ModellingofSteamTurbineanditsGoverningSystem

DrMSRMurty

GENERATORCONTROLS

Grid

Reference

ST : steam turbineG : generator

SV : stop valve

CV

SVSteam

Speed

Power

GOVERNINGSYSTEM G

ST

CV contr. valve valve

Fig. 1 STEAM TURBINE GOVERNING SCHEME

SPEED

+

Valve

Position

SET

POINT -

Mechanical

Power

+GOVERNOR TURBINE ROTOR

INERTIA

Fig2GOVERNINGSYSTEMFUNCTIONALBLOCKDIAGRAM

GoverningControlsystem

SpeedSensing :Mechanical(Flyballtype),Hydraulic(Pump),Electric(toothedWheelpickup)

MWTransducerforPower Processing :Hydromechanical,Electrohydraulic,DigitalElectrohydraulic

Amplification:Hydraulicamplifiersinvariousstages

Actuation:HydraulicServomotor

TG Unit Operating Modes:Isolated : S Open : Speed changes when gov valve is adjustedInterconnected : S Closed or grid connected: Speed is unaffected

TGUnitOnGrid:LoadControl

TGSpeed/Frequencydoesnoteffectgridfrequency

Aftersynchronization,changeinvalveopeningcannotchangeTGunitspeedbutchangesonlyPowerOutput

InfiniteInertiaBusGRID

ControlRequirements Startup: To control machine speed forproper synchronization

Normal Operation: To Control MW and Toparticipate in the control of system frequency

Emergency: Load Rejection/Circuit BreakerOpeningTo restore speed deviation quickly withoutmuch transient overspeed

Governingsystem:Technology

MechanicalHydraulicControl(MHC) Electro HydraulicControl(EHC)Sensing,Processing,primaryamplificationusingelectroniccircuitry

Transistorversion(BHEL/KWUSiemensIskamaticmodules)

DigitalElectro HydraulicControl(DEHC)Microprocessorbased

Processing

Decideshowvalvepositionshouldbechangedwhenspeedchanges

Objective:Minimumupsetsinthesystem SpeedController,LoadController:structureandtuningdecidetransientperformance

Droopcharacteristic(4%or5%):importantgoverningparameter

ElectronicControllerFeatures

SeparationofSpeedControlandLoad(Power)Controlfunctionswithseparateprocessingphilosophy

SpeedController:Proportional Derivativeaction:AnticipatoryControl

LoadController:Proportional Integralaction InteractionwithATRS andTSE

SimpleSpeedgoverningsystem

/Gate

Command

GateOil

ServomotorSENSING

PROCESSING AMPLIFICATIONANDACTUATION

SETPOINT

Steam

Actuation of Valve (Servomotor)

Hydraulic Amplification

Speed & MW

PrimaryAmplification

Sensing

Processing

Electro-hydraulicConverter

ControlValve

EH

HYDRAULICPART

ST

G

ELECTRONICPART

Fig 3 ELECTRO HYDRAULIC GOVERNOR SCHEME

VALVEOPENINGCOMMAND

Load

Speed

+

+LoadRef.

SpeedRef.

SPEEDCONTROLLER

(PDP)

LOADCONTROLLER

( P I )

SELECTIONLOGIC

(MIN MAX)

Fig 4 SPEED CONTROLLER AND LOAD CONTROLLER IN EHG

DroopCharacteristic

SpeedChange

TurbinePowerChange

=(Load)

ValveOpeningChange

SteamFlowChange

Load%

SpeedOrFrequency

104%

100%

0 50 100

52Hz

50Hz

Frequency(Hz)

50

524% Drop

Load0% 50% 100%

Fig 6 REGULATION OR DROOP CHARACTERSTIC

Time (Sec)

Speed(%)

Unstable

Oscillatory (Hunting)

Fig 7 TYPICAL SPEED HUNTING TRANSIENT

ValveOpening

Dead band or insensitive zone

Speed / frequency

Fig. 8 DEAD BAND CHARACTERISTIC

STEAMTURBINESCHEMEWITHHPANDIPCONTROLS

Reheater

IPCV

HPCVSteam

Condenser

LPTHP

T

IPT G

R H

TransferFunctionofSteamVolume

Steam Vessel

Steam inflow Steam

outflow

Steam pressure

1----------(1 + TV. s)

Wi W o

Functionalblockdiagramofturbinegoverningsystem

TURBINECONTROLLER

29Feed forward provision

KSVSSpeed Controller: Proportional Derivative

Load Controller : Proportional Integral

Load

Load Ref

SpeedRef SPEED

CONTROLLER

LOADCONTROLLER

SELECTIONLOGIC

EH

To Hyd.Amplifier

speed

Ks(1+VsTs.S)(1 + Ts . S)droop

KPL + 1TILS

KS

PI

t

Output(Y)

t

PDP

K

Output(Y)

