A Distributed Control Method for Power Module Voltage Balancing of Modular Multilevel Converters

This paper focuses on the distributed control method of the power module (PM) capacitor voltage balancing in modular multilevel converters (MMCs). The large number of PMs imposes great computational challenge on the capacitor voltage balancing in MMC based high voltage direct current (HVDC) transmission system. In this paper, a distributed voltage balancing method is proposed to reduce the computational load of control strategy of MMCs with hundreds of PMs per arm. With this method, all PMs in one arm of MMC are grouped first. The PMs in each group is balanced independently by voltage sorting and arm current polarity. The sorting all the PM voltages in this arm is avoided. Meanwhile, a close-loop control strategy is also introduced to balance the average PM voltage of each group dynamically. Simulation results on a +/- 350 kV/ 1000 MW MMC based HVDC system show the effectiveness of the proposed distributed control method.