This is just an update on where the – dare I call it research? – stands. Was going to have it ready to go on-line at midnight, but after work chores took a good bit longer than anticipated. Anyway, after looking at lightning arresters, I started on Geomagnetic Induced Currents, with the idea of calculating induced currents. This has led to quite a rabbit hole. One head scratcher, previously noticed, is the factor of ground resistance in GIC. It turns out that GIC isn’t just induced current in conductors; it can essentially produce “ground loops.”
Ground loops are notorious for all sorts of bad things, from audio systems to surge suppression. It comes from difference in electric potential between grounds. Voltage is difference in potential. If two grounds are at the same potential, you don’t have a flow of current. But if two grounds are at a different electric potentials, you get a flow of current from one ground to the other. That’s why single point grounding is recommended for surge suppression, otherwise you can get current flow between different grounds.
It’s also why just walking up to a downed power line can electrocute you. Soil has resistance, which causes a drop in voltage the further you get from a downed power line. Resistance varies depending on soil and other factors, but you have lower electrical resistance than dirt. This means one foot can be at one electrical potential while the other is at a different one. This causes electricity to flow up one leg and down the other. That’s one reason we wear special electrically insulated overshoes while doing line work.
Basically GIC can cause a great bit ground loop. In 2000, a solar storm caused 300 amps on the neutral at a transformer in Sweden. There was a loop formed by the ground to the transformer, up through the lines, and down through the ground of another transformer.
This raises questions I haven’t sorted out. For one thing, with the exception of high voltage DC, US transmission is AC three-phase without a neutral, what’s called three-wire Delta. Most US distribution transformers provide power in another configuration known as Wye, sometimes called Star, which does have a neutral. This sort of connection is called Delta-Wye. I’m going to have to look more into the propagation of GIC through this sort of set-up: I might be focusing too much on the distribution side and missing the obvious.
Another question is why this current didn’t trip protection devices on the transmission side of things, isolating the transformers. In come cases it does, and this had me looking at things backwards.
When discussing GIC, I was thinking that it induced current on the lines rather than through ground loops. I would be very surprised if it didn’t do both. The crucial thing is I assumed that all GIC would be on the conductors, like lightning, and not through a ground loop. This means my touting of line protection devices, which do indeed operate during a solar storm, might not address all of the problem. Consider a lightning arrester mounted on power line near a transformer. Ideally, a lightning strike on the power line causes the lightning arrester to bleed off over-voltage before it reaches the transformer. But if a GIC ground loop trips a lightning arrester, it could flow through the equipment to get to the arrester if they’re at different potentials. That’s not good.
Since my initial premise, while not completely invalid, didn’t address the ground loop issue with GIC, right off the bat that increases the likelihood that I’m wrong about a Carrington Event causing little structural damage to the grid. There’s other things I want to look into, such as something we dealt with involving protection devices and current on the neutral back when we ran some huge portable generators, and I want to get a better handle on this whole ground loop through transmission line thing. But I may have already succeeded in proving myself wrong.
I need to look at this closer, though, instead of going “Oh, well, I was wrong.” What if my understanding of this is wrong? Then there are things like a lightning arrester connected to the same grounding grid in a substation. This means the transformer and lightning arrester is at the same potential. Does this mean that, if the arrester opened, GIC would flow through it, bypassing the transformer? If so, why didn’t that happen in Sweden in 2000?
Also, I came across an interesting presentation on GIC and types of transformers. Not all transformers are made the same, and there’s different core configurations, with a surprisingly low percentage susceptible to damage from GIC. That could mean that a Carrington Event wouldn’t create widespread structural damage, but for different reasons than I thought. So much to look at.
In the meantime, I’m thinking of some posts on the basics of the grid in North America. There’s things the general public isn’t aware of. Like the road crew we came across after a recent storm pulling downed lines that normally carried 14,400v out of the road. Fortunately for them, the line was dead, but there’s no way you can tell just by looking. We don’t know what they were thinking, but it probably wasn’t those wires normally carried voltage over seven times higher that was used in electric chairs.