A supplemental merger agreement filed Tuesday with the North Carolina Utilities Commission "addresses state regulatory issues raised by the market power mitigation proposal filed with the FERC on March 26, as well as clarifying provisions included in the previous settlement agreement," Duke Energy and Progress Energy said.

The agreement, as reported in a statement from the companies, says they will not ask retail customers to pay for costs associated with five merger steps. The agreement is with the Public Staff, the arm of the NCUC that represents consumers.

"This agreement is an important milestone to move the merger process forward," said Jim Rogers, chairman, president and CEO of Duke Energy. "We look forward to hearing from the FERC on our mitigation plan so we can have this proposal and related issues considered by the NCUC as soon as possible."