Allete Clean Energy is selling the 48MW expansion to the 107.5MW Thunder Spirit wind farm in North Dakota to Montana-Dakota Utilities.
Thunder Sprit 2 will add 16 machines to the 48-turbine first stage, which Montana-Dakota bought from Allete in 2015.
Commercial operations at the $85m expansion, which will be located near Hettinger, are scheduled for late 2018.
Montana-Dakota, which is owned by MDU Resources Group, initially signed a power purchase agreement for the electricity from the project with an option to buy the wind farm.
MDU Resources Group president and chief executive David Goodin said: “This project will be a great cost-effective addition to Montana-Dakota's generation portfolio.”