UNDERWOOD, N.D. — An electric company will upgrade a transmission line that transports power from central North Dakota to a town outside Minneapolis.

Maple Grove-based Great River Energy is planning to invest $130 million in the CU HVDC line, which stands for high voltage direct current. The line moves 73 percent of the cooperative's power supply nearly 450 miles (724 kilometers) from Underwood, N.D., to Buffalo, Minn., the Bismarck Tribune reported.

The upgrade is necessary to continue producing power at Coal Creek Station, North Dakota's largest coal-fired power plant, said project manager Greg Schutte.

"It's an investment in the station and an investment in Coal Creek Energy Park," Schutte said.

The company is making the improvements to two stacks on either end of the line that convert the power for transport.

"We had inklings the stations were getting too old," Schutte said, so the utility performed a life assessment on the system. "We found out we had some issues."

The power plant and transmission system will see a 74-day outage starting in March, running at half power for all but three days of total shut down. Contractors during that time will gut and reconstruct the two stacks.

"We're basically just keeping a shell," Schutte said. "That's a huge outage for us."

The system will see a 7-megawatt efficiency gain because it will be water-cooled rather than air-cooled. The utility currently has to power 1,000-horsepower worth of fans that push air through the system.

Schutte said there are only five transmission lines like the CU HVDC line, which he said is one of the oldest. The line serves about 500,000 customers in Minnesota and Wisconsin, and runs nearly 100 percent of the time, according to Schutte.