Fundamentals of Power System Protection PDF

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Power System Protection Fundamental Notes...

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1.1FundamentalsofPowerSystemProtection

The purpose of an Electric Power System is to generate and supply electrical energy to consumers.Thepowersystemshouldbedesignedandmanagedtodeliverthisenergytothe utilizationpointswithbothreliabilityandeconomically. The capital investment involved in power system for the generation, transmission and distributionissogreatthattheproperprecautionsmustbetakentoensurethattheequipment not only operates as nearly as possible to peak efficiency, but also must be protected from accidents. Thenormalpathoftheelectriccurrentisfromthepowersourcethroughcopper(oraluminium) conductorsingenerators,transformersandtransmissionlinestotheloadanditisconfinedto this path by insulation. The insulation, however, may break down, either by the effect of temperatureandageorbyaphysicalaccident,sothatthecurrentthenfollowsanabnormalpath generallyknownasShortCircuitorFault. AnyabnormaloperatingstateofapowersystemisknownasFAULT.Faultsingeneralconsistof shortcircuitsaswellasopencircuits.Opencircuitfaultsarelessfrequentthanshortcircuitfaults, andoftentheyaretransformedintoshortcircuitsbysubsequentevents.

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1.2.ConsequencesofoccurrenceofFaults

Faults are of two type Shortcircuitfault‐current Opencircuitfault‐voltage

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Intermsofseriousnessofconsequencesofafault,shortcircuitsareoffargreaterconcernthan opencircuits,althoughsomeopencircuitspresentsomepotentialhazardstopersonnel. Classification of short circuited Faults • Threephasefaults(withorwithoutearthconnection) • Twophasefaults(withorwithoutearthconnection) • Singlephasetoearthfaults  Classification of Open Circuit Faults SinglePhaseopenCircuit Twophaseopencircuit Threephaseopencircuit

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Consequences

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Damagetotheequipmentduetoabnormallylargeandunbalancedcurrentsandlow voltagesproducedbytheshortcircuits

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Explosionsmayoccurintheequipment’swhichhaveinsulatingoil,particularly duringshortcircuits.Thismayresultinfireandhazardousconditionstopersonnel andequipments.

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Individualgeneratorswithreducedvoltageinapowerstationoragroupof generatorsoperatingatlowvoltagemayleadtolossofsynchronism,subsequently resultinginislanding.

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Riskofsynchronousmotorsinlargeindustrialpremisesfallingoutofstepand trippingout.

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The general layout of a protection system may be viewed as given in the following figure 

 Trip circuit

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1.3Componentsofaprotectionsystem Protectionsystemsusuallycomprisefivecomponents:     

Currentandvoltage transformersto step down the high voltages and currents of the electricalpowersystemtoconvenientlevelsfortherelaystodealwith Protectiverelaystosensethefaultandinitiateatrip,ordisconnection,order; Circuitbreakerstoopen/closethesystembasedonrelayandautoreclosercommands; Batteriestoprovidepowerincaseofpowerdisconnectioninthesystem. Communicationchannelstoallowanalysisofcurrentandvoltageatremoteterminalsofa lineandtoallowremotetrippingofequipment.

Forpartsofadistributionsystem,fusesarecapableofbothsensinganddisconnectingfaults. Failuresmayoccurineachpart,suchasinsulationfailure,fallenorbrokentransmissionlines, incorrectoperationofcircuitbreakers,shortcircuitsandopencircuits.Protectiondevicesare installedwiththeaimsofprotectionofassets,andensurecontinuedsupplyofenergy.  1.4ZonesandtypesofProtectionsystem  1.4.1ZonesofProtectionsystem  •Anelectricpowersystemisdividedintoseveralzonesofprotection.Eachzoneofprotection, containsoneormorecomponentsofapowersysteminadditiontotwocircuitbreakers. •Whenafaultoccurswithintheboundaryofaparticularzone,thentheprotectionsystem responsiblefortheprotectionofthezoneactstoisolate(bytrippingtheCircuitBreakers)every equipmentwithinthatzonefromtherestofthesystem. •ThecircuitBreakersareinsertedbetweenthecomponentofthezoneandtherestofthe powersystem.Thus,thelocationofthecircuitbreakerhelpstodefinetheboundariesofthe zonesofprotection. •Differentneighboringzonesofprotectionaremadetooverlapeachother,whichensurethat nopartofthepowersystemremainswithoutprotection.However,occurrenceofthefaultwith intheoverlappedregionwillinitiateatrippingsequenceofdifferentcircuitbreakerssothatthe minimumnecessarytodisconnectthefaultyelement

  1.4.2TypesofProtection(PrimaryandBack‐upProtection)  1.4.2.1PrimaryProtection – The primary protection scheme ensures fast and selective clearing of any fault within the boundariesofthecircuitelement,thatthezoneisrequiredtoprotect.PrimaryProtectionasa ruleisprovidedforeachsectionofanelectricalinstallation.  However,theprimaryprotectionmayfail.TheprimarycauseoffailureofthePrimaryProtection systemareenumeratedbelow. 

