeusedaspartoftheinstallation.Therateatwhichthevoltagedeclinesdependsupon:thedemand currentoftheload,durationofthedischarge,chemicaldesignandtypeofcells,numberandsizeof platesineachcell,batterystateofchargeatbeginningofdischarge,ageofbatterycellsand temperatureofcells.Thecapacityofthebatteryisbasicallyitsabilitytosupplyagivencurrentfora givenperiodoftimeatagivencelltemperaturewithoutgoingbelowaminimumvoltage(batteriesare ratedinamperehouratagivendischargerate).Stationarybatteriesareusuallyratedfor8hour,3hour, 1hour,1minutedischarge.Theamperehourratingissimplytheproductofthedischargeinamperes multipliedbythegivendichargetimeperiod.Fortheleadacidbatterythepositiveplatesare:thepasted (Faure)platewhichcomprisesofalatticeworkmetallicgridwiththeopenningsfilledwithleadoxide paste.Thegridmaybemadeupofleadantimonyorleadcalcium.Thesecondtypeisthemultitubular plates,whichuseporousplatestocontaintheleadoxide.Thegrid(leadantimony)isarowofspines extendingfromthetopbartothebottomcapbar.Poroustubesfilledwithleadoxide(powedered)with thegridformsthepositiveplate.ThisdesignprovidesmoreAHofcapacitypercubicfootofbattery volumeatmoderateratesofdischarge.Thethirdandlasttypeistheplantetypewhichisconsideredto havethelongestlifeexpectancyofallleadacidstationarybatterydesigns.Thepositiveplateconsistsof agrid(leadantimony)oflargeareawiththinlayersofleadoxide.Suchplateshavecomplexdesigns withcircularopenningswherecorrugatedleadribbonsarerolledintospiralribbons.Thenegative platesirrelevantofthetypeofthepositveplatesarebuiltwithpastedplatedesign.Metallicspongelead isusedonthenegativeplates.Thenegativegridforthemultitubularandplante(positiveplates)is madeofleadantimony,withthepastedplateitiseitherleadcalciumorleadantimony.Thegridof alloysantimonyorcalciumservesbothpurposesgivesphysicalsupportandstrengthtothesoftlead andactsasanelectricconductor.Thegridachievesandretainsaphysicalshapeandconductsthe currenttoallpartsofmaterial. Forthenickelalkalinebatteriestherearetwotypesofplates,thepockettypeandthesinteredtype.The pockettypeisusedforbothpositiveandnegativeplates.Theactivematerial(nickelhydratepositive andcadmiumspongenegativeplusaddittivestohelpconductivity)issandwichedbetweentwo perforatedstrips(nickelplatedsteel).Thestripsarecrimpedtogetherandthisassemblyisplacedina Ushapeframe.Afterintermeshingthepositiveandnegativetheinsulatorpinsareputinplace,through theframeandplates.Theseelementsarethenputinacontainerandthecellcover(withventcapand appropriateholeforterminalpoles)isinstalled.Therearethreecommonratings:high(discharge shorterthan1hour),mediumrate(dischargeshorterthan4hrs)andlow(thebatterywillsupposedly carryloadsforupto20hrs). Forleadacidbatteries,theelectrolyteisasolutionofdilutedsulphuricacid.Whenthebatteryisfully charged,thepositiveplateisleadperoxideandthenegativeoneisspongelead.Thespecificgravityof theelectrolyteismaximumatstartofdischargeandthespecificgravitygraduallydecreasesasdicharge occurs.Specificgravityforstationarybatteriesusedforswitchgearapplications,controlandemergency lightingisapproximately1.210.Todeterminethestateofchargeofthebattery,thegravityreadingis

