Repairing Underground Power Cables Is Nearly Impossible

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. I don’t remember exactly how old I was when I  came across this story, but I do know that it was   one of the very first times that I realized how  awesome infrastructure and engineering could be.   I figure if it had such a big impact on  me, that it’s a story worth retelling,   especially because there’s a recent update at  the end. Maybe it will inspire others to be more   interested and engaged in their constructed  environments like it did for me (which is   basically my entire goal with these videos).  I’m Grady, and this is Practical Engineering.   In today’s episode, we’re discussing the  Scattergood-Olympic Underground Transmission Line. How do you get electricity from where  it’s generated to where it’s used?   That’s the job of high voltage transmission  lines. Electrical power is related to the product   of the voltage and current in a transmission line.  If you increase the voltage of the electricity,   you need less current to deliver the same  amount of power, so that's exactly what we   do. Transformers at power plants boost the voltage  before sending electricity on its way (usually in   three separate lines, called phases), reducing  the current, and thus minimizing energy wasted   from the resistance of conductors. High voltages  make electrical transmission more efficient, but   they create a new set of challenges. High voltage  electricity is not only extremely dangerous, but   it also tends to arc through the air (which is not  a great insulator) to the other phases or grounded   objects. The conventional solution is to string  these lines overhead on towers. This keeps them   high enough to avoid contact with trees and human  activities, but the towers serve a second purpose.   They keep enough distance between each line so  that electrical arcs can’t form between them. Unfortunately, stringing high voltage lines  overhead isn’t always feasible or popular   with the local residents, especially in dense  urban areas. That was true in the 1970s when   engineers in LA were deciding how to expand  their transmission system and deliver power   from the Scattergood power plant to the  Olympic substation near Santa Monica. So,   they tried something that was relatively  new and innovative for the time:   they ran the line underground. Three  230 kilovolt lines, one for each phase,   would deliver enormous amounts of electricity over  the approximately 10 mile or 16 kilometer distance   in West Los Angeles, powering hundreds  of thousands of homes and businesses.   However, putting high voltage lines below the  ground created a whole new set of challenges. When strung across towers, the conductors  at this voltage each require around 10 feet,   or 3 meters of clearance to avoid arcs. The  air is the insulator doing the job of keeping   electricity constrained within the conductors. So  how do you take those three high voltage phases   and cram them into a single, small pipe running  underground? Well, you need a better insulator   than just air. One of the more popular options of  the time was to use high pressure, fluid filled   cables. This design starts with installation  of a steel pipe below the ground with access   vaults spaced along the way. Copper conductors are  surrounded with many layers of paper insulation.   Next, a protective layer of wire called  skid wire is spiralled around each one   to protect the paper from damage and allow for  easy sliding along the pipe during installation.   The conductors are pulled through  the steel pipe using massive winches   and then spliced together at each vault.  Once the steel pipe is fully welded closed,   it’s slowly filled with a non-conductive  oil known as liquid dielectric. This oil impregnates the paper insulation around  each conductor to create a highly insulative layer   that prevents arcs from forming, even with the  conductors sitting mere inches apart from one   another and the surrounding steel pipe. At the  same time, the oil works as a heat sink to carry   away heat generated from losses in the conductors.  It is critical that the oil completely saturates   the paper insulation and fills every nook and  cranny within the pipe. Just like a hole in the   plastic insulation around an extension cord, even  a tiny bubble in the oil can create a place for   arcs to form because of the extreme voltages. So,  the oil inside the pipe is pressurized (usually   around 14 times normal atmospheric pressure or  over 200 PSI) to ensure that no bubbles can form. The rating of a transmission line (in other words,  how much power it can deliver) is almost entirely   based on temperature. All conductors (with rare  exceptions) have some resistance to the flow of   electric current, and that creates heat which will  eventually damage the conductors and insulation   if it builds up. The more heat you can remove,  the more power you can push through the line.   That’s a major benefit of pipe-type oil-filled  cables: they’re surrounded by a gigantic liquid   heat sink that can be circulated to keep the  temperature down and prevent hot spots from   forming in the lines. At each end of the  transmission line is a plant filled with   pumps and tanks to pressurize - and often to  circulate - the dielectric oil in the pipe. This particular transmission line in LA  circulated the oil in six-hour cycles.   At the end of each cycle, the pumps reverse  to move the fluid in the opposite direction   through the pipe. Some systems are  different, but for the Scattergood line,   this pumping is a slow process. You’re not trying  to pump all the fluid from one end of the line   to the other, but rather simply get it to move a  short distance along the line to average out the   temperatures and minimize the possibility  of any single section from overheating.   However, even at that slow speed, you can’t  just switch the flow direction in an instant. I have a video all about a phenomenon called  fluid hammer, and you can check that out if   you want to learn more after this, but I’ll  summarize here. Moving fluid has momentum,   and rapidly changing its velocity can create  dangerous spikes in pressure. Water hammer can   be a problem in residential homes when taps  or valves within washing machines close too   quickly. You might hear a pipe knocking against  the wall, or in worse cases, you might completely   rupture a line. However, in large pipelines  that can contain enormous volumes of fluid,   reversing a pump can be the equivalent of  slamming a freight train into a brick wall.   