Paralleling AC Generators:

Most electrical distribution systems and power grids have more than one AC generator operating at one time.  Generally, two or more generators are operated in parallel in sequence to increase the available power. Three conditions have to be met prior to paralleling (or synchronizing) AC generators.

Their terminal voltages have to be equal.  One of the AC generators could be picked up as a reactive load to the other AC generator if the voltages of the two AC generators are not equal. This causes high currents to be exchanged among the two machines, distribution system damage or probably causing generator.

Those frequencies have to be equal.  A mismatch in frequencies of the two AC generators will cause the generator along with the lower frequency to be picked up as a load on the other generator (a condition referred to as "motoring").  This could cause an overload in the generators and the distribution system.

Their outcome voltages must be in phase. A mismatch in the phases will cause huge opposing voltages to be developed. The worst case mismatch would be 180° out of phase, resulting in an opposing voltage among the two generators of double the output voltage. This high voltage could cause damage to the generators and distribution system due to high currents.

In During  paralleling  operations,  voltages  of  the  two  generators  which  are  to  be  paralleled  are denotes by the use of voltmeters.  Frequency matching is accomplished by the use of output frequency meters. A Phase matching is accomplished by the use of a synchroscope, a device which senses the two frequencies and produce an indication of phase differences and a associate comparison of frequency variations.