Underground Electric Transmission Installations Gaining Traction

By Jeff Griffin, Senior Editor | June 2010 Vol. 65 No. 6
Two jack and bore underground duct installations for Progress Energy in St Petersburg, Fla. Photo: Brierley Associates

Quanta Services has several companies with core competency in underground transmission engineering and construction, says John Wilson, president of the electric power division of Quanta Services, Houston. Quanta companies currently have several projects containing underground transmission lines under construction. One of those is EHV Power, Gormley, Ontario, Canada.

Trend to underground
EHV President Steve Burks, says the trend to place more power transmission lines underground is generally positive.

“There are many drivers,” Burks believes, “including improved cost factors and less challenging technology that can accommodate higher voltages and is easier to install. The current economy has imposed a temporary lull, but the envelope for opportunity is better than in the past for placing transmission lines underground. Most continue to be short, averaging about two miles with the longest about 20 miles.”

Obviously there are significant differences between constructing aerial and underground transmission infrastructure. Because it requires insulation, underground cable has much larger diameter than uninsulated aerial cable and is, therefore, more difficult to transport and handle.

“Size of dielectric cable is a function of load,” says Burks. “A 345 kV cable will be five to six inches in diameter. A reel containing one conductor typically will be 12 to 14 feet in diameter and weigh 40 tons. That’s one reel of cable containing one conductor, and a project will require three of them. Splices will be made every 1,200 to 1,800 feet, and they must be perfect. Obviously a lot of space is needed to handle and allow room for splices.”

Duct banks in which cable is placed must be insulated to dissipate heat.

“The duct banks must be of a thermal grade concrete to provide a thermal envelope to accommodate the load of the cable that will be placed in it,” says Burks. “Heat is the enemy of underground cable, and if the operating temperature becomes too hot, the insulation will break down.”

Placement of duct banks and construction of manholes and vaults requires extensive excavation in areas where traffic and normal activities would be disrupted.

“Excavation for ductwork will be at least three-to-four feet wide and four-feet deep,” Burks explains. “If the cable route must go deeper, the system has to be designed for the deepest part; the conductor must be designed to accommodate the deepest portion of the route, because heat dissipation lessens with depth.”

Often cable routes are in easement already containing other utilities which must somehow be avoided or relocated.