The present invention comprises the following: a three-phase three-level inverter (1); three single-phase five-level inverters (2) which are connected in series to the phase outputs of the three-phase three-level inverter (1); and pulse width control means (8) that for each phase supply a gate pulse to the three-level inverter (1) and to the single-phase five-level inverter (2) of the corresponding phase. A state-transitioning means (84) determines, on the basis of changes in voltage level created by the pulse width control means (8), the output of the switching leg of the three-level inverter (1) and the output of the switching legs outside and inside of the single-phase five-level inverter, and the state-transitioning means makes any of three outputs of the switching legs of the three-level inverter (1) and the outside and inside of the single-phase five-level inverter zero or positive when the voltage level is positive, makes all these three outputs of the legs zero or negative when the voltage level is negative, and makes all these three outputs of the legs zero when the voltage level is zero.
展开▼
