The shutdown of a nuclear plant near San Diego helped halt a massive blackout last month that cascaded through Southern California, Arizona and Mexico and left millions without power.

But the Sept. 8 outage could have been worse if safety equipment near Southern California Edison's San Onofre Nuclear Generating Station hadn't detected trouble and shut down the reactors, said Stephanie McCorkle, a spokeswoman for the California Independent System Operator Corp. (ISO).

"If it didn't separate, then we could have impacted the rest of the California grid," McCorkle said in an interview. "The nuclear plant did what it was supposed to do; it could have been worse."

Relays in the switchyard separated failing power lines between Southern California Edison and San Diego Gas & Electric and protected customer load in Northern California, McCorkle said (Greenwire, Sept. 12). At least one of the two reactors has since been brought back online.

But utilities probing the blackout have yet to reveal what triggered the outage, why safety equipment on the grid didn't stop the problem from spreading and whether better standards are needed to ensure grid operators can communicate during such crises.

The 500-kilovolt Southwest Powerlink tripped at 3:27 p.m. local time when an Arizona Public Service (APS) employee was doing line maintenance, but it's not clear whether the line work started the outage.

The probe is also focusing on the 11 minutes after the Southwest Powerlink failed, when as many as 23 "events," or power outages, transmission failures and generation failures occurred on five separate grids, McCorkle said.

Federal and regional reliability agencies, utilities and grid operators are constructing a timeline of events on the day of the blackout, when temperatures and electricity demand soared. Generation in Mexico may have tripped offline 20 seconds after Southwest Powerlink was lost for completely unrelated reasons, McCorkle said.

The Federal Energy Regulatory Commission (FERC) and the North American Electric Reliability Corp. (NERC) are conducting a separate joint inquiry. McCorkle said it could be months before the reports on the outage are released (Greenwire, Sept. 12).

"This is a very, very complex blackout to unravel and dissect," McCorkle said. "We've lost the Southwest Powerlink several times in the past. Losing the transmission path doesn't take down the grid, so there are other events that happened."

Scant reaction time for ISO

Anjan Bose, a professor of electrical engineering at Washington State University, said power from the failed 500-kilovolt line surged onto parallel lines running from Arizona to California that could handle only lower voltages.

"Because it was such a hot day, there was a lot of power pulling on the 500-kV line, and when that tripped, that power went onto the other, lower-voltage lines," he said in an interview. "Usually that's not enough to carry that."

San Diego, which doesn't have sufficient local generation, imports power through transmission lines reaching into Arizona and Northern California, Bose said. Those lines became overloaded and tripped offline, he said.

A lag of about 11 to 12 minutes occurred between the outage of the 500-kilovolt line and the rest of the power grid because it took time for the safety equipment on connecting power lines to detect the anomalies, he said.

"What happens is the lower-voltage lines get overloaded, and that sticks around for some time. And then the automatic protection takes it out before the line burns out," he said. "It took about 11, 12 minutes before enough of these lines got taken out when San Diego got isolated and there wasn't enough power there to meet the load and it just blacked out."

It's not clear, Bose said, why utilities didn't manually or automatically shed loads or create localized brownouts to stop the outage from spreading.

Inquiries may reveal whether the correct procedures were in place, whether grid operators reacted correctly and whether automatic relays should have sensed the frequency anomaly and shed load, Bose said.

The region's grid operators had little real-time data about what transmission lines or power plants were tripping, McCorkle said. The inability to receive an early warning left the ISO only minutes to use protocols and bring up generation, she said.

"If we know what's tripping off in other balancing authorities ... it can certainly tell us the severity of the issue," she said. "You need to see this stuff, and you need to see it quickly. You need it to come in as fast as electricity travels. And that didn't happen that day."

Transmission needs

Experts say building up the transmission system and local generation in Arizona and Southern California could add extra protection against a future outage.

McCorkle said the region will be better off once San Diego Gas & Electric builds the 120-mile Sunrise Powerlink transmission line from Imperial County to San Diego (Greenwire, June 21). The $2 billion project is scheduled to be completed by mid-2012.

"While it is unclear if the Sunrise Powerlink would have helped the situation on September 8, the ISO identified the need for additional transmission in Southern California years ago," she said. "It's a notorious area of the North American electric grid where transmission has been deficient."

Bose agreed the area between Arizona and Southern California is notorious for its insufficient transmission. Having more high-voltage transmission in the corridor could have prevented such an outage, he said.

But Jim Sahagian, Sempra Energy's vice president of commercial development, said a tripped line, not a lack of capacity, caused the problem.

"If you think of it like you have energy flowing in from different sources, and one of the main arteries where that energy is flowing into a load center gets stopped, you still have demand, and if other sources aren't able to keep up with it, you get a sag," he said.

Nonetheless, he said, grid upgrades will help.

"The San Diego area is a major metropolitan area that doesn't have that many external transmission lines coming into it," he said. "I think the Sunrise Powerlink is certainly going to help make it a more reliable region in terms of energy supply."

Reporters Anne C. Mulkern and Debra Kahn contributed.