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Arizona power company baffled by events that led to outage

September 9, 2011 |  1:20 pm

Click here for more photos of the blackout. Officials from Arizona's largest electric utility said Friday they are still baffled by the chain of events that resulted in massive blackouts across Arizona, California and parts of Mexico.

APS has said they believe an employee at the North Gila substation tripped a 500-kV transmission line that runs from Arizona to Southern California. They are investigating how that happened and the subsequent failure of safeguards that should have prevented major outages.

PHOTOS: Blackout leaves millions without power

“We’re struggling,” said Daniel Froetscher, Vice President of Energy Delivery for APS. “We have to take a hard look at the system design and figure out exactly what happened.... We don’t know the underlying causes.”

Early Thursday, APS officials noticed an issue with a series capacitor at the North Gila substation outside the city of Yuma, Froetscher said. The capacitor — a piece of equipment about the size of a small car that helps the utility manage voltage — wasn’t functioning properly. APS personnel were dispatched to take it offline.

Typically, the utility can shut down an individual capacitor and reroute power without any disruption of service, Froetscher said. But this time, something went wrong. After the North Gila capacitor was taken offline, the 500-kV line that runs through the substation went down.

At that point -- 3:27 p.m. -- the grid should have compensated for the loss of the 500-kV line, which runs from Yuma to the Imperial Valley and San Diego, Froetscher said.

“The intent is for the system to automatically open and close different breakers and relays and switches to reroute power from the line that was lost to other lines to continue to provide service,” he said. “Transmission lines do go out of service on an unplanned basis; it’s not an unusual event.... Most times when a line goes out unexpectedly, the system performs exactly as it’s expected to and customers never know the difference.”

For about 10 minutes, the system seemed to be working properly. But by 3:38 p.m., residents in Yuma began to lose power.

From there, outages cascaded across the Southwest, causing interruptions for utilities in Southern California, Froetscher said. But like APS, utilities in those areas typically use a variety of power sources to compensate for the loss of a line. It's unclear why those protocols didn’t function properly.

Froetscher said it's possible there were errors made by APS both in the work done on the capacitor and in how it reacted to the outage.

“The bigger issue that will be the focus of many people’s work, is what really was the contributing cause to the outage in Yuma and the subsequent outages in California?” he said. “It would be awful speculative and premature at this point for me to guess.”

Rebecca Wilder, a spokeswoman for the Arizona Corporation Commission, which regulates electric utilities in the state, said the agency was monitoring the APS investigation.

“We can’t speculate as to what caused the widespread outage,” she said.

Stephanie McCorkle, a spokeswoman for the California Independent System Operator, which operates most of the state’s electric grid, insisted that it’s too early to even theorize about the cause of the San Diego power outage or its size.

“Someone who comes to conclusions quickly doesn’t know what he is talking about,” she said, noting that a similar massive outage struck parts of California on Aug. 10, 1996, with the loss of a 500-kV high-voltage transmission line from the Pacific Northwest that is similar to the Southwestern Power Link that went down Thursday in Arizona.


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-- Sam Allen in Los Angeles and Marc Lifsher in Sacramento

Photo: APS North Gila substation is the site where an employee tripped a 500-kV transmission line. Credit: Nicole Santa Cruz / Los Angeles Times