If a large AC synchronous generator, like one from a hydro dam, were to receive overdemand from the grid, would it start losing frequency or losing voltage?

I know they have several safeguards to keep this from happening, but if these safe guards were not in place, what would be expected to happen?

Edit: Supposing there is only this generator in the whole grid.

  • \$\begingroup\$ The whole of the grid will attempt to supply the increase in demand from that thing that is attached to the grid. The energy will be drawn from the grid and some part of that demand it will be supplied by all the major contributors to that grid. \$\endgroup\$ – Andy aka Jan 20 '15 at 13:02
  • \$\begingroup\$ @Andyaka read my edit \$\endgroup\$ – kurast Jan 20 '15 at 13:08
  • \$\begingroup\$ "Supposing there is only this generator in the whole grid" - then it's not really a grid. \$\endgroup\$ – Andy aka Jan 20 '15 at 13:12

It depends on whether the weak link is the generator itself or the prime mover.

If the construction of the generator limits its power output — the resistance of the wire, the strength of the magnetic field, etc. — then the voltage will sag, even while the torque on the shaft remains within the capability of the prime mover, so the frequency will not change appreciably.

On the other hand, if the prime mover is undersized with respect to the torque that the generator can produce, then it will slow down, and both the frequency and the voltage will drop.


The generator will "fall behind" the network in relation to phase, and this will remove some of the load from it. Of course, if it falls too far behind then problems can occur. And if the rest of the network cannot pick up the load then the frequency will drop. It's a bit dangerous for the voltage to drop (after regulation in the switch yards) since that will cause motor loads to work harder and hence increase current demand.

I recall seeing a TV show maybe 50 years ago talking about, I think, Hoover dam, and they mentioned that the clock on the wall (synced to 60Hz) could fall behind several seconds during the day and they would catch up overnight. I don't know if this is still the case.

  • \$\begingroup\$ I've read that clocks falling behind could happen with any clock synchronized to the AC frequency, not just one specifically at the hoover dam. \$\endgroup\$ – Random832 Jan 20 '15 at 19:46
  • 1
    \$\begingroup\$ @Random832 - Right. The clock at the dam was in sync (more or less) with all the clocks on the grid. \$\endgroup\$ – Hot Licks Jan 20 '15 at 19:53

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