To avoid spikes in pressure which could damage  equipment or rupture the pipe, the pumps at either   end of the Scattergood-Olympic line would spend  the last hour in the six-hour cycle slowing the   oil down, providing a smooth transition to flow  in the opposite direction for the next cycle. Circulating the dielectric oil helps to keep  the temperature within the pipe consistent   along the line, but can’t control how that average  temperature changes over time. Transmission lines   don’t deliver a constant current. Rather the  current depends on the instantaneous electricity   demand which changes on a minute by minute  basis depending on the devices and equipment   being turned on or off. When demands fluctuate,  the current in a transmission line changes,   and so the amount of heat in the line increases  or decreases accordingly. As you might know,   many materials expand or contract with changes  in temperature, and that’s true for the copper   conductors used in underground transmission lines.  When these lines expand within the outer pipe,   they often move and flex in a process  called thermal mechanical bending or TMB.   If not carefully designed, these bends can become  tighter than the minimum bending radius of the   cable, exceeding the allowable stresses within the  material. Over hundreds or thousands of cycles of   TMB, the paper insulation around each conductor  can begin to soften or tear, eventually leading   to a dielectric breakdown (in other words,  arcs and short circuits). TMD can also pull   larger diameter splices into narrower sections  of the pipe, causing them to rub and abrade. That’s what happened in 1989 to the  Scattergood-Olympic line. But before   the LA Department of Water and Power could  repair the fault, first they had to find it.   Locating a fault in an underground line is  half-art/half-science, and there are many   interesting types of equipment that can be used.  They tried to use ground-penetrating radar along   the line, but they couldn’t identify the fault.  They also tried time-domain reflectometry - a   method of transmitting a waveform through the  cable and measuring the reflections - but the   results weren’t conclusive. They also used  a device called a thumper which introduces   impulses of high voltage into the cable. When  this impulse reaches the fault, it causes an   electrical arc which can be heard as a thump above  the ground, usually aided by a handheld detector   with a microphone and digital filters. Going from  one extreme in technology to the opposite, the   crews used car batteries and voltmeters to take  measurements of the conductor’s resistance between   tap points to precisely identify the location  of the fault within Mr. McMahon’s neighborhood. Once found, the challenge of repairing  the faulted cable could begin.   How do you fix an insulated conductor inside  a steel pipe bathed in high-pressure oil? With   liquid nitrogen, of course. Pumping all the oil  out of the pipe before the repair wasn’t feasible.   It couldn’t be stored and reused after the  project because that process would introduce   contaminants that would reduce the oil’s  insulative properties. They also couldn’t   dispose of it and replace it with new oil,  because the stuff’s expensive and it would take   a long time to get in such an incredible quantity,  potentially extending the very expensive downtime.   Even more importantly, relieving the  oil pressure from the rest of the pipe   could allow gas bubbles to form inside the layers  of paper insulation, potentially damaging them   and creating new places for faults to form. The  clever solution they used was to freeze the oil   using liquid nitrogen, which is usually around  -200C or -320F, creating solid plugs on either   end of the section to be repaired. This allowed  the rest of the pipe to remain under pressure. Losing these plugs would be a catastrophe,   creating an eruption of high-pressure  oil and spilling huge quantities of it   into the environment, so the repair crew  had liquid nitrogen companies on call across   California as contingency to ensure that the oil  could be kept frozen for the duration of the fix. Unfortunately, after taking x-rays  along the entire length of the line,   they realized that many of the cables’ splices  were in danger of experiencing a similar fault   due to thermomechanical bending. After coming  to the conclusion that this wasn’t going to   be a quick fix, the Department of Water and  Power decided to drain the entire line of oil   and implement preventative measures  while it was already down for repairs.   Aluminum collars were installed at key locations  along the pipe to constrain the thermal movement   of the cable. This was done in a semi-clean  environment with air handling and cleanliness   requirements to prevent contaminants from finding  their way into the pipe. After many months,   and tens of millions of dollars worth of  downtime, the trucks and crews finally pulled   out of Tom’s neighborhood, and the underground  transmission line was finally brought back online. There’s an update to Tom’s story to bring  us to modern times. The Scattergood-Olympic   line’s troubles didn’t end with the work in 1989.  LA’s routine testing showed that the insulation   was continuing to degrade, and outages on  the line were significantly disrupting the   reliability of their transmission network  across the city. In 2008, the Department of   Water and Power began developing a replacement  project, this time using newer cable insulated   with polyethylene instead of high pressure oil.  After 10 years of planning, environmental permits,   public meetings, design, and construction,  the project was completed in 2018.   The original transmission line is still in place  and can be used as a backup if it's ever needed. As a part of my research for this story, I spoke  to Tom on the phone. He told me that shortly after   his writeup spread across the early internet, it  was sent to a teletype machine within one of the   offices of the LA Department of Water and Power,  providing some higher-up within the organization   a neatly printed version that may or may not  still be hanging on a wall somewhere downtown.   Huge thanks to Tom for taking the time all  those years ago to share his enthusiasm   for large-scale infrastructure, thanks to Jamie  Zawinski for preserving the story on his blog,   and thank you for watching.  Let me know what you think.

