It's common today to see lower-voltage distribution lines that connect to homes and businesses buried directly in the ground using less invasive construction methods. However, high-voltage transmission lines require greater infrastructure. Placing transmission lines underground is a practice generally used only when there is no viable overhead corridor. Underground line present significant challenges due to the environmental disturbances, additional repair time and the much higher installation and repair costs. The Public Service Commission determines when it is appropriate to put transmission lines underground. In such a case, specific construction measures are necessary for safe and reliable operation of the line.
Commonly asked questions and answers
Will ATC agree to bury the transmission line if the municipalities pass an ordinance?
No. As a public utility, we are responsible for both least-cost planning and system reliability. Burying transmission lines at a cost 4 - 10 times higher than overhead are costs that are passed on to all ratepayers in our service area. Utility construction methods and associated costs are solely under Public Service Commission jursidiction. The rate impacts of underground construction extend beyond the project area. If ATC places transmission lines underground based on a local ordinance requirement, citizens outside of the project area would also be responsible for the cost, but have no representation in the decision. This is one of many reasons why major utility projects are subject to state, rather than local, approval. In submitting proposals to the PSC, we are mindful of the impact of our projects on electricity bills for all consumers we serve.
Why doesn't ATC place electric transmission lines underground?
Underground lines are considerably more costly and environmentally invasive than overhead lines. For this reason, the state Public Service Commission rarely supports or approves underground construction of transmission lines. It is ATC’s responsibility to consider many factors, including cost and environmental impacts, when proposing new electric transmission lines. The most affordable industry standard is overhead power lines.
Isn't it better to place lines underground?
It is important to note there are pros and cons to building lines overhead and underground. While there are aesthetic benefits of placing transmission lines underground, those benefits are offset by the drawbacks. In addition to cost, the key difference between underground and overhead lines is that it typically takes more time to locate, diagnose a problem and repair an underground transmission line. The difference in repair time is best characterized in hours or days rather than weeks or months. In heavily congested urban areas (where underground tunnels usually exist for placing public facilities), underground lines are sometimes the only option to navigate around tall buildings when no viable overhead corridor exists.
Is it more expensive to put transmission lines underground?
Yes. Transmission lines supply electric power to large areas serving large numbers of people, and they are much more expensive to build. The extra cost of placing these lines underground is quite significant. An equivalent underground transmission line can cost several times more than the cost of an overhead transmission line.
Why does it cost so much more?
Transmission lines can provide enough electricity to power whole cities and are much more technically complex and material intensive. The design, installation and maintenance costs are all higher for underground lines. Installation costs for underground transmission lines can be several times those of an overhead line. Factors influencing the cost usually are site-specific and include the following:
- Route considerations, including right-of-way, easement and permitting costs, and whether the line will be placed in a road right-of-way.
- Terrain and obstacle considerations, including other underground utilities, streams and railroad crossings, embankments, bridges, major streets and highways, traffic, and soil conditions such as wetlands, bedrock and hazardous materials.
- Permitting considerations, including traffic and lane restrictions, noise, time of day and other construction considerations.
- Design considerations, such as mitigating soil thermal characteristics.
What are the challenges in putting the lines underground?
The placement of transmission lines underground requires specific engineering construction measures to ensure the safe and reliable operation of the line. Because a single transmission line circuit requires three wires, each much be installed in an individual pipe. The three pipes are encapsulated in thermal concrete and surrounded by special thermal backfill materials. These facilities require significant trenching of at least five feet in depth and width. Because the repair of failed underground lines can be costly, environmentally disruptive and time-intensive, underground construction design includes the installation of a spare pipe that can be used to replace a damaged cable or pipe without reopening the entire trench. The underground design also must accommodate a dedicated fiber optic cable for operation of line protection and control devices, which protect the system during faults and other anomalies.
What are the operational challenges of underground transmission lines?
All electric lines produce heat and therefore have a limit on the amount of power they can carry to prevent overheating. Underground lines cannot dissipate heat as well as overhead lines. Factors such as the type or surrounding soil conditions, adjacent underground utilities and the depth of installation all affect the wire’s ability to dissipate heat. Lower thermal ratings for underground transmission lines mean they do not have as much flexibility as overhead lines to carry heavy volumes of power on hot summer days. Once lines are constructed underground, we have little or no flexibility to upgrade the facilities to respond to changes on the system. Overhead lines offer us the ability to replace wires and make other changes to allow the facilities to carry more electricity if necessary.
Does that mean transmission lines cannot be placed underground?
Engineering complexities, cost differentials, and environmental costs are generally significant. For this reason, placing transmission line underground is an option only when a suitable corridor cannot be identified such as in city/urban areas or near airports.
For higher voltage transmission lines (i.e., 345-kilovolt), undergrounding is not an option because at that voltage the lines cannot dissipate heat and become capacity bottlenecks on the system, defeating the purpose of building them. Less than 1 percent of ATC’s 9,100-mile network of transmission lines is underground.
Why can't ATC pay the cost difference?
As a regulated utility, ATC is required to explore least-cost options when proposing new transmission lines. Because the costs associated with new and existing transmission lines are passed along to retail electric customers in their monthly electric bill, the Public Service Commission of Wisconsin authorizes the construction of underground construction only when there are no other viable overhead corridors.
How does ATC address concerns related to transmission line aesthetics?
As homeowners and residents ourselves, who live in communities served by ATC, we share the desire for an aesthetically pleasing community. During the routing and siting process for new transmission lines, we will look first at possible routes that follow existing utility corridors, roadways, and railroads. We then look at other linear features such as recreation trails, property lines and fence lines. We also look for opportunities to place smaller distribution lines on the same poles to reduce the clutter of multiple lines in an area.
Once the route is selected, we work with community and landowners on the placement of individual poles and, to the extent possible, work around lines of sight that may affect the appearance of an area. Further, we ask communities to help us by sharing their concerns, asking questions, and providing input on the siting, installation and pole design for our facilities. We know that our facilities have impacts, but our goal is to site our lines in the least objectionable and intrusive way possible.
|
