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NERC Emphasizes Need to Address Faults in Inverter-Based Resources

The North American Electric Reliability Corp. is reiterating the need for owners and manufacturers to address latent solar power system faults in the wake of a widespread loss of solar generation in southwestern Utah this spring.

During the morning of April 10, 2023, nine solar photovoltaic facilities failed to ride through a normally cleared fault on a 345-kilovolt transmission circuit, resulting in an unexpected loss of 921 megawatts of generation, which is categorized as a Category 1 event in the NERC Event Analysis Process.

The event was “the first major widespread solar loss to occur in the Western Interconnection outside of California,” NERC said in its 2023 Southwest Utah Disturbance report.

While no generation tripped because of the transmission outage, supervisory control and data acquisition data shows that aggregate solar photovoltaic output in the PacfiCorp-East region in southwestern Utah dropped “significantly,” according to the NERC report.

The abnormal response from multiple solar photovoltaic facilities was caused by the protection and controls within each facility responding to the bulk power system fault in an unreliable manner.

The disturbance occurred at 08:51 a.m. Pacific time, right about the time when aggregate solar photovoltaic output reached its peak for the day. Synchronous generation, wind, and solar photovoltaic resources comprised 42 percent, 31 percent, and 26 percent of total generation prior to the disturbance, respectively, NERC said.

No notable changes in net load quantities attributable to distributed energy resource tripping were observed nor were there any abnormal performance issues identified with the wind and synchronous generation fleet in the PacifiCorp-East area, NERC said.

Nonetheless, the magnitude of the solar reduction, greater than 900 MW, was “significant” as a large percentage of aggregate generation, over 57 percent of the PacifiCorp-East solar photovoltaic fleet output, was unexpectedly lost, NERC said, adding that is “the most concerning attribute of this event, particularly with growing levels of solar PV in the PACE footprint and neighboring areas.”

NERC noted that PacifiCorp-East has received about 45 gigawatts of interconnection requests for its 2022 cluster studies and is planning to connect an additional 3.8 GW of generation, including 1.6 GW of solar, in the next three years. Peak load in the PacifiCorp-East region is about 13 GW.

In the report, NERC reiterated “the strong need for inverter-based resource performance issues to be addressed by Generator Owners (GOs) in a timely manner.”

“GOs are often not addressing performance issues that latently exist within the existing fleet,” the report said. “All of the causes of abnormal performance in this event have been previously documented by NERC in past reports; however, actions were not taken either by the GOs or by the inverter original equipment manufacturers (OEM) to mitigate these known risks.”

NERC said its Project 2023-026 is addressing the reliability risk by “requiring analysis and mitigation of unexpected or unwarranted protection and control operations from inverter-based resources following the identification of such a performance issue.”

The NERC report also reiterated the need for “a comprehensive ride-through standard in lieu of NERC PRC- 024-3,” adding that “Project 2020-027 is currently addressing this risk issue by replacing PRC-024-3 with a performance-based ride-through standard that ensures generators remain connected to the [bulk power system] during system disturbances.”

NERC said the project “remains a top priority” to address “persistent inverter-based resource performance issues and the elevated risk” to the bulk power system reliability posed with “the rapidly changing resource mix.”

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