What are the conditions for parallel operation of three phase Alternators?

To synchronise AC generators, several important factors must be checked
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a)      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.

b)      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.

c)      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.