Grid-connected stability analysis and control strategy study of grid-forming converters considering frequency coupling

The Grid-forming converters' stability is examined to address the issue of the risk of instability of Grid-forming converters under a strong grid. To improve the grid-connected stability of Grid-forming converters, a simplified design approach based on virtual impedance is suggested. The concept of sequential impedance that takes frequency coupling into consideration is used to investigate how frequency coupling affects the output impedance characteristics. Using the Nyquist criteria and output impedance characteristics, Grid-forming converters' stability under different grid strengths is investigated, and the mechanism of grid-connected instability is shown. Based on the amplitude relationship between the grid impedance and the output impedance of Grid-forming converters, a quantitative design approach for virtual impedance is proposed, and the theoretical validity of the design is confirmed. Investigations are conducted into how the virtual impedance control technique affects Grid-forming converters' output impedance properties and how it enhances grid stability. The grid-connected Grid-forming converters simulation platform based on Virtual Synchronous Generator control is constructed, and the simulation results demonstrate the precision of the stability analysis theory and the efficacy of the control approach suggested in the research.