Final blackout report by European operators confirms event was caused by multiple factors, recommends improving oversight and coordination

Is the report based on sound data and methods?

“Generally speaking, yes. My initial impression is that the report is based on a broad technical foundation and an appropriate methodological approach for analysing a large-scale power disturbance. It incorporates operational chronology, system records, dynamic analysis, a review of protection behaviour, and an assessment of the response of various network and generation components. Furthermore, I consider it methodologically significant that it does not seek a single or simplified explanation, but rather presents the incident as the result of several concurrent factors.

That said, it should be emphasised that the report itself acknowledges certain limitations in the information available and the absence of specific records. I therefore regard it as a technically sound and useful document, but not as a reconstruction that is absolutely definitive in every detail.”

What new insights does it provide?

“In my view, the main new insight is that the report moves beyond a mere chronological description of what happened towards a more developed causal interpretation. In other words, it does not limit itself to reconstructing the sequence of events, but rather attempts to explain the technical mechanisms involved in the blackout and how they were linked together. I find it particularly significant that the report focuses on voltage stability, the actual capacity for reactive power control and the interaction between different elements of the system, rather than reducing the explanation to a single variable. It also provides a more detailed insight into the role of prior oscillations, successive generation disconnections and the coordination between the transmission grid, distribution network and different generation technologies”.

What data would you highlight?

“I would highlight, above all, five main ideas. Firstly, that the report does not identify a single cause, but rather a combination of technical factors that converge in a highly complex incident. Secondly, that a central part of the explanation shifts towards voltage behaviour and the system’s ability to withstand rapid disturbances, rather than towards simplified interpretations based exclusively on inertia.

Thirdly, the report highlights the importance of effective reactive power margin and voltage control as key elements in containing situations of this kind. Fourthly, it analyses the sequence of generation disconnections and how these contributed to exacerbating the incident’s progression. And, fifthly, it highlights that in increasingly complex power systems with a growing presence of resources connected via power electronics, monitoring, coordination and technical requirements are becoming ever more important”.

Are there any significant limitations to bear in mind?

“Yes, and I believe it is important to highlight them to avoid exaggerated interpretations. The first is that the report itself acknowledges that not all records were available at the same level of detail, so some conclusions must be read with the caution befitting an investigation of this nature. The second is that this document should not be used as if it were a simple attribution of responsibility or a definitive ruling on a single party. It is a technical analysis of a highly complex event, not a legal or punitive assessment. The third is that the report should not be turned into evidence in favour of single-cause explanations, whether to blame a specific technology or to deny the technical challenges posed by the transformation of the electricity system. The value of the report lies precisely in showing that the incident was systemic and requires a broad technical analysis".

What practical recommendations can we highlight based on this report?

“From a first reading, I would highlight several practical recommendations. The first would be to continue strengthening the system’s voltage control capacity and the effective availability of reactive power, as the report suggests these elements were particularly relevant during the incident. The second would be to review and refine protection criteria and their coordination, so that the system can respond more robustly to rapid and complex disturbances. The third would be to improve operational visibility and real-time monitoring of the behaviour of the various connected resources, including those playing an increasing role in the current electricity mix.

The fourth would be to continue adapting the system’s technical and operational requirements to a context in which power electronics and the transformation of the generation fleet are becoming increasingly significant. And the fifth, perhaps the most general, would be to interpret this report as an opportunity for technical learning and for strengthening the system’s resilience, rather than as a basis for simplified or excessively polarised interpretations.

Overall, my initial assessment is that the ENTSO-E report provides a valuable technical basis for better understanding a highly complex incident. I find it particularly useful because it helps steer the debate towards issues of stability, control, coordination and adaptation of the electricity system, which will become increasingly relevant in the future. Precisely for this reason, I believe it should be read rigorously and with caution, avoiding both oversimplifications and overly categorical conclusions.”

My assessments are based on the executive summary (Management Summary) of the Final Report (Final Report) drawn up by the Expert Panel for the investigation of incidents on the electricity grid (classified as severity 2 and 3 according to the ICS (Incident Classification Scale) methodology for ENTSO-E (European Network of Transmission System Operators for Electricity).

Although the Executive Summary evaluated here summarises the main findings and conclusions drawn by the experts, it omits certain details that are expected to be included in the full report. It is therefore a preliminary assessment, pending a more detailed analysis of the complete final report.

