"The focus of the investigation is to ensure the safe operation of the refinery, to determine whether last week's incident damaged any equipment, and if so, ensure that there are appropriate repairs made," said Cal/OSHA spokeswoman Erika Monterroza.
No Valero workers were injured in the incident, but Cal/OSHA does occasionally probe refineries after significant safety incidents.
The state's investigation comes as local air regulators conduct their own investigations into Valero's flaring.
Since Friday's outage, the Bay Area Air Quality Management District has issued six notices of violation to the refinery for excess emissions and public nuisance.
And, for the first time in 18 months, Solano County is investigating the refinery.
Hazardous materials specialists at the county's Department of Resource Management interviewed Valero officials about the incident on Wednesday, according to its director, Terry Schmidtbauer.
"The main thing is, how does a plant lose complete power?" Schmidtbauer said in an interview Wednesday.
Pacific Gas & Electric says the outage began shortly after crews took one of two transmission lines offline to complete upgrades.
Valero says it does not have a backup power source because it never got a permit to create a co-generation unit -- an addition that the company says would increase its carbon footprint.
The last time Solano County conducted a probe of Valero was following a petroleum release in December 2015 after an above-ground tank at the facility became corroded.
The county has inspected the refinery five times in the last 2½ years, said Schmidtbauer.
For Friday's incident the company is required to file a follow-up report to the Governor's Office of Emergency Services within 30 days, said Robb Mayberry, a Cal OES spokesman.
Meanwhile, Benicia officials are conducting a top-to-bottom review of their emergency responses after the outage revealed weaknesses in the way the city communicates with its residents about potential disasters.
Environmentalists say the flaring is another reason for stricter oversight of the oil industry.
"The black clouds of pollution from this latest accident are a vivid reminder of how dangerous these refineries are to nearby communities," said Hollin Kretzmann of the Center for Biological Diversity.
"This dirty industry has a long and disturbing history of violating our clean air protections," said Kretzmann. "We need to crack down on these accident-prone refineries immediately and rapidly shift toward cleaner, safer energy."
The outage is expected to increase the cost of gasoline in the Bay Area, according to Denton Cinquegrana, chief oil analyst at the Oil Price Information Service, who's been watching gasoline market trades in reaction to Valero's shutdown.
"Prices have been falling off as of late, but now they're starting to move back up based on the event," Cinquegrana said. "I do believe that you're going to start to see that kind of make its way downstream and out to the street level."
Flaring incidents at Chevron's Richmond refinery, the Phillips 66 facility in Rodeo and Shell's Martinez plant sent local gas prices up in January.