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Reliable But Costly
Recent trends in distribution line undergrounding.
Following a large storm causing a lengthy power outage, one can bet that media coverage will turn to at some point to underground lines. Future outages will be averted, presumably, without overhead lines exposed to falling tree limbs. And children will play safer absent the danger of contact with live wires, exposed above ground.
But just as there are pros and cons to overhead distribution lines, the same can be said for undergrounding, starting with cost.
"Building overhead lines is very cost-effective," said Frank Alonso, P.E., manager, transmission line engineering, for Leidos Engineering (Orlando, Fla.), who also has experience with distribution lines.
"The material is less expensive, the insulation requirements are significantly less, and the maintenance is relatively inexpensive compared to underground," he said. Another benefit is that, when an outage occurs, it can almost always be located quickly. "Overhead lines are also very easy to tap into," he said.
Yet overhead lines can prove vulnerable even in good weather.
According to R. John Miner, president of Collaboration Unlimited (Austin, Tex.), a utility consulting firm, overhead lines are subject to intermittent outages that result from shorts, such as those caused by tree branches or squirrels. "In these instances," he notes, "the power may only be off for a few seconds, but it can be a problem for certain commercial and industrial customers with critical expensive equipment that automatically shuts down even if power is off for only a second or two."
Undergrounding indeed can bring benefits. Three of the most important are reliability, safety and aesthetics.
A five-year study of underground and overhead reliability for North Carolina's IOUs (Duke Energy, Progress Energy and Dominion) found that the frequency of outages on underground systems was 50 percent less than for overhead systems.
Underground lines provide better public safety, since there are no exposed downed lines or poles falling on homes or vehicles or within easy access of children or unsuspecting adults. "Everything that is energized and has a shock hazard is not only underground, but also insulated and enclosed, preventing human touch," said Collaboration Unlimited's Miner.
Also, an underground system is more aesthetically pleasing. One tangible benefit of this is that property values tend to be higher in neighborhoods with underground lines than in comparable (age, location, housing structure, etc.) neighborhoods with overhead lines.
Costs and Benefits
Various studies have found that the cost to build new underground distribution lines is five to ten times more expensive than new overhead construction, depending on where they are built. A 2009 Edison Electric Institute (EEI) study found that the cost of burying existing overhead distribution lines ranged from $80,000 a mile in rural areas to $2.1 million a mile in urban areas.
Are customers willing to fund such costs? The 2013 edition of an annual study published by the EEI, titled "Out of Sight, Out of Mind: An Updated Study on the Undergrounding of Overhead Power Lines," found that