As ISO-NE starts moving forward with its work to update resource capacity accreditation rules in New England, the region’s energy stakeholders are urging it to cast a wide net and not commit to an approach too soon.

The grid operator in the last few weeks has said it’s leaning toward a marginal approach to capacity accreditation, using a concept called Marginal Reliability Impact (MRI). That’s in contrast to an average approach that accredits resources based on their share of their class’s total reliability contribution. (See ISO-NE Starts its Capacity Accreditation Journey.)

At this week’s NEPOOL Markets Committee meeting, Advanced Energy Economy warned ISO-NE not to rush into a decision, highlighting challenges with the marginal approach and advocating for broader consideration.

“Marginal accreditation is a novel approach and presents potential challenges as a replacement to the current capacity accreditation regime,” AEE’s Caitlin Marquis said in a presentation to the committee.

Among those challenges: It could result in different compensation to resources that provide the same total reliability benefit to the system and be more sensitive to accurate modeling of the region’s resource mix.

Also, even though the marginal method is often cited as having clearer entry and exit signals for resources, Marquis said, accurate signals don’t always facilitate efficient decisions if they’re still highly variable.

“Average versus marginal is a significant decision that should not be rushed; before moving forward with marginal, we should fully consider challenges and address shortcomings,” Marquis said in her presentation.

That could include exploring alternative or hybrid approaches, she said.

Also at the meeting, Ben Griffiths of LS Power raised concerns about the ability of a marginal accreditation method, which is an effective load-carrying capability (ELCC) measurement, to accurately measure the contributions of thermal resources.

“Proposals to apply ELCC-like accreditation mechanisms to thermal resources can obscure economic choices and may solidify the status quo by muting price signals,” Griffiths said in his presentation.

ELCC works for variable renewables because their performance is mostly determined by factors outside their control, Griffiths said. That’s not the case for thermal resources, which are more governed by economic conditions and operational choices, he argued.

The “class-based ELCC/MRI approach necessarily lumps good and poor performers into one class, which reduces downside risk for poor performers, and limits accreditation value for good ones,” Griffiths said. A preferable approach would be to refine a unit-specific accreditation method like PJM’s unforced capacity, which he said is a “reasonable starting point.”

ISO-NE is still early in what will be a yearlong-plus process of developing an update to capacity accreditation.

At the meeting this week, the RTO’s Feng Zhao put forward new details about how its conceptual design for an MRI would work, with a promise of more design information to come in the next few months.