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Reducing the power loss and handling short-circuit faults in HVDC systems are crucial for economical and reliable operation of modular multilevel converters (MMC). This paper proposes a loss reduction method for AC voltage-boosted FBSM-MMC by controlling the second harmonic component in the circulating current. The relationship between converter loss and second harmonic circulating current (SHCC) is first derived for AC voltage-boosted FBSM-MMC. From the analysis, it can be seen that the converter loss can be reduced by injecting the optimal SHCC into the arm current. The topology comparison between traditional FBSM-MMC and AC voltage-boosted FBSM-MMC is carried out to show the benefit of this method. Simulation results using PSCAD/EMTDC confirm the feasibility and validity of the proposed loss reduction method for AC voltage-boosted FBSM-MMC.