Technical Feasibility of Underground AC Transmission Cable Grid

[mbrooke]

Steal them from other sites and paste them. Go crazy!

Wait, you can do that?

 

[LarryFine]

Wait, you can do that?

No, I was just kidding.




Sometimes, you have to post it as an image using its URL.

 

[mbrooke]

No, I was just kidding.




Sometimes, you have to post it as an image using its URL.

To complicated for me... but I'll see what I can do.

 

[Hv&Lv]

You can never forget cost...

https://www.tep.com/wp-content/uploads/TEP-138-UG-Report-Rev.-0-signed.pdf

 

[electrofelon]

You can never forget cost...

https://www.tep.com/wp-content/uploads/TEP-138-UG-Report-Rev.-0-signed.pdf

in that report, there are some good figures. Looks like the cable is $120 per foot. They also give a figure of about 1 million $ per mile for construction of a new overhead 138-kv line.

 

[drcampbell]

Us hams call it Hardline

Curiously enough, they call it "Flex" and also offer a line of rigid transmission line. It bolts together like high-pressure flanged steam pipe.

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[synchro]

They also have rectangular waveguide which has no center conductor as coaxial waveguide or cable does. It will only propagate signals above a certain "cutoff" frequency, whereas coaxial (i.e., TEM mode) will propagate all the way down to DC.
Optical fiber is a circular dielectric waveguide which also has an associated cutoff frequency below which electromagnetic waves will not propagate.

 

[mbrooke]

Couldn't they keep cost down by direct burrying that cable? You know, 138kv URD?

 

[Hv&Lv]

Couldn't they keep cost down by direct burrying that cable? You know, 138kv URD?

The pipe isn’t the problem or the driving factor for cost.
Pipe is relatively cheap.
You don’t want to bury this type of wire 3’ deep and walk away.

 

[mbrooke]

The pipe isn’t the problem or the driving factor for cost.
Pipe is relatively cheap.
You don’t want to bury this type of wire 3’ deep and walk away.

Why can't that be done? Burrying 6 feet and walking away?

 

[Hv&Lv]

Why can't that be done? Burrying 6 feet and walking away?

Think.

basically your lifeline to several hundred thousand customers is installed without supplemental protection.
Someone digs, bores, or drills into it.
Your major asset is too unprotected. This isn’t an hour fix.
NC had a drill go through a line like this and it was in a sound. You would think no one would be anywhere close to it.

Took MONTHS to fix

 

[mbrooke]

Think.

basically your lifeline to several hundred thousand customers is installed without supplemental protection.
Someone digs, bores, or drills into it.
Your major asset is too unprotected. This isn’t an hour fix.
NC had a drill go through a line like this and it was in a sound. You would think no one would be anywhere close to it.

Took MONTHS to fix

Well, you'd have multiple circuits. So loss of one is not an issue.

Protection adds cost.

Direct burial with a red ribbon on top makes it a lot cheaper.

 

[drcampbell]

... NC had a drill go through a line like this and it was in a sound. You would think no one would be anywhere close to it. ... Took MONTHS to fix

Underground ... out of sight ... easily forgotten or misplaced.

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[mbrooke]

I know you said "cost aside", but it's hard to get past the cost difference between underground and overhead. If you've ever done a cost estimation for a new MV utility feed where overhead vs underground was evaluated it is quite significant at times. Even for small and short MV services it can be tens of thousands of dollars for underground vs next to nothing for overhead.

I'd probably agree that outage frequencies may go down... but the duration of the outages when they do occur could go in the opposite direction, so your outage-hours could be relatively the same or worse. And now everything was just more expensive.

Disagree, with a networked or dual feed approach the duration is eliminated. These often have auto transfer on the primary gear:

 

[bwat]

Disagree, with a networked or dual feed approach the duration is eliminated. These often have auto transfer on the primary gear:

So now we're talking underground everywhere and everything networked or dual feeds. Cost is obviously a factor in the way things are, but if we're playing the game where it is not, why stop there though. We could concrete encase everything and have detailed GIS maps on every inch that is always 100% accurate.

 

[gadfly56]

Well, you'd have multiple circuits. So loss of one is not an issue.

Protection adds cost.

Direct burial with a red ribbon on top makes it a lot cheaper.

So, how deep would you have to go to make it unlikely someone would jam a backhoe into a 500kv line? If there was a fault to ground from one of these feeders, how wide an area would develop a lethal step potential?

 

[Jraef]

The pipe isn’t the problem or the driving factor for cost.
Pipe is relatively cheap.
You don’t want to bury this type of wire 3’ deep and walk away.

My neighborhood has 11kv underground feeds, they buried them under the sidewalks about 4’ down. The underground transformer for a big chunk of my neighborhood is in my yard and is basically under water whenever it rains. Apparently one on the connectors leaked one winter and allowed water to seep into the insulation where it eventually migrated down into the conductor. The result was an explosion that lifted the sidewalk concrete up about a foot in the air. It took a crew of 4 guys 4 days to fix it and test it, not counting the concrete crew that replaced the sidewalk. I was working from my home office at the time and of course was without power, so my entertainment was that I watched their progress over the days. That convinced me that underground is not the answer to reliability.

 

[mbrooke]

So, how deep would you have to go to make it unlikely someone would jam a backhoe into a 500kv line? If there was a fault to ground from one of these feeders, how wide an area would develop a lethal step potential?

I wouldn't worry about depth, because in any case you shouldn't go digging without having the area marked out prior.

 

[mbrooke]

My neighborhood has 11kv underground feeds, they buried them under the sidewalks about 4’ down. The underground transformer for a big chunk of my neighborhood is in my yard and is basically under water whenever it rains. Apparently one on the connectors leaked one winter and allowed water to seep into the insulation where it eventually migrated down into the conductor. The result was an explosion that lifted the sidewalk concrete up about a foot in the air. It took a crew of 4 guys 4 days to fix it and test it, not counting the concrete crew that replaced the sidewalk. I was working from my home office at the time and of course was without power, so my entertainment was that I watched their progress over the days. That convinced me that underground is not the answer to reliability.

Radial always leaves a lot to be desired.

BTW, 4 days is incompetence, I've seen underground repairs done way faster.

 

[electrofelon]

I wouldn't worry about depth, because in any case you shouldn't go digging without having the area marked out prior.

There's a lot of things people do that they shouldn't do, I don't see that changing anytime soon!

Also locates can be off. Friend of mine hit a 15 kV line last fall, the locate didn't mark it for some reason.