By Amanda Durish Cook

MISO is hoping to avoid grid instability by possibly requiring inverter-based generation seeking to enter the interconnection queue to provide a specific set of calculations and documentation.

Under the plan, the owner of an inverter-based resource would be required to supply MISO its short-circuit ratio at the point of interconnection before completing an application. The RTO is also contemplating having a project owner either submit a manufacturer statement showing the inverter can operate stably or an Electromagnetic Transients Program (EMTP) study report confirming stable operation. Any project owner unable to prove stable operation will either have to add equipment to raise the short-circuit ratio or reduce the size of the project.

MISO interconnection engineer Warren Hess said the RTO will disallow use of its “momentary cessation” of active power output from inverter-based resources in order to prevent them from tripping offline unnecessarily. In that case, every generation resource would have to adhere to NERC’s PRC-024-2 standard, which requires generator owners to set their protective relays to ensure generating units remain connected during defined frequency and voltage excursions.

But stakeholders on an Oct. 16 Planning Subcommittee conference call said MISO might be requiring too much of inverter-based customers too early in the queue process. Some said the RTO should consider asking for short-circuit ratio values later in the queue process because those values will change as projects drop out of the queue. Consultant Roberto Paliza said such information should be provided at the end of the queue’s definitive planning phase, adding that MISO should make it clearer what performance standards it requires of inverter-based generation.

But Hess said that short-circuit calculations are relatively easy to provide once customers know the locations of their interconnections. He said MISO wants to avoid entering projects into the queue that ultimately cannot perform without causing harm.

“We are going to be here for guidance to help to calculate short-circuit ratios and coordinate with the applicable transmission owners. Since interconnection customers are deciding where to connect on the system, they should be responsible to work with the transmission owners to get short-circuit ratios for their inverter-based interconnection,” MISO Resource Interconnection Planning Manager Neil Shah said.

“Not doing anything is not an option,” agreed MISO Manager of Resource Interconnection Arash Ghodsian.

MISO staff also promised to work with interconnection customers and transmission owners to gather information and provide guidance on new interconnection requirements.

“This is going to be a two-way street,” Ghodosian said.

Staff said they plan to present draft Tariff language on the possible requirements at the Planning Subcommittee’s December meeting.