E283C5.docx
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E283C5.docx
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E283C5
CHAPTER5–SYNCHRONOUSGENERATOR
Summary:
1.SynchronousGeneratorConstruction
2.TheSpeedofRotationofaSynchronousGenerator
3.TheInternalGeneratedVoltageofaSynchronousGenerator
4.TheEquivalentCircuitofaSynchronousGenerator
5.ThePhasorDiagramofaSynchronousGenerator
6.PowerandTorqueinSynchronousGenerator
7.MeasuringSynchronousGeneratorModelParameters
8.TheSynchronousGeneratorOperatingAlone
-TheEffectofLoadChangesonaSynchronousGeneratorOperatingAlone.
9.ParalleloperationofACGenerators
-Theconditionsrequiredforparalleling
-Thegeneralprocedureforparallelinggenerators
-Frequency-powerandVoltage-ReactivePowercharacteristicsofasynchronousgenerator.
-Operationofgeneratorsinparallelwithlargepowersystems
-Operationofgeneratorsinparallelwithothergeneratorsofthesamesize.
10.SynchronousGeneratorRatings
-TheVoltage,SpeedandFrequencyRatings
-ApparentPowerandPower-FactorRatings
-SynchronousGeneratorCapabilityCurve
1.SynchronousGeneratorConstruction
ADCcurrentisappliedtotherotorwinding,whichthenproducesarotormagneticfield.Therotoristhenturnedbyaprimemover(eg.Steam,wateretc.)producingarotatingmagneticfield.Thisrotatingmagneticfieldinducesa3-phasesetofvoltageswithinthestatorwindingsofthegenerator.
“Fieldwindings”appliestothewindingsthatproducethemainmagneticfieldinamachine,and“armaturewindings”appliestothewindingswherethemainvoltageisinduced.Forsynchronousmachines,thefieldwindingsareontherotor,sotheterms“rotorwindings”and“fieldwindings”areusedinterchangeably.
Generallyasynchronousgeneratormusthaveatleast2components:
a)RotorWindingsorFieldWindings
a.SalientPole
b.NonSalientPole
b)StatorWindingsorArmatureWindings
Therotorofasynchronousgeneratorisalargeelectromagnetandthemagneticpolesontherotorcaneitherbesalientornonsalientconstruction.Non-salientpolerotorsarenormallyusedforrotorswith2or4polesrotor,whilesalientpolerotorsareusedfor4ormorepolesrotor.
Non-salientrotorforasynchronousmachine
Salientrotor
Adccurrentmustbesuppliedtothefieldcircuitontherotor.Sincetherotorisrotating,aspecialarrangementisrequiredtogetthedcpowertoitsfieldwindings.Thecommonwaysare:
a)supplythedcpowerfromanexternaldcsourcetotherotorbymeansofslipringsandbrushes.
b)Supplythedcpowerfromaspecialdcpowersourcemounteddirectlyontheshaftofthesynchronousgenerator.
Slipringsaremetalringscompletelyencirclingtheshaftofamachinebutinsulatedfromit.Oneendofthedcrotorwindingistiedtoeachofthe2slipringsontheshaftofthesynchronousmachine,andastationarybrushridesoneachslipring.
A“brush”isablockofgraphitelikecarboncompoundthatconductselectricityfreelybuthasverylowfriction,henceitdoesn’tweardowntheslipring.Ifthepositiveendofadcvoltagesourceisconnectedtoonebrushandthenegativeendisconnectedtotheother,thenthesamedcvoltagewillbeappliedtothefieldwindingatalltimesregardlessoftheangularpositionorspeedoftherotor.
Someproblemswithslipringsandbrushes:
-Theyincreasetheamountofmaintenancerequiredonthemachine,sincethebrushesmustbecheckedforwearregularly.
-Brushvoltagedropcanbethecauseofsignificantpowerlossesonmachineswithlargerfieldcurrents.
Smallsynchronousmachines–useslipringsandbrushes.
Largermachines–brushlessexcitersareusedtosupplythedcfieldcurrent.
Abrushlessexciterisasmallacgeneratorwithitsfieldcircuitmountedonthestatoranditsarmaturecircuitmountedontherotorshaft.The3-phaseoutputoftheexcitergeneratorisrectifiedtodirectcurrentbya3-phaserectifiercircuitalsomountedontheshaftofthegenerator,andisthenfedtothemaindcfieldcircuit.Bycontrollingthesmalldcfieldcurrentoftheexcitergenerator(locatedonthestator),wecanadjustthefieldcurrentonthemainmachinewithoutslipringsandbrushes.Sincenomechanicalcontactsoccurbetweentherotorandstator,abrushlessexciterrequireslessmaintenance.
Abrushlessexcitercircuit:
Asmall3-phasecurrentisrectifiedandusedtosupplythefieldcircuitoftheexciter,whichislocatedonthestator.Theoutputofthearmaturecircuitoftheexciter(ontherotor)isthenrectifiedandusedtosupplythefieldcurrentofthemainmachine.
Tomaketheexcitationofageneratorcompletelyindependentofanyexternalpowersources,asmallpilotexcitercanbeused.
