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Report ID: IB-GRID-INC-20250428-TECH-v0.9
Date Compiled: May 5, 2025
Subject: PRELIMINARY Technical Analysis - System Collapse Event, Iberian Peninsula Grid, 2025.04.28

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ATTN: Joint REE-REN Incident Response Task Force; ENTSO-E System Operations Committee Liaison
FROM: Ad-Hoc Technical Analysis Working Group (TAWG) - Grid Stability & Dynamics Section
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1. INTRODUCTION & SCOPE

This document constitutes the initial technical assessment by the TAWG regarding the catastrophic system collapse event that occurred on the synchronous power grid of mainland Portugal and Spain on April 28, 2025. The disturbance initiated at approximately 12:33 CEST (Central European Summer Time) / 11:33 WEST (Western European Summer Time / Lisbon Local), leading to a rapid, widespread blackout across the entire Iberian Peninsula – unequivocally the most severe grid failure in its operational history.

The event unfolded within mere seconds, characterized by an extreme loss of generation capacity. Telemetry indicates Spain alone experienced a generation drop estimated at 15 GW (representing roughly 60% of its active demand at the time) over an interval of approximately five seconds. This resulted in the immediate interruption of power supply to tens of millions of consumers across both nations. Transient effects, including brief outages and frequency deviations, were also registered in adjacent areas, notably parts of Southern France, confirming the event's significant impact boundary. Critically, operational logs confirm the Iberian grid became electrically isolated (islanded) from the rest of the Continental Europe (CE) synchronous area during the initial phase of the disturbance.

Grid operators (Red Eléctrica de España - REE; Redes Energéticas Nacionais - REN) and designated investigators, including this working group, are actively analyzing high-resolution operational data retrieved from SCADA systems, Phasor Measurement Units (PMUs where available), digital fault recorders (DFRs), and standard logging systems. Key data sets under review include, but are not limited to: load curves (aggregate and regional), generation output profiles (by technology type and major plant where possible), system frequency records (multiple points across Iberia and relevant CE nodes), voltage profiles, and real-time power flow measurements across major internal transmission corridors and cross-border interconnectors (especially Spain-France and Spain-Morocco ties).

The primary objective of this preliminary analysis is to rigorously evaluate plausible hypotheses regarding the initiating cause(s) and cascading mechanisms of this extreme event. This assessment is strictly evidence-based, focusing solely on the interpretation of electrical and power system operational data. All technically credible scenarios – ranging from conventional component failures and operational errors to deliberate external interference and rare natural phenomena – are examined against the observed grid behavior.

Public statements, media speculation, or unverified official hypotheses are noted only where they directly intersect with, corroborate, or contradict the technical evidence under review. Crucially, no potential hypothesis is dismissed prematurely based solely on external denials, lack of public confirmation, or perceived sensitivity, provided it remains consistent with the physical data signatures recorded during the event sequence. The goal is a data-centric assessment aimed at determining how well each potential scenario aligns with the measured dynamics of the Iberian power system during the critical moments leading to, and during, the collapse. Subsequent sections detail the evaluation of each primary hypothesis based on this analytical framework.

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EXECUTIVE SUMMARY (PRELIMINARY)

This document provides an initial technical assessment of the catastrophic grid collapse experienced across the Iberian Peninsula (Portugal/Spain) on April 28, 2025, commencing approx. 12:33 CEST (11:33 PT). The event resulted in a near-total blackout, with Spain losing ~15 GW generation within seconds (~60% demand). Minor impacts observed in Southern France. Analysis based strictly on available operational data (ENTSO-E, TSO telemetry - load, generation, frequency, flows). External statements/speculation noted only for context against technical evidence. All plausible hypotheses evaluated based only on electrical data signatures. Likelihood estimates are preliminary, based on current data.

