How an underground high-voltage power line works

Let’s say that you want to run 500 megawatts of power from a power plant to a distribution station in a large city, and you can’t use huge overhead power lines and towers. You need to bury the wires. How hard is that? And what would you have to do if it needed repair? This article spells it out:

Cuts and jumpers

From the article:

Several days ago a very large number of trucks and men from the Los Angeles Department of Water and Power descended on my neighborhood. They removed large sections of Pershing drive to a depth of 15 feet or so over a stretch of about a city block. I assumed they had a problem with a water main or something.

When they started building semi-permanent structures over the holes I knew something really big was up. When the large trucks full of strange power tools, mega-welding machines, breathing equipment, and racks of test equipment came I started wondering. Driving by a couple nights ago (11 PM), I noticed that the pace hadn’t slowed - they were at it 24 hours a day.

My curiosity got the best of me yesterday when they brought in the giant tanks full of liquid nitrogen. LN-2 for the DWP? I parked my car and played the lookie loo.

It turns out they have a problem with an underground wire. Not just any wire but a 230 KV, many-hundred-amp, 10 mile long coax cable. It shorted out. (Lotta watts!) It feeds (fed) power from the Scattergood Steam Plant in El Segundo to a distribution center near Bundy and S.M. Blvd.

To complicate matters the cable consists of a copper center conductor living inside a 16 inch diameter pipe filled with a pressurized oil dielectric. Hundreds of thousands of gallons live in the entire length of pipe. Finding the fault was hard enough. But having found it they still have a serious problem. They can’t afford to drain the whole pipeline - the old oil (contaminated by temporary storage) would have to be disposed of and replaced with new (pure) stuff which they claim takes months to order (in that volume). The cost of oil replacement would be gigantic given that it is special stuff. They also claimed the down time is costing the costing LA $13,000 per hour. How to fix it and fast?

The article offers a complete look at underground power transmission. After you read the article, you will realize why these towers are so common:

Although inspecting and repairing the towers isn’t trivial either: