Electric power system - Wikipedia

An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the ... Electricpowersystem FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Networkofelectricalcomponentdeployedtogenerate,transmit&distributeelectricity PartofaseriesonPowerengineering Electricpowerconversion Voltageconverter Electricpowerconversion HVDCconverterstation AC-to-ACconverter DC-to-DCconverter Rectifier Inverter Electricpowerinfrastructure Electricpowersystem Powerstation Electricalgrid Interconnector Demandresponse Electricpowersystemscomponents Ringmainunit Grid-tieinverter Energystorage Busbar Busduct Recloser Protectiverelay vte Asteamturbineusedtoprovideelectricpower Anelectricpowersystemisanetworkofelectricalcomponentsdeployedtosupply,transfer,anduseelectricpower.Anexampleofapowersystemistheelectricalgridthatprovidespowertohomesandindustrieswithinanextendedarea.Theelectricalgridcanbebroadlydividedintothegeneratorsthatsupplythepower,thetransmissionsystemthatcarriesthepowerfromthegeneratingcenterstotheloadcenters,andthedistributionsystemthatfeedsthepowertonearbyhomesandindustries. Smallerpowersystemsarealsofoundinindustry,hospitals,commercialbuildings,andhomes.Asinglelinediagramhelpstorepresentthiswholesystem.Themajorityofthesesystemsrelyuponthree-phaseACpower—thestandardforlarge-scalepowertransmissionanddistributionacrossthemodernworld.Specializedpowersystemsthatdonotalwaysrelyuponthree-phaseACpowerarefoundinaircraft,electricrailsystems,oceanliners,submarines,andautomobiles. Contents 1History 2Basicsofelectricpower 3Componentsofpowersystems 3.1Supplies 3.2Loads 3.3Conductors 3.4Capacitorsandreactors 3.5Powerelectronics 3.6Protectivedevices 3.7SCADAsystems 4Powersystemsinpractice 4.1Residentialpowersystems 4.2Commercialpowersystems 5Powersystemmanagement 5.1Faultmanagement 5.2Maintenanceandaugmentation 5.3Frequencyandvoltagemanagement 6Notes 7Seealso 8References 9Externallinks History[edit] AsketchofthePearlStreetStation In1881,twoelectriciansbuilttheworld'sfirstpowersystematGodalminginEngland.ItwaspoweredbytwowaterwheelsandproducedanalternatingcurrentthatinturnsuppliedsevenSiemensarclampsat250voltsand34incandescentlampsat40volts.[1]However,supplytothelampswasintermittentandin1882ThomasEdisonandhiscompany,TheEdisonElectricLightCompany,developedthefirststeam-poweredelectricpowerstationonPearlStreetinNewYorkCity.ThePearlStreetStationinitiallypoweredaround3,000lampsfor59customers.[2][3]Thepowerstationgenerateddirectcurrentandoperatedatasinglevoltage.Directcurrentpowercouldnotbetransformedeasilyorefficientlytothehighervoltagesnecessarytominimizepowerlossduringlong-distancetransmission,sothemaximumeconomicdistancebetweenthegeneratorsandloadwaslimitedtoaroundhalfamile(800m).[4] ThatsameyearinLondon,LucienGaulardandJohnDixonGibbsdemonstratedthe"secondarygenerator"—thefirsttransformersuitableforuseinarealpowersystem.[5]ThepracticalvalueofGaulardandGibbs'transformerwasdemonstratedin1884atTurinwherethetransformerwasusedtolightupfortykilometres(25miles)ofrailwayfromasinglealternatingcurrentgenerator.[6]Despitethesuccessofthesystem,thepairmadesomefundamentalmistakes.Perhapsthemostseriouswasconnectingtheprimariesofthetransformersinseriessothatactivelampswouldaffectthebrightnessofotherlampsfurtherdowntheline. In1885,OttóTituszBláthyworkingwithKárolyZipernowskyandMiksaDériperfectedthesecondarygeneratorofGaulardandGibbs,providingitwithaclosedironcoreanditspresentname:the"transformer".[7]ThethreeengineerswentontopresentapowersystemattheNationalGeneralExhibitionofBudapestthatimplementedtheparallelACdistributionsystemproposedbyaBritishscientist[a]inwhichseveralpowertransformershavetheirprimarywindingsfedinparallelfromahigh-voltagedistributionline.Thesystemlitmorethan1000carbonfilamentlampsandoperatedsuccessfullyfromMayuntilNovemberofthatyear.[8] Alsoin1885GeorgeWestinghouse,anAmericanentrepreneur,obtainedthepatentrightstotheGaulard-GibbstransformerandimportedanumberofthemalongwithaSiemensgenerator,andsethisengineerstoexperimentingwiththeminhopesofimprovingthemforuseinacommercialpowersystem.In1886,oneofWestinghouse'sengineers,WilliamStanley,independentlyrecognizedtheproblemwithconnectingtransformersinseriesasopposedtoparallelandalsorealizedthatmakingtheironcoreofatransformerafullyenclosedloopwouldimprovethevoltageregulationofthesecondarywinding.