Demystifying Grid-forming Inverter Large-signal Stability Assessment with Equivalent-circuit Models

Classical transient stability assessment methods, such as the equal area criterion and Lyapunov energy function-based approaches, have long been used to analyze large-signal stability of power systems. However, when applied to grid-forming (GFM) inverters, these methods fail to account for key inverter-specific dynamic behaviors, including damping effects, reactive power versus voltage interactions, and the influence of current limiters. In recent literature, an alternative equivalent circuit-based energy function approach has been proposed to provide a more accurate transient stability assessment of GFM inverters. In this paper, we demonstrate how this circuit-based approach emerges from fundamental energy function principles and show how it addresses key limitations of classical transient stability assessment methods. Numerical simulations validate the effectiveness of the equivalent circuit-based energy function in characterizing the transient stability of GFM inverters.