A draft report by ColumbiaGrid’s gas-electric interdependencies study team shows that the transmission system serving the Interstate 5 corridor encompassing the Seattle, Wash., and Portland, Ore., metropolitan areas would be able to withstand an interruption to the natural gas used for generating electricity in the region west of the Cascade Mountains.
The study was undertaken to explore whether the area’s transmission system, particularly the West of Cascades North (WOCN) and West of Cascades South (WOCS) transmission interfaces, would be able to serve the area adequately with generation from more distant resources – including the Columbia River system and areas east of the Cascade Mountains, and California – if natural gas supplies to local gas fired generation were interrupted.
The WOCN and WOCS interfaces deliver remote resources from east of the Cascade Mountains to west-side loads and typically load served most heavily during unusually cold winter weather events.
Studies were conducted with all natural gas-fired plants in the I-5 corridor area assumed to be out of service, with the exception of those that have dual fuel capability. About half of the area’s 4,400 MW of gas-fired generators have the capability of switching to an alternate fuel, meaning that an interruption of the gas supply would require the importation of more than 2,000 MW of electricity from remote sources.
To model worst-case conditions for the cross-Cascades paths, the study assumed replacement generation from eastern Washington would be used to replace lost generation west of the Cascades.
Because there is currently no clear guidance from NERC’s reliability standards or the Western Electricity Coordinating Council’s (WECC) criteria regarding the conditions that the transmission system would need to endure during natural gas curtailment conditions, the study team developed its own benchmark.
The study team determined that the curtailment of natural gas to all of the plants in the I-5 corridor would be considered to be a single contingency; therefore, the system would only need to survive an additional single transmission element outage to be deemed adequate.
According to the draft report, the transmission system performed acceptably from a voltage stability, steady state voltage, and thermal loading perspective in scenarios where those natural gas plants that do not have dual fuel capability in the I-5 corridor were forced out of service. The conclusion was dependent on the assumption that those plants can operate on alternative fuel, primarily diesel, for the hours needed during the day to meet expected peak winter loads and that such operation could continue for several subsequent days.
“It is important to note that there is no reason to believe that the natural gas supply system is somehow inadequate to serve its customers or that gas curtailments are expected to be any more likely in the future,” the report stated, noting that its purpose was a “what if” exploration of possible scenarios.
The draft report will be modified with comments provided by stakeholders and other interested parties. Comments are due Dec. 7. A third draft of the report will be posted for discussion and potential approval during the next study team meeting Dec. 20.