For decades, the cable that makes up portions of the nation’s electrical distribution systems has routinely been placed underground, out of sight where it is more secure and at less risk of damage by weather or accidents that can knock down aerial lines and disrupt service. Power lines have been buried in most new residential developments since the 1970s, and many buildings in office parks, educational facilities and other institutions are served by underground power and communications distribution lines.
On the other hand, most high-voltage transmission lines remain overhead, traveling cross-country suspended from steel towers. Technical issues, primarily related to heat generated by high-voltage lines, the significantly higher cost of burying cable and transmitting comparable amounts of energy, are primary factors that have limited the amount of underground transmission cable.
However, that is changing, and a growing number of projects contain segments of underground transmission lines.
Today, there is a significant amount of 23-kilovolt (kV) underground transmission cable in the United States and a growing number of transmission systems that include underground 345-kV cable. Infrastructures in Europe and Asia include segments of underground transmission cable up to 500 kV.
A primary reason is the development of cable using plastic for insulation, which has higher thermal tolerances than paper, oil-impregnated paper and fluid-filled pipes for insulating electric cable. Utility providers, project planners, engineers, developers and communities are more open to considering underground segments of projects because of this newer cable that can effectively and dependably carry high voltages underground, a narrowing gap between underground costs compared to overhead construction, and a growing list of successful underground transmission installations.
Many factors influence consideration of underground transmission lines, said Brian Dorwart, Brierley Associates, Bedford, N.H. As an engineer, Dorwart has experience designing trenchless projects that include underground transmission segments, and he currently is designing shore landings for cable runs crossing the Hudson River between New Jersey and New York and directional drilling river crossings for power transmission installations in Jacksonville, Fla., and Malden, Mass.
“The security and reliability of underground infrastructure, along with regulatory issues and public demand, are factors,” Dorwart said. “Cost ultimately may be the deciding factor, so it is important to note that the gap between underground and aerial construction costs has narrowed. The common rule of thumb once was that underground was 20 times more expensive than aerial. Today, the difference generally used is that underground costs 10 times more.”
Aside from cost, it is generally accepted that underground cable is more secure than overhead cable.
“Acts of nature can cause outages, and these days, [there’s] the possibility ... a terrorist strike can cause grid failure,” Dorwart said. “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 said. Regulators are providing feedback from the public to power companies regarding overhead issues, such as electromagnetic impact as a cause of cancer as well as improved aesthetics of landscapes without overhead cables.
Ground conditions and available space are considerations when determining whether cable will be overhead or underground. Acquisition of new rights-of-way already are a problem in many areas.
“With alternative energy production, such as wind farms and solar, the production is typically a long way from the consumer and from a grid-connection point,” Dorwart said. “Therefore, these energy sources often need very expensive infrastructure investments to reach connection nodes to the grid. As these projects usually are ‘green,’ they may have significant political and regulatory pressure to place transmission lines underground. In such instances, regulators can have a significant impact on a project by imposing onerous restrictions that can price a tight project out of underground and back to overhead.”
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 (PE) plastic was first used to insulate power cables up to 5 kV in 1942, said Sara Susach, international account manager, energy division Southwire Co., Atlanta, a U.S. company that manufactures cross-linked polyethylene (XLPE) cable.
High-voltage cable and components for underground applications are produced to rigid standards to ensure even distribution of voltage stresses. Conductor shields, insulation and insulation shields must be absolutely clean with smooth interfaces and tightly bonded.
A summary of information provided by Southwire, a U.S. manufacturer of crosslinked polyethylene cable, describes the process in its ISO-certified facilities:
Super-clean compounds are processed in a Class 1000 clean room. Compounds are applied over the conductor with a true triple extruder head, the cleanest, state-of-the-art method of making cable. Automated scanners monitor thickness and concentricity for precise dimensional control.
A dry nitrogen atmosphere in the curing zone protects cable from moisture, and computer-controlled time temperature profiles produce optimum electrical and mechanical properties.
Finished cable is tested well beyond specified voltage range and partial discharge tested to guarantee no cable with insulation defects leaves the factory. Certified test reports document testing.