1.Currentorvoltagesupplytotherelay. 2.D.C.trippingvoltagesupply 3.Protectiverelays 4.Trippingcircuit 5.CircuitBreaker 1.4.2.2Back‐upProtection Back‐up protection is the name given to a protection which backs the primary protection wheneverthelaterfailsinoperation.Theback‐upprotectionbydefinitionisslowerthanthe primaryprotectionsystem.Thedesignoftheback‐upprotectionneedstobecoordinatedwith thedesignoftheprimaryprotectionandessentiallyitisthesecondlineofdefenceafterthe primaryprotectionsystem. 1.5ProtectionSystemRequirementsandsomebasicterminologiesused •Thefundamentalrequirementsforaprotectionsystemareasfollows: 1.5.1 Reliability: It is the ability of the protection system to operate correctly. The reliability featurehastwobasicelements,whicharedependabilityandsecurity.Thedependabilityfeature demandsthecertaintyofacorrectoperationofthedesignedsystem,onoccurrenceofanyfault. Similarly, the security feature can be defined as the ability of the designed system to avoid incorrectoperationduringfaults.Acomprehensivestatisticalmethodbasedreliabilitystudyis required before the protection system may be commissioned. The factors which affect this featureofanyprotectionsystemdependsonsomeofthefollowingfewfactors.

a)QualityofComponentused b)Maintenanceschedule c)Thesupplyandavailabilityofsparepartsandstocks d)Thedesignprinciple e)Electricalandmechanicalstresstowhichtheprotectedpartofthesystemissubjected to. 1.5.2Speed:Minimumoperatingtimetoclearafaultinordertoavoiddamagetoequipment. Thespeedoftheprotectionsystemconsistsprimarilyoftwotimeintervalsofinterest.

a)TheRelayTime:Thisisthetimebetweentheinstantofoccurrenceofthefaulttothe instantatwhichtherelaycontactsopen. b)TheBreakerTime:Thisisthetimebetweentheinstantofclosingofrelaycontactsto theinstantoffinalarcextinctioninsidethemediumandremovalofthefault. 1.5.3 Selectivity: This feature aims at maintaining the continuity of supply system by disconnectingtheminimumsectionofthenetworknecessarytoisolatethefault.Theproperty of selective tripping is also known as “discrimination”. This is the reason for which the entire systemisdividedintoseveralprotectivezonessothatminimumprotionofnetworkisisolated withaccuracy.Twoexamplesofutilizationofthisfeatureinarelayingschemeareasfollows a)Timegradedsystems b)Unitsystems 1.5.4.Sensitivity:Thesensitivityofarelayreferstothesmallestvalueoftheactuatingquantity atwhichtherelayoperatesdetectinganyabnormalcondition.Incaseofanovercurrentrelay, mathematicallythiscanbedefinedastheratiobetweentheshortcircuitfaultcurrent(Is)and therelayoperatingcurrent(Io).ThevalueofIo,shouldnotbetoosmallorlargesothattherelay iseithertoosensitiveorslowinresponding. 1.5.5Stability:Itisthequalityofanyprotectionsystemtoremainstablewithinasetofdefined operatingscenariosandprocedures.Forexamplethebiaseddifferentialschemeofdifferential protectionismorestabletowardsswitchingtransientscomparedtothemoresimpleandbasic MerzPriceschemeindifferentialprotection. 1.5.6Adequacy:Itiseconomicallyunviabletohavea100%protectionoftheentiresystemin concern. Therefore, the cost of the designed protection system varieswiththe criticalityand importanceoftheprotectedzone.Theprotectionsystemformorecriticalportions isgenerallycostly,asallthefeaturesofagoodprotectionsystemismaximizedhere.Butasmall motor can be protected by a simple thermally operated relay, which is simple and cheap. Therefore,thecostoftheprotectionsystemshouldbeadequateinitscost. 1.6 Coordination Relay protection coordination means that downstream devices (breakers/fuses) should activate before upstream devices. This minimizes the portion of the system affected by a fault or other disturbance. At the substation level, feeder breakers should trip before the main breaker. Likewise, the downstream breakers should trip before the substation feeder supplying the panel

1.7 Need of protection scheme in power system Electricalpowersystemoperatesatvariousvoltagelevelsfrom400Vto400kVorevenmore. Electricalapparatususedmaybeenclosed(e.g.,motors)orplacedinopen(e.g.,transmission lines).Allsuchequipmentundergoabnormalitiesintheirlifetimeduetovariousreasons.For example,awornoutbearingmaycauseoverloadingofamotor.Atreefallingortouchingan overheadlinemaycauseafault. Alightningstrike(classifiedasanactofGod!)cancauseinsulationfailure.Pollutionmayresultin degradationinperformanceofinsulatorswhichmayleadtobreakdown.Underfrequencyorover frequencyofageneratormayresultinmechanicaldamagetoit'sturbinerequiringtrippingofan alternator.Evenotherwise,lowfrequencyoperationwillreducethelifeofaturbineandhence itshouldbeavoided. It is necessary to avoid these abnormal operating regions for safety of the equipment. Even moreimportantissafetyofthehumanpersonnelwhichmaybeendangeredduetoexposureto livepartsunderfaultorabnormaloperatingconditions.Smallcurrentoftheorderof50mAis sufficient to be fatal! Whenever human security is sacrificed or there exists possibility of equipmentdamage,itisnecessarytoisolateandde‐energizetheequipment.Designingelectrical equipmentfromsafetyperspectiveisalsoacrucialdesignissuewhichwillnotbeaddressedhere. Toconclude,everyelectricalequipmenthastobemonitoredtoprotectitandprovidehuman safetyunderabnormaloperatingconditions.Thisjobisassignedtoelectricalprotectionsystems. Itencompassesapparatusprotectionandsystemprotection....