Iberian blackout| Update
460 450 440 430 420 410 400 390
Upper voltage limit, Spain
Upper voltage limit, Portugal
Events at 12:32:57, 12:33:16 and 12:33:17 CEST Loss of generation was experienced in the Granada, Badajoz and Sevilla regions, amounting to an estimated total of about 2200 MW. The first event was due to the tripping of a generation transformer, resulting from a problem on the lower voltage side, in the area of Granada. This transformer connected different generation facilities (PV, wind and solar thermal) to the transmission grid and at the time was injecting 355 MW.
12:30:00 pm 12:31:30 pm 12:32:20 pm 12:33:10 pm
Voltage evolution after 12:30 CEST in the main 400 kV transmission substations in Spain and Portugal. Phasor measurement unit (PMU) data from Red Eléctrica and REN. Image: ENTSO-E
In order to damp these oscillations, the operators in the control rooms of the relevant TSOs took several mitigating measures (as defined in the established operation protocols): Lines were switched on to decrease system impedance and improve generator stability. Fixed power operation mode was set up on the HVDC link between Spain and France as this is an effective measure for mitigating oscillations.
The power flow between Spain and France was reduced, as an additional countermeasure to decrease the Iberian centre of inertia angle relative to the rest of the Continental Europe power system.
In addition, shunt reactor manoeuvres were carried out to recover voltages, which reached low values briefly during the oscillations. Several electricity producers connected to the Spanish transmission system confirmed that these oscillations were clearly visible at their generators. The second oscillation in the half hour prior to the blackout occurred between 12:19 and 12:22 CEST. This was also an inter-area oscillation, with a dominant frequency of 0.21 Hz, corresponding to what the Expert Panel describes as the well- known East–Centre–West Continental mode. The mode shapes of the second oscillation with its oscillation vectors aligned in the same direction, clearly demonstrate the characteristics of an inter-area oscillation, according to the Expert Panel. In fact, the Panel notes, the whole Iberian Peninsula oscillates in a coherent way and similar amplitude against the rest of the continental European grid.
This second oscillation was effectively mitigated through further countertrading measures, which again reduced power flows between Spain and France, and also with the coupling of internal power lines in the south of Spain. Following the second oscillation, the voltage was within the range 390–420 kV, before increasing again, but still within the operational voltage range for the transmission network. At that moment, Spain’s international scheduled exchanges – all in the export direction – were 1000 MW to France, 2000 MW to Portugal and 800 MW to Morocco.
Sequence of events during the incident
Around the time when the net active power exchange position of Spain began to decrease, the voltage started to rise. The Expert Panel is examining if and to what extent the rise in voltage is related to the following possible causes (among others): (i) the reduction of reactive power absorption by generators which decreased their active power when operating with a fixed power factor; and/or (ii) the reduction of reactive power absorption by transmission lines, associated with their loading; and/or (iii) increased reactive power injection from distribution systems. This potential correlation could have been exacerbated, says the Expert Panel, by the fact that the reduction of active power (and hence of reactive power absorption) took place mainly in the southern part of the system, leading to a longer low-loaded transmission path for exports towards France. According to the Expert Panel, the currently available preliminary data indicates the following sequence of events taking place after the increase of voltage between 12:32:00 and 12:32:57 CEST. Prior to this sequence of events, the voltage in the transmission network was below the upper operational limit, the Expert Panel notes.
5000 4800 4600 4400 4200 4000 3800 3600 3400 3200
The Expert Panel notes that the tripping of this transformer and consequently the loss of infeed options also explains the disconnections of some of the generation units connected to the transformer due to over-frequency, with others tripping due to overvoltage, as reported by the operators of the power plants.
The second event included trips of PV and solar thermal facilities connected to two 400 kV transmission substations, in the Badajoz area, with a total interrupted injection of around 720 MW.
The third event included several trips, in different areas, in less than one second: wind farms in Segovia and Huelva; photovoltaics in Badajoz, Sevilla, Caceres and Huelva; solar thermal in Badajoz; and other generators in a variety of locations. This amounted to more than 1100 MW.
The causes of these three events are still under investigation, notes the Expert Panel. As a result of these events a voltage increase was observed in Spain, leading to a similar increase in Portugal, while the frequency decreased.
Events between 12:33:18 and 12:33:21 CEST Voltage in southern Spain increased sharply, and consequently also in Portugal.
The over-voltage triggered a cascade of generation losses that caused the frequency of the Spanish and Portuguese power system to drop.
Event at 12:33:19 CEST
The power systems of Spain and Portugal started to lose synchronism with the European system.
435 430 425 420 415 410 405 400 395
12:32:00 12:32:15 12:32:30 12:32:45 Apr 28, 2025
Evolution of voltage at the Carmona substation (Spain) and of the net active power exchange position of Spain (ES) in the minute preceding the blackout. PMU (phasor measurement unit) data from Red Eléctrica. Image: ENTSO-E
www.modernpowersystems.com | July/August 2025 | 9 12:33:00
kV
Active power exchange ES (MW)
Voltage (kV)
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