comparedwiththefullchargevalueandtothespecificgravitydropofaparticularcellsizeataspecific dischargerate.Thereadinghastobecorrectedtotheambienttemperatureattimeofmeasurement,if otherthan25degC.FortheNiCadtheelectrolyteisasolutionofpotassiumhydroxidedilutedin waterwithnormalspecificgravityof1.16to1.19at25degC.Whenthebatteryisfullychargedthe positiveplate,nickelhydrate,ishighlyoxidizedandthenegativeplateisspongemetalliccadmium. Afterdischargetakesplacethepositiveplatereducestoloweroxidewhilethemetalliccadmiuminthe negativeplateoxidizes.Thespecificgravityoftheelectrolytecannotbeusedtoindicatethestateof chargeofthebattery.Thespecificgravityreadingswillvaryfromnormalratingwhentheelectrolyte temperatureislowerorhigherthan25degC,whenthesolutionleveldropsbelowthenormalorthe batteryhasbeeninserviceforlongtime. Thechargerisastaticrectifier(scr),itsfunctionistochangethesinglephaseorthreephaseinput(120, 208,240,480,600V60HZ)toad.c.outputsuitableforchargingthebatteryandmaintainninga constantvoltagethroughoutthebattery'sloadrange.Topreventtheselfdischargephenomena(standing loss)thechargermaintainsafloatchargethatcontinuouslymonitorsandcorrectsfortheseinternal losses. Relays:Everysystemissubjecttoshortcircuitsandgroundfaults,thatshouldberemovedquickly.The mostcommonrelayforS.C.protectionistheO/Crelay.Ashortcircuitonanelectricsystemisalways accompaniedbyacorrespondingvoltagedip(anoverloadwillcauseamoderatevoltagedrop).A voltagerestrainedorvoltagecontrolledO/CrelayisabletodistinguishbetweenO/Landfault conditions. Directionalovercurrent:consistsofatypicalO/Cunitandadirectionalunit,whicharecombinedto operatejointly,forapredeterminedphaseangleandmagnitudeofcurrent.Sucharelayoperatesonly forcurrentflowtoafaultinonedirectionandwillbeinsensitivetocurrentflowintheopposite direction. Directionalpowerrelays:comesinsingleorthreephaseversionsandtheyworkonthewattmeter principle.Thecontacts(inelectromechanicalconstruction),movableandfixed,getincontactatapre determinedvalueofpower.Itcouldbeusedfordirectionaloverpower,tooperateifexcessenergyflows outofanindustrialplantintotheutility.Itcanalsobeusedtosenseanunderpowerconditionand separatetwosourcesoperatinginparallel. Differentialrelays:thebasicprincipleofoperationforsuchrelaysisthecontinuouscomparisonoftwo ormorecurrentquantities.Whenafaultoccurs,theresultingdifferentialcurrentwillcausetherelayto operate.Differentialprotectionschemesforgenerators,motors,twowindingtransformerbanksand busesarecommoninindustrialplants.Insubtransmissionanddistributionlevels,differentialprotection isusedwithpowertransformersandinbusprotectionschemes.Itprotectsagainstabnormalitieswithin azoneandshouldbeinsensitivetofaultsoutsidethiszone(throughfaults),overexcitationorduring energizationorstartingconditions. Groundfaultrelayingcanbeanyofthefollowingconfigurations:residuallyconnected,directsensing orzerosequence(vectorialsummation). Asinglewindowtypecurrenttransformerismountedinsuchawaytoencircleallthreephase conductorsofincomingoroutgoingcircuits.For3phase,4wirecircuits,theneutralisalsorunthrough

thesensorandthesecondaryofthissensorisconnectedtoanO/Crelay.Forneutralrelaying(direct sensing),acurrenttransformerisintheneutralgroundingcircuitandconnectedtotheO/Crelay.The synchrocheckrelayisusedtoverify,whentwoalternatingcurrentcircuitsarewithinthedesiredlimits offrequency,voltageandphaseangle,topermitthemtooperateinparallel. Thesynchronizingrelaymonitorstwoseparatesystemsthataretobeparalleled,initiatingswitching whenthefollowingthreeconditionsaremet:thevoltagedifferenceofthetwosystemsandthe frequencydifferencearewithinthepredeterminedrange.Thephaseanglebetweenthetwosystems