Wis. Stat. § 18.56: Revenue obligations.

The commission may authorize, for any of the purposes described in s. 18.53 (3), the issuance of revenue obligations. The revenue obligations shall mature at any time not exceeding 50 years from the date thereof as the commission shall determine. The revenue obligations shall be payable only out of the redemption fund provided under s. 18.561 (5) or 18.562 (3) and each revenue obligation shall contain on its face a statement to that effect. A revenue obligation may contain a provision authorizing redemption, in whole or in part, at stipulated prices, at the option of the commission and shall provide the method of redeeming the revenue obligations.