CAISO’s Western Energy Imbalance Market (WEIM) last week took the unusual step of rescinding a rule change and associated functionality that it never actually implemented.

At their monthly joint meeting Thursday, the WEIM Governing Body and the CAISO Board of Governors approved an ISO staff request to withdraw a 2020 tariff revision that would have shifted the WEIM’s market deadline for submitting base schedules from 40 minutes before a delivery hour (T-40) to 30 minutes (T-30) before. The shorter timeline was intended to accommodate energy products in Bonneville Power Administration (BPA) power purchase agreements that can be scheduled after the T-40 deadline. BPA joined the WEIM in 2022. (See CAISO Floats EIM Base Schedule Rule Changes.)

CAISO’s implementation of the request always was contingent on the ISO’s ability to accommodate the change without compromising its performance in solving the real-time market, Danny Johnson, ISO market design sector manager, explained to the two governing boards.

But Johnson said testing showed CAISO “is unable to support this functionality when considered in conjunction with other real-time market enhancements,” specifically the flexible ramping product refinements that went live in the market in February.

“That initiative … increases both the reliability and the efficiency of the real-time markets through procuring flexible deliverable capacity to meet net load uncertainty, and implementing that functionality required additional computational time,” he said. “Once that was implemented, we determined we would not be also able to implement a base schedule submission deadline at T-30.”

By that time, the ISO also had determined that BPA could fully participate in the WEIM without instituting the scheduling changes.

“We largely attribute their ability to participate successfully to the tagging and scheduling practices of BPA’s WEIM neighbors,” Johnson said.

Johnson said BPA was disappointed when CAISO signaled its intent to withdraw the rule change, but also understood the technical constraints around implementing it.

In comments to CAISO, NV Energy said it had hoped to use the additional time created by the T-30 deadline to manage the variability of the net load uncertainty now included in the WEIM resource sufficiency evaluation done ahead of every delivery hour. But the utility also did not object to withdrawal of the rule change.

“Management recognizes this concern, and we’re committing to [working] with both NVE and all stakeholders to better understand the concerns on the newly implemented net load uncertainty requirements,” Johnson said.