Shunt-Wound Generators in Parallel. A and B of Fig. 8-59 are two similar generators feeding the same bus bars C and D. If A tends to take more than its share of the total load, its voltage falls and more load is automatically thrown on B.
Also, if the driver of one of the generators slows down to
stop, the emf of the machine falls until the other generator starts to drive it
as a motor. This continues until its driver takes over again.
The external characteristics of the two machines are shown
in Fig. 8-60. At voltage E, the currents in the generators are Ia and Ib, and
the line current is Ia # Ib. To make machine A take more of the load, its
excitation must be increased to raise its characteristic curve.
If a 1000-kW generator and a 500-kW machine have the same
regulation curves, the machines will divide the load according to their
respective capacities, as shown in Fig. 8-61.
Compound-Wound Generators in Parallel. A and B of
Fig. 8-62 are two compound-wound machines. If A tends to take more than its
share of the load, the series excitation of A increases, its voltage rises, and
it takes still more of the load.
Thus, the operation is unstable. If this continues until A
takes all the load and the voltage of B drops to the point that A reverses the
current in B, B will be driven as a motor. With the reversed current in the
series field of B it becomes a differentially compounded motor, and the series
weakens the flux to speed up the motor.
This may progress to a point at which the unit may be
damaged mechanically and electrically. To prevent this, a bus bar of large
section and of negligible resistance, called an equalizer bus, is connected
from e to f (Fig. 8-62).
Points e and f are then practically at the same potential.
Therefore, the current in each series coil is independent of the current in its
particular generator, is inversely proportional to the resistance of the coils,
and is always in the same direction.
When a single compound generator has too much compounding, a
shunt in parallel with the series field coils will reduce the current in these
coils and so reduce the compounding.
When compounded generators are operating in parallel using
an equalizer bus, the current in the series field coils depends only on the
resistance of the coils and a shunt connected across one of them is actually
across all of them, reducing the compounding of all but not disturbing the
relative compounding between the machines.
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