FERC on Tuesday accepted a MISO tariff revision that promises an annual update of unforced capacity-to-intermediate seasonal accredited capacity ratio the RTO uses to determine supply ahead of its capacity auction (ER23-1223).

The ratio disrupted MISO’s first seasonal capacity auctions and delayed the opening of its offer window by about a month. (See 1st MISO Seasonal Auctions Yield Adequate Supply.)

FERC said the grid operator’s pledge to calculate the ratio on an annual basis is reasonable and “provides greater notice to market participants regarding the timing” of its calculation.

“We expect that the schedule MISO develops with its stakeholders will incorporate sufficient time to work with market participants to validate and confirm [seasonal accredited capacity] values before finalizing the ratio,” the commission said. “We encourage MISO to continue working with its stakeholders to improve its processes from lessons learned.”

The grid operator’s calculation of the systemwide ratio in December produced an incorrect value. A computer error that counted previously excused maintenance outages against some planning resources undervalued their contributions.

This year, MISO and its Independent Market Monitor decided against reworking the ratio ahead of the spring capacity auction. They reasoned that the oversight wouldn’t harm reliability, there wasn’t enough time to rerun numbers, and market participants had already relied on the inaccurate ratio to enter bilateral supply contracts outside of the voluntary auction.

However, FERC found MISO in violation of its tariff and issued a show-cause order that had staff rehashing the calculation and delaying its first seasonal capacity auctions. (See FERC Terminates MISO Show-cause Order.)

After the ordeal, the grid operator updated its tariff to state that it will calculate the ratio on a standardized timeline, despite the determination requiring multiple rounds of market participants’ data submission and staff’s review and confirmation. MISO said its pledge to run the ratio annually is part of its lessons learned in moving to a seasonal capacity environment. It didn’t specify when it plans to publish the ratio, saying it will settle on dates with stakeholders and include them in a business practice manual.

Commissioner James Danly said in a partial dissent that MISO should commit to a more specific timeline in its tariff and name dates.

“Given that there is so much at stake in the inputs to the Planning Resource Auction (PRA), the date of the annual establishment of the systemwide … ratio for each planning year is fundamental to the mechanics of the market,” he wrote. “This ratio ultimately informs load serving entities and resources of their accredited capacity in advance of the [PRA]. While there could be debate on this, I believe that the rule of reason compels us to require the date’s inclusion in the tariff rather than the business practice manual.”