Page 253 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS GENERATORS

12.16 CHAPTER TWELVE

This problem can be avoided by ensuring that each of the three phases has the same
voltage magnitude and phase angle as the conductor to which it is connected. To ensure
this match, these paralleling conditions must be met:
1. The two generators must have the same rms line voltages.
2. The phase sequence must be the same in the two generators.
3. The two a phases must have the same phase angles.
4. The frequency of the oncoming generator must be slightly higher than the frequency of
the running system.
If the sequence in which the phase voltages peak in the two generators is different (Fig.
12.20a), then two pairs of voltages are 120° out of phase, and only one pair of voltages
(the a phases) is in phase. If the generators are connected in this manner, large currents
will flow in phases b and c, causing damage to both machines.
The phase sequence problem can be corrected by swapping the connections on any two
of the three phases on one of the generators.
If the frequencies of the power supplied by the two generators are not almost equal when
they are connected together, large power transients will occur until the generators stabilize
at a common frequency. The frequencies of the two generators must differ by a small
amount so that the phase angles of the oncoming generator will change slowly relative to
the phase angles of the running system. The angles between the voltages can be observed,
and switch S can be closed when the systems are exactly in phase.
1

The General Procedure for Paralleling Generators

If generator G is to be connected to the running system (Fig. 12.20), the following steps
2
should be taken to accomplish paralleling:
1. The terminal voltage of the oncoming generator should be adjusted by changing the field
current until it is equal to the line voltage of the running system.
2. The phase sequences of the oncoming generator and of the running system should be the
same. The phase sequence can be checked by using the following methods:
a. A small induction motor can be connected alternately to the terminals of each of the
two generators. If the motor rotates in the same direction each time, then the phase
sequences of both generators are the same. If the phase sequences are different, the
motors will rotate in opposite directions. In this case, two of the conductors on the
incoming generator must be reversed.
b. Figure 12.20b illustrates three lightbulbs connected across the terminals of the switch
connecting the generator to the system. When the phase changes between the two
systems, the lightbulbs become bright when the phase difference is large and dim
when the phase difference is small. When the systems have the same phase sequence,
all three bulbs become bright and dim simultaneously. If the systems have opposite
phase sequence, the bulbs get bright in succession.
The frequency of the oncoming generator should be slightly higher than the frequency
of the running system. A frequency meter is used until the frequencies are close; then
changes in phase between the the generator and the system are observed.
The frequency of the oncoming generator is adjusted to a slightly higher frequency to
ensure that when it is connected, it will come on-line supplying power as a generator,
instead of consuming it as a motor.

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