Minimum Loss Discontinuous Pulse-Width Modulation Per Phase Method for Three-Phase Four-Leg Inverter

The three-phase four-leg inverter (TPFLI) provides unbalanced voltage to a load or injects unbalanced current to a grid to compensate unbalanced current. However, it has a high switching loss because of the fourth leg. This paper presents a discontinuous pulse-width modulation (DPWM) method that minimizes the per-phase switching loss of the TPFLI. The discontinuous three-dimensional space-vector pulse-width modulation (3D SVPWM) can be implemented in the TPFLI by injecting the same offset voltage as the conventional DPWM of three-phase three-leg inverter (TPTLI). However, the conventional DPWM is unsuitable for TPFLI because of its unbalanced current. We demonstrate that the discontinuous phase can be chosen according to the offset direction when a voltage reference vector is specified. Based on this result, a new per-phase minimum-loss discontinuous PWM strategy is developed and compared with the conventional DPWM. This method can also be applied to the TPTLI and implemented by injecting an offset voltage generated using the inverter phase voltage reference and phase current. Furthermore, the lifespan of a TPFLI can be extended by preventing deterioration of a specific leg. The validity of the proposed method is verified through simulation and experiments.