CARMEL, Ind. — MISO is holding to its plan to enact a widescale marginal capacity accreditation while announcing this week that it will swap risky hours for peak load to calculate its reserve margin requirements.

Officials at a July 11-12 Resource Adequacy Subcommittee (RASC) meeting said as part of MISO’s move to a probabilistic, direct loss-of-load accreditation for most of its resources, it will identify periods that have the highest potential for reliability risks in its loss-of-load modeling and set requirements from them. That process is set to replace MISO’s current practice of margin requirements established on peak load.

MISO also proposed a three-year transition to the direct loss-of-load accreditation, which will be based on generator performance during predefined tight operating conditions. The grid operator hopes to file the changeover with FERC in October or November. (See MISO Accreditation Impasse Persists at Workshop; MISO Stakeholders Debate Capacity Accreditation, RA.)

MISO’s Davey Lopez said staff will reach out to market participants in the coming months with accreditation results under a direct loss-of-load approach. He said MISO is working with Astrapé Consulting to estimate accreditation trends into the future under a transformed fleet. MISO plans to use results from its annual Regional Resource Assessment to publish forward-looking accreditation and planning reserve margin requirement estimates. (See MISO: 200 GW in New Capacity Necessary by 2041.)

“We will only make a filing after you all have seen both the…accreditation and the notional trend of what accreditation will look like under a different resource mix,” Executive Director of Market and Grid Strategy Zak Joundi pledged. He said MISO will build in its filing how it will share accreditation data from “a future-looking standpoint.”

Joundi said it makes sense for MISO to leverage the annually updated Regional Resource Assessment to predict the fleet mix MISO will be accrediting.

Joundi also said though MISO’s reserve margin calculations will be adjusted to focus on risky hours, they still will incorporate seasonal peak loads and still will solve to meet them.

“It’s just signaling that’s not where we’re seeing risk happening,” Joundi explained of MISO’s new calculation route.

So far, the accreditation change will not apply to load-modifying resources. Lopez said MISO plans to address LMR accreditation later.

MISO officials are wedded to the direct loss-of-load accreditation as stakeholders continue to have qualms with the lowered capacity credits for most resources and eventual near-zero capacity credits for solar generation that the design is likely to produce within a decade.

Stakeholders’ motion in spring to oppose a marginal approach to capacity accreditation passed with 31 members in favor, six voting against and eight abstaining from the email vote.

MISO’s Dustin Grethen said he “invited people to think of” MISO’s accreditation philosophy as what capacity is actually earned, versus the cruder, nameplate capacity-minus-forced outages MISO previously employed for its thermal resources.

During the May Resource Adequacy Subcommittee meeting, Joundi said MISO and stakeholders already have been debating accreditation design elements for the better part of two years.

“The way we landed on the proposal on the table was not by luck,” Joundi said, adding that MISO staff underwent months of analysis on the most beneficial accreditation design for the system. “We believe the current proposal…meets where we need to be to be ready for the future and is the most appropriate.”

Stakeholders pushed back on the timeline, saying that though discussions were held on accreditation concepts, MISO only settled on a draft design since early 2023.

Lopez said it just makes sense that accreditation should be directly derived from loss-of-load expectations.

“They’re in the same currency,” he told stakeholders at the May RASC.

MISO Independent Market Monitor David Patton said that MISO must continue its effort to assign realistic capacity accreditation to all units, despite stakeholder protest. (See MISO Accreditation Impasse Persists at Workshop.)

“There’s a lot of folks behind me that aren’t going to like an efficient accreditation regime because these resources are expensive to build, but if we’re not honest about that, we’re going to accredit resources that have no hope of meeting the planning margin,” Patton said during the spring MISO Board Week.

Patton said without an honest accreditation method, MISO runs the risk of not having “the resource base that we need to keep the lights on.”