2011 Southwest blackout
[5] The Southwest Power Link, a single circuit 500 kV transmission line, connected San Diego Gas & Electric (SDG&E, a subsidiary of Sempra Energy) to the Arizona Public Service (APS) system via the North Gila substation near Yuma, AZ[Note 1] No other 500 kV pathways connect SDG&E and adjacent utilities, but Path 44, a set of five separate 230 kV transmission lines, connected SDG&E to Southern California Edison (SCE) through the switchyard of San Onofre Nuclear Generating Station.
Western Area — Lower Colorado (WALC) typically exported power to each through analogous 161 kV lines to the Hoover Dam.
Across the border, Comisión Federal de Electricidad (CFE)'s Baja California system (connected to SDG&E, but isolated from the main Mexican electrical grid) was already operating at maximum generation capacity.
[6]: 20, 24, 26 Grid managers maintain stability through the "n-1" criterion: the system should remain within operating limits even if a single component fails.
Because the subsequent collapse took place over the next 11 minutes, the WECC took no action during the event to develop alternate transmission paths, generation schemes, or load distributions.
[6]: 30, 33 With the Southwest Power Link disconnected, the electricity flow from Arizona reconfigured itself in accordance with Kirchhoff's circuit laws.
This current overloaded two transformers at IID's Coachella Valley substation, which prepared to trip offline to prevent damage.
[6]: 31, 40–41 At 3:35 pm, WALC's system also began to struggle, first disconnecting from the Southwest Power Link at Gila, and then manually shedding load to the north at Parker.
[6]: 44–45 After these events, WALC's system stabilized, and would continue to supply power to Blythe in the Palo Verde Valley following the blackout.
At this point, a blackout was inevitable: the Path 44 lines could not sustainably carry so much current, and there was no time to tell the southern utilities to reduce load.
[6]: 49, 102 Without the La Rosita generators, current along Path 44 would exceed the San Onofre switchyard's automatic relay settings.
[10] According to investigators at the US Federal Energy Regulatory Commission and the North American Electric Reliability Corporation, problems included: Five utilities were affected: SDG&E, serving San Diego County and parts of southern Orange County and Riverside County; Imperial Irrigation District, 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).
[12] Freeways in the region experienced extreme traffic congestion, especially on the I-5 and I-15 corridors between southeastern Greater Los Angeles and the San Diego area's North County.
[14] The outage also caused some sewage pumping stations to fail, resulting in contaminated beaches and potentially unsafe water supplies in several areas.
