2011 Southwest blackout
The 2011 Southwest blackout, sometimes referred to as the Great Blackout of 2011, was a widespread power outage that affected large areas of Southern California as well as western Arizona and northern Baja California and Sonora. It occurred on Thursday, September 8, 2011, beginning at about 3:38 pm PDT. It was the largest power failure in California history.
The electric transmission system owned and controlled by San Diego Gas & Electric (SDG&E, a subsidiary of Sempra Energy) is connected to Arizona by the Southwest Power Link, which is a single 500 kV power line (afterwards, SDG&E built a second line, but this connects in the Imperial Valley and the Arizona section is still a single line). There is no connection at the 500 kV level between SDG&E and either Southern California Edison (SCE) or the Los Angeles Department of Water and Power (LADWP). Rather, they are both connected to the San Onofre Nuclear Generating Station (SONGS). The Imperial Irrigation District (IID) has a 92 kV system connected to both.
Five utilities were affected: SDG&E, serving San Diego County and parts of southern Orange County and Riverside County; IID, serving the Imperial Valley; the portion of Comisión Federal de Electricidad (CFE), Mexico's electric utility, serving Baja California; Arizona Public Service (APS); and the Western Area Power Administration's Lower Colorado system (WALC). The blackout left nearly seven million people without power, including 1.4 million customers in San Diego County and 1.1 million customers in Mexico. The outage was the result of 23 distinct events that occurred on 5 separate power grids in a span of 11 minutes. Federal, regional and local officials are investigating what happened and why the outage cascaded the way it did. APS's North Gila Substation reported power loss at 3:27 pm PDT. Within seconds a portion of a Mexico power plant shut down, but there is no indication that Arizona impacted Mexico. Units 2 and 3 at SONGS automatically tripped offline due to a "grid disturbance" which initiated the plant's Emergency Feed Actuation System (EFAS).
The hardest hit region of the blackout, the San Diego-Tijuana metropolitan area, was essentially brought to a standstill. Surface streets became gridlocked due to the loss of traffic signals, and the San Diego skyline went dark. The San Diego Trolley system was shut down as there was no power to operate signal lights and related functions, and trains were stopped in Los Angeles for the same reason. Citizens in Tijuana and in inland areas like the Coachella Valley stayed outdoors late into the night to escape the heat. Freeways in the Southern California megalopolis experienced extreme clogging, especially on the I-15 and I-5 corridors between southeastern Greater Los Angeles and the San Diego area's North County. One hospital was left without power for two hours when its backup generator failed. Only a few areas in the region, such as Blythe, California, in the Palo Verde Valley, were not affected by the outage.
Eleven hours after the outage began, power was restored to 694,000 of the affected customers, and by 4:30 am on September 9 power was restored to all customers, although the system was described as "still fragile". As a precaution, all public schools in San Diego County and the Capistrano Unified School District in southern Orange County were closed on September 9. Most major universities and community colleges, as well as all federal courts in San Diego, closed for the day as well.
The outage caused significant losses to restaurants and grocery stores, which were forced to discard quantities of spoiled food; perishable food losses at grocery stores, eating establishments and households were estimated at $12 million to $18 million. The outage also caused some sewage pumping stations to fail, resulting in contaminated beaches and potentially unsafe water supplies in several areas. As a precaution, in some neighborhoods, residents were told to boil their water or use bottled water for several days after the outage. Due to the failure at the sewage pumping stations, diesel generators have been installed at five pumping stations.
Question of association with terrorism
The outage occurred days before the tenth anniversary of the September 11 attacks, and hours before the United States Department of Homeland Security warned of a potential terrorist attack leading up to the anniversary, so that a first reaction to the blackout was to wonder if it might be the result of an attack. However, the Federal Bureau of Investigation and SDG&E ruled out terrorism early in their investigation, and no subsequent evidence was found to suggest that the outage was anything other than accidental.
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Due to a mistake by a technician, a 500 kilovolt line was cut between APS's Hassayampa and North Gila substations in Arizona. This transmission line is part of the Southwest Power Link. With this missing, the 500 kV system went from near San Diego to near Yuma, but didn't connect to anything else.
APS thought that they could have the line reconnected quickly (it was not broken, it just needed the right switch closed). In fact, the line opening had caused a large phase difference in the grid. Due to this phase difference, the line could not be connected.
Most of the power rerouted to going via the Los Angeles area through the SONGS switchyard. At this point SDG&E's system was taking more power from SONGS than it was producing, however this would get worse over time as things progressed.
The phase shift also caused big trouble on IID's and WALC's systems. In less than one minute, two of IID's transformers had overloaded and disconnected. This caused severe low voltage on WALC's system. Several minutes later, a third transformer tripped off, causing the bulk of IID's system to be disconnected from SCE to the north. This caused drastic voltage problems which resulted in a loss of about half of IID's load as well as some generation.
Similar transformer overloads caused the Yuma area to be disconnected from WALC's system to the north. Now the only supply was the 500 kV line to Imperial Valley. One more transmission line tripped out, which was the last connection east of SONGS, between WALC's system to the north and SDG&E's system, CFE's system, and the Yuma area to the south.
Now what remained of the IID system had only one connection to what remained of the 500 kV system. This line overloaded as well. Instead of just cutting that line, their scheme commanded two generators in Mexico to go off-line. This was to solve a problem that no longer existed, and in fact didn't do anything useful, however it did make the overall problem worse. Then the line did trip off, and most of IID's remaining load was lost.
Now the power through SONGS was very high (around 170%), and a "safety net" system built in to the power plant operated, and cut off SDG&E. Now SDG&E's system, CFE's system, and the Yuma area were totally separated. This island was very deficient in generation and rapidly spun down. Load shedding throughout this system operated in rapid fire, but some generation was lost as well, and it was not enough. In seconds, this system broke into three islands, all of which then collapsed.
As it happens, both SONGS units shut down, although this didn't affect anything.
On April 27, 2012, the US Federal Energy Regulatory Commission and the North American Electric Reliability Corporation issued a joint report analyzing the technical details of the blackout and giving 27 findings and recommendations to prevent a recurrence. The report found problems in operations planning and situational awareness.
Some of the findings are:
The event showed that the system was not in an "N-1" state. Utilities are required to operate the system so that the failure of one component can not cause instability, separation, or cascading.
Next-day plans largely did not match the system at the time. For example, some utilities had plans based on a "heavy summer" scenario. This is the worst case in terms of load, but it does not consider that several generating stations may be out of service for maintenance.
Components with a voltage of 92 kV, which is a large part of IID's system, were not included in simulations. The simulations included components with a voltage of 100 kV or higher. Typically lower-voltage components (e.g. 69 kV) can be ignored.
Protection schemes were not considered in their effect on the bulk power system. These are supposed to be reviewed; in fact, some of them were not really considered in terms of their effects. The "S line" scheme was intended to protect one of IID's transformers, but in fact that transformer was not overloaded. The fact that it disconnected generation was not helpful, and without this the blackout would not have occurred. Also, the results of the SONGS separation scheme were not really considered. It was thought to be for an extreme case that was unlikely to really occur.
The report gives some comments on large phase-angle differences in the power grid, and what can be done to detect and deal with such problems. According to their power flow simulations, it would have been difficult to reconnect the transmission line even with a significant shift in generation. In the event, there was not sufficient time to fix things this way.
There is also included some comparison with the 2003 Northeast blackout. Many of the same factors contributed to both events.
- 1996 Western North America blackouts
- California electricity crisis
- Northeast blackout of 2003
- Path 46, also called West of Colorado River, Arizona-California West-of-the-River Path (WOR)
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