The final report on the April 28, 2025, Iberian Outage has dropped, and the answer to what caused this blackout is — it’s complicated.

The European Network of Transmission System Operators for Electricity (ENTSO-E) said in its March 20, 2026 report that there was no simple, single cause for the power outage that struck continental Spain and Portugal last year, but was rather a confluence of issues.

Power oscillations, gaps in voltage and reactive power control, differences in voltage regulation practices, rapid output reductions and generator disconnections in Spain, and uneven stabilization capabilities each had a share of the blame, according to ENTSO-E.

The panel of 49 experts also issued their recommendations for how to prevent a similar incident in the future. Namely, stronger operational practices, better monitoring and closer coordination between power system operators and stakeholders. They went on to say their inquiry shows the need for better-fitting regulatory frameworks to help support the evolving European power grid.

The April 28 blackout was a surprise due to its severity, suddenness and the absence of any clear, immediate cause. It was the most serious outage on the European power grid in more than 20 years, and there have not been any others like it, according to ENTSO-E.

The report was written with the assistance of experts from Germany’s Amprion and BNetzA, Ireland’s EirGrid, Belgium’s CREG and Elia, Denmark’s Energinet, Hungary’s MEKH and Mavir, Spain’s Red Electrica and BNetzA, Portugal’s ERSE and REN, France’s CRE and RTE, Greece’s SEleNe CC, Switzerland’s ElCom and Swissgrid, Italy’s ARERA and Terna, Sweden’s EI, and Romania’s ANRE and Transelectrica.

“The blackout occurred after an uncontrolled, rapid rise in system voltage and loss of voltage control on a day with multiple concurrent phenomena, accelerated by rapid generation output reductions and disconnections, leading to voltage instability and cascading generation disconnections in Spain,” according to the management summary of ENTSO-E’s report.

After the event hit, Portugal and Spain’s transmission system operators (TSOs) REN and Red Electrica sprang into action with their respective restoration plans. There was also a small area of Southern France near the Spanish border affected, and France’s RTE also initiated its procedures and protocols to restore voltage levels in the French power grid.

The outage struck at 12:33 a.m. on April 28 Central European Summer Time (CEST). System restoration in Portugal was completed by 1:22 p.m., April 29, and in Spain, the transmission system was fully restored by 4:00 on the same day.

ENTSO-E categorized the event as a Scale 3, or major incident, where the operation of part or all of the transmission system is terminated.

Timeline

The morning hours on the area in question saw a ramp-up of available renewables, which led to lower prices on the day-ahead market and to Spain’s exports reaching 5 GW in total. Grid operators saw oscillatory events in the morning, but no significant oscillations with amplitudes above 20mHz were detected until 12:03 p.m.

However, during the half-hour before the blackout, a pair of oscillations hit, with power, voltage and frequency swings in the Continental Europe Synchronous Area. In response, grid operators cut exports from Spain to France and changed the operating mode of the high-voltage direct current (HVDC) link between France and Spain.

“While these measures effectively mitigated the oscillations, their nature led to an increase in voltage in the Iberian power system,” according to the report.

At one minute out from the blackout, the voltage of the Iberian power system at the 400 kV level was below 420 kV and no notable oscillation with amplitude higher than 20 mHz could be observed.

At around the same time, several power plants decreased their generation by about 500 MW and there was an increase in net load on the distribution grids of about 317 MW. The report states the reasons for some of these events is still unknown.

About a half a minute before the outage, transformers in substations began tripping. These events created step changes in voltage. More trips hit, causing the loss of 727 MW from photovoltaic and thermal solar power facilities. After this, several more trips hit and disconnected more solar and wind assets from the grid, leading to a total loss of about 928 MW.

Some of these trips occurred due to overvoltage protection, but the panel was not able to establish the cause of most of these trips, according to the report. It does note, however, that overvoltage protection settings at some generation units were set below the voltage limits established in accordance with the applicable requirements.

The loss of these units without adequate compensation for the loss of reactive power absorption by other resources capable of controlling reactive power led to increased voltages not only in Spain but also in Portugal. Congruently, the frequency decreased.

Between 12:33:18 and 12:33:21, the voltage in southern Spain increased sharply, and also in Portugal. This overvoltage triggered a cascade of power generation losses, and consequently, the Spanish and Portugese power grids’ frequency fell.

Over the same interval of seconds, Spain and Portugal’s power systems lost synchronism with the rest of the Continental Europe, and automatic load shedding kicked in. Emergency safeguards were unable to halt the outage, however protective devices did prevent a further loss of synchronism deeper into the European grid.

From then on, Spain and Portugal were effectively islanded. The power imbalance continued to rise, and frequency fell further as a consequence.

At 12:33:23, the HVDC lines still sending power from Spain to France tripped, and the Iberian power system was electrically separated, and all system parameters collapsed. France was only marginally affected by this incident.

Investigating Fixes

In its report, ENTSO-E found the incident evolved via a combination of voltage fluctuations and oscillatory phenomena, which in turn led to widespread power plant disconnections in Spain followed by a cascade of overvoltage disconnections and culminating in the loss of synchronism of the Iberian system with the Continental Europe Synchronous Area.

There was very little time to act to prevent some of the factors that triggered the outage from worsening because the root causes of it unfolded within a matter of seconds, according to the report’s conclusions chapter.

Broadly, the report says the European grid needs to be more resilient due to factors such as greater use of renewable energy, increasing electrification, further market integration and geopolitical risks — all of which put greater strain on grid stability.

The recommendations section calls for ENTSO-E to develop a guideline of good practice on voltage support means and studies on voltage stability.

It also says grid operators and power plans should use voltage control mode whenever possible, and explore the possibility of a centralized or zone-based voltage regulation.

In the medium term, grid operators should assess system needs that will appear in real-time and design the system to operate during unexpected events through a combination of static and dynamic reactive power resources. This could include shunt reactors and capacitors, STATCOMs, and other solutions.

Grid operators should also work to make the harmonized operating voltage range foreseen at the Continental European level is uniformly applied across the continent by removing deviations that allow operation outside this range. This would have an added benefit of reducing wear and tear on power grid components.

To address harmful oscillations, grid operators should “define the required level of damping for inter-area oscillations, collect damping measure roadmaps from TSOs, ensure their consistency, and verify that the combined measures are sufficient to meet the targeted minimum damping level.”

To support this, the experts call for improving the monitoring and detection framework by using existing Phasor Measurement Units (PMUs), oscillographs and available power quality monitoring devices. Implementing automatic detection solutions and effective early warning systems would help resolve oscillations in a timely fashion.

The report also calls for examining the settings and factors by which power plants connect and disconnect, with an eye toward aligning with the highest capabilities of grid users connected downstream. Complying with minimum timing requirements and voltage thresholds will help prevent unintended disconnections.

Type A power modules, such as the small-scale solar PV assets, should be made capable of stable use without disconnection from the grid. The report finds that these units can be sensitive to high-voltage episodes even when transmission-side voltage is within nominal limits.

Grid operators also need to put controls in place after any protective trip or unexpected disconnection so they can assess whether the actions of grid users during real events match with the applicable withstand capability requirements. At least 10 disconnections in the Iberian Outage were not in line with requirements, the report states.

ENTSO-E also believes the Continental Europe Synchronous Area should create snapshots of its common grid model promptly after an unexpected event like an outage. This will allow better models and simulations to be built in the future.

Black-start tests should be made periodically, preferably every 3 years or after a major event takes place. Where possible, black-start tests should also consider the expected static and dynamic behavior of system users during the planned island operation.