There was little indication that a blackout would soon cascade across the Iberian Peninsula when dawn broke this past April 28th. In both Lisbon, Portugal, and Madrid, Spain, springtime temperatures were expected to top out in the low 20s Celsius (70s in Fahrenheit), with ample sunshine and none of the extreme weather typically associated with blackouts.
Yet at exactly 12:33 p.m., approximately 55 million people across Spain, Portugal, and a small portion of France lost power, grounding airplanes, halting public transport, and triggering the deployment of tens of thousands of police officers across Spain to prevent possible looting and disorder. News of the blackout quickly grabbed global headlines, in large part because widespread power outages are exceedingly rare in Europe. The last significant outage to hit Spain was in 2006, when 10 million customers across Europe were impacted, including in Madrid and Barcelona.
The blackout also sparked immediate speculation about its cause. Some observers initially raised the possibility that a cyber-attack had brought the grid down, which Spain’s grid operator, Red Electrica, quickly ruled out as a cause. Critics of wind and solar power were also quick to blame renewables for the sudden loss of power, claiming, among other things, that an overabundance of solar resulted in a supply and demand imbalance that triggered the blackout.
Clearly, it is important to understand what caused the blackout on the Iberian Peninsula and to use that knowledge to both identify measures to prevent future outages and pinpoint gaps in understanding that must be addressed by rigorous research. Daniel Brooks, senior vice president, energy delivery and customer solutions at EPRI asked the following questions:
- What are the lessons that we should learn from this?
- Where are the gaps in the existing understanding and capabilities, as we have systems that continue to evolve and emerge into much more dynamic systems that are highly inverter-based?
- How should we work together to make sure that we have the capabilities to operate these systems to the reliability and resilience that we must have for the electric sector as we go forward?
These are not questions that can be answered quickly in the immediate aftermath of a blackout. In fact, significant data collection and analysis are necessary to understand the causes of an outage of this scale properly.
Sean McGuinness, an EPRI senior technical executive based in Dublin, Ireland, who specializes in blackout investigations and grid protection, says thousands of measurements from across Spain and Portugal were necessary to begin a thorough analysis. “There are log files that need to be collected. All of them need to be time synchronized and verified against each other,” McGuinness said. “You really do need a comprehensive country-wide set of measurements to verify you know exactly how it started, exactly what happened next, and exactly the cause and effect.”
A Clearer Picture Emerges
In June, Red Electrica released a report detailing its findings about the causes of the blackout and the grid operator’s recommended actions to avoid future outages. Investigations and research into the causes and best next steps will undoubtedly continue. Still, this report and other publicly available data help move beyond speculation to a more concrete understanding of the factors that resulted in the outage.
Iberia was in what system planners refer to as “shoulder season,” a temperate time of year when electricity demand is moderate and grids are typically less stressed. It’s also a season when operators perform planned maintenance. On April 27, multiple 400-kilovolt transmission lines were out of service in Spain, and studies conducted the day before identified 10 synchronous generators that would be needed to support voltage control during daylight hours. When one of those units declared itself unavailable that evening, it wasn’t replaced.
As April 28 began, morning loads ramped steadily to 25 gigawatts in Spain and 8 gigawatts in Portugal. With clear skies, solar PV generation surged to over 20 gigawatts by midday. Some hydro was pumping in preparation for evening peaks, and nuclear generation was stable. Wind output was relatively low. Overall, the grid appeared to be in a relatively benign state.
But early on the morning of the 28th, small oscillations began to emerge. Oscillations are fluctuations in power and voltage that occur when different parts of the grid briefly fall out of sync, often caused by generation and consumption imbalances across regions. By 10:30 a.m., one oscillation caused a noticeable four-kilovolt swing on a major 400-kilovolt line. These oscillations are not inherently dangerous—European operators deal with them routinely—but the fact that they kept recurring was a troubling sign. Around 11:00 a.m., they returned with increased magnitude and duration, lasting several minutes at a time and requiring operators to intervene and stabilize the system.
“Oscillations are not uncommon, and usually the system can handle them,” McGuinness said. “But by late morning, they weren’t going away. That was unusual.”
To mitigate under-voltage concerns during these oscillations, operators began switching out shunt reactors. Shunt reactors act like sponges, soaking up reactive power to keep voltages stable. But those actions had a side effect: increasing system voltage. With solar output already high and transmission lines lightly loaded, the additional capacitive VARs raised voltages further, setting the stage for what was to come.
A Quick-Moving Collapse
Just after 12:30 p.m., a power plant in southern Spain tripped offline. Usually, such an event is absorbed by the grid’s reserves without any problems. But shortly afterward, a second generating unit disconnected. Within seconds, a third event occurred, involving one or more large plants. This final loss pushed the system past its limit.
“There were sufficient reserves to handle two gigawatts of generation loss,” said McGuinness. “But once the third plant tripped, the system had no remaining flexibility.”
The cascading effects were nearly instantaneous. Voltage began to spike, and frequency declined as more generation shut down. The interconnection between France and Spain, already relatively weak, was automatically severed to protect the broader European grid. Spain and Portugal were islanded, and a blackout engulfed the entire Iberian Peninsula.
