(KPAX) NorthWestern Energy has filed its formal request with the Montana Public Service Commission to increase its share by 25% in the Colstrip coal plants for the cost of $1.

The Montana-based utility is planning to buy out co-owner Puget Sound Energy's stake of 185 megawatts in Colstrip's units 3 and 4, increasing its total share in the plant to 55 percent. Puget Sound is shifting away from coal.

The document, which is more than a thousand pages, was filed Thursday in Helena. The Public Service Commission is scheduled to discuss it next Tuesday during a work session.

NorthWestern first announced its intentions in December as the Public Service Commission was considering its proposed 20-year resource plan. NorthWestern officials say they desperately need new power resources to feed growing regional demand, which will help offset price fluctuations in the energy market.

Other power producers have made moves to shift away from coal.

Environmental groups such as the Sierra Club have criticized the plan, saying that ratepayers may be getting the power cheap, but they'd also be buying high maintenance and operating costs for the aging units in a market that is swinging away from coal.

Colstrip sells most of its power to the Pacific Northwest, where the appetite for fossil fuels has soured in recent years. Puget Sound Energy has been looking to get out of the plant to meet renewable energy standards in Washington state, and two other Washington-based co-owners, Avista Corp. and PacifiCorp, both plan to pull out this decade for the same reason.

Colstrip is co-owned by six utilities: NorthWestern, Puget Sound Energy, Avista, PacifiCorp, Portland General Electric and Talen Energy. The power plants two oldest units, 1 & 2, shut down at the beginning of this year.