Channel: Practical Engineering

Views: 2,505,154

Keywords: Electric Transmission Line, Tom McMahon, Los Angeles, 1989, underground transmission line, voltage, electrical arcs, Copper conductor, paper insulation, skid wire, liquid dielectric, heat sink, pressurize, fluid hammer, thermal mechanical bending, TMB, fault, ground-penetrating radar, time-domain reflectometry, thumper, liquid nitrogen, polyethylene, LA Department of Water and Power, Jamie Zawinski, practical engineering, engineer, Grady, civil engineering, Scattergood-Olympic

Id: z-wQnWUhX5Y

Length: 13min 34sec (814 seconds)

Published: Tue Sep 21 2021

Reddit Comments

[u/5UMBUDDY]

How awesome engineering is, and how the infrastructure affects everyone, these are outstanding themes. And everyone depends on electricity and clean water therefore depends on engineers.

[u/dinosaurs_quietly]

I can’t believe I watched the whole video on the issues of oil insulated conductors just for him to say “oh and by the way that technology isn’t used anymore”.

[u/ElectricMan324]

An important topic. I'm in the industry and have many conversations with people about "why dont we just put all power lines underground?". This video explains a lot of the issues.

[u/Honest_Cynic]

Interesting story, and reminds us how little was on the internet in the early days, thus all the eyeballs on this story. Remember the fear of high-voltage power lines causing brain cancer several decades ago, based on a Swedish study? I think the conclusion was that it was a non-causal, i.e. a "casual", correlation. It caused concerned mothers to avoid buying a house near overhead lines. I wonder if any went so far as the investigate such hidden underground power lines.

Overhead lines must have the ceramic insulators periodically washed of dust, especially in California where dust builds up during the 7 months of no rain. Techs spray them from a distance with de-ionized water. If they used regular water, they could get a fatal arc.

I read that many transmission lines were buried in southern Arizona, as an attempt to avoid the lightning of their monsoon season. Studies found that they were hit just as often as overhead lines. The lightning found them underground, leaving a 1/4" D path of "glass" where it arc'ed thru the sand. Of course it was harder to find and repair the damage. Those were normal plastic-insulated wires, not oil covered.

[u/taintedblu]

Really does seem like an enormous pain in the ass. Fantastic video, super informative!

[u/GoofAckYoorsElf]

One does not simply repair an underground power line

Boromir (probably)