Former TVA power chief Bob Steffy often cited the tale of his uncle’s ax to explain the longevity of TVA’s coal plants.

“That ax had four blades and three handles, but it was still the same old great ax,” Mr. Steffy quipped.

Like the ax of Mr. Steffy’s uncle, many of TVA’s aging coal plants have been rebuilt and upgraded. Four of TVA’s 11 coal plant sites are more than a half century old, although workers have replaced many of the parts in those plants several times.

To TVA, those changes were a part of routine plant maintenance. But to federal regulators in the final year of the Clinton administration in 1999 and 2000, such changes were seen as making the plants “new sources” that must meet stricter standards for new plants.

The debate over what constitutes routine maintenance of an old plant and what is essentially building a new coal plant generated one of the biggest and most expensive legal battles ever for coal-fired power generators over the past decade.

Critics of TVA and the Bush administration contend that the battlefield so far has proved unfairly advantageous for TVA.

Because of its ownership by the federal government, TVA ended up not facing the same legal action taken against more than a dozen private utilities and public cooperatives.

“TVA is being held to a lower standard than any other utility company in America,” said John Walke, clean air director for the Natural Resources Defense Council. “There’s nothing right about that.”

Federal law requires new power generators to install the best-control technology. But existing plants are exempt from such “new source” standards and are grandfathered under older, less stringent requirements.

When the 1990 clean air amendments were adopted, Cleveland, Tenn., native and former Senate staffer Will Calloway said members of Congress figured the old and dirtier coal plants eventually would be phased out. But so far they have not “and a lot of us are very disappointed.”

TVA was among the utilities initially cited by EPA during the Clinton administration for violating new source standards by making major modifications at older coal plants without installing best control pollution technologies.

As a sister federal agency, the U.S. Environmental Protection Agency issued an administrative order to TVA during the Clinton administration. But after a four-year legal battle that was finally settled by the U.S. Supreme Court, TVA was able to dismiss the initial EPA action.

Because the Bush administration didn’t file a lawsuit subsequently against TVA on the same charges in federal court, the federal utility is not charged by the government with any “new source” violations, according to those involved in the case.

Other private utilities and public co-ops, however, have settled with EPA and agreed to install more pollution controls. In October 2007, for instance, American Electric Power agreed to spend more than $4.6 billion and make improvements at 16 of its coal-fired plants in the largest environmental enforcement settlement in history.

Mr. Walke claims TVA “was the poster child” for new source violations. In one case, TVA built a rail line to deliver major equipment to refurbish one of its plants, Mr. Walke said.

But as a federal utility, “TVA got off scot-free” from any government action, he said.

But TVA President Tom Kilgore insists that TVA’s plant modifications were legal and TVA lawyers successfully defeated the case on procedural grounds in court.

“Like everybody else, we defend ourselves,” Mr. Kilgore said. “We think we’re right on how we pursued the modifications to those plants.”

The National Parks Conservation Association and the Sierra Club also sued TVA in 2001 for new source violations, but that case has yet to come to trial.

Mr. Walke said TVA also could face a renewal of the new source violation from the next president.

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