The fire Wednesday night at the Donald C. Cook Nuclear Plant in southwest Michigan also resulted in a brief activation of the site's emergency plan, the plant's owner, American Electric Power Co., said in a news release.
A security officer was treated for smoke inhalation.
The transformer, which is outdoors and adjacent to the plant, is used to increase the voltage of the plant's generator for more efficient long-distance transmission of electricity.
When the transformer failed, the plant's operating system automatically shut down the Unit 1 reactor, which was operating at full power. All safety systems responded appropriately, and the reactor was not damaged, AEP said.
The plant's fire brigade extinguished the fire in 35 minutes, and the plant notified the Nuclear Regulatory Commission and state and local emergency-response agencies, AEP said.
The fire's cause remained under investigation.
