Federal energy regulators on Tuesday blamed inadequate planning and lack of grid coordination for a massive blackout last year that started with the loss of a single transmission line at an Arizona substation and quickly affected millions of people in Southern California and Mexico's Baja California.
The Sept. 8 outage that lasted through the night shows the need for close communication between major utilities and smaller operators, and for everyone to update their emergency settings, officials said.
The blackout was caused by many of the same problems that triggered a 2003 outage that cut power to 50 million people in the Midwest and the Northeast, regulators said in a teleconference from Washington.
It was another reminder that U.S. transmission lines remain vulnerable to cascading power failures despite the safeguards that are in place.
The system is built to quickly compensate for glitches _ like the loss of a single line _ but a lack of understanding of what was happening between neighboring operators caused the problem in Arizona to balloon into something much bigger than it should have been, according to the joint report by the Federal Energy Regulatory Commission and North American Electric Reliability Corp.
When a maintenance worker caused the loss of the line operated by Arizona Public Service, it immediately overloaded adjacent systems stretching to California and northern Baja California.
The outage knocked out traffic lights, causing gridlock on roads in the San Diego area. Two reactors at a nuclear power plant along the California coast went offline after losing electricity, and nearly 3.5 million gallons of sewage spilled into the ocean, closing beaches in San Diego.
People on both sides of the U.S.-Mexico border spent the night struggling to fall asleep in high temperatures that reached 115 degrees in desert areas. There were no reported deaths or injuries related to the outage.
"This report highlights the growing need for more coordination of grid operations in the West," FERC Chairman Jon Wellinghoff said in a statement.
Federal officials conducted an eight-month inquiry to determine the cause.
During an 11-minute time span after the Arizona transmission line tripped, some outside operators "observed changes in flows into their systems but were unable to understand the cause or significance of these changes and lacked sufficient time to take corrective actions," according to the report.
Regulators said a separate process looks at whether any utility might have been in violation of rules, which could result in a hefty fine. That process is confidential and regulators declined to say whether any utilities were being investigated in the 2011 blackout.
Gerry Cauley, president and CEO of the North American Electric Reliability Corp., said the outage underscored the complexity and interdependence of the North American electric system and the importance of information sharing and system studies.
In 2005, a major outage struck the Los Angeles metropolitan area. That same year, Congress required utilities to comply with federal reliability standards for the electric grid instead of doing self-regulation. Layers of safeguards and backups were intended to isolate problems and make sure power keeps flowing.
But some of those safeguards _ such as measures that automatically kick facilities offline when there is an overload of power _ also may have been set too strictly in this case and need to be revised, the regulators said.
As lines were knocked out, power flows jumped to levels that matched or exceeded the standards and led to the San Onofre Nuclear Generation Station being taken offline, which caused San Diego and much of the surrounding county to go dark.
Mexico also took power plants offline to contain the blackout.
Federal regulators found if the emergency settings had allowed for a higher level of power flows, it could have given operators time to mitigate the problem and the outage could have affected a much smaller area.
Similar relay problems contributed to the 2003 blackout, said NERC Senior Vice President Dave Nevius.
Steve Berberich, CEO of the company that operates the state's wholesale power system, the California Independent System Operator, said many of the issues pointed out in the report were identified by a task force involving the affected utilities and are already being addressed.
"I think it's a good report," he said. "I think it has confirmed many of the things that we already knew."
Berberich said the company is looking at changing the emergency settings on the transmission line at San Onofre. He said the relay is designed to prevent blackouts in San Diego from spreading to Los Angeles, and vice versa. The emergency settings also protect equipment. Nothing was damaged in the 2011 outage.
Officials, though, are looking at what can be done so emergency settings do not knock systems offline unnecessarily.
The grid operator has said it is taking additional steps to prevent power outages in the San Diego and Los Angeles areas this summer. Federal regulators say keeping the lights on will be challenging if both reactors remain off line through the hot months.