Parallel Operation Of Two Alternators

The operation of connecting an alternator in parallel with another alternator or with common Bus-Bars is known as synchronizing. Generally, in power transmission & distribution many alternators operate parallel in synchronization with the bus bars. Thus, the alternators remain connected to a live system of constant voltage & frequency. It is never advisable to connect a stationary alternator to live bus-bars, because stator induced e.m.f being zero, a short circuit will result. For proper synchronizing of alternators, the following three conditions must be satisfied

a)The terminal voltage of the incoming alternator must be the same as bus-bar voltage.
This can be indicated by a volt meter (V).
b)The speed of the incoming machine must be such that its frequency (PN/120) equals bus-bar frequency.
This can be measured by a frequency meter (Hz).
c)The phase of the alternator voltage must be identical with the phase of bus-bar voltage. It means that the switch must be closed at (or very near) the instant the two voltages have correct phase sequence.
This can be measured by synchronizing lamps or Synchronoscope.