2011 Southwest Blackout on paper

Ten years ago, a widespread power outage affecting the southwestern region of US and the northern part of Baja California, Mexico occurred. Particularly affected was the San Diego area as well as Imperial Valley, and Tijuana in Mexico. It was considered the largest outage in history to affect Southern California.
At the time of the 2011 outage, the San Diego area only had one 500 kV transmission line which originated in Arizona providing the majority of electrical power to the region. There were (and still is) no other interconnections at the 500 kV transmission level between San Diego Gas & Electric (SDG&E) and other adjacent utilities like Southern California Edison (SCE). Rather, the only other major route for power to the San Diego region is Path 44, a set of five separate 230 kV transmission line circuits which interconnected with SCE through San Onofre Nuclear Generating Station. A 500kV line interconnecting SCE with an existing SDG&E power line which ran along the Interstate 15 corridor to Escondido through the N. San Diego county community of Rainbow was proposed in 2001 but opposition from residents of the areas the new transmission line would be routed through resulted in the project being abandoned a few years later.
A mistake during maintenance at the North Gila Substation near Yuma Arizona lead to the accidental shutdown of an incoming 500kV line from the Hassayampa Substation located by the Palo Verde Nuclear plant in Tonopah, Arizona. With that line disconnected, most of the power to the San Diego region was then rerouted through SCE's transmission system. At that time, the San Diego region was importing more power than what could be supplied through San Onofre's Switchyard and this would progressively get worst throughout the event; eventually resulting in a safety system activating which disconnected Path 44 lines leading into San Diego.

[@paultaylorbikes]

Very nicely explained. NERC/FETC put out a report and lesson learned