It has many advantages that using modular multilevel converter based railway static power conditioner（MMC-RPC） to govern the power quality problems of the traction power supplying system. However

the structure topology of MMC-RPC may cause some problems such as circulating current which should be suppressed. Firstly

combined with the compensation characteristics of MMC-RPC and the analysis of the compensation principle of traction power supplying system

the mechanism of the circulating current is analyzed after establishing the mathematical model of the single phase MMC. Secondly

a circulating current suppression controller of the single phase MMC-RPC system is proposed and designed using the proportional-integral-complex integral controller based on the theoretical analysis. Finally

a simulation model is constructed in Matlab/Simulink to validate the effectiveness of the proposed control strategy. Calculation and simulation results both point out that there are even-order circulating current in the single phase MMC-RPC which cause the fluctuations of DC voltage and DC current.