[9]Usingthisknowledgehebuiltamulti-voltagetransformer-basedalternating-currentpowersystemservingmultiplehomesandbusinessesatGreatBarrington,Massachusettsin1886.[10]Thesystemwasunreliablethough(dueprimarilytogenerationissues)andshort-lived.[11]HoweverbasedonthatsystemWestinghousewouldbegininstallingACtransformersystemsincompetitionwiththeEdisoncompanylaterthatyear.In1888,WestinghouselicensedNikolaTesla'spatentsforapolyphaseACinductionmotorandtransformerdesigns.TeslaconsultedforayearattheWestinghouseElectric&ManufacturingCompany'sbutittookafurtherfouryearsforWestinghouseengineerstodevelopaworkablepolyphasemotorandtransmissionsystem.[12][13] By1889,theelectricpowerindustrywasflourishing,andpowercompanieshadbuiltthousandsofpowersystems(bothdirectandalternatingcurrent)intheUnitedStatesandEurope.Thesenetworkswereeffectivelydedicatedtoprovidingelectriclighting.DuringthistimetherivalrybetweenThomasEdisonandGeorgeWestinghouse'scompanieshadgrownintoapropagandacampaignoverwhichformoftransmission(directoralternatingcurrent)wassuperior,aseriesofeventsknownasthe"warofthecurrents".[14]In1891,Westinghouseinstalledthefirstmajorpowersystemthatwasdesignedtodrivea100horsepower(75 kW)synchronouselectricmotor,notjustprovideelectriclighting,atTelluride,Colorado.[15]OntheothersideoftheAtlantic,MikhailDolivo-DobrovolskyandCharlesEugeneLancelotBrown,builtthefirstlong-distance(175 km)high-voltage(15kV,thenarecord)three-phasetransmissionlinefromLauffenamNeckartoFrankfurtamMainfortheElectricalEngineeringExhibitioninFrankfurt,wherepowerwasusedtolightlampsandrunawaterpump.[16][9]IntheUnitedStatestheAC/DCcompetitioncametoanendwhenEdisonGeneralElectricwastakenoverbytheirchiefACrival,theThomson-HoustonElectricCompany,formingGeneralElectric.In1895,afteraprotracteddecision-makingprocess,alternatingcurrentwaschosenasthetransmissionstandardwithWestinghousebuildingtheAdamsNo.1generatingstationatNiagaraFallsandGeneralElectricbuildingthethree-phasealternatingcurrentpowersystemtosupplyBuffaloat11kV.[9] Developmentsinpowersystemscontinuedbeyondthenineteenthcentury.In1936thefirstexperimentalhighvoltagedirectcurrent(HVDC)lineusingmercuryarcvalveswasbuiltbetweenSchenectadyandMechanicville,NewYork.[17]HVDChadpreviouslybeenachievedbyseries-connecteddirectcurrentgeneratorsandmotors(theThurysystem)althoughthissufferedfromseriousreliabilityissues.[18][17]Thefirstsolid-statemetaldiodesuitableforgeneralpoweruseswasdevelopedbyErnstPresseratTeKaDein1928.Itconsistedofalayerofseleniumappliedonanaluminumplate.[19] In1957,aGeneralElectricresearchgroupdevelopedthefirstthyristorsuitableforuseinpowerapplications,startingarevolutioninpowerelectronics.Inthatsameyear,Siemensdemonstratedasolid-staterectifier,butitwasnotuntiltheearly1970sthatsolid-statedevicesbecamethestandardinHVDC,whenGEemergedasoneofthetopsuppliersofthyristor-basedHVDC.[20] In1979,aEuropeanconsortiumincludingSiemens,BrownBoveri&CieandAEGrealizedtherecordHVDClinkfromCaboraBassatoJohannesburg,extendingmorethan1,420 kmthatcarried1.9GWat533kV.[17] Inrecenttimes,manyimportantdevelopmentshavecomefromextendinginnovationsintheinformationandcommunicationstechnology(ICT)fieldtothepowerengineeringfield.Forexample,thedevelopmentofcomputersmeantloadflowstudiescouldberunmoreefficientlyallowingformuchbetterplanningofpowersystems.Advancesininformationtechnologyandtelecommunicationalsoallowedforeffectiveremotecontrolofapowersystem'sswitchgearandgenerators. Basicsofelectricpower[edit] Animationofthree-phasealternatingcurrent Electricpoweristheproductoftwoquantities:currentandvoltage.Thesetwoquantitiescanvarywithrespecttotime(ACpower)orcanbekeptatconstantlevels(DCpower). Mostrefrigerators,airconditioners,pumpsandindustrialmachineryuseACpowerwhereasmostcomputersanddigitalequipmentuseDCpower(digitaldevicespluggedintothemainstypicallyhaveaninternalorexternalpoweradaptertoconvertfromACtoDCpower).ACpowerhastheadvantageofbeingeasytotransformbetweenvoltagesandisabletobegeneratedandutilisedbybrushlessmachinery.DCpowerremainstheonlypracticalchoiceindigitalsystemsandcanbemoreeconomicaltotransmitoverlongdistancesatveryhighvoltages(seeHVDC).[21][22] TheabilitytoeasilytransformthevoltageofACpowerisimportantfortworeasons:Firstly,powercanbetransmittedoverlongdistanceswithlesslossathighervoltages.Soinpowersystemswheregenerationisdistantfromtheload,itisdesirabletostep-up(increase)thevoltageofpoweratthegenerationpointandthenstep-down(decrease)thevoltageneartheload.Secondly,itisoftenmoreeconomicaltoinstallturbinesthatproducehighervoltagesthanwouldbeusedbymostappliances,sotheabilitytoeasilytransformvoltagesmeansthismismatchbetweenvoltagescanbeeasilymanaged.