GIC: Low Voltage/High Current?

It’s starting to look like Geomagnetic Induced Currents wouldn’t have enough voltage for electric utility lightning arresters to kick in. That’s not to say they would be minuscule. One figure, for a Carrington level event that missed the earth some years ago, had 26 volts per kilometer in upper latitudes near the poles. That might not sound like much, but that’s 2,600 volts over 100 km (62 mi), and 13,000 volts over 500 km (311 mi). Now consider that transmission lines range from 46,000 volts to 500,000 volts. Compared to 500 kv, 13 kv isn’t much. Everything is relative.

This would mean a Carrington Event would trip fuses, reclosers, and equipment that senses voltage on the neutral, but lightning arresters wouldn’t bleed it off. I had thought, from descriptions of what happened to telegraph equipment in 1859, that the voltage would be higher. In short, my initial premise, that lightning arresters would protect the grid from GIC, looks wrong.

It also means that the spark gap arresters in place in 1859 wouldn’t have protected telegraph lines at all. When you consider that telegraphs in 1859 operated on 100v – 160v, and with milliamp current, it’s no wonder that up to several thousand volts caused sparks, shocks, and fires at telegraph stations and on poles. And yet it probably wasn’t enough to cross spark gaps and dissipate to ground.

Does this mean we don’t have to worry about transformer damage? On the contrary, this is starting to sound like my nightmare of a solar event that doesn’t trigger protection devices but still does a number on transformers. For while voltage might be lower than transmission lines, GIC can carry hefty amps. Factor in GIC distorting sine waves, and it isn’t good. Eddy currents in transformer cores can cause spot heating and other bad things.

If I’m reading some papers correctly, it seems that older transformers, with what’s called a shell core, are most susceptible to GIC damage. Fortunately, these are in the minority, at maybe 19%. Other types that aren’t quite as susceptible, could also be damaged, but aren’t as likely to fail. Even though my initial premise is wrong, it looks like it wouldn’t knock out most substation transformers on the grid.

Have to think more about it, along with mitigation strategies. Would cutting power to the grid during the event minimize transformer damage by minimizing eddy currents? Possibly, but at this point I don’t know. That’s just a guess – but so was my assumption that lightning arresters would protect against GIC.

That’s where it stands now. Still work in progress, but I’m fairly confident that I was wrong about GIC and lightning arresters.

What else about GIC do I have wrong?