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Electric Transmission Submarine Cables Damaged in Straits of Mackinac

The cables tripped offline about 30 seconds apart Sunday evening, April 1

American Transmission Co. last week took the unprecedented step to shut down two submarine cables in the Straits of Mackinac that electrically connect the Upper Peninsula to lower Michigan as the result of yet-undetermined damage.

The cables tripped offline about 30 seconds apart Sunday evening, April 1. A patrol of the overhead elements of the system between Point Lebarbe in St. Ignace and the McGulpin Riser Station in Mackinac City showed no damage. The submarine cables, which contain a mineral-based fluid for insulation, were monitored overnight and subsequently determined to be leaking. Pressure on the system was reduced to minimize the fluid leak as maintenance, environmental and operations personnel worked to locate the compromised section of the cables on Monday, April 2. Investigations included aerial patrols over the Straits, cable testing and system reconfiguration options.

Extreme weather conditions, including icing in the channel and on shore, hindered the damage investigation and contributed to ATC’s decision to shut down the cables this morning, April 3. As a result, the two cables cannot be repaired and have been rendered permanently inoperable. ATC will be determining the condition of other cables in the Straits.

“It was an extraordinary set of circumstances, but ultimately, the decision to shut down the cables had to be made,” said Mark Davis, ATC chief operating officer. “We will continue to investigate the cause of the incident, determine any necessary remediation efforts and continue communicating with the appropriate regulatory agencies.”

ATC has notified the National Oceanic and Atmospheric Administration, Environmental Protection Agency, U.S. Coast Guard, U.S. Fish & Wildlife Service, Michigan Department of Environmental Quality, Michigan Department of Natural Resources, and the Michigan Public Service Commission of its decision to shut down the electrical cables.

ATC owns and operates most of the electric transmission grid in the Upper Peninsula. The system continues to operate normally at this time. ATC is coordinating with the Midcontinent Independent System Operator and Midwest Reliability Organization to determine short-term and long-term solutions.

American Transmission Co. has created a website for information about the damage to two electric power lines under the Straits of Mackinac last week. The website provides an overview of the situation, ATC’s infrastructure and response.

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