[21] Solidstatedevices,whichareproductsofthesemiconductorrevolution,makeitpossibletotransformDCpowertodifferentvoltages,buildbrushlessDCmachinesandconvertbetweenACandDCpower.Nevertheless,devicesutilisingsolidstatetechnologyareoftenmoreexpensivethantheirtraditionalcounterparts,soACpowerremainsinwidespreaduse.[23] Componentsofpowersystems[edit] Supplies[edit] Themajorityoftheworld'spowerstillcomesfromcoal-firedpowerstationslikethis Allpowersystemshaveoneormoresourcesofpower.Forsomepowersystems,thesourceofpowerisexternaltothesystembutforothers,itispartofthesystemitself—itistheseinternalpowersourcesthatarediscussedintheremainderofthissection.Directcurrentpowercanbesuppliedbybatteries,fuelcellsorphotovoltaiccells.Alternatingcurrentpoweristypicallysuppliedbyarotorthatspinsinamagneticfieldinadeviceknownasaturbogenerator.Therehavebeenawiderangeoftechniquesusedtospinaturbine'srotor,fromsteamheatedusingfossilfuel(includingcoal,gasandoil)ornuclearenergytofallingwater(hydroelectricpower)andwind(windpower). Thespeedatwhichtherotorspinsincombinationwiththenumberofgeneratorpolesdeterminesthefrequencyofthealternatingcurrentproducedbythegenerator.Allgeneratorsonasinglesynchronoussystem,forexample,thenationalgrid,rotateatsub-multiplesofthesamespeedandsogenerateelectriccurrentatthesamefrequency.Iftheloadonthesystemincreases,thegeneratorswillrequiremoretorquetospinatthatspeedand,inasteampowerstation,moresteammustbesuppliedtotheturbinesdrivingthem.Thusthesteamusedandthefuelexpendeddirectlyrelatetothequantityofelectricalenergysupplied.AnexceptionexistsforgeneratorsincorporatingpowerelectronicssuchasgearlesswindturbinesorlinkedtoagridthroughanasynchronoustiesuchasaHVDClink—thesecanoperateatfrequenciesindependentofthepowersystemfrequency. Dependingonhowthepolesarefed,alternatingcurrentgeneratorscanproduceavariablenumberofphasesofpower.Ahighernumberofphasesleadstomoreefficientpowersystemoperationbutalsoincreasestheinfrastructurerequirementsofthesystem.[24]Electricitygridsystemsconnectmultiplegeneratorsoperatingatthesamefrequency:themostcommonbeingthree-phaseat50or60 Hz. Therearearangeofdesignconsiderationsforpowersupplies.Theserangefromtheobvious:Howmuchpowershouldthegeneratorbeabletosupply?Whatisanacceptablelengthoftimeforstartingthegenerator(somegeneratorscantakehourstostart)?Istheavailabilityofthepowersourceacceptable(somerenewablesareonlyavailablewhenthesunisshiningorthewindisblowing)?Tothemoretechnical:Howshouldthegeneratorstart(someturbinesactlikeamotortobringthemselvesuptospeedinwhichcasetheyneedanappropriatestartingcircuit)?Whatisthemechanicalspeedofoperationfortheturbineandconsequentlywhatarethenumberofpolesrequired?Whattypeofgeneratorissuitable(synchronousorasynchronous)andwhattypeofrotor(squirrel-cagerotor,woundrotor,salientpolerotororcylindricalrotor)?[25] Loads[edit] Atoasterisagreatexampleofasingle-phaseloadthatmightappearinaresidence.Toasterstypicallydraw2to10ampsat110to260voltsconsumingaround600to1200wattsofpower. Powersystemsdeliverenergytoloadsthatperformafunction.Theseloadsrangefromhouseholdappliancestoindustrialmachinery.Mostloadsexpectacertainvoltageand,foralternatingcurrentdevices,acertainfrequencyandnumberofphases.Theappliancesfoundinresidentialsettings,forexample,willtypicallybesingle-phaseoperatingat50or60 Hzwithavoltagebetween110and260volts(dependingonnationalstandards).Anexceptionexistsforlargercentralizedairconditioningsystemsasinsomecountriesthesearenowtypicallythree-phasebecausethisallowsthemtooperatemoreefficiently.Allelectricalappliancesalsohaveawattagerating,whichspecifiestheamountofpowerthedeviceconsumes.Atanyonetime,thenetamountofpowerconsumedbytheloadsonapowersystemmustequalthenetamountofpowerproducedbythesupplieslessthepowerlostintransmission.[26][27] Makingsurethatthevoltage,frequencyandamountofpowersuppliedtotheloadsisinlinewithexpectationsisoneofthegreatchallengesofpowersystemengineering.Howeveritisnottheonlychallenge,inadditiontothepowerusedbyaloadtodousefulwork(termedrealpower)manyalternatingcurrentdevicesalsouseanadditionalamountofpowerbecausetheycausethealternatingvoltageandalternatingcurrenttobecomeslightlyout-of-sync(termedreactivepower).Thereactivepowerliketherealpowermustbalance(thatisthereactivepowerproducedonasystemmustequalthereactivepowerconsumed)andcanbesuppliedfromthegenerators,howeveritisoftenmoreeconomicaltosupplysuchpowerfromcapacitors(see"Capacitorsandreactors"belowformoredetails).