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

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.

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.

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: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: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.

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?

[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.