An improved control system for modular multilevel converters featuring new modulation strategy and voltage balancing control

Modular multilevel converter becomes one of the most promising high power converters thanks to their modular structure design, distributed dc capacitors, improved power quality, and no separate dc voltage sources required. However, capacitor voltage balancing remains one major challenge. This paper proposes a new voltage balancing control associated with an improved pulsewidth modulation (PWM) method. Specifically, a new mathematical model and control structure with multi-loops including a new balancing control loop between the upper and lower arms are presented. The capacitor voltages can be well balanced without adding unnecessary switching to any SMs and voltage sorting algorithm. Moreover, the control and modulation are more accurate and the ac power quality can be improved. Simulations and experimental results have verified the effectiveness of proposed methods.