[28] Afinalconsiderationwithloadshastodowithpowerquality.Inadditiontosustainedovervoltagesandundervoltages(voltageregulationissues)aswellassustaineddeviationsfromthesystemfrequency(frequencyregulationissues),powersystemloadscanbeadverselyaffectedbyarangeoftemporalissues.Theseincludevoltagesags,dipsandswells,transientovervoltages,flicker,high-frequencynoise,phaseimbalanceandpoorpowerfactor.[29]Powerqualityissuesoccurwhenthepowersupplytoaloaddeviatesfromtheideal.Powerqualityissuescanbeespeciallyimportantwhenitcomestospecialistindustrialmachineryorhospitalequipment. Conductors[edit] Partiallyinsulatedmedium-voltageconductorsinCalifornia Conductorscarrypowerfromthegeneratorstotheload.Inagrid,conductorsmaybeclassifiedasbelongingtothetransmissionsystem,whichcarrieslargeamountsofpowerathighvoltages(typicallymorethan69kV)fromthegeneratingcentrestotheloadcentres,orthedistributionsystem,whichfeedssmalleramountsofpoweratlowervoltages(typicallylessthan69kV)fromtheloadcentrestonearbyhomesandindustry.[30] Choiceofconductorsisbasedonconsiderationssuchascost,transmissionlossesandotherdesirablecharacteristicsofthemetalliketensilestrength.Copper,withlowerresistivitythanaluminum,wasoncetheconductorofchoiceformostpowersystems.However,aluminumhasalowercostforthesamecurrentcarryingcapacityandisnowoftentheconductorofchoice.Overheadlineconductorsmaybereinforcedwithsteeloraluminiumalloys.[31] Conductorsinexteriorpowersystemsmaybeplacedoverheadorunderground.Overheadconductorsareusuallyairinsulatedandsupportedonporcelain,glassorpolymerinsulators.Cablesusedforundergroundtransmissionorbuildingwiringareinsulatedwithcross-linkedpolyethyleneorotherflexibleinsulation.Conductorsareoftenstrandedfortomakethemmoreflexibleandthereforeeasiertoinstall.[32] Conductorsaretypicallyratedforthemaximumcurrentthattheycancarryatagiventemperatureriseoverambientconditions.Ascurrentflowincreasesthroughaconductoritheatsup.Forinsulatedconductors,theratingisdeterminedbytheinsulation.[33]Forbareconductors,theratingisdeterminedbythepointatwhichthesagoftheconductorswouldbecomeunacceptable.[34] Capacitorsandreactors[edit] AsynchronouscondenserinstallationatTemplestowesubstation,Melbourne,Victoria ThemajorityoftheloadinatypicalACpowersystemisinductive;thecurrentlagsbehindthevoltage.Sincethevoltageandcurrentareout-of-phase,thisleadstotheemergenceofan"imaginary"formofpowerknownasreactivepower.Reactivepowerdoesnomeasurableworkbutistransmittedbackandforthbetweenthereactivepowersourceandloadeverycycle.Thisreactivepowercanbeprovidedbythegeneratorsthemselvesbutitisoftencheapertoprovideitthroughcapacitors,hencecapacitorsareoftenplacednearinductiveloads(i.e.ifnoton-siteatthenearestsubstation)toreducecurrentdemandonthepowersystem(i.e.increasethepowerfactor). Reactorsconsumereactivepowerandareusedtoregulatevoltageonlongtransmissionlines.Inlightloadconditions,wheretheloadingontransmissionlinesiswellbelowthesurgeimpedanceloading,theefficiencyofthepowersystemmayactuallybeimprovedbyswitchinginreactors.Reactorsinstalledinseriesinapowersystemalsolimitrushesofcurrentflow,smallreactorsarethereforealmostalwaysinstalledinserieswithcapacitorstolimitthecurrentrushassociatedwithswitchinginacapacitor.Seriesreactorscanalsobeusedtolimitfaultcurrents. Capacitorsandreactorsareswitchedbycircuitbreakers,whichresultsinmoderatelylargestepchangesofreactivepower.Asolutiontothiscomesintheformofsynchronouscondensers,staticVARcompensatorsandstaticsynchronouscompensators.Briefly,synchronouscondensersaresynchronousmotorsthatspinfreelytogenerateorabsorbreactivepower.