The modular multilevel converter (MMC) has been widely researched and applied in high-voltage direct-current (HVDC) transmission. MMC three-phase bridge arm energy imbalance is caused by power fluctuations in the three-phase unbalanced alternating current (AC) grid

which generates internal circulating current. In light of the three-phase unbalanced DC transmission system

the internal circulating current mechanism of MMC is derived and analyzed. According to the characteristics of internal circulating current

a compound control method combining proportional-integral (PI) control and repetitive control is proposed for circulating current suppression based on positive and negative sequence in a double synchronous rotating coordinate system. This method is suitable for both three-phase balanced and three-phase unbalanced systems. Meanwhile

in order to improve the waveform quality of the direct current (DC) transmission system under the three-phase unbalanced condition

the negative sequence current compensation method based on dq decoupling control is introduced. The simulation results have verified that no matter whether the AC system three phases are balanced or not

the control strategy always has good performance in circulating current suppression

while ensuring the balance of the system’s three-phase AC. This work is supported by the State Key Program of National Natural Science Foundation of China (No. 51237002).