Impedance modeling and stability analysis of VSG considering frequency coupling and frequency bands distribution

Impedance modeling serves as an effective approach for investigating wideband oscillation issues. The grid-forming(GFM) system can provide inertia support for the renewable energy system. At present, the modeling and analysis of impedance characteristics are mainly aimed at the grid-following(GFL) system. It is essential to develop a comprehensive wideband impedance model for GFM systems and investigate their stability, particularly to elucidate the mechanisms behind oscillations. This study incorporates the mirror frequency effect (MFE) into the analysis. Firstly, the sequence impedance model is established using harmonic linearization approach. Building upon this foundation, the oscillation mechanism of system is revealed through the interaction of the impedance characteristics of the grid and the converter. Subsequently, the control parameters of individual loops are evaluated, and the frequency spectrum is segmented into distinct bands. The influence of each link on the impedance characteristics across various frequency ranges is analyzed, and the dominant factors affecting the impedance characteristics of different frequency bands are revealed.