Parallel Plant Operation
If the load of a single generator becomes so large
that its rating is exceeded, you should add another
generator in parallel to increase the power available for
the generating station. Before two ac generators can be
paralleled, the following conditions have to be
1. Their terminal voltages have to be equal.
2. Their frequencies have to be equal.
3. Their voltages have to be in phase.
When two generators are operating so that the
requirements are satisfied, they are said to be in
synchronism. The operation of getting the machines
into synchronism is called synchronizing.
Generating plants may be operated in parallel on
an isolated bus (two or more generators supplying
camp or base load) or on an infinite bus (one or more
generators paralleled to a utility grid).
One of the primary considerations in paralleling
generator sets is achieving the proper division of load.
That can be accomplished by providing the governor
of the generator with speed droop. That would result in
a regulation of the system. The relationship of
REGULATION to LOAD DIVISION is best
explained by referring to a speed versus load curve of
the governor. For simplicity,. we will refer to the
normal speed as 100 percent speed and full load as 100
percent load. In the controlled system, we will be
concerned with two types of governor operations:
isochronous and speed droop.
The operation of the isochronous governor (0
percent speed droop) can be explained by comparing
speed versus load. as shown in figure 3-3. If the
governor were set to maintain the speed represented by
line A and connected to an increasing isolated load, the
speed would remain constant. The isochronous
governor will maintain the desired output frequency,
regardless of load changes if the capacity of the engine
is not exceeded.
The speed-droop governor (100 percent speed
droop) has a similar set of curves. but they are slanted.
as shown in figure 3-4. If a speed-droop governor were
connected to an increasing isolated load, the speed
would drop (line A. fig. 3-4) until the maximum engine
capacity is reached.
Now lets imagine that we connect the speed-
droop governor (slave machine) to a utility bus so large
that our engine cannot change the bus frequency (an
Figure 3-3.Isochronous governor curve.
Figure 3-4.Speed-droop governor curve.
infinite bus). Remember that the speed of the engine is
no longer determined by the speed setting but by the
frequency of the infinite bus. In this case, if we should
change the speed setting, we would cause a change in
load, not in speed. To parallel the generator set, we are
required to have a speed setting on line A (fig. 3-4). at
which the no-load speed is equal to the bus frequency.
Once the set is paralleled. if we increase the speed
setting to line B. we do not change the speed. but we
pick up approximately a half-load. Another increase in
speed setting to line C will fully, load the engine. If the
generator set is fully loaded and the main breaker is