Find and Repair a 230kV 800Amp Oil-Filled Power Cable Fault

scattergood01Have you ever wondered what guys like I do, when we’re not telling you that you need to do some completely unaffordable thing to keep your house wiring safe? We’re telling the utilities the same thing.

I ran across this the other day, talking about fixing an underground cable from a powerplant in California. It also highlights one of the reasons why a fair number of us are not fond of underground, no matter how much prettier you think it makes the landscape. 🙂

How do you fix a shorted cable ? Not just any cable. An underground, 3-phase, 230kV, 800 amp per phase, 10 mile long one, carrying power from a power station to a distribution centre. It costs $13,000 per hour in downtime, counting 1989 money, and takes 8 months to fix. That’s almost $75 million. The Los Angeles Department of Water and Power did this fix about 26 years ago on the cable going from the Scattergood Steam Plant in El Segundo to a distribution center near Bundy and S.M. Blvd. [Jamie Zawinski] posted details on his blog in 2002. [Jamie] a.k.a [jwz] may be familiar to many as one of the founders of Netscape and Mozilla.

To begin with, you need Liquid Nitrogen. Lots of it. As in truckloads. The cable is 16 inch diameter co-axial, filled with 100,000 gallons of oil dielectric pressurised to 200 psi. You can’t drain out all the oil for lots of very good reasons – time and cost being on top of the list. That’s where the LN2 comes in. They dig holes on both sides (20-30 feet each way) of the fault, wrap the pipe with giant blankets filled with all kind of tubes and wires, feed LN2through the tubes, and *freeze* the oil. With the frozen oil acting as a plug, the faulty section is cut open, drained, the bad stuff removed, replaced, welded back together, topped off, and the plugs are thawed. To make sure the frozen plugs don’t blow out, the oil pressure is reduced to 80 psi during the repair process. They can’t lower it any further, again due to several compelling reasons. The cable was laid in 1972 and was designed to have a MTBF of 60 years.

Finding out the location of the fault itself was quite a feat. It involved time-domain reflectometry (inconclusive), ultrasound, and radar (didn’t work) and then using an Impulse Generator-Tester (Thumper) which got them pretty close to the defective segment. What pinpointed the problem was a bunch of car batteries and some millivoltmeters. They hooked up car batteries to both ends, tapped the cable at several points and knowing the drops and resistance of the cable, got within a few feet of the fault. Finally, X-Ray equipment was brought in. Sure enough, they could see the cable shorting against the steel wall of the pipe. Cutting open, and closing it all up, required certified welders spending up to 8 hours on each section to avoid damage to the paper insulation. The welders placed their thumbs 3 inches away from the seams they were welding, and stopped when it got warm to touch, allowing it to cool off before starting again.

The failure was attributed to “TMB”, short for Thermal Mechanical Bending. TMB causes the cable to wiggle in place due to load surges. This eventually causes insulation failure due to abrasion against the pipe and separation of the many layers of paper tape. They repaired the short, put aluminum collars in most of the joints to hold the splices in place, and have added a load management scheme to reduce the current peaks. Apparently, the fix wasn’t good enough. According to this Wikipedia article, “the 315 megawatt capacity Scattergood Steam Plant (Unit 3) to West Los Angeles (Receiving Station K) 230 kV line is having to be replaced after only 45 years of operations, due to multiple failures within this rather long single-circuit, oil-filled, “pipe type” cable.”

Find and Repair a 230kV 800Amp Oil-Filled Power Cable Feels Like Mission Impossible | Hackaday.

TDR’s are one of the most useful diagnostic tools ever, they pay for themselves quite quickly but it’s nearly impossible to convince bean counters that think Radio Shack sells useful meters that a $2K plus tool, that doesn’t fix anything, and occasionally isn’t good enough is justified. Heck, I haven’t even quite convinced myself yet. Thumpers work (sometimes) on the principle of “letting all the smoke out”. It’s much easier to find a broken something than a cracked one, after all. For the rest, if you’re interested follow the links.

It’s part of the reason than the electrical trades are often so fascinating to be in.

And there’s this, from his comment stream, showing how sometimes we manage to get authorized to buy a new widget.

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About NEO
Lineman, Electrician, Industrial Control technician, Staking Engineer, Inspector, Quality Assurance Manager, Chief Operations Officer

6 Responses to Find and Repair a 230kV 800Amp Oil-Filled Power Cable Fault

  1. 🙂

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  2. the unit says:

    I imagine the technology required to fix that problem covered quite a few disciplines of expertise, experience, and learning. Of course we in a real world wouldn’t know that without a community organizer lecturing us. I just passed out on very hot day…”somebody call a paralegal.” Or maybe a MBA. 🙂

    Liked by 1 person

    • NEO says:

      Yes, quite, I’m sure an MBA can fix it. 🙂

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      • John Doe says:

        What is so interesting about your commentary is that you really have no idea how poorly it comes across. You don’t know how bad you are. For many programs, MIT comes to mind right off the bat, engineers are the largest group of undergraduate majors, It has become the preferred major for more and more businessmen, as the technical aspects of running more and more complicated modern businesses require much higher mathematics than the layperson. The problem solving skills are also readily transferable.

        For example an MBA who is also an engineer might recognize the use of a freeze seal as a typical method of containing a pressurized system without releasing pressure for maintenance, in fact an MBA who is an engineer might have seen one applied in numerous circumstances. But this is what comes from experience in living a life less ordinary. I am sure you find it fascinating, as would a young man just starting his engineering career, but after a decade or so, it is a complex but routine maintenance issue.

        Have you even considered that the reason you cannot achieve much is that you can’t reach 100% because you’re two slow, two slow?

        Like

  3. Tubeway Army – Gary Numan : Are Friends Electric ?

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  4. Pingback: My Article Read (8-2-2015) | My Daily Musing

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