Why Aren't Powergrids Underground?

jonging asks: "It is common knowledge that an underground power grid is less susceptible to the effect of a large thunderstorm. The American Transmission Company cites numerous reasons why it (and other power companies I assume) do not bury their transmission lines underground (e.g. environmental concerns, cost of installation and repair, etc.). Exactly how detrimental are underground transmission lines to the environment? Wouldn't the time spent without a power outage generate more than enough revenue to offset initial costs? Aren't the need for repairs in cities with successful underground power grids rare?" The linked article goes into extensive detail about the disadvantages in initial costs of putting in underground lines, but doesn't go into any detail about the maintenance costs of either option. With storms getting worse and worse (Maryland, DC and Northern Virginia have weathered torrential downfalls this week), might underground lines prove more resistant to storm-related power outages?

Comments

It costs money?

[NotQuiteReal]

That is all I have to say.

Sure, it would be nice to put it underground, but it costs more that way...

Re:It costs money?

[tomhudson]

Long term it still costs more.

Its a lot harder to maintain buried conduit. Plus, there's the problems of accumulated gases in any piping you lay down, plus drainage, plus trash/dirt/crap accumulation at the manholes.

Look what happens when buried conduit deteriorates - the resulting fire is nasty because its more concentrated than in the open air.

Re:It costs money?

[Technician]

There is a big cost diffrence if putting in a subdivision and burying the 7200 volt line into the subdivision transformers and burying a 500,000 transmission line. Safety is also a concern. Which line would you rather hit with a backhoe?

On a high tension line, the capacitance per foot is much higher for a buried line than for an overhead line. For long distance feeding this capacitive load adds greatly to the power loss in the line. Burried is OK in New York City, but forget it for the grid. There are too many losses. Putting the 2 top grounded lines above the high tension lines have greatly reduced lightning strikes to the power conductors and their resulting outages from damaged insulators and substation equipment.

Disclaimer.. My father was a substation operator for Bonniville Power Administration. I've seen the MegaVar meters on some long lines.

Re:It costs money?

['nother poster]

Well, I know people who work in the power generation and distribution business in the midwest U.S. Arcs cause resets, too many resets, and the circuits are taken out and it takes serious human intervention to bring them back in. This costs time and money. I find it shocking that there are utilities that scrimp on a few links of insulator to lengthen the path to ground through the dust buildup so that they don't have periodic fault to ground or fault to phase events. Those can get expensive fast. Once again, if any area experiences more than a few of these events and the plant engineers don't schedule a fix on the next maint outage they should be fired. Dumping an 18.8 kV feeder sucks. Dumping a 46 kV feeder is a major pain. Dumping a 230kV trans line will inconvenience lots of customers and will cost a fortune to cycle and bring back up. A utility doesn't let that happen more than about two times before heads roll. About the only thing that would piss off the management worse would be doing something really stupid and getting a 600MW alternator kicked out of the grid and having to spin it back up and sync it back in.

Re:It costs money?

[ray-auch]

I would actually like to see some figures to back that up.

http://www.highland.gov.uk/NR/rdonlyres/CBDD84F3-3 326-4DB9-AAA1-44537AE9B885/0/text.pdf

Has a lot of good info on underground vs. overhead for proposed new powergrid in Scotland.

Estimates of lifetime cost ratios (table 8 at the end of the document) are between 6.9 - 10.2 for traditional fluid-filled cable and still 4.9 - 7.8 for newer (and arguably less proven) XLPE insulation technology.

Also, this is recent tech which you would use to build your grid _now_ - go back a couple of decades and the difference was much larger. At Dinorwig - http://en.wikipedia.org/wiki/Dinorwig_power_statio n - they had to run the first few miles of cable underground because of national-park restrictions, and there you're looking at water-cooled cable requiring acutal cooling stations (size of small house) every couple of miles. While it's very impressive to see an entire power station underground, with no visible power lines, it was definitely not cheap to do it that way.

Bottom line is that the overhead option is using a few feet of air to get its insulation for free, and it's always tough to compete with free.

Re:It costs money?

[the_xaqster]

One of the main problems with underground cables is locating faults. Most faults are caused by water seeping into the cable via a damaged insulator. When enough water has seeped in, the cable shorts and blows the breaker. Unfortunatly, this also dries the cable out nicely, which means that testing for the fault becomes a problem. The best method for locating these faults is to switch bits of cable in and out, and narrow down which section it is, then dig it up.

And how do the insulators get damaged? One way that happens more than most people would admit is it gets clipped by someone digging up something else. Say you are digging up the gas pipe in the street. If you just nick the electricity cables insulation, would you tell your boss so he can get the electric company out to replace the cable, delaying your work by hours, or are you just going to throw some dirt over it, so no-one will be able to tell?

I have worked for 2 Electric companies, so I know a little about this.

Re:It costs money?

[tomhudson]

Ithink it shouldn't be to hard to design those tunnels in a way where you can use robots like in the sewage systems.

Really? The reality is that sewage work is still done by humans, not "robots". There's no way that a "robot" can dig up a street to replace a broken water main or sewer pipe, and the fibreglass inserts/patches are NOT a long-term fix when a pipe breaks.

