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Full-bridge submodule (FBSM) capacitance of an FBSM-based modular multilevel converter (FB-MMC) can achieve a significant reduction with boosted ac voltages, which makes it a promising candidate in voltage-source-converter-based high-voltage-direct-current scenarios in terms of cost reduction and dc short-circuit fault tolerance. With consideration of third-order harmonic voltage injection (THVI), the characteristics of the FBSM voltage fundamental frequency and second-order harmonic fluctuations are analyzed in detail under steady-state and single-line-to-ground fault conditions. Both the optimal phase and amplitude of the injected third-order harmonic voltage are derived constrained by the same modulation margin, based on which the optimized THVI method is proposed. Under the same fluctuation conditions, the comparison of energy storage requirements between the FB-MMC with and without the THVI method is also carried out. The feasibility and validity of the proposed THVI method are verified by simulations on a ±200-kV FB-MMC system and experiments on a laboratory-scale platform. With the same fluctuations as the FB-MMC without THVI, both the simulation results and the experimental results indicate that the FBSM capacitance of the one applying the proposed THVI method can be reduced by about 16%.
Zhao et al. (Wed,) studied this question.