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Tuesday, October 30, 2012

During outages, issue of buried power lines surfaces

EDITOR’S NOTE: This story was written after the 2011 October snowstorm, but the issue of buried power lines has arisen again following Hurricane Sandy.

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With New Hampshire facing its third multiday, widespread power outage in less than three years, this question arises, as it always does in such situations: Why don’t we just bury power lines, the way natural gas lines are buried, so they can’t be hit by trees?

The answer, according to 11 studies done in eight states and summarized in a 2010 report by Edison Electrical Institute, is that burying distribution lines (the ones on your street) or transmission lines (the big ones on towers) is far more expensive than it seems, but is not as reliable as might be expected.

The report did indicate that burying lines is certainly possible. The nation has roughly 6 million miles of distribution lines, and 18 percent are underground.

(Fewer than 1 percent of the big transmission lines are buried, and most are mid-sized 138-kilovolt systems or smaller. Cost and time, and the need for backup systems, are the main obstacle.)

Buried distribution lines usually exist in crowded cities and in new developments, where it is much cheaper to bury lines during construction, as compared to doing it after the fact. They are rare in rural areas or established subdivisions.

In storms, the report says, underground systems are vastly superior, a fact which has been the “catalyst of every state’s underground study.” Widespread blackouts lead to increased interest in the topic in Florida, Texas, Oklahoma, Virginia, North Carolina and Maryland, among others.

They also produce some day-to-day cost savings, such as reducing the need for expensive, and controversial, tree-trimming.

But underground lines have vulnerabilities that overhead lines don’t, notably to water. Flooding or underground seepage, can raise havoc in buried pipes. Tree roots and shifting ground are also issues, as are inadvertent breaks caused by digging (although burying removes the issue of vehicle accidents taking down poles).

All in all, the study said “underground electric infrastructure has only a slightly better reliability performance than overhead electric systems.”

A key word is “infrastructure,” since electrical systems need transformers, fuses and other devices to control and transit power. Putting the systems underground is more difficult than it seems and makes for a less flexible layout, increasing the chance that something will go wrong in normal operation.

Finding and repairing problems in buried lines can be an order of magnitude more costly, and take much more time than finding and repairing problems in overhead lines. That’s also a problem in daily operation, “as visual inspection is impossible, making it more difficult and costly to maintain and repair.”

The really big problem, though, is the cost of converting systems.

Even in Florida, which is flat with sandy soil, utilities estimate it would cost an average of $150,000 per mile to replace overhead infrastructure with buried ones. In Virginia and North Carolina, where the Appalachian Mountains are prominent, costs approached a whopping $3 million a mile in spots.

In most states, including New Hampshire, these costs would have to be covered by ratepayers, adding several thousand dollars a year to bills.

By contrast, PSNH said the 2008 ice storm added $60 million in unexpected costs, which translated to less than $6 a month in extra cost for residential customers.

David Brooks can be reached at 594-6531 or dbrooks@nashua telegraph.com.