The commission shall not have outstanding at any one time bonds and notes for any of its purposes in an aggregate principal amount exceeding two hundred million dollars, excluding bonds and notes issued to refund outstanding bonds and notes; provided, however, that the limitation contained in this section shall not apply to any bonds or notes of the commission which are secured, directly or indirectly, by first mortgage loans which have been insured or guaranteed by an agency or instrumentality of the United States, or any bonds or notes of the commission which at the time of issuance are rated not lower than “Aa” by Moody’s Investors Service, Inc., in the case of long-term obligations or rated Moody’s “Investment Grade I” by Moody’s Investors Service, Inc., in the case of short-term obligations; or rated not lower than “AA” by Standard & Poor’s Corporation in the case of long-term obligations or rated “SP-1+” by Standard & Poor’s Corporation in the case of short-term obligations, or the equivalent ratings by Moody’s Investors Service, Inc., or Standard & Poor’s Corporation in the event the ratings described in this section are changed.