A carrier-based PWM method for neutral-point ripple reduction of a 3-level inverter

The purpose of neutral-point voltage balancing algorithms in a 3L-NPC topology is to eliminate the DC and AC voltage unbalance of top- and bottom-side dc-link voltages. The DC unbalance problem can cause the output current distortion; therefore, this problem has to be solved. The AC unbalance problem means the ripple component of top- and bottom-side dc-link voltages exists. In applications using the NPC topology, a large dc-link capacitor value is required to mitigate the AC unbalance. As a result, the size and cost of applications become large and expensive. Therefore, this paper focuses on the compensation to mitigate the AC unbalance of the neutral-point voltage when a carrier based PWM method is applied. In this paper, the process of the optimal compensation value calculation is discussed and the effectiveness of the proposed compensation method is verified by using the simulation.