Spain’s voltage control was insufficient at time of April blackout

The Government of Spain said the total blackout in the Iberian Peninsula, which occurred on April 28, was caused by overvoltage, with several factors contributing to the crash. Notably, the system run by the country’s TSO Red Eléctrica de España lacked sufficient voltage control. Deputy Prime Minister Sara Aagesen even said the point of no return could have been avoided if voltage control action had been taken earlier.

In a long-awaited document, a government committee that investigated the April 28 collapse of the Iberian electricity network ruled out that a cyberattack caused it. The panel analyzed more than 300 gigabytes of data related to the total blackout, which was one of the worst ever in Europe.

“In 49 days, practically half the timeframe established by the EU, the committee has provided a rigorous and verified diagnosis that will allow us to strengthen the electricity system, a solid foundation on which we can work to design rapid responses to prevent this from happening again. Next week’s Council of Ministers will approve several relevant measures,” said Third Vice-President of the Government of Spain and Minister for Ecological Transition and Demographic Challenge Sara Aagesen.

The cybersecurity investigation, the largest ever undertaken in the country, did identify vulnerabilities that could expose networks or systems to future risks, she asserted.

The blame game is continuing as citizens and businesses are demanding accountability for the massive damage. The European Network of Transmission System Operators for Electricity (ENTSO-E) issued a preliminary report two weeks after the incident.

Overvoltage caused the blackout, according to the new analysis. The committee attributed it to multiple factors. The system had insufficient voltage control capacity, there were frequency oscillations, and power plants were disconnected, “in some cases in an apparently improper manner,” the document reads.

Renewables accounted for 82% of power generation mix just before blackout

The Iberian grid crashed at 12:33. Restoration began with energy from France and Morocco and with self-starting hydroelectric plants in the Duero basin and other locations, which formed energy islands. By 22:00, nearly 50% of demand in Spain was met, reaching 99.95% by 7:00 the next day.

At 12:30 on April 28, renewable energy sources accounted for 82% of the electricity generation mix, followed by nuclear power (10%). Gas plants had a 3% share, coal contributed 1%, while cogeneration and waste amounted to a combined 4%.

Data show a drop in solar generation as prices at the power exchange were going negative, and it coincided with a rise in voltages

There was significant voltage volatility in the transmission system in the morning on the day of the blackout, the document’s authors noted, pointing out that such a situation was also registered on April 22 and 24.

The rise in voltages between 10:30 and 11:10 coincided with a drop in solar generation, probably due to the power market signals, as wholesale prices went negative, the report adds. At the same time, the direction of the exchange with France switched from exports to imports.

Voltage control fleet failed to contain chain reaction

At 12:03, there was an atypical frequency oscillation, by 0.6 hertz, causing large voltage fluctuations for 4.42 minutes. Another one, of 0.2 hertz, occurred at 12:16, followed by an equivalent one at 12:19.

Red Eléctrica de España, the transmission system operator (TSO), conducted mitigation measures, which contributed to the rise in voltages, the committee underscored.

Aagesen said the point of no return could have been avoided if voltage control action had been taken earlier. The government controls 20% of the company, which is listed on the Bolsa de Madrid stock exchange.

At 12:32, voltage began to rise rapidly and steadily, and numerous progressive disconnections of generation facilities were recorded. The names of all power plants in the document are blacked out.

A number of units responsible for voltage control produced reactive power, the opposite of what they were supposed to

The chain reaction could not be contained, as each disconnection contributed to further voltage increases, the analysis showed. A drop in frequency resulted in the loss of synchronization with France, tripping the interconnection with the rest of the continent.

The committee stressed that the number of synchronous plants regulating voltage on the day of the incident was the lowest since the beginning of the year. One of the 10 units that Red Eléctrica scheduled the day before experienced an outage on the same afternoon, and the TSO didn’t replace it in time, the analysis reveals.

Moreover, several units in the group did not respond adequately to the TSO’s instructions to reduce the voltage. Some even produced reactive power, the opposite of what was required, contributing to the issue, the committee added.

Some power plants went offline too early

There were disconnections of the generating power plants that occurred before the voltage thresholds in the 400 kV system were exceeded (380 kV and 435 kV), the report finds.

Among the committee’s recommendations is to allow asynchronous installations to apply power electronics solutions to manage voltage fluctuations. The panel proposed boosting demand, flexibility, storage and interconnection capacities.

Photovoltaics with grid-forming inverters, storage can contribute to voltage control

Photovoltaics are already capable of controlling voltage, but regulations did not allow the application of the technology, according to the Spanish Photovoltaic Union (UNEF), Portuguese Renewable Energy Association (APREN), SolarPower Europe, Global Solar Council and Global Renewables Alliance.

In a joint statement that they issued as a reaction to the report, they called for accelerated investment in grid resilience and system flexibility – especially through grid-forming inverters and battery storage.

The associations recalled that Spain ranked 14th last year in Europe in new battery capacity. Less than 250 MWh came online and nearly all were smaller-scale batteries, not at a utility level. It compares to 9 GW of solar power capacity that the country added in 2024.