Electrical Power Systems

312 Electrical Power Systems
(b) Reduced block diagram of governor
ig. 12.7: Governor with load reference control for adjusting
frequency-load relationship.
rom the practical point of view, the adjustment of load reference set point is accomplished
by operating the ‘‘speed-changer motor.’’ ig. 12.8 shows the effect of this adjstment. amily of
parallel characteristics are shown in ig. 12.8 for different speed-changer motor settings.
ig. 12.8: Effect of speed-changer setting on governor characteristics.
The characteristics shown in ig.12.8 associated with 50 Hz system. Three characteristics
are shown representing three load reference settings. At 50 Hz, characteristic A results in zero
output, characteristic B results in 50% output and characteristic C results in 100% output.
Therefore, by adjusting the load reference setting (u) through actuation of the speed-changer
motor, the power output of the generating unit at a given speed may be adjusted to any desired
value. or each setting, the speed-load characteristic has a 6% droop; that is, a speed change of
6% (3Hz) causes a 100% change in power output.
12.9 TURBINE MODEL
All compound steam turbine systems utilize governor-controlled valves at the inlet to the high
pressure (or very high pressure) turbine to control steam flow. The steam chest and inlet piping
to the steam turbine cylinder and reheaters and crossover piping down stream all introduce
delays between the valve movement and change in steam flow. The mathematical model of the
steam turbine accounts for these delays.
igure 12.9 (a) shows a schematic diagram of a tandem compound single reheat steam
turbine and ig. 12.9 (b) shows the linear transfer function model of the tandem compound