Unitil keeps lightning strikes at bay; 40 miles of new static wire; redirects bolts from above.
Lightning supposedly never strikes twice in the same place, but when it comes to static wire, that adage goes out the window.
Static wire runs above high-voltage electrical transmission lines and redirects static electricity buildup and lightning strikes away from circuits to prevent damage to utility equipment and prevent outages.
Think of it as a horizontal lightning rod, says Thomas J. Murphy, manager of environmental compliance and business continuity for Unitil Corp.
The utility company started a three-year, $1 million project this week that will replace 40 miles of static wire running through Fitchburg, Lunenburg and Townsend.
After the ice storm in December, 2008, an outside agency evaluated the aluminum static wire for Unitil, he said, and found areas of the static line were worn from age and failing. The lifespan of static wire is 40 to 50 years and Unitil last replaced it in the 1960s, he said.
"There are 20 million cloud-to-ground lightning strikes in the U.S. every year," he said. "To the New England region there are, on average, two to five cloud-to-ground strikes per square kilometer per year. If you extrapolate that over the course of the 40-mile line, that is in the low hundreds for the number of hits it takes in a given year."
The static wire only runs above 69,000-volt lines or higher - not distribution lines - and protects the conductors underneath and redirects the lightning to the ground, he said. But, lightning strikes are not the only concern.
"When we experience electrical storms - the kind when your hair stands on end-it is from electromagnetic fields electrifying the atmosphere," he said. "The leading trailers that are seen in photos of lightning cause a collection of static electricity."
During a storm, the cloud is negatively charged and the ground is positively charged, he said. When the charges are big enough there is a potential for a connection to be made.
"Eventually they will connect and at that point there is an actual lightning bolt and massive discharge," he said. "But, with the movement of a storm across an area there is a higher potential for trailers creeping up more than lightning strikes. The static wire prevents the conductors from taking a hit and taking out transformers and bigger equipment along the lines and surges into costumers' homes and equipment."
James M. McGrath, manager of transmission line engineering at National Grid, said his company's approach to static wire maintenance is different than Unitil's because National Grid owns over 2,500 miles of line throughout New England.
"We have a much larger system," he said. "We have an inspection and maintenance program we operate on a regular basis. We use foot patrols where we walk the lines every five years and conduct aerial inspections every six months. We fly the entire system every six months to see if anything warrants further assessment and a comprehensive inspection."
He said inspections are done using helicopters with high resolution cameras and conductors are taken down for sampling and testing. "Based on that we make a decision to replace the wire," he said. "A lot of factors can influence the lifespan of the wire. The static wire itself can last 50 years. For the best value for our ratepayers we may let it go beyond that if it is not ready for replacement. There may be an area with conditions that lets the wire last longer."
Mr. Murphy said Unitil crews have started work on the project on will finish this year in October in Townsend. Work won't resume again until July 2012, he said, because it is too difficult to navigate the lines in winter and most vernal pools are dried up by July, he said.
Also to minimize the impact on the environment, swamp mats are placed over wetlands where work is done. Additionally, crews attach temporary supports mounted to the poles to run the wire through. Mr. Murphy said once a significant length of the old wire has been transferred to those supports, the new wire is attached to one end of the old wire and pulled through them to the other end, preventing crews from having to drag the wire across sensitive environmental areas. It is then attached to the poles.
Crews will finish static wire replacement in Lunenburg and two-thirds of the project in Townsend this year and about 15 percent in Fitchburg, prioritizing replacement of the most worn wire first, said Mr. Murphy.
Next year crews will finish replacement in Fitchburg, he said, and then the rest of Townsend in 2013.
"We went through and surveyed the system in 2009 and prioritized the areas that are worn out more and had shown signs of failure during past storms," he said. "We are watching weather forecasts for storm events so in case lightning hits, we can have the static wire backup on the line and running. It's a balancing act between safety for the crews and reliability. Our intent is for there to be no impact to customers' power flow, if all goes well."
Mr. Murphy said residents should stay away from areas where crews are working since they are working on high-voltage lines.
Ongoing updates on the project area available at www.unitil.com.
CUTLINE: Static Wire
PHOTOG: T&G Staff/STACEY ARSENAULT
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|Publication:||Telegram & Gazette (Worcester, MA)|
|Date:||Jul 16, 2011|
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