The Department of Energy has for the third time extended the deadline for negotiating a federal loan-guarantee offer with the owners of Plant Vogtle in Georgia. The department offered up to $8.3 billion in loan guarantees for the expansion project in 2010, but details of the arrangement were never finalized. "There continues to be constructive dialogue ... and we are cautiously optimistic as we work with the DOE," said Jeannice M.W. Hall, a spokeswoman for Southern Co. The new deadline for a final agreement is Sept. 30.