ORRINGTON, Maine — Central Maine Power Co. plans to upgrade its power grid after nearly 40 years, and officials are keeping town selectmen informed about the massive construction project that could begin later this year, if approved at the state and federal levels.

“A lot has changed since 1971,” which is when the current line was put into service, Kay Rand, of Bernstein Shur Governmental Solutions of Augusta, told town leaders Monday during a presentation about CMP’s expansion plan, dubbed the Maine Power Reliability Program.

The proposed $1.5 billion upgrade would double the capacity of the electric grid’s approximately 485-mile backbone between Orrington and Eliot, where it connects to Newington, N.H.

The project would rebuild the existing 345-kilovolt transmission line on the eastern side of the corridor, add a new 345-kilovolt transmission line where the current line is located, and add a smaller 115-kilovolt transmission line on the western side of the corridor.

Maine’s population has increased by 32 percent since 1971, and has shifted south to southern, eastern and coastal Maine, Rand said.

“Where the electricity is used has changed significantly and we’ve also doubled” usage over the last four decades, she said.

The proposed expansion project stretches across 13 counties and 78 cities and towns, and plans are for around 98 percent of the new construction to be adjacent to the current power grid, Rand said.

CMP’s project is under review by the Maine Public Utilities Commission and requires approval from the U.S. Army Corps of Engineers, the state’s Department of Environmental Protection, the Saco River Corridor Commission and the State Planning Office, as well as all the individual municipalities.

“We’ve got about 35 approvals so far,” Rand said, referring to towns along the route.

About 4.8 miles of the proposed expansion is in Orrington, and the Fields Pond Road substation also would need improvements, but they would not change the footprint of the facility, Rand said.

Around “$9.7 million is safe to use as an estimate,” she said, referring to the project costs in Orrington. “It’s a significant investment here in Orrington.”

If all goes as planned, shovels for the Maine Power Reliability Program would be in the ground in July, and the project would take around 4½ years to complete and require an average 2,100 employees, Rand said. Locally, construction work wouldn’t begin until spring 2012, when work on the new line is scheduled, and would be complete in two years, she said.

Initial plans were presented to the Orrington Planning Board last week.

“This project is really about keeping the lights on here in Maine and making sure the system is strengthened,” Rand said.