Xcel Energy Inc. has gone to court defending its large coal-fired generating plant in Becker, Minn., against claims that its emissions harm visibility in Minnesota and Michigan national parks.

Minnesota's largest power company filed papers in U.S. District Court late Tuesday to intervene in an environmental lawsuit alleging lax enforcement of the federal Clean Air Act by the U.S. Environmental Protection Agency.

"To protect our customers' interests in clean, reliable and reasonably priced power, we are now asking the court to let us participate in the litigation," Xcel said in a statement Wednesday.

The lawsuit, filed in December by six environmental groups, accuses the agency of not enforcing air pollution rules on two 1970s-era generators at Xcel's Sherco power plant in Becker. The U.S. Interior Department considers the power plant a source of haze over Voyageurs National Park in Minnesota and Isle Royale National Park in Lake Superior off the Michigan coast. The EPA has asked for the case to be dismissed.

Xcel, in its motion to intervene, said the company could face new, expensive pollution-control requirements if the National Parks Conservation Association and other groups win the lawsuit. Xcel has estimated the upgrade cost at $350 million and says it's already spending $50 million on new controls.

The company, which supplies power to 1.2 million customers in the state, said that extensive studies conducted in the 1980s by an EPA consultant concluded that no visibility impairments in Voyageurs could be traced to specific sources, including Sherco. Xcel said state and federal agencies have approved the company's compliance with haze regulations.

"Both national parks are located more than 200 miles (in the case of Isle Royale, almost 300 miles) from the Sherco facility," Xcel's motion papers say. "Numerous other industrial and electric generating facilities with air emissions are located between the two parks and Sherco and are much closer to the parks."

Haze, which is linked to smokestack emissions of nitrogen oxides, has long been subject to special regulation under the Clean Air Act to protect pristine natural areas from visual pollution.

A court hearing on the motion is scheduled in two weeks.

David Shaffer • 612-673-7090 Twitter: @ShafferStrib