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AutomaticGenerationControl
DrMSRMurty
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AutomaticGenerationControl(AGC)
Automaticfrequencyregulation by
governingsystemsofindividualturbine
generatorsand
AutomaticGenerationControl(AGC)orLoadfrequencycontrol(LFC)systemofthe power
system.
InEnergyManagementsystem(EMS)attheEnergyControlCenter(ECC)
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AGC
AGC components
Loadfrequencycontrol (LFC)
EconomicDispatch(ED)
InterchangeScheduling(IS) AGCisalsoreferredas
SystemControl
LoadDispatch
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GeneratorTurbineGovernorBehavior
Generation(MechanicalPower) Load(ElectricalPower)imbalanceresultsinchangeinmachinespeed,
frequencyandpowerflow
Machineelectromechanicaldynamicsisdescribedby
swingequation
Asinglegeneratorandloadisanalyzedandthen
generalizedtolargesystem
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GeneratorTurbineGovernorBehavior
Pm
Pl
PmPl=M[d/dt]
Forsmallchangesinparameters
Pm Pl=M[d( )/dt]
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GeneratorTurbineGovernorBehavior
1/(Ms)
A
sustained
load
generation
imbalanced
would
lead
to
acontinuouschangeinfrequency!!
Pm(s)
Pl(s)
(s)
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Loadresponsetofrequencychange
ForRotatingcomponentsofloadtherealpowerincreases
withfrequency Pl(s)= Pl(s)+D (s)
Pl(s)nowisanincipientloadchange(amotorstarts)D (s)representstheresponsethattheadditionalloadcauses
frequencytodrop,allmotorsslowdown,andsoloaddropsas
D (s)
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GeneratorTurbineGovernorBehavior
Pm(s) Pl(s)D (s) =sM (s)
(s)=[ Pm(s) Pl(s)]/(Ms+D)
1/(Ms+D)
Pm(s)+
Pl(s)
(s)
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GeneratorTurbineGovernorBehavior
Measuresspeed(frequency)andadjustsvalvesto
changegeneration
Frequencydrops=>Raisegeneration
TheGovernor
Pm
Pl
Pe
Speed
Governor DesiredGeneration
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GeneratorTurbineGovernorBehavior
Pm
Pl
Pe
Speed
Governor
DesiredGeneration
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GeneratorTurbineGovernorBehavior
SteadyStateResponse
Steadystateerror
Usingenergybalance
Pl D (1/R) = 0
Load Load Generation
Change Response Changefrom
Governor
= Pl/(D+1/R)
TypicalR=0.05pu (5%factoryset)
For P=1,D=1,R=0.05 =1/21= 0.0476pu
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SingleTurbineGeneratorwithload
Forachangeinload,speed/frequency
changes(withgenerationremainingunchanged):
[Pm
Pl]=M[d/dt ] RotorInertiaEquation
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Pm
PlTurbine Gen
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SpeedChangeduetoloadimbalance
Thegoverningsystem senseschangeinspeed
andadjustssteamcontrolvalve(gate)sothatmechanicalpower(Pm) matcheswiththechangedload(Pl).
Thechangeinfrequency()atsteadystatecanbedescribedusingtheDROOPequationintermsofchangeinload( Pl)andafactorR
calledspeedregulationordroop. = [ Pl](R) Droopequation
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SingleTurbineGeneratorwithload
[Pm Pl]=M[d/dt ] RotorInertiaEquation = [ Pl](R) Droopregulationequation
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Pm
PlTurbine Gen
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LOADDROPRESPONSE
Load
100%
Time(sec)
t
100%
80%
Speed
(%)
Speeddoes
not
return
To100%
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ProportionalControl:Droop
RotorInertia_
+
Load
Generation
SpeedReference
Speed
+
Controlactionstopswhenthepowererrorhaszerovalue
Speederror
Speederrorpresentatsteady
state
Steadystate:
Generation=Load,but
Machinespeeddifferent
fromSpeedsetpoint
Proportional
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1.0
1.0
Power(p.u)
Speed(p.u)
DroopCharacteristic
0.5
1.02
0.0
1.04
1.0p.uor
100%change
0.04p.u
or4%
change
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NEEDFORSUPPLEMENTARYCONTROL
Speedvariationstopsbutatadifferentsteady
value. Thespeedhoweverhastobebroughtbackto
theoriginalvalueforwhichspeed/load
referencehastobeadjustedeitherbythe
operatororbyasupplementarycontrol
system
called Load
Frequency
Control
(LFC)
system
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+
Pref
-
CombinedMechanical
Power
+Composite Governor Composite Turbine
Power SystemInertia
BLOCK DIAGRAM SHOWING POWER SYSTEM FREQUENCY VARIATION
Total Elec. load
Frequency
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Set point
GeneratorPower
Frequency
Total Generation
TotalLoad
Primary regulation
Other m/c
ToOtherMachines
Set point AreaFreq-
uency
Secondary
regulation
-
- +
++
+
AUTOMATICLOAD REQUENCY
CONTROLLER
Governor Turbine GRIDINERTIA
Fig 7 AUTOMATIC LOAD RFEQUENCY CONTROL SYSTEM
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EnergyManagement
System(EMS)-AutomaticGenerationControl (AGC)
ElectroHydraulicGovernor(EHG)
ElectroHydraulicGovernor(EHG)
ElectroHydraulic
Governor(EHG)
ElectroHydraulicGovernor(EHG)
Turbine-Generator(TG)
Turbine-Generator(TG)
Turbine-Generator(TG)
Turbine-Generator
(TG)
Set Point
Set Point
Set Point
Set Point
Frequency (f)
f
f
f
SYSTEM CONTROL
CENTER (SCC)
HYDRO POWER PLANTS
Telemetry
------..
