KNOXVILLE (AP) — Standard & Poor's has downgraded the Tennessee Valley Authority's credit rating to AA-plus, but TVA says the downgrade is "not expected to have a material impact."

TVA Chief Financial Officer John Thomas said the utility's top credit rating otherwise has been reaffirmed by Moody's Investors Service Inc. Fitch rating service has yet to change its TVA bond rating.

Thomas said the fundamental financial strength of TVA is unchanged. He said TVA understands that as a government-owned enterprise, a change in the rating of U.S. government securities can impact TVA.

TVA said in a statement Tuesday that it receives no taxpayer dollars and its debt is not part of the national debt.