Coming of Age: Rejuvenating Underground Cables

When rehabilitating aging URD infrastructure, many utility providers forego cable replacement and opt for rejuvenation

Medium-voltage underground cable is designed to be used and not seen. Padmount electrical transformer boxes containing and connected by underground residential distribution (URD) cable are a ubiquitous sight throughout residential neighborhoods, spaced on average 330 feet apart. Consumers who live in the community are generally unaware of the jumble of cables that each box comprises, and they rarely need to consider whether the URD itself is in an adequate state of repair. Most utility providers, on the other hand, are in a constant state of responding to aging cable and the threats it represents.

Over time, utility companies face significant challenges for addressing deteriorating URD conditions. URD cables are most commonly degraded when moisture diffuses into the cable's dielectric layer, gradually diminishing the cable's insulative properties. This condition, called water treeing because of the tree-shaped structure observed when the degraded cable is viewed microscopically, is the most common contributor to URD reliability issues. When the insulation on the cable connecting two transformers degrades to a point of failure, the lights go out in the entire neighborhood.

Aging URD cables are a growing problem in communities around the world, disrupting customers and causing business challenges for utility providers. But in most cases, the traditional remedy for URD cable failure—taking the impacted cable out of service and putting new cable in its place—has proven to be unfeasible. When cables fail, the resulting outages and the replacement work required to restore power create logistical problems that are usually unpredictable and expensive—costs that must be absorbed by the provider, the customer, or both. Meanwhile, customers often experience multiple outages as the providers install new cable, often disrupting the customers' property and landscaping in the process.

In comes rejuvenation. Instead of replacing the cables, utilities fix the cable with this process. Following are images of the rejuvenation process along with some details about the technology. Also, look for Mountain View Electric Assocation's use of cable rejuvenation to be featured in the October print issue of T&D World.

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