SpeedController LoadController

Parametersinfluencingtheperformance

RotorInertia Droop Speed/LoadControllerparameters Deadband Valvecharacteristics IPTurbinecontrol Pressurecontrolmodes

GovernorRegulationorDroop

4%Droop:4%SpeedChangewillcause100%changeinPowerOutput(Gain:25)

Droopisnecessaryfori)Sharingofloadii)Ensuringclosedloopstability:Lowervalueofdroopincreasesgainandmakesthesystemoscillatory

GoverningSystemResponse

Responsetimesareimportant:delayincorrectioncancausetransientspeedrisehighandtriptheturbine

Stabilityofgoverningsystemdependsonprocessingalgorithm(PI,PID,PDPetc.,)andonsystemparameters&timeconstants

Performancespecifications:LoadRejection

Rejectiontozeroloadfromanyload Speedshallbereturnedtothesetpointasmaybemodifiedbyspeeddrooporregulation

Nomorethanoneunderspeeddeviationexceeding5%

Nomorethanoneoverspeeddeviationexceeding5%afterinitialoverspeeddeviation

Performancespecifications:Sustainedconditions

Steadystategoverningspeedband:Notmorethan0.3%(atnoloadoranyload).Alsocalledspeedstabilityindex.

Steadystategoverningloadband:Notmorethan0.4%(at5%speeddroop)Alsocalledpowerstabilityindex.

StabilityIndex:Degreeofstability

Judgedbythemagnitudeofsustainedoscillationsofspeedandpoweroutputfromtheturbinethatareproducedbythegovernorsystem

Stabilityindexillustratestheregulatingperformanceforthegovernorandturbine

GovernorDeadbandillustratestheperformanceforthegovernoralone

LoadRejection

GoverningSystemPerformancecanbejudgedbyfullloadrejectionbehavior:TransientSpeedRise(TSR),hunting

EmergencyGovernorshouldnotgetactivated Influencingparameters:RotorAccelerationTime(Ta),Droop,SpeedControllergains,Incrementaldroop

LOADREJECTIONRESPONSE

Load

100%

Time(sec)

t

100%0%

Speed(%)

TSR(6 10%)

5%Droop

3FW

SH1

QDRUM

WW

BOILER

SH2

G

Q

150ata540C

HTP

IPT

COND

RH

.

.

.

FlueGas

Fuel

Air

DESH

InteractionwithBoilerControls

GOVERNOR

MasterPressureControl

DrumLevelcontrol

TemperatureControl

Spray

ExtractionSteamPressure

STEAM

CONDENSER

LPCVLOAD

SPEED

HPT

LPT

G

GOV,SYSTEM

HPCV

TO PROCESS

EXTRACTION TURBINE CONTROL SCHEME

StreamGenerators

LP

Process

VHP HP MPPRDS

Process ProcessProcess

HRSGs

UBs

G

TypicalSteamandPowerSystem

Headers

107/ 510 45/ 400 20/ 340 5.5/ 220

Kg/Cm2 o C

C

G

GAS TURBINE

GSteam Turbine

SteamandPowersystemDynamics

Steam System

Power System

Fuel System

Upsets in one system can influence the other

ModellingandSimulation

Instantaneousresponse

AssumesuddenLoadReductionandinstantaneouschangeingeneration

1.0

.8

Powerp.u Load,PL

Time(Sec.)

GenerationPg

Frequency

Time(Sec.)

50Hz

RotorInertia

Atsteadystate:TurbineTorque(Pm)=LoadTorque((Pel)

Duringtransient:Speed=((Pm Pel)/Ta)dt

Ta=Accelerationtimeorinertiaconstant(functionofmomentofinertia)

Typicalvalues:Ta=9 12sec

Acceleration/Deceleration

me

Pg

Deviationareaisindicativeofacceleration/deceleration

PL

PL me

Pg

DelayduetothecumulativeeffectsofGovernor,hydraulicpassages,turbine

n or f

t

Frequency or SpeedvariationDue to the above :

GovernorResponse

SettlingSpeedhigher

Xp

Hydraulicrelaystimeconstant

Inflow(Qi) Xp

Qi=volumetric

displacementofoil

=Ap.dxsm/dtKpXp=Apsxsm

Xsm

Xsm1

Tsm.S Xp

bLevergain

HPTURBINETIMECONSTSANT

T4=Steammassinsideturbine

MassflowthroughHPturbineKg/Sec207.4VolumexDensity=0.844(m3)(1/0.02337) =36.114

T4=36.114/207.4=0.17Sec.

VHPincludes: Volumeininletportionupto1st stage.

allpipingconnections blading

IPTurbine:0.27Sec

LPTurbine:0.47Sec

TReheaterr :10to20Sec.

NALCO

NALCO

GNFC Bharuch

GNFC Bharuch

GNFC Bharuch

GNFC Bharuch