TOLEDO, Ohio—Ohio utility FirstEnergy Corp. says it will delay the September closings of three coal-burning plants in northeast Ohio for three years so the company can upgrade some transmission lines, at the instruction of electric grid manager PJM Interconnections.

The three coal plants that now will remain open until 2015 are in Cleveland, Eastlake and Ashtabula. They only will operate when demand increases, and Akron-based FirstEnergy still plans to close the coal units once the transmission lines, mostly in northern Ohio, are improved, company spokesman Mark Durbin said.

The company said the closings of coal-burning operations in Adrian, Pa., and Williamsport, Md., along with three in West Virginia—Albright, Rivesville and Willow Island—remain on track for closing by September.

PJM Interconnections, which oversees the power grid in Ohio and a dozen other East Coast states, reviewed plans for closing the FirstEnergy plants and told the company it needed to upgrade its transmission lines before shuttering the coal-burning units in northeast Ohio, Durbin said. PJM wanted the three Ohio plants to stay online as a backup to ensure enough power generation during the upgrades, Durbin said.

FirstEnergy is ridding itself of nine coal-fired power plants because of new environmental regulations the U.S. Environmental Protection Agency announced last December. The new standards are aimed at reducing emissions of mercury and other toxic pollution.

The company has taken steps at several of its coal-burning plants to make them cleaner for the environment. It has said that once the closings are completed, nearly all of its power will come from low emission sources.

An Associated Press survey found that the EPA changes were likely to result in mothballing dozens of power-generating units in the Midwest and in the coal belt of Kentucky, West Virginia and Virginia.

The cost of improving transmission lines, substations and transformers will cost between $700 million and $900 million, Anthony Alexander, FirstEnergy's chief executive officer, said during a call with analysts Tuesday, The (Cleveland) Plain Dealer reported.

FirstEnergy's electric system has 6 million customers in Ohio, Pennsylvania, New Jersey, West Virginia, Maryland and Virginia. Coal and nuclear power plants generate about 80 percent of the company's output.