Impedance Modeling of Modular Multilevel Matrix Converter Considering the Differ-frequency Coupling Harmonics

The modular multilevel matrix converter (M3C) is the core equipment of the low frequency transmission system (LFTS) which connects two AC systems with different frequency directly. The conventional multi-harmonic linearization method used for MMC that using the fundamental frequency as the frequency interval cannot accurately describe the harmonic characteristics of M3C. In this paper, based on the harmonic analysis of M3C, a multi-harmonic vector model in frequency domain is established, which includes dual fundamental frequencies. Based on that, the multi-harmonic small signal models of power stage and control loops are analyzed, and the sequence impedance model of M3C is developed including the effects of the submodule dc voltage dynamics and the differ-frequency coupling harmonics. Finally, the accuracy of the M3C impedance model is verified through time-domain simulations and sweeping experiments.