Unit 3 to be down for some time

"In this instance you had electrical equipment on the generator that is disassembled, being repaired," TVA spokes-man Gil Francis said. "It will be reassembled, and the unit will come back in service. They will do that as quickly as they can, but the No. 1 priority is to do it safely."

When operational, Unit 3 produces 1,155 megawatts of power, enough for about 650,000 homes. Because the unit is down, the Tennessee Valley Authority must find replacement power. "We've got interconnections to the marketplace to buy replacement power if needed, particularly when it's cheaper," Francis said.

"Or we can use other resources, whether it's combustion turbine, other coal plants, hydro plants, Raccoon Mountain. It's such a large system we have that flexibility."

Raccoon Mountain is an energy storage facility near Chattanooga. TVA pumps water to an elevated pool during non-peak hours, then releases the water to generate hydroelectric power during hours of peak demand. Unit 3 automatically shut down Dec. 31 when the main turbine generator received a load reject signal of unknown cause. All safety systems responded properly to the signal. A load reject signal causes the system to "think" that the generator has lost the attached load.

It is similar to the tires of a speeding vehicle losing contact with the road. With the load gone, there is a risk that the engine will overaccelerate, damaging the engine. Similarly, if TVA's safety systems failed, a false load reject signal could cause the turbine and attached generator to speed up to dangerous levels.

The repairs, according to one TVA spokesman, are focused on Unit 3's exciter. The exciter provides a source of direct current to the generator, which allows the generator to produce alternating current electricity. The alternating current electricity connects to the power grid through an output circuit breaker.

Shutdowns during August forced TVA to purchase high-priced power produced by natural gas turbines, but Francis said a January shutdown is unlikely to impact rates.

Power purchases in January are generally less expensive than in August because most systems are not running at full capacity. Francis said TVA does not keep statistics indicating how the length of the Unit 3 outage compares to other unplanned outages in TVA history.

"There is no list of unplanned outages where I can just pull it," he said. "Each outage is different." He said TVA uses unplanned outages to perform maintenance unrelated to the reason for the outage.

"You manage that outage to the best of your ability to take care of all the other issues that are pending," Francis said.