Common-Mode Voltage Mitigation for Three-Phase Hybrid NPC Inverter with Flying-Capacitor Leg

Abstract

The classical neutral point clamped (NPC) 3-level has been with great success in the past few decades, but is facing the problems of neutral point voltage fluctuation and common-mode voltage (CMV). This paper deals with these issues using a hybrid NPC inverter, which consists of an extra flying-capacitor (FC) leg. The FC is switching between 1010 and 0101 modes to keep the neutral point voltage and the ratio between two modes can be controlled to balance the charging current from the NPC inverter. The modulation-based strategies for common-mode voltage suppression are proposed in this paper. When the modulation ratio is less than 1, the constant CMV modulation with only medium voltage vectors and a specific zero vector can immensely reduce the CMV. When the modulation ratio is greater than 1 (<1.15), the strategy of CMV reduction will be used to attenuate the peak CMV of the inverter system. With the active switching of the FC leg, the neutral point voltage can be balanced in each switching cycle even with the new modulation in the NPC converter. Simulation and experimental results are provided to validate that the proposed strategies can achieve good performance in reductions of CMV and noise in wide modulation ratio.