Testing Relay at 70hz

With the normal grid power system the short answer is unlikely as the frequency varies by a few Hertz (generally less), even under the most arduous conditions. However as this is for a generator there may be a possibility that when the generator is in islanded mode (ie- not connected to the grid) the generator frequency may vary wildly during some starting, heavy load dropping or failure events. It is hard to say based on the information given.

Thanks. I would like to know the pratical case of the 70hz operation of generator (thermal/hydro) in cases like sudden load change, etc. How much time does other controls in the plant take to resume the frequency back to 60hz? is running to 70hz possible?

It really depends on the control gear. Large load changes to a single generator can result in large momentary frequency changes, but generally large amounts of generators are connected together which reduces the effect on the grid in general of one of the generators output frequency from drifting too much.

As for running at 70Hz, I don't think this is possible in a normal case. In normal operation I would have expected the generator frequency to vary only by say a few Hz, maximum. I have a feeling that the overfrequency protection (or generator relay) will drop out the generator if the frequency ever gets close to 70Hz (which it should only do in the event of a fault).

I would advise checking the generator and other protection literature to see if this is the case (and at what frequency the generator protection should be operating at, which I think is less than 70Hz), as your generator relay may never see this condition that caused it to lockup. If it did and the relay locked up, would the generator relay (or other protection devices monitoring the generator or generator relay) still shut down the generator safely? If so, I don't see a big problem.