Cost differential of hung vs. buried power lines

In Florida and the Gulf Coast, we’ve endured a lot of hurricanes in the past couple of years (in case you haven’t heard :wink: ), always accompanied by power blackouts, some of which last for months. The blackouts usually* happen because the wind blows down trees and the trees knock down power lines – which wouldn’t happen if the lines were buried, which many power lines are. So why don’t they bury all the power lines? Well, as Maureen Johnson said in Heinlein’s To Sail Beyond the Sunset, “To any question that begins, ‘Why don’t they . . .?’ the answer is usually, ‘money.’” What does it cost to bury power lines? Presumably it costs more than stringing them from poles – but by how much, in terms of cost-per-mile? Would it be worth the cost to bury all power lines in hurricane-prone regions?
*Of course, a really disastrous hurricane like Katrina can affect the power plants themselves, e.g., by flooding.

You also run into problems burying cables in areas that flood often, and IIRC you can’t do it at all if the water table is too high. That pretty much means all of Florida and most of Louisiana.

:frowning:

I guess we’re screwed, and shall remain so.

Worth it to who? For the investor owned utilities, there is no payback to burying more lines. It is almost always something that gets mandated to them. It really costs them little to have a section of the grid out for a few weeks, since the areas that are out of service are typically low use areas anyway.

What does it cost to bury power lines? The answer is… it depends.

Are you trenching in small house lines, direct burying small distribution lines to neighborhood boxes, running medium voltage wires in conduit between substations, or running high voltage cross country transmission lines? What is the type of ground that needs to be covered? Rocky? Sandy? Populated? Farms?

While that is true, it doesn’t help you out much. Burying small lines is generally more money but not cost prohibitive. Burying medium sized lines is usually required in most new developments around here. It is more money, but is usually mandated by the city and developers have to put up with it. Oddly enough, my feeling is that it is done more for appearances than reliability.

Burying large cross country transmission lines would be enormously expensive.

Wires that are on poles are typically not insulated, they are just bare copper. When you bury them you are required to purchase insulated wire (and the insulation gets pretty thick on higher voltages.)

And, lastly… people never dig into overhead high voltage lines with backhoes. Sloppy operators do occasionally dig into buried lines, with shocking consequences.

I would question the assertion that wires cannot be buried in high water table areas. All wires and connections are waterproof. Junctions can be made in above ground enclosures. No particular reason that it couldn’t be done (other than it costs more money and the power companies will never do it voluntarily as there is no payback for them.)

This thread from last summer addresses the issue in more detail. (Note: the water table and flooding don’t necessarily cause problems; underground lines are placed in sealed conduits, at least where there is a chance of water seepage if not always.

I live in south louisiana. Down where the houses that didn’t wash away flooded (mine flooded). Buried lines in wet soil (our water table is only a couple of feet down) aren’t a problem. It is done all the time. Cost varies widely of course. From conversations I have heard, around here a buried line costs about twice as much as open air lines. They are mandated by the government for subidivisions. The cost is borne by the developer (and of course that means the home buyer)-the power company doesn’t pay for the initial installation. They do carry the cost of maintenance. They are done mostly for escestetic reasons, not reliability. It doesn’t take long to put up overhead lines. Those places that have them might have waited a couple of days more, but not much. Of course this is talking about suburban densities. Out in the country there are places that still don’t have power. For Katrina restoration of power around here was governed mostly by the time it took to rebuild the substations. Hopefully the power was out by the time the water hit, but can you imagine the show when a 10 foot wall of water washes over a ground-level substation? I do know that it took them weeks to rebuild the substation. They had to replace all the transformers and cable.

Running high voltage transmission lines underground is fabulously expensive.

You have a choice of plain old burying them where there is no built up area, or you may well have to construct an underground duct in city areas.

Cables have to be jointed, and wherever these joints occur, you need to build a joint box, which is usually a concrete box which will be around 20 feet long by around 10 feet wide, and this will have to be at a depth where it is unlikey to be drilled into, and it will have a removeable lid, so the ground above cannot be used for anything else, meaning the utility will have to purchase it.

Insulation on such cables is generally of pressurised oil, which means that the cable must brak surface from time to time so that it can be connected to the pumping equipment, and of course this has to be in a secure structure, whihc takes up more land.

Where high voltage cables run close alongside roads they have to be placed in ducting as there is a tendency for the cable to creep along the direction of traffic flow due to the vibrations transmitted and over a period of time it will lead to failure of the cable.
Ducting is very expensive, and you can’t allow any other structure to be placed above it, so it means you have to purchase the land.

Wherever the cable breaks surface, or goes underground from the main transmission lines, you need more land and more secure structures, along with services, often this will mean instrumentation, and this pushes up the cost more.

Cables underground also have other inherant electrical properties that require more equipment to offset these effects, such as ground capacitance.

Undgerground cable have to be physically much larger in construction that the overhead types, one is because the insulation is not air, but must be a carefully made cable serving, and they cannot dispose of heat as easily as overhead cables, so you need thicker cables to be able to carry the current without generating this heat.
The pressurised oil that serves as the insulator for underground cables is also used to carry heat away.

Repair of these cables is far more expensive, involving lots more safety barriers, digging, perhaps a new jointing box to be made where a cable has completely blown out, however such cables are also very much more reliable, it pays to over-engineer underground cables rather than have to repair them.

Bear in mind that in this context I’m talking of cables operating well over 45kV.

Those cables that operate without the necessity of pressurised oil insulation are very much cheaper to install and operate, but they are still hundreds of times more expensive than their overhead counterparts.

This was widely discussed after the Quebec ice storm in Jan. 1998. The power to Montreal comes from hydro-electric installations about 1000 miles away and the power is transitted at something like 730kV. Of the five power lines coming into the city, four fell and fifth was almost literally hanging by a thread. They were designed for 40 mm of ice (about 1.6") and there was actually 100 mm (4"). The bottom line was that it was entirely unfeasible to bury them, but it was certainly feasible to strengthen the towers (which were what actually fell; the lines themselves held up). I hope this has been done.

Heh. Joints and joint boxes. You whacky Brits with your crazy “English”.
(This side of the ocean it’s splices and splice boxes.)