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 138,000-volt 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-kV circuits in the Straits, approximately four miles in length. The new circuits will be comprised of two, three-phase 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:

This animation depicts the process and schedule for the cable removal and replacement:

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.

Click and scroll on the linked StoryMap below to learn more about this project:

Project certified in MISO’s 2018 Transmission Expansion Plan:Winter 2018
Apply for permits from U.S. Army Corps of Engineers / Michigan's Dept. of Environment, Great Lakes & Energy:Winter 2019-20
Remove portions of submarine cables 5 & 6:August - September 2020
Anticipated approvals from ACOE/EGLE:April 2021
Remove existing submarine cables 1-6:May - July 2021
Install and energize first new submarine cable:May - July 2021
Install and energize second new submarine cable:August - September 2021
In service:October 2021
Project complete:Spring 2022

Which agency authorized ATC to move ahead with this project?

Following the 2018 incident that significantly damaged our submarine cables, ATC worked with the Midcontinent Independent System Operator 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 along with other regulatory agencies to proceed with plans and permitting for this repair project.

How was ATC able to maintain an operable electric circuit following the 2018 cable damage?

Typically three cables comprise one circuit. Prior to April 2018, ATC operated a double circuit, 138-kV volt line, (six energized high-voltage cables) that ran across the lakebed of the Straits of Mackinac. Three of the cables were destroyed by an anchor strike in April 2018Within weeks of the incident, the undamaged cables were reconfigured to create a single circuit. Visit the event response page to learn more. 

What is the process for inspecting and removing the cables?

In August and September 2020, two de-energized and unburied submarine cables approximately three miles in length will be removed, over the course of approximately 30 days. The cables and protective concrete matting near the shorelines will first be inspected underwater by a Remote Operating Vehicle (ROV). Underwater divers will then inspect and mark the cables and protective concrete matting with buoys.

A crew from ATC’s contractor Durocher Marine will then hoist the concrete matting atop the cables onto the barge. Underwater divers will cut and cap the cables and hoist the cut sections into an environmentally protective bin aboard the barge.

What are the next steps for this project?

While the unburied sections of two cables are removed in August and September of 2020, the buried sections of cables near the north and south Michigan peninsula shorelines will occur in spring 2021. Following that, a new, three-phase 138-kV cable will be installed and energized, providing a temporary double circuit with the existing, operating circuit. In summer 2021the remaining cables will be removed in a similar manner as in 2020. Following that removal, the second and final three-phase cable will be installed and energized.

How will ATC protect the new cables from a potential future impact?   

Protecting the environment and shielding the cables from potential impact are a significant priority for ATC. MISO’s Board of Directors approved the project in its Midwest Transmission Expansion Plan in 2018 with the condition of ensuring these criteria. In partnership with our contractor, LS Cable Systems America, Inc.ATC’s project team plans to deploy an underwater sled that will create a trench for the new cables. The trenched cables would be protected within the lakebed from marine equipment. In some locations across the waterway, protective rock barrier may also be installed atop the cables. 

What is ATC doing to protect the waters with the new cables that will be installed?

Protecting the environment is paramount. Engineering advancements have evolved since the original cables were installed, 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. 

ATC applied for environmental permits to complete the cable removals from the U.S. Army Corps of Engineers and the Michigan Dept. of Environment, Great Lakes & Energy in early 2020. We received permits from both agencies for this work in April 2020.We anticipate receiving permission in fall 2020 to install the new cables in 2021.

How will ATC monitor the functionality of the new cables?

The two, new three-phase cables will incorporate fiber optic strands which will be installed within each of the high-voltage cables to allow for acoustic sensing and temperature monitoring. 

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.