A new modular multilevel conversion structure with passive filter minimization

The modular approach to converter design for high-power ac drives, proposed in the early nineties, has proved to be a very effective solution to face the industry demand for reliable high-power motor drives. Using this solution, commercial equipment is able to supply medium-voltage motors by series connecting single-phase low-voltage H-bridge modules, delivering high quality waveforms. A three-phase diode rectifier, with a dc-link filter capacitor, generally feeds each H-bridge. Unfortunately, the capacitor must be bulky and expensive to compensate for the large low-frequency harmonic current component that is drawn by the single-phase load. Here, a new conversion structure is proposed, where well-known converter stages are combined in order to obtain a significant size and stress reduction of the dc-link capacitor. Simulation and experimental results confirm the validity of the proposed approach