AC Side Impedance Modeling of Grid-Tied Modular Multilevel Converter Using Double Closed Loop Control Strategy

Impedance based criterion is proved to be efficient for stability analysis of interconnected system, yet few existing impedance models of grid-tied modular multilevel converter (MMC) take outer control loop and internal dynamics into consideration simultaneously, which may lead to wrong analysis results. After analyzing the relations between inner and outer loop, this paper establishes the AC side impedance model with DC voltage/power control as outer loop through multi-harmonic linearization. The simulation results indicate that the outer loop has significant impacts on the impedance. The established impedance model can precisely reflect the characteristics of internal dynamics and outer loop, which provides basis for accurate analysis and effective improvement of the stability of MMC.