Re: How do power plants keep the A/C frequency in phase with each other?

Hi Alec!

I think the stuff about a master sync is not true, but I'm not an expert in this, so the Madsci admin might want to find a utility grid engineer for the real story.

The physicist's answer: AC electric generators, when connected together don't need any phase synchronization. They keep in phase naturally because it requires a huge torque to push them out of phase. Here's how to think about it. First connect only two AC generators to the grid, and drive them both with turbines. If you now stop driving one of them (you disconnect it from its turbine), then that generator will be driven as a motor by the other one. If its bearings have low friction, then it will coast at zero phase, neither putting out nor taking in any energy.

If you then apply a powerful brake to that coasting generator's shaft, its phase will drift slightly backwards, and the generator will take in energy from the grid (it starts running as a motor.) If you then remove the brake, the generator goes back to zero phase. Reconnect the turbine and start driving that generator, and it will not spin any faster, but it's phase will change slightly forwards. It takes lots of torque from the turbines to do this, and it will inject a huge energy flow into the common distribution grid.

Perhaps the part about satellite communication refers to the way they determine the rate of power that each generator supplies to the grid? The load sharing? The flow of energy being injected by each generator is adjusted by controlling its phase relative to the other generators. However, it's the generator's output power which is the important variable, not the phase. To adjust the phase, just turn up the oil valve, ( or add more coal, or pull the control rods out of the nuke reactor a bit.)

Finally, when an unused generator is brought online, its phase must first be exactly matched to the phase of the generators local grid connection. No satellites are needed, since the phase matching is between the generator and the terminals of the big switch which connects it. A person, or more probably an automatic system, must first set the generator's phase so it is almost exactly the same as the phase of the grid. When the relative phase is zero, then the big switch can be closed, then the turbine power can be cranked up high.