What led to the historic collapse of the electricity system in the Iberian Peninsula?

What happened on Monday, April 28, 2025, in Spain and Portugal will certainly be included in history books dealing with European energy strategies and the development of the electricity system.

The complete shutdown of infrastructures essential to the fundamental functions of civilized society, affecting some 60 million people across a vast geographical area, attracted the attention not only of the Spanish and Portuguese citizens, but also of the population and decision-makers of the entire European Union.

 

Is this possible elsewhere?

Yes, it is possible. In highly complex electricity systems, disturbances can sometimes be widespread, and there are many examples from the past of rolling blackouts or even total power outages affecting large areas (e.g., Texas, USA, February 2021 – winter storms and extreme cold left 4.5 million households without power for days; 2006 Western European blackout, when 15 million households were left without power for several hours; 2003 Italian blackout, when a power line between Switzerland and Italy failed, leaving 50 million people without power for about half a day; Western Hungary, December 2014, when freezing rain destroyed power lines and brought down poles, causing power outages in some areas for days).

Obviously, we do not like these system failures because they cause major economic damage, but they are not entirely avoidable. At the same time, it is crucial to identify the specific cause of this incident on the Iberian Peninsula, as we can only avoid similar situations in the future if we know the cause of the actual problem and if learn from it.

 

What are the main characteristics of the Spanish and Portuguese electricity systems?

Portugal has a population of approximately 10 million, Spain has a population of 48 million. Portugal has approximately 20,000 MW of power plant capacity installed in its system, while Spain has approximately 120,000 MW, the distribution of which by energy source is shown in Figure 1. Half of Spain’s capacity comes from conventional power plants, a quarter from solar energy, and a quarter from wind energy. There are no nuclear power plants among the conventional capacities in Portugal, while Spain has seven nuclear power plant units with a total capacity of 7,100 MW.

An important feature of the Iberian Peninsula compared to the internal parts of the continental European electricity grid (e.g. Central Europe) is that this area is essentially a “dead end”: it is connected to the Moroccan network by a single submarine cable to the south, but to the French system by only two 220 kV, two 400 kV and one direct current cable (see Fig. 2). In this sense, it is more isolated than the interior of Europe, which means that it is less able to rely on cross-border capacities when dealing with operational disruptions.

 

 

 

What was the state of the Iberian Peninsula’s electricity system before and during the blackout?

We know the Spanish system’s planned load schedule for April 28, 2025 from the “day-ahead” schedule prepared on the previous day. This is shown by the black curve in Figure 3. This was the plan for Monday.

In contrast, the system load shown by the red curve was realized, meaning that on Monday afternoon, the system load fell to 10,000 MW instead of being around 25,000 MW. According to official Spanish reports, this 15,000 MW was lost on the generation side.

The Spanish authorities reported that around 12:30 p.m., within about a minute and a half, two major generators dropped out (disconnected from the grid), resulting in a total loss of 15,000 MW generation capacity from the system. This is clearly visible in the red curve. Losing 60% of the system load is very difficult to manage in an electricity system, so it is no surprise that the end result was what we saw in the news. Incidentally, the 15,000 MW was most likely due to the loss of solar power plants in southwestern Spain and their disconnection from the grid. This led to the shutdown of several conventional power plants due to a significant deviation from the nominal grid frequency, but we will come back to this later.

 

The Portuguese actually suffered much more, because their system is connected only to the Spanish electricity system, and as a result of the disruption spreading from the Spanish grid, the Portuguese system completely “collapsed,” with its output falling to virtually zero, as shown by the red curve in Figure 4 below. The Portuguese system load remained close to 0 MW for about 6 hours.

 

It is interesting to observe the flows across the Portuguese-Spanish border shown in Figure 5: while Portugal exported 1500-2300 MW of electricity to Spain in the early hours of Monday morning, the flow reversed at 9 a.m. and remained so until the time of the blackout. At that point, the systems disconnected and electricity flows between the two countries ceased for almost a day.

