Modular multilevel converter(MMC)is very suitable for medium voltage and high voltage wind power generation system due to its advantages in many aspects. However

the capacitor voltage of each sub-module(SM)fluctuates widely in low frequency large current condition of the generator

which leads to large capacitors of the SMs. A novel method is proposed to suppress the capacitor voltage fluctuation of the SM with the circulating current injection based on the negative sequence double frequency circulation controller. And the basis is given for choosing the initial phase of the injected circulating current and the choice of SM capacitance based on the method. For the increase of the double frequency circulation with this strategy

a set of tradeoff coefficients characterizing the effect of voltage suppression relative to the increase of circulating current is presented. It is pointed out that the coefficient should be chosen according to the system requirements to meet the overall performance. Simulation results based on MATLAB/Simulink verifies the effectiveness of the proposed control strategy.