What are the conditions for parallel operation of alternators?

Figure: Parallel operation of shunt generators.

To synchronize AC generators, several important factors must be checked:

• Equal terminal voltages- The terminal voltage of the incoming machine must be approximately equal to bus-bar voltage. This is obtained by adjustment of the incoming generator’s field strength.

• Equal frequency- The frequency of the incoming machine must be equal to those bus-bars. This is obtained by adjustment of the incoming generator’s prime-mover speed.

• Phase voltages in proper phase relation- The phase of the incoming machine voltage must be the same as that of the bus-bar voltage relative to the load i.e. the phase voltages of the incoming machine and the bus-bar should be in phase opposition. This implies that there will be no circulating current between the windings of the alternators already in operation (the bus-bars) and the incoming machine.

Note: The above figure shows the shunt generator No.1 is connected to the bus-bars and delivering load. The shunt Generator No.2 is connected in parallel to the Generator No.1. When the load on the generator No.1 increases beyond its rated capacity, immediately the second shunt generator operate and wish the first generator to come across the raised load demand.

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