Final ENTSO-E report sheds light on 'multiple factors' that caused Iberia blackout

The European Network of Transmission System Operators – Electricity (ENTSO-E) has released its final report on the causes of the blackout that wiped out the power grids in Spain and Portugal on 28 April 2025.

According to the report, 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 rapid voltage increases and cascading generation disconnections, resulting in the final blackout event.  

Based on these findings, ENTSO-E’s Expert Panel set out recommendations addressing each factor to help prevent similar events in the future. These include strengthened operational practices, improved monitoring of system behaviour and closer coordination and data exchange among power system actors. 

The findings of the investigation also underscore the need for regulatory frameworks to adapt to support the evolving nature of the power system.  

Interestingly, ENTSO-E’s report notes that the energy sector is innovation-intensive, and technical rules may become outdated or ineffective over time. 

Therefore, regular and proactive monitoring by authorities should involve relevant market stakeholders and aim to align the applicable requirements with current (and, where possible, expected future) technical system needs and market conditions.  

Additionally, says the report, regular monitoring should specifically focus on newly applicable legislation and its implementation, where the effects may be uncertain. 

The incident on April 28 was Europe’s most severe blackout incident for the power system in over two decades. Now, ENTSO-E’s investigation has concluded what the root causes were, setting out a list of recommendations to improve system resiliency. 

Root causes: voltage control

The vast majority of root cause details are laid out in relation to voltage control, with recommendations primarily directed to grid operators. According to the report, any indication that the grid is in a weakened state can create a sense of urgency to act before any other incident occurs, and as such this might avoid rapid voltage changes, if actions can be taken sufficiently fast before the next incident.

ENTSO-E says it will develop a guideline of good practice on voltage support and studies on voltage stability. TSOs are to assess that a plan for the realisation of sufficient voltage support is in place and remains adequate or is updated where necessary.

ENTSO-E’s investigation identified recurrent mismatches between expected reactive power provision and the actual real-time performance of some system users, reducing the effectiveness of voltage control during fast voltage variations.

Such mismatches, says the report, were not unique to the blackout on 28 April but had occurred before in the Spanish system. Thus, ensuring sufficient, visible, and effectively usable reactive power resources is essential to manage rapid voltage changes and maintain system stability. 

For this, in the years ahead, the report recommends TSOs assess the system needs that will appear in real-time and technically design the system so that it can operate in the face of expected uncertainties through an appropriate combination of static and dynamic reactive power resources. 

Additionally, in operational planning processes and in real-time, TSOs should account for the occurrence of rapid voltage slopes (stemming from fast-changing active and reactive power flows) by ensuring sufficient and effective margins in dynamic reactive power resources and appropriate alarm settings.

More on the April 28 blackout:
Iberia blackout caused by domino effect of failings in milliseconds
Report reveals timeline of Iberia blackout catastrophe

On automatic activation of reactive power assets, ENTSO-E’s investigation identified that a substantial reactive power capacity from shunt reactors was available but not activated during the voltage rise preceding the blackout. 

Approaches to such situations should be analysed in detail, says the report, and a common European approach should be considered where appropriate.

On voltage range, the report says that TSOs should ensure that the harmonised operating voltage range foreseen at the European level is effectively applied across the continent, by avoiding or removing deviations that allow operation outside this range. 

Finally, the report states that rapid variations in power injected into the transmission or distribution grid by power plants, storage systems, and consumers (for example, a rapid reduction in PV injection following negative prices) can lead to abrupt changes in power flow in the transmission grid. 

Such behaviours increase system stress and can contribute to greater TSO imbalances and defence plan activations, particularly when power factor is fixed.

Root causes: oscillations and disconnections 

The final two root causes listed by the report with recommendations are power system oscillations, i.e. fluctuations in power, voltage, and frequency within an electrical grid, and disconnections. 

On the former, the report calls for a framework to improve the damping of inter-area oscillations in the Continental Europe Synchronous Area. According to ENTSO-E’s analysis, the Continental Europe Synchronous Area has a low-damped inter-area mode that can directly or indirectly affect its operational security. 

Moreover, the evolution of this Area, (indicated by fewer synchronous generators, more inverters, and extensions with new countries’ connections, for example) could lead to new inter-area modes with low damping.

Additionally, the report says that high-quality, real-time measurement data is needed to reliably detect and localise oscillations and to tune, validate, and continuously monitor system-wide damping controls. 

On disconnections, ENTO-E’s analysis points to sensitivity to local protection settings (such as overvoltage thresholds measured away from the point of connection or instantaneous trips without delay) that can unnecessarily disconnect generation when voltages oscillate or rise transiently. 

In some cases, the protection appears to diverge from the requirements. The report says that TSOs and DSOs should assess the adequacy and consistency of the protection settings for generation units and evacuation grid elements, such as transformers below which several power-generating and storage modules are connected, including distribution-connected and distributed energy resources. 

This verification should ensure that withstand capabilities – the maximum stress a system can withstand without damage or failure – are aligned with the highest capabilities of grid users connected downstream, with explicit checks on over or undervoltage thresholds, and minimum time delays. 

Renewables and responses

Although renewables were not widely referenced in the recommendations, the report does highlight the sensitivity of small-scale PV generation to high-voltage episodes, even when transmission-side voltage remains within acceptable limits. 

The data that ENTSO-E’s Expert Panel received from PV system manufacturers during the investigation showed that many inverters tripped due to overvoltage during certain periods on 28 April. The panel also identified an important role of the reconnection time of the non-observable PV systems in the loss of generation.

However, the report adds, its Expert Panel was unable to conduct an in-depth investigation and says it is considering to continue and intensify the exchange with DSOs and inverter manufacturers.

Commenting on the report on social media platform Linkedin was Ember’s Chris Rosslowe, Senior Energy Analyst, Europe: "The official investigation into the Iberia blackout has concluded that multiple factors contributed to a unique type of system failure. Put simply, cascading over-voltage, made worse by insufficient reactive power control. 

“Spain's reliance on renewable energy was not the cause, contrary to many claims. Rather than a vulnerability, wind and solar are helping to shield Spanish power prices from the current gas price driven by the Middle East conflict, demonstrating the value of homegrown clean power for Europe."

Also responding to the report, which calls for widespread reform, was Spanish TSO Red Eléctrica, which fervently defended its role during the blackout: “Red Eléctrica did not fail,” the TSO said.

The TSO added that many of the recommendations laid out by the report are already implemented or in the process of being implemented in Spain. 

“The conclusions of this report, prepared by 49 European experts, show that there is no breach or conduct attributable to Red Eléctrica that could be the cause of the power outage on April 28,” said the company, which is the sole TSO of the Spanish electricity system."