At about 10:20 a.m., an inexplicable voltage dip caused an electrical switching station to catch fire, Tesoro spokesman Mike Marcy said.
The refinery reported a partial power outage at about 10:30 a.m., which caused several units to shut down quickly and flares to be activated, the Contra Costa Hazardous Materials Program reported.
Power was lost to several of the processing units, which turn crude oil into refined fuels, Marcy said.
An employee suffered minor injuries after using a fire extinguisher to put out the electrical fire. He suffered a minor burn on his hand and was treated at the scene before returning to work, Marcy said.
Officials issued a Level II warning because of the amount of smoke coming from the flares.
"It looked like a smudge on the horizon above the flare," Marcy said.
The warning serves as an advisory to let people know there is an incident at the refinery and to warn those with respiratory conditions to take precautions, but it does not require anyone to shelter in place.
Hazardous materials specialists and refinery personnel monitored air quality near the plant during the incident, but did not detect any off-site impact, Contra Costa Hazardous Materials Program Specialist Mike Wedel said.
Power was partially restored at about 11:10 a.m., and with the ensuing flaring, the county warning level was downgraded to Level I at about 11:50 a.m., Marcy said.
The refinery is continuing to operate at reduced levels, but customer supply of transportation fuels will not be impacted, Marcy said.
The partial outage today followed a total power outage that happened about a month ago at the same refinery.
The outage, which happened Nov. 10, sent a huge plume of black smoke into the air over the refinery and prompted a shelter-in-place warning for residents in Avon, Clyde and North Concord.
Contra Costa Hazardous Materials Program Director Randy Sawyer, however, said that no health impacts were reported as a result of that incident.
The Golden Eagle refinery, located at 150 Solano Way between Martinez and Concord, is the second-largest oil refinery in Northern California and has a crude oil capacity of 166,000 barrels per day.
The plant produces gasoline, diesel, heavy fuel oils, liquefied petroleum gas and petroleum coke, according to the company's website.
Tesoro Corp. is headquartered in San Antonio, Texas.