Pre-failure vented water tree (left) and bow-tie water tree (right)

Post-failure vented water tree

Failures - water trees
Insulation of medium voltage underground residential distribution (URD) power cables age from a phenomenon called water treeing. Vented water trees are those that are connected to the conductor shield or the insulation shield. Because the shield is source of water and ions, vented trees generally grow larger than their bow-tie tree cousins. As the water trees grow longer the dielectric strength of the insulation degrades. At some point the voltage in the conductor of the cable can no longer be contained and an electrical tree is initiated. Once the electrical tree starts from routine operations it will likely propagate quickly until a hole is blown across the width of the insulation and the lights go out. While there are other causes of cable failure, water trees are the root cause of the majority of underground distribution cable reliability issues. The vented and bow-tie water trees pictured nearby both come from desert geographies. At cable depth there is almost always water – everywhere. To learn about the ubiquity of water and how it permeates through cables check out a reprint of an IEEE Electrical Insulation Magazine article titled, " The Molecular Thermodynamics of Water in Direct-buried Power Cables".

Choices – rehabilitation
When a buried electric cable fails a circuit owner must choose between one of the 3•Rs:

• Repair the fault with a splice and return the cable to service.
• Replace the cable that failed with a new cable.
• Rejuvenate the cable after repairing the fault and return the cable to service.

The last 2•R’s are the rehabilitation choices. The first R, Repair, is not a sustainable option, because re-failure rates are very high. Repairing a single foot of a 328 ft (100 m) cable segment does not address the other 99.7% of the cable which was installed at the same time, experienced the same operational history, and was subjected to the additional indignity of transient over-voltages, which accompany every ground fault. Replacement is at least twice as capital intensive as rejuvenation, so the optimum tactic is to treat those cables which can be treated and replace those few that cannot be treated. When a group of cables (perhaps sharing the same vintage) reach a certain age the failure rate increases year-after-year and a proactive strategy is required to maintain a targeted reliability.

Strategies – Distribution Hierarchy of Needs
A framework for the prioritization of underground distribution reliability budget is provided by the nearby chart which was inspired by Maslow’s Hierarchy of human needs. A complete discussion of the subject is available as a reprint of the paper "Underground Distribution Reliability: The 5•Ps."

In short, the cables at the bottom of the pyramid should command the first available resources. Only when the reliability issues of cables at a given level are satisfied should incremental funding be expended on the level immediately above. Ironically, the top-most triangle has been mistakenly targeted by many circuit owners for rejuvenation funding ahead of the more critical levels below. This unintentional misallocation of resources has resulted in strategic spending decisions, which have not provided the highest reliability with the greatest capital efficiency.