Modelling of Steam Turbine and its Governing System

Dr M S R Murty

GENERATOR CONTROLS

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

Fig 2 GOVERNING SYSTEM FUNCTIONAL BLOCK DIAGRAM

Governing Control system

• Speed Sensing :Mechanical (Fly ball type), Hydraulic (Pump), Electric (toothed  Wheel pick up)

• MW Transducer for Power• Processing : Hydro‐mechanical, Electro‐hydraulic, Digital Electro‐hydraulic

• Amplification: Hydraulic amplifiers in various stages

• Actuation: Hydraulic Servomotor

TG Unit Operating Modes:Isolated : S Open : Speed changes when gov valve is adjusted

Interconnected : S Closed or grid connected: Speed is unaffected

TG Unit On Grid: Load Control

• TG Speed/Frequency does not effect grid frequency

• After synchronization, change in valve opening cannot change TG unit speed but changes only Power Output

Infinite Inertia BusGRID

Control Requirements• Start‐up: 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

Governing system: Technology

• Mechanical Hydraulic Control (MHC)• Electro‐ Hydraulic Control (EHC)Sensing, Processing, primary amplification using electronic circuitryTransistor version (BHEL/ KWU Siemens Iskamatic modules)

• Digital Electro‐ Hydraulic Control (DEHC)Microprocessor based

Processing

• Decides how valve position should be changed when  speed changes 

• Objective : Minimum upsets in the system• Speed Controller, Load Controller : structure and tuning decide transient performance

• Droop characteristic ( 4 % or 5%): important governing parameter 

Electronic Controller Features

• Separation of Speed Control and Load (Power) Control functions with separate processing philosophy

• Speed Controller: Proportional‐ Derivative action: Anticipatory Control

• Load Controller: Proportional‐ Integral action• Interaction with ATRS and TSE

Simple Speed governing system

/Gate

Command

GateOil

ServomotorSENSING

PROCESSING AMPLIFICATION AND ACTUATION

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

Droop Characteristic

Speed Change

Turbine Power  Change

= (Load)

Valve Opening Change

Steam Flow Change

Load %

Speed Or Frequency

104 %

100 %

0 50 100

52 Hz

50 Hz

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

STEAM TURBINE SCHEME WITH HP AND IP CONTROLS

Reheater

IPCV

HPCVSteam

Condenser

LPTHPT

IPT G

R H

Transfer Function of Steam Volume

Steam Vessel

Steam inflow Steam

outflow

Steam pressure

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

Wi W o

Functional block diagram of turbine governing system

TURBINE CONTROLLER

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)

Speed Controller Load Controller

Parameters influencing the performance

• Rotor Inertia• Droop• Speed /Load Controller  parameters• Dead band• Valve characteristics• IP Turbine control• Pressure control modes

Governor Regulation or Droop

• 4 % Droop : 4 % Speed Change will cause 100 % change in Power Output ( Gain : 25)

• Droop is necessary fori) Sharing of loadii) Ensuring closed loop stability : Lower value of droop increases gain and makes the system oscillatory

Governing System Response

• Response times are important : delay in correction can cause transient speed rise high and trip the turbine

• Stability of governing system depends on processing algorithm ( PI, PID , PDP etc.,) and on system parameters & time constants 

Performance specifications: Load Rejection

• Rejection to zero load from any load• Speed shall be returned to the set point as may be modified by speed droop or regulation

• No more than one under speed deviation exceeding 5 % 

• No more than one over speed deviation exceeding 5 % after initial over speed deviation

Performance specifications: Sustained conditions

• Steady state governing speed band: Not more than 0.3 % ( at no load or any load). Also called speed stability index.

• Steady state governing load band: Not more than 0.4 % ( at 5 % speed droop) Also called power stability index.

Stability Index: Degree of stability

• Judged by the magnitude of sustained oscillations of speed and power output from the turbine that are produced by the governor system

• Stability index illustrates the regulating performance for the governor and turbine

• Governor Deadband illustrates the performance for the governor alone 

Load Rejection 

• Governing System Performance can be judged by full load rejection behavior : Transient Speed Rise (TSR), hunting

• Emergency Governor should not get activated• Influencing parameters : Rotor Acceleration Time ( Ta), Droop, Speed Controller gains, Incremental droop

LOAD REJECTION RESPONSE 

Load

100%

Time(sec)

t

100%0%

Speed(%)

TSR(6 ‐ 10%)

5% Droop 

3

FW

SH1

QDRUM

WW

BOILER

SH2

G

Q

150 at a    540ºC

HTP

IPT

COND

RH

.

.

.

Flue Gas

Fuel

Air

DESH

Interaction  with Boiler Controls

GOVERNOR

Master PressureControl

Drum Levelcontrol

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

Typical Steam and Power System

Headers

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

Kg/Cm2 o C

C

G

GAS TURBINE

GSteam Turbine

Steam and Power system Dynamics

Steam System

Power System

Fuel System

Upsets in one system can influence the other

Modelling and Simulation

Instantaneous  response

• Assume sudden Load Reduction and instantaneous change in generation

1.0

.8

Powerp.u Load, PL

Time (Sec.)

Generation Pg

Frequency

Time (Sec.)

50 Hz

Rotor Inertia

• At steady state :Turbine Torque (Pm) = Load Torque( (Pel)

• During transient :Speed = ∫((Pm – Pel) / Ta ) dt                                         

• Ta = Acceleration time or inertia constant (function of moment of inertia) 

Typical values: Ta = 9 ‐ 12 sec 

Acceleration/ Deceleration

→  me

Pg

Deviation area is indicative ofacceleration/ deceleration

PL

PL →  me

Pg

Delay due to the cumulative effects of Governor, hydraulic passages,turbine

n or f

→ t

Frequency or SpeedvariationDue to the above :

Governor Response 

Settling Speed higher 

‐

Xp○ ○○

Hydraulic relays time constant

Inflow(Qi) α ΔXp

Qi = volumetric      

displacement of oil

= Ap . dxsm/dtKpΔXp = Ap sxsm

Xsm

ΔXsm1

Tsm .SΔ Xp

bLever gain

HP TURBINE TIME CONSTSANT

T4  =            Steam mass inside turbine

Mass flow through HP turbine   → Kg / Sec 207.4 

Volume x Density= 0.844 (m3) (1 / 0.02337) = 36.114

T4  = 36.114 / 207.4 = 0.17 Sec.

VHP includes  : ‐ Volume in inlet portion up to 1st stage.‐ all piping connections‐ blading

IP Turbine    :  0.27 Sec

LP Turbine   :  0.47 Sec

TReheaterr :  10 to 20 Sec.

NALCO

NALCO

GNFC Bharuch

GNFC Bharuch

GNFC Bharuch

GNFC Bharuch