PARALLEL OPERATION OF ALTERNATORS

Alternators are connected in parallel to
(1) increase the output capacity of a system beyond that of a single unit,
(2) serve as additional reserve power for expected demands, or
(3) permit shutting down one machine and cutting in a standby machine without interrupting power distribution. When alternators are of sufficient size, and are operating at different frequencies and terminal voltages, severe damage may result if they are suddenly connected to each other through a common bus. To avoid this, the machines must be synchronized as closely as possible before connecting them together. This may be accomplished by connecting one generator to the bus (referred to as bus generator), and then synchronizing the other (incoming generator) to it before closing the incoming generator’s main power contactor. The generators are synchronized when the following conditions are set:

1. Equal terminal voltages. This is obtained by adjustment of the incoming generator’s field strength.

2. Equal frequency. This is obtained by adjustment of the incoming generator’s prime-mover speed.

3. Phase voltages in proper phase relation.