Modular multilevel converter based high voltage direct current (MMC-HVDC) transmission systems may increase the short-circuit current and affect the AC fault clearing of circuit breakers. To address this issue, this paper analyzes the influence of the MMC on the short-circuit current when a symmetrical or asymmetrical fault occurs in the AC system. Firstly, the influence mechanism of the MMC on the short-circuit current under symmetrical AC faults is studied. It is found that the short-circuit current magnitude at the fault point is mainly affected by the q-axis output current of the MMC. For the singlephase line-ground fault, the three-sequence equivalent circuit of the MMC is analyzed, and the calculation method of the three-sequence short-circuit current is presented. It is observed that the negative-and zerosequence currents at the valve side of the MMC are both zero, and the positive-sequence current is determined by MMC operating conditions. Besides, the short-circuit current magnitude under the singlephase line-ground fault is also mainly related to the q-axis output current of the MMC. Based on these results, an additional controller is proposed to reduce the short-circuit current under the symmetrical and asymmetrical faults. Finally, the effectiveness of the proposed mechanism and suppression control is verified by PSCAD/EMTDC simulations.

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