Power systems protection, part 1

download Power systems protection, part 1

If you can't read please download the document

description

Introduction, circuit breakers,load break switches, current transformers, potential transformers, test switches, terminal blocks, control wires, stationary batteries and relays

Transcript of Power systems protection, part 1

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.