If two power plants feed AC voltage/current into the same electricity grid, how do we make sure they are in the same phase to not cancel out each other?

If you want to do it manually it can be done using a spinning dial that tells you the frequency of the incoming generator compared to the generator already online, as mentioned by someone earlier. You want the incoming generator to have a slightly higher frequency so that the dial spins slowly in a clockwise direction. When the dial reaches approximately 2 minutes to 12 the incoming generator can be connected. When they are both synchronised they are in phase with each other and become locked. All new switchboards today have a power management system that does the synchronisation for you, but now and then manual synchronisation is done. This is purely for training purposes should the automatic system fail.

I believe most older systems have a safety feature that does not allow for synchronisation when they are out of phase.

What happens if you manage to synchronise two generators out of phase with each other and there are no safety systems? Generators are basically electric motors. So they will alternate between delivering power and receiving power. They will however not do this for long, as they will literally tear each other apart. I've heard stories about this kind of thing, and the result is total destruction.

/r/askscience Thread