Synchronization of Two Generators

To synchronize AC generators several important factors must be the same.
1- The phase rotation of both generator systems must be the same.

2- The AC voltages of both generators should be equal.
3- The frequency of the incoming generators must match when synchronized ( in practice the frequency of the incoming generator is 1 to 2 Hz higher than that of the online generator. This can be observed with lights or by using a system chroscope.)

4- The speed and output voltage of the incoming generators are slightly higher to prevent it from becoming a load to the system when it is connected.

for more information see here too.

http://cr4.globalspec.com/thread/57066#newcomments