An electric generator is operated under conditions in which rapid off-loading may occur. A generator control unit (GCU) employs a detector to detect rates of change of output of the generator. In response to a rapid change of output, e.g. an off-loading, an overvoltage protection system is activated. Excess energy stored in an excited winding is directed into an impedance circuit thus precluding overvoltage that may have been produced by the excess energy.