Diode-Clamped Multilevel Converters: a practicable way to balance dc-link voltages

The converter topologies identified as Diode Clamped Multilevel (DCM) or, equivalently as Multi Point Clamped (MPC), are rarely used in industrial applications, owing to some serious drawbacks involving mainly the stacked-bank of capacitors that constitutes their multilevel DC-link. The balance of the capacitor voltages is not possible in all operating conditions when the MPC converter possesses a passive front-end. On the other hand, in AC/DC/AC power conversion, the back-to-back connection of a multilevel rectifier with a multilevel inverter allows the balance of the DC-link capacitor voltages and, at the same time, it offers the Power Factor Correction capability at the mains AC input. An effective balancing strategy suitable for MPC conversion systems with any number of DC-link capacitors is presented here. The strategy has been carefully studied to optimize the converter efficiency. The simulation results related to a high power conversion system (up to 10 MW) characterized by four intermediate DC-link capacitors are shown.