How to Keep the Lights On: Comparing Above-and-Below Ground Power Lines
The old adage says that Thomas Edison did not fail a thousand times when creating the lightbulb - he instead discovered a thousand ways how not to create a lightbulb. The issue we now face as a country is how to keep the lights on. During the most recent ASCE Report Card, the U.S. energy system scored a D+, stating that “…most electric transmission and distribution lines were constructed in the 1950s and 1960s with a 50-year life expectancy”.
According to the same report card, America needs to spend $177 billion over the next eight years in order to bring our energy system up to the level it should be. For electrical transmission and distribution lines there are typically only two ways to spend that money: underground or overhead.
Underground Transmission and Distribution Systems
Underground systems are an interconnected web of conduits and manholes that transport electric power cables from substation to substation and from substations to customers. Typically underground systems are located in more urban areas and can be several miles in length.
One of the main benefits of using an underground system is that underground power lines are not subject to the same weather impacts, such as wind and ice, as an overhead system. This generally results in a lower number of outages due to the underground system. The Out of Sight, Out of Mind 2012 study looked at three different indices to measure reliability of overhead and underground systems and determined that “…the underground electrical system contributes a smaller percentage to the overall outage numbers experienced by customers.”
That study’s title touches on another reason residents and customers tend to prefer underground systems— manhole covers are less of an eyesore than fifty-foot tall poles. They’re also compact, housing more power cables with a smaller footprint. A typical underground duct bank spaces two or three inches between conduits, as opposed to wire-to-wire spacing of four to seven feet for an overhead line.
However, underground systems are not without their faults. As mentioned in the ASCE Report Card, we need to spend $177 billion in upgrades to our power systems. The fastest way to spend that money is by building more underground systems. In a paper published by the Public Service Commission of Wisconsin it was estimated that the “…cost for constructing underground transmission lines ranges from 4 to 14 times more expensive than overhead lines of the same voltage and distance.” In order to invest more money into their systems, power companies need to charge their consumers more.
As with any service, power companies generally like to keep their consumers happy. This isn’t always easily accomplished when installing or rehabbing underground systems. Typically installed within public right-of-ways or roadways, underground systems generally take longer to install than overhead ones and cause a greater disruption to traffic patterns. Because going underground is not a new idea (sewers and water lines have been underground for nearly a hundred years), underground power systems are the late comers and now need to find space within an already cramped corridor.
Overhead Transmission and Distribution Systems
Overhead systems are what generally come to mind when talking about electrical power: the power poles that span for miles along highways and country roads. Typically installed in open areas, overhead electrical systems can run for several hundreds of miles and for this reason they are generally used for high voltage cross-country power lines.
Overhead transmission and distribution systems are more budget friendly than underground systems. Utility companies are not only saving money on the materials—poles are relatively inexpensive when compared to an underground vault—but also on labor. Overhead transmission and distribution systems are installed quicker than underground systems. If you think about the construction of an overhead line, the only excavation needed is for a pole every couple of hundred feet, compared to excavating a trench for several hundred feet for an underground line.
Overhead systems are easily tapped and expanded upon for many of the same reasons. In order to expand upon an underground network you would need to excavate the existing system and install new duct. With overhead systems, extensions can be accomplished by simply adding more poles.
As with underground systems, overhead systems do have their drawbacks. The main limitation of an overhead system is weather. Wind, ice, and even heat can all play a role in the reliability and overall performance of an overhead system. Utility poles are also subject to damage caused by external factors like car accidents and animals. Underground systems resolve many of these common risks, but are still susceptible to weather events like flooding.
Overhead lines can also be subject to more regulations. The Federal Aviation Administration (FAA) imposes height regulations for any structures and wires near airports, railroad companies and federal, state and local municipalities may also regulate the minimum height for wires crossing or running parallel to road ways, railroad tracks and navigable water ways. While underground lines are regulated with regards to clearances to other utilities, these clearances are generally less strict than those for overhead lines. There may also be regulations with regards to proximity to residential areas.
The court of public opinion can be the downfall for any project, and overhead lines are no different. The general public views overhead transmission poles as unsightly as opposed to the monuments to modern engineering they are. Many local and state agencies will require that all new electric lines be placed underground within more urban areas.
There is no silver bullet, or one size fits all, method to deciding which system, whether overhead or underground, to use. Puerto Rico’s power system was a primarily overhead system and was devastated by Hurricane Maria. Most of the island is still without power and there is no definitive answer as to whether or not an entirely underground system would have been able to continue serving power during the hurricane. On the flip side of the same coin, both Houston and Miami have primarily underground power systems that also experienced massive outages due to flooding during hurricanes Harvey and Irma. Over 2 million customers lost power across Texas, Ohio, and the northeast because of hurricane Harvey. Hurricane Irma left nearly 8.2 million customers without power across the southeast. While power has been restored to nearly all customers affected by these two storms, more reliable energy systems may have reduced the length and number of outages. Power companies need to decide on case by case basis which system will work best for their area. According to ASCE, there is a $177 billion dollar deficit that needs to be spent in order to get America’s energy system to the level it should be. The next question is how to spend it, and that is no light decision.
About the Author:
Tony Sanganetti is a civil engineer at LaBella Associates. He has 3 years of experience in transmission and distribution engineering. A primary focus of his experience is on underground 34.5kV transmission systems working in urban environments. Connect with him on Linked In.
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