What factors must be met connecting identical generators in parallel?

Answer 1

The following is for three phase manual paralleling of two generators. Today's control systems will sense and do this operation automatically but the general process is the same. There are a couple of factors that have to be met when paralleling generators. The process of paralleling generators is called synchronization. What this means is that both generators have to be in phase with each other and running at the same speed. The generator that is running is called the lead generator and the one that you are trying to parallel is called the lag generator. There are two types of instrumentation used in synchronising, one is a light bank and the other a bit more sophisticated is a synchroscope meter. The synchroscope indicates whether the lag generator is faster, slower or in phase with the bus. The lag generator is brought up to speed just under the RPM of the lead generator. The synchroscope will be revolving very slowly in a clockwise rotation at this point. When the hand on the synchroscope reaches the 11 o'clock position the switch is thrown to bring the generator on to the bus tying the two generators together.

The light system of paralleling generators has lamps hooked in series and connected between the hot lead of the lead generator and the hot lead of the lag generator. When one generator is running faster than the other one, the lights will blink off and on as the generators come in and out of phase with each other. Adjusting the throttle of the lag generator and bringing it closer to synchronise speed will cause the lights will start to pulsate slower and slower until they go off for a period of seconds and then on for a period of seconds. At this point the two generators are electrically paralleled. Count the time between on off on of the lights. At half the off time (zero voltage) throw the switch and the generators will lock together, the two generators will be running in parallel with each other. Manually to keep them paralleled requires a bit of work, as the faster generator will try and take the load and motor the other one. This is where a governor is a great asset. If the generators are tied together out of phase two things could happen. The least problematic would be the generators breaker will trip. The second and most dangerous is if the lag generator is just a little bit out of phase when the switch is thrown, then the generator will be jerked into phase lock as momentary heavy current flows between the two generators. The sudden loading of the generator has been known to break crankshafts of the engine driving it.