The failure of Spanish grid operator REE to calculate an adequate energy mix and maintain voltage levels has been identified as the primary cause for the massive blackout that affected Spain and Portugal on 28 April 2025, as reported by Reuters.
The findings were revealed in a government investigation released on Tuesday, 17 June.
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The comprehensive probe also identifies the failure of old power plants, including nuclear and gas-fired facilities, to maintain appropriate voltage control on the day of the blackout.
REE’s inability to absorb a sudden increase in voltage was highlighted by Spanish Energy Minister Sara Aagesen as the factor that triggered a cascade of generation disconnections.
Aagesen told a news briefing in Madrid: “The system did not have sufficient voltage control capabilities, either because they were not sufficiently programmed, or because those that were programmed did not adequately provide what was required by the standard, or a combination of both.”
REE, which is partly state-owned, did not have enough thermal power stations switched on when the voltage surge caused a chain reaction leading to the power outage.
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By GlobalDataAagesen stated: “REE told us that they made their calculations and estimated that (switching on more thermal plants) was not necessary at this time. They only set it for the early hours of the day, not the central hours.”
Voltage surges are typically caused by issues such as lightning strikes, equipment malfunctions or overall grid instability.
The government investigation revealed that on the day of the outage, grid instability was detected before the problem had occured.
Aagesen said: “Power plants should have controlled voltage and many of them were economically remunerated to do so. They did not absorb all the reactive power that was expected.”
Utilities lobby Aelec, representing major electricity suppliers such as Iberdrola and Endesa, concurred with the findings that pinpointed voltage control deficiencies.
“Despite having sufficient resources to guarantee voltage control, REE opted to manage voltage with limited synchronous capacity and an unbalanced geographical distribution, which left the system in a vulnerable situation,” Aelec stated.
REE has not responded to requests for comments regarding the findings.
The government announced its intention to propose measures aimed at enhancing the grid and improving its capacity to regulate voltage within the system. It also plans to advocate for further integration of the peninsula with the European grid.
Aagesen had revealed in May that the blackout affecting Spain and Portugal was triggered by a sudden loss of power generation at the Granada substation. This initial failure was compounded by subsequent issues at the Badajoz and Seville substations.
