Black swan or failure. What we still need to know about the blackout

Almost two weeks after the blackout that left the entire Iberian Peninsula without power, the cause, or causes, are still unknown. The preliminary survey released by ENTSO-E (the association of European network operators that is investigating the incident) reveals that there were two fluctuations in the synchronization of the European network before the major event. But they were controlled. At the time of the blackout, there were no fluctuations and the network was operating within normal bands, when part of the energy production in Spain fell, uncontrollably unbalancing the Iberian system. It all happened in less than 30 seconds.

The reconstruction of those seconds and what happened before and after could take some time, warned the Spanish Prime Minister. Pedro Sánchez justified the delay with the need to investigate 756 million data points collected from the operators of the Spanish system — which could take at least three months — and promised full transparency .

Some experts have described the event as a “black swan”, that is, the result of an accumulation of factors or a combination of variables that no one predicted and therefore could not have been avoided. However, at the same time, evidence has emerged of recent failures in the Spanish electricity grid that have been undervalued. And reports are emerging warning of risks and the need to invest more that had been shelved. The epicentre is in Spain, but Portugal cannot escape the crisis of confidence that has hit the electricity system because it is, for better or for worse, highly integrated into the Spanish grid.

Energy independence and security of supply have been signalled by the commercial separation of the two networks after the blackout. The decision was also political (REN took over coordination with the Government) during the election campaign, but it has costs because energy made in Portugal is more expensive. There has been an announcement of a limited return to commercial exchanges, but the Portuguese network manager demands more information about what happened on the other side of the border.

Neither REN nor the Spanish Electricity Network (REE) have yet released any assessment or investigation into what happened on April 28. The French grid operator, RTE (a minor player in this crisis), has published an exhaustive 20-question explainer .

The president of APE (Portuguese Energy Association) acknowledges that it is “strange” that the cause is still unknown, when in other blackouts it was possible to find out almost immediately what went wrong and the reasons why “the protection and control mechanisms did not respond as they should have”. But it is important to understand, stresses António Coutinho, that the system we have today is very different to the one we had 25 years ago when the famous stork incident occurred. And there is a great interest (in the energy world) in knowing what happened.

In addition to the causes, the president of APE argues that it is important to learn lessons that are not limited to the electricity sector. “We cannot ignore a good crisis to improve the system and increase resilience.”

It is known with almost 100% certainty that the origin of the problem was in Spain and in the electricity transmission grid, but we still do not know precisely where it started. For example, it was said (by the Spanish authorities) that the plants that automatically shut down were located in the southwest of Spain — Extremadura and Castile —, but it is not known where, or which plants went down. Although it is assumed that a significant part of these plants were solar power plants, since this was the technology that was clearly dominating the energy mix.

In the preliminary report released this Friday, ENTSO-E once again refers to the south of Spain as the location of the plants that were disconnected.

There is information that the nuclear power plants that were connected were automatically deactivated (for protection), but it is not clear what their role was.

It is also known that the failure of the transmission network resulted from a sudden drop in the injection of energy production. According to the association of European operators, 2,200 megawatts (a figure higher than the 1,500 MW initially reported) that were being injected in Spain disappeared from the network. There were no production failures in Portugal or Spain. As a result of this event, the frequency dropped and the voltage rose in the networks of Spain and Portugal.

When the frequency reached 48 hertz, synchronization in the electrical grid requires 50 hertz, which automatically activated the protection plans of the Spanish and Portuguese systems, turning everything off and leaving no room to isolate the problem and stop the contagion from spreading to Portugal.

According to the timeline released by ENTSO-E , the 2,200 MW of power — a large part of Spain's consumption at the time — went out in 20 seconds. After six seconds — between 12:33:18 and 12:33:24 (Spanish time), the electricity system completely collapsed in the Iberian Peninsula and the interconnection with France was interrupted.

Another consolidated piece of information is that Spain had a strong export surplus at the time of the blackout, supplying electricity to France, Portugal and Morocco because there was a lot of solar energy entering the grid at low prices. According to ENTSO-E, Portugal was the country that was receiving the most energy — 2,000 MW. France was receiving 1,000 MW and Morocco 800 MW.

