System Strength Evaluation Framework for the Irish Power System

Short Circuit Ratio (SCR) and Equivalent SCR (EqSCR) have long been used to classify locations as “strong” or “weak,” where strong systems are less prone to voltage instability after disturbances.

However, as inverter-based resources (IBRs) increasingly dominate power systems, these traditional metrics are proving inadequate. High shares of IBRs introduce complex dynamics that SCR alone cannot capture, such as harmonic interactions, non-fundamental frequency components, and sub-synchronous torsional effects. This calls for a re-evaluation of system strength concepts and metrics.

Ireland exemplifies this challenge. Leading the energy transition, Ireland achieved 40% renewable electricity by 2020 and now targets 80% by 2030. Meeting this goal requires integrating 9 GW of onshore wind, 8 GW of solar, and 5 GW of offshore wind, alongside revised planning and operational policies. Currently, during new generator connection process, Minimum Fault Level (MFL) or SCR is used as a proxy for Minimum System Strength (MSS) for Grid Code compliance assessment. Yet, these simple metrics fail to reflect the nuanced behaviour of systems with high IBR penetration.

Operationally, EirGrid and SONI rely on global constraints—System Non-Synchronous

Penetration (SNSP) and Minimum Units On (MUON)—to ensure sufficient synchronous generation for stability in the All-Island Power system. To further increase renewable integration, new operational strategies will focus on regional and local constraints rather than global ones. This shift requires a new system strength policy and a regional inertia framework to replace SNSP and MUON. Consequently, there is a need for screening techniques that go beyond SCR to identify risks during planning and operations.

This paper outlines efforts to redefine system strength for Ireland and Northern Ireland and proposes updated metrics informed by research and practical experience. These metrics aim to provide a more comprehensive assessment of system strength across planning and operational contexts. Benchmarking, desktop reviews, and demonstrations using small-scale RMS studies on an All-Island system model validate these metrics. The studies show that the proposed metrics behave as expected under varying system conditions, offering a more reliable basis for decision-making in a high-renewable future.