Stability Analysis of Current-Limited Grid-Forming Inverters with Frequency Stabilization: An Equivalent Impedance Approach

The rapid deployment of inverter-based resources (IBRs) in modern power grids aims to integrate renewable energy, yet the prevalence of grid-following (GFL) inverters raises stability concerns due to their dependency on a stiff grid. This has led to increased attention to grid-forming (GFM) inverters, which generate internal reference signals and can synchronize with the grid, essential for transitioning to IBR-dominated grids. However, challenges remain in understanding GFM inverters' capabilities and limitations in stabilizing grids with fewer synchronous machines, particularly regarding transient stability during disturbances. Inverters' strict overcurrent limits demand effective current limiting for protection, but current limiting risks instability as it prevents GFM inverters from meeting the power injection set by primary control leading to instability. This instability mechanism necessitates frequency stabilization methods to enhance inverter transient behavior, especially during prolonged disturbances. Since frequency stabilization methods rely on the current limiting conditions, understanding the interactions between current limiters and frequency stabilization methods is crucial. To fill the gap, this paper proposes an equivalent impedance-based model to analyze the interactions between current limiters and stabilization methods and their effects on GFM transient stability. The model, validated through numerical models and experiments, provides insights for improving GFM stability and reliability during grid disturbances.