Acknowledging that natural gas transmission pipeline pressures will remain at reduced levels in San Francisco and the peninsula area to its south this winter, Pacific Gas and Electric Co. (PG&E) last Wednesday kicked off a targeted advertising and bill discount incentive campaign to get customers in those areas to reduce their energy consumption in December. It is all part of precautions being taken after the fatal Sept. 9 San Bruno transmission pipeline rupture.

State regulators and the San Francisco-based combination utility have been concerned that the lower pressures on three transmission lines running through the San Bruno area could cause noncore customers to be curtailed (see NGI, Nov. 1). However, with various mitigation measures, including the shareholder-funded added conservation incentive program, PG&E thinks the curtailments may be averted.

“Any customer who can save more gas compared to their December use the previous two years will get a $25 pre-paid debit card as a means of thanking them for making a difference at an important time,” a PG&E spokesperson told NGI. The program will be confined to residential customers in San Francisco and San Mateo Counties; noncore customers threatened with potential curtailments are already taking steps to lessen their usage.

At stake is a collective noncore gas load of 200-225 MMcf/d, representing 25% of PG&E’s overall gas demand in San Francisco and San Mateo Counties, the utility spokesperson said.

In January and February, PG&E will go back to its regular, companywide winter gas savings programs offering 10% bill credits for customers who can lower the amounts of energy they save in the two full winter months compared to usage in those months the previous year.

In the immediate aftermath of the San Bruno pipeline tragedy, PG&E lowered pressures in the transmission pipelines running through the peninsula. The pressures were subsequently lowered further at the direction of the California Public Utilities Commission (CPUC) and they remain at the 300 psi level.

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 recent completion of the Trans Bay power transmission cable at the bottom of San Francisco Bay. The direct current 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 CPUC earlier this year, 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.”

High-pressure gas pipelines typically operate at pressures between 100 and 1,040 psig (pounds per square inch gauge), with pressures varying with the size and physical characteristics of the pipelines. Federal limits for transmission pipelines such as Line 132 are set at 400 psi, and PG&E has used 375 psi as its operating limit, but with the lower pressure the maximum volumes of gas are reduced also, so when winter peak-demand hits the line is limited in how much more gas it carry.

PG&E said the utility has been working with the CPUC to take various steps to mitigate the lower pressure limitations. “We’re continuing to serve our customers in San Francisco and on the peninsula at the lower operating pressures. We will not raise the pressure on any of the lines until our assessments are complete and we have discussed all of the ramifications of those reviews with the CPUC and local government officials.”

Through adding valves and other modifications to the three pipelines to increase their “operational flexibility,” the spokesperson said PG&E is hoping to stave off the need for curtailments. “We’ve offered interties and installed additional isolation valves, pressure regulators and other devices.”

The utility is feeling more confident going into December, the spokesperson said, because for the past seven or eight days (through last Wednesday) California experienced colder-than-normal conditions, and so far PG&E has not had to resort to any curtailments.

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