On May 1, 2012, FERC and NERC released a joint report detailing the events that led to the 2011 blackout in the Southwestern United States that left 2.7 million customers without power.  The report called for transmission operators and balancing authorities to improve coordination and planning.

The report details how the blackout started with the loss of a single 500 kV transmission line owned by Arizona Public Service (“APS”).  The loss of this line was not the sole cause of the blackout, but it did trigger a series of cascading events that culminated in the blackout.  The report explains that the system had safety measures in place to deal with the possibility of losing a large transmission line, but this line tripped on a day during peak demand hours.  Coincidentally, San Diego and Mexico were experiencing lower levels of generation.  Once the APS line was lost, the power was redistributed to lower voltage systems.  The redistribution caused large voltage deviations that led to equipment overloads.  Three overloads were on major 230/92 kV transformers at the Coachella Valley (“CV”), Ramon substations, Western Electricity Coordinating Council (“WECC”) Path 44, located south of the San Onofre Nuclear Generating Station (“SONGS”).

The flow redistributions and resulting overloads led to more shut-downs of transmission lines and generating units.  This eventually caused automatic load shedding throughout Southern California and Baja in a relatively short time span.  Just before the blackout, all of the power to San Diego and parts of Arizona and Mexico were redistributed to Path 44.  The excessive loading on Path 44 initiated an intertie separation scheme at SONGS and eventually resulted in the complete blackout of San Diego and Baja.

The report identified the following failures in “operations planning and real-time situational awareness:”

  1. Failure to update external networks in next-day study models;
  2. Lack of Real-Time External Visibility;
  3. Failure of WECC, affected transmission operators, and balancing authorities to consistently recognize the adverse impact sub-100 kV facilities can have on bulk power system reliability;
  4. Failure to recognize Interconnection Reliability Operating Limits and establish valid system operating limits; and
  5. Failure to study and coordinate special protection systems.

The report also made several recommendations for WECC to improve planning and preparing for external contingencies.  WECC is the regional entity designated by NERC to works to improve the reliability of the bulk power system.

A copy of the joint NERC-FERC report on the 2011 blackout is available here.