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March 4, 1999

Power outages are result of economic trade-offs, UW researcher says

Most of the wind-induced power outages that left 250,000 Puget Sound-area homes in the dark Wednesday morning could have been avoided. But someone – most likely electrical utility customers – would have to pay for it.

Up to 90 percent of power outages result from problems in local power lines, and a quarter of those are caused by trees. Installing power lines underground rather than stringing them overhead between utility poles could solve that problem. But it costs ten times more and inevitably would be paid for by utility customers, says Richard Christie, a UW professor in the Department of Electrical Engineering who has spent the past decade researching cost-effective ways to improve the reliability of electrical power distribution.

“In most cases, the public utility commissions or city councils making the decisions determine that the added reliability of underground power distribution isn’t worth the expense,” says Christie. “And until a storm comes along and knocks out power all over the place, most ratepayers are inclined to agree.”

Similar economic trade-offs are made in staffing levels for maintenance and repair crews. The single largest factor in determining how long it takes for power to be restored in a particular area is the number of crews in the field, Christie says. When a limb or tree falls on a power line, it causes a short that triggers a circuit breaker to shut down power to all customers on that local distribution feeder. Repair crews are dispatched to find and fix the cause of the short and to operate local switches to restore power to customers. But there aren’t enough crews to go around during storms, which, according to Christie, are responsible for about 25 percent of all power outages.

Utilities could hire more maintenance and repair personnel to reduce the amount of time people are without power during storms, but the crews would be under-utilized the rest of the time. Remote controlled circuit breaker switches also could be used to more quickly restore power, Christie says, but again they cost far more than manually operated switches.

Instead, utilities have hit upon a decidedly low-tech and relatively inexpensive solution: tree-trimming. Utilities spend more than a billion dollars a year nationwide on tree-trimming campaigns aimed at reducing the number of power lines downed or damaged during storms. Even this involves some element of cost-benefit analysis, however. Since 1992, Christie has been working with Snohomish Public Utility District to determine how much to invest in tree trimming before the return, in terms of reduced tree-related power outages, begins to diminish.

“This is nothing more than people with chain saws and bucket trucks out there cutting down branches, but it can add up to millions of dollars,” says Christie. “Utilities don’t want to spend too little on trimming and get hit with excessive repair costs from tree-related power outages, but they don’t want to spend too much and have little to show for it. My understanding is that our research already has paid for itself many times over in terms of operational savings.”

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For more information, contact Christie at (206) 543-9689 or christie@ee.washington.edu.