30 ILCS 350/11
Bonds may be refunded or advance refunded upon such terms as the governing body may set in accordance with this Act, for such term of years, not in excess of the maximum term of years permitted by applicable law for the bonds to be refunded, and in such principal amount, all as may be deemed necessary by the governing body. Revenue bonds may be issued to refund general obligation bonds or alternate bonds issued under this Act. General obligation bonds shall not be issued to refund revenue bonds or alternate bonds except as expressly permitted by applicable law. Any redemption premium payable upon the redemption of bonds may be payable from the proceeds of refunding bonds which may be issued for the purpose of refunding such bonds, from any other lawfully available source or from both proceeds and such other sources.
30 ILCS 350/11