PURPOSE: To prevent distortion at a rotor section due to thermal imbalance by sustaining high speed turning drive of a rotor, until inner cooling liquid is evaporized, upon tripping of a drive unit.

CONSTITUTION: Upon function of an overcurrent relay 14 due to short circuit in the main circuit of a generator, for example, liquid helium supply to a superconducting field winding is interrupted and a generator circuit breaker 13 and a field circuit breaker 16 are tripped. Upon confirmation of tripping of the generator circuit breaker 13 and the field circuit breaker 16, a drive stop command is provided. At this time, a high speed rotation turning command is provided on condition that the liquid helium exists in the superconducting field winding and the temperature of the field winding is low, and a turning unit 19 keeps the rotational speed of the rotor of a superconducting generator 11 at such level as the liquid helium is brought centrifugally into tight contact with the field winding thus maintaining a uniform liquid level around the rotor.