The Iberian Blackout of 28 April 2025 and Frequency Stability in Power Systems with High Renewable Energy Source Penetration: Dynamics, Comparisons, and Policy Implications

The Iberian power system disturbance of 28 April 2025 exposed the vulnerability of converter-dominated systems with high renewable energy source (RES) penetration to rapid frequency decline and voltage-reactive instability. This paper reconstructs the second-by-second event sequence using publicly available waveform data, estimates system inertia using ROCOF-based and aggregate-unit methods, and examines the interaction between voltage-reactive stress and frequency response. Results show that the disturbance originated in the voltage/reactive domain through overvoltage-triggered disconnections, which amplified ΔP and accelerated frequency decay to a nadir of approximately 48.5 Hz.

Estimated inertia was in the range of 2.21-2.71 s. The constrained operational time window, driven by voltage-reactive events, limited the effectiveness of UFLS and FCR services. Application of the Inertia Adequacy Index (IAI ≈ 0.11) and its voltage-aware extension (IAI-V ≈ 0.005) confirms that the voltage-reactive channel closed the operational window before frequency reserves could respond.

Cross-event comparison with the UK 2019 and ERCOT 2021 blackouts demonstrates divergent initiation modes. The findings indicate that inverter-dominated regions require harmonized grid-forming capabilities, locational VAR/FFR procurement, and enforceable inertia floors.