The Spanish government has concluded that April’s unprecedented nationwide blackout was triggered by a cascade of failures in voltage control, oscillations in the system, and the premature disconnection of several power plants.
A report presented today to the Council of Ministers by vice president and ecological transition minister Sara Aagesen found the 28 April grid collapse had a “multifactorial origin” and was not caused by a cyberattack, as some had initially speculated.
The investigation, led by the national electricity crisis committee, found that the incident began with abnormal voltage instability and system oscillations that developed into a chain reaction of overvoltage events.
These led to the disconnection of generation facilities in multiple provinces, ultimately causing the peninsular grid to shut down.
“Within just 49 days, the committee has delivered a rigorous diagnosis that will allow us to strengthen the system,” Aagesen told ministers.
The first phase of the event saw atypical oscillations in frequency-up to 0.6Hz-starting at 12:03, which continued for over four minutes.
Efforts by the system operator to mitigate the oscillations, including reducing flows to France and altering grid configurations, inadvertently increased overall voltage.
The second phase, starting at 12:32:57, involved rapid and sustained voltage increases that caused widespread generator trips in Granada, Badajoz, Segovia, Huelva, Seville, Cáceres and elsewhere.
The final phase, between 12:33:18 and 12:33:30, saw a chain of further disconnections, resulting in a frequency collapse and the loss of synchronisation with the rest of Europe.
Restoration efforts were swift, thanks to cross-border flows from France and Morocco and black-start hydro capacity in the Duero basin. By 22:00, 50% of electricity demand had been met, rising to 99.95% by 07:00 the next morning.
Despite this successful recovery, the report identified several failings.
Synchronous plants scheduled for voltage control did not perform adequately, with some exacerbating the problem by producing reactive power when they should have been reducing it, according to the report.
The number of synchronous plants in operation on the day was the lowest of the year, it added.
Some generators disconnected before voltage exceeded regulatory thresholds of 380kV to 435kV.
In some cases, protective mechanisms meant to mitigate system stress instead worsened it, discharging lines further and contributing to even higher voltages.
“The system had sufficient generation capacity to manage the event,” the report noted. “What was lacking was control-either because the resources were not properly scheduled, or those that were scheduled did not respond as required.”
To prevent a recurrence, the committee has issued a series of recommendations.
These include enhanced supervision of generator compliance with system operator commands
They also include strengthening voltage control and anti-oscillation capabilities
Fast-tracking implementation of the PO 7.4 regulation to enable asynchronous installations to manage voltage using power electronics is another recommendation.
Increasing grid flexibility and demand-side responsiveness, boosting storage capacity and interconnection with France and reviewing regulations for adjustment services and technical restrictions are also suggested.
In cybersecurity, no evidence of malicious intrusion was found. However, the report recommends accelerated adoption of EU cybersecurity rules, segmented network design, and real-time threat detection systems.
The committee also highlighted difficulties in obtaining information from several system agents and will refer specific cases to the CNMC for possible administrative proceedings.
“Transparency and confidentiality have guided this process,” the report stated. “But where necessary, further action will be taken.”