A new report from Europe’s electric grid regulator has revealed new details about the continent’s power system during April’s mass outages in Spain and Portugal, but insight into the causes of the blackout will have to wait for a follow-up report to be issued in 2026.
The “Grid Incident in Spain and Portugal on 28 April 2025” document, issued Oct. 3 by the European Network of Transmission System Operators (ENTSO-E), constitutes “a factual record to transparently inform stakeholders and governance bodies” and not an assignment of blame for the mass outages. ENTSO-E is an association of 40 transmission system operators (TSO) spanning 36 European countries.
The outage began the afternoon of April 28 and left the entire population of Spain and Portugal, as well as parts of France, without power for up to 18 hours. Spain’s government and grid operator Red Electrica released separate reports in June concluding the blackouts occurred because traditional synchronous generation could not provide adequate control of high voltage resulting from frequency oscillations exacerbated by a faulty power plant controller.
Reviewing those reports, U.S. experts — including NERC Chief Engineer Mark Lauby — said the U.S. grid was unlikely to suffer similar challenges because of reliability requirements put in place by FERC and NERC. Lauby said NERC would have to wait for ENTSO-E’s report “to gain any [further] insights into the incident. (See Lauby Says U.S. ‘On the Right Track’ After Iberian Blackout.)
An expert panel comprising representatives of affected and non-affected transmission system operators (TSOs), national regulatory authorities, regional coordination centers (RCCs) and the European Union’s Agency for the Cooperation of Energy Regulators wrote the report. ENTSO-E set up the panel May 12 to review the outages as required by the EU’s Incident Classification Scale methodology after an event is classified as a Scale 3 incident, meaning termination of operation of part or all of a TSO’s transmission system.
Red Electrica, Redes Elétrica Nacional and Réseau de Transport d’Électricité — the TSOs for Spain, Portugal and France, respectively — provided input and suggestions for specific chapters but did not act as primary authors for the chapters on their regions. The report’s authors wrote that this was “to ensure the neutrality of the reports delivered by the expert panel.”
‘Systems Collapsed’
According to ENTSO-E’s report, at 9 a.m. the day of the incident, Spain’s electric grid began to display increasing variability in voltage. These variations were not significant until shortly after 10:30 a.m., when voltage briefly exceeded 430 kV in part of the 400-kV transmission network.
When the outages began at 12:32 p.m., the voltage of the 400-kV network was below 420 kV and no oscillations with amplitude higher than 20 MHz were observed. Between 12:32:00 and 12:32:57, 208 MW worth of distributed wind and solar generators in southern Spain tripped offline, while distribution grids experienced a rise in net load of about 317 MW. The report’s authors did not identify a cause of this increase but theorized it “might be due to the disconnection of small embedded generators [of less than] 1 MW or to an actual increase in load,” or both.
Major disconnection events occurred from 12:32:57 to 12:33:18 in the Granada, Badajoz, Sevilla, Segovia, Huelva and Cáceres regions, leading to an additional loss of at least 2 GW of generation. The reason for most of these trips is not known, though the report attributed an unspecified amount to over-voltage protection.
No generation trips had been observed in Portugal or France up to this point. That changed between 12:33:18 and 12:33:21, when a sharp voltage increase in southern Spain bled over into Portugal, triggering “a cascade of generation losses that caused the frequency of the Spanish and Portuguese power system to decline.” Both countries’ grids began to lose synchronism with the rest of the European power system at 12:33:19.
At that point, the automatic load shedding and system defense plans of Spain and Portugal were activated but could not prevent the ongoing collapse of the Iberian grid. At 12:33:20, the AC interconnection to Morocco tripped due to underfrequency, and a second later, protection devices disabled the AC overhead lines between France and Spain. Finally, the HVDC lines transmitting power from Spain to France tripped at 12:33:23 and “all system parameters of the Spanish and Portuguese electricity systems collapsed.”
The French grid was “marginally affected,” according to the report. France experienced load loss of about 7 MW, and one nuclear power plant tripped offline during the incident.
A review of RCC data indicated the grid was considered secure at the time of the event and no major issues were known. No congestion had been detected on the Iberian transmission network, and the available production capacity was believed to be sufficient for expected consumption.
In an email to ERO Insider, Lauby wrote that NERC is reviewing ENTSO-E’s factual report and waiting for the regulator to issue its final report on the root causes of the outage, which is being developed by the same panel and expected in the first quarter of 2026. Lauby called this timeline “typical” for detailed system studies.
“Overall, the lessons learned have not yet changed — that is, to ensure the actions taken to manage oscillations do not exacerbate the ability [to] manage voltage on the system [and] that generating resources should be enabled to provide dynamic voltage support,” Lauby wrote.
