At the time of the blackout, three separate, parallel transmission pathways connected Southern California to generating stations in Arizona. Excess power was then sold on to Baja California , which had disabled MW worth of generation equipment for maintenance. In an overload event, the San Onofre switchyard was designed to disconnect the offending lines, but such disconnection had never occurred before.
Neither the Western Electricity Coordinating Council WECC , in charge of the system's reliability; nor the California Independent System Operator CAISO , which coordinates the state's power generation, had either studied the effects of a disconnection or established alarms to indicate that the breakers were at risk of operating.
Grid managers maintain stability through the " n -1 " criterion : the system should remain within operating limits even if a single component fails. Unless many components are run close to their operating limits, this practice typically ensures grid operators have enough time to react to failure.
Because IID did not require active human scrutiny of the n -1 condition at the time, IID did not attempt to redistribute power and reduce the risk. The outage was the result of 23 distinct events that occurred on 5 separate power grids in a span of 11 minutes. The technician sent to disconnect the capacitor bank accidentally skipped a critical step which involved bypassing the transmission circuit around the capacitors.
Attempting to disconnect the capacitor bank under load resulted in an arc forming across the switches as they were being opened. At pm, the arcs from two phases crossed, resulting in a phase-to phase short which caused an upstream oil circuit breaker to trip, protecting the substation from damage. However, a large transfer of charge that occurred when the line opened caused one of the transmission lines to shift out of phase , inhibiting reconnection.
