Neutral-point voltage control for grid-connected three-level inverters using a discontinuous pulse width modulation

This paper proposes a method for neutral-point voltage balancing control for a three-level inverter that uses discontinuous pulse width modulation (DPWM). DPWM is the method that only 2 phases are involved to switching among 3 phases. In the case of unbalanced neutral-point voltage in inverter using DPWM, the conventional neutral-point voltage control method adding offset voltage to the reference voltage has restriction due to the small vector in the non-switching period. Furthermore, if the offset voltage is comparatively big value, the conventional method may have problem distorting output voltage because discontinuous switching period would be overlapped by another phase's discontinuous switching period. The proposed method is implemented by adjusting the discontinuous pulse width of in positive and negative cycle. The method maintains low switching loss and effective voltage balance, without the need for complex calculations. Simulation results confirm the feasibility and effectiveness of the proposed method.