HURON — NorthWestern Energy plans to start construction in July on $84 million in upgrades to its natural gas-fired power plant near Huron.

The company said it will use modern industrial combustion engine technology to replace its existing natural gas and diesel fuel-fired turbines, including one unit that dates back to 1961. The plant will now be capable of producing 58 megawatts of power, and NorthWestern said the upgrades will be more efficient and include mitigation measures to reduce noise from the plant. The plan is for the upgrades to be completed by the end of 2021.

NorthWestern Energy issued a request for proposals on the project in spring 2019 for 60 megawatts of flexible capacity resources. It chose Caterpillar to provide the reciprocating engine supplies and Minnesota-based Fagen as the engineering, procurement and construction contractor.

The upgrades were part of the company’s updated Electricity Supply Resource Procurement Plan presented to the South Dakota Public Utilities Commission in 2018. The company has nearly 64,000 electricity customers in 110 communities in eastern South Dakota, geographically ranging in areas from Aberdeen and Huron to Mitchell and Yankton.

Historically, the Huron plant has been available to provide peaking energy, contingency reserves and for reliability support. In the 2018 plan report from NorthWestern Energy, it said that Huron Generating Station Unit 1 was the oldest in the NorthWestern fleet in South Dakota, and operated on average for 40 hours per year. The age of the unit has made obtaining replacement parts difficult, the report said, and also lacked functional fire suppression and vibrating monitoring systems and was “a prime candidate for retirement.”

NorthWestern Energy spokeswoman Jo Dee Black said customers won’t notice a change in how their electricity is delivered once the change is made.

“This is one of those resources that is always available,” she said. “It’s the type of capacity that is always ready and isn’t dependent on other factors like the strength of the wind or the sunlight, and can be balanced out with renewables.”

Overall, the peaking gas makes for a tiny portion of Northwestern’s Energy resource mix. According to the 2018 plan report, about half of NorthWestern’s mix came from coal, with wind accounting for 29%, and peaking gas/diesel responsible for 0.5%, with the rest coming from market sources.

The project is also expected to be constructed on a smaller footprint at the existing Huron Generation Station, which is located east of the city.