Unlike in Portugal, the French grid’s protection systems worked. There was an automatic shutdown of power plants in south-west France (including a nuclear reactor) and the operator activated power cuts to large industrial and some domestic consumers to reduce demand. These cuts lasted less than half an hour. France managed to contain the damage before it contaminated its grid and the rest of the European grid. A substantial part of this success was due to the relatively low interconnection between the two countries, which limits the amount of energy that France can buy from Spain given the size of the market.

In Portugal, the interconnection with Spain supplies more than 20% of total national consumption and at the time of the blackout Portugal was purchasing almost a third of its needs. With all the doors open, the Portuguese network was caught in a chain of automatic system disconnections — from the power plants to the substations — that lasted seconds. There was no room for a load shedding that would have saved islands in the network, nor even time for any intervention by the manager.

Although most experts argue that Portugal would always be more vulnerable even if it were not importing, REN suspended imports for the 10 days following the blackout, in the name of supply security and awaiting further responses from its Spanish counterpart. The interconnection is operational, but the doors only began to open on Thursday.

But if the interconnection left the door wide open to the tsunami that was coming, it also helped to restore the service. Although Portugal had to use the two power stations that allow the system to start up autonomously from scratch — Castelo de Bode, in the center, began supplying the network at 4:11 pm and Tapada do Outeiro, in the north, started up at 5:26 pm — the restoration of power at the end of the day was aided by the invaluable interconnections with Spain from Trás-os-Montes and the Algarve, which began to be resumed at 6:36 pm.

In Spain, where the autonomous start-up of power plants did not work, the interconnection with France and Morocco was essential to restore the energy supply to the grid at the end of the day of the blackout. The ENTSO-E report confirms that the re-energization of the Spanish system began as early as 12:44 with the resumption of part of the interconnection with France and 20 minutes later with Morocco.

“Interconnections allow us to increase the economic efficiency and resilience of the system”, but at the same time “when I am interconnected I am exposed” and this means that “I have support” if I have problems, and that I am vulnerable to the problems of my neighbour, stresses António Coutinho. Portugal has demonstrated that it has the capacity to function without interconnection, but “we are not more resilient, nor more efficient” (from an economic point of view we are paying more), concludes the president of APE.

Another fact that has already been identified is the existence of an unusual number of disturbances in the stability of the Spanish electricity grid, which are believed to have resulted from fluctuations in the frequency level prior to the blackout. According to the Spanish press, on April 22, almost 20 cascading disconnections of lines and photovoltaic plants were recorded in just one minute. This succession of incidents led to localized drops in supply that affected a refinery in Cartagena and the high-speed train network in Madrid.

REE considered these anomalies to be normal, adding that their combination caused voltage fluctuations that were resolved within a few minutes. These fluctuations were attributed to changes in flow with Portugal, a drop in photovoltaic production and maintenance work on the interconnection with France. In its response to El Mundo, the Spanish grid manager said that these voltage fluctuations were resolved within 5 minutes.

On the day of the blackout, three frequency instability incidents were identified that caused power failures to enter the grid. The European operators' association confirms that the European electricity grid felt these two disturbances in the continental European synchronization area, but that it recovered stability. One of these incidents occurred 19 seconds before the major event — the 22 gigawatts that fell in a few seconds because the power plants automatically shut down to protect themselves from the band oscillations — that took down the entire grid.

But it is not clear what role these disruptions played in the massive outage that occurred, since the system was able to digest them. Nor is it known what specific event precipitated the massive power loss that occurred at 12:33 (Spanish time). Nor has a cause-and-effect relationship been established with the incidents that affected power in localized segments of the grid in the previous days.

The available information indicates that when the incident occurred, solar was the clearly dominant energy source in the Spanish production mix and represented, together with wind, 70% of supply. This is a significant level, but it is not unprecedented and has already been seen on other days in 2024 and 2025. In other words, the French network concludes in its explainer, this is not enough to attribute blame for the blackout.