voltageiszero,takingintoconsiderationtheoperatingtimeoftheswitchingdevices. Pilotwirerelays,operateontheprincipleofcomparingtheconditionsattheterminalsoftheprotected line.Therelayswilloperateifthecomparisonindicatesafaultinternallyontheline,theyare insensitivetoexternalfaults.Thisschemeisusedwhentielineshavetobeprotected,eitherbetweenthe industrialsystemandtheutilitysystemorbetweenmajorloadcentreswithintheindustrialplant. Mi>Voltagerelayscanbeclassifiedaccordingtotheirreasonofoperation(i.e.,overvoltage, undervoltageorboth,voltageunbalance,reversephasevoltageorexcessivenegativesequencevoltage). Under/overvoltagerelaysarefoundinthefollowingcircuits:capacitorswitchingcontrol,a.c.&d.c. overvoltageprotectionforgenerators,automatictransferofpowersupplies,loadsheddingonU/Vand U/Vprotectionformotors. Voltageunbalance(comparingtwosources),anexamplefortheapplicationofsucharelayiswiththe voltagerestrainedrelays,whentheP.T.fuseblows.Thisisseenasafaultbythevoltagerestraintrelay. Theuseofbalancerelayscanblocktheoperationoftherestrainedrelay. Reversephasevoltagerelaysareusedtodetectreverseconnectionsinthreephasecircuits,feeding motors,generatorsortransformers. Negativesequencevoltagerelaysareusedtodetectsinglephaseconditions,aslongasthesensingP.T. isontheloadsideoftheopenedphase. Negativesequenceovercurrentrelaysareusedforsinglephaseprotection.Thelocationofthecurrent transformerwithrespecttotheopenedpointisinsignificant. Distancerelayscomeinthefollowingtypes:theMHOtype,impedance,reactance,MHOor admittance,OHMorangleimpedance,offsetMHO,modifiedimpedance,complexcharacteristicstype, ellipticalcharacteristicsandquadrilateraltype.Theymeasurevoltage,currentandtheratioisexpressed intermsofimpedance.Theimpedancecanrepresenttheequivalentimpedanceofageneratororlarge synchronousmotororatransmissionline.TheMHOrelayisusedtodetectthelossoffieldof synchronousgeneratorsandmotors. Frequencyrelayssenseunderoroverfrequencyconditionsduringsystemdisturbances.Theusual applicationoftheserelaysis:toselectivelydroptheload,basedonthefrequency,inordertorestore normalsystemstability,splittingupagridbyopeningtielinestopreventcompletesystemcollapse,for thegeneratorsandauxiliariesprotection,whenfrequencysupervisioncanpreventturbinesanddrive damagesandforisolatingsmallsystemshavingtheirowngenerationfromthemainsystem. Temperaturesensitiverelaysusuallyoperateinconjunctionwithtemperaturedetectingdevices.These devicescanbeclassifiedintoRTD(resistancetemperaturedetectors)andthermocouples.Theyare locatedintheequipmenttobeprotected(embeddedinthestatorwindingorthebearingsofthemotor

orgenerator).Thetemperaturedetectorscanhave10,100or120ohmandisconnectedinabridge configuration,withthetemperaturesensitiverelayconnecteddiagonallyacrossthebridge.Replicatype temperaturerelayshavetheiroperatingcharacteristicscloselymatchingtheheatingcurveofthegeneral purposemotorcurves(inthelightandmediumoverloadzones),thustheyareusedforoverload protectionofmotorsinthemediumvoltagerange. Multifunctionrelays:Theserelaysaremicroprocessorbasedandprovidemorethanoneprotection functionandevensomeindicationsandmetereddata.Theycanbeclassifiedinrtofeederprotection units,inductionmotorsprotectionandsynchronousmotorsprotectionunits.Theyhavetheprovisionof beinginterrogatedandadjustedremotelythroughtheircommunicationportsandthelocalnetworkthey areconnectedto.