[35]StaticVARcompensatorsworkbyswitchingincapacitorsusingthyristorsasopposedtocircuitbreakersallowingcapacitorstobeswitched-inandswitched-outwithinasinglecycle.Thisprovidesafarmorerefinedresponsethancircuit-breaker-switchedcapacitors.Staticsynchronouscompensatorstakethisastepfurtherbyachievingreactivepoweradjustmentsusingonlypowerelectronics. Powerelectronics[edit] ThisexternalhouseholdACtoDCpoweradapterusespowerelectronics Powerelectronicsaresemiconductorbaseddevicesthatareabletoswitchquantitiesofpowerrangingfromafewhundredwattstoseveralhundredmegawatts.Despitetheirrelativelysimplefunction,theirspeedofoperation(typicallyintheorderofnanoseconds[36])meanstheyarecapableofawiderangeoftasksthatwouldbedifficultorimpossiblewithconventionaltechnology.Theclassicfunctionofpowerelectronicsisrectification,ortheconversionofAC-to-DCpower,powerelectronicsarethereforefoundinalmosteverydigitaldevicethatissuppliedfromanACsourceeitherasanadapterthatplugsintothewall(seephoto)orascomponentinternaltothedevice.High-poweredpowerelectronicscanalsobeusedtoconvertACpowertoDCpowerforlongdistancetransmissioninasystemknownasHVDC.HVDCisusedbecauseitprovestobemoreeconomicalthansimilarhighvoltageACsystemsforverylongdistances(hundredstothousandsofkilometres).HVDCisalsodesirableforinterconnectsbecauseitallowsfrequencyindependencethusimprovingsystemstability.PowerelectronicsarealsoessentialforanypowersourcethatisrequiredtoproduceanACoutputbutthatbyitsnatureproducesaDCoutput.Theyarethereforeusedbyphotovoltaicinstallations. Powerelectronicsalsofeatureinawiderangeofmoreexoticuses.Theyareattheheartofallmodernelectricandhybridvehicles—wheretheyareusedforbothmotorcontrolandaspartofthebrushlessDCmotor.Powerelectronicsarealsofoundinpracticallyallmodernpetrol-poweredvehicles,thisisbecausethepowerprovidedbythecar'sbatteriesaloneisinsufficienttoprovideignition,air-conditioning,internallighting,radioanddashboarddisplaysforthelifeofthecar.Sothebatteriesmustberechargedwhiledriving—afeatthatistypicallyaccomplishedusingpowerelectronics.Whereasconventionaltechnologywouldbeunsuitableforamodernelectriccar,commutatorscanandhavebeenusedinpetrol-poweredcars,theswitchtoalternatorsincombinationwithpowerelectronicshasoccurredbecauseoftheimproveddurabilityofbrushlessmachinery.[37] SomeelectricrailwaysystemsalsouseDCpowerandthusmakeuseofpowerelectronicstofeedgridpowertothelocomotivesandoftenforspeedcontrolofthelocomotive'smotor.Inthemiddletwentiethcentury,rectifierlocomotiveswerepopular,theseusedpowerelectronicstoconvertACpowerfromtherailwaynetworkforusebyaDCmotor.[38]TodaymostelectriclocomotivesaresuppliedwithACpowerandrunusingACmotors,butstillusepowerelectronicstoprovidesuitablemotorcontrol.Theuseofpowerelectronicstoassistwiththemotorcontrolandwithstartercircuits,inadditiontorectification,isresponsibleforpowerelectronicsappearinginawiderangeofindustrialmachinery.Powerelectronicsevenappearinmodernresidentialairconditionersallowareattheheartofthevariablespeedwindturbine. Protectivedevices[edit] Mainarticle:powersystemprotection Amultifunctiondigitalprotectiverelaytypicallyinstalledatasubstationtoprotectadistributionfeeder ThePowersystemscontainprotectivedevicestopreventinjuryordamageduringfailures.Thequintessentialprotectivedeviceisthefuse.Whenthecurrentthroughafuseexceedsacertainthreshold,thefuseelementmelts,producinganarcacrosstheresultinggapthatisthenextinguished,interruptingthecircuit.Giventhatfusescanbebuiltastheweakpointofasystem,fusesareidealforprotectingcircuitryfromdamage.Fuseshoweverhavetwoproblems:First,aftertheyhavefunctioned,fusesmustbereplacedastheycannotbereset.Thiscanproveinconvenientifthefuseisataremotesiteorasparefuseisnotonhand.Andsecond,fusesaretypicallyinadequateasthesolesafetydeviceinmostpowersystemsastheyallowcurrentflowswellinexcessofthatthatwouldprovelethaltoahumanoranimal. Thefirstproblemisresolvedbytheuseofcircuitbreakers—devicesthatcanberesetaftertheyhavebrokencurrentflow.Inmodernsystemsthatuselessthanabout10 kW,miniaturecircuitbreakersaretypicallyused.Thesedevicescombinethemechanismthatinitiatesthetrip(bysensingexcesscurrent)aswellasthemechanismthatbreaksthecurrentflowinasingleunit.Someminiaturecircuitbreakersoperatesolelyonthebasisofelectromagnetism.Intheseminiaturecircuitbreakers,thecurrentisrunthroughasolenoid,and,intheeventofexcesscurrentflow,themagneticpullofthesolenoidissufficienttoforceopenthecircuitbreaker'scontacts(oftenindirectlythroughatrippingmechanism).Abetterdesign,however,arisesbyinsertingabimetallicstripbeforethesolenoid—thismeansthatinsteadofalwaysproducingamagneticforce,thesolenoidonlyproducesamagneticforcewhenthecurrentisstrongenoughtodeformthebimetallicstripandcompletethesolenoid'scircuit. Inhigherpoweredapplications,theprotectiverelaysthatdetectafaultandinitiateatripareseparatefromthecircuitbreaker.Earlyrelaysworkedbaseduponelectromagneticprinciplessimilartothosementionedinthepreviousparagraph,modernrelaysareapplication-specificcomputersthatdeterminewhethertotripbaseduponreadingsfromthepowersystem.Differentrelayswillinitiatetripsdependingupondifferentprotectionschemes.Forexample,anovercurrentrelaymightinitiateatripifthecurrentonanyphaseexceedsacertainthresholdwhereasasetofdifferentialrelaysmightinitiateatripifthesumofcurrentsbetweenthemindicatestheremaybecurrentleakingtoearth.Thecircuitbreakersinhigherpoweredapplicationsaredifferenttoo.Airistypicallynolongersufficienttoquenchthearcthatformswhenthecontactsareforcedopensoavarietyoftechniquesareused.Oneofthemostpopulartechniquesistokeepthechamberenclosingthecontactsfloodedwithsulfurhexafluoride(SF6)—anon-toxicgaswithsoundarc-quenchingproperties.Othertechniquesarediscussedinthereference.[39] Thesecondproblem,theinadequacyoffusestoactasthesolesafetydeviceinmostpowersystems,isprobablybestresolvedbytheuseofresidualcurrentdevices(RCDs).Inanyproperlyfunctioningelectricalappliance,thecurrentflowingintotheapplianceontheactivelineshouldequalthecurrentflowingoutoftheapplianceontheneutralline.Aresidualcurrentdeviceworksbymonitoringtheactiveandneutrallinesandtrippingtheactivelineifitnoticesadifference.[40]Residualcurrentdevicesrequireaseparateneutrallineforeachphaseandtobeabletotripwithinatimeframebeforeharmoccurs.Thisistypicallynotaprobleminmostresidentialapplicationswherestandardwiringprovidesanactiveandneutrallineforeachappliance(that'swhyyourpowerplugsalwayshaveatleasttwotongs)andthevoltagesarerelativelylowhowevertheseissueslimittheeffectivenessofRCDsinotherapplicationssuchasindustry.EvenwiththeinstallationofanRCD,exposuretoelectricitycanstillprovefatal. SCADAsystems[edit] Inlargeelectricpowersystems,supervisorycontrolanddataacquisition(SCADA)isusedfortaskssuchasswitchingongenerators,controllinggeneratoroutputandswitchinginoroutsystemelementsformaintenance.Thefirstsupervisorycontrolsystemsimplementedconsistedofapaneloflampsandswitchesatacentralconsolenearthecontrolledplant.Thelampsprovidedfeedbackonthestateoftheplant(thedataacquisitionfunction)andtheswitchesallowedadjustmentstotheplanttobemade(thesupervisorycontrolfunction).Today,SCADAsystemsaremuchmoresophisticatedand,duetoadvancesincommunicationsystems,theconsolescontrollingtheplantnolongerneedtobeneartheplantitself.Instead,itisnowcommonforplantstobecontrolledwithequipmentsimilar(ifnotidentical)toadesktopcomputer.Theabilitytocontrolsuchplantsthroughcomputershasincreasedtheneedforsecurity—therehavealreadybeenreportsofcyber-attacksonsuchsystemscausingsignificantdisruptionstopowersystems.[41] Powersystemsinpractice[edit] Despitetheircommoncomponents,powersystemsvarywidelybothwithrespecttotheirdesignandhowtheyoperate.Thissectionintroducessomecommonpowersystemtypesandbrieflyexplainstheiroperation. Residentialpowersystems[edit] Residentialdwellingsalmostalwaystakesupplyfromthelowvoltagedistributionlinesorcablesthatrunpastthedwelling.Theseoperateatvoltagesofbetween110and260volts(phase-to-earth)dependinguponnationalstandards.Afewdecadesagosmalldwellingswouldbefedasinglephaseusingadedicatedtwo-coreservicecable(onecorefortheactivephaseandonecorefortheneutralreturn).Theactivelinewouldthenberunthroughamainisolatingswitchinthefuseboxandthensplitintooneormorecircuitstofeedlightingandappliancesinsidethehouse.Byconvention,thelightingandappliancecircuitsarekeptseparatesothefailureofanappliancedoesnotleavethedwelling'soccupantsinthedark.Allcircuitswouldbefusedwithanappropriatefusebaseduponthewiresizeusedforthatcircuit.Circuitswouldhavebothanactiveandneutralwirewithboththelightingandpowersocketsbeingconnectedinparallel.Socketswouldalsobeprovidedwithaprotectiveearth.Thiswouldbemadeavailabletoappliancestoconnecttoanymetalliccasing.Ifthiscasingweretobecomelive,thetheoryistheconnectiontoearthwouldcauseanRCDorfusetotrip—thuspreventingthefutureelectrocutionofanoccupanthandlingtheappliance.Earthingsystemsvarybetweenregions,butincountriessuchastheUnitedKingdomandAustraliaboththeprotectiveearthandneutrallinewouldbeearthedtogethernearthefuseboxbeforethemainisolatingswitchandtheneutralearthedonceagainbackatthedistributiontransformer.