What made this event so startling wasn’t just its scale, but its speed. According to McGuinness, blackouts typically unfold over tens of seconds or even minutes. This one unfolded in as little as five seconds. “The system went from stable to blackout faster than anyone expected. That’s created a lot of questions.”
A More Complex Grid
Many questions have focused on the role of inverter-based resources (IBRs)—including solar PV and battery systems—which now supply an increasingly large share of electricity in Spain and Portugal. Unlike traditional synchronous generators, like gas and coal plants, which rely on large rotating masses, IBRs are governed by power electronics, which inject power based on programmed logic.
“This isn’t about blaming renewables,” said Eamonn Lannoye, managing director of EPRI Europe. “But how they connect to and behave on the grid is different. And those differences matter, especially when the system is under stress.”
One key issue was how voltage was managed on the grid. Many of Spain’s wind and solar plants were set up to operate in a fixed way that didn’t allow them to adjust voltage when the system was under stress. A new rule that would let them help stabilize voltage was planned for later in 2025, but it hadn’t been put into action yet when the blackout happened.
That inability to adjust voltage likely contributed to the severity of the event. Without the ability for IBRs to help regulate voltage, each plant that tripped meant not only a loss of megawatts but also of reactive power. This created a feedback loop: voltage rose, causing more plants to disconnect, which in turn caused voltage to rise further.
Distribution Grid Impacts
When voltage levels suddenly jumped, those changes quickly rippled down into local power lines and neighborhoods. In some areas, the sharp swings caused rooftop solar panels and other distributed energy resources (DERs) to shut off to protect themselves automatically. As a result, electricity that had been supplied locally by these systems suddenly had to come from the main grid instead, adding even more strain to an already unstable system.
“Fast voltage swings on the transmission system don’t stay there,” said McGuinness. “They’re reflected in the distribution grid. And if the distribution grid was already elevated, even small changes can push devices out of their protection limits.”
At the same time, some of the usual tools used to stabilize the grid weren’t available or couldn’t respond fast enough. Pumped storage hydro plants—which act like big batteries—were already running or couldn’t be brought online quickly. Some natural gas plants that could have helped were offline and needed over an hour to start up, which was too slow to make a difference. Meanwhile, possible load-shedding may have made things worse by lowering the amount of electricity flowing through power lines, which can cause voltage to rise even further in certain situations.
Lessons Learned and the Road Ahead
Despite the unprecedented speed and scale of the blackout, system operators restored power quickly. Portugal’s grid was fully energized within 12 hours. Spain’s was back up and running in under 23 hours.
While it was encouraging that power was restored so quickly, the blackout still revealed serious challenges in how the grid was operated. For example, some of the steps taken by system operators—like turning specific power lines and voltage control equipment on or off, and following standard safety procedures—followed standard protocol. However, when combined, those actions left the system too fragile, with not enough cushion to handle unexpected problems.
“From a procedural standpoint, operators did what they were trained to do,” McGuinness said. “But the system was so close to its edge that any additional disturbance pushed it into collapse.”
Several lessons from the Iberian blackout are already informing industry discussions and research. Chief among them is the need for improved voltage control, both in real time and across planning and operational timeframes. That means reconsidering how static and dynamic reactive power resources are balanced and ensuring more inverter-based systems can respond to the system’s voltage needs.
The outage also highlights the importance of protection coordination across the entire system. Developments such as the first generator tripping while voltages were still within normal range suggest a need to review and refine protection settings and protocols, especially for rooftop solar and other behind-the-meter systems.
Lannoye also emphasizes that grid resilience isn’t just about adding hardware. “We need to look at the full lifecycle—from planning to commissioning to operations—and find the procedural and regulatory gaps,” he said. “Sometimes it’s a matter of how assets are used, not just whether they exist.”
Grid codes and market designs may also need to evolve. For example, Spain is transitioning from 60-minute to 15-minute market intervals. Under certain price conditions, some solar plants curtailed output to avoid negative pricing, taking both watts and VARs offline. Those kinds of natural responses to market signals can have real-time consequences on physical grid stability.
Planning for Complexity
What makes the Iberian blackout such an important inflection point is that it occurred under seemingly normal conditions. There was no storm, no cyberattack, no extreme demand. It was a blue-sky day. And yet, a series of small factors aligned to create a continent-scale blackout in under 30 seconds.
“This wasn’t a high-stress day,” said McGuinness. “That’s exactly why it’s so important to understand. If it can happen then, it can happen anytime.”
As EPRI and other research organizations continue their analysis, the focus is shifting toward building new tools, simulations, and best practices for operating grids in a world of fast dynamics, decentralized resources, and highly automated systems.
Some of those efforts are already underway . Others will emerge as more is understood. However, one thing is certain: the lessons from April 28 will shape how modern power systems are designed, operated, and regulated in the years ahead.
EPRI Technical Experts:
Eamonn Lannoye and Sean McGuiness
For more information, contact techexpert@eprijournal.com.