Generation Signals(MW)
System Frequency
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Load Frequency ControlLFC Implementation
FrequencyMeasuredAt a centralLocation Tie line flows(MW)
DesiredFrequency
Net Interchange
ACE
Filters K AllocationTo PlantsOther Considerations
PrefTo Units
Economic Dispatch SeverityActual Unit Movement Unit Energy BalanceMinimum Movement Response Rate Time error
~ every 4 sec
~ every 4 sec
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MultipleGeneratorsandAreas
Pm1
P1l
Pe1
Pm2
Pl2
Pe2
jX
Area1orGen1 TieLine Area2orGen2
Ptie
Nowlookattwogeneratorsorareasconnectedbyalineornetwork
IfloadchangesinanyareahowdofrequenciesandlinepowerPtiechange?
Wewillwanttorestorebothtonominalvalue
Asimplemodelforthelineisjustaseriesinductivereactance
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MultipleGeneratorsandAreas
Pm1
P1l
Pe1
Pm2
Pl2
Pe2
jX
Area1orGen1 TieLine Area2orGen2
Ptie
QualitativeResponseLoadincreaseinarea1
Area1frequencydrops
Area1voltagephaseanglefallsbehindare 2
Ptie decreases(stabilizesArea1frequency,dragsdownarea2)Area2frequencydrops
Bothgovernorsraisegeneration
SteadystateachievedatalowerfrequencyandPtie
Area1assistsArea2inmeetingtheloadincrease;frequencydropislower
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AreaControlError(ACE)
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TIELINE BIAS CONTROL.Inthiscontrolstrategy
eachareaofaninterconnectedsystemtriestoregulateitsareacontrolerror(ACE)tozero,
where:
Differencebetweentheactual(a)andthe
scheduled(s)
net
interchange
on
the
tie
lines.
Frequencyerror
Systemnaturalresponsecoefficient
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ACEGeneration
ACE>0,DECREASEGeneration
ACE
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LoadFrequencyControl Governorsensurethatfrequencyisrestoredtonear
nominal Thishappensirrespectiveoflocationof
load/generationchange
ThepurposeofLFCistoreallocategenerationso
Systemwidefrequencyisrestored
EachareameetsitsobligationLoad+Interchange
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LoadFrequencyControl Definition
AreaControlError(ACE)
ACE= NetInterchange+ f
NetInterchange=Interchangeerror
=Scheduled Actual
f= =frequencydeviation
=frequencybias(puMW/pufrequency)
Definitionissometimeswrittenwithnegativesignonbothterms
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LoadFrequencyControl BasicIdea
ACE>0 decreasegeneration
ACE
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LoadFrequencyControlPropertiesofACE
Aslongasonefrequencybias 0
IfallareashaveACE=0
then =0andall NetInterchange=0
DrivingACEtozerorestoresfrequencyandinterchange
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LoadFrequencyControlPropertiesofACE 1
Twoareas(lossignored)
ACE1= NetInterchange + 1
ACE2= NetInterchange + 2
=(ACE1+ACE2)/(1+ 2)
=0ifACE1=ACE2=0and 1+ 2 0ThenInterchangeerrorisalsozero
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LoadFrequencyControlPropertiesofACE 1
Twoareas(lossignored)ACE1= NetInterchange + 1
ACE2= NetInterchange + 2
=(ACE1+ACE2)/(1+ 2)
=0ifACE1=ACE2=0and 1+ 2 0
ThenInterchangeerrorisalsozeroReasonablevaluesof1, 2 willwork
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LoadFrequencyControlPropertiesofACE 2
Choose i=Di+1/RiIdeally
Net
Interchange
=
(Di+1/Ri)
PliNetInterchange+Di+1/Ri) =Pli
ACEi= PLi !!!!!!!!
