According to news reports, the blaze created thick, black smoke that could be seen for miles at a Georgia Power plant in the Atlanta suburb of Smyrna.No one was injured in the fire.
John Sell, a Georgia Power spokesman, said a transformer malfunctioned, spilling about 5,500 gallons (21,000 litres) of mineral oil over a switchyard that caught fire.
Firefighters put out the blaze, but some oil spilled into the nearby Chattahoochee River, he said.
Hazardous materials crews were expected to be on the scene throughout the night collecting oil from the water.
"We think as much as 200 gallons (760 litres) might be in the river," Sell said. He said the company had placed a boom in the river and pads in drains and other areas around the plant to prevent any more oil from spilling.
Georgia Power is a unit of Atlanta-based Southern Co .