Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 143

Frequency is thus an appropriate signal for monitoring the balance between elec-
trical supply and demand. Consequently, as seen in Figure 5.4, the turbine-governor
adjusts the steam valve position to control the mechanical power output. The
governor also monitors the rotor speed, w , which is used as a feedback signal to
control the balance between the mechanical input power and the electrical output
power. The traditional speed-sensing device would have been a Watt centrifugal
governor. However, with a desire for a high speed of response and accuracy in
speed and load control, these mechanical governors have been replaced by electro-
hydraulic equivalents. Each generating unit in the power system will possess its
own turbine-governor control system. In order that the units can operate in parallel,
the speed versus power output characteristic of each unit requires droop , which
implies that a decrease in machine speed should accompany an increase in gener-
ated output. Figure 5.5 illustrates the idealised governor characteristic of a large
steam turbo-alternator.
If f ref is defined as the reference frequency for the unit, i.e. the frequency at
which the generating unit provides no electrical output, then, as the frequency falls
from this level, the unit will begin to generate dependent on the slope, or droop ,of
the characteristic. The droop is defined as the percentage increase in speed
(or frequency) following full-load rejection, and a value of 4-5 per cent is typical.
An important feature of this arrangement is that the reference frequency, f ref , and
hence the main steam valve position, can be changed quite independently of
any observed variation in the machine speed. Thus the output of the unit can be
controlled precisely, independent of the system frequency.
Under normal conditions, each generating unit will operate under free gover-
nor action, whereby an increase/decrease in the system frequency is interpreted as a
decrease/increase in the system load. Each unit will move up/down its respective
droop characteristic until a new equilibrium is reached, such that generation equals
demand. Against a nominal target of 50 Hz, system frequency should normally be
maintained within reasonably tight limits - in Great Britain the defined range is

f ref

Increase
unit output

System frequency

Decrease
unit output

0 P max

Electrical power (MW)

Figure 5.5 Governor droop characteristic