Page 252 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS GENERATORS

SYNCHRONOUS GENERATORS 12.15

During normal operation, it is desirable to maintain the voltage supplied to the load
constant even when the load varies. The terminal voltage variations can be corrected by
varying the magnitude of E to compensate for changes in the load. Since E K and
A A
remains constant, E can be controlled by varying the flux in the generator. For example,
A
when a lagging load is added to the generator, the terminal voltage will fall. The field
resistor R is decreased to restore the terminal voltage to its previous level. When R
F F
decreases, the field current I increases. This causes the flux to increase, which results in
F
increasing E and, therefore, the phase and terminal voltage.
A
This process is reversed to decrease the terminal voltage.
PARALLEL OPERATION OF AC GENERATORS

In most generator applications, there is more than one generator operating in parallel to supply
power to various loads. The North American grid is an extreme example of a situation
where thousands of generators share the load on the system.
The major advantages for operating synchronous generators in parallel are as follows:

1. The reliability of the power system increases when many generators are operating in
parallel, because the failure of any one of them does not cause a total power loss to
the loads.
2. When many generators operate in parallel, one or more of them can be taken out when
failures occur in power plants or for preventive maintenance.
3. If one generator is used, it cannot operate near full load (because the loads are changing),
then it will be inefficient. When several machines are operating in parallel, it is possible
to operate only a fraction of them. The ones that are operating will be more efficient
because they are near full load.

The Conditions Required for Paralleling

Figure 12.19 illustrates a synchronous generator G supplying power to a load with another
1
generator G that is about to be paralleled with G by closing the switch S . If the switch is
2 1 1
closed at some arbitrary moment, the generators could be severely damaged and the load
may lose power. If the voltages are different in the conductors being tied together, there will
be very large current flow when the switch is closed.

FIGURE 12.19 A generator being paralleled with a running power system.

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