The storm that lights the sky pushes current into power lines
A geomagnetic storm begins as charged gas thrown off the sun. When it reaches Earth it moves the magnetic field around the planet, and a moving magnetic field drives electric current through anything long and metal underneath it. High voltage transmission lines are long and metal. The current driven into them is called a geomagnetically induced current, and it is the wrong kind of current for grid equipment to carry, so transformers run hot and protective switches read a healthy grid as a broken one and disconnect it.
What it did to Quebec in 1989
On March 13, 1989, the Hydro Quebec grid went dark in the middle of the night and stayed dark for more than 9 hours. The gas that caused it had left the sun 3 days earlier. According to Natural Resources Canada, 7 pieces of voltage control equipment on the La Grande network tripped one after another in less than a minute. That split the network and took 9,450 megawatts of generation off a system carrying 21,000 megawatts. The rest of the grid then collapsed in 25 seconds. Voltage on the 735 kilovolt lines fell out of balance by 15%.
Both figures are from the Canadian space weather forecast centre. The network split first, then the rest of the grid collapsed in 25 seconds.
Show the numbers
| Generation lost at once | 9,450 |
| Whole Hydro Quebec system | 21,000 |
How often a storm this size arrives
The NOAA Space Weather Prediction Center rates geomagnetic storms from G1, minor, to G5, extreme. According to that scale, a G5 storm can cause widespread voltage control problems, and some grid systems may experience complete collapse or blackouts. A G4 is expected to make some protective systems mistakenly trip key parts of the grid offline. G5 is also the rare one. NOAA counts about 4 in a solar cycle, against 1,700 at the mildest level.
Long run averages published by NOAA. The G5 bar is 4, so it renders as a sliver with the number printed above it. That is the finding, not a fault in the chart.
Show the numbers
| G1 minor | 1,700 |
| G2 moderate | 600 |
| G3 strong | 200 |
| G4 severe | 100 |
| G5 extreme | 4 |
The 2024 storm was bigger and the grid held
The largest geomagnetic disturbance in more than 20 years reached North America from May 10 to 12, 2024, and the lights stayed on. The difference was warning. The North American Electric Reliability Corporation, which sets the reliability rules for the grid, records that the forecast centre called reliability coordinators on a dedicated hotline 6 hours before the storm arrived. Operators scheduled extra generation, cancelled maintenance on transmission lines, and watched the induced currents as they climbed.
We're really buckling down here.
Brent Gordon, chief of the space weather services branch at the NOAA Space Weather Prediction Center. Source 5.
What broke anyway
Transformers raised high temperature alarms on their top oil. Induced current went past the safe limit on the transformers that connect generators to the grid, so operators cut generator output to protect them. A harmonic filter tripped on a high voltage line. A solar farm and a battery bank saw their voltage swing. The affected areas were mostly the United States Northeast, the mid Atlantic, and western Canada. None of that is a blackout. The equipment that failed in Quebec failed with no notice. In 2024 the same physics reached a grid that had been told it was coming.