Abstract:

 This paper introduced the topological structure and working principle of modular multi-level converter （MMC）, and presented a new control method for MMC-HVDC （modular multi-level converter based high voltage direct current） based on voltage vector composition. The switching frequency of sub-module was high in the traditional control method for MMC, so the switching loss was also high. In the proposed control method, the switching frequency of sub-module could be reduced to power frequency, so the module switching loss was greatly reduced. Meanwhile, the voltage balance control strategy of module and the closed-loop control system of converter were designed for the control method, in which the module voltage could obtain good control along with power frequency switching. Simulation results show that, the switching frequency of module is lower, and the voltage fluctuation range is small, in the proposed control system, which can realize the closed-loop control of power, as well as has great value in theoretical research and engineering application.

Key words: voltage source converter based high-voltage direct current （VSC-HVDC）, modular multi-level converter （MMC）, voltage vector, switch frequency, capacitor voltage balance