Root causes of Iberian blackout | Update
A perfect storm of interacting factors
ENTSO-E has published its final report on the 28 April Iberian blackout, with a focus on root causes, of which many are identified. A key factor was the non-effectiveness of voltage control within the Spanish power system. It was the most severe blackout in the European power system in over 20 years, and, being attributable to overvoltage, the first of a kind
The final report (472 pages) of the 49-member Expert Panel on the Iberian blackout, published 20 March 2026, identifies its root causes and outlines recommendations to strengthen the resilience of Europe’s interconnected electricity system. The investigation concludes that the blackout resulted from a combination of many interacting factors, including oscillations, gaps in voltage and reactive power control, differences in voltage regulation practices, rapid output reductions and generator disconnections in Spain, and uneven stabilisation capabilities. These factors led to fast voltage increases and cascading generation disconnections, resulting in the blackout.
Based on these findings, the Expert Panel sets out recommendations addressing each of the factors identified in the report to help prevent similar events in the future. These include strengthened operational practices, improved monitoring of system behaviour and closer co-ordination and data exchange among power system actors. The findings of the investigation also underscore the need for regulatory frameworks to adapt in order to support the evolving nature of the power system. The 28 April 2025 blackout was a first of its kind event, and the recommendations aim to strengthen system resilience with solutions that are already technologically deployable. The blackout highlighted how developments at the local level can have system-wide implications and underlines the importance of maintaining strong links between local and European system behaviour and co-ordination, while ensuring that market mechanisms, regulatory frameworks and energy policies remain aligned with the physical limits of the system.
The Panel has determined that the incident began at 12:32:00 on 28 April 2025. The system’s
The role of inertia: the Expert Panel’s view
“Analysis of the impact of inertia indicates that, even with significantly higher inertia values, the loss of system synchronism would not have been avoided, considering the sequence of events. This can be explained by the rapid reduction in synchronising torque in the Iberian Peninsula (due to cascading generator tripping) that led the system to rapidly reach the point of no return.”
Figure 1. After 12:32:00, the voltages rose at numerous nodes. Source: ENTSO-E
solar photovoltaic generation was similar to previous days, while wind generation was more variable but within the ranges observed in previous days.
The morning hours of 28 April 2025 were characterised by increasing renewables generation, which led to lower prices on the day- ahead market and to Spain’s exports reaching 5 GW in total. From approximately 09:00, the voltage variability in Spain began to increase, albeit without significant fluctuations, until shortly after 10:30, when the voltage in a part of the 400 kV transmission network briefly approached – but did not exceed – 435 kV.
With the exception of one node (Olmedilla), the voltages in the 400 kV network remained below 435 kV during the period before the incident. Oscillatory behaviour was noted in the morning, but no significant oscillations were detected until 12:03.
The voltage levels on the 400 kV network in Portugal remained within the normal range of operation (380 kV to 420 kV), which differs from the ranges allowed in Spain.
An analysis of the geographical distribution of generation and consumption in Spain reveals that electricity production in the south- west of the country was very high relative to consumption in that region. This results in high electricity transit flows from the southwest of Spain to the surrounding areas.
During the half hour preceding the blackout, two main periods of oscillations – power, voltage and frequency swings – were observed in the Continental Europe Synchronous Area (CE SA), the first of which took place from 12:03 to 12:08. The analysis indicates that this oscillation had a local character classified as converter-driven
instability phenomenon primarily affecting the Spanish and Portuguese power systems with a dominant frequency of 0.63 Hz. The second oscillation occurred between 12:19 and 12:22 as an inter-area oscillation, with a dominant frequency of 0.2 Hz, corresponding to the East-Centre-West continental mode. To damp these oscillations, the operators in the control rooms of the relevant TSOs implemented several mitigating measures, such as reducing exports from Spain to France, coupling internal power lines in the South of Spain, and changing the operating mode of the HVDC link between France and Spain. While these measures effectively mitigated the oscillations, their nature led to an increase in voltage in the Iberian power system. At 12:32:00 the voltage in the 400 kV part of the Iberian power system was below 420 kV and no notable oscillations could be observed. After 12:32:00, the voltages at numerous nodes rose, as shown in Figure 1. Around 12:30, several generators changed their active power output. From 12:32:00 to 12:32:48, concurrent with the voltage rise shown in Figure 1, the active power output of large RES (Renewable Energy Sources) generators (greater than 5 MW) in Spain decreased by approximately 500 MW. The RES power plants maintained a fixed power factor, meaning that changes in active power also affected the generators’ reactive power output.
Additional events (mainly generation-related) occurred from 12:32:00 onwards. Between 12:32:00 and 12:32:57, 208 MW of identified distributed wind and solar generating units in northern and southern Spain experienced either a fast downwards change of operating point
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