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Modular multilevel converter (MMC) is a highly attractive topology for medium and high voltage power conversions without transformers. The main problem is the voltage balancing on floating capacitors. This paper presents a solution using phase-disposition carrier PWM method to solve the voltage balancing of a novel MMC, which is derived from the generalized multilevel inverter topology and introduced in earlier papers. In this novel MMC topology, a phase bridge is divided into three parts: upper bridge, lower bridge and middle cell. All floating capacitor voltages are balanced by selecting proper basic cells operating at either “1” mode, PWM mode or “0” mode according to their instantaneous capacitor voltages and the bridge current direction. The proposed method can be extended for any number of series connected basic cells and voltage levels. A laboratory prototype was built and the validity of the proposed method is verified by simulation and experimental results.
Date of Conference: 12-16 Sept. 2010