There is provided an adjustable speed generator motor system with full power converter (FPC system) in which two 3-level converters (1, 8) are connected back-to-back on the DC side as frequency converters, and by providing a mode in which a fly-wheeling diode is forced to conduct a current independently for three phases according to the current polarity when overcurrent is detected in an AC system-side 3-level converter (1) and a mode in which the fly-wheeling diode is forced to conduct a current independently for three phases according to magnitude relation of the absolute value of a 3-phase AC current in a generator motor-side 3-level converter (8) and the current polarity of each phase when an imbalanced value between positive and negative DC capacitor voltages (15, 16) exceeds a threshold, the FPC system is capable of continuing stable operation regardless of an occurrence point of a system-side fault and an accident mode.