Early in the afternoon of 28 April 2025, as far as more than 50 million residents of Spain and Portugal knew, their electrical grid was working fine. But at 12:33 pm Central European Time (CET), the power went out across the two countries and also in a sliver of southern France served by the same grid (
Fig. 1) [
1]. One writer living in Madrid likened the ensuing events to “the beginning of the end of the world” (
Fig. 2) [
2]. Traffic gridlocked, trains and subways halted, cell phone and internet service went down, stores and businesses shuttered [
1-
3]. In just the first few hours of the black-out, Madrid’s firefighters responded to more than 200 emergencies, many involving people stuck in elevators [
4]. Airlines canceled around 500 flights, stranding about 80 000 travelers [
5]. In Spain, as many as 167 people, mainly elderly women, died because of the effects of the blackout, according to estimates based on historical mortality rates [
6].
Efforts to restore the power started shortly after the outage began, and by early the next day 90% of Spain and a large part of Portugal had regained electrical service [
7]. Even before the lights came back on, however, speculation about the cause of the blackout was rife. Some so-called pundits seized the opportunity to blame renewables, which provide about 43% of Spain’s electricity, claiming that they undermined grid reliability [
8]. Another much-discussed possibility was a lack of electrical inertia—again the result of Spain’s extensive deployment of renewables—that prevented the grid from damping oscillations in the frequency of its alternating current, eventually causing power plants to disconnect [
8]. A cyberattack was another early suspect, and chatter on the internet blamed a mysterious phenomenon known as induced atmospheric vibration [
8,
9].
What happened on that day is now clearer, thanks to investigations by the Spanish government and the country’s grid operator Red Eléctrica, as well as the findings of an expert panel convened by the European Network of Transmission System Operators for Electricity [
8,
10,
11]. “The issue did not originate with renewables,” said Ali Mehrizi-Sani, professor of electrical and computer engineering at Virginia Polytechnic Institute and State University in Blacksburg, VA, USA. Nor was the blackout the result of a cyberat-tack, inadequate grid inertia, or some nebulous atmospheric event. Instead, the investigations concluded, the trigger was a rapid voltage increase in the Spanish system that was not corrected, leading to a cascade of power plant disconnections and then a grid collapse [
10,
11]. Portugal’s grid, which connects to Spain’s, also shut down as a result. So far, however, the investigations have left one critical issue unresolved, said Mani Venkatasubramanian, professor of electrical engineering at Washington State University in Pullman, WA, USA. “The question remains—why did the voltage rise in the beginning?”
As in other electrical system failures, the outage involved many different factors and “some quite complex interactions,” said Keith Bell, professor of electronic and electrical engineering at the University of Strathclyde in the United Kingdom. The information presented in the investigative reports provides Spain with a to-do list for improving its electrical system. The findings may also help other countries that are trying to maintain grid resilience while integrating more renewable sources into their energy mix. Renewables “add a layer of complexity” to grid operation and management, said Deric Tilson, a senior nuclear energy innovation analyst at the Breakthrough Institute, an environmental research think tank based in Oakland, CA, USA. Given that power demand will likely soar in the near future, largely from increased use of artificial intelligence [
12], maintaining grid reliability will become even more difficult, said Venkatasubramanian. “The grid is going to be challenged in the next ten years,” he said.
Even though the electrical grid is, in Tilson’s words, “the largest complex system ever made by humans,”catastrophic failures like the one in Spain and Portugal are rare. One famous example is the August 2003 outage that left about 50 million people without power in the eastern United States and Canada for up to 7 h [
13]. And in 2021, a severe winter storm knocked out power for almost 70% of Texas residents for nearly two days [
14]. The last incident of this scale in Europe occurred in 2003, when Italy’s electrical grid collapsed, said Bell.
One reason that grids usually keep delivering electricity is that they are designed to “control voltage and frequency 24/7,” said Mehrizi-Sani. Electrical inertia, which comes mainly from spinning machines such as generators in nuclear and fossil fuel-burning plants, is key to managing grid frequency. If the frequency begins to fall below 60 Hz in the United States (or 50 Hz in Europe), that is a sign power demand exceeds supply [
15]. If the frequency sinks below 59.3-59.5 Hz in the United States, grids begin disconnecting users, a process known as underfrequency load shedding [
15]. However, electrical inertia slows the frequency decrease, giving generating plants time to kick in, adding more power to the system and boosting the frequency [
15].
If restorative measures fail, however, frequency can continue to decrease and ultimately cause the grid to collapse [
15]. Some early discussion about the cause of the Spanish blackout zeroed in on renewables because typical inverter-based components, including most wind and solar facilities as well as grid-scale battery systems, lack inertia [
15,
16]. Without specifically designed control actions, they cannot help control frequency—so-called grid-forming inverters that can help check frequency fluctuations are just beginning to come online [
16]. As some countries have increased their use of renewables, they have compensated for the loss of inertia from conventional sources by installing devices designed to provide it, including gigantic flywheels and huge rotating cylinders known as synchronous compensators or synchronous condensers [
17,
18]. Mainland Spain had none of these devices [
17]. That deficiency, however, proved irrelevant during the blackout [
8]. “This incident was not caused or exacerbated by a lack of inertia in the grid,” said Victor Becerra, professor of power systems engineering at the University of Portsmouth in the United Kingdom.
