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Reducing the capacitance of submodules (SM) and handling short-circuit faults in HVdc systems are crucial for economical and reliable operation of modular multilevel converters (MMC). This paper proposes an SM capacitance reduction method for full-bridge submodule (FBSM)-MMC, which combines circulating current injection and ac voltage boosting. An optimized circulating current control method considering the limitation of fixed semiconductor current rating is introduced. By adding second-harmonic voltage to the reference arm voltage, the capacitor voltage ripple can be controlled to be a targeted value under all operating conditions. To further decrease the SM capacitor voltage ripples, the negative voltage state of FBSM is fully utilized to boost the ac voltage under fixed dc voltage with the modulation index up to 1.414. Consequently, the capacitance requirement for ac voltage boosting FBSM-MMC can be significantly reduced as compared to the conventional FBSM-MMC. Experimental results confirm the feasibility and validity of the proposed SM capacitance reduction method.

Improved Design and Control of FBSM MMC With Boosted AC Voltage and Reduced DC Capacitance