Repairing Underground Power Cables

When we were kids, overhead power cables were a constant fixture in the neighborhood. Not only were they the bane of our kites, but they also had a tendency to fail during storms leaving us in the dark. These days lots of cables go underground — safer for kites and harder to storm damage. On the other hand, if they do need repair, it is a major operation, as [Practical Engineering] discusses in a recent post you can watch below.

In the story, a large underground distribution cable — the Scattergood-Olympic transmission line — had a failure in the late 1980s. The 10 mile line has three high-voltage phases and when the line was created, running high voltage lines underground was a bit exotic.

One big problem with burying cables is a need to minimize the size of the trench, while keeping the cables far apart so they don’t arc. The video covers how the transmission line used oil to insulate the conductors.

In 1989, the power cable had an insulation problem and one of the biggest challenges was to find the location of the problem. Several approaches didn’t work but they finally located the approximate location.

In the end, the repair turned out to take longer than expected because they found latent failures and decided to fix them all at one time. The oil-filled cable continued to have problems, though, and at the end of the video, you’ll hear the story about how the power company finally used new wiring to replace the old cables.

The power system is one of those things you don’t often think about until it doesn’t work. It is interesting to see all the technology that goes into doing something common — moving electricity over a wire — at a giant scale.

This isn’t the first time we’ve mentioned this engineering repair. But if you think burying cable underground is audacious, try sinking one in the ocean back in the 1850s.

15 thoughts on “Repairing Underground Power Cables

  1. I wonder what a vertical shift during an earthquake of say 2 inches or more would do to such a line? Or do they put the line inside another much wider pipe? (even though that pipe isn’t filled with oil)

    1. A few years back, I was working in Ft. Collins and the city was contracting several outfits to upgrade a lot of stuff in the older section of the Downtown area. They were removing most of the over head lines and poles as buildings were being restored, newer buildings were being constructed where the costs of repair or the feasibility of repairs were above and beyond practical limits. All of the new lines were being installed in heavy PVC conduits. There were so many tubes running everywhere from earlier construction and new tubes and lines being buried. Many time as the crews were digging to expose the older line and adding new, digging equipment would damage pipes. Pipes would have be to cut and re done to allow branching for other power leads, additional cables were being pulled through and small cables were being pulled out and replaced with larger lines. It was a total mess and a long drawn out process to say the least. It caused traffic issues that resulted in more accidents. Many streets had to be completely closed down and people that were living in areas of construction had real issues with parking to the point the city was simply could not deal with it. There just wasn’t any place to go. A lot of people that had access to alleys were forced to take down fences in there back yards and park their cars in their yards. The alleys were never intended for that most of the places nor were the back yards with all the old growth trees and shrubs. Most people that had small garages faced the streets and many of those were too small for modern cars, or were being used to store stuff and not able to use the driveways was another problem. Before I went to Ft. Collins I was living in western S.D. and commuted all over where many power lines were being removed in areas that sustained frequent high winds and heavy snow and ice accumulations. Along the north south highway, there is a line running from east of the Black Hills all the way into NW Neb. with many branches and some substation boosters. There is also a high tension line that runs @ 230KV with 1.2K amps of 3 phase running from a Hydro plant on the Missouri River near the Montana border with N.D. all the way into Kansas for a major supply line to five states in conjunction with a few other lines to power the regional grid. There is much advantage to removing overhead lines, economical and ecological. Many area lines have attachments on the top of poles to prevent large prey birds and others from landing and building nests on the poles. There is not simple cheap way around getting power from place to place which is a big reason in itself for people to invest in private renewable power production. This is about the only solution to solving grid load problems. More power being produced locally to off set usage over the entire grid. It is also the only way we will be able to planet and environment. There will still be issues of toxic materials but it would be much easier to deal with that during production of the solar panels and wind turbines. Nuclear waste is not going away no matter what. As for dealing with power lines around fault lines they can be brought above ground in those areas so damage is reduced.

      1. Been there and done that. Worked on large underground installations and not a day went by that we did not hit unlocated cables, pipe, conduits, etc. Also had a lot of supposedly abandoned spurs which were actually energized. Dangerous business. I was always scolding guys for leaning against backhoes and excavator while digging. Good way to get lit up for sure.

