FERC said it needs more explanation behind MISO’s plan to accredit resources based on a combination of their projected availability and historical performance during periods of high system risk.

The commission handed MISO a deficiency letter July 25 concerning several aspects of its proposed direct loss of load capacity accreditation method and gave it 30 days to respond (ER24-1638).

Under the proposed method, generators’ capacity credits would be determined by a two-step process that marries historical performance of individual generators with a probabilistic performance during simulated loss-of-load events. (See MISO: New Capacity Accreditation Filing Imminent.)

First, MISO would calculate a probabilistic, resource-class average accreditation using its loss-of-load modeling. It would tailor resource class-level accreditations to individual generators based on their availability during both normal operating conditions and high-risk hours, including hours containing low margins or hours with an emergency event in place. MISO plans to give greater weight to hours that contain emergency or near-emergency conditions in the ensuing accreditation.

Most resources’ credited capacity would shrink under the new method. Resources would be divided by fuel type: gas, coal, hydro, nuclear, energy storage, pumped storage, wind and solar. MISO said the new process would satisfy both a prospective and retrospective approach to accreditation and wants it in place for the 2028/29 planning year.

But FERC wanted to know if MISO would consider deliverability limits in either the individual or resource-class level accreditation calculations. It asked whether a resource is required to obtain full deliverability rights to receive the maximum capacity accreditation and asked if the accreditation would differentiate between resources interconnected at MISO’s basic, unguaranteed energy resource interconnection service or the higher-quality, firm network resource interconnection service.

FERC was also interested in how the 1,950-hour limit that MISO intends to use in its probabilistic model for high-risk hours would help it take the best measure of resource availability.

MISO proposed to gauge resource availability using the riskiest 65 hours, or 3% of a season, across 30 weather years in its loss-of-load modeling. The 1,950 hours include all the times when loss of load occurs and then draw on hours when available generation comes within 3% of load or less.

However, that limit does not kick in if MISO’s modeling shows more than 1,950 hours when loss of load occurs. The RTO said it did not want to “dilute” real loss-of-load risk in its accreditation.

FERC asked how MISO would factor load forecast error and effective margin into the weighting calculation for risky hours to capture future uncertainty.

The RTO should also explain how it will model and dispatch storage with the new method, FERC said, pointing out that stakeholders had asked it to delay the filing of the proposal until it can improve its loss-of-load modeling of storage.

FERC said MISO needs to justify its strategy to use resources’ planned outages to decrease the capacity availability of resource classes in its probabilistic model. It pointed out that the RTO currently allows exemptions for planned outages in its accreditation.

The commission asked after MISO’s criteria for establishing resource classes, including the operating characteristics and any quantitative thresholds it looks for to sort resources.

And FERC questioned MISO placing oil, gas and dual-fuel resources in the same resource class. It asked MISO if there was a minimum number of megawatts or individual resources it requires before forming a new resource class. The commission appeared to suggest that it perceived operating differences between dual-fuel, oil and gas resources.

Finally, the commission was interested in knowing more about how MISO would handle instances when a market participant disputes the class their resource is categorized into. It also requested MISO’s final deadline for making resource-class level accreditation calculations in the event that resource classes change by more than 3% and at least 30 MW.