A distributed voltage balancing method for modular multilevel converter

The great number of power modules (PMs) imposes great computational challenge on the voltage balancing of modular multilevel converter (MMC) as the sorting of the PM capacitor voltages is usually adopted. This paper presents a distributed control method for PM capacitor voltage balancing in MMC. With this method, all PMs in one arm are grouped and the sorting algorithm of the voltage balancing in each group is implemented in decentralized control unit. To keep the voltage balancing among different groups, a close-loop control system is proposed, using the average capacitor voltage of each group as the feedback. With this method, the computational load is reduced compared to the conventional method while the voltage balancing effect and the switching frequency are almost the same. Real-time experimental results on a ±350 kV/1000 MW MMC based on RT-LAB (OP7020 and OP5607) verify the feasibility and effectiveness of the proposed method.