When it comes to both overhead and underground distribution and transmission lines, all may not be as it seems. That is, while there has been a distinct trend toward the installation of more underground distribution lines, there are a number of factors that dictate the continued use of (and even new installation of) overhead lines.
Conversely, while conventional wisdom has seemed to suggest that overhead transmission lines will always be the norm (except in certain heavily congested urban areas where underground is a necessity), recent developments in underground transmission line equipment are adding significantly to the appeal of line burial.
Distribution lines come down
Following a large storm that results in power outages from downed power lines, one can bet that at least one media outlet will suggest that the local utility should invest in underground distribution lines. That way, the lines will not be susceptible to falling trees, limbs or telephone poles, and it may prevent future outages. For good measure, such stories usually add that underground lines are safer because the potential for people to contact live wires above ground is eliminated.
These stories offer some valid points. There are a number of potential drawbacks to overhead distribution lines. The major one is that they are susceptible to outages from falling trees and limbs, especially during storms. In addition, every once in awhile, errant drivers will plow into poles.
Furthermore, overhead lines are subject to intermittent outages that result from shorts, typically caused by tree branches or animals. In these instances, the power may only be off for a few seconds, but it wreaks havoc for commercial and industrial customers with critical and expensive equipment that automatically shuts down if power is off for any length of time.
Overhead lines pose potential safety problems. Downed lines are a deadly hazard. Utilities also occasionally have to deal with some negative public opinion related to the unappealing aesthetics of overhead lines.
According to Mark Rectenwald, industrial and utility projects manager for electrical contractor Hatzel & Buehler, Pittsburgh, overhead lines require nearby tree trimming, which is not only costly for the utilities but also often problematic for property owners.
“Tall trees that require trimming are always a sore spot for property owners,” he said.
The positives of undergrounding complement the negatives of overhead lines. Increased reliability is one of the most significant supporting factors for undergrounding. Outage frequency on underground systems is less than it is for overhead systems because of the vulnerability of overhead lines and poles.
Public safety is a second benefit of underground lines. Because the lines are underground, they are isolated from damage due to falling debris or an accident.
Third, an underground system is more aesthetically pleasing. Property values tend to be higher in neighborhoods with underground lines versus the lines overhead.
Fourth, while undergrounding is more expensive than overhead, it is cost-effective in certain instances.
“Almost all new subdivisions are underground,” said Randy Williams, president of Rocky Mountain Contractors Inc., Helena, Mont. “Without any buildings in the way, it is relatively inexpensive to install underground, and landowners prefer it to the unsightly overhead lines. Also, long rural lines are many times cheaper to plow in cable instead of building overhead lines.”
Rectenwald discussed the soundness of underground wiring.
“It is more practical to install underground services at the onset of building in an area, as opposed to going into an area and trenching through landscaped lawns, trees, shrubbery, driveways and sidewalks,” he said.
While Rocky Mountain Contractors finds underground to be less expensive in many rural areas, this isn’t the case everywhere.
“Overhead is still the choice for rural applications,” said Jacob Grice, P.E., project manager—overhead line, New River Electrical Corp., Westerville, Ohio, and Cloverdale, Va. “We do a lot of work in Ohio for electric cooperatives, and the majority of that is still overhead because you are moving along long distances with low customer density.”
Overhead lines have their benefits. Primarily, they are very cost-effective to build. The material is less expensive, the insulation requirements are significantly less, and the maintenance is relatively cheaper compared to underground. When outages occur, they can almost always be located quickly. Finally, overhead lines are very easy to tap into.
In addition, there have been some improvements in overhead reliability in recent years that are also encouraging utilities to keep overhead lines in their strategic arsenals into the future. As a result of directives from the Federal Energy Regulatory Commission (FERC) and the North American Electricity Reliability Corp. (NERC), more utilities are focusing efforts on “hardening” their overhead lines, such as reinforcing poles, becoming more aggressive with tree trimming, and using smart grid technology to more quickly identify and address overhead outages. These initiatives are indeed costly but not as costly as undergrounding everything that is currently overhead.
