Coordinated Operation and Stability Limits of Grid-Forming and Grid-Following Inverters in Power System Restoration

As power systems transition from synchronous generation towards inverter-based resources (IBRs), these resources will increasingly need to provide black-start and power system restoration services. This paper evaluates the transient interactions and stability limits of coordinated grid-forming (GFM) and grid-following (GFL) inverter operation during a bottom-up restoration process. Using a parallel–sequential restoration strategy, restoration of the main transmission backbone and formation of a local restoration island with high GFL penetration proceed in parallel, while the energisation events within each restoration path are carried out sequentially, in a modified IEEE 9-bus test system. Electromagnetic transient (EMT) simulations are used to identify the maximum tested stable operating point of the restoration island as GFL capacity is progressively increased. The simulation results demonstrate that, for the studied system and operating conditions, a single 10 MW GFM anchor can maintain stable island operation with up to 110 MW of aggregate GFL capacity, corresponding to a GFM-to-GFL capacity ratio of 1:11. When the aggregate GFL capacity is increased to 120 MW, corresponding to the next tested ratio of 1:12, the restoration island becomes unstable, with the observed instability associated with loss of synchronisation in the GFL phase-locked loops (PLLs). At the 1:11 operating point, the corresponding equivalent short-circuit ratio (ESCR) is 2.5, indicating weak-grid conditions. Finally, the 1:11 restoration island is successfully resynchronised with the main grid, with active power, frequency, and PCC voltage recovering within approximately 1.5 s following the switching events.