The earnings of the electric plant or distributing system shall be applied and used in payment of warrants and interest thereon issued in connection with operation of any such plant or system, and also in payment for alterations, improvements, additions or extensions and for redemption and retirement of outstanding bonds, together with interest thereon, and shall be expended only in connection with and for improving such plant or system and not for other municipal purposes, except as otherwise provided in ORS § 225.270.