Circulating current control of double-star chopper-cell modular multilevel converter for HVDC system

Modular multilevel converter is regarded as a promising technology in high voltage application, such as off-shore wind farm system. This paper proposes control technique for modular multilevel converters, aiming at suppressing the AC components in the circulating current. Specifically, an additional proportional-resonant control loop is designed to regulate the AC component of the circulating current to zero. The proposed method can effectively suppress the AC circulating current even in the unbalanced/fault condition. Simulation results for a 10MVA, 25KV DC system are provided to demonstrate the feasibility of the proposed method.