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

“Acts of nature can cause outages, and these days the possibility of a terrorist strike can cause grid failure,” says Dorwart. “Aerial lines are exposed to hurricanes, ice and snow, wind and other natural disasters and overhead cables can be easily accessed for sabotage. Underground infrastructure is far less vulnerable to such risks and, thus, may be considered more reliable. For infrastructure serving military bases and other institutions of national importance, underground lines are more secure.”

There also are instances in which going underground simply is perceived as the “right” thing to do, Dorwart adds. Regulators are providing feedback from the public to power companies regarding overhead issues such as electromagnetic impact as a cause of cancer and improved aesthetics of landscapes without overhead cables.

Regardless of why a segment of transmission cable is being installed underground, a primary reason that option is available is development and evolution of high-voltage power cable using plastic for insulation.

Polyethylene plastic was first used to insulate power cables up to 5 kV in 1942, says Sara Susach, international account manager, energy division Southwire Company, Atlanta, a U.S. company that manufactures XLPE cable.

“By 1955, PE was used in cable up to 35 kV,” Susach continued. “In the 1950s, GE successfully crosslinked PE which helped its use as cable insulation gain widespread acceptance. In the 1980s, European and Japanese companies made great strides in development of XLPE with installations of 230 kV and even short 275 and 500 kV installations in Japan. Utilities liked XLPE cable for its absence of dielectric fluid and the inherent simplicity of the extruded cable systems, and the 1990s saw increasing numbers of 138 and 230 kV installations.

Diagram of buried conduit installation. (Illustration courtesy of Southwire)

“Today XLPE cable is the dominating cable for HV and EHV underground power cable.”

Susach says to date, more than 4 million feet of Southwire HV XLPE cable has been installed in underground transmission systems in the U.S., and several other cable manufacturers also provide XLPE cable for U.S. projects.

Steady progress
Underground cable typically is installed in duct banks with splices made inside manholes. Terminations are made at substations or riser poles. There are single-point, cross-bonded and multi-point bonding options.