Reducing DC Voltage Ripples of MMC by Injecting Circulating Current under Imbalanced Grid Conditions

Modular multilevel converter (MMC) is regarded as the most competitive solution for high-voltage direct current (HVDC) transmission using voltage source converters (VSCs). Circulating current suppressing strategies based on proportional resonant (PR) controllers are widely used in MMC. Although this kind of strategies can effectively eliminate all the circulating current components in MMC-HVDC, it will result in dc-line voltage ripples under unbalanced grid conditions and thus deteriorates system performance and stability. To solve this problem, this paper develops and analyzes the model of the dc voltage fluctuation of MMC under imbalanced grid voltage accurately and proposes an optimized circulating current control method. Compared with the traditional circulating current suppressing control based on PR controller, the optimized circulating current control method which reduces the double frequency ripples of dc voltage. Simulations in PSCAD/EMTDC verify the effectiveness of the proposed control strategy.