A study of circulating current in MMC based HVDC system under an unbalanced grid condition

This paper shows a study about circulating current of modular multilevel converter (MMC) based a high voltage direct current (HVDC) system under some unbalanced grid conditions. A basic structure of the MMC HVDC system is introduced and its system dynamics is analyzed. Based on the analysis, the modeling of circulating current and its control method under balanced or unbalanced grid condition are described. From the point of view in circulating current control, two kinds of power control methods are analyzed and their performances compared. In addition, in order to apply the power control methods, some practical considerations are proposed and described. Several simulation results performed in the PSIM software verify the proposed analysis and consideration of the power control methods under various grid conditions.