April blackout was first caused by overvoltage, EU experts say

The power outage that caused a blackout across Spain and Portugal on April 28 was caused by a "cascading overvoltage" that began in the south of Spain and rapidly spread across the Iberian peninsula.

This is according to a preliminary report published on Friday by a group of experts from the European Network of Transmission System Operators for Electricity (ENTSO-E), which is leading the investigation into the "unprecedented incident."

The factual report describes the conditions before the outage, and the sequence of events during the incident and the restoration of power, but does not assign blame or responsibility.

The report downplays the role of renewable energy sources in causing the incident.

The panel of 45 experts is expected to publish a final report early next year.

Excessive voltage

On April 28, 2025 at 12:33 CEST, the power systems of Spain and Portugal experienced a blackout. A small area in France, close to the border with Spain, was also affected for a limited duration.

The report concludes that this was the most serious electrical incident in Europe in the past twenty years.

According to the data presented, the cause was an imbalance between supply and demand in the system, as well as voltage and frequency fluctuations that triggered a "cascading" disconnection of power generation plants in southwestern Spain.

The report indicates that a series of unprecedented "overvoltages" occurred – never before recorded in Europe – and that the speed with which they spread prevented the existing prevention and recovery plans from stopping the blackout.

"The central factor in the collapse was the system's inability to control the magnitude of the voltage. Voltage is a parameter that must be managed locally, near where the problem arises," said Damian Cortinas, chair of ENTSO-E's board.

The report states that the conditions of the Spanish and Portuguese electrical systems on the day of the incident were those of a "typical spring day," with high supply from photovoltaic and wind energy, the generation of which was concentrated in the southwestern part of Spain.

However, the experts avoided blaming renewables as the cause of the blackout and instead emphasized the "loss of control" over voltage and the system's inability to regulate the "cascading overvoltage."

Avoiding blame

The report does not aim to "point fingers" or assign blame, Cortinas said, since that is the responsibility of the authorities in each country, he added.

"These are the facts, but we are not assigning any responsibility or fault. That is not our job," he repeated at the report's presentation on Friday. 

The factual report will be followed by the final report, expected in the first quarter of 2026, which will include an in-depth analysis of the events of April 28, establish the root causes of the incident, and include recommendations for improving the resilience of the European power system.

Lack of information

Cortinas explained that the investigators "need time" to analyze the event, and lamented the difficulties in investigating the incident – not only due to its "unprecedented nature," but also because of obstacles in accessing information from the involved parties.

According to the investigators, Spain's grid operator Red Eléctrica obtained consent for data from 33 generation companies and distributors (including Iberdrola, Endesa, and Naturgy), while eight others withheld their consent.

Sequence of Events

The experts' analysis describes April 28 as a “typical spring day,” marked by a sharp increase in renewable generation that drove down market prices and led to higher electricity exports from Spain.

The report notes voltage increases began around 10:30am, with "significant" fluctuations still within operational limits – Spain's voltage ceiling is 435 kV, while Portugal's is 420 kV.

More extreme oscillations occurred around 12:03pm, when the system came under significant "stress," according to the experts.

In the half-hour before the blackout, the electrical grid experienced its most violent swings, especially at 12:19pm. Operators managed some disturbances, but ultimately could not curb the voltage surge that led to system collapse.

That collapse began at 12:32pm, when intense oscillations triggered a rapid voltage spike, cascading into massive disconnections. Between 12:32:57 and 12:33:18, multiple power plants went offline, shedding around 2.5 GW of renewable output across solar and wind installations in southern and central Spain.

At 12:33:21, the Iberian network lost synchronization with the rest of Europe as protective systems disconnected cross‑border links to prevent further damage.

Some disconnections were conducted deliberately as safeguards, but most still lack a definitive explanation. Restoration efforts brought Portugal's grid back online by 00:22 on April 29, and Spain's by 4am, with experts calling the recovery "extremely rapid."