28 April 2025 · Spain and Portugal
At 12:32 on a sunny Monday the voltage on Spain's high-voltage grid began to climb. By 12:33:27 the mainland grids of Spain and Portugal had collapsed. This is how it happened, replayed from the grid's own measurements and the official investigation.
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12:00
Peninsular Spain is drawing 25 gigawatts, and four-fifths of the electricity generated is renewable, most of it solar. Spain is exporting: about 1 GW to France, 2 GW to Portugal and 0.75 GW to Morocco.
The trace under the clock is the grid frequency measured in Freiburg, Germany. Spain, Portugal and most of continental Europe run on one synchronous grid, so a disturbance in Iberia shows up there too.
12:03
A forced oscillation at 0.63 Hz appears, mostly inside Iberia, and lasts about five minutes. To damp it, Red Eléctrica switches extra 400 kV lines in, takes shunt reactors out and asks France to accept less power. Each of those measures also pushes voltage up.
12:19
Now the whole continent sways, east against west, at 0.2 Hz, the frequency swinging by up to 200 millihertz. It fades after three minutes. Then voltages start rising towards the top of the allowed range.
12:32:00
The investigators date the start of the incident to this second. Voltage rises at substation after substation. Solar and wind plants run at a fixed power factor and do nothing to hold it back, and several conventional plants absorb less reactive power than expected.
12:32:57.220
A transformer that carries solar and wind power out of a substation in Granada trips on overvoltage protection. 355 MW are gone. The frequency dips to 49.94 Hz.
12:33:16.443
Twenty seconds later the line carrying a solar farm's output in Badajoz drops out with 582 MW, then a solar plant nearby with 145 MW. Those plants were absorbing reactive power; with them gone, voltage jumps again, to 440 kV at Cáceres.
12:33:17
Wind and solar plants in Segovia, Seville, Badajoz, Huelva and Cáceres disconnect within 650 milliseconds, 928 MW in all. By 12:33:18 more than 2.5 GW is gone and the 400 kV grid in the south reaches 451 kV. More solar plants trip, each one pushing the voltage higher for the rest.
12:33:19.620
Iberia's frequency falls away from the rest of the continent and the two systems lose synchronism. Power on the French border swings wildly, from 3.8 GW coming in to 5.6 GW going out within a second. Automatic defences shed pumped-storage plants, then whole blocks of demand.
12:33:21.535
Morocco disconnected a second ago. Now the last AC line to France opens on loss-of-synchronism protection, which stops the collapse spreading north. Iberia is alone at 48.5 Hz, and the frequency is falling faster every moment. Within two seconds three nuclear units trip.
12:33:27
Mainland Spain and Portugal have no electricity. The Balearic and Canary Islands keep theirs. In Freiburg, 1,500 km away, the continental frequency sinks to 49.84 Hz and takes about ten minutes to get back to 50.
12:33 to 07:00
This is Spain's electricity demand in five-minute steps, against the same Monday and Tuesday a week later. Restarting a grid from nothing means building islands around power stations that can start without power, then stitching them together. France and Morocco lent voltage across the border.
Demand: Red Eléctrica, REData, national demand in real time. The 1.8 GW left after 12:35 is almost entirely the Balearic and Canary Islands. Milestones: ENTSO-E final report and the Spanish government's committee report.
The internet went with it
How many of each province's internet address blocks answered a ping, compared with the same morning. Each row is one province. Bright means online, dark means unreachable. Batteries and backup generators kept part of the network up for a while, so it faded over the first hour, then came back with the power.
Data: IODA, Georgia Institute of Technology, active probing of /24 address blocks every 10 minutes. Provinces with fewer than 20 responsive blocks are left out. One probing round at 19:50 that dropped everywhere at once, islands included, is treated as a measurement fault and smoothed over.
What went wrong
The European investigators found no single root cause, but a chain. A fast, uncontrolled rise in voltage in southern and central Spain made generators disconnect to protect themselves. Every disconnection pushed the voltage higher still, and within seconds the frequency collapsed with it.
Spain's voltage control could not absorb the rise. Solar and wind plants ran at a fixed power factor and did not respond to voltage. Several conventional plants did not deliver the reactive power expected of them, and the rules set no dynamic requirement and no penalty. Shunt reactors were switched by hand; only 58% of their capacity was connected at 12:32.
Many generators' overvoltage protection was set in ways that did not match the requirements. Spain's 400 kV grid may run up to 435 kV, which left little margin before plants were allowed to disconnect. The defence plans did what they were designed to do, but they were built to stop a collapse of frequency, not one driven by voltage.
Portugal's transmission grid was fully energised again at 00:22 on 29 April. Spain's was at about 04:00, and 99.95% of Spanish demand was supplied at 07:00.