London-based National Grid PLC purchased KeySpan for $7.3 billion in cash Monday. KeySpan, which operates the power transmission system for the Long Island Power Authority, brings electricity to more than one million customers in Nassau and Suffolk counties. The deal, which creates the third-largest energy delivery utility in the United States, should not affect Long Island customers. CEOs for both companies say the takeover will improve power plants. They also say they do not have plans to sell any of the plants. National Grid PLC hopes to complete the takeover of the northeast's largest gas-distribution company by early next year.