Safety First: Rotor Cracks Shut Plant

Tuesday, August 14, 2012 @ 06:08 AM gHale

A hydroelectric power plant on Raccoon Mountain shut down after finding cracks in rotors, Tennessee Valley Authority officials said.

The plant near Chattanooga has provided hydroelectric power for 34 years, but went out of service in March. The utility hopes they can repair one of the four generators within two years, said TVA Vice President John McCormick. He called the other three “900,000 pounds of scrap.”

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The German firm that designed the units told TVA to check the rotors for cracks after a similar plant in Austria splintered in late 2009. There are two similar plants in Virginia and California facing the same problem.

TVA officials said the problem is a design flaw in the 34-year-old, $310 million plant.

The utility took the entire plant offline as soon as a contractor’s alert and its own checks found the rotor cracks in one after another of all four generators there.

But the plant’s outage was kept quiet until last Friday, when TVA President and Chief Executive Tom Kilgore told financial analysts the utility had brought two of its idled coal units back this summer to make up for Raccoon Mountain’s lost peak-power generation.

“We brought them back because … we found cracks in the rotors so we’ve taken all four of [the pumped storage units] offline,” Kilgore said.

Eventually they will replace the rotors poles and rim of all four units, but not until the 2014 and 2015 time frames, according to TVA officials.

“These are parts you can’t get at Home Depot,” said TVA spokesman Travis Brickey.

Eight feet long, three feet wide, two feet deep and about 7,000 pounds, each of the 24 rotor poles in each generator, as well as the cracked rotor rims, must be custom built. The materials alone will weigh 450 tons, McCormick said.

The generators pulled water from the river — 7 million gallons a minute — and pumped it up through a cavernous concrete tunnel through the middle of the mountain to fill a large manmade lake above them.

Then with the flip of a switch, that water flowed back down through those generators to make power on its way back to the Tennessee River Gorge below.

Before they found the cracks, the plant saw use nearly every day, and it served as an important grid balancer for TVA.

Like all hydropower, the electricity at Raccoon Mountain is extremely cheap power to produce.

TVA said the plant’s constant push and pull of water made it work like a giant storage battery.

The plant pumped water up the mountain during the hours when consumers were using the least power.

Then when families came home and cranked up the heat or the air conditioning, TVA would send that water tumbling back down the mountain’s 1,000-foot tunnel to stroke four house-sized generators.

The flow of the water spun the turbines to rotate a shaft inside an electromagnetic coil.

The plant provided a “dependable” average of 1,652 megawatts of electricity a day — enough to power nearly a million homes.

Then the process would repeat over and over.

It could generate electricity for up to 22 hours before needing to draw water up again for 28 hours to fill the mountaintop reservoir.

McCormick said if there is a silver lining to TVA’s newest cloud, it may be the timing of Raccoon Mountain’s outage.

The utility can take advantage of the present low cost of gas and TVA’s new gas-fired plants to somewhat balance the added price of more expensive generation or buying power from other utilities to meet the peak demands normally handled by Raccoon Mountain.

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