EVENT OVERVIEW (Data Confirmed)

Timestamp: 2025-04-28, ~12:32:xx - 12:33:xx CEST initiation.
Nature: Rapid, cascading collapse of Iberian synchronous grid.
Magnitude: ~15 GW generation loss (Spain) in <5 seconds. Tens of millions affected. Iberian grid separation from CE grid confirmed.
Key Signature: Severe under-frequency event across Iberia post-initiating disturbance.

HYPOTHESIS EVALUATION (Based on Grid Operational Data)

Hypothesis 1: Transmission Interconnection Failure (Spain–France Interconnector)

Estimated Probability: ~15% (Low-Moderate likelihood as primary trigger)

Supporting Evidence (Grid Data):
Grid Separation Confirmed: Iberian system islanded from CE grid during event sequence. REE reported ES-FR tie disconnection co-incident with disturbance. This isolation is a known precursor to frequency collapse in stressed islanded systems.
Cascade Plausibility: Loss of a major interconnector can trigger instability. Spain exporting ~250-1700 MW to FR pre-event. Loss of export path could cause initial over-frequency -> generator trips. REE official account mentions ES-FR disconnection leading to collapse. Reported two-step disturbance (1.5s apart) could map to sequential loss of interconnector elements.

Contradictory Evidence (Grid Data):
Flow Magnitude vs Impact: Pre-fault export (<1.7 GW) seems insufficient to directly cause ~15 GW generation loss. Pure interconnector trip (loss of load) implies initial over-frequency due to surplus generation in Spain. Observed event was severe under-frequency (generation deficit).
Disturbance Origin: Reports indicate origin in Southwest Spain (high solar concentration), geographically distant from FR border interconnections. Suggests interconnector trip was likely a consequence of internal instability (protective relay action due to frequency/power swings), not the root cause.
French Grid Stability: France experienced only minor perturbation, suggesting fault was internal to Iberia, not on the interconnector itself (which would likely cause larger disturbance in FR). FR later reversed flow to support ES recovery.

Analysis: Interconnector failure alone doesn't explain scale or frequency direction. Separation did happen and was critical for the full collapse (prevented external support). Likely a secondary event triggered by internal Iberian instability. Low-moderate probability as the initiating cause. Explains extent more than origin.

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Hypothesis 2: Internal Generation Trip or Failure (Major Supply Loss within Iberia)

Estimated Probability: ~40% (Moderate-High likelihood as the root cause)

Supporting Evidence (Grid Data):
Abrupt Generation Loss: Data confirms massive, near-instantaneous generation drop inside Iberia. REE preliminary report notes two events, 1.5s apart, initiating the collapse. Spain online generation plummeted (~25 GW to <12 GW). Portugal showed similar sync'd drop. Timeline/magnitude strongly suggests multi-unit generation trip.
Location Consistent: REE points to SW Spain (high solar PV concentration) as origin. Large solar farm trips (potentially due to shared transmission fault) consistent with observed GW deficit. Subsequent frequency dive aligns with known cascade mechanisms.
"El Cero" Phenomenon: ENTSO-E/TSO data show near-total disconnection of all generation types (nuclear, hydro, thermal, renewables) as frequency collapsed below protection thresholds (e.g., <49 Hz). Supports initial large generation loss triggering cascading trips.
Load Drop Matches Supply Loss: Recorded ~13-15 GW load drop directly mirrors generation unavailability. Consistent with uncontrolled supply loss, not gradual fault or managed shedding. Matches PM quote of "15 GW in 5 seconds".

Contradictory Evidence (Grid Data):
Requires Underlying Trigger: Simultaneous failure of multiple independent generators is unusual. Suggests a common cause (e.g., fault on critical transmission corridor/substation in SW Spain taking out multiple connected plants). Generation trip may be a symptom of a network fault.
Protection Protocols Activated: Some analysis suggests widespread renewable plant disconnections were protective responses (consequence) to the already-collapsing frequency, not the initial cause. Need to differentiate initiating trip(s) from subsequent protective actions. Initial trigger might have been non-generator fault (voltage, short circuit).

