1.School of Electrical and Electronic Engineering, North China Electric Power University, Beijing 102206, China;2.Electric Power Research Institute of State Grid Jiangsu Electric Power Co., Ltd., Nanjing 210096, China

In the DC distribution system based on modular multilevel converters (MMCs), the voltage fluctuations of the DC bus will generate the circulating currents in the converters. To study the generation mechanism of the circulating current, the voltage fluctuation and internal circulating current of the sub-module are deduced mathematically combined with the switching function. The conduction law of circulating current amplitude and frequency is obtained, and the mathematical expression of circulating current is proposed. To eliminate the influence of the circulating current, an adaptive circulating current suppression controller based on fast total least squares-estimating signal parameter via rotational invariance techniques (TLS-ESPRIT) and proportional integral vector proportional integral (PI-VPI) is proposed. By changing the modulation function of MMC, the bridge arm voltage is adjusted to follow the voltage fluctuation of the DC bus, which is able to suppress the circulating current. The simulation is carried out on MATLAB/Simulink platform. The results verify the correctness of the influence mechanism of DC voltage fluctuations on the MMC. Furthermore, the proposed control strategy can suppress the amplitude of the circulating current to 3.2%~4.5% of its original value.