An overvoltage protection device for protecting a variable-speed power system which is connectable to a grid system on a high-voltage side of a main transformer, the variable-speed power system including a winding induction machine implemented by a doubly-fed alternator in which a stator winding is connected to a circuit on a low-voltage side of the main transformer and including an AC exciter which is connected to a circuit of a rotor winding of the winding induction machine, the overvoltage protection device comprising: an overvoltage suppression device which includes a short-circuit device including thyristors that each are connected between phases of the rotor winding of the winding induction machine and which is configured to connect respective phases via the short-circuit device or via the short-circuit device and a resistor; and a controller configured to perform reset control to forcibly release a short-circuit state of the short-circuit device by applying a reverse voltage to each of the thyristors configuring the short-circuit device from the AC exciter, characterized in that the overvoltage protection device further comprises: a detection unit (27, 27', or 27") configured to detect a prescribed phenomenon that occurs when a failure has occurred on the high-voltage side of the main transformer and when a failure has not occurred on the low-voltage side of the main transformer or inside the main transformer or to detect that the prescribed phenomenon has not occurred; and a control unit (L1, L2, F1, F2, SW1, SW2) configured to determine, based on a detection result of the detection unit (27, 27', or 27"), whether the reset control to forcibly release the short-circuit state of the short-circuit device by applying the reverse voltage to each of the thyristors configuring the short-circuit device from the AC exciter is to be permitted to be performed. an overvoltage suppression device which includes a short-circuit device including thyristors that each are connected between phases of the rotor winding of the winding induction machine and which is configured to connect respective phases via the short-circuit device or via the short-circuit device and a resistor; and a controller configured to perform reset control to forcibly release a short-circuit state of the short-circuit device by applying a reverse voltage to each of the thyristors configuring the short-circuit device from the AC exciter, characterized in that the overvoltage protection device further comprises: a detection unit (27, 27', or 27") configured to detect a prescribed phenomenon that occurs when a failure has occurred on the high-voltage side of the main transformer and when a failure has not occurred on the low-voltage side of the main transformer or inside the main transformer or to detect that the prescribed phenomenon has not occurred; and a control unit (L1, L2, F1, F2, SW1, SW2) configured to determine, based on a detection result of the detection unit (27, 27', or 27"), whether the reset control to forcibly release the short-circuit state of the short-circuit device by applying the reverse voltage to each of the thyristors configuring the short-circuit device from the AC exciter is to be permitted to be performed. a detection unit (27, 27', or 27") configured to detect a prescribed phenomenon that occurs when a failure has occurred on the high-voltage side of the main transformer and when a failure has not occurred on the low-voltage side of the main transformer or inside the main transformer or to detect that the prescribed phenomenon has not occurred; and a control unit (L1, L2, F1, F2, SW1, SW2) configured to determine, based on a detection result of the detection unit (27, 27', or 27"), whether the reset control to forcibly release the short-circuit state of the short-circuit device by applying the reverse voltage to each of the thyristors configuring the short-circuit device from the AC exciter is to be permitted to be performed.