Optimized Design and Control for Hybrid MMC With Reduced Capacitance Requirements

Reducing the capacitance of the sub-modules (SMs) and handling dc short-circuit faults are critical for the economical and reliable operation of modular multilevel converters (MMCs) in high-voltage direct current (HVDC) transmission systems. To deal with these issues, the optimized design and control for the hybrid MMC are proposed in this paper. First, the capacitor voltage fluctuations of the hybrid MMC with second-order circulating current and third-order common-mode voltage injection are analyzed. On this basis, the modulation index, required number of half-bridge SMs and full-bridge SMs, and SM capacitances are optimally designed with consideration of the operation constraints of the hybrid MMC, respectively. Further, an optimized control strategy is proposed to ensure the stable operation of the converter and to provide good dynamic performance. The capacitance requirements of the hybrid MMC with optimized design can be reduced to about 38% of those of the hybrid MMC with conventional design. The correctness of the theoretical analysis and the effectiveness of the proposed optimized design and the control strategy have been validated by the simulation and experimental results.