Circulating current suppressing strategy for MMC-HVDC in the stationary frame

Proportional resonant (PR) controllers/repetitive controllers (RC) based circulating current suppressing strategies in the stationary frame are widely used in MMC because of their simpleness and effectiveness. Unfortunately, they cannot be directly used in MMC-HVDC systems since they can result in dc-line voltage ripples and thus deteriorate system performance and stability under unbalanced grid conditions. This paper studies the problem in detail and then proposes corresponding improved circulating current suppressing strategies for MMC-HVDC, in which newly designed zero-sequence circulating current controller is employed. The proposed strategies can effectively eliminate both circulating currents and dc-line voltage ripples for MMC-HVDC systems under balanced and unbalanced grid conditions. Hence, they greatly enhance the fault ride-through capability of MMC-HVDC systems without adding any control cost or the number of controllers. The theoretical analysis and the proposed strategies are both verified by an established MMC-HVDC simulation model in MATLAB/Simulink.