A simple control strategy for balancing the DC-link voltage of neutral-point-clamped inverter at low modulation index

This paper proposes a simple control strategy based on the discontinuous PWM (DPWM) to balance the DC-link voltage of three-level neutral-point-clamped (NPC) inverter at low modulation index. It introduces new DPWM methods and one of them is used for balancing DC-link voltage. The current flowing through the neutral point of the DC-link causes the fluctuation of the DC-link voltage of the NPC inverter. The proposed DPWM method changes the path and duration time of the neutral point current and it makes the overall variation of the DC-link voltage zero during a sampling time of the reference voltage vector. Therefore, by using the proposed strategy, the voltage of the DC-link can be balanced fairly well and its ripple is also reduced significantly. Moreover, comparing with conventional methods that have to perform the complicated calculation, the proposed strategy is very simple. The proposed DPWM method is verified by experiment.