Since1/Ri>>DiweknowDi+1/Riprettywell
ACEmeasuresarealoadchange shouldgiveusgoodcontrol
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FromLFCtoEconomicAllocation
Time
MW
Governor
~seconds
LFC
~minutes
Unitspickupload capacity
LFCdistributesbasedonresponse
Economicdispatch
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AGCScheme
FromGraingerandStevensonJr)
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FromLFCtoEconomicAllocation
Time
MWEconomicdispatch
Coal
Gas
Prefchanges
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TheEconomicdispatchproblem Given
Nunitsonline
SystemLoad+Loss
EqualsAreaNetGeneration NetInterchangewhenACE~0
Determine MWallocation(Schedule)foreachUnit
MinimizeFuelandVariableO&Mcost
Constraints Aconstrained Unitcapacity,Reserves optimizationproblem
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TheEconomicdispatchproblemMinimize
Pi
CT=C1(P1)+C2(P2)++CN(PN)
P1+P2++PN=PT
Pimin Pi Pimax i=1,2,,N
Ci(Pi)=Fuel+VariableO&Mcost($/H)uniti
Pi =NetMWoutputUnitI
Pimin,Pimax=MinandMaximumCapacityUniti
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LoadVariationwithfrequencyMotorloadinparticularisaffectedbyfrequency
Whenfrequencydrops,motorsslowdown,producelesswork,andconsumelessenergy
Frequencydropsby1%,motorloadwilldrop3%.
Nonmotorresistiveloadgenerallyremainsconstant.
Thenetforbothoftheaboveisageneralruleof
thumb:
+/ 1%changeinfreq.=+/ 2%changeinload
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Deadband
Anadditionalfeaturedisplayedbygenerators.
Deadband istheamountoffrequencychangeagovernormustseebeforeitstartstorespond.
Deadband wasreallyanaturalfeatureofthe
earliestgovernorscausedbygearlash(loosenessorslopinthegearmechanism)
Deadband servesausefulpurposebypreventinggovernorsfromcontinuouslyhuntingas
frequencyvarieseversoslightly
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Policy oftheNERC
Generatorswithnameplateratingsof10
MWor
more
must
have
governors
installed.
Governorsshouldprovide5%droop.
Deadband onall
governors
must
be
set
to
+/ 0.036Hz(on60Hzsystem)
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FRC FrequencyResponseorFrequencyResponse
Characteristic(FRC)isthechangeinfrequencythatoccursforachangeinloadresource
balanceinacontrolareaorinterconnection
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Example:
Ifageneratorof1,000MWislostsomewhereinacontrolarea,frequencywilldecline.
Theactualamountofdeclinewilldependon: Characteristicsoftheload(howmuchmotor
load) Thetotalgovernorresponseavailable Numberofgeneratorsonline
Theirrelativeloading Theirgovernorsettings
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Graphshowsfrequencyexcursions
vs.generationloss
Linerepresentstheaverage
frequencyresponseof1,500
MW/0.1Hz.
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Graphshowsfrequencyexcursions vs.generationloss
350eventsweretrackedinWECC from1994to2002 Linerepresentstheaverage frequencyresponseof1,500
MW/0.1Hz. Note:ThetotalFrequency ResponseinanInterconnection isthesumoftheresponses fromallcontrolareaswithinthe Interconnection
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Asmentioned,FrequencyResponseCharacteristic(FRC)istheactualresponseprovidedbycontrolareasforaparticularsetofevents.
ControlareasuseAutomaticGeneratorControl(AGC)systemstomeettheirminutetominuteobligationstoservetheirinternalload.
Whenanexcursionhappensexternaltoacontrolarea,there
shouldbeanimmediateoutflowfromthecontrolareatoarrestfrequencydecline.
Theoutflowitselfisfromloadrejectionandgovernor response.
InordertopreventAGCfromfightingthisnaturalfrequency
support,aBiastermisaddedtotheACEequation.
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