In a power conversion system having a DC power supply which has an almost fixed
DC voltage to ground and a predetermined stray capacitance to ground and outputs
a DC power, and a power converter which converts the DC power output from the DC
power supply into an AC power by switching, the phase of an AC voltage to ground
generated in the DC power supply by switching is detected, the phase of an AC ground
fault current that flows through the stray capacitance to ground and the ground
fault resistance of the DC power supply is detected, and the difference between
the two detected phases is compared with a predetermined threshold value, thereby
determining a DC ground fault in the DC power supply. With this arrangement, the
ground fault state of the DC power supply can more accurately be detected. In addition,
the operation efficiency of the power conversion system can be increased.
