geomagthrowaway

This isn't true. Your electronics might be fried by a flare if they were in space, but the geomagnetic field changes due to a flare will not induce currents in any conductor shorter than several tens of kilometers. I think you are thinking of nuclear EMPs which could affect electronics?

The problem is not directly with the wires, its the addition of the quasi-DC induced currents in an AC transformer. Even the addition of a small DC current can lead to unwanted harmonics or in extreme cases spot heating (this is what you think of when people say 'knocking out a transformer').

If you want a very in-depth review: https://doi.org/10.1002/2016SW001501

I think you have it the wrong way around. Geomagnetically induced currents (GICs) are more prevalent in high-voltage wires because they have low DC resistance. The longer a line, the larger the induced voltage. High-voltage lines can extend up to 1,000s of km, as they are most efficient. Then its basically V=IR. So low-resistance wires and long distances = larger GICs.

The danger is not from the cables burning out (extreme GIC events can lead to a couple of hundred Amps, which shouldn't be a problem for the cables), but the addition of the DC current in an operational transformer will push it into core saturation leading to nasty effects. Twisted/untwisted cables don't come into it.

Source: postdoc who models Carrington-scale events, and did my PhD on GICs in a regional power network.