By Rich Heidorn Jr.

In the first 16 months after its creation in 2017, NERC’s Inverter-Based Resource Performance Task Force produced four disturbance reports, two alerts, several webinars and a reliability guideline. Its latest creation could be considered its Greatest Hits.

“I refer to it as a comprehensive guide for all things inverters,” NERC’s Rich Bauer said during a webinar Friday on the new publication, Improvements to Interconnection Requirements for BPS-Connected Inverter-Based Resources.

The task force began disseminating a 98-page inverter performance guideline a little over a year ago.

“Our recommendation to transmission owners is you need to be familiar with the guideline and incorporate what’s in [it] in your interconnection requirements,” Bauer said. “We got a lot of feedback from the industry, and they said, ‘What are the critical pieces in the performance guideline that we should include in our interconnection agreements?’ That’s really what spawned this latest guideline.”

Although many of the task force’s recommendations apply to balancing authorities, reliability coordinators or transmission operators, they are generally addressed to the TO, which is responsible for maintaining interconnection requirements under reliability standard FAC-001 and for conducting interconnection studies under FAC-002.

The new 52-page publication seeks to cover the growing number of inverter-based generator interconnections that fall between the distribution system and the bulk electric system (BES) threshold (capacity of 75 MVA and voltages of 100 kV) that is subject to NERC’s standards. The task force says “the vast majority of solar PV plants connected to the BPS [bulk power system], totaling over half the capacity, are not considered BES and are therefore not subject to NERC reliability standards.”

“Maybe it’s connected at less than 100 kV, but it’s more than 75 MVA. Maybe it connects at 69 or 92 kV. Or maybe it connects at greater than 100 kV, but it’s less than 75 MVA. Those are [what] I call the ‘tweener’ interconnections,” said Bauer, NERC’s associate director of reliability risk management.

“While each individual resource may not have a substantial impact to the BES, the overall response, behavior and control of these resources impact BPS reliability and stability,” the guideline says. “It is therefore critical to have consistency across the generating fleet.”

If widely adopted, the guideline could act as a de facto standard. Although interconnections outside the BES definition are not directly subject to NERC standards, the task force realized it could still influence them indirectly: The guideline includes 18 recommendations for interconnection requirements and six for modeling.

The task force said the guidelines will serve as a “bridge” to the Institute of Electrical and Electronics Engineers’ P2800 effort to standardize the performance and capability of newly interconnecting BPS-connected inverter-based resources (IBRs). Completing the standard and getting it adopted by relevant authorities is expected to take about two years.

“A significant portion of [the 2018 performance guideline] looks a lot like an equipment specification, and NERC standards [have] never really delved into the area of providing equipment specifications,” Bauer said. “We recognized this fairly early on and … that’s what spawned this IEEE 2800 project.”

Unlike a NERC standard, which would cover legacy equipment, the new guideline is intended to apply only to new and “materially modified” interconnections involving IBRs. IBRs are not “brand-new” to the bulk system, said NERC’s Ryan Quint, citing wind farms installed since the 1980s.

“There is a lot of legacy equipment out there. Not all of that the legacy equipment can meet the recommended performance that we’re seeking today,” said Quint, senior manager of advanced analytics and modeling.

Grid operators need more granular data because of the growing penetration of IBRs, he said.

“Clarity and consistency [are] needed for inverter-based resources because their electrical response and behavior when they’re connected to the grid is dominated by controls rather than physics,” Quint said. “The inverters, the plant-level controllers, the communication — all the stuff that interacts to make the plant behave the way it does — is all based on a control system … that can be [programmed] to some extent to meet certain performance requirements and ensure reliability of the grid. … The goal is not to have expensive, overbearing requirements for inverter-based resources. It’s just to bring clarity to what the grid really needs.”

Monitoring

In producing reports on the August 2016 Blue Cut Fire, the October 2017 Canyon 2 fire, and the 2018 Angeles Forest and Palmdale Roost disturbances, the task force discovered that some of the IBR facilities “lacked the data we really needed to analyze those events in great detail,” Bauer said.

He said investigators need high-resolution, time-synchronized data. “We actually recommend where we would take measurements at. Of course, we would want a measurement at the point of interconnection, but there’s also a really big need to see data down in the plant … down at the individual inverter level.”

Weak Grid

Bauer said a recent incident in the U.K. has highlighted the need to understand how inverters affect the grid’s short-circuit strength.

The U.K. grid lost 800 MW of generation, shedding load to 1 million customers, after the voltage-regulating control on a very large wind facility became unstable.

“Make sure you study all potential configurations on your system to ensure that you can’t end up in a weak grid situation, because that’s in essence what happened in the U.K.,” Bauer said. “They had some planned maintenance outages, which put them in a weak grid situation … and that’s what caused their control to go unstable.”

“Weak grid” concerns are on the mind of engineers in New England, ISO-NE associate engineer Brad Marszalkowski said. “Lately we’ve been doing [electromagnetic transient modeling] for every interconnection request because they’re all inverter-based resources,” he told the webinar. “We see a lot of these weak grid issues.”

Be Proactive

Jeff Billo, ERCOT’s senior manager of transmission planning and the task force’s vice chair, also offered some lessons learned during the webinar.

ERCOT, which has 22 GW of wind and almost 2 GW of utility-scale solar, could add another 10 GW of wind and about 4 GW of solar in the next 15 months, if all the signed interconnection agreements backed with financial security postings come to fruition.

The Texas grid operator recently created a new task force in response to interconnection requests for “multi-hundred-megawatt” batteries, Billo said. (See “TAC Approves Task Force to Study Battery Energy Storage,” ERCOT Technical Advisory Comm. Briefs: Sept. 25, 2019.)

Billo cited Texas’ IBR growth “to emphasize how important the work is at IRPTF to ERCOT and why we’re taking this as seriously as we are.”

ERCOT has banned IBRs from using momentary cessation since 2015, he said. “However, we were surprised when we got the survey responses back from the second NERC alert last year. There were some of the newer solar resources that were using that momentary cessation. We’ve identified that we need to clarify our requirements.

“We don’t want to wait until we start seeing problems on our grid before we start to implement some of these requirements,” he continued. “We’ve found it’s really important to be proactive in getting the requirements out, because once you have a problem on your system, it’s really too late. It’s near impossible to go back and retrofit the equipment that’s already in the field.”