| Iberian blackout, six months on
without significant variations until shortly after 10:30, when the voltage in a part of the 400 kV transmission network briefly approached – but did not exceed – 435 kV.
Voltages in the 400 kV network remained below 435 kV during the period before the incident. No significant oscillations were detected until 12:03. During the half hour preceding the blackout, two main periods of oscillations – power, voltage, and frequency swings – were observed in the Continental Europe Synchronous Area (CE SA), the first of which took place from 12:03 to 12:08. Analysis indicates that this oscillation had a local character primarily affecting Spanish and Portuguese power systems with a dominant frequency of 0.63 Hz. The second oscillation occurred between 12:19 and 12:22 as an inter- area oscillation, with a dominant frequency of 0.21 Hz, corresponding to the East-Centre-West continental mode. In order to damp these oscillations, the operators in the control rooms of the relevant TSOs took several mitigating measures, such as reducing the export from Spain to France, coupling of internal power lines in the South of Spain, and changing the operation mode of the HVDC link between France and Spain. While these measures mitigated the oscillations, they also led to an increase of voltage in the Iberian power system, the expert panel says.
At 12:32:00 – the starting point of the incident for the purposes of the expert panel report – the voltage of the 400 kV Iberian power grid was below 420 kV and no notable oscillations were being observed.
Several significant generation trips occurred from 12:32:00 onwards, the expert panel report says. Between 12:32:00.000 and 12:32:57.000, there was a loss of 208 MW identified as distributed wind and solar generators in northern and southern Spain, as well as an increase in net load in the distribution grids of approximately 317 MW, which might have been due to the disconnection of small embedded generators < 1 MW (mainly rooftop PV) or to an actual increase in load or to a combination of both. The reasons for these events are not known, the expert panel remarks.
From 12:32:57.000 until 12:33:18.020, major disconnection events occurred in the regions of Granada, Badajoz, Sevilla, Segovia, Huelva, and Cáceres, which resulted in an additional loss of generation of at least 2 GW (analysis of frequency deviation suggests a loss of some 2.2 GW). This phase of major disconnection events started some milliseconds after 12:32:57 with the tripping of a generation transformer in the region of Granada due to the activation of over-voltage protection in the 220 kV side of a 400/220 kV transformer, which connects several generation facilities (photovoltaic, wind and concentrating solar power (CSP)) to the transmission grid. The transformer was injecting 355 MW into the grid and the voltage on the 400 kV grid was 417.9 kV at this time.
The next event consisted of two sets of trips, resulting in an additional loss of around 725 MW of PV and CSP facilities connected to two 400 kV transmission substations in the area of Badajoz. In the first substation, an evacuation line tripped at
3000 2500 2000 1500 1000 500 0
As of 12:33:18 CEST:
• Loss of 2.5 GW of generation in Spain • No disconnections in Portugal
Estimated loss of approximately 525 MW in Spain
445 440 435 430 425 420 415 410 405
Apr 28, 2025
Three major tripping events during the incident, “before the point of no return”: 1: Milliseconds after 12:32:57 CEST, tripping of a generation transformer in the region of Granada, due to the activation of overvoltage protection. The transformer was injecting 355 MW into the grid at this time and voltage was 417.9 kV on the 400 kV transmission side.
2: Trips of PV and CSP facilities connected to two 400 kV transmission substations, in Badajoz, with a total interrupted injection of around 725 MW.
3: Several trips between 12:33:17 CEST and 12:33:18 CEST, which led to disconnection of wind and solar generation in Segovia, Huelva, Badajoz, Sevilla and Caceres, amounting to a total of around 930 MW (or more than 1100 MW on the basis of observed frequency variation).
Source: ENTSO-E
Overvoltage results in a cascade of disconnections of generators
490 480 470 460 450 440 430 420 410 400 390 380 370 360 350 340 330
50.1 50.0 49.9 49.8 49.7 49.6 49.5 49.4 49.3 49.2 49.1 49.0 48.9 48.8 48.7 48.6 48.5 48.4 48.3 48.2 48.1 48.0 47.9 47.8 47.7 47.6 47.5 47.4
Time [UTC +02] Carmona (ES) - frequency Bassecourt (CH) - frequency Upper voltage limit (435 kV) Carmona (ES) - voltage magnitude Lower voltage limit (360 kV)
Grid frequency and voltage as recorded in the Carmona (Spain) and Bassecourt (Switzerland) substations during the incident (data from Red Electrica, Swissgrid). 4: 12:33:19 CEST. Decrease of frequency on Iberian Peninsula and loss of synchronism with rest of Continental Europe. System defence plans (automatic load shedding) were activated in Spain and Portugal but unable to stop the blackout due to its overvoltage nature
5: 12:33:21 CEST. Disconnection of all AC lines from Spain to Morocco and France Source: ENTSO-E
www.modernpowersystems.com | October 2025 | 21
Voltage [kV]
Cumulative sum of tripped generation [MW]
Frequency [Hz]
Voltage [kV]
12:32:00
12:32:15
12:32:30
12:32:45
12:33:00
12:33:15
12:32:50 12:32:55 12:33:00 12:33:05 12:33:10 12:33:15 12:33:20 12:33:35 12:33:30 12:33:35
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