Parallel Operation:
DC Generators are required to operate in parallel supplying a common load when
the load is larger than the capacity of any one machine. In situations where the load is small
but becomes high occasionally, it may be a good idea to press a second machine into operation
only as the demand increases. This approach reduces the spare capacity requirement and its
cost. In cases where one machine is taken out for repair or maintenance, the other machine
can operate with a reduced load. In all these cases two or more machines are connected to
operate in parallel.
Shunt Generators:
The parallel operation of two shunt generators is similar to the operation of two storage
batteries in parallel. In the case of generators, we can alter the external characteristics easily
while it is not possible with batteries. Before connecting the two machines the voltages of
the two machines are made equal and opposing inside the loop formed by the two machines.
This avoids a circulating current between the machines. The circulating current produces
power loss even when the load is not connected. In the case of the loaded machine, the
difference in the induced emf makes the load sharing unequal.
The figure shown below shows two generators connected in parallel. The no-load EMFs are made
equal to E1 = E2 = E on no load; the current delivered by each machine is zero. As the load
is gradually applied a total load current of I ampere is drawn by the load. The load voltage under these conditions is V volt. Each machine will share this total current by delivering
currents of I1 and I2 ampere such that I1 + I2 = I.
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| Shunt Generators in Parallel |
Series Generators:
Series generators are rarely used in industry for supplying loads. Some applications
like electric braking may employ them and operate two or more series generators in parallel. The figure shown below shows two series generators connected in parallel supplying load currents of I1 and I2.
If now due to some disturbance, E1 becomes E1 + ∆E1 then the excitation of the machine I
increase, increasing the load current delivered. As the total current is I the current supplied
by machine II reduces, so also its excitation and induced emf. This machine I take a greater
and greater fraction of the load current with machine II shedding its load. Ultimately the
current of machine II becomes negative and it also loads the first machine. Virtually there is
a short circuit of the two sources, the whole process is thus highly unstable. One remedy for a problem like this is to make the two fields immune to the circulating current between the
machines.
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| Series Generators in Parallel |
Compound Generators:
The parallel operation of compound machines is similar to shunt generators. Differential
compounding would produce a drooping external characteristic and satisfactory
parallel operation is made easy. But most of the generators are used in the cumulatively
compounded mode. In such cases, the external characteristics will be nearly flat making the
parallel operation more difficult. By employing an equalizer connection for the series windings
this problem can be mitigated.
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| Compound Generators in Parallel |
