Energy Function Based Transient Stability Analysis of Grid-Following Inverters in Bulk Power Systems

The growing integration of grid-following inverters (GFLIs) into bulk power systems poses significant challenges to transient stability, particularly under large disturbances. Unlike synchronous generators, GFLIs lack inherent inertia and rely on phase-locked loops (PLLs) for grid synchronization, introducing non-linear dynamics and potential instability. This paper presents an energy function-based framework for analyzing the transient stability of GFLIs with explicit consideration of PLL dynamics. A nonlinear state-space model of the GFLI system is formulated and equilibrium points are systematically identified. Lyapunovbased energy functions are then constructed to characterize the domain of attraction (DoA) around equilibrium points, enabling quantitative stability assessment. The proposed framework is validated on a single-inverter infinite-bus (SIIB) test system, where simulations demonstrate its effectiveness in tracking system trajectories through the energy function and evaluating transient stability margins. The results establish a foundation for the development of robust control strategies to ensure stability in future inverter-dominated power systems.