Virtual Submodule Applied to the Modular Multilevel Converter

Abstract:
The design phase of a modular multilevel converter (MMC) needs to be supported by suitable tools for the submodule loss evaluation. This paper proposes a fast MMC cell loss calculation based on the virtual submodule (VSM) concept. Compared to other proposed tools, the impact of the circulating current control is directly taken into account and different modulation schemes can be easily compared. To verify the proposed concept, the results are compared with the losses obtained from a switched model with closed-loop control, where the analytical MMC key waveforms are approached in the steady state. The proposed method provides a great flexibility and a significant reduction of the simulation / computational time otherwise needed to evaluate SM losses under various operating conditions.