NOVELTY - A converter controller (205) controls the power converter (204) for alternating current (AC) excitation and the system side power converter. A system fault detector detects a voltage drop of the electric power system, an overcurrent of a direct current (DC) or AC component of the stator current. A stator current detector detects a current passing through the stator. A counter allows the power converter for AC excitation to start excitation of the doubly-fed generator after detection of a system fault and when the direct current component of the stator is lower than a predetermined value.    USE - A wind power generation system.    ADVANTAGE - By detecting and reactivating the attenuation of a stator current and the direct current component of the stator current, the output control of the reactive current during a fault can be correctly started. By operating the converter even during the system fault, the converter can be operated using the voltage of the generator, the charging current for the DC voltage can be obtained from the generator, and the operation can be continued with the direct current maintained even during power failure for a long time. Switching to stator current control during a system fault enables the stator current to be controlled at a predetermined value. If an overcurrent occurring when a system voltage is suddenly recovered exceeds a protection level of a converter, a gate is stopped and reactivated, thus a correct supply of a correct reactive current is provided when recovery from a fault is performed When a system voltage drops, especially around a voltage of zero, the phase detection of control is locked to follow the phase signal calculated from the phase and the frequency before the system voltage drop. This reduces the frequency fluctuation when the system voltage drops to zero, and preventing an overcurrent when the system voltage is recovered.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method of controlling a power converter.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of a circuit configuration of the electric power system and a wind power generation device.    Wind power generation system (200)    Generator (201)    Wind turbine (202)    Converter (204)    Converter controller (205)