All experts agree that a strong exposure of the electrical system to renewable production, particularly solar and wind, generates less inertia in the electrical grid, which in turn increases the phenomena of desynchronization in the grid. This is a consequence of the technology, the IBR (non-synchronous generators), which connects renewable power to the grid without generating the inertia that allows frequency fluctuations to be absorbed, unlike the connection devices of traditional power plants that generate this inertia.

As expert and consultant António Vidigal explains in a LinkedIn post, “during periods of low inertia, a sudden large imbalance in generation and consumption power, due, for example, to a large generator suddenly leaving the grid, causes a large instantaneous frequency deviation. On the other hand, during times of high inertia, the same generator tripping causes a small frequency deviation.”

The data already on the table suggests that this phenomenon played a role in the Iberian blackout. However, as similar situations have been overcome by the system in the past, it is not yet clear what was different this time and caused the glass to overflow.

Among other theories put forward to explain what was different on 28 April is an error in production forecasts that underestimated the amount of renewable energy that would enter the grid and a regional asymmetry between the regions that were producing (the provinces in south-west Spain) and those that were consuming the most. This is a characteristic of the Spanish system that increases pressure on the grids and on frequency balances, as cables have to be very long to connect consumption and production, crossing areas with no demand. In this context, greater control of the voltage level is advisable.

But could the system's lack of inertia constitute a blind spot in the energy debate? The question was posed by RTE. The French operator's response is self-serving, given that the current system in France is marked by a strong nuclear bias.

The French company refers to a 2021 report in which it argues that an electricity system that relies solely or almost entirely on renewables is riskier from a technical feasibility point of view than a system that maintains a significant nuclear fleet. A thesis that is supported by a RTE director on Linkedin.

The problem is known and there are mechanisms that allow the system's lack of inertia to be mitigated or compensated, either through batteries or by keeping conventional power plants (including hydroelectric power plants) connected to the grid even when they are not producing. Here, after the Sines coal-fired power plant was disconnected, a synchronous compensation service contract was signed with Alqueva, with the aim of continuously adjusting reactive energy compensation and stabilizing the grid, allowing the latter's voltage to be controlled.

But as more wind and solar come onto the grid and conventional ones go off, the risk increases and managing inertia becomes a critical system variable.

Although the much-talked-about exposure of the Iberian system is attributed to high injections of renewables, the truth is that there are countries with greater penetration of green energy that have managed to increase the resilience of their systems. This is the case of Scandinavia, but here the intense network of interconnections between markets gives it an advantage compared to the energy island of Iberia, says António Coutinho. Ireland, a literal island, is another good example highlighted by the effort made in the resilience of the network.

This will probably be the most investigated blackout in recent European history, especially when the contagion effect that interconnections can cause is also at stake. In the integrated European market, a local blackout assumes systemic importance and for this reason there will be an investigation conducted by the association that brings together European network operators, ENTSO-E, and conducted by independent experts. One of these experts is the Portuguese Albino Marques , who will be responsible for organising the regional group for continental Europe. Albino Marques was the person in charge of coordinating REN's electricity system until 2021. He is currently a consultant for REN and coordinator of ENTSO-E.

The investigation will be conducted in two phases. In the first phase, the panel will collect and analyze all available data on the incident, reconstruct the April 28 event and determine the causes of the blackout in a report that will be made public. The second phase will include making recommendations to help prevent future incidents.

Spain and Portugal have three months to submit all the requested data and these experts have up to six months to produce a report. Entities such as the association of European regulators and the European Commission will also assess what happened.

In parallel, internal investigations are underway in Spain, with the creation of an intra-governmental committee that could take between three and six months to determine the causes. More than 750 million pieces of data provided by operators and registered in the systems are being combed through and no hypothesis has been ruled out, not even that of a cyber attack.

In Portugal, and after an initial preliminary survey, REN will have to submit to ERSE (Energy Services Regulatory Authority) a full report on the blackout within 20 days — a deadline that may be extended — that answers several questions: cause of the supply interruptions and their justification; consequences of the interruptions, namely, the number of customers affected, the geographical areas affected and the energy not supplied or not distributed; actions to restore service, procedures adopted, difficulties encountered and communication strategy.

The Government also requested audits of transport, communications and aviation regulators to understand how these critical systems responded.

