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Modelling of Steam of Steam Turbine and its Governing System Dr M S R Murty
GENERATOR CONTROLS
SV : stop valve
SV Steam
CV contr. valve valve ST : steam turbine G : generator
CV
GOVERNING SYSTEM Reference
S T
Speed
Power
Fig. 1 STEAM TURBINE GOVERNING SCHEME
Grid G
SET POINT
+
-
GOVERNOR
TURBINE
+
ROTOR INERTIA
Valve Position
Mechanical Power
Fig 2 GOVERNING SYSTEM FUNCTIONAL BLOCK DIAGRAM
S P E E D
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 Bus GRID
Control Requirements • Start‐up:
To control machine speed proper synchronization
for
• Normal Operation:
To Control MW and To participate in the control of system frequency
• Emergency:
Load Rejection/Circuit Breaker
Opening To restore speed deviation quickly much transient overspeed
without
Governing system: Technology • Mechanical Hydraulic Control (MHC) • Electro‐ Hydraulic Control (EHC) Sensing,
Processing, primary amplification using electronic circuitry
Transistor
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
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 for i) Sharing of load ii) 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%
Speed (%)
TSR
0%
(6 ‐ 10%) t
5% Droop
100%
Time(sec)
Interaction with Boiler Controls DESH SH1
Drum Level control
SH2 150 at a 540ºC
Spray
DRUM Q
Temperature Control
3
FW WW
Fuel
. Flue . Gas .
Air
BOILER
Master Pressure Control
GOVERNOR
Q
RH
H T P
IP T
G
COND
STEAM HPCV
L P T
H P T
G
SPEED LOAD Extraction Steam Pressure
GOV, SYSTEM
LPCV CONDENSER
TO PROCESS EXTRACTION TURBINE CONTROL SCHEME
Typical Steam and Power System Headers Stream Generators
VHP
HP
PRDS
MP
LP
UBs
Steam Turbine
G G
HRSGs
C
G GAS TURBINE
Process
Process
107/ 510
45/ 400
Kg/Cm2
o
C
Process
20/ 340
Process
5.5/ 220
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 Power p.u
1.0
Frequency Load, PL 50 Hz .8 Generation Pg
Time (Sec.)
Time (Sec.)
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
Deviation area is indicative of acceleration/ deceleration Pg
PL
→ me
Delay due to the cumulative effects of Governor, hydraulic passages,turbine Pg
PL
→ me
Frequency or Speed variation Due to the above :
n or f Settling Speed higher
→
Governor Response
t
Hydraulic relays time constant ○
○
○
Xp
Xsm
Inflow(Qi) α ΔXp Qi = volumetric displacement of oil = Ap . dxsm/dt KpΔXp = Ap sxsm Δ Xp
1 ‐
Tsm .S
b
ΔXsm
Lever 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.