Besides, you've overlooked the installation costs. It can easily be 100x more expensive to run a wire underground than overhead. Overhead - 2 cherry pickers, 5 guys, a few spools of wire, a day, and a couple of blocks are rewired (they just upgraded all the wiring on the street 2 blocks over last week - took 2 days because of the trees. On the other hand, 3 years ago they did a major upgrade along about 40 blocks - in one day - with a larger crew of cherry-pickers and support vehicles). Underground - backhoes, loaders, dump trucks, flatbed, concrete saw, gravel, conduit, manholes, manhole covers, asphalt repaving, cement mixer, sidewalk repair, 2 weeks, easily 30-40 people involved (gas, sewer, waterworks also have to be coordinated).

Then there are the transformer rooms (since you can't just hang them from a pole) - concrete pads, etc. Money money money.

Re:It costs money?

[mothlos]

I can personally attest to the problems of buried electrical cable. I am a dispatcher for a local power company. About 3/4 of our grid is above ground with the rest earth buried (not tunnel buried as in some cities). Buried cables account for fully half of our line failures. The most common issues are the result of earth shifting and water seepage. Repair of underground cable requires extra equipment and manpower to locate and excavate to fix problems. Also, there is a danger of damaging underground cables and pipes maintained by other utilities. Utilities spend a lot of money locating their underground infrastructure for each other.

Simple physics

[loony]

Each cable that transports AC is subject to drain by the capacity the parallel lines themselves represent. The closer the wires, the higher the capacity. At about 30 km on a regular high voltage cable, you reach a point where the reactive power drain reaches the maximum power the cable can transport - the cable is saturated without draining a single watt at the end.

DC does not have this issue however then you have all the problems that killed Edison's original DC power distribution in favor of Telsa's AC distribution.

Peter.

Re:Simple physics

[coyote-san]

Are you sure that isn't inductance?

Anyway, we're almost certainly talking about different things. Nobody is suggesting burying long-distance high-tension lines. Just the last half-mile or so. That's enough to eliminate the visual clutter and keep the neighborhood from losing power after a tree limb breaks, etc.

Higher transmission losses with UG lines...

[Ellis D. Tripp]

Underground powerlines would suffer higher capacitive losses than overhead lines, and losses between the generating plant and the user would be power that the utility company can't (directly) bill for.

With all the public concern about EMF exposure, the situation would be made much worse when all those distribution transformers move from 40' up a pole to concrete pads at ground level. And then there is the everpresent problem of "backhoe fade"...

Re:Higher transmission losses with UG lines...

[peragrin]

actually this is the big point. Power companies can massively undersize wires that are traveling in free air.

Three conductors in free air 15 feet off the ground the power companies can run a #2 sized cable for 200 amps. Yet that same wire underground needs to be 4/0 or 250 MCM which is several times larger.

The cost of goods to run lines over head is considerable less even if you take into account storms trashing it. Just from a dollar point of view you can competely rebuild a surface grid two or three times for the cost of doing it once underground. Digging costs that much more. Digging near roads is even worse.

I think it makes long term sense to go underground but I do see the cost advantages of going above. Plus the union can hire more people.

You prove the point

[PizzaFace]

Downtown Washington rarely has power outages because the power lines are underground.

Footpaths

[MavEtJu]

Here in Australia, or at least large (for big values of large) amounts of it, it's all above ground too. Where I'm coming from, the Netherlands, it's all below ground.

When I discuss it with the people here, they give me all kind of reasons why it should be above ground (limited but not only to unable to quickly repair, the famous cable cut from people digging and, believe it or not, the people who are doing the repairs now would be jobless).

Just a quick glance about how it could be done and you'll see that it would be quite a trick anyway: All footpaths in Australia are large blocks of concrete or asphalt, and the nice small tiles you see in shopping centers are also just laying above a concrete layer. Opening up that would be a major++ operation. Compare it to the Netherlands where all footpaths (and most of the bicyclepaths) are just 30x30 cm tiles laying on top of yellow or black sand, you'll see that it has a historical tradition to put things underground and have them easily accessible.

Official report from Edison Electric Institute

[philgross]

We could continue to debate this endlessly, but maybe you could save time and just read the official report?

I'll also mention that 4 of the 5 NYC boroughs (Manhattan, Brooklyn, Queens, and the Bronx) have their electricity distribution almost entirely below ground. It was a massive investment, but it was long ago.

Re:Water

[Technician]

pets (and people) getting electrocuted from lines that were buried 40 or more years ago and were now corroding or fraying.

We have learned from our mistakes. All newer high voltage buried cable is coaxal in design. The hot conductor is surrounded by a grounded jacket. A fault shorts the cable to the grounded jacket tripping the overcurrent protection instead of putting lots of voltage to the ground.

The situation in Ukraine

[Zx-man]

As weird as it may sound, quite a number of small towns here, in Ukraine have their powergrids (mostly) underground. It is so because in the 90s it was not uncommon for every piece of cable/wiring to get stolen sortly after being installed. So, back in day it used to be financially effective. Now with crime rates down it, probably, would not be worth the price, thought. But it stays as it was.

Or downtown DC is close the the Whitehouse...

[Zadaz]

Why this is modded +5 informative shows how bad the average person's logic is. Unless you pegged my sarcasm meter (which is possible) it seems more plausible that keeping the nations government stably powered is a more significant factor than the placement of the lines.

Power has gone out at least once a year for the past five years to my (Downtown San Francisco) neighborhood. Due to underground power lines. A couple months ago an underground substation exploded and burned the hell out of a woman walking on the sidewalk. A couple years ago directly in front of my apartment a short underground ignited flammable (sewer) gases which blew the manhole covers 40 feet in the air (And the power out for the whole day). No one was hurt, but one of the covers did go most of the way through a car.

My UPS gets a good work out.