Unified Selective Harmonic Elimination Control for Four-Level Hybrid-Clamped Inverters

A unified selective harmonic elimination (SHE) control for four-level hybrid-clamped (4L-HC) inverters is presented in this article. With this unified strategy, all four-level switching patterns and the corresponding switching angles can be obtained simultaneously by solving one group of unified four-level SHE equations. Therefore, the optimal switching pattern of each modulation index with the designed optimization goal can be evaluated, and the best overall output performance is achieved. In order to ensure the proper operation of the 4L-HC inverter, the voltages across the three-phase flying capacitors and three dc-link capacitors must be controlled and balanced at one-third of the dc-link voltage. The proposed voltage control method is based on redundant switching states and introducing a slight variation to the precalculated switching angles, which extends or limits the conduction time of capacitors depending on voltage deviation and current direction. The effect of switching angle variations on the harmonic performance is also studied. Simulation and experimental results are presented to confirm the validity of the unified model and the proposed voltage-balancing strategy. Comparisons with phase-shifted pulsewidth modulation (PWM) are also presented to demonstrate that the implemented SHE-PWM could significantly reduce the switching frequency while keeping the capacitor voltages well balanced within the expected band limits.