Five contractors were working in an underground penstock, or pipeline, at the Cabin Creek Station, where the fire broke out. Xcel said the workers climbed to a safe distance from the blaze and were communicating with plant operators by radio.
The 48-inch diameter pipe was opened to get air into the pipe, Xcel spokeswoman Ethnie Groves told reporters at a news conference.
Firefighters from the Clear Creek County Fire Authority were battling the blaze. The plant was shut down for routine maintenance at the time, the company said. The cause of the fire was not immediately known.
The pipeline is between 1,500 and 2,000 feet (460 and 610 meters) below ground and carries water from a reservoir to power turbines inside the plant.
The power station, which is west of Denver, has two turbines and a generating capacity of 324 megawatts.