By Amanda Durish Cook

American Transmission Co. (ATC) has restored one of two underwater circuits connecting Michigan’s Upper Peninsula with the lower part of the state following a month-long outage for the damaged lines.

ATC reconfigured and combined three undamaged submarine cables to form one circuit across the Straits of Mackinac. The company owns two 138-kV circuits across the peninsulas, consisting of three submarine cables apiece. One cable in each circuit was damaged on April 1, and both circuits were taken offline after they leaked a toxic, petroleum-based fluid used for insulation into the water.

MISO had been using an alternative transmission route through Wisconsin to transmit energy to the Upper Peninsula while both circuits were down. (See Wis. Tx Picks up Slack After Upper Peninsula Outage.)

ATC said it tested the new configuration and has been operating the reworked circuit since May 1.

The company said the cables were possibly damaged by “vessel activity” in the lake, spilling fewer than 600 gallons of fluid insulation. The U.S. Coast Guard has initiated an investigation into the incident and is still reporting low risk to the public and wildlife.

The damaged cables have been “soldered, capped, sealed and returned to the bottom of the Straits,” ATC said. Company spokesperson Jackie Olson confirmed the cables were rendered “permanently inoperable.”

ATC said it is making plans to construct two new circuits in the Straits, this time using a solid dielectric insulator instead of a liquid-based insulation. If the new circuits are approved, ATC will permanently decommission all six fluid-filled insulating cables, the company said, though it did not release a timeline or cost estimate for the possible new project.

“Our planning team is hoping to secure internal approval for such a project in the next several weeks,” Olson said.

MISO spokesman Mark Adrian Brown said while the RTO was pleased to see restoration of ATC’s submarine connection, no reliability issues arose during the outage.

“While other routes also serve the U.P., the restoration is an important step for added reliability and greater redundancy of the power grid,” Brown told RTO Insider.

Brown also said ATC worked closely with MISO throughout the outage to ensure reliability as it performed subsea inspections and determined a course of action.

“This connection is essential for reliability for the eastern U.P. and the northern portion of lower Michigan,” said ATC Chief Operating Officer Mark Davis in a statement. “We were able to maintain reliability by implementing conservative operating procedures during the month the connection was lost, but re-establishing this powerline will give us greater flexibility and an added measure of reliability to help us keep the lights on.”

Davis thanked the Coast Guard and other groups that helped monitor and minimize the incident, including the EPA, the Michigan Department of Environmental Quality, county emergency managers, local native tribes, the National Oceanographic and Atmospheric Administration and U.S Fish and Wildlife.

“The coordinated response helped minimize impacts to the environment and local community,” Davis said.