For grids to remain stable, voltage also needs to remain within narrow, lower and upper limits [
8]. If voltage rises too high, short circuits can occur and loads can trip, or disconnect from the grid, to prevent damage. One way that conventional power generators, such as nuclear and fossil fuel plants, and renewable facilities can curb voltage changes is by adding or removing a component of power known as reactive power that helps the grid deliver the active power that runs electrical devices and equipment [
8]. Increasing the amount of reactive power in the grid can push the voltage up, and reducing the amount can drive the voltage down. However, reactive power “can lead to under-voltage or overvoltage if not handled properly,” said Venkatasubramanian. Inadequate amounts of reactive power contributed to the 2003 outage in the northeastern United States and Canada [
13].
The three investigative reports laid out, sometimes in second-by-second detail, the events in Spain on 28 April 2025. Two periods of grid fluctuations preceded the blackout. Starting at 12:03 pm CET, the voltage and frequency began to oscillate rapidly [
11]. Grid operators managed to tamp down these swings within five minutes, reducing the amount of power that Spain exported to France, and initiating other procedures [
11]. The second period of oscillations began at 12:19 CET, but grid operators also quickly brought these fluctuations under control [
11]. However, “one of the consequences of the [corrective] procedures was that voltage on the network got higher,” Bell said.
That increase in voltage “quite likely” contributed to what happened next, said Bell. At 12:32 pm CET, voltage in the grid began to rise rapidly. In some parts of the system, it climbed to more than 460 kV, well above the upper limit of 435 kV [
11]. Around this time, generation facilities started to trip. Why some of these facilities disconnected from the grid remains uncertain, but some did so to prevent damage from the soaring voltage [
11].
Reactive power “was the critical factor” in the grid’s collapse shortly afterward, said Venkatasubramanian. Red Eléctrica had contracted with several conventional generation plants to help limit voltage changes, said Becerra. However, these plants did not properly perform the service and thus did not absorb enough reactive power to lower the voltage [
19]. “The inadequacy of these generators to control the voltage became one of the key causes of the problem,” Becerra said.
The reports from Spanish government and Red Eléctrica disagree about who is to blame for the outage. The government claims that Red Eléctrica incorrectly predicted the mix of energy sources required on that day and thus did not have enough thermal plants online just before the blackout, leaving the grid vulnerable to a voltage increase [
19]. However, Red Eléctrica denies that charge and places the blame entirely on the power plants that did not kick in to help moderate the voltage [
20].
In any case, experts say, several steps might have prevented the blackout or reduced its severity—and might forestall a repeat. For one thing, inverter-based renewable facilities could have helped tamp down voltage by absorbing reactive power, noted Becerra, but the existing Spanish rules for grid operation, drafted 25 years ago, only permitted conventional sources to intervene. “Inverter-based renewable energy is an important element in preventing future incidents like this,” he said. Spain has already upgraded its operating rules to allow renewables to help control voltage [
21].
Spain and Portugal might also have gained from greater inter-connectedness with their neighbors [
8]. At the time of the black-out, they were able to share about 2.8% of their power with France and Morocco—the European Union (EU) currently recom-mends a value of 10%, which will rise to 15% in 2030 [
22]. Stronger connections with other countries could have buffered some of the fluctuations that led to the blackout, said Becerra. Spain and Portugal are constructing a new high-voltage line that will allow their grids to exchange an additional 2200 MW with France—but the two countries still will not meet the EU’s recommended inter-connection percentage [
22].
Additional grid-scale battery storage might also have helped to stabilize voltage by quickly absorbing or injecting power, said Mehrizi-Sani. In other countries such as the United Kingdom, these battery systems have proven their ability to keep the grid operating [
23]. But Spain boasts less grid-scale battery storage than US states such as Texas and California [
24].
Some of the lessons from the blackout are specific to Spain and Portugal. The United States does not need to change its rules so that solar and wind facilities can help regulate grid voltage—they are already required to do so [
25]. But some of the lessons apply to other countries as well. For example, Spain and Portugal are not the only European countries that have not met the EU’s interconnection target of 10%—six others, including France, Italy, and the Netherlands, also fall short [
26].
The outage also suggests that despite extensive research and modeling, experts still do not fully grasp the complexities of grid function. The voltage surge came as a surprise, said Venkatasubramanian. Other outages, including the 2003 North American blackout, featured falling voltages as demand out-stripped generation. “We have studied and prepared for voltage decreases, not for the cascading over-voltage seen here,” he said. That suggests researchers need to dig deeper into how different power sources interact and what keeps the grid resilient—and what can cause it to fail. “We need to have a better understanding of what to be ready for,” Venkatasubramanian said.
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