Neutral point voltage control of hybrid NPC four-level inverter with auxiliary switched legs

Neutral point clamped (NPC) four-level inverter has been widely used as a mature technology in fields such as motor drive and photovoltaic applications, but suffering from the neutral point voltage (NPV) fluctuation problem. This paper presents the control of neutral point voltage of the hybrid NPC four-level inverter with two auxiliary switched legs. The flying capacitor auxiliary switched (FC) leg is capable of balancing the neutral point voltage during each switching cycle, and the hybrid four-level inverter can maintain NPV balance without sacrificing degrees of freedom. This paper introduces the control method of the FC leg to eliminate the neutral voltage fluctuation and the carrier cascade method of the three-phase leg. Simulation results are provided to verify that the hybrid NPC four-level inverter has better neutral balancing capability than the regular NPC four-level inverter, with the fluctuation of capacitor voltage reduced by more than 90%, and the power quality of the system is significantly improved.