Decoupled Modulation With Common-Mode Load-Voltage Control for Three-Phase Four-Leg Three-Level Inverter

Three-phase four-leg four-wire (3P4L4W) three -level (3L) inverters have the ability to supply both balanced and unbalanced loads. This letter establishes common-mode (CM) and differential-mode (DM) circuit models for the 3P4L4W 3L inverter. It is revealed that the 3L phase-leg CM voltage is determined by the voltage balancing control (VBC) for the split dc bus voltages, that the DM load voltages are subject to the DM voltage control for the 3L phase leg, and that the CM load voltage is subject to the control for the fourth phase leg. On this basis, a decoupled modulation is proposed where the 3L phase legs are modulated to attain VBC and closed-loop DM load-voltage control, whereas the fourth phase leg is independently modulated to realize the closed-loop CM load-voltage control. The proposed work has been experimentally verified, showing that the 3P4L4W 3L inverter with this decoupled modulation scheme can provide well-balanced ac load voltages and low total harmonic distortion for any type of ac loads: balanced, unbalanced, linear, and nonlinear.