A control device includes: a phase error calculation unit to calculate a phase error between a phase of a voltage command for the power converter and a phase of a point-of-interconnection voltage; and an isolated system determination unit to determine whether the AC power system has shifted from an interconnected system to an isolated system, based on the phase error and a magnitude of the point-of-interconnection voltage, the interconnected system being the AC power system connected to a generator, the isolated system being the AC power system not connected to the generator. The isolated system determination unit determines that the AC power system has shifted from the interconnected system to the isolated system, when at least a first condition is met that the phase error is outside a reference range and the magnitude of the point-of-interconnection voltage is greater than a first voltage threshold.