PURPOSE: To ensure the protection of a generator against overvoltage, by judging that the feedback signal for an exciter is found lost in case the terminal voltage of the generator is below the specified value, even if the field current of the generator is above the specified value.

CONSTITUTION: A voltage detection circuit 10 compares the voltage feedback signal detected through a potential transformer 4 with the specified value, by which a contact 11 will be closed and an auxiliary relay 12 excited, if the detected voltage value is below the specified value. On the other hand, a field current detection circuit 14 compares the field current signal detected through a shunt 7 with the specified value, by which a contact 15 will be closed and an auxiliary relay 16 excited, if the detected field current value is above the specified value. If the contacts of auxiliary relays 12 and 16 are connected in series and both contacts are closed, the signal will be supplied to a generator trip circuit as the loss of generator voltage feedback signal for an exciter.