A new arm voltage balancing technique for the control of modular multilevel converters

The Modular Multilevel Converter (MMC) is a relatively new converter topology which has been favoured for high voltage DC transmission due to its performance, flexibility in control and its modular and scalable design which allow it to be used over a wide variety of system voltage levels and power ratings. The converter consists of a number of cascaded half bridge cells with isolated capacitors to make up an upper and lower arm in each phase. A control requirement of these converters is to maintain voltage balance between all capacitors in the converter. This paper proposes a new arm voltage balancing method to maintain capacitor voltage balance with largely varying arm powers while maintaining AC and DC output performance. The proposed technique is confirmed in computer simulation.