The existing control for modular multilevel converter(MMC)is mainly direct current control based on the rotating dq coordinate system by controlling the upper and lower arms together and symmetrically. This requires strictly symmetrical operation on the DC side of MMC. A separate DC polar control for MMC is proposed to effectively improve the operating characteristics of the DC asymmetric operation. The circuit of MMC is redefined and remolded to get the relation for the power transmitted from AC to DC side. Based on the power relation

the separate DC polar control designed controlling the arm current

active and reactive power of upper and lower arms respectively

with the effect of arm circulation current suppression. And a definition of the voltage offset rate is proposed. With some corrections to the control and MMC configuration

DC asymmetric operation for MMC can be realized by the separate DC polar control

the over voltage level of DC asymmetric fault also can be apparently reduced. Time-domain simulation studies based on PSCAD/EMTDC are performed to verify the proposed control scheme and its effectiveness.