Analysis of Stability Stationary Point for Virtual Parameters of Grid-forming Converter Integrated Power Systems Based on Small-signal Model

The power grid is often assumed as an infinite bus in the research of grid-forming (GFM) converters. However, with increasing integration of renewable energy resources, the inertia and damping of actual power grids dominated by synchronous generators (SGs) are continually decreasing. The integration of large-scale GFM converters will inevitably interact with SGs in low-frequency oscillation (LFO) mode. In this paper, the small-signal model for LFO analysis of two-machine system is firstly established. The explicit analytical expressions for the dominant poles and damping ratio of LFO mode in different scenarios of two-machine system are derived using classified discussion approach with unique contributions obtained: 1) The existence of stability maximum/minimum stationary point for virtual damping/inertia of the GFM converter is proved, where the selected corresponding parameters will lead to the best/worst system stability. 2) The impact of the GFM converter on the system stability under different power grid strength is proved with a straightforward mathematical approach. 3) The theoretical analysis is extended to a generalized multi-machine system. The analytical results indicate that there exist stability stationary points for the virtual parameters of any GFM converter in any LFO mode. Finally, case studies are established to verify the validity of the analysis.