Stability Analysis of MMC Considering Internal Dynamics Based on Equivalent Impedance Model

The modular multilevel converter (MMC) has become a competitive solution for the grid integration of energy, due to its benefits, including superior efficiency and low distortion. Nevertheless, its complicated internal dynamics, like circulating current and submodule capacitor voltage ripples have a great influence on operation stability. To analyze the stability, this paper applies impedance-based analytical approach based on a new equivalent model that considers internal dynamics. Unlike existing analysis, this paper (i) represents the effect of internal dynamics on the output characteristics by the equivalent series impedances, and (ii) focuses on the effect of equivalent impedance on stability, rather than overall impedance. Therefore, the proposed method offers a more intuitive and accurate analysis of the effects of internal dynamics on stability with fewer repetitive modeling processes. As a result, it can effectively obtain the analytical formula of resonant frequencies with potential threats to operating stability and analyze the mechanism behind oscillation in low-frequency band caused by MMC internal dynamics. Finally, detailed simulations and experiments verify the efficacy of the proposed method and stability assessment results.