The nuclear power plant near Richland shut down unexpectedly at 6:51 a.m. Friday.
The Columbia Generating Station's main power transformers automatically disconnected from the grid, and the plant then automatically shut down, said John Dobken, spokesman for Energy Northwest.
The 1,207-megawatt plant is the only nuclear power plant in the Pacific Northwest and produces enough electricity to power a city the size of Seattle.
"The plant responded as designed" and is stable, Dobken said. There is no risk to the public.
Officials were working on Friday to determine the exact cause of the problem and thoroughly understand the issues involved.
By evening they had isolated the issue to one of the plant's transformers. It has three plus a spare.
No estimate of when the plant will return to full power has been made public.
Bonneville Power Administration, which distributes electricity from the nuclear power plant, "is well positioned to respond to the loss of generation," said BPA spokesman Kevin Wingert. "We are taking steps to ensure no impact to customers."
The plant last had a unexpected shutdown, or scram, in August.
An earlier unexpected shutdown in December 2016 was related to the electricity distribution system.
Cold weather caused the loss of the 500-kilovolt line connecting the nuclear plant's main output transformers to the substation.