Page 161 - Steam Turbines Design, Applications, and Rerating

Governors and Control Systems 141

1. Steam conditions constant
2. Speed changer set at rated conditions
3. Turbine under no external control device

% Speed regulation
(r/min @ zero output) − (r/min @ rated power output)
=
(0.01) (r/min at rated output)
Most steam turbines have some speed droop—i.e., there occurs a drop
in speed as the load is applied to the turbine. A governor that has no
droop, i.e., constant speed, with load changes, is termed an isochronous
governor. A NEMA, Class D governor is basically an isochronous gover-
nor. Its speed vs. load relationship is shown in Fig. 8.3a.

8.2.2 Speed variation
Speed variation, expressed as a percentage, is the total magnitude of
speed change, or fluctuations from the set speed. This characteristic
includes dead band and sustained oscillation.

% Speed variation
(change in r/min above set speed) + (change in r/min below set speed)
=
(0.01) (rated speed)

8.2.3 Dead band
Dead band is a characteristic of the speed-governing system that is
commonly known as wander. It is the insensitivity of the speed govern-
ing system that is defined as the total speed change during which there
is no resultant change in the position of the governing valves to com-
pensate for the speed change.

8.2.4 Stability
Stability is the measure of the ability of the speed-governing system to
position the governor-controlled valves so that sustained oscillations of
speed are not produced during a sustained load demand, or following a
change to a new load demand. A stable governor would conform to the
speed vs. time relationship depicted in Fig. 8.3b.
Extreme oscillation about the set point is commonly called hunting.

8.2.5 Speed rise
The governor must be capable of catching the speed increase when the
load is dropped instantaneously and return the system to the set point.
Speed rise is defined as the maximum momentary increase in speed