 

The next two figures show the power plant production data broken down by energy source for the two countries concerned.
In Spain, before the start of the disruption, total production was 32,368 MW, with nearly 19,340 MW coming from solar power plants, 3,416 MW from wind farms, 3,172 MW from hydroelectric power plants, 2,460 MW from fossil fuels, nuclear units 3,384 MW, and biomass power plants 356 MW (see Fig. 6). As mentioned above, the collapse of the system brought production below 10,000 MW in Spain, with the grid disruption affecting almost the entire country.

 

As Figure 7 shows, apart from biomass power plants, virtually all units were out of operation in Portugal, and the country was unable to import from Spain, so although the outage most likely originated in Spain according to official statements, Portugal was more severely affected. A complete six-hour blackout in a country of 10 million people is a very serious event, so it is no wonder that it received so much attention. As Spain has a population approximately five times larger than Portugal, the consequences in Spain will be greater in terms of both the area affected and the number of people affected, as well as in economic terms overall.

Finally, in Figure 8, let us look at flows across the Spanish-French border, because I believe this network connection played an important role in managing the disruption and ensuring a rapid restoration of services. The fact that both the Spanish and Portuguese systems were restored by Tuesday morning is a very significant achievement, and all those involved deserve credit for this.

On Monday, April 28, at dawn, Spain’s ENTO-E database showed a total output of approximately 2,500 MW to France. This decreased to 1,500 MW by 7 a.m. During the disruption, the flow at the border ceased, and France assisted the Spanish system with a capacity of between 500 and 2,000 MW until 11 p.m. on Monday, when the electricity flow at the French-Spanish border effectively ceased for several hours.

 

What could have caused the disruption?

This blogpost cannot and does not seek to answer that question. There is simply not enough information available at this moment. The Spanish and Portuguese authorities will carry out their investigations (using the large amount of data available to them) and draw their own conclusions. Hopefully we will learn the results soon.

I think it is important that the possibility of a cyberattack has been ruled out. It is also important that the initial claim that the incident was caused by extreme weather conditions has been dismissed. This was very strange from the very beginning, as it was easy to verify that there were no extreme temperatures on that day (in summer, temperatures in central Spain are much higher).

Operational disruptions caused by electromagnetic vibrations from power lines cannot be ruled out, but there are currently no circumstances that would justify this, and it is difficult to imagine that there could have been mechanical vibrations in the power lines that exceeded the mechanical effects of a strong windstorm, which these power lines have obviously withstood many times before. It seems much more likely that a (solar) power plant or substation failure triggered a chain of events that escalated to such a large scale.

It is also conceivable that some of the transmission lines were already operating permanently above their rated power and voltage levels, a generating unit or a network element failed, causing power flows to be transferred to other lines, which became even more overloaded, causing the initial local disturbance to escalate into a widespread cascade. We will find out when we receive the official investigation results.

 

What happened at the nuclear power plants?

Half of Spain’s nuclear capacity was operational during the grid disruption. These units shut down normally when the grid collapsed, the diesel generators started up normally and the units were brought to a safe state in accordance with regulations. We are not aware of any anomalies or malfunctions. Nuclear power plants are designed to withstand transient events that occur during a grid collapse, so this could not and did not cause any significant problems. In such cases, the nuclear power plant units are completely disconnected from the grid, and all necessary safety systems are powered by diesel generators, which was also the case here. The nuclear power plants did not place any extra load on the grid, as they are able to supply their own electricity in such cases.

 

What should be accomplished, and what are the implications at the level of the electricity system?

