Differences Between Parallel and Synchronization of Generators

If you need to run a generator in parallel with another power source (either a single generator or a set of generators running in parallel or the utility power), you need to synchronize it first.

- MS

People often talk about "synchronizing", it sounds "clever" but the job is really "paralleling". To parallel two generators without damaging them or " making the lights flicker" one must a) adjust the voltages to be identical [practically, within 2% is good) b) adjust the frequency of the "in-coming" generator until it is very close to that of the "running" machine (for manual connection within 0.1 Hz, that is when the "Synchonising lamps" go dark - bright - dark in 10 seconds or longer is practical, it may be called a "10 second slip-cycle"). c) operate the breaker close switch when the voltage difference between the two generators is zero (which is the point at which the generators voltages are actually "in-phase). One can use a voltmeter to indicate when the voltage difference is zero, but it must not have too slow a response! Getting the breaker close within 0.1 second or so of the voltage zero is usually good enough.

To continue (sorry, I had to go out for a while!) - when the generators have been paralleled, the real "synchronization" will begin. When the generators were paralleled, they were not synchronous [running at exactly the same speed/frequency and phase] because the rotational speeds differed by about 0.1 Hz equivalent. There is a "natural " behaviour of AC generators coupled by inductors that the current flow between them, when there is a phase difference, creates a mechanical torque which pulls the machine back towards phase coincidence ("synchronising torque"). The rotors of the two generators will actually swing in angle relative to each other, the one with the greater speed swinging ahead at first. After time, resistive losses will damp the swinging and if the load and governors stayed ideally steady the rotors would settle into a steady phase relation at a common speed/frequency and a true synchronism of frequency and phase. In real systems, load is always changing, governor responses are never quite the same (even on sets of the same type) and there is always some "swinging" of generator rotors relative to each other. These swing frequencies are about 1 to 3 Hz.