Excitation System & capability curve of synchronous generator

Excitation systems perform control and protective functions essential to the satisfactory performance of the power system.

The amount of continuous reactive power a generator can supply is restricted by various limits. In the over-excitation region limits are imposed by rotor heating or amount of field current and second is the stator current. In the under excitation region the limits are imposed by load angle. So in steady state the generator should always operate within this region and the loci of the various limiters are called the capability curve of the generator.

1. Excitation
&
Capability Curve
of a Synchronous Generator
FB/TRAINING/E&I/GEN

2. Typical Configuration of a Power Plant

3. Excitation
•Thebasicpurposeofanexcitationsystemistoprovidedirectcurrent
tothesynchronousmachinefieldwinding
•Thebasiccontrolfunctionofexcitationsystemwithrespectto:
Generator:Tosupplyandadjustthefieldcurrentsoastomaintain
constantterminalvoltageandrespondtotransientdisturbancewithfield
forcinginaccordancewithgeneratorcapability.
Powersystem:Torespondquicklytoimprovetransientstabilityand
alsotomodulatefieldtoimprovepowersystemstability.
•Theexcitationsystemmusthaveameansofmeasuringgenerator
statorvoltageandcurrent.

33. Capability Curve
of
Synchronous
Generator

34. •TheCapabilityCurveofaSynchronousGeneratordefinesa
boundarywithinwhichthemachinecanoperatesafely.
•ItisalsoknownasOperatingChartsorCapabilityCharts.The
permissibleregionofoperationisrestrictedtothefollowing
pointsgivenbelow.
TheMVAloadingshouldnotexceedthegeneratorrating.Thislimitis
determinedbythearmatureofthestatorheatingbythearmature
current.
TheMWloadingshouldnotexceedtheratingoftheprimemover.
Thefieldcurrentshouldnotbeallowedtoexceedaspecifiedvalue
determinedbytheheatingofthefield.
Forsteadystateorstableoperation,theloadangleδmustbelessthan
90degrees.Thetheoreticalstabilitylimitofthestablecondition
occurswhenδ=90⁰.
GENERATOR CAPABILITY DIAGRAM

35. GENERATOR CAPABILITY DIAGRAM

36. GENERATOR CAPABILITY DIAGRAM
Armature current limit
Field current limit
Prime Mover Limit
End region heating limit
(1)ArmatureCurrentLimit/StatorCopperLoss(statorheating):Themaximumallowable
heatingofthestatorsetsamaximumphasecurrentI
Aforthemachine.It’sequivalentto
setamaximumapparentpowerforthemachine.(Powerfactorisirrelevant).
P
SCL=3I
A
2
R
S
(1)FieldCurrentLimit/RotorCopperLoss(rotorheating):Themaximumallowableheating
oftherotorsetsamaximumfieldcurrentI
Fforthemachine.It’sequivalenttoseta
maximumE
Aforthemachine.P
RCL=I
F
2
R
F
(3)Prime-mover’sPowerLimit:Theactivepoweroutputislimitedbytheprimemover
capability to a value within the MVA rating. P =|Esinδ|
Thelimitisrelatedtothemechanicalinputandtheabilityofthegeneratorto
electromagneticallycreateatorqueequalandoppositetothedrivingmechanicaltorque.
DEFINES THE OPERATING ZONE OF
A SYNCHRONOUS GENERATOR IN A
P-Q PLANE

37. END REGION HEATING LIMIT
(4)Thelocalizedheatingintheendregionofthearmatureimposes
athirdlimitontheoperationofasynchronousmachine.
This limit affects
the capability of
the machine in the
under excited
condition.

38. •Atagivenexcitation,ifmechanicaltorqueincreases,rotor
accelerates,increasingδandelectromagnetictorque.
•Thisnegativefeedbackcontinuesuntilelectromagneticand
mechanicaltorquesbalance.
•However,ifgeneratorisoperatingwithδcloseto90°whenrotor
speedsup,δincreasespast90°,electromagnetictorquefallsand
positivefeedbackoccurs,causingrotortoacceleratefurther,pull
outofsynchronismandresultinzerooutputpowerandpossibly
catastrophicfailure.Thestaticstabilitylimitissetatδ=90°.
GENERATOR CAPABILITY DIAGRAM
Power = 3 V
phI Cos φ= 3 V
phE Sin δ/ X.
The power or torque can be thought of as
cross product of two electromagnetic
fields or a function of the sine of angle
between Vand E.

39. •A capability diagram is a plot of complex power S=P+jQ
•its curve can be derived back from voltage phasor diagram of the Syn.
Gen.
SYNCHRONOUS GENERATOR

40. •capability curve must represent power limits of
generator, hence there is a need to convert the
voltage phasorinto power phasor.
•P=3 VφIAcosθ
•Q=3VφIAsinθ
•S= 3VφIA
•Reminding Pmax= 3 VφEA/ Xs
•The conversion factor to change scale of axes from
V VAis 3 Vφ/ Xs
SYNCHRONOUS GENERATOR

41. •P=3 VφIAcosθ= 3 Vφ/ Xs(XsIAcosθ)
•Q= 3VφIAsinθ= 3 Vφ/ Xs(XsIAsinθ)
•On voltage phasor diagram, origin of phasor diagram is at –
Vφon horizontal axis, so origin on power diagram is:
•Q = 3Vφ/Xs(-Vφ)=-3Vφ^2/Xs
•Field current ~ machine’s flux & flux ~ EA=kφω
•Length corresponding to EAon power diagram:
•DE=-3 EAVφ/ Xs
•IA~ XsIA, and length corresponding to XsIAon power
diagram is 3 VφIA
SYNCHRONOUS GENERATOR

42. FUNDAMENTALS

43. •The corresponding power units
SYNCHRONOUS GENERATORFUNDAMENTALS

44. FUNDAMENTALS
The rotor heating limit sets |E|max The stator heating limit sets |I |max.

45. FUNDAMENTALS

46. FUNDAMENTALS
Thefinallimitisrelatedtothemechanicalinputandtheabilityofthegenerator
toelectromagneticallycreateatorqueequalandoppositetothedriving
mechanicaltorque

47. FUNDAMENTALS

48. FUNDAMENTALS

49. •Generator capability curve a plot of P versus Q
GENERATOR CAPABILITY DIAGRAM

50. GENERATOR CAPABILITY DIAGRAM

51. Operational Limit

52. Capability Curve

53. GENERATOR CAPABILITY DIAGRAM

54. SYN. GENERATOR RATING
•Inanybalanceddesign,thethermallimitsforthe
fieldandarmatureintersectatapoint,which
representsthemachinenameplateMVAand
powerfactorrating.
•CapabilityDiagramgivesinformationaboutfull
loadrotor(excitationcurrent)&maximumrotor
angleduringsteadystateP.F.

55. SYNCHRONOUS GENERATOR
Capability Curve of KP
•A Gen. is rated 15MW, 6300V, 50 Hz, Y connected, 3000 rpm. at
18.75MVA at 0.8 PF lagging.
•It has a synchronous reactance of Xd=219.6 % per phase
•The friction and windagelosses are LkW, and core losses are M kW
Sketch capability curve for this generator.

56. Answer ?

57. Presented By:
Er. ManojKumar Maharana
HOD Electrical
Feedback Power O&M Services Ltd.