Mitchell said TVA was alerted to the fire at the automated power plant at the dam about 6 a.m., and the generator was automatically shut down.
Mitchell described the blaze as an electrical arcing fire that started when wires got too hot.
She said the generator would be shut down for several days while repairs to the wiring in the switchgear room are made.
Mitchell said there would be no service interruption as a result of the outage. A 50-megawatt generator can supply enough electricity to power about 40,000 homes.