Is Equal Area Criterion Applicable for Transient Stability Assessment of Grid-Forming Inverters?

As grid-forming (GFM) inverters are installed in modern power systems, a key question is whether traditional transient stability assessment methods can be applied towards them. This paper critically evaluates the well-known equal area criterion and Lyapunov energy function-based methods towards the analysis of GFM systems. Though GFM inverters can exhibit superficially similar swing dynamics compared to synchronous generators, their damping properties are markedly different. In particular, GFM units exhibit consistent damping over all timescales and frequencies whereas the effect of machine damping is predominantly seen in steady-state. The lossless behavior of machines during transients underpins the aforementioned classical methods and limits their use towards GFM dynamics without proper modifications. This paper is focused on highlighting this issue so that researchers understand the limitations of existing stability assessment methods and are motivated to formulate new approaches tailored for inverter-dominated grids. Detailed theoretical analysis and simulations of a single-inverter-infinite-bus system reveal how non-conservative damping impacts transient stability margins.