Mains frequency stability
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gmarsh
diyAudio Member

Join Date: Apr 2004
Location: Halifax, NS
Quote:
 Originally posted by eeka chu How are the mains frequencies aligned? Say I own a power plant that feeds into a shared line. I have a load of three phase generators and I want to transmit the maximum amount of power from the generators to the grid. I would want to get the phase of my generators as carefully aligned with the grid as possible. Is there some kind master clock, do they align with each other or does each station just emit it's power and then have it aligned at some other point? I imagine the second could allow for the phases to drift slightly, which would result in needless power loss.
I did a course on this in EE. Lets see if I can jog my memory...

In a power plant, the generators run synchronous with the power grid - the stators are connected (via transformer, disconnects, etc) to the power grid, which means that the magnetic field in your 3 phase generator is rotating at exactly 60hz / # of poles, which means that your stator spins at exactly 1800, 1200, 600, 300, whatever... RPM. You need to align the speed (frequency), phase and voltage of your generators with that of the grid before you connect them, otherwise bad stuff happens. But once they're connected, the amount of power that your generator puts out is determined by the rotor field.

The frequency of the grid is determined by the amount of supply, the amount of demand, and the inertia of every synchronous generator and motor connected to the grid.

If the instantaneous amount of power fed into the grid is higher than the demand, then the extra power has to go somewhere - it speeds up every generator and motor connected to the grid. So the 60Hz frequency of the grid increases! But since the power grid is so massively huge and has probably millions of tons of whirling generator attached to it, this occurs *very* slowly. Also the great majority of loading on a power grid is resistive, so the voltage on the grid will increase slightly and the power output will go up anyway - so the frequency increase effect tends to be damped.

So how do they sync the power grid to 60Hz? I don't actually know how it's coordinated, but it's done by changing the amount of power delivered onto the power grid.

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