Analysis: Strong evidence points to internal generation loss as the primary driver. Sequence likely: 1) Initial fault (equipment?) causes large gen trip(s) in SW Spain. 2) Massive supply deficit -> rapid frequency drop. 3) Interconnector trips (isolation). 4) Cascading trips of remaining generation due to under-frequency -> "El Cero". High probability (~40%) for this general mechanism. Exact initiating event (what caused first trip?) remains key question. Dovetails with Hypothesis 3 (Low Inertia).

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Hypothesis 3: System Instability Due to Low Inertia (Renewables-Dominated Grid)

Estimated Probability: ~20% (Moderate contributor; likely critical amplifying factor)

Supporting Evidence (Grid Data & Context):
High Non-Synchronous Penetration: Event occurred midday, peak solar. Solar PV reported ~50-60% of Spain's mix. High share of inverter-based resources (IBRs) = low system rotational inertia. REE Feb 2025 report explicitly warned of this risk (disconnections under disturbance). System state was inherently less resilient to frequency swings.
Rapid Frequency Collapse: Observed speed/depth of frequency fall (<49 Hz almost immediately) suggests insufficient inertia to damp the disturbance. A ~15 GW loss causing such rapid decay points to low system kinetic energy. RoCoF likely exceeded design limits. Comparisons to previous incidents (e.g., 2021 EU event) show this was far more severe.
Expert Commentary: Wide consensus that low inertia exacerbated the event. RBC analysis cited "complacency" in solar-heavy grid lacking storage. Engelaar (Neara) noted low inertia amplifies oscillation risks. SMC summary confirms low inertia -> faster disturbance propagation. Limited interconnection also restricted external inertia support.

Contradictory Evidence (and Mitigating Factors):
No Spontaneous Instability: Grid was stable prior to 12:32 event despite similar high-renewable conditions on other days. Low inertia is a vulnerability, needs a trigger. Not an initiating cause in itself.
Protections Did Operate (partially): UFLS activated (load dropped), indicating system attempted defense. Was simply overwhelmed by scale/speed of imbalance. Renewable trip-offs were per safety protocols, arguably preventing equipment damage. Fundación Renovables argues renewables disconnection was consequence, not cause.
Operator Experience: Iberian TSOs manage high renewables daily. Remedial actions for low inertia exist. Suggests system could handle state if not for massive initial trigger.

Analysis: Low inertia highly likely a critical contributing factor, explaining why the initial event cascaded so catastrophically. Reduced system's ability to absorb the shock. Explains the speed and severity of the collapse. Assign ~20% probability as primary cause (less likely than specific fault), but acknowledge its near-certain role in amplifying the outcome. Future mitigation likely needs inertia solutions (synthetic inertia, storage).

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Hypothesis 4: Deliberate Sabotage or Cyberattack

Estimated Probability: ~10% (Low likelihood based on current electrical evidence, but investigation ongoing)

Supporting Evidence (Circumstantial & Security Context):
Unprecedented Scale: Extreme rarity/severity invites speculation of non-accidental cause. Official reactions (emergency protocols, PM contacts NATO, High Court terrorism probe) show sabotage/attack considered seriously at highest levels. Targeting grid at known vulnerability point (peak solar/low inertia) would be strategic.
Potential for Coordinated Failure: Near-simultaneous events (SW Gen loss + Interconnector trip) could theoretically be achieved by coordinated physical sabotage or cyber intrusion tripping multiple elements. Sophisticated attack could mimic technical fault signatures.
Lack of Obvious Physical Cause (so far): If investigation finds no damaged equipment explaining the initial fault, suspicion of non-physical trigger (cyber) might increase. Rapid restoration without reports of major component destruction is slightly unusual for such a large physical event.

