PURPOSE: To enable smooth operation of a synchronous machine after restoration of an auxiliary power by preset ting a set point of a static voltage setter after restoration of an abnormal auxiliary power.

CONSTITUTION: An automatic voltage regulator 4 feeds an output to a phase controller 6 of a thyristor rectifier 7 according to a deviation of a generator output voltage Vg from a set point Vref. A static voltage setter 5 gives an up/down counter 5c an up/down U/D command from a raising/lowering control circuit 5a. The up/down counter 5c counts up or down a clock pulse of a clock- pulse generator circuit 5b according to said command. An output of the up/down counter 5c is outputted as a set point Vref. On the other hand, if an auxiliary power of the static voltage setter 5 is restored to the original state after it has been broken, a value set at a presetting circuit 5f by an initialization circuit 5e is set at the up/down counter 5c.