AUGUSTA — The Maine Public Utilities Commission has approved a tweaked version of Central Maine Power’s proposed $1.4 billion power grid upgrade.

Maine’s largest electric utility plans to build a new 345,000-volt transmission line from Orrington to Newington, N.H., doubling capacity of the grid’s backbone. It contends the improvements, the first major upgrade since 1971, are necessary to maintain stability of the power grid beyond 2012.

Commission Chairwoman Sharon Reishus said after today’s vote that she’s confident the upgrade “will allow Maine to meet its electric transmission needs for the next 10 years.”

Maine ratepayers will pay 8 percent of the cost; the rest will be picked up by other users of the New England power grid.