Contradictory Evidence (from Operations & Investigation):
No Direct Evidence: TSOs (REE, REN) report no indication of cyberattack based on preliminary checks. No public forensic evidence (malware, intrusion logs, physical tampering residue) released. Sequence matches known technical failure modes under low inertia.
Technical Causes More Common: Historically, major blackouts overwhelmingly stem from technical/operational issues, not attacks. Required attack sophistication (multi-point, synchronized) would be significant. No credible claims of responsibility.
Normal Protection Operation & Restoration: Grid protection systems functioned (isolated faults, tripped generators preventatively). Restoration proceeded via standard protocols without reported interference. Suggests operators retained control, inconsistent with ongoing attack scenario aimed at maximizing damage/duration.
Official Downplaying: After initial caution, officials (EU Council Pres, ES govt) leaned towards technical causes pending full investigation, citing lack of attack evidence.

Analysis: Electrical data signatures are currently indistinguishable from a major technical fault cascade. No positive evidence supports attack hypothesis at this time. Probability assessed as low (~10%) based purely on grid behavior. Cannot be fully ruled out pending deep forensic investigation (cyber logs, physical site checks). Remains a sensitive area requiring thorough verification.

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Hypothesis 5: Exotic Atmospheric or Geophysical Phenomena

Estimated Probability: ~5% (Very low likelihood; lacks supporting data)

Supporting Evidence (Initial Reports/Observations):
Early "Atmospheric Vibration" Reports: Initial media reports cited REN sources blaming "extreme temperature variations" causing 400kV line oscillations ("induced atmospheric vibration") in Spain. Implied unusual physical stress on lines.
Geomagnetic Storm (Remote Possibility): GICs from solar storms can cause grid issues. However, no significant space weather alerts for mid-latitude Europe on Apr 28. Mentioned for completeness of "exotic" natural causes.
Unusual Nature Prompted Speculation: Out-of-the-ordinary event led to considering out-of-the-ordinary causes.

Contradictory Evidence (Meteorological & Operational):
No Unusual Weather Confirmed: AEMet (Spain Met Office) explicitly stated no abnormal meteorological phenomena detected. Weather reported as normal/calm across Iberia. Directly refutes "extreme temperature variation" claim.
REN Retraction/Clarification: REN later distanced itself from the atmospheric vibration cause, suggesting misattribution or premature speculation. Lack of continued support from TSOs undermines hypothesis.
Lack of Precedent/Mechanism: "Atmospheric vibration" without storm conditions is not a recognized common cause of major blackouts. Expected physical evidence (line damage, oscillation data) not reported. Unlikely to cause precise 1.5s two-event sequence.
Expert Skepticism: Widespread doubt among external experts regarding atmospheric cause due to lack of weather evidence and known mechanisms.

Analysis: Exotic atmospheric/geophysical hypothesis lacks credible supporting evidence. Initial reports appear retracted or unsubstantiated. Meteorological data contradicts the premise. Considered highly improbable (~5%). Focus remains on internal grid technical factors.

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PRELIMINARY CONCLUSION (Based on Current Data)

The most probable root cause scenario, based solely on technical grid data available to date, involves a significant internal disturbance within the Spanish grid, likely originating from a physical fault (e.g., substation, transmission line) in the Southwest region. This fault appears to have led to an immediate, large-scale loss of generation (potentially multiple solar farms connected via the faulted infrastructure).

This initial large generation deficit occurred when the Iberian system was operating under low inertia conditions (due to high solar PV penetration). The low inertia critically amplified the disturbance, causing frequency to plummet much faster and deeper than the system could withstand via standard protections (like UFLS).

The Spain-France interconnector trip was almost certainly a consequence of this internal instability, isolating Iberia and preventing external frequency support, thereby sealing the fate of the islanded system.

The subsequent "El Cero" (total generation collapse) was a result of cascading protective tripping of remaining generators (thermal, hydro, nuclear, other renewables) as frequency fell below critical safety thresholds.

Probability Apportionment (Rough Estimate - Overlaps Exist):
Internal Grid Fault / Large Generation Trip Scenario (incl. Low Inertia Amplification & Interconnector Trip Consequence): ~80-85% combined likelihood.
Deliberate Sabotage / Cyberattack: ~10% (pending further forensic investigation).
Exotic Natural Phenomena: ~5% (largely discounted based on current data).