Pacific Gas and Electric Co. erroneously ordered a reduction in energy supply from Ares Owners Holdings LP's Panoche Energy Center in Fresno County, Calif. — a mistake the California ISO said factored into its call for rolling blackouts on Aug. 15.

CAISO also identified the breakdown of AltaGas Power Holdings (U.S.) Inc.'s Blythe Energy Project I in Riverside County, Calif., on Aug. 14 as a contributing cause of that day's rolling blackouts.

In an Aug. 17 meeting, CAISO management briefed the grid operator's Board of Governors on the blackouts, noting the ramp down of one gas-fired power plant and the outage of another without naming the facilities. Consumer groups and others subsequently called for CAISO to identify the plants at issue, and that call was answered Sept. 11.

"After legal and operational review, the ISO is providing the background information about the two units referred to during the Board of Governors meeting and data on all units that were on partial or full outage at any point in time during the intense heat wave of Aug. 13-16," the grid operator said in a fact sheet on its outage report.

CAISO said the 494-MW Blythe facility was operating at full capacity until it went offline due to "plant trouble" on Aug. 14 at 2:57 p.m., and the 412-MW Panoche plant, for which Pacific Gas and Electric, or PG&E, acts as scheduling coordinator, unexpectedly ramped down from 394 MW to about 146 MW at 6:13 p.m. the following day.

"The ISO has been informed by the scheduling coordinator that the unit ramped down quickly because of an erroneous dispatch from the scheduling coordinator to the plant," the grid operator said.

PG&E spokesman James Noonan in a Sept. 15 email said "PG&E does not know if the error resulted in rotating outages."

Noonan also said CAISO decides whether utilities must implement rotating outages based on numerous factors, including the status of hundreds of generating sources in California as well as thousands of megawatts of energy imported from outside the state.

"The dispatch message in question was intended to bring a different power plant to its full generating capacity, but was sent in error to the Panoche Energy Center," Noonan said. "PG&E remained in close coordination with the California Independent System Operator throughout the statewide energy capacity event, and when the error was highlighted, took the steps to correct it immediately."

In a company statement accompanying Noonan's email, PG&E said the erroneous verbal dispatch order resulted in ramping down Panoche's output for less than 30 minutes. "The drop represented roughly 0.5% the total CAISO load of 44,913 MW for that time period," PG&E stated.

The plant then was immediately ordered to return to full generating capacity, where it stayed for the rest of the day, the utility said.

PG&E is a subsidiary of PG&E Corp.