This morning, power was restored to almost 4 million residents in Arizona, Southern California, and northern Mexico after a major blackout swept through the area yesterday afternoon. The blackout lasted for approximately 12 hours. It not only forced San Diego’s airport to close, but also caused the automatic shutdown of two nuclear power plants – San Onofre (near San Diego) and Diablo Canyon (near San Luis Obispo).

The widespread power outage started yesterday at approximately 3:30pm, when an Arizona Public Service (APS) employee was completing some maintenance work on a substation near Yuma, Arizona and tripped a 500 kilovolt (kV) line. This managed to knock out power across parts of Arizona, Southern California and Mexico, raising concerns regarding the safeguards that were supposed to prevent this type of blackout from extending through such a large geographic area. According to San Diego Gas and Electric President, Mike Niggli:

“To my knowledge this is the first time we’ve lost an entire system.”

The automatic shutdown of the San Onofre nuclear power plant appears to have resulted from a “high current trip” that caused the automatic detachment of the power plant from the transmission grid for safety reasons. Presumably, this trip was caused by the surge in demand when the 500 kV Arizona line went down. When Southern California could no longer receive power from Arizona, the demand for power from the San Onofre plant likely surged, causing the voltage at the plant to drop as the power demand surged. The sudden nature of this shift indicated a problem in the system, causing the plant to detach from the grid to protect itself. This triggered an automatic shutdown of the plant to maintain the plant’s physical integrity. It is estimated that the plant will be down for at least 2 days.

Specific details on the shutdown at the Diablo Canyon nuclear power plant have not yet been released. But, it is likely that the same demand surge that impacted the San Onofre plant to shutdown affected the Diablo Canyon facility as well.

The following photo was sent to me from a friend in San Diego. It shows a rare sight - San Diego without any lights, except from the airport's runways, which stayed lit throughout the evening.


Photo Credit:

1. Picture of transmission lines and pole by Ken Lund and used under this Creative Commons License.

2. Photo of San Diego during blackout, showing airport runway lights (c) John McCarthy and used with his permission.