ROCHESTER, N.Y. — Spanish power utility Iberdrola SA says it has closed on the sale of three natural gas distribution subsidiaries in Connecticut and Massachusetts, netting about $918 million to help finance a major upgrade of transmission lines linking Canada and New England.

The company’s Central Maine Power unit is leading the $1.4 billion project to add or upgrade 500 miles of lines to bolster capacity and reliability in Maine, New Hampshire and Massachusetts.

Iberdrola also said today it invested about $73 million in infrastructure near Ithaca, N.Y., and is spending $55 million on upgrades near Corning, N.Y.

Connecticut Natural Gas Corp., Southern Connecticut Gas Co. and Berkshire Gas Co. were sold to New Haven, Conn.-based UIL Holdings Corp. They serve a total of about 370,000 customers.