At 12:38 on Monday 28 April 2025 the electrical network of the entire Iberian peninsula went simultaneously in blackout, leaving many regions without electricity until late at night. The causes of the accident are not yet completely clear, and usually events of this type require months for a complete analysis. The most probable hypothesis is a significant cantor of the frequency of the network, combined with a low inertia of the system at that specific moment.

This is because our electrical network works with alternating current (AC): this means that the tension ranges continuously between two values ​​with a frequency that, for Europe, is set at 50 Hz (50 oscillations per second). All devices connected to the network (loads) operate within an extremely reduced tolerance (about ± 0.015 Hz). To avoid damage, the network is equipped with protection systems that automatically disconnect loads and generators if they detect too high or too low frequency.

How the energy system works

It was these systems that caused yesterday’s blackout. But how do you keep the frequency stable? Under normal conditions, traditional electrical power plants have turbines with rotational kinetic energy synchronized with network frequency. If less energy enters the network than the question, the frequency tends to decrease. The inertia of the turbines means that this drop takes place slowly, allowing the system to activate valves that regulate the influx of steam (in thermoelectric plants) or water (in hydroelectric), stabilizing the frequency. This mechanism allows you to quickly absorb any imbalances between supply and energy supply.

What went wrong yesterday? Yesterday, in Spain, over 70 percent of energy came from solar and wind plants, which generally do not use synchronized turbines with the network (the terrestrial wind generators, in fact, are mainly asynchronous). This makes the network more vulnerable to sudden imbalances. The exact cause of the prediction remains unknown. A fire on transmission lines could hardly cause such a large blackout.

Some graphs of the monitors of the Spanish energy network show a rapid increase in sun production immediately before the blackout, but this contrasts with the preliminary explanation provided by the Spanish network operator, which attributes the cause to a loss of generation located in the south-east of the country. A rare atmospheric phenomenon, known as “induced atmospheric vibration”, may have caused losses in the transmission lines, generating a sudden overhang of the frequency visible also in the European network.

The protection systems reacted promptly, detaching loads and causing a general blackout. Spain was also disconnected from the European network, causing a temporary blackout even in the French Basque region, which was currently exchanging energy with Spain.

In France, however, the Blackout lasted only a few minutes and already around 1:30 pm the Spanish northern regions had been reconnected, starting again thanks to imports from France. The rest of Spain remained without the current for the whole day, but fortunately the event took place in April and not in the summer, avoiding critical situations due to the heat.

The success of the “black start” of the network was positive, the restoration of the production of energy completed in less than 24 hours, a result not obvious given the high dependence on intermittent sources. A significant detail: Spanish nuclear power plants have activated the emergency procedure by automatically turning off as their cooling pumps depend on external nutrition. Backup systems worked correctly, avoiding accidents. However, at the moment, the nuclear plants have not yet been restarted, and the Spanish network is currently fueled mainly by hydroelectric and gas.

There is a first explanation for the blackout in Spain (and there were warnings)