Presenting the incident report to Spain’s Council of Ministers last week, Minister for Ecological Transition and the Demographic Challenge Sara Aagesen said that a ‘multifactorial’ failure of voltage control had led to the hours-long power outage. The grid failure hit just after midday on 28 April, affecting nearly the entire Iberian Peninsula, including parts of Portugal.

The report paints a stark picture of how the grid unravelled in just 12 minutes. Voltage instability had been mounting for days prior to the incident. At 12:03 on 28 April, an ‘atypical’ 0.6 Hz oscillation destabilised the grid. REE took mitigating actions, such as reducing power flow from France and increasing grid meshing, but these measures inadvertently increased voltages further. Within half an hour, facilities across at least seven provinces were disconnected, and by 12:33 the system collapsed entirely.

The resulting blackout caused widespread disruption. Schools, offices and transport hubs were shut, while some people were trapped in lifts and trains, particularly in remote areas.  

However, the report found that recovery began swiftly, aided by hydro plants and power imports from Morocco and France. By 10 pm nearly 50% of the country’s supply was back online, and by 7 am the next day, 99.95% of coverage had been restored.

The report ruled out cyberattacks and said that the high share of renewables in the energy mix (41%, according to the 2024 Statistical Review of World Energy) had not contributed to the outage.  

A number of procedural shortcomings were cited and the report has recommended a sweeping set of reforms. Key among these are stricter supervision of voltage control obligations, new protection protocols against overvoltages and enhanced use of power electronics in asynchronous plants under Operational Procedure 7.4. The report also calls for fast-tracking European cybersecurity rules and expanding interconnection capacity with France and other neighbouring countries.

A resilience package to strengthen the grid is to be presented at the next Council of Ministers, said the government.  

Spanish grid operator REE published its own investigation and analysis of the situation, which it also described as multifactorial, but did not appear to accept responsibility for the incident.  

Instead, it partly blamed conventional power generation sources (CCGT, coal, nuclear and large hydro) for not compensating quickly enough to the voltage changes, as they are required to do. It said: ‘Generation subject to Operating Procedure 7.4 failed to comply with its dynamic voltage control obligations, leading to higher-than-expected voltage levels on the system. Furthermore, voltage excursions—both upward and downward—tend to be more pronounced due to this noncompliance. Generators typically respond only when voltage deviations become significant, suggesting that their response is primarily driven by internal plant protection mechanisms.’  

It also blamed a single solar plant: ‘The 0.6 Hz frequency oscillation at 12:02 pm may have originated in a photovoltaic plant in the province of Badajoz [Extremadura]. This oscillation reappeared minutes later, and on a third occasion, just before the appearance of a 0.2 Hz inter-area oscillation. It is believed that the plant's oscillation led to measures being taken to dampen the oscillations the system was experiencing, which reduced its voltage control margins.’

The report also blamed disparities in the way the grid works across interconnectors, and said that Spanish and Portuguese actions were out of sync.

REE also stated: ‘The incident was NOT caused by an inertia problem in the system,’ referring to the grid’s ability to withstand frequency volatility through the use of large rotating masses such as turbogenerators from conventional power. It continued: ‘The incident was caused by a voltage problem and the cascading tripping of renewable generation plants, as indicated. Greater inertia would have caused the frequency drop to be slightly slower, but due to the massive loss of generation due to voltage, the system would not have been recoverable.’

The REE report compiled a list of recommendations, which among others included specific investment in grid infrastructure ‘to continuously and dynamically control voltage, in addition to generators, using synchronous compensators or STATCOMs, instead of relying solely on discrete devices such as reactors or capacitors.’

A third Spanish organisation, a trade association of Spanish electrical utilities, Aelec, published its own view of events that disputed the conclusions of REE, which it blames for the blackout.  

It said: ‘The report of the Government Committee for the analysis of the electricity crisis of April 28 has confirmed that sufficient means were not enabled to control the voltage in the electricity system. The responsibility for having these means lies with the system operator, Red Eléctrica de España (REE).

‘It is worrying that the system operator does not assume its technical responsibilities, contradicting the conclusions of the government report, and attributing the origin of the blackout to small generation disconnections. If the synchronous resources necessary to deal with generation failures and control voltages had been available, as is usually the case in a well-managed electricity system, the system would not have collapsed.

‘Therefore, to claim that everything was done correctly and that a partial disconnection of generation alone justifies a widespread blackout, is reputational damage for the Spanish electricity sector and generates unnecessary uncertainty for the industry as a whole.

‘We would like to reiterate that the plants managed by our member companies operated normally during the day of 28 April. All of them complied in a timely manner with the orders of the system operator, reacted to the oscillations and increases in voltage observed, and acted as established. The real cause of the blackout was not the performance of the plants, but the manifest insufficiency of units dispatched by the operator to safely cover all the needs of the system in terms of voltage control and operational stability.’

Renewables trade associations were quick to respond to the report’s publication, emphasising that it had stated renewables were not the cause of the blackout, as had been speculated in the immediate aftermath of the power outage.  

A joint statement issued by SolarPower Europe said: ‘Let us be clear: solar PV was not the cause of the blackout… Going forward, the Iberian blackout must be a moment of learning. Solar PV already has the capacity to control voltage, but regulations did not allow its application.’

It continued: ‘This is a call for accelerated investment in grid resilience and system flexibility – especially through grid-forming inverters and battery storage. These technologies are already available and are key to supporting stable voltage levels, managing variability and delivering renewable-powered energy security.’

According to the joint statement, Spain ranked 14th in Europe in terms of battery installation last year, with less than 250 MWh of new batteries, compared to 9 GW of new solar. Nearly all new installations were smaller-scale batteries, not at utility level. ‘Pre-blackout, the Spanish market was already responding to this clear need for more battery storage and flexibility,’ said the statement. ‘In 2025, Spain is expected to climb to become a top-five European battery market, thanks to the utility-scale battery segment revival. Now it is for authorities to ensure that the sector is able to deliver,’ it continued.

Noting that the International Energy Agency forecasts solar will be the world’s largest source of electricity by 2033, the joint statement concluded: ‘Solar, renewables and electrification remain the way forward for Europe, and the world.’

This sentiment was echoed by Sarah Brown, Europe Programme Director of energy think tank Ember. ‘Renewables played a crucial role in restoring system stability and getting the power back on so quickly,’ she pointed out. She added: ‘This event reinforces what we already knew. As power systems evolve, enhanced grid optimisation and flexibility are essential for resilience.’