The high costs of underground
As previously noted, underground lines are generally more expensive to install than overhead lines. Various studies have found that the cost to build new underground lines is three to 10 times more expensive than new overhead construction, depending on where they are built. Of course, costs are greatest in urban areas, where contractors must battle significant amounts of infrastructure, including pavement and other utility lines and equipment.
“One of the major concerns is all of the other underground utilities along the entire length of the excavation,” Rectenwald said.
Costs are also high in existing suburban areas because of property restoration, such as grass, sidewalks, driveways and roads.
Another downside to underground distribution is the potential susceptibility to water damage and related insulation deterioration. If they are not designed properly, underground facilities can be vulnerable to water damage, especially from flooding. In addition, while underground cable, equipment, connectors and so forth are designed to be submersible, water vapor can cause problems. While the design may account for this, water vapor can still penetrate and cause damage if combined with certain defects in the cable, some of which may be microscopic. Chemical changes can occur in the insulation material, which can lead to cable failure over time.
Yet another concern with undergrounding relates to the duration of outages. Underground outages take longer to repair because contractors need more time to locate the faults. In addition, once underground outages are located, a significant amount of time is required for trenching, cable splicing and re-embedment.
Even though undergrounding offers a number of benefits, they are not enough to cause a wholesale rush among utilities to bury all of the lines.
Just as underground distribution lines are more expensive than overhead distribution lines, underground transmission lines are more expensive than overhead transmission, and usually the cost difference is significant. A 2011 paper published by the Public Service Commission of Wisconsin reported that a typical new 69-kilovolt (kV) overhead single-circuit transmission line costs approximately $285,000 per mile, while a new 69-kV underground line costs approximately $1.5 million.
Of course, underground transmission line costs are significantly greater in urban areas because of the existing infrastructure.
“The most significant challenges to underground transmission are excavation through city streets, generally 8 to 10 feet deep, and encountering obstructions,” Rectenwald said.
While a growing shortage of skilled labor is a problem in all areas of utility work, it is particularly pronounced when it comes to installing underground transmission lines, according to David Lindsay, senior project manager for New River Electrical Corp., Cloverdale, Va.
“The U.S. labor market has a limited supply of skilled trade labor for the installation of cable and accessories,” he said. “Proper installation of 230+kV joints and terminations is a highly skilled job requiring intensive training. As application of the technology continues to grow, labor constraints may become critical for large projects.”
Still, while underground transmission costs are greater in urban areas than in rural, it is the urban sites where underground transmission is often a necessity. In rural areas, underground transmission may be considered a luxury.
“Underground transmission is a rarity as compared to overhead in total circuit miles in the U.S.,” Lindsay said. “However, the amount of HV/EHV [high-voltage/extra-high-voltage] cable being installed has continued to increase over the past five to 10 years. Growth in the use of cable is coming from revitalized and growing urban centers, as well as increased use in suburban and extra-urban areas.”
“There will always be a need for underground transmission around large cities to route transmission power between substations in close proximity to the cities.”
Another trend encouraging the growth of underground transmission is the improving quality of the cables, an issue that was once considered very problematic. Furthermore, availability of this higher quality cable is increasing, and costs are decreasing.
“The U.S. has seen two new greenfield manufacturing plants open dedicated to HV/EHV extruded [XLPE] cables, and a third manufacturer announced a significant capacity expansion,” Lindsay said. “This is a very large investment in manufacturing capacity to support a market viewed by many to be on the verge of long-term growth.”
So, while the mainstream media seems to continue to emphasize the growing popularity of underground distribution lines, distribution lines will run overhead many years to come. However, while significant growth in underground transmission lines seemed like a pipe dream years ago, such a trend has recently become much more of a possibility, and even a reality.