Stability-Augmented Optimal Control of Hybrid PV Plants with Very High Penetration of Inverter-Based Resources (SAPPHIRE)

The Stability-Augmented Optimal Control of Hybrid PV Plants with Very High Penetration of Inverter-based Resources (SAPPHIRE) project team developed and demonstrated advanced modeling, control, and monitoring tools to address the stability challenges of operating power systems with high shares of inverter-based resources (IBRs). At the system level, SAPPHIRE created a stability-constrained scheduling framework that incorporates frequency-stability-constrained alternating current optimal power flow formulations. This framework allows operators to co-optimize cost and stability requirements in both island and bulk power systems. At the plant level, the project advanced grid-forming (GFM) control technologies for hybrid photovoltaic plants (HPPs), enabling them to enhance small-signal stability by flexibly transitioning between operating modes. The team also developed complementary modeling of battery degradation and secondary frequency regulation to ensure economic and reliable operation while an HPP is providing grid services. Meanwhile, SAPPHIRE delivered significant innovations in real-time monitoring techniques to enhance grid stability. For example, the project team developed measurement-based real-time inertia estimation for both ambient data and probing signal approaches. These methods were validated through simulations and power-hardware-in-the-loop experiments at the National Laboratory of the Rockies' Flatirons Campus. We established and successfully applied a new oscillation event analysis framework to investigate a real 19-hertz oscillation event on Kaua'i Island. By combining field data, full electromagnetic transient (EMT) modeling of island grids, and EMT-based small-signal stability analysis techniques, the project identified the root causes and validated mitigation strategies, including converting grid-following to grid-forming inverters. The project team successfully conducted the following three field demonstrations in Hawaii: Grid stability enhancement with GFM HPPs on Kaua'i Island showed improved frequency quality after inverter conversion to GFM mode by leveraging 2 years of continuously collected data from the island's grid. Real-time inertia estimation was implemented and demonstrated using a photovoltaic (PV)-battery plant to inject inertia probing signals to Kaua'i Island. The Kaua'i Island Utility Cooperative grid operated with extremely high solar and hydro energy (100%) for 3 consecutive hours, with 88.5% from solar PV and 11.5% from hydropower, demonstrating secure system performance enabled by GFM HPPs. We delivered 13 peer-reviewed journal papers, 16 conference papers, and 32 invited presentations, and we contributed to 4 industry-led technical reports from the North American Electric Reliability Corporation, the Energy Systems Integration Group, and the Institute of Electrical and Electronics Engineers Power System Dynamic Performance Committee. We also generated two software records for inertia-monitoring tools. Overall, SAPPHIRE demonstrated that HPPs equipped with advanced GFM controls can provide essential stability services, mitigate oscillation risks, and enable reliable operation of islanded grids powered entirely with major energy from solar and water. These outcomes provide actionable pathways for utilities, system operators, and industry stakeholders as they plan for high-IBR futures.