Minnesota's 71-MW Thomson hydropower plant back on line after two-year rehab

Thomson Hydropower Project

Minnesota Power's 71-MW Thomson hydropower plant is generating energy for the first time since being knocked out of service by record flooding more than two years ago.

The plant, located on the St. Louis River southwest of Duluth, was flooded in June 2012. The flood waters were the larges ever recorded at Thomson and flooded the plant's six turbine generators, overtopped Thomson reservoir and breached a portion of an earthen dike at the forebay.

The flooding also washed out roads and caused mudslides, Minnesota Power said, limiting access to the powerhouse for months.

The US$90 million restoration project is now largely complete, the utility said, and included the reconstruction of the forebay canal, electrical and mechanical rehabilitation, upgrades to the water conveyance system and construction of additional spillway facilities at Thomson's main dam.

"We're proud to report that the largest contributor in our hydro fleet is back up and running," Minnesota Power COO Brad Oachs said. "It took a natural disaster to knock it off line, but Thomson Hydro's intrinsic value necessitated its repair and refurbishment.

The project's original forebay embankment -- constructed in 1905 -- was reconstructed and upgraded to meet standards established by the Federal Energy Regulatory Commission (FERC) and include the building of a 2,900-foot-long sheet pile wall and reinforced concrete emergency spillway capable of handling future flooding.

Meanwhile, each of the plant's six generating units suffered "significant" damage in the flood, leading to the disassembly, cleaning, inspection and refurbishment of each. Minnesota Power said insurance covered the repairs to the damaged equipment and electrical systems -- much of which was also upgraded during the rehabilitation project.

"Thomson is our oldest facility, an iconic piece of our company history, and through this investment, remains an important part of our energy future, providing customers with low-cost, renewable energy for years to come," Oachs said. "We believe it can generate carbon-free energy for another hundred years."

Portions of the repair project not covered by the company's insurance will translate to a slight increase in its customers' rates, the utility said, pending approval by the Minnesota Public Utilities Commission. The increase is expected to translate to about US$1 per month for the average residential customer.

For more rehabilitation and upgrade news, visit here.

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