Unipolar Hybrid Frequency PWM Strategy With Power Matching for Asymmetric Cascaded H-Bridge Multilevel Inverters

The asymmetric cascaded H-bridge (ACHB) multilevel inverter can generate higher voltage levels with fewer power devices and dc supplies. However, conventional modulation strategies face challenges in simultaneously eliminating power backflow and achieving balanced output power among cascaded units. In response, this article proposes a double-tier power matching (DPM) hybrid frequency modulation strategy for a nine-level ACHB inverter with a dc-link voltage ratio of 2:1:1. In the upper tier power matching scheme, an improved staircase modulation for the high-voltage unit ensures power balance between high-voltage unit and low-voltage units by regulating the pulse width of the high-voltage unit. In the lower tier power matching scheme, a novel level-shift modulation is employed for the low-voltage units, utilizing carrier rotation with intervals of half a fundamental period to achieve power balance within a single fundamental period. Additionally, in order to prevent power backflow, the proposed strategy undergoes further optimization. This optimization also leads to a substantial reduction in both the implementation complexity of the DPM strategy and the switching losses in the inverter. Ultimately, this strategy is validated by both simulation and experimental results.