1. We definitely need to see the results of the Spanish investigation. Further conclusions can be drawn from that.
2. It is certain that in systems with high solar energy capacity, fluctuations in production place additional technical requirements on the system. This must be taken into account continuously.
3. The resilience and robustness of the network is a key issue. During the green transition, many countries have failed to make the necessary upgrades to their networks, which increases the vulnerability of the systems. If a transmission system operates in a permanently overloaded state, this is certainly not beneficial and will increase its vulnerability to network disturbances. This problem is an ongoing challenge for distribution network operators, transmission system operators and governments in all countries.
4. It is extremely important to establish and develop appropriate monitoring systems so that we are aware of the actual state of the system.
5. In conventional power plants, steam and gas turbines are large rotating masses that are extremely important for handling small transients. The rotating mass of turbines gives the system high inertia, which increases robustness. Photovoltaic power plants do not have this inertia. From the point of view of the stability of the electricity system, it is essential that there is sufficient rotating mass in the system. It is also required that there are power plants which provide flexibility. Due to their technology, solar and wind power plants cannot meet these requirements.
6. An important lesson from the Spanish blackout is that although large-scale network disruptions are rare in Europe, they cannot be ruled out. Moreover, with the spread of volatile, weather-dependent renewable sources, the system may be more vulnerable to problematic system states. We know that many high-voltage transmission line construction and network development projects have been delayed or postponed in European countries due to a lack of public support or other reasons. The development of the electricity system is inevitable if decarbonization continues, which will result in significant changes in the energy mix and system characteristics. During network development, particular attention should also be paid to improving the monitoring and remote control capabilities of network elements.
7. A stress test of the electricity systems of European countries is needed. It must be determined to what extent the vulnerability factors that contributed significantly to the Spanish-Portuguese blackout are present in the systems of other European countries.
8. There was a total system blackout in Portugal, but a significant part of the Spanish network also went down. The key question is how to restart the system from “total darkness.” This is obviously documented in the operating manuals of system operators and large power plants in every country, but whether the plans actually work in a real-life situation is another question. For this reason, it is essential to check the black start capability of conventional power plants, including nuclear power plants. This should also be part of a stress test covering European countries so that if a similar malfunction occurs in another country, it is possible to return to normality as quickly as possible from this very unfavorable system state, which causes great damage to the general public and the economy.

What should individuals and households do?

1. As we have seen in recent days, it is useful to be prepared for power outages. Not because we want them to happen, but because they do happen from time to time, and their impact is less negative if we are prepared for them.
2. If you have a battery-powered torch, candles, matches, some canned food, a few litres of water and some cash in your household, this can help you get through such an event lasting a few hours or a few days. In winter, alternative heating without electricity may also be an important consideration.
3. A major problem in such situations is the uncertainty caused by a lack of information. We could say: “When there’s power, everything works. Without it, nothing works.”, or in a more compact way: “Power on, life on. Power off, game over.” I mean without electricity, there is no internet, no social media, no mobile network, no TV… So it is useful to have a radio at home that runs on batteries or a rechargeable battery. That way, we can follow official announcements from the authorities. It is easier to get through a difficult period if we know what the real situation is and what the prospects are.
4. When buying an electric car, it is worth paying attention to whether it has an option that allows external devices to be connected to the car battery. The battery of a modern electric car can store 60-80 or even close to 100 kWh of electricity, which can cover the daily electricity needs of an average household (8-20 kWh) for several days. It can be a great help if we can at least run the refrigerator from such a battery, if we can recharge the batteries of smaller appliances (torches, radios, mobile phones).

But let’s be optimistic and hope that these events will not become regular occurrences in Europe.

 

Note: The source of data for all figures was the ENTSO-E database, but the figures were created by us. My colleague Bence Biró assisted me in preparing the figures, for which I am very grateful.

Prof. Dr. Attila Aszódi

Full professor at the Institute of Nuclear Techniques of Budapest University of Technology and Economics. Currently, he is the dean of the Faculty of Natural Sciences at the Budapest University of Technology and Economics (Hungary). He took part in several projects of the International Atomic Energy Agency and the European Commission. He has more than 300 published scientific articles. His research interest is reactor safety, thermal hydraulics of nuclear reactors, Computational Fluid Dynamics, Particle Image Velocimetry, nuclear power plants, energy policy and sustainability.

The opinions expressed on this page are those of the authors. They do not purport to reflect the opinions or views of the European Nuclear Society.