Overhead vs. Underground Power: Why Do We Use Both Locations?
- Feb 7, 2022 11:12 pm GMT
Much of the local grid infrastructure in the United States relies on overhead and underground power lines and distribution circuits. Neither location has become the gold standard. Both have benefits and drawbacks that make a combination of the two more practical than one or the other.
These benefits are why American grid infrastructure typically uses both locations for power cables, overhead and underground.
Installation of Overhead and Underground Lines
The primary advantage of overhead lines is cost. Burying a cable is more expensive than constructing it aboveground in almost all situations. The average underground line will cost anywhere between one to two times as much as an overhead line.
According to one 2011 paper from the Public Service Commission of Wisconsin, “a typical new 69 kV overhead single-circuit transmission line costs approximately $285,000 per mile as opposed to $1.5 million per mile for a new 69 kV underground line (without the terminals).”
The price differential for a 138 kV line is even more pronounced, with the overhead line costing around $390,000 per mile and the underground line costing around $2 million per mile.
The cost differences between an underground and overhead line can vary wildly due to various factors — like local infrastructure, soil quality and the accessibility of the site.
If the soil is easy to dig and the team will face few obstacles — like phone or water lines — the overall cost of installing the underground cables may be lower than average. However, the more difficult it is to safely and quickly dig in the area, the more expensive installing underground lines will become.
Overhead lines can also be constructed much faster. Shorter construction time comes with various advantages. Labor costs will likely be lower, and the process will be less disruptive to nearby communities.
Faster construction also significantly reduces the amount of runoff produced by the work, as dirt will not be as disturbed and kicked up by heavy machinery and workers on the job site.
The higher cost of underground circuits can make them difficult to justify in lightly populated areas. Cables in a rural community may serve just a few customers, meaning that the cost per customer can become extremely high.
In addition, overhead and underground lines come with different but significant safety risks.
Heights and the operation of aerial lifts may threaten workers installing overhead lines. As with most types of heavy equipment, training and the right PPE will make people who will be in, on or around the aerial lift much safer.
Underground lines require heavy digging equipment. Obstacles like water and telephone lines can also create new safety risks if struck. As with aerial lifts, training and PPE can help keep workers safe during the installation process.
All workers installing power lines will face a few of the same risks — like the potential for electrocution. Best practices that mitigate common construction hazards are necessary to keep these workers safe.
The Aesthetic of Overhead Lines
Often, underground lines are the only option for areas where aesthetics are a major concern, despite the difficulties of digging and burying cables.
Overhead circuits can be fairly ugly, especially if installed without careful attention to design and organization. They can increase the number of cables, junctions and poles in an area, making the system more cluttered and less visually appealing.
Construction companies can employ strategies to make overhead lines more visually appealing. Minimizing the number of utility poles and using aesthetically pleasing materials, like fiberglass or steel, can make for a prettier overhead system than one that relies on a large number of wood poles.
Underground lines are often preferred in urban areas, even though digging there is often challenging and disruptive. Nature resorts, national parks and wildlife refuges that require power may also use underground lines to improve local aesthetics and minimize the disruption to area wildlife.
The Operation, Maintenance and Repair of Power Lines
Overhead lines are typically cheaper to maintain and operate, though they are more vulnerable to extreme weather and wind than underground cables.
On average, overhead lines also typically last several decades longer than underground versions before needing to be replaced. However, the life span estimates of both may vary.
Fault-finding and repair for overhead lines are typically faster, resulting in shorter power disruptions and improved uptime. The same is not true for underground cables, which often require a more involved and less speedy fault-finding and repair process.
If the problem’s location is not immediately obvious, the repair crew must employ various fault-finding techniques to determine where the line has failed. Some strategies, like cable thumping, can add more strain and potentially damage insulation and shorten life span.
Overhead lines can also withstand overloads more readily than underground lines due to their thermal capacity. Cables that are buried rather than exposed to air will radiate heat less quickly, making them more susceptible to overheating during an overload.
The thermal conductivity of local soil can have impact the thermal capacity of an underground line, potentially making it more or less vulnerable to overloads. Certain types of substrate may have extremely undesirable thermal qualities. Cables may fail much faster than the average underground line.
However, underground cables are much closer to their return path, so they will also have a lower reactance. This means less voltage drop, allowing for longer, uninterrupted lengths of cable.
Overhead vs. Underground Power Lines: The Good and the Bad
Overhead lines are typically cheaper to install and maintain, but they aren’t always the best option. Underground cables are out of the way and more aesthetically pleasing than their counterparts. They may also be the only option in certain areas with no space for aboveground versions.
Workers installing both types of lines will face significant safety risks during construction. Training, PPE and good working practices will help mitigate these hazards and ensure a smooth process.
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