Underground transmission lines

ATC’s practice of placing transmission lines on overhead structures is rooted in our responsibility to consumers to build projects as cost efficiently as possible. While many lower voltage, local electric distribution lines are placed underground, particularly in newer neighborhoods, almost all high-voltage electric transmission lines are proposed as overhead for three general reasons: cost, repair time and environmental considerations.

The Public Service Commission of Wisconsin 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?

Installation costs for underground transmission lines can range from 2.5-10 times the cost of an equivalent overhead line. As a regulated utility, ATC is required to explore low-cost options when proposing new transmission lines since costs associated with new and existing transmission lines are passed on to retail electric customers in their monthly electric bill. Utility construction methods and associated costs are solely under PSC 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 does ATC rarely place electric transmission lines underground?

Transmission lines are rarely constructed underground, largely due to the additional time involved for repairs and higher installation and repair costs. For this reason, the state PSC 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 may be offset by 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.

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 2.5-10 times those of an overhead line. Factors influencing the cost usually are site-specific and include the following:

Routing

Right-of-way, easement and permitting costs and whether the line will be placed in the road right-of-way

Terrain and obstacles

Other underground utilities, streams and railroad crossings, embankments, bridges, major roads, traffic and soil conditions

Permitting

Traffic and lane restrictions, noise, time of day and other construction restrictions

Design

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 conduit. The three conduits 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 conduit 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 that they can carry. Underground lines cannot dissipate heat as well as overhead lines. Factors such as the type of surrounding soil, adjacent underground utilities and the depth of installation all affect the wire’s ability to dissipate heat.

New underground lines can have higher thermal ratings than outdated overhead lines they are replacing; however, ATC has far less flexibility to make improvements as needed on underground lines. When lines are above ground, ATC can replace the wire or make other improvements to its carrying capacity without significant disruption. This means that ATC can respond to unforeseen circumstances, such as a change in the electric demand forecast or a change in power flow on the network, much more easily on overhead lines.

Does that mean transmission lines cannot be placed underground?

Transmission lines are rarely constructed underground, largely due to the additional time involved for repairs and higher installation and repair costs. The PSC is responsible for determining 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.

Because of these issues, underground lines tend to make sense only where there is no viable overhead corridor, such as in densely populated urban areas or in the vicinity of airports. Approximately 1 percent of ATC’s 9,400-mile transmission network is located underground, all of which are 138-kV or lower voltage.

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 PSC 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.