Speculations about the massive power outage in Spain and Portugal

1. Event Overview

On April 28 (approximately 12:30 local time, Madrid time), an unprecedented large-scale blackout occurred across the Iberian Peninsula—Spain, Portugal, and parts of southern France. The grid instantly lost over 50% of its load, and most cities gradually restored power after about six hours. However, full recovery was not completed until the morning of April 29. During this period, railways, aviation, hospitals, communications, and retail systems experienced widespread paralysis.

2. Publicly Available Authoritative Information

1. Preliminary Findings by Spain's National Grid (REE)

• Scale and Timeline of Power Loss: Approximately 15 GW was lost within 5 seconds starting at 10:33 GMT, with effective power dropping from 27,000 MW to less than 13,000 MW.
• Trigger Point: All 400 kV interconnection lines with France were disconnected in an extremely short time, forcing the Iberian grid into "island operation." The protection system quickly cut off additional lines, triggering a cascading collapse.
• Excluded Factors: No signs of cyberattacks, operational errors, or extreme weather.

2. Announcement by Portugal's Transmission Operator (REN)

• Scope of Impact: Nationwide blackout affecting approximately 6.2 million households starting at 11:33 (Lisbon time). REN initiated a phased black-start plan, restoring 424 substations overnight before achieving 90% power supply.

3. EU and National Actions

• The EU Energy Directorate has commissioned an independent expert team to submit a technical traceability report within six months and provide improvement recommendations by September 2026.
• France urgently supplied power to Spain to assist with the black-start process.

3. Suspected Technical Trigger Mechanisms

1. "Island Effect" and Interconnection Vulnerability

The Iberian Peninsula has only four main AC/DC interconnections with the European continent, with a total capacity of ≈3 GW, accounting for 2%–3% of installed capacity—far below the EU's 2030 target of 15% interconnection. Once interconnection lines trip, the system cannot quickly obtain inertia and frequency support from neighboring countries, leading to a rapid frequency drop.

2. Possible Forms of Large-Scale Generation Disconnection

• Renewable Side: During midday, extreme solar irradiation in southern Spain pushed photovoltaic output close to its peak. If a UHV bus station or inverter group failed, several GW of PV power could vanish within seconds.
• Conventional Side: During the same period, four nuclear reactors in Spain were operating at minimum power. The safety authority confirmed the reactors were safely shut down and awaiting grid reconnection, suggesting at least some nuclear units also tripped due to low-frequency protection.
• Concurrent Factors: If interconnection lines disconnected first, the instantaneous power gap would amplify, potentially triggering additional generator trips and worsening the drop.

3. Frequency Collapse and Cascading Protection

Under the ENTSO-E 50 Hz framework, the grid allows a conventional swing of ±0.2 Hz. A 15 GW gap corresponds to an initial slope of −500 mHz/s, far exceeding the Iberian system's inertia response limit. Even under-frequency load shedding (UFLS) could not prevent the frequency from falling below the "system collapse threshold" of 47.5 Hz, forcing a full black-start process.

4. Officially Excluded Causes

5. Unresolved Key Questions

1. Location of the Initial Fault Unit
   • Industry speculation focuses on the 400 kV collection station in Extremadura or the Pyrenees DC converter station, but waveform analysis and relay protection data have not been released.
2. Protection Settings and Renewable Penetration
   • High shares of PV and wind power reduce system inertia, potentially making current protection settings too conservative and leading to excessively long cascading trip chains.
3. Structural Defects in Interconnection Lines
   • The INELFE submarine cable between France and Spain underwent maintenance earlier this year, but whether insulation defects exist remains unverified.

6. Comprehensive Inference—"Multi-Trigger, Single Weakness" Model

Most Likely Scenario: A critical 400 kV interconnection line or DC converter station failed due to equipment issues → All lines to France disconnected simultaneously or progressively → The Iberian grid instantly lost external support → During midday with high PV output and low system inertia, a 15 GW-scale "supply-side disconnection" or equivalent gap occurred → Frequency collapse and cascading disconnection unfolded, causing a nationwide blackout in Spain/Portugal.

This inference aligns with excluded factors and shares high similarities with the mechanisms of the 2003 Italy blackout and the 2021 Texas low-frequency event.

7. Lessons and Long-Term Responses

1. Accelerate Cross-Border Interconnection
   • The Bay of Biscay HVDC and Aragon-Occitanie AC link projects have faced prolonged early-stage debates, requiring EU intervention to overcome French local resistance.
2. Enhance Local Inertia and Dynamic Support
   • Provide "synthetic inertia" through pumped storage, compressed air energy storage, and synchronous battery converters.
3. Renewable Grid Integration Protection Strategies
   • Reassess low-voltage ride-through capabilities and fast power support curves for large-scale PV/wind farms.
4. Diversify Black-Start Plans
   • Develop parallel "zonal autonomy" and "cross-region relay" black-start scripts to reduce reliance on a single system-wide approach.
5. Public Emergency and Communication Redundancy
   • Traditional FM/AM radio became the primary information channel during the event, exposing vulnerabilities in 5G, high-speed rail, and payment systems. These must be integrated into national resilience planning.

8. Follow-Up Focus

• Official Technical Investigation Report: Expected to be released in Q4 2025, potentially identifying whether the Pyrenees DC station was the primary cause.
• Frequency Waveform and Event Simulation: Spain's grid operator has committed to sharing waveforms on the ENTSO-E platform for academic research, warranting close tracking.
• EU Energy Mutual Assistance Legislation: France and Germany have exchanged views on cost-sharing mechanisms for emergency power transmission support. If passed, the Iberian region would directly benefit.

Conclusion

As of April 30, authorities have not determined the final cause of the incident. Public data from multiple operators and independent experts suggest this blackout was likely triggered by a critical interconnection line/converter station failure, amplified by the Iberian Peninsula's "weak interconnection + high renewables" structural vulnerability, rapidly escalating into a system-wide frequency collapse and cascading blackout. Cyberattacks, extreme weather, and human error have been preliminarily ruled out, but details await further investigation. This event highlights the urgency of EU regional interconnection, grid resilience, and coordinated energy transition governance, serving as a critical warning for global grids with high renewable penetration.

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