A high tech wire act

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A high tech wire act

A high tech wire act

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The A127/B128 West is a 115 kilovolt, 67-mile, “double-circuited” (two lines attached to a single set of structures) overhead line that runs from the Harriman hydroelectric station in Vermont to central Massachusetts.  The lines, which were originally constructed in the 1920s, are connected to six substations and at peak times serve about 150 megawatts of customer load.

In 2007, we determined that the existing wire on the lines needed to be replaced for several reasons:  they were aging, their capacity needed to be increased to keep pace with growing demand, and clearance to ground needed to be increased to ensure public and employee safety.

With so many issues to address in one project, Transmission Line Engineering decided to analyze different kinds of  conductor (the wire on the transmission line that carries the electricity) to determine the best option to resolve all of these issues, as cost-effectively as possible.

The team, led by Jess Farrell, lead engineer, reviewed the following characteristics of several types of conductor:

-physical capabilities such as strength, sag resistance, ability to withstand extreme weather
-number of structures that would have to be replaced because of the new conductor
-cost to install – including materials, structures, labor, the conductor itself
-environmental/community impacts such as construction duration and visual impact
-long-term costs such as required maintenance and duration of conductor lifespan

Ultimately, the team selected a relatively new type of conductor manufactured by 3M, called Aluminum Conductor Composite Reinforced, or ACCR for short.

ACCR conductor is one of many conductors that are categorized as “high-temperature, low-sag” (HTLS) conductors. HTLS conductors are generally utilized when more power is needed on an existing line and the size of a traditional conductor to replace the existing would create the need for a number of structure replacements or conductor clearance-to-ground issues. HTLS conductors are also used in long spans where the weight of the conductor can significantly increase the cost of the structures supporting it.

ACCR differs from typical conductor in that it has an aluminum core instead of a steel one.  It’s also different than other aluminum-core conductors because its outer covering, the one that conducts electricity, is made from a super-hardened, very strong, but lightweight aluminum alloy as opposed to heavier materials such as steel or nickel alloy. According to 3M, ACCR has the same strength as similar size steel-core conductors, but is much lighter and sags less. It also retains its performance over decades of high temperature use, and is stable in a wide range of environmental conditions.

According to Jess, it’s more expensive than regular conductor, “But in this particular installation it should perform as well as other types of conductor and it significantly reduced the number of existing structures we would have had to replace.  This made the project much more constructible, reduced the potential amount of environmental impacts, and was the lowest-cost option.  In other words, the company and our customers will save money over the long term without sacrificing reliability.”

The first commercial installation of ACCR was done in 2001, with the first major installation in 2005. In the US, utilities such as Xcel Energy, Arizona Public Service, Allegheny Power, Silicon Valley Power and Alabama Power have installed this conductor in both experimental and project-specific applications. We first used ACCR on the National Grid system in New York on the 115 kilovolt Walck Road – Huntley line along with another competing conductor technology, Aluminum Conductor Composite Core (ACCC).

Including the A127/B128 West reconductoring project, a total of approximately 1,000 miles of ACCR have been installed in the US. To date, the A127/B128 West reconductoring project is the longest installation of ACCR conductor in the world.

“We’re always looking for opportunities to incorporate innovative materials and technologies into our system,” Jess said.  “We were faced with multiple challenges to overcome on this project, so the upfront investment in research on new conductor options paid off for National Grid and for our customers.”

Transmission Line Engineering will continue to look for projects where ACCR and other HTLS conductors could prove beneficial along with exploring other ways to expand our innovation horizons.

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