A grid with more renewables must evolve technologically, which requires more investment. But that’s not all. The level of demand for flow management is much higher. Not only because renewable power is intermittent and unpredictable, but also because it is decentralized across thousands of units. The electrification of consumption, especially in industry, requires increasing and, at the same time, dispersing the capacity to receive and supply energy. To do this, it is necessary to rely more on system services that enable a defense shield to be activated against the volatility of renewables, which involves, for example, having a dispatchable plant (hydro or thermal) ready to start up, even if it ends up not producing if it is not needed. All of this costs money and can impact prices.

It is up to the grid operators, REE in Spain and REN in Portugal, to propose the investments they consider necessary for the resilience of the grid. These plans are reviewed by regulators because there are tariff impacts (in Portugal, ERSE issues an opinion), but the final decision lies with governments. It is also up to the system operators to warn of security of supply risks and indicate what is necessary to minimize these risks.

The network operator's responsibility in this type of event is divided into two areas. The first is operational and involves understanding whether REE and REN managed the situation appropriately, whether they processed all available information, whether they assessed the risks and whether they did everything to overcome them.

Reports that have already appeared in the Spanish press seem to indicate that there were signals that were ignored or not taken into account by REE, such as the aforementioned oscillation disturbances that affected the frequency level days before.

Then there is a more structural axis that primarily involves REE and REN, but also the entities with planning and regulatory responsibilities in the sector. And, of course, the Government, which has more influence in the Spanish market than in Portugal. The management of the electricity grid is chosen by the State, which owns 20% of the company. The current president, Beatriz Corredor, who has been the target of much criticism, came from the Sanchez government. On the day of the blackout, the Spanish Prime Minister took on the leading role and was personally at the REE dispatch centre during the system recovery operations.

Luís Montenegro already went to the REN dispatch center, but the person who was coordinating (and later explaining) how the system was being reestablished was the administrator of the network manager, João Conceição, who was the face instead of the president, Rodrigo Costa, who was away from the country on the day of the blackout.

The challenges of energy transition in transmission and distribution networks are a hotly debated topic in Portugal and Spain, and there are European guidelines to increase resilience. However, the blackout has brought to the surface other concerns that were not always seen as a priority. This is because all of this costs money and these investments are remunerated by electricity tariffs, which means higher prices for consumers. This is a consequence that the government and regulators are trying to avoid by arguing that renewables allow prices to be lowered. This is true in terms of production, but not in terms of the networks.

In the aftermath of the blackout, the Spanish press echoed a REE report that was submitted in January to the Directorate-General for Energy, which acknowledged that there were areas in the infrastructure that could become critical with greater integration of renewables and less conventional production (hydro and thermal). This development required reinforcing the grid protection systems, as they might not detect faults and respond to resolve them. In statements to El Mundo, the ministry responsible for energy explained that the investments identified were part of a five-year plan and that REE had not signalled any urgency in implementing them.

Here, the supply security monitoring report, prepared based on information provided by REN, indicates that Portugal already establishes minimum technical requirements for the connection of generating centers of a certain size, in order to ensure the correct functioning of the system in the face of a reality marked by “an increasing number of operating hours with limited frequency and voltage control capacity, as well as very low inertia”.

For the president of the Portuguese Energy Association, the blackout demonstrated the need to invest more in frequency regulation systems. “I can’t want to have renewable energy and then not adapt the entire electrical system”, stresses António Coutinho. It also showed how dependent we are on energy for the most basic things. And this dependence will only grow because electrification has been the main way to decarbonize the economy. The power outage also revealed vulnerabilities in critical systems and essential services, among others.

António Coutinho points to the example of traffic lights, which, although not an essential service, can seriously disrupt society when they fail at critical points. Another example is service stations, which supply fuel for generators that keep the lights on when the electrical system fails, and which also stop working when there is no electricity. There is also the case of mobile phone operators' antennas, which have batteries that last only two hours, which has proven to be clearly insufficient.

One aspect that should be taken into account is that it is increasingly cheaper to install the capacity to produce and store distributed energy and that even electric cars can play this role (if they are charged). The expert concludes: “we have to look at energy use in a way that ensures the functioning of society and this implies having more redundancies”.