To investigate the low-frequency capacitor-voltage fluctuation of modular multilevel converter（MMC）

we established a low-frequency operation-control strategy of MMC. Firstly

we established the mathematical model of MMC

and researched the capacitor-voltage fluctuation of MMC from the point of energy flow. It is pointed out that the deviation of the bridge arm energy is inversely proportional to operating frequency

causing that the system cannot run normally under low frequency. To solve this problem

we put forward a control scheme

which adds high frequency square-wave component to the arms’ circulating current and the output voltage simultaneously to suppress capacitor-voltage fluctuation and ensure the bridge arm energy balance of MMC. The theoretical analysis shows that when using the square-wave method

the peak circulating current will decrease by 50% than that by using the sinusoidal-wave method proposed in the previous work. The experimental results demonstrate that the fluctuation of modular capacitor voltage is restrained effectively and the low frequency drive system has good performance

demonstrating the effectiveness of the control scheme.