Section 21. Any city or town having bonds outstanding under this chapter, acting by its municipal light board, when authorized by a two-thirds vote as defined in section one of chapter forty-four, may issue refunding bonds for the purpose of paying bonds issued by it or on its behalf under sections eleven to twenty-two, inclusive, at maturity or upon acceleration or redemption, subject to the approval of the department under this chapter. The refunding bonds may be issued in sufficient amounts to pay or provide the principal of the bonds being refunded, together with any redemption premium thereon, any interest accrued or to accrue to the date of payment of such bonds, the expenses of issue of the refunding bonds, the expenses of redeeming the bonds being refunded, and such reserves for debt service or other capital or current expenses from the proceeds of such refunding bonds as may be required by a trust agreement or resolution securing bonds or notes. The refunding bonds may be issued not more than five years prior to the maturity or redemption date of bonds being refunded. The issue of refunding bonds, the maturities and other details thereof, the security therefor, the rights of the holders thereof, and the rights, duties and obligations of the city or town in respect of the same shall be governed by the provisions of this chapter relating to the issue of bonds other than refunding bonds insofar as the same may be applicable, but no bonds shall be refunded to a date later than the refunded bonds could have matured hereunder.