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AGC APPLICATION

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    AutomaticGenerationControl

    DrMSRMurty

    1

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    AutomaticGenerationControl(AGC)

    Automaticfrequencyregulation by

    governingsystemsofindividualturbine

    generatorsand

    AutomaticGenerationControl(AGC)orLoadfrequencycontrol(LFC)systemofthe power

    system.

    InEnergyManagementsystem(EMS)attheEnergyControlCenter(ECC)

    2

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    AGC

    AGC components

    Loadfrequencycontrol (LFC)

    EconomicDispatch(ED)

    InterchangeScheduling(IS) AGCisalsoreferredas

    SystemControl

    LoadDispatch

    3

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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

    12

    Pm

    PlTurbine Gen

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    SpeedChangeduetoloadimbalance

    Thegoverningsystem senseschangeinspeed

    andadjustssteamcontrolvalve(gate)sothatmechanicalpower(Pm) matcheswiththechangedload(Pl).

    Thechangeinfrequency()atsteadystatecanbedescribedusingtheDROOPequationintermsofchangeinload( Pl)andafactorR

    calledspeedregulationordroop. = [ Pl](R) Droopequation

    13

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    SingleTurbineGeneratorwithload

    [Pm Pl]=M[d/dt ] RotorInertiaEquation = [ Pl](R) Droopregulationequation

    14

    Pm

    PlTurbine Gen

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    LOADDROPRESPONSE

    Load

    100%

    Time(sec)

    t

    100%

    80%

    Speed

    (%)

    Speeddoes

    not

    return

    To100%

    15

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    ProportionalControl:Droop

    RotorInertia_

    +

    Load

    Generation

    SpeedReference

    Speed

    +

    Controlactionstopswhenthepowererrorhaszerovalue

    Speederror

    Speederrorpresentatsteady

    state

    Steadystate:

    Generation=Load,but

    Machinespeeddifferent

    fromSpeedsetpoint

    Proportional

    16

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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

    17

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    NEEDFORSUPPLEMENTARYCONTROL

    Speedvariationstopsbutatadifferentsteady

    value. Thespeedhoweverhastobebroughtbackto

    theoriginalvalueforwhichspeed/load

    referencehastobeadjustedeitherbythe

    operatororbyasupplementarycontrol

    system

    called Load

    Frequency

    Control

    (LFC)

    system

    18

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    19

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    20

    +

    Pref

    -

    CombinedMechanical

    Power

    +Composite Governor Composite Turbine

    Power SystemInertia

    BLOCK DIAGRAM SHOWING POWER SYSTEM FREQUENCY VARIATION

    Total Elec. load

    Frequency

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    21

    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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    22

    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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    23

    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)

    26

    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

    40

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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

    51

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    Example:

    Ifageneratorof1,000MWislostsomewhereinacontrolarea,frequencywilldecline.

    Theactualamountofdeclinewilldependon: Characteristicsoftheload(howmuchmotor

    load) Thetotalgovernorresponseavailable Numberofgeneratorsonline

    Theirrelativeloading Theirgovernorsettings

    52

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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

    54

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    Asmentioned,FrequencyResponseCharacteristic(FRC)istheactualresponseprovidedbycontrolareasforaparticularsetofevents.

    ControlareasuseAutomaticGeneratorControl(AGC)systemstomeettheirminutetominuteobligationstoservetheirinternalload.

    Whenanexcursionhappensexternaltoacontrolarea,there

    shouldbeanimmediateoutflowfromthecontrolareatoarrestfrequencydecline.

    Theoutflowitselfisfromloadrejectionandgovernor response.

    InordertopreventAGCfromfightingthisnaturalfrequency

    support,aBiastermisaddedtotheACEequation.

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