        One way I have seen fault areas dealt with is to use a much larger conduit and kind of coil the cable a bit inside of the pipe. This creates some stored slack. I have also worked with spring loaded splices. Essentially you have two terminals connected by two big coaxial coil springs (coiled in opposite directions so that you don’t create a giant inductor). It allows for some shock loading of the cable.

        As far as repairs go. It is all about access points. Usually you don’t do a splice where there is not a manhole or access point (possible but not recommended since the buried splice is hard to monitor and service). Repair is usually replacing a section from manhole to manhole. There is also a move to flexible conduit and direct burial. Much lower install cost since directional boring is commonly used and you can steer around obstacles. However it is less maintainable in the event of a fault and location is a bit more difficult since it can snake a bit and there are a lot less vaults which lowers install cost but limits future access.

        Monitoring these cables is not difficult with things like Time Domain Reflectometry, pressure, insulation testers and such. It just takes the discipline to actually set up an effective maintenance program. These things fail so rarely that it is common to just forget about them until a problem comes up. The power grid also needs better redundancy so things can be taken off line without causing a major outage.

    2. Yes, there is quite a bit of extra space there for several reasons. It is easier to pull cable in larger conduit (less contact area when the cable is much smaller than the pipe, better heat dissipation and distribution (less localized hot spots). There is also slack built in at points along the route. You have to account for conduit shift as well as the temperature expansion of the cables. A couple of inches of displacement is probably not a problem, a lot more than that is a major problem. Good surveys can tell you where this is likely to happen. Interfaces between dissimilar materials (rock vs sand, dry vs wet) are good indicators of movement points.

      1. +1, I was thinking of the ’89 quake in CA where the expressway’s upper deck collapsed onto the lower deck. Clearly that one moved a lot of stuff (although the deck might have been just shaking, not actual shifting)

  2. A stretch or squeeze would be worse because this is what failed in the first place. Thermal expansion and contraction made the splices move out of their roomier spaces and short out on the rest of the slimmer pipe. Just what was in that oil? PCB’s?

    Reminds me of wax coated paper caps. Oh, the smell of hot tubes and wax caps.

    Coaxial pipe would help if it could even out the thermal changes and monitor any oil leak. It sure would have stopped the latest oil spill on the west coast!

  3. Ireland’s state electricity company the ESB had a whistleblower go public with the news that oil filled power cables were leaking huge quantities of oil which is really bad enviromentally. the esb was unable to fix the leaks so kept topping them up instead. iirc they weren’t happy with being outed and fired the sqealer.

  4. The best way to monitor these cables is with pressurized oil or air. This is common in communications cables. You apply pressure to one end and monitor at various points along the way. The positive pressure on the cable also helps prevent ingress of moisture vapor and liquids. Another complication with underground power cable is that the resistance of the cable creates heat. In aerial lines this dissipates rapidly into the air, underground this can be a big problem. Overall though underground cable is a big win in terms of lifespan, maintenance and reliability.

    1. Single phase underground line also generates a bunch of capacitive current.

      Underground cable can be an improvement operationally as it removes overhead clearance problems and many weather related faults. It is definitely not an improvement in terms of lifetime. Overhead lines typically last 10 years or more longer than underground.

    2. > Overall though underground cable is a big win in terms of lifespan, maintenance and reliability.

      Can you be more specific with your general statement in regards to AC vs. DC, different voltage levels (LV 1100kV[1] or MS 3-30kV, HS 60-110kV, HöS 220-1150kV [2]), urban distribution / land transmission, etc. or cite a source for that?

      All I could find in the last ~10 minutes indicates mostly the opposite[3]. (mixed bag of arguments not applicable to all types of aerial/underground cables.)
      sth. from the PlasticPipe industry (so kinda biased)

      .. and so on.

      [3] (and other variations)

  5. Very interesting stuff. As for being audacious back in the day, everything is audacious when you are first one to do it. In 150 years they will likewise marvel at how we made self-driving cars (failures and all) with such primitive technologies.

    1. I actually thought the ocean telegraph cable was WAY ahead of it’s time, can you imagine doing that in a ship while rocking back and forth? Even the ability to construct a cable to withstand ocean abrasion must have been something big, but in 1858? I get it was a telegraph cable, but still. They apparently use submarines now to lay cable – I did not know that.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.