Improved pulse-width modulation and capacitor voltage-balancing strategy for a scalable hybrid cascaded multilevel converter

This study re-introduces a hybrid multilevel converter that is capable of operating independent of load power factor and modulation index plus is applicable to medium- to high-voltage application. These features are achieved by adopting a new modulation strategy that utilises third harmonic subtraction in order to: increase the modulation index linear range, have a smaller capacitor size and smaller footprint, have zero net power exchange between the cell capacitors, and the load is guaranteed for all operating conditions. This study establishes an optimal magnitude of third harmonic to be subtracted by the H-bridge cells to ensure proper converter operation. The validity of the presented modulation strategy and capacitor voltage balancing are confirmed by simulation and experimentation on a scaled-down model and prototype. The scalability of the hybrid multilevel converters to high-voltage application is demonstrated using simulation, including start-up and shut down without the need for auxiliary circuitry.