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

A simple control strategy is proposed 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 applied to balancing the DC-link voltage. The current flowing into the neutral point of DC-link capacitors causes the fluctuation of neutral point potential in the NPC inverter. The proposed DPWM method changes the path and duration time of the neutral point current, which makes the overall variation of it zero during the sampling time of the reference voltage vector. Therefore, using the proposed strategy, the neutral point potential of DC-link capacitors can be balanced fairly well and its ripple is also reduced significantly. Moreover, compared with conventional methods that have to perform the complicated calculations, the proposed strategy has been very simple. The proposed DPWM method has been verified by experiment.