Unit 2 Load Frequency Control ppt.ppt

Unit 2 Load Frequency Control
bASIC pRINCIPLE

1. •Introduction
•Reasonsforconstantfrequency
•MethodsofLoadfrequencycontrol
•LFC problemin SingleAreaPowerSystem
•LoadFrequencyControlofTwoAreaSystem
•AutomaticGenerationControl (AGC)

2. •Inanelectricpowersystem,LoadFrequencyControl(LFC)isa
systemtomaintainreasonablyuniformfrequency,todividetheload
betweenthegenerators,andtocontrolthetie-lineinterchange
schedules.
•Thechangeinfrequencyissensedwhentherotorangle∂is
changed.
•Theerrorsignalsaretransformedintorealpowercommandsignal,
whichissenttoprimemovertocallforanincrementinthetorque.
•Theprimemoverthenbringschangeinthegeneratoroutputbyan
amountwhichwillchangethevaluesofwithinthespecified
tolerance.

3. •Thespeedofthealternatingcurrentmotorsdependsonthefrequencyofthepower
supply.Therearesituationswherespeedconsistencyisexpectedtobeofhigh
order.
•The accuracy of the electric clocks are dependent on the frequency
of the supply.
•Ifthenormalfrequencyis50Hertzandthesystemfrequencyfallsbelow47.5
Hertzorgoesupabove52.5Hertzthenthebladesoftheturbinearelikelytoget
damagedsoastopreventthefailingofthegenerator.
•DuetothesubnormalfrequencyoperationtheblastoftheIDandFDfansinthe
powerstationsgetreducedandtherebyreducethegenerationpowerinthethermal
plants.

5. •Theloadfrequencycontrolstrategieshavebeensuggestedbasedonthe
conventionallinearControltheory.Thesecontrollersmaybeunsuitableinsome
operatingconditionsduetothecomplexityofthepowersystemssuchasnonlinear
loadcharacteristicsandvariableoperatingpoints.
•Undernormaloperatingconditioncontrolleraresetforsmallchangesinload
demandwithoutvoltageandfrequencyexceedingtheprespecifiedlimits.Ifthe
operatingconditionchangesbyanycause,thecontrollermustbereseteither
manuallyorautomatically.Theobjectiveofloadfrequencycontrolleristoexert
thecontrolofffrequencyandatthesametimerealpowerexchangeviaoutgoing
transmissionline.

6. •Thespeedgovernoristhemainprimarytoolfortheload
frequency
control(LFC).
•Figureshowsaschematicarrangementofaspeed
governingsystemusedonsteamturbinestocontrolthe
outputofthegeneratortomaintainconstantfrequency.
•Thespeedgoverningsystemconsistsofthefollowingparts
.
1)Speedgovernor
2)Linkagemechanism
3)Hydraulicamplifier
4)Speedchanger

8. Thisistheheartofthesystemwhichsensesthechangeinspeed(frequency).Asthe
speedincreasestheflyballsmoveoutwardsandthepointBonlinkagemechanism
movesdownwards.Thereversehappenswhenthespeeddecreases.
2.Hydraulicamplifier
•ItcomprisesapilotvalveandmainpistonLowpowerlevelpilotvalvemovementis
convertedintohighpowerlevelpistonvalvemovement.
• Thisisnecessaryinordertoopenorclosethesteamvalveagainsthighpressure
steam.

9. •Itprovidesasteadystatepoweroutputsettingfortheturbine.
•Itsdownwardmovementopenstheupperpilotvalvesothatmoresteamisadmittedtothe
turbineundersteadyconditions(hencemoresteadypoweroutput).
•Thereversehappensforupwardmovementofspeedchanger.
3. Linkage mechanism
•ABCisarigidlinkpivotedatBandCDEisanotherrigidlinkpivotedatThislink
mechanismprovidesamovementtothecontrolvalveinproportiontochangein
speed.
•Italsoprovidesafeedbackfromthesteamvalvemovement

10. Model of Speed GoverningSystem
•Assumethatthesystemisinitiallyoperatingundersteadyconditions—the
linkagemechanismstationaryandpilotvalveclosed,steamvalveopenedbya
definitemagnitude,turbinerunningatconstantspeedwithturbinepoweroutput
balancingthegeneratorload.Lettheoperatingconditionsbecharacterizedby
•Weshallobtainalinearincrementalmodelaroundtheseoperatingconditions.
•LetthepointAonthelinkagemechanismbemoveddownwardsbyasmall
amountΔ
yA.Itisacommandwhichcausestheturbinepoweroutputtochange
andcanthereforebewrittenas

