Balancing Control of Neutral-Point Voltage for MVPWM-Controlled Three-Level T-Type Inverter

Medium-voltage vector pulse width modulation (MVPWM) has been applied for grid-connected three-level T-type inverter in transformerless photovoltaic generation. MVPWM benefits zero leakage current, but it is weak in balancing the neutral-point voltage of the inverter due to medium-voltage vectors used only. In discontinuous PWM (DPWM) controlled inverter, the maximum or minimum phase is clamped to positive or negative rail. Thus, the generated P-type or N-type small-voltage vectors are able to balance the neutral-point voltage. This paper integrates DPWM into a carrier-based MVPWM to improve voltage balancing. In the balancing condition, the inverter is simply modulated by MVPWM. The inverter is able to switch to DPWMMAX or DPWMMIN mode to accomplish voltage balancing. During the balancing duration, the leakage current is further suppressed by eliminating high variation of common-mode voltage existing in DPWMMAX and DPWMMIN. Simulation results are provided to validate the proposed method.