A Simplified PWM Strategy for Three-Level Converters on Three-phase Four-wire Active Power Filter

Presently, the neutral-point potential offset is an inherent problem for three-level converters on three-phase four-wire APFs. The conventional 3-dimentional space vector pulse width modulation (3D-SVPWM) strategy is adopted as a normal solution to the problem, but the selection of space vector sectors for four-wire converter is both abstract and complicated in practice. This paper proposes a new PWM strategy accordingly to simplify the selection process and the calculation of duration. Compared to the conventional 3D-SVPWM, the proposed method locates the sector of optimal vector directly with the polarity of three-phase reference voltage. Then, by applying voltage-second balance rule, it can perform a fast calculation of the duration and generate PWM signals to control the switching devices' on/off. As an approach to correcting the imbalanced DC-link capacitor voltage, this strategy reconstructs the switching sequence. A portion of state O duration is disassembled into states P and N to suppress the neutral-point potential offset. The proposed strategy is validated with a range of experiments. The results indicate that this simplified PWM strategy can effectively reduce the computational burden, and also the DC-link capacitor voltage is well controlled.