11. •whereΔP
Cisthecommandedincreaseinpower.
•ThecommandsignalΔP
C(i.e.Δ
yE)setsintomotionasequenceofevents—thepilot
valvemovesupwards,highpressureoilflowsontothetopofthemainpistonmovingit
downwards;thesteamvalveopeningconsequentlyincreases,theturbinegenerator
speedincreases,
i.e.thefrequencygoesup.Letusmodeltheseeventsmathematically.
•TwofactorscontributetothemovementofC:
….(1)
….(3)

12. •ThemovementΔy
Ddependinguponitssignopensoneoftheportsofthepilotvalve
admittinghighpressureoilintothecylindertherebymovingthemainpistonand
openingthesteamvalvebyΔy
E.Certainjustifiablesimplifyingassumptions,which
canbemadeatthisstage,are:
•Inertialreactionforcesofmainpistonandsteamvalvearenegligiblecomparedto
theforcesexertedonthepistonbyhighpressureoil.
•Becauseof(i)above,therateofoiladmittedtothecylinderisproportionaltoport
openingΔy
D.
•Thevolumeofoiladmittedtothecylinderisthusproportionaltothetimeintegralof
Δy
D,.ThemovementΔy
Eisobtainedbydividingtheoilvolumebytheareaofthe
cross-sectionofthepiston.Thus
….(4)

13. •ItcanbeverifiedfromtheschematicdiagramthatapositivemovementΔy
Dcausesnegative
(upward)movementΔy
EaccountingforthenegativesignusedinaboveEq.
•TakingtheLaplacetransformofaboveEqs.(2),(3),and(4)weget
•EliminatingΔY
C(s)andΔY
D(s),wecanwrite
….(5)
….(6)
….(7)

14. •Where
•Equation (8) is represented in the form of a block diagram in Fig.

15. Turbine Model:
Let us now relate the dynamic response of a steam turbine in terms of changes in power
output to changes in steam valve opening Δy
E. Figure 8.4a shows a two stage steam
turbine with a reheat unit.
The dynamic response is largely influenced by two factors, (i) entrained steam between
the inlet steam valve and first stage of the turbine, (ii) the storage action in the reheater
which causes the output of the low pressure stage to lag behind that of the high pressure
stage.
Thus, the turbine transfer function is characterized by two time constants. For ease of
analysis it will be assumed here that the turbine can be modelled to have a single
equivalent time constant.
Figure 8.4b shows the transfer function model of a steam turbine. Typically the time
constant T
tlies in the range 0.2 to 2.5 sec.

17. AdjustmentofGovernorCharacteristics
•Thecontrolofsystemfrequencyandloaddependsuponthe
governor of the prime movers.
•The below figure shows the characteristics of the speed governor system

18. •Forstableoperationofgenerator,thegovernorsaredesignedto permitt the
speed to drop as the load is increased .
•Then slop of the curve represent the speed regulation R.
•Governors typically have a speed regulation of 5-6% from zero to full load.

19. Where

NL
=SteadystatespeedatNo-
Load

FL=Steadystatespeed at fullload

0
=Ratedspeed
Iftwoormorealternatorswithdropinggovenorcharacteristicsareconnectedtoa
powersystemtherewillbeuniquefrequencyatwhichtheywillsharealoadchange.

20. •Assme,twogeneratorunitswithdropingcharacteristicsas
showninabovefigure.
•LettheinitialfrequencyofboththeunitisfwithpoweroutputP1andP2whena
loadincreasestoPitcausestheunitstoslowdown/.
•Thegovernorincreasestheoutputuntiltheyreachanewcommonoperating
frequencyf.

21. •Anextendedpowersystemcanbedividedintoanumberofloadfrequencycontrol
areasinterconnectedbymeansoftielines.Withoutlossofgeneralitytwoarea
caseconnectedbytie-lineisconsidered.Thecontrolobjectivesareasfollows:
(1)Each controlarea as for as possibleshould supply its own load
demand and power transfer through tie line should be on mutual
agreement.
(2)Both control areas should controllable to the frequency control.

22. •Atwoareasystemconsistsoftwosingleareasystemsconnected
throughapowerlinecalledtie-line.Eachareafeedsitsuserpool,and
thetielineallowselectricpowertoflowbetweentheareas,because
bothareasaswellasthepowerflowonthetie-line.Forthesame
reason,thecontrolsystemofeachareaneedsinformationaboutthe
transientsituationinbothareastobringthelocalfrequencybacktoits
steadystatevalue.Informationaboutthelocalareaisfoundinthetie
linepowerfluctuations.Therefore,thetie-linepowerissensed,and
theresultingtie-linepowersignalisfedbackintobothareas.Itis
convenientlyassumedthateachcontrolareacanberepresentedbyan
equivalentturbine,generatorandgovernorsystem.

