Circulating Current Suppression for MMC-HVDC Systems with Asymmetric Arm Impedance

The modular multilevel converter (MMC) has been a highly promising topology in the high-voltage direct-current (HVDC) transmission area, where each arm of the MMC may consist of hundreds of series-connected submodules and an inductor. Due to its parameter inaccuracy, component aging, and so on, the component parameter in different arms of the MMC may be different, which may cause circulating current in the MMC-HVDC transmission system, and result in current deterioration, power losses, and electromagnetic interference, etc. In this paper, the circulating current suppressing (CCS) in the MMC-HVDC system, due to asymmetric arm impedance, is analyzed. Based on the mathematical analysis, a method of using an auxiliary circuit is proposed for the MMC to realize the CCS and improve the performance of the MMC-HVDC system. Simulation studies are conducted with PSCAD/EMTDC in the HVDC system, which confirms the feasibility of the proposed method.