The final report published by ENTSO-E is based on the factual report prepared by the same panel and published by the same body on 3 October 2025. Thus, this report is primarily based on the same data as the factual report, although it adds some specific data that was not available in the previous report. Furthermore, it includes two new chapters of great relevance. The new Chapter 4 delves deeper into the analysis carried out to identify the causes of the incident. Meanwhile, the new Chapter 9 presents the findings of the investigation and the panel’s recommendations to prevent a recurrence of the situation.

In general terms, this report highlights that the incident was not due to a one-off failure or a single cause, but rather points to a structural problem in the provision and control of ancillary services, particularly voltage regulation, in a system with increasing penetration of converter-based generation. It is observed that existing technical and economic mechanisms are no longer aligned with the system’s actual needs.

Is the report based on sound data and methods? 

The report expressly states that the Expert Panel has requested from the affected transmission system operators (TSOs), as well as distribution system operators (DSOs) and other relevant network stakeholders (large generators, large consumers, inverter manufacturers and owners of self-consumption installations) all the data necessary to carry out a forensic analysis of the incident, including meter readings, trip orders, alarm logs, etc.

However, some data that could be relevant could not be obtained, such as, for example, the tripping of certain generators prior to the blackout (the response from the owners of the installations is that they do not record such events). This may constitute a limitation on the report’s conclusions.

What new insights does it provide?

Although the description of the events and the data presented are not significantly new, the added chapters are of great interest. The new Chapter 4 analyses in detail the phenomena observed and their impact on the incident, whilst Chapter 9 proposes measures and actions aimed at preventing or mitigating similar situations in the future.

What results/data would you highlight?

I believe the experts’ conclusions regarding the causes of the incident are highly significant; these can be summarised as follows:

1. The (synchronous) generators responsible for providing voltage support to the system were not operating as expected. The report notes that several generators failed to meet the reactive power reference in at least 75% of the hourly samples, which is the compliance rule. However, the reason for this situation is not clarified.
2. Renewable generation plants were operating with a fixed power factor. Consequently, their reactive power exchange was linked to active power, thus limiting their ability to participate flexibly in voltage regulation. Furthermore, the operating framework did not impose sufficient restrictions on the rate of change (ramps), which can exacerbate transients and interactions with voltage regulation.
3. Small-scale generators connected to distribution networks made a limited effective contribution to overall voltage control, which proved insufficient and uncoordinated with the system’s needs. Furthermore, many of them were suddenly disconnected when the inverters’ overvoltage protections were triggered, drastically increasing the net demand that had to be met by the rest of the system.
4. Certain devices relevant to voltage control, such as shunt reactors, required manual operation, which entails decision-making and operating times incompatible with the control requirements demanded by the incidents that occurred (events with dynamic components significantly faster than manual response times).
5. The permissible voltage range for transmission (400 kV) in the Spanish system is very wide (greater than in neighbouring countries). This allows operation at high voltages, which reduces the operational margins against overvoltages.
6. The existence of two oscillation episodes prior to the blackout is confirmed: one forced by power converter-based generators (0.63 Hz), and another natural inter-area oscillation (0.2 Hz), which are indicative of a dynamic weakness in the system. Their exact role in the incident is unclear, but the panel notes that some generators did not have, or did not have properly adjusted, the systems that could have dampened them (PSS: Power System Stabilizer).
7. The system entered an operational state in which, although contingency plans were implemented in a timely and appropriate manner, it was no longer possible to prevent the system from collapsing.
8. Although the system restoration plan was effective, experts have identified certain problems associated with the start-up of specific generation units and the stable maintenance of certain electrical areas. One of the main problems identified is the lack of information and management capacity regarding generators connected to the distribution network.

From the above observations, it can be concluded that:

1. Although it is not clear why some generators were not providing the required reactive power support for voltage regulation, the researchers emphasise that the regulatory framework in force at the time of the incident did not provide for penalties for failure to provide reactive power support, which is indicative of a structural weakness in that framework. Furthermore, the protocol did not establish dynamic specifications, i.e., regarding response speed. In other words, the report identifies that the system does not incorporate the value of reactive power or dynamic control. In the current context of the electricity system, this misalignment with the system’s actual needs reduces the robustness of voltage control and increases the risks of instability.
2. The report highlights instability phenomena associated with power converters. Whilst it is not stated that these were the sole cause of the incident, it has been observed that these phenomena interacted with other generation equipment in the area, revealing limitations in the design and coordination of the system’s dynamic control.
3. The operational requirements of renewable energy facilities and distributed resources are playing an increasingly significant role, and their role in the overall regulation of the system must be thoroughly reviewed.
4. The sequence of events that took place led to a situation in which, with the defence mechanisms available, it was no longer possible to prevent the collapse. Contingency plans must be reviewed to avoid reaching critical situations such as the one that occurred.
5. The incident demonstrates that traditional voltage control and auxiliary service schemes have significant limitations for modern electrical systems dominated by power electronics-based generation.