[42] Therehavebeenanumberofminorchangesovertheyearstopracticeofresidentialwiring.Someofthemostsignificantwaysmodernresidentialpowersystemsindevelopedcountriestendtovaryfromolderonesinclude: Forconvenience,miniaturecircuitbreakersarenowalmostalwaysusedinthefuseboxinsteadoffusesasthesecaneasilyberesetbyoccupantsand,ifofthethermomagnetictype,canrespondmorequicklytosometypesoffault. Forsafetyreasons,RCDsarenowofteninstalledonappliancecircuitsand,increasingly,evenonlightingcircuits. Whereasresidentialairconditionersofthepastmighthavebeenfedfromadedicatedcircuitattachedtoasinglephase,largercentralisedairconditionersthatrequirethree-phasepowerarenowbecomingcommoninsomecountries. Protectiveearthsarenowrunwithlightingcircuitstoallowformetalliclampholderstobeearthed. Increasinglyresidentialpowersystemsareincorporatingmicrogenerators,mostnotably,photovoltaiccells. Commercialpowersystems[edit] Commercialpowersystemssuchasshoppingcentersorhigh-risebuildingsarelargerinscalethanresidentialsystems.Electricaldesignsforlargercommercialsystemsareusuallystudiedforloadflow,short-circuitfaultlevels,andvoltagedropforsteady-stateloadsandduringstartingoflargemotors.Theobjectivesofthestudiesaretoassureproperequipmentandconductorsizing,andtocoordinateprotectivedevicessothatminimaldisruptioniscausedwhenafaultiscleared.Largecommercialinstallationswillhaveanorderlysystemofsub-panels,separatefromthemaindistributionboardtoallowforbettersystemprotectionandmoreefficientelectricalinstallation. TypicallyoneofthelargestappliancesconnectedtoacommercialpowersysteminhotclimatesistheHVACunit,andensuringthisunitisadequatelysuppliedisanimportantconsiderationincommercialpowersystems.Regulationsforcommercialestablishmentsplaceotherrequirementsoncommercialsystemsthatarenotplacedonresidentialsystems.Forexample,inAustralia,commercialsystemsmustcomplywithAS2293,thestandardforemergencylighting,whichrequiresemergencylightingbemaintainedforatleast90minutesintheeventoflossofmainssupply.[43]IntheUnitedStates,theNationalElectricalCoderequirescommercialsystemstobebuiltwithatleastone20 Asignoutletinordertolightoutdoorsignage.[44]Buildingcoderegulationsmayplacespecialrequirementsontheelectricalsystemforemergencylighting,evacuation,emergencypower,smokecontrolandfireprotection. Powersystemmanagement[edit] Powersystemmanagementvariesdependinguponthepowersystem.Residentialpowersystemsandevenautomotiveelectricalsystemsareoftenrun-to-fail.Inaviation,thepowersystemusesredundancytoensureavailability.OntheBoeing747-400anyofthefourenginescanprovidepowerandcircuitbreakersarecheckedaspartofpower-up(atrippedcircuitbreakerindicatingafault).[45]Largerpowersystemsrequireactivemanagement.Inindustrialplantsorminingsitesasingleteammightberesponsibleforfaultmanagement,augmentationandmaintenance.Whereasfortheelectricgrid,managementisdividedamongstseveralspecialisedteams. Faultmanagement[edit] Faultmanagementinvolvesmonitoringthebehaviourofthepowersystemsoastoidentifyandcorrectissuesthataffectthesystem'sreliability.[46]Faultmanagementcanbespecificandreactive:forexample,dispatchingateamtorestringconductorthathasbeenbroughtdownduringastorm.Or,alternatively,canfocusonsystemicimprovements:suchastheinstallationofreclosersonsectionsofthesystemthataresubjecttofrequenttemporarydisruptions(asmightbecausedbyvegetation,lightningorwildlife).[47] Maintenanceandaugmentation[edit] Inadditiontofaultmanagement,powersystemsmayrequiremaintenanceoraugmentation.Asoftenitisneithereconomicalnorpracticalforlargepartsofthesystemtobeofflineduringthiswork,powersystemsarebuiltwithmanyswitches.Theseswitchesallowthepartofthesystembeingworkedontobeisolatedwhiletherestofthesystemremainslive.Athighvoltages,therearetwoswitchesofnote:isolatorsandcircuitbreakers.Circuitbreakersareload-breakingswitcheswhereasoperatingisolatorsunderloadwouldleadtounacceptableanddangerousarcing.Inatypicalplannedoutage,severalcircuitbreakersaretrippedtoallowtheisolatorstobeswitchedbeforethecircuitbreakersareagainclosedtoreroutepoweraroundtheisolatedarea.Thisallowsworktobecompletedontheisolatedarea.[48] Frequencyandvoltagemanagement[edit] Beyondfaultmanagementandmaintenanceoneofthemaindifficultiesinpowersystemsisthattheactivepowerconsumedpluslossesmustequaltheactivepowerproduced.Ifloadisreducedwhilegenerationinputsremainconstantthesynchronousgeneratorswillspinfasterandthesystemfrequencywillrise.Theoppositeoccursifloadisincreased.Assuchthesystemfrequencymustbeactivelymanagedprimarilythroughswitchingonandoffdispatchableloadsandgeneration.Makingsurethefrequencyisconstantisusuallythetaskofasystemoperator.