A control strategy of modular multilevel converter based on arm current direct control is proposed to control the AC current

DC current and arm circulating current. Through the theoretical analysis

the relationship between the transmission power of the upper/lower arm and the arm current is obtained. The inner-and outer-loop control system is designed to control the upper/lower arm independently. Respectively

the current of three-phase upper/lower arm is transformed into dq coordinate system

and the command values of each phase current of the upper/lower arm are obtained. Then

the arm circulating current suppressor is used to realize active/reactive power closed-loop control of the converter. The control strategy is validated on the 31-level flexible HVDC power transmission physical experiment platform. The results show that the proposed control method has the advantages of high accuracy and high efficiency suppression of arm circulating current in the converter.