What practical recommendations can we make or highlight based on this report?

The recommendations and proposed measures are included in the full report, but are not present in the executive summary analysed. However, based on the analysis of the main causes of the incident identified by the investigators, the need to review and update the operating procedures relating to the provision of voltage regulation services stands out, as does the role of renewable generators in the electricity system.

Although the incident had multiple causes, the panel suggests that the event was not a mere anomaly, but a manifestation of the structural limitations of the current design of the electricity system. However, with the appropriate analysis, the operational framework can be adapted to meet the new grid requirements arising from the increased presence of power converters.

Summary

The report by the ENTSO-E expert panel concludes that the blackout was not the result of a single failure, but rather the outcome of multiple factors that highlight structural limitations of the current electricity system. In particular, deficiencies are identified in voltage regulation, associated with insufficient reactive power contribution from conventional generators, the operation of renewable plants with limited control capacity, and poor coordination of distributed resources. Added to this is the existence of poorly damped dynamic phenomena, including oscillations prior to the incident and possible interactions associated with power converters. The report also highlights shortcomings in the regulatory framework, which did not adequately incentivise voltage support nor set out stringent dynamic requirements. As a result, the system reached an operational state in which, even though the defence mechanisms functioned correctly, it was not possible to prevent the collapse. Overall, the incident highlights the need to adapt the design and operation of the electricity system to an environment with high penetration of power electronics-based generation.

Is the report based on sound data and methods?

“The report is based on the ICS (Incident Classification Scale) method, established by ENTSOE to, amongst other objectives, systematically analyse the causes of incidents that may occur in the electricity grid. Consequently, the incident that occurred in the Iberian system has been analysed by a panel of 49 experts in accordance with this method. The data analysed consisted mainly of that requested from both transmission and distribution network operators”.

What new insights does it provide?

“The report concurs with that produced by Red Eléctrica in June 2025 in that the blackout was caused by a voltage problem, which led to a cascade disconnection of several generators due to their protection systems tripping. Without delving into the causes behind the voltage rise, the ENTSOE panel points to the ineffectiveness of voltage control in the Spanish electricity system as the key reason why the blackout could not be prevented.”

What results or data would you highlight?

“Voltage control in the system failed for three reasons:

• Some conventional generators capable of dynamic voltage control did not perform this function as required.

• Renewable generation does not provide voltage control.

• The System Operator did not have time to activate the manually operated compensation elements, some of which had been deactivated by a previous undervoltage event.

Furthermore, the report notes an inappropriate setting of the trip values for the overvoltage protection systems.

Regarding the frequency oscillation that occurred subsequently, it is emphasised that, whilst the loss of generation led to an increase in system frequency, the loss of synchronism could not have been avoided, even if the system’s inertia had been greater, given the speed at which a point of no return was reached.”

What practical recommendations can we make or highlight based on this report?

“It appears that the full report identifies a series of recommendations in Chapter 9, which are not mentioned in the summary provided. As Red Eléctrica has already pointed out, one of the first actions should be to review the voltage control operating procedure (P.O. 7.4).”

Is the report based on sound data and methods?

‘Yes, the report has compiled all possible data, and where data could not be obtained (because it does not exist, or had been withheld or not shared), it has been simulated using a realistic model. The report has also analysed technologies similar to those currently in use to identify their response and behaviour. This means the results and conclusions are underpinned by a strong foundation of data and detailed analysis.’

What new insights does it provide?

‘It doesn’t really offer any major new insights compared to the October reports, which already presented some initial analyses. It reinforces the message that everything happened because it was a combination of problems that pushed the electricity grid to its limit.’

What data would you highlight?

"The key conclusions are that renewable energy sources were not to blame, although they can provide more support if required by the operator; that a system with greater inertia would have faced the same problem; and that there were issues with voltage and reactive power regulation. Regulations must be updated and there must be greater coordination between the active players in the electricity grid (as well as aligning the response of the protection systems); the electricity grid is a reliable and safe system according to the parameters and models used (planning and operation); and that the procedures implemented were those established".

What practical recommendations can we make based on this report?

‘One of the critical points is the coordination of protection systems, the control of low-power renewable energy sources (below 1 MW), and the need to dynamically update voltage regulation.

Furthermore, it reinforces the fact that it is possible to operate with high levels of renewables, but that regulation and the provision of ancillary services and grid support must be extended to all generation, whatever the type.’