What Synchronization Actually Requires
Paralleling a generator onto a live bus or onto the utility isn't a matter of just closing a breaker once the generator is running — it requires that the incoming generator's output matches the bus it's about to be connected to on three separate conditions simultaneously: voltage magnitude, frequency, and phase angle (and phase rotation, which must match by design of the wiring, not moment-to-moment). Closing the breaker before all three match forces two different AC sources — each with its own instantaneous voltage waveform — into a hard electrical connection, and the difference between those waveforms doesn't gently resolve itself. It shows up as a large circulating current the instant the contacts close.