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Great River Energy HVDC Project Manager Greg Schutte will lead the $130 million update of the power cooperative's CU HVDC transmission line converter stations, slated to start in the spring. Behind Schutte are crates containing new cooling towers that will switch the converter system from being air cooled to water cooled. Bismarck Tribune

North Dakota coal plant to upgrade transmission system that carries power to 500,000 people

UNDERWOOD, N.D. — Greg Schutte compares Great River Energy's current transmission system to an 8-track tape and the improvements being made as upgrading to the latest iPhone.

The CU HVDC line, which stands for high voltage direct current, was put in service 40 years ago in 1978.

It's an extremely important line to GRE because it moves 73 percent of the cooperative's power supply 436 miles from Underwood to Buffalo, Minn., west of the Twin Cities, and serves about 500,000 customers across Minnesota and parts of Wisconsin.

The power cooperative is preparing to invest $130 million in the line, which Schutte said is imperative to continue producing power at the state's largest coal-fired power plant, Coal Creek Station.

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

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

The main concern is with the thousands of valves located within the conversion equipment, all of which are oil lubricated. The newer technology changes that, making the valves fireproof and reducing risk of failure.

So in 2015, GRE awarded a contract to ABB, a Swedish company, to replace the system.

Starting in March, the power plant and transmission system will go through a 74-day outage, running at half power for all but three days of total shut down. In that time, ABB will gut and reconstruct the two 65-foot stacks that convert the power for transport across the line.

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

GRE began the process seven or eight years ago and, in the past couple years, has devoted more than 20,000 internal engineering hours to making the conversion run smoothly.

A 350-by-100-foot building is being constructed on site to serve as a staging area as the equipment is shipped from overseas. The contractors will pre-assemble as much as possible.

"They want to be really focused once the outage starts," GRE spokesman Lyndon Anderson said of crews that will be running 24/7.

More than 100 union contractors will be on site.

"It's the biggest project on our books," Anderson said.

Along with the valves, the computers that control the system, "the brains of it," are being replaced, according to Schutte.

The components that make up the system will be reduced by 70 percent, which means less moving parts to maintain.

The system also will see a 7-megawatt efficiency gain because it will be water cooled rather than air cooled. Currently, GRE has to power 1,000-horsepower worth of fans that force air through the system. With the updates, they can sell that power rather than using it.

"That's nothing to shake your head at either," Schutte said.

Once the project is complete, the staging building will become a shop for the line and substation maintenance crew. ABB will stay on site for a 90-day trial operation after the outage.

Schutte said there are only five transmission lines in the United States like the CU HVDC line and it's one of the oldest. The only remaining one that will need updating is Minnesota Power's Square Butte, HVDC line, which also runs through North Dakota between the Minnkota Power Cooperative's Milton R. Young Station and Duluth, Minn. Schutte predicts that line is about four years behind GRE's for updates.

GRE's line has been extremely reliable, running nearly 100 percent of the time, according to Schutte. Without the updates, it was predicted that reliability would drop off next year and the cooperative wanted to be ready for it.

The last major development by GRE was the building of the Spiritwood Station, which had a $437 million price tag. The cooperative's DryFining technology installed at Coal Creek cost about $285 million.

Other area transmission projects have involved new construction. Basin Electric Power Cooperative recently finished a 345-kilovolt line from Beulah to Grassy Butte and Tioga at a cost of $300 million, according to Basin spokesman Curt Pearson.

Mark Hanson, a spokesman for Montana-Dakota Utilities, said MDU is splitting the cost of a $240 million to $300 million 345-kilovolt line between Ellendale and Big Stone City, S.D., with Otter Tail Power Cooperative.

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