(1)    When operating notes are authorized, there shall be established in the state treasury or with a trustee if so required in the authorizing resolution, an operating note redemption fund separate and distinct from every other fund, which may contain separate and distinct accounts for each particular operating note issue.

   (2)   The operating note redemption fund shall be expended and all moneys from time to time on hand therein are irrevocably appropriated, in sums sufficient, only for the payment of principal and interest on operating notes giving rise to it and premium, if any, due upon refunding or early redemption of such operating notes, and for the payment due, if any, under an agreement or ancillary arrangement entered into under s. 18.73 (5) with respect to such operating notes.

   (3)   Moneys of the operating note redemption fund may be commingled only for the purpose of investment with other public funds, but they may be invested only as provided in the authorizing resolution. All such reinvestments shall be the exclusive property of such fund and all earnings on or income from such investments shall be used in meeting principal and interest payments on operating notes issued.

   (4)   There shall be transferred, under s. 20.855 (1) (a), a sum sufficient for the payment of the principal, interest and premium due, if any, and for the payment due, if any, under an agreement or ancillary arrangement entered into pursuant to s. 18.73 (5) with respect to operating notes giving rise to it as the same falls due. Such transfers shall be so timed that there is at all times on hand in the fund an amount not less than the amount to be paid out of it during the ensuing 30 days or such other period if so provided for in the authorizing resolution. The commission may pledge the deposit of additional amounts at periodic intervals and the secretary of the department may impound moneys of the general fund, including moneys temporarily reallocated from other funds under s. 20.002 (11), in accordance with the pledge of revenues in the authorizing resolution, and all such impoundments are deemed to be payments for purposes of s. 16.53 (10), but no such impoundment may be made until the amounts to be paid into the bond security and redemption fund under s. 18.09 during the ensuing 30 days have been deposited in the bond security and redemption fund.