[49]Evenwithfrequencymaintained,thesystemoperatorcanbekeptoccupiedensuring: equipmentorcustomersonthesystemarebeingsuppliedwiththerequiredvoltagereactivepowertransmissionisminimised(leadingtomoreefficientoperation)teamsaredispatchedandthesystemisswitchedtomitigateanyfaultsremoteswitchingisundertakentoallowforsystemworks[50] Notes[edit] ^SimplyreferredtointheliteratureasR.Kennedy[7] Seealso[edit] Powersystemsimulation References[edit] ^"GodalmingPowerStation".EngineeringTimelines.Retrieved3May2009. 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^"ANovelbutShort-LivedPowerDistributionSystem".IEEE.1May2005.Archivedfromtheoriginalon25June2007.Retrieved2008-05-25. ^Guarnieri,Massimo(2018)."SolidifyingPowerElectronics".IEEEIndustrialElectronicsMagazine.12:36–40.doi:10.1109/MIE.2018.2791062.hdl:11577/3271203.S2CID 4079824. ^GeneWolf(1December2000)."ElectricityThroughtheAges".Transmission&DistributionWorld. ^abAllAboutCircuits[Onlinetextbook],TonyR.Kuphaldtetal.,lastaccessedon17May2009. ^RobertoRudervall;J.P.Charpentier;RaghuveerSharma(7–8March2000)."HighVoltageDirectCurrent(HVDC)TransmissionSystemsTechnologyReviewPaper"(PDF).WorldBank.{{citejournal}}:Citejournalrequires|journal=(help)(alsohereArchived3March2016attheWaybackMachine) ^NedMohan;T.M.Undeland;WilliamP.Robbins(2003).PowerElectronics:Converters,Applications,andDesign.UnitedStatesofAmerica:JohnWiley&Sons,Inc.ISBN 0-471-22693-9. ^Chapman,Stephen(2002).ElectricMachineryandPowerSystemFundamentals.Boston:McGraw-Hill.pp. Chapter4.ISBN 0-07-229135-4. ^Chapman,Stephen(2002).ElectricMachineryandPowerSystemFundamentals.Boston:McGraw-Hill.pp. Chapters6and7.ISBN 0-07-229135-4. ^Electricityaroundtheworld,ConradH.McGregor,April2010. ^Whatareamps,watts,voltsandohms?,HowStuffWorks.com,31October2000.Lastaccessed:27June2010. ^Chapman,Stephen(2002).ElectricMachineryandPowerSystemFundamentals.Boston:McGraw-Hill.pp. Chapter11.ISBN 0-07-229135-4. ^Briefpowerqualitytutorialsforengineers,PSL,accessed21August2010. ^MarshallBrain,"HowPowerGridsWork",howstuffworks.com,1April2000. ^PracticalApplicationsofElectricalConductors,StefanFassbinder,DeutschesKupferinstitut,January2010. ^NavalEngineeringTrainingSeries(Figure1.6),U.S.Navy(republishedbytpub.com),2007. ^Conductorampacity,AllAboutCircuits,TonyR.Kuphaldtetal.,2000. ^Grigsby,Leonard(2007).ElectricPowerGeneration,Transmission,andDistribution.CRCPress2007.pp. Chapter14.ISBN 978-0-8493-9292-4. ^B.M.Weedy,ElectricPowerSystemsSecondEdition,JohnWileyandSons,London,1972,ISBN 0-471-92445-8page149 ^SwitchingCharacteristicsofThyristorsDuringTurn-OnArchived7July2012atarchive.today,[electricalandelectronics.org],April9,2009. ^"Air-conditionerManufacturerChoosesSmartPowerModules".PowerElectronicsTechnology.31August2005.Retrieved30March2016. ^Calverley,H.B.;Jarvis,E.A.K.;Williams,E.(1957)."Electricalequipmentforrectifierlocomotives".ProceedingsoftheIEE-PartA:PowerEngineering.104(17):341.doi:10.1049/pi-a.1957.0093. ^http://ocw.kfupm.edu.sa/user/EE46603/Circuit%20Breakers.pdf[bareURLPDF] ^HowdoesanRCDwork?Archived15February2010attheWaybackMachine,PowerBreaker,accessedon14-Mar-10. ^Report:hackonUkraine'spowergrid,KimZetter,WIRED,March3,2016. ^"TheMENSystemofEarthing"(PDF).ElectriciansNewsletterNo.1.OfficeofEnergy(WA):2.May2001.Archivedfromtheoriginal(PDF)on10March2011.Retrieved30Dec2010. ^"Emergencylightinganessentialservice". ^"CommercialLoads—Part2".ecmweb.com.25March2010.Retrieved6April2018. ^AviationKnowledge(2016).BoeingB747-400FCBT#31ElectricalSystem-OverviewandACPower.{{citeAVmedia}}:CS1maint:url-status(link) ^Lutfiyya,H.L.,Bauer,M.A.,Marshall,A.D.(2000)."FaultManagementinDistributedSystems:APolicy-DrivenApproach".JournalofNetworkandSystemsManagement.8(4):499–525.doi:10.1023/A:1026482400326.S2CID 41004116.{{citejournal}}:CS1maint:usesauthorsparameter(link) ^FaultManagementinElectricalDistributionSystems(PDF).FinalreportoftheCIREDWorkingGroupWG03FaultManagement(Report).1998.S2CID 44290460.Archivedfromtheoriginal(PDF)on27February2020. ^GauravJ(2018).DifferencebetweenCircuitbreakerandIsolator.Archivedfromtheoriginalon12December2021. ^S.Stoft.PowerSystemEconomics.IEEEPress,2002. ^PowerSystemRequirements(ReferencePaper)(PDF)(Report).AEMO.2020. Externallinks[edit] IEEEPowerEngineeringSociety PowerEngineeringInternationalMagazineArticles PowerEngineeringMagazineArticles AmericanSocietyofPowerEngineers,Inc. 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