A novel power unit of a cascade unit comprises a DC voltage driver unit, a voltage lifting chopping part and an inverting part. The DC voltage drive of the power unit adopts a three-phase bridge type uncontrolled rectifier system, and two controllable IGBTs and two fast diodes are added at the DC side, thus forming a DC voltage lifting chopping circuit which can control the voltage at the DC link and broaden the scope of AC output voltage amplitude; an inverter adopts IGBT single-phase full bridge controllable output, and a fault bypass circuit is added at the output terminal of the inverter; and a rectifier transformer is of common wiring multi-winding type, and all secondary windings have the same wiring mode. A voltage lifting half-bridge and a neutral point in a filter capacitor set are connected with the neutral point in a harmonic current circuit to provide a channel for injecting zero sequence current so as to achieve the sinusoid of high power factor rectification and the input current.