A Novel Inductance and Capacitance Selection Method for Modular Multilevel Converters Based on Modulation Margin Considerations

The arm inductance and the submodule capacitance have an enormous impact on the operating performance and investment of modular multilevel converters (MMCs). However, conventional methods for selecting inductance and capacitance suffer from quite a few drawbacks, such as the possibility of overmodulation, the tendency to lead to excessive design margin, and the need for repetitive trials. In this article, a concept of modulation margin is introduced to quantify the overmodulation risk. Subsequently, the relationship between key design requirements (including capacitor voltage stress and avoidance of overmodulation) and circuit parameters is comprehensively investigated by steady-state analysis, through which three parameter design rules that have hardly been reported in the existing literature are revealed. Building upon that, a novel inductance and capacitance selection procedure based on modulation margin considerations is developed for the MMC. The proposed method can avoid overmodulation and excessive design margin as early as in the initial design. Hence, time-consuming and repetitive trials, simulations, and checks can be significantly reduced, which facilitates the simplification of the parameter design process and the realization of a safe and economical design. A design example of a three-phase MMC prototype is presented to verify the feasibility of the proposed method.