Almost two months after the blackout, the sector now has three technical reports offering broadly consistent perspectives on general aspects, but significant differences regarding the root causes of the incident and the technical and operational evaluation of prior decisions.
This week began with the release of an independent technical report commissioned by AELEC and produced jointly with Compass Lexecon and INESC TEC.
The report concludes that the main cause of the blackout was the lack of available conventional generation—particularly in the south of the Iberian Peninsula—which left the system without sufficient dynamic voltage control capacity.
Moreover, it highlights that operational actions taken by Red Eléctrica de España (REE) in the minutes leading up to the incident—including the reconnection of lines and changes in the interconnection with France—further worsened an already critical situation.
The report also points to abnormal voltage fluctuations in the days prior and suggests that the system was operating in a structurally unstable manner due to operational planning that failed to comply with the requirements of Operational Procedure 3.2.
20250623_InformeBlackoutKey comparisons
Firstly, regarding the issue of insufficient conventional generation on the day in question, both the AELEC and Government Committee 28-A reports agree: the level of thermal generation available for dynamic voltage control was the lowest of the year, creating a critical vulnerability in the system.
Both reports assert that this situation contravenes the regulatory obligations set out in Operational Procedures 7.4 and 3.2, which assign responsibility to the system operator—REE—to ensure sufficient resources are available.
By contrast, REE [Review REE report] states in its own report that initial operating conditions were “normal” and claims that the available generation met the minimum requirements for stable system operation according to its technical assessment at the time.
Another major point of divergence lies in the evaluation of the operational manoeuvres carried out just before the blackout. AELEC and Committee 28-A both criticise the reconnection of previously disconnected transmission lines—some out of service for several days—arguing that these actions caused an “immediate and significant increase” in voltage, exacerbating the instability. Committee 28-A’s report specifically stresses that these decisions directly worsened the system’s instability.
REE, however, maintains that the manoeuvres were necessary in order to control critical oscillations identified minutes before the blackout. According to the operator, although these actions temporarily raised voltage levels, they were “essential and justified under existing regulatory procedures”.
The management of oscillations in the international interconnection also reveals major discrepancies.
REE contends that the operational shift in the interconnection with France (switching to constant DC power mode) was a necessary response to observed oscillations. However, the Government Committee argues that this change may have worsened the situation, particularly since REE failed to revert to the previous mode as it had done in similar prior events.
AELEC partially supports this criticism, suggesting that the decision merits further scrutiny.
On the matter of prior fluctuations, all three reports technically agree on their severity and relevance as early warning signs before the incident.
All reports confirm that “numerous significant high-voltage events” were recorded at least ten days before the blackout. However, REE maintains that despite these episodes, “the overall system conditions remained acceptable within regulatory limits”.
In summary, the fundamental differences between the three reports do not lie in the technical facts themselves—on which there is broad consensus—but in the operational interpretation of those facts and the technical decisions made, or not made, by the system operator.
The applicable regulatory frameworks, particularly Operational Procedures 3.2 and 7.4 and Royal Decree 413/2014, are critical to assessing these discrepancies from a strictly technical and professional standpoint.
Operational Procedure 3.2 establishes the responsibility of the system operator to schedule sufficient generation to ensure system security and stability, including through the application of technical constraints.
Operational Procedure 7.4 outlines the requirements for voltage control capacity, assigning conventional generation the role of providing dynamic regulation—something that renewable technologies, in their current design, cannot guarantee.
Royal Decree 413/2014, meanwhile, regulates the connection and operational conditions of renewable installations, setting limits on their ability to respond to disturbances.
These regulatory frameworks are essential to technically assess whether the operational decisions made before and during the blackout were consistent with legal requirements regarding security of supply.
A rigorous, objective analysis of these discrepancies is crucial to improving protocols and preventing future incidents of this nature. Regardless of the outcomes of any future legal proceedings, it is clear that the technical and operational lessons from the 28 April blackout must be urgently integrated into the management of the Spanish electricity system.
