The Secretary of Energy, acting by and through the Administrator, shall operate and maintain the Federal transmission system within the Pacific Northwest and shall construct improvements, betterments, and additions to and replacements of such system within the Pacific Northwest as he determines are appropriate and required to:

(a) integrate and transmit the electric power from existing or additional Federal or non-Federal generating units;

(b) provide service to the Administrator's customers;

(c) provide interregional transmission facilities; or

(d) maintain the electrical stability and electrical reliability of the Federal system: Provided, however, That the Administrator shall not construct any transmission facilities outside the Pacific Northwest, excepting customer service facilities within any contiguous areas, not in excess of seventy-five airline miles from said region, which are a part of the service area of a distribution cooperative which has (i) no generating facilities, and (ii) a distribution system from which it serves both within and without said region, nor shall he commence construction of any major transmission facility within the Pacific Northwest, unless the expenditure of the funds for the initiation of such construction is specifically approved by Act of Congress.