Interaction Energy-Based Transient Stability Framework for Multi-Grid-Forming Inverter Systems

As grid-forming (GFM) inverters replace synchronous machines, classical transient stability tools fall short in capturing their nonlinear dynamics-particularly under currentlimited conditions. This paper introduces a two-bus framework that enables analytical characterization of transient stability in multi-inverter systems. By aggregating parallel-connected GFM inverters into an equivalent source, we derive fault-induced angle dynamics and identify critical clearing conditions. A key contribution is the formulation of an interaction energy term that quantifies power exchange between inverters, showing how electrical distance and parameter mismatches-in inertia, damping, and power reference-affect angle divergence and stability margins. The newly developed analytical expressions reveal that the interaction energy increases with the severity of the mismatch and is mitigated by a greater electrical distance between the inverters. Numerical simulations in PLECS validate the proposed analytical formulation.