Neutral-point-clamped inverter with improved voltage waveform and control range

A new control method for neutral-point-clamped (NPC) inverters based on closed-loop regulation of the line-to-line output voltages is proposed. The method uses independent hysteresis comparator controllers to regulate the direct and quadrature axis components of the three phase output voltages. Main problems of NPC inverters such as neutral point potential variations and distortion at low input references are solved by the simple control scheme even when low-speed devices are used. In addition, the control method offers a wider control range and fast response. Computer simulations and experimental results are given to confirm the validity of the proposed control scheme