Nebraska Power Review Board authorization shall not be required for the following: (1) Construction of microwave facilities to be completed in a reasonable time where the district has, prior to April 24, 1978, entered into binding contractual obligations with respect to such facilities; (2) Extensions of fifty miles or less of any single portion of a microwave system if such portion was in existence on or before April 24, 1978; or (3) The addition of new equipment to modernize or upgrade but not to extend microwave facilities.