Design Analysis and Capacitance Reduction of Hybrid Modular Multilevel Converters Under Boost AC Mode

This article offers an in-depth analysis of the hybrid modular multilevel converter (MMC) working steadily under boost ac mode and proposes an improved capacitance design approach reducing its overall capacitance without affecting the performance. This enables more cost-effective and space-efficient applications of hybrid MMC in distribution networks. First, considering the uneven energy distributions, an analytical model is established to estimate the nonlinear capacitor energy variations of the two different types of submodules in the hybrid MMC with higher simplicity and adaptability. It quantitatively reveals that different energy variations have inherently occurred between the two types of submodules in the hybrid MMC under boost ac mode. Second, by fully utilizing this characteristic, the submodule capacitances are designed separately to avoid unnecessary oversize and minimize the overall capacitance. Based on a comprehensive evaluation of the proposed design, the overall capacitance of the hybrid MMC can be reduced by at least 36% compared with the existing method. Moreover, the proposed design has much lower power losses when the same capacitance is fulfilled by full-bridge MMC. Finally, the effectiveness of the proposed method is validated both by simulations and experiments.