Environmental Engineering Reference
In-Depth Information
3.3.4 Multigenerator System
The two-generator load-sharing paradigm can be extended to explain the behaviour of a
typical power system where a very large number of generators are interconnected through a
transmission network to feed a multiplicity of loads. If all generators have the same standard
governor droop characteristic and a common set point, then they will all share load in propor-
tion to their ratings. In fact, for the economic reasons illustrated in the example of Figure 3.5
and discussed at length in Chapter 7, generators with the lowest cost of energy production
are run with their governor set points adjusted at full output (e.g. characteristic bb in Figure
3.5). As such generators are incapable of providing increments in power their governors are
unresponsive to demand increases. A number of generators that are less economic to operate,
or for other reasons to be explained later, are run with governor set points that results in them
operating at part load. Such generators are capable of responding to demand increases. It can
be shown that in such well integrated systems and to a good approximation all the generators
with
active governors
can be lumped together into one very large generator having a rating
equal to the sum of the ratings of the constituent generators and possessing the typical 4%
droop due to the combined governor action of all the machines. This
equivalent
generator
has a rating several times the value of the typical rating of generators on the network. The
reason for this will be explained later. The
f - P
characteristic of such an equivalent generator
is shown by aa in Figure 3.6, where
f
is the system frequency at which the present steady
demand
P
d
on the equivalent generator is supplied. Because of the large size of this equivalent
generator, aa intercepts the
P
axis far to the right. Suppose that an additional generator is
connected to the network where the 4% droop characteristic is given by bb. At the system
frequency
f
the additional generator injects into the system a power
P
. The power balance
principle now demands that the system frequency rises by an amount such that the power
injection by the equivalent generator is reduced by
Δ
Δ
P
. The fi gure shows that this requires a
very small frequency rise
f
. It can be concluded that in a large interconnected power system,
an increase or decrease of power from a single generator will have a small effect on
frequency.
A power system that is fed by active governor generation capacity overwhelmingly larger
than the rating of a single generator is known as an
infi nite bus
. The frequency of such a
Δ
a
b
f
P
d
Δ
f
f
b
a
P
Δ
P
Δ
P
Figure 3.6
Frequency power characteristic of an infi nite bus
