Limitations in Impedance Reshaping of Grid Forming Converters for Instability Prevention

Under a weak grid scenario, Grid Forming (GFM) converters have superior impedance-based small-signal stability compared to Grid Following (GFL) converters. The underlying assumption in defining a weak grid has traditionally been consideration of the inductive behaviour of the power grid load. In this paper, it is shown that, in presence of a non-inductive grid load, impedance-based small-signal instability is unavoidable in specific condition for Virtual Synchronous Machine (VSM)-based GFM. To prevent such instability, the classical way is to reshape the impedance through controller re-tuning in the frequency range of concern. For this purpose, the impedance reshaping capability of GFM is analysed and sensitivity analysis is performed with different control parameters. The simulation results are presented to verify the risk of unavoidable impedance-based small-signal instability due to very limited impedance reshaping capability of the GFM controller.