Apilotexciterisasmallacgeneratorwithpermanentmagnetsmountedontherotorshaftanda3-phasewindingonthestator.Itproducesthepowerforthefieldcircuitoftheexciter,whichinturncontrolsthefieldcircuitofthemainmachine.Ifapilotexciterisincludedonthegeneratorshaft,thennoexternalelectricpowerisrequired.
Abrushlessexcitationschemethatincludesapilotexciter.Thepermanentmagnetsofthepilotexciterproducethefieldcurrentoftheexciter,whichinturnproducesthefieldcurrentofthemainmachine.
Eventhoughmachineswithbrushlessexcitersdonotneedslipringsandbrushes,theystillincludetheslipringsandbrushessothatanauxiliarysourceofdcfieldcurrentisavailableinemergencies.
2.TheSpeedofRotationofaSynchronousGenerator
Synchronousgeneratorsarebydefinitionsynchronous,meaningthattheelectricalfrequencyproducedislockedinorsynchronizedwiththemechanicalrateofrotationofthegenerator.Asynchronousgenerator’srotorconsistsofanelectromagnettowhichdirectcurrentissupplied.Therotor’smagneticfieldpointsinthedirectiontherotoristurned.Hence,therateofrotationofthemagneticfieldinthemachineisrelatedtothestatorelectricalfrequencyby:
3.TheInternalGeneratedVoltageofaSynchronousGenerator
Voltageinducedisdependentuponfluxandspeedofrotation,hencefromwhatwehavelearntsofar,theinducedvoltagecanbefoundasfollows:
Forsimplicity,itmaybesimplifiedtoasfollows:
4.TheEquivalentCircuitofaSynchronousGenerator
ThevoltageEAistheinternalgeneratedvoltageproducedinonephaseofasynchronousgenerator.Ifthemachineisnotconnectedtoaload(noarmaturecurrentflowing),theterminalvoltagewillbeequivalenttothevoltageinducedatthestatorcoils.Thisisduetothefactthattherearenocurrentflowinthestatorcoilshencenolosses.Whenthereisaloadconnectedtothegenerator,therewillbedifferencesbetweenEAandVφ.Thesedifferencesaredueto:
a)Distortionoftheairgapmagneticfieldbythecurrentflowinginthestatorcalledarmaturereaction.
b)Selfinductanceofthearmaturecoil
c)Resistanceofthearmaturecoils
d)Theeffectofsalientpolerotorshapes.
Wewillexplorefactorsa,b,andcandderiveamachinemodelfromthem.Theeffectofsalientpolerotorshapewillbeignored,andallmachinesinthischapterareassumedtohavenonsalientorcylindricalrotors.
ArmatureReaction
Whentherotorisspun,avoltageEAisinducedinthestatorwindings.Ifaloadisattachedtotheterminalsofthegenerator,acurrentflows.Buta3-phasestatorcurrentflowwillproduceamagneticfieldofitsown.Thisstatormagneticfieldwilldistortstheoriginalrotormagneticfield,changingtheresultingphasevoltage.Thiseffectiscalledarmaturereactionbecausethearmature(stator)currentaffectsthemagneticfield,whichproduceditinthefirstplace.
Refertothediagramsbelow,showingatwo-polerotorspinninginsidea3-phasestator.
(a)ArotatingmagneticfieldproducestheinternalgeneratedvoltageEA.
(b)Theresultingvoltageproducesalaggingcurrentflowwhenconnectedtoalaggingload.
(c)ThestatorcurrentproducesitsownmagneticfieldBSwhichproducesitsownEstatinthestatorwindings.
(d)ThefieldBSaddstoBRdistortingitintoBnet.ThevoltageEstataddstoEA,producingVφattheoutputofthephase.
(a)Thereisnoloadconnectedtothestator.TherotormagneticfieldBRproducesaninternalgeneratedvoltageEAwhosepeakcoincideswithdirectionofBR.Withnoload,thereisnoarmaturecurrentandEAwillbeequaltothephasevoltageVφ.
(b)Whenalaggingloadisconnected,thepeakcurrentwilloccuratananglebehindthepeakvoltage.
(c)Thecurrentflowinginthestatorwindingsproducesamagneticfieldofitsown.ThisstatormagneticfieldBSanditsdirectionaregivenbytheright-handrule.ThestatorfieldproducesavoltageofitsowncalledEstat.
(d)With2voltagesand2magneticfieldspresentinthestatorwindings,thetotalvoltageandthenetmagneticfieldare:
Howcantheeffectsofarmaturereactiononthephasevoltagebemodeled?
-ThevoltageEstatliesatanangleof90°behindtheplaneofIA.
-ThevoltageEstatisdirectlyproportionaltothecurrentIA.
IfXisaconstantofproportionality,thenthearmaturereactionvoltagecanbeexpressedas:
Therefore:
Thus,thearmaturereactionvoltagecanbemodeledasaninductorinserieswiththeinternalgeneratedvoltage.
Self-inductanceandResistanceoftheArmatureCoils
Ifthestatorself-inductanceiscalledLA(reactanceisXA)whilethestatorresistanceiscalledRA,thenthetotaldifferencebetweenEAandVφis:
WhereXS=X+XA
Thefullequivalentcircuitisshownbelow:
Adcpowersourceissupplyingtherotorfieldcircuit,whisismodeledbythecoil’sinductanceandresistanceinseries.InserieswithRFisanadjustableresistorRadjwhi
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