At the Prototype reactor site I qualified on there was a pretty impressive tale of a student who managed to try to bring a turbogenerator onto the electrical distribution system approximately 180 degrees out-of-phase.
The resultant current surge generated enough force and torque to physically lift the turbogenerator off of the deck before the breaker tripped. The amazing thing is that somehow the steam piping didn't rupture in the process.
Next year the aptly-named "sync check" modification was installed to prevent shutting the circuit breaker unless the two buses had approximately equal phase.
In the '80s, I built synchroscope controllers for Westinghouse's Nuclear Systems Division that were controlled via a main-frame computer. These were used in simulators (each reactor facility has to have a training simulator that's identical to the real control room) and were specifically used to train operators to match the phase of generators they were bringing on-line.
From what I can remember, the tolerance was about +/-5% and within that range, the generators would lock to each other. If one generator was putting out AC power that was ahead of the other, the current would cause the generator to behave like a motor and speed up until it matched. The other 350 degrees of the phase dial were much more dangerous ... explosions, physical damage, etc.
The resultant current surge generated enough force and torque to physically lift the turbogenerator off of the deck before the breaker tripped. The amazing thing is that somehow the steam piping didn't rupture in the process.
Next year the aptly-named "sync check" modification was installed to prevent shutting the circuit breaker unless the two buses had approximately equal phase.