American Transmission Co. projects

Project website for a Wisconsin-based company that owns and operates the high-voltage electric transmission system that powers communities in portions of the Upper Midwest.

Straits Cable Replacement Project

Project overview

Michigan’s Upper Peninsula and lower Michigan have been connected electrically for decades by two underwater 138-kilovolt transmission lines, comprised of six cables. These connections are critical to electric reliability in the eastern U.P. and northern lower Michigan. ATC will remove the six underwater cables and re-establish two new 138,000-volt circuits in the Straits, approximately four miles in length. The new circuits will be comprised of two, three-conductor submarine cables containing solid dielectric insulation.

Project need

In April 2018, ATC’s transmission lines were damaged when an anchor severed two of the cables and severely damaged a third. The three undamaged cables were reconfigured to form a single transmission line, allowing ATC to restore an electrical connection between the U.P. and lower Michigan. More information about the incident can be found at:

With only one operating circuit currently in service across the Straits, there are risks to electric system reliability and maintenance in the region when other transmission lines are out of service – either planned or unplanned. Two new underwater transmission lines are needed to maintain adequate electric reliability and operating flexibility. Permits will be required from the United States Army Corps of Engineers, Michigan’s Department of Environment, Great Lakes and Energy, and other local entities.

The cost of the project is estimated to be $105 million.

Certified in MISO’s 2018 Transmission Expansion Plan:Winter 2018
Apply for ACOE/EGLE permits:Winter 2019-20
Construction start:Spring 2021
In service:Winter 2021

Which agency authorized ATC to move ahead with this project?

ATC worked with the Midcontinent Independent System Operator following the 2018 incident to determine an appropriate solution for maintaining electric reliability in the Upper Peninsula. In its December 2018 Transmission Expansion Planning Report, MISO recommended the replacement of the two electric circuits in the Straits as soon as practical to ensure the continued reliable service to the Upper Peninsula. ATC is now implementing a recovery plan that was authorized by MISO and we continue to work with the Michigan governor’s office to proceed with plans and permitting for this repair project.

Why isn’t ATC proceeding with plans to locate the cables in a tunnel with other utilities?

While a tunnel may be the right solution for other utilities, it is not the right solution for ATC due to multiple concerns including safety, timing, practicality and economic issues. We have a responsibility to maintain electric reliability in Michigan and received authorization from MISO in December 2018 to do so, which is why we are proceeding now with this project.

How will ATC protect the cables from being damaged in the future?

ATC is in the process of determining the appropriate protective measures for the cables and is gathering input from the Michigan Department of Environment, Great Lakes and Energy, Michigan Dept. of Natural Resources, Michigan Public Service Commission, the U.S. Army Corps of Engineers, Midcontinent Independent System Operator, U.S. Coast Guard, tribal nations and others.

Will the new cables contain dielectric fluid?

Engineering advancements have evolved since the current cables were installed in 1974 and 1990, and the new cables will use cross-linked polyethylene, a type of plastic, for insulation. They will not contain dielectric fluid and will not leak if damaged.

How many soil borings will occur and what does ATC hope to learn from them?

Up to seven marine soil borings and five land-based borings will occur along the northern and southern shorelines of the Straits, and up to 31 marine vibratory corings will occur beneath the lake bed surface. ATC will use the soil boring information to determine the final design and sizing of the cables and the cable burial depth.

What environmental impact will the soil borings have on the lake bed and shorelines?

Due to the nature of the soil borings and vibratory corings, any impacts to the sediment will be minimal. We expect no impacts to the fisheries from this geotechnical exploration.

Who will pay for the project?

Like all of ATC’s reliability-driven projects, the project cost will be distributed amongst the utilities in ATC’s service area over the life of the infrastructure. Around 10 percent of a utility customer’s bill in ATC’s service area pays for transmission service, which includes transmission projects like this.