The modular multilevel converter (MMC), renowned for its flexibility and efficiency, has been extensively applied in DC distribution networks to meet the diverse power supply demands of distributed energy resources and loads. However, in medium-voltage DC systems, the limited number of MMC submodules restricts the number of output levels, resulting in degraded voltage waveform quality and increased harmonic distortion. To address these issues, this paper proposes a novel Multi-Level Modular Multilevel Converter (MLMMC), which achieves voltage level multiplication through the coordinated operation of high-voltage and low-voltage submodules. To ensure stable voltage balance between submodule capacitors, the mechanism of voltage stability is analysed, and an appropriate voltage-balancing modulation strategy tailored to the MLMMC is developed. Finally, the proposed scheme is verified through simulations in MATLAB/Simulink. The results demonstrate that an MLMMC composed of seven high-voltage and three low-voltage submodules achieves output performance equivalent to a conventional MMC with twenty-four submodules. Moreover, the output remains stable even when the configuration ratio between high- and low-voltage submodules varies within a certain range.
Xie et al. (2026) studied this question.