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BASICS OF GOVERNING

P K CHAKRABORTY

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Functions of Governing System

To control the initial run up and synchronization of themachine

To control the turbine load when running in parallel withother generating sets.

To contain the speed rise within acceptable limits in caseof the unit getting disconnected from the load

To provide protective gears to ensure the safe operationof the turbine

Thus the individual TG governing system imply a need to

withstand a full load rejection safely

provide appropriate contributions to systemfrequency control.

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SPRING

DISC

TURBINE

G

E

NE

R

A

T

O

R

CONTROL

VALVE

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TURBINE

CONTROL

VALVE

GEN

M M M M

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TURBINE

ROTOR

WORM

GEAR

SPRING

FLY WEIGHT

FIXED

FULCRUM

CONTROL

VALVE

SLEEVE

V

0

maxn1

n3

n2

H

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8 BAR

2.5 MM 25 NB LINE

DRAIN

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TRANSIENT SPEED RISE

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TRANSIENT SPEED RISE

INCREASE OF SPEED ON FULL LOAD THROW OFF

7 TO 9 % OF RATED SPEED

RATED SPEED 3000 RPM 3210 TO 3270 RPM

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Governing Of Large Reheat MachinesOn reheat machines it is necessary to control not only the hp

valves but also the interceptor (IP) valves because on a fullload rejection the stored steam in the reheater and theassociated pipe lines could be sufficient to overspeed themachine to destruction. The characterstics of the IP valvesare such that they usually remain fully open over thenormal low range of the machine and the governing actionis achieved by throttling of the HP valves. This operatingregime is known as HP governing & is achieved by giving afixed bias to the IP valves.

However, the reheater control valves comes intoaction during certain abnormal conditions like fast loadrejection or low load conditions with excessively highreheater pressure, with the help of over riding controlsprovided by features like load sheding relay, IP trimmingdevice etc.

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What is Droop?

% of frequency change ( % of RPMchange) will lead to full travel of

Governing valves or full load change.

5% droop means: 5%*50 Hz= 2.5 Hz

Will change 500MW. In terms of outputtotal change should take place is 100%

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