Abstract: In the case of asymmetric faults in high voltage direct current（HVDC） transmission system，

modular multilevel converter-battery energy storage system（MMC-BESS）has a longer state of charge（SOC）

balancing process，uneven allocation of resources，easy to occur over modulation and other security problems，a

new SOC equalization control strategy for MMC-BESS under unbalanced network pressure was presented. Two

different equalization control strategies were proposed for interphase，upper and lower bridge arms and submodules.Firstly，by using the output energy of the bridge arm as the feedback quantity to adjust the output power of the phase unit and the upper and lower bridge arms，the DC component and the fundamental frequency component in the bridge arm circulation were regulated to reach the SOC rapid equilibrium between the phases and the bridge arms. Secondly，restriction was added to the proportional gain of SOC balance between submodules，and then the judgment function was used. Finally，the experimental shows that the proposed control strategy has better control performance and response speed than the conventional direct SOC equalization control strategy in engineering.

      Key words: modular multilevel energy storage converter；state of charge（SOC）；asymmetric fault；

circulating current