Environmental Engineering Reference
In-Depth Information
3.3.2 The Governor
The steam fl ow from the boiler to the turbine of a large conventional fossil fuelled generator
is regulated by a valve of considerable size and weight. For the turbogenerator to respond
quickly to a requirement to increase or decrease its output, this valve has to be opened or
closed as quickly as possible. This is achieved through a hydraulic actuator. The control signal
for the hydraulic actuator is provided by the governor. In the past governors were of mechani-
cal nature but modern generators are fi tted with electronic governors. The governor's function
is to measure the rotational speed of the generator, to compare it to the reference value (50
or 60 Hz) and, based on the error signal, to instruct the hydraulic actuator to open or close
the steam valve.
Figure 3.4 shows the action of the governor of a single generator supplying a load with
50 Hz AC. Governors are designed to operate as proportional control systems; i.e. an error
must be present between the set point frequency and the actual frequency for the governor
to alter the fuel or steam supply. This proportional control is characterized by the line in
Figure 3.4 which has a fall or
droop
of 4% across the operational range. This 4% value
has been found through extensive experience to be appropriate for stable governing and is
widespread. Governors have the inbuilt facility that allows an operator to adjust the frequency
at which the characteristic intercepts the frequency axis, known as the
set point.
For the
line aa the set point is 52 Hz. With this set point, the system frequency is 52 Hz when the
generator is supplying no load and is reduced incrementally to 50 Hz when the load increases
to rated generator power. Changing the set point to 51 Hz moves the line to bb. Set point
adjustment is of considerable importance because it allows power system operators to decide
how the demand is shared by the generators on the grid, but more will be given on this
later.
f(Hz)
a
52
b
a
50
b
48
0
100%
p
Figure 3.4
Frequency/power governor characteristic
