[jcalvinowens]

DC current on long transmission lines saturates mains transformer cores, turning the high voltage primary into a short circuit at the peaks of the "unlucky" side of the AC waveform.

https://ieeexplore.ieee.org/document/9652509

> When CMEs strike the magnetic field of the Earth which provides protection against the fast-moving plasma becomes compressed and results in a varying magnetic field on the ground. This magnetic field variation generates a geoelectric field on the Earth’s surface and leads to Geomagnetically Induced Current flow through man-made technology. This GIC exhibits a very low-frequency quasi-Direct Current (DC) (less than 1 Hz) with amplitudes of 10–15 A and up to 300 A peak current for 1–2 minutes that flows along conductors and technological infrastructure. The power transformers which are connected by transmission lines are the most affected by GIC events. The GIC enters from the neutral ground point of the star-connected (wye) transformer windings and divides equally among the phases. When the GIC flows through the transformer windings, a DC magnetic flux is generated in the core, whose magnitude depends on the GIC flow magnitude. This DC flux is then superimposed on the AC flux in such a way that the asymmetrical saturation takes place in the magnetic cores of the transformers (half-cycle saturation) and increases their reactive power consumptions critically.

> During saturation phenomenon, transformers draw an extremely large asymmetrical exciting distorted current that is rich in even and odd harmonics. These harmonics can trigger the relays improperly, overheating the generators and transformer’s windings and cores, leading to unstable operations of the power system and could result in long-term damage to the system’s components. These effects may turn into catastrophic failures (i.e., permanent damage or blackouts) if they persist for a few minutes. The 1989 geomagnetic storm in North America that lead to severe economic losses is an example of such catastrophic consequence due to the effects of GIC.