HOUSTON, April 30 (Reuters) - Southern Co's Mississippi Power unit has defended its request to build a $2.2 billion advanced coal plant in Kemper County, Mississippi, against opponents who want state regulators to halt a review of the project.

Southern's smallest utility unit wants the Mississippi Public Service Commission to certify the need for a 582-megawatt integrated gasification combined-cycle (IGCC) power plant to be built by 2013 in Kemper County.

The utility, which serves 189,000 customers in the southeastern corner of the state, wants commission action by August so it can buy equipment and meet a deadline to qualify for nearly $500 million in federal incentives.

But the costly Kemper County IGCC project has drawn opposition from the Mississippi Attorney General as well as the Sierra Club and competing power producers.

In a filing late on Wednesday, Mississippi Power disputed arguments raised by the AG's office that the commission lacks authority to approve the IGCC project and to allow the utility to charge customers for some costs before plant construction.

The Mississippi commission is scheduled to hold a hearing May 5 on motions to suspend the certification process.

The utility said the commission's authority to issue certificates of need "is extremely broad" and that limits raised by Attorney General Jim Hood and other parties would create delay and "potentially jeopardize a utility's ability to meet its obligation to efficiently and reliably serve its customers," the filing said.

Mississippi Power also said that opponents' claims that the Baseload Act, passed in 2008, puts many conditions on the commission's ability to consider new generation are wrong. "Their motive is clearly to delay MPC's certificate proceeding in hopes of killing" the project, the filing said.

The 2008 law allows utilities to include some pre-construction costs to reduce overall costs passed on to customers. Mississippi Power said its plan will save customers $163 million over the life of the IGCC plant.