As an aftereffect of the deadly San Bruno natural gas transmission pipeline blast in September, gas curtailments remain a possibility for Pacific Gas and Electric Co.'s (PG&E) largest customers in San Francisco and the peninsula region to the south, but a separate electricity milestone in the Bay Area Monday has helped ease that threat.

The issue has hovered as part of PG&E's ongoing state regulator-mandated activities in the aftermath of the 30-inch diameter pipeline rupture because of the continuing need to operate the failed pipeline (Line 132) and two similar transmission pipelines bringing supplies through the peninsula to San Francisco at reduced pressures (300 pounds per square inch gauge), meaning smaller volumes of gas can be summoned on cold winter peak-demand days (see Daily GPI, Oct. 27).

A lone natural gas-fired generation plant, Potrero Hill, operated by a unit of Atlanta-based Mirant Corp. in San Francisco represents nearly 60% of the interruptible gas load in the city, and it can be shut down nearly completely now without jeopardizing electricity reliability because of the completion of the Trans Bay power transmission cable at the bottom of San Francisco Bay. The direct current (DC) cable is a new conduit for delivering up to 400 MW to the city beginning this winter, assuming everything goes smoothly in integrating the line into the state grid operator's system.

A recabling project by PG&E and the submarine Trans Bay project mean the electric system capability will be "greatly enhanced," PG&E told the California Public Utilities Commission (CPUC) earlier this fall, meaning that if gas curtailments are required, they may not have to affect as many customers on the peninsula, since San Francisco's power plant should absorb a lot of the gas cuts.

An initial assessment filed with the CPUC by the utility indicated that with colder average temperatures and continuing to operate the peninsula pipelines at reduced pressures, curtailments are almost a certainty. However, those assessments also indicated that some raising of pressures and the cabling projects beefing up San Francisco's electricity infrastructure could ease the extent of gas shutoffs that eventually are needed.

"Because the Potrero plant's Unit 3 is 57% of the noncore load in San Francisco, and Unit 3 can be curtailed without impacting electricity supply, PG&E has begun working with both the [state grid operator] and Mirant to explore the potential to voluntarily curtail Unit 3 prior to other noncore [gas] customers," PG&E told state regulators in late October. "This would significantly reduce the likelihood of other noncore curtailments."

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