10 October 2021

Why Can't You Just...

Why can't you just put high voltage transmission electrical cables underground? Repairing Underground Power Cables Is Nearly Impossible

The solutions to problems always look easy if you don't know any of the details of the science. This is especially true if you are looking at projects that are 50 years old, that were built with the best technology of the day.

This story is in relation to the Scattergood-Olympic transmission line in Los Angeles, which was built in the 1970s. Three, 230 kilovolt transmission lines connect the Scattergood power station and Olympic substation in Santa Monica. It is 10 miles from the power station to the substation.

On an autumn evening in 1989, Tom McMahon noticed some unusual construction getting started in his Los Angeles neighborhood. As more and more trucks began showing up with bizarre power tools, test equipment, and tanks of liquid nitrogen, his curiosity got the better of him and he had to take a look. He learned that a high voltage underground transmission line had experienced a fault, costing the City tens of thousands of dollars per hour in lost capacity and downtime. Over the next few months, he got more acquainted with the project manager for the repair, and he shared all the fascinating details of what he learned in a series of messages on his company’s mailing list. Those messages spread like wildfire across various bulletin boards, lists, and forums of the early internet.

How the underground cable was designed. The problems that had developed by 1989. How those problems were identified, and the plan to fix them. The larger set of problems, and the wholesale fix. The problems that developed in 2016, and how they were repaired in 2018.

People will think, "OK, we have the fix today, we can put these cables underground with no problem." Really. You know that. You know that no problems will develop over the next 50 years. It must be nice to be so sure. I imagine the people who changed the material specification of the O-Rings on the space shuttle were sure, before an O-ring failed and destroyed a space shuttle, and I imagine that the people who changed the formula of the insulating foam on the space shuttles were sure, before several of the heat shield tiles on a shuttle were damaged by falling foam on liftoff, which destroyed a second shuttle. In both those cases, it was a small change. "What could go wrong?"

In the case of the Scattergood-Olympic transmission line it was heat, or more specifically, it was thermo-mechanical movement. "How can heat destroy metal?" A question you might ask if you don't actually know anything about materials science.

This is the video that accompanies the link above. Repairing Underground Power Cables Is Nearly Impossible

As you see in the video, just inspecting them to determine where a fault might be isn't that easy either

4 comments:

  1. Thanks for that enlightening post . I was much in favor of burying power lines till I learned its a bad idea with current materials .

    Not many people can say they changed someones mind on the Internet but I think you managed it.

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  2. I think the preferred method of burying lines now is concrete duct, with insulated and shield transmission lines (basically giant coax. That avoids the problem of the oil (Hmm, was that the old PCB-ladened stuff used in old transformers? That stuff's a mother to deal with and dispose of.) But that method can cost 15 times as much as overhead lines.

    https://www.xcelenergy.com/staticfiles/xe/Corporate/Corporate%20PDFs/OverheadVsUnderground_FactSheet.pdf

    ReplyDelete
    Replies
    1. In this case, putting in the concrete conduit would have been too expensive and unpopular with the Californians, so they installed the massive coax with boring machines. I expect someone with a backhoe will dig one up in about 10 years.

      Delete
    2. Ah,Kalifornica: "Hey, let's fill a ten mile pipe with carcinogens pressurized to 200 PSI to protect out lawns. We be green!"

      Delete

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