When a transmission line in Arizona carrying bulk power to San Diego went out Sept. 8, the weak links in the transmission system between Arizona and San Diego were facilities operated by Imperial Irrigation District. Twelve percent of the power that the 500 kV Hassayampa-North Gila transmission line normally carries was rerouted onto the district’s 92 kV system, overloading transformers and other facilities, according to a federal investigative report into the causes of the ensuing blackout that 11 minutes later left 2.7 million utility customers without power in Southern California, Arizona, and northern Mexico. “The weaknesses may have been in the Coachella Valley,” said Steve Berberich, California Independent System Operator chief executive officer, commenting on the federal inquiry. Imperial Irrigation District general manager Kevin Kelley maintained the district’s facilities in the Coachella Valley “performed as they should have and were, in fact, needed to prevent serious damage from excessive power flows surging through the system.” He added, though, that in the wake of the outage the district has raised the trip settings on the facilities to allow for higher flows during emergency conditions and also is adding another transformer at its Coachella Valley Substation to beef up capacity But what happened after the Coachella lines shut down, according to the May 1 report by the Federal Energy Regulatory Commission and North American Electric Reliability Corp., was that rather than containing the outage, a never before employed San Onofre Nuclear Generating Station separation scheme spread the blackout. Ultimately, according to the report, it was that automated separation that triggered the massive spread of the outage from Imperial’s jurisdiction into heavily-populated San Diego and northern Baja California all the way to Yuma, Arizona. “There’s a need for more study about the purpose of that [separation scheme] and how it works with other parts of the system,” said Heather Polzin, FERC lead investigator on the report. After Imperial’s facilities failed, the amount of power flowing down to San Diego along Path 44 increased to the point that the line exceeded the trigger level under the automated separation regime, according to the report. While Path 44--which consists of five separate 230 kV lines south of San Onofre--can carry 3,100 MW of power, it’s programmed to automatically shut down when the flow exceeds 2,500 MW for more than a brief time, cutting off the flow of power from the north to San Diego. “We are looking at the separation scheme,” said Berberich. He said that after what happened last September the grid operator is examining modifying the scheme to either move the separation point further north, or modify the power level that triggers separation from its current 2,500 MW limit. If the separation point was moved north, Berberich explained, the nuclear plant would continue to supply San Diego after separation. Had that occurred, there’s a chance the area would not have suffered the massive outage since after it was separated from Path 44 it did not have enough local generation capacity to meet demand and quickly went dark, as did Mexico and Yuma. They also were in part being fed by power flowing along Path 44 after losing power from the Hassayampa-North Gila transmission line. Once in San Diego, some of the power from Path 44 flowed on lines along the U.S.-Mexican border all the way from the coast to Yuma. The federal report indicated that another way to have avoided the spread of the blackout would have been for the grid operator to shed load in San Diego after the cascade of events started, anticipating that it might trigger separation. The problem was, however, that the Western Electricity Coordinating Council had no way of knowing when flows were approaching the separation trigger level to call for action by CAISO. Meanwhile, CAISO operators, according to the federal report, “were primarily concerned with returning flows on Path 44 to below the path rating of 2,500 MW, but believed they had 30 minutes to do so.” In the end, though, the scheme allowed only minutes before the breakers on the five transmission lines automatically opened, shutting off the flow of power to the south.