Study on neutral-point voltage balance of 3-level NPC inverter in 3-phase 4-wire system

Problem of neutral-point voltage unbalance is a key issue for 3-phase 4-wire 3-level NPC inverter. This paper analyzes the relationship between the neutral point voltage and the neutral point current. Neutral-point voltage problem is transformed into neutral-point current problem. Then value of neutral-point current under balanced and unbalanced load condition is calculated. According to the mathematical analysis, a neutral point voltage control strategy is investigated. The simulation and experimental results verify control method.