In addition to providing cable and connection components for underground power applications, Southwire provides feasibility studies and engineering; underground system design; cable pulling, splicing, and termination services; as well as commissioning and testing. In addition, the cable manufacturer provides turnkey services, including civil work, construction management, fault locating and maintenance.
“By 1955, PE was used in cable up to 35 kV,” Susach said. “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.
Today XLPE cable is the dominating cable for high-voltage (HV) and extra-high-voltage (EHV) underground power cable.
To date, more than 4 million feet of Southwire HV XLPE cable has been installed in underground transmission systems in the United States, Susach said. For underground infrastructure, XLPE cable is nonobtrusive; immune to weather; highly reliable with high energy efficiency and high emergency rating; and has low electromagnetic field (EMF), inductance and impedance.
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 multipoint bonding options.
“The underground cable industry has seen the amount of high-voltage underground transmission lines increase as cable technology has advanced and reliability improved,” said John Rector, Black & Veatch associate vice president and project manager. Black & Veatch works in engineering, construction management and constructing power infrastructure containing underground transmission lines. The global engineering, consulting and construction company is based in Overland Park, Kan.
“In the late 1980s, we were installing 115-kV underground transmission lines,” Rector said. “In the early ’90s, Black & Veatch installed the first commercial 230-kV underground transmission project in Orlando, Fla. In the years that followed, the number of underground installations grew, and the reliability of these systems became evident, more power companies and planners became comfortable with the idea of placing 230-kV transmission lines underground.”
More 230-kV cable was installed; then came a short segment of 345-kV cable.
Additional short 345-kV projects followed, and then came the 26-mile Middletown-Norwalk project in Connecticut, one of the cable industry’s landmark projects demonstrating the viability of ever advancing high-voltage technology.
“Every time we increase the voltage level of these cable systems, we prove the reliability of this technology,” Rector said. “Europe and Asia for some time have been installing 400-kV and 500-kV systems. The U.S. utilities are a relatively conservative group, which, I think, is a good thing. High-voltage cable technology is moving rapidly toward 500 kV in this country. Very soon we will see 500-kV underground transmission lines in the United States, probably in California or the East Coast.”
The need for added capacity in congested areas where overhead lines either are not permitted or are difficult to permit will spur more growth in underground transmission, Rector said. And a strong indicator that underground transmission will continue to grow, he said, is that a new plant for manufacturing high-voltage dielectric cable opened in the United States in late 2009, making two such facilities within the United States to serve North America.
The skill sets for installing underground transmission are quite specialized and demanding. The higher the voltage, the larger diameter of the cable, complicating logistics of transporting and handling of cable supplies. High voltages are unforgiving, and there is little margin for error during installation.
Some electrical contractors are tackling these projects. The Quanta Services organization includes electrical contractors with core competencies in underground transmission engineering and construction, said John Wilson, president of the electric power division of Quanta Services, Houston. Wilson said Quanta companies currently have several projects containing underground transmission lines under construction.
The trend for placing more power transmission lines underground is generally positive, said Steve Burks, president of EHV Power, Gormley, Ontario, a Quanta Services company specializing in underground power transmission.
“There are many drivers,” Burks said, “including improved cost factors and less challenging technology that can accommodate higher voltages and is easier to install. The economy has imposed a temporary low, 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.”
Many factors set construction of underground transmission lines apart from aerial infrastructure. With its insulation, underground 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,” Burks said. “A 345-kV cable will be 5 to 6 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 required 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,” Burks said. “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 3- to- 4-feet wide and 4-feet deep,” Burks said. “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 that already contains other utilities, which must somehow be avoided or relocated.
Manholes or vaults must be constructed along the cable route. Each section of cable is pulled into and out of vaults where connections are made and future maintenance will be performed.
“Civil construction costs—excavation, duct banks, restoration—-often are 50 percent or more of project costs,” Burks said, adding that, “from a contractor’s perspective, most of the risk of a project is on the civil side.”
Burks said underground transmission work is highly specialized, and only a few contractors are qualified for the work.
“However, the landscape of contractors is changing,” he said. “Cable improvements are reducing the degree of specialization required.”
GRIFFIN, a construction and tools writer from Oklahoma City, can be reached at email@example.com.