The nuclear power plant near Richland reconnected to the regional power grid Thursday morning after shutting down unexpectedly six days ago.
It is the Northwest's only commercial nuclear power plant. It provides some of the electricity for 92 Northwest utilities, equivalent to the power that would be used by a city the size of Seattle.
The investigation into the issue that caused the Columbia Generating Station to scram, or unexpectedly shut down, continues.
There has been no threat to public health or safety.
One of the plant's main power transformers automatically disconnected from the transmission system following a grid disturbance May 18, according to Energy Northwest, the public agency that operates the plant.
The transformer's protection system detected an issue and tripped, which caused the main generator also to trip and the plant to stop operating.
A part of the transformer that tripped has been sent for analysis, said John Dobken, Energy Northwest spokesman.
No information has been released on how many days it will take the plant to return to full production. It has 1,207 megawatts of gross capacity.
The downtime was used to perform maintenance on plant equipment, including working on valves, which cannot be done when the plant is operating.
The plant's most recent previous scram was in August, when plant operators shut the plant down after an air removal valve in the plant's turbine building closed. It caused a loss of vacuum pressure in the system that turns steam back into water for reuse of the plant.
That led to a brief decrease in the water level in the reactor.
Before that, the plant shut down unexpectedly in December 2016. The scram was in response to a problem caused by cold weather at the Bonneville Power Administration's nearby Ashe Substation.