Dynamic Issues in Power System Restoration With Synchronous Generators and Grid-Forming Inverters

The integration of renewable energy sources with synchronous generators (SGs) and inverter-based resources (IBRs) is essential for enabling a decarbonized power grid. However, as renewable penetration increases and conventional SGs are displaced, the system becomes more susceptible to large-scale disturbances, as evidenced by the April 2025 blackout in Spain. This underscores the need for improved coordination between SGs and IBRs during power system restoration, particularly in its early stages. This study investigates the dynamic behavior of generator startup during restoration in mixed SG–IBR systems, with a focus on grid-forming (GFM) IBRs. Using electromagnetic transient (EMT) simulations, interaction challenges between SGs and GFM-IBRs are analyzed to identify solutions that enhance restoration reliability. A multi-loop droop (MLD) control framework is designed to examine the effects of droop gain, current-limiting strategies, system loading, and GFM-IBR placement in the cranking path. In addition, a comparative analysis of three GFM control strategies, MLD, dispatchable virtual oscillator control (dVOC), and virtual synchronous machine (VSM), is performed to evaluate their performance during IBR startup in a multi-machine scenario. Results reveal that voltage loop bandwidth, droop gain, and current-limiting techniques significantly impact SG–IBR dynamic interactions. The findings emphasize the critical role of GFM controller selection and tuning in achieving reliable restoration in mixed-generation power systems.