24. •Sometimes,loadonthesystemisincreasedsuddenlythenthe
turbinespeeddropsbeforethegovernorcanadjusttheinputofthe
steamtothenewload.
•Asthechangeinthevalueofspeeddiminishes,theerrorsignal
becomessmallerandthepositionofthegovernorgetclosertothe
pointrequiredtomaintaintheconstantspeed.
•Onewaytorestorethespeedorfrequencytoitsnominalvalueisto
addanintegratorontheway.
•Astheloadofthesystemchangescontinuouslythegenerationis
adjustedautomaticallytorestorethefrequencytothenominalvalue.
Thisschemeisknownasautomaticgenerationcontrol.
•Inaninterconnectedsystemconsistingofseveralpools,theroleof
theAGCistodividetheloadamongthesystem,stationsand
generatorssoastoachievemaximumeconomyandreasonably
uniformfrequency.

26. •Introduction
•Importanceofvoltagecontrol
•Methodsofvoltagecontrol
•Shuntcompensation
•Seriescapacitor
•Synchronouscondenser
•Tapchangingtransformer
•Autotransformertapchanging
•Boostertransformer

27. •Transmission of power from generating stationto
consumers.
•Constant voltage for satisfactory operation.
•Variation cause unpredictable operation or mal
functioning.
•Cause : change in load at supply side.
•Load ,
voltage due
to
volta
ge drop in alternator synchronous impedence,
transmission impedence, transformer impedence,
feeders and distributors.
•Prescribe limits : +-6% of declared voltage
•Voltage regulating device at suitable places.

28. •Lightingload:
Lampcharacterisicisverysensitivetochangeinvoltage.
•belowlimit20%
•Abovelimit50%
inilluminationpower.
inlife oflamp.
•Inductionmotor:
•involtagesaturationofpole
•involtagereduces startingtorque.
•DistributionTransformer:
•Duetoheating,ratingreduces.

29. •Usedat morethanonepoint dueto
•Desirabledrop intransmissionanddistribution.
•Dissimilarloadcharacteristics.
•Individualmeansofvoltagecontrolforeach circuit.
•Devicesusedat
•Generatingstation
•Transformerstation
•Feedersifdropexceedsthelimit

30. 1.Shuntcompensation
2.Seriescapacitor
3.Synchronouscondenser
4.Tapchangingtransformer
5.Autotransformertapchanging

31. ShuntReactor:
Usedtocompensateeffectlinecapacitance
limitvoltageriseonopencircuitorlightload
increaseseffectiveZ
C
Theyare connectedeither:
directlytothelines attheends
Tertiarywindingseasilyswitchedas VARvary.
Inlonglinesto overcomeferrantieffect.
ConnectedtobusbarwithoutC.Bforswitching.

32. Shunt Capacitor :
•Supply leading reactive power and boost the voltage
as loading of current reduces.
•Switching substation inductive load absorb
inductive current of lower P.F.
•They are connected either:
•H.V. bus
•Tertiary winding of transformers
•Advantage :
•low cost and flexibility of installation.
•Disadvantage :
•Q is proportional to (voltage)^2. So output reduces.

33. Seriescapacitor:
Connectedinserieswithline.
Usedtoreduce inductivereactanceoflineso reductionof
I
2Xloss
characteristicimpedanceZ
C
Reactivepowerproducedincreases withincreasingpower
transfer.
Application:
improvepower transfercapacity.
voltageregulation

35. A synchronous machine running without a prime mover or a
mechanical load
Depending on field excitation, it can either absorb or generate VARs
With a voltage regulator, it can automatically adjust VAR to maintain
constant voltage
Started as an induction motor and then synchronized
Normally connected to tertiary windings of transformers
Unlike a SVC, a synchronous condenser has an internal voltage
Speed of response not as fast as that of an SVC

36. •Off load Tap changing transformer :
•Position of tap number of turns output voltage.
•Stud 1 : min value
•Stud 5 : max value
•Light load primary voltage = alternator voltages and movable arm is
placed at stud 1.
•Load dropso movement of stud.

37. •OnloadTapchangingtransformer:
•widelyused so no interruptionofsupplyvoltage
•Secondarydividedintotwoparallelpathsocurrentdivided.
•Tapchangingoperationisperformedone afterother.
•Disadvantages:
•Voltage surgeduetohighvoltagedrop.
•Numoftapings=2 *voltagesteps.

38. •Midtappedautotransformer isused.
•Connectedwithonesideofline,divided into twoparts.
•Oddswitchesandevenswitches
•Normaloperationnodrop
•Tapchanginghigh droplargecirculatingcurrentflow
controlbyreactor.