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NERC warns solar PV operators of inverter issues during grid disturbances

The North American Electric Reliability Corp. ( NERC) warned that disturbances to the electric power grid due the loss of inverter-based resources warrants a handful of actions that generator owners must take before the end of June.

NERC issued the Inverter-Based Resource Performance Issues Alert on March 14 and sent it to generator owners of bulk electric system solar photovoltaic (PV) generating resources. 

The alert came after NERC analyzed what it said were “multiple large-scale disturbances” in 2021 and 2022 involving the widespread loss of inverter-based resources. These events led to “abnormal performance” across solar PV generating resources on the bulk electric system. 

NERC said those “systemic performance issues” could lead to potential widespread outages if they were to persist. 

Two disturbances in Odessa, Texas, resulted in what NERC characterized as abnormal performance across several solar PV resources.

It said that as the penetration of inverter-based resources continues to increase, “it is paramount that any performance deficiencies with existing and future generating resources be addressed in an effective and efficient manner.”

The document released by NERC contained a handful of recommendations for specific actions to be taken. It said that entities registered as generator owners are “required to acknowledge receipt and respond to” a series of questions by June 30. 

NERC analysis showed that some solar PV resources exhibit systemic fault ride-through deficiencies. These led to their “unexpected and abnormal” performance during grid disturbances. NERC said it also had observed that control settings installed in the field “do not always match” what was studied during the interconnection process. 

NERC said it plans to issue a subsequent alert in the third quarter regarding modeling issues.

In the first documented instance, a single-line-to-ground (Phase A) fault occurred on May 9, 2021, on a generator step-up (GSU) transformer at a combined-cycle power plant near Odessa, Texas. The fault was caused by a failed surge arrester at a gas-fired combustion turbine during startup for testing. 

A follow-up incident report by NERC said the circuit breaker for CT1 operated and cleared the fault within three cycles. The #2 unit experienced a partial trip followed by a run back for a total loss of 192 MW. 

The fault caused voltages in the area to drop to 0.72 pu at the 345 kV connecting station for the generation facility, 0.84 pu at a nearby 138 kV station, and as low as 0.54 pu at a 69 kV bus near Alpine, Texas. Voltage in the area recovered to near pre-disturbance levels “very quickly” (within a couple electrical cycles) after the fault cleared.

NERC said that in addition to the generation loss at the combined cycle plant, a number of solar PV and wind plants connected to the bulk power system also exhibited active power reductions caused by the fault event. It said that none of the affected inverter-based resources were tripped by the fault itself. Rather, it said that “all reductions” were due to inverter-level or feeder-level tripping or control system behavior within the resources.

In the second instance, on June 4, 2022, a surge arrestor failed at a synchronous generation facility, also in Odessa. It caused a B-phase-to-ground fault on the 345 kV system. The fault cleared in three cycles, disconnecting part of the plant that was carrying 333 MW. 

Other units in the plant unexpectedly tripped for an additional immediate loss of 202 MW. A separate synchronous generation facility in South Texas, more than 450 miles away, lost an additional 309 MW. 

In total, 844 MW of synchronous generation tripped at the time of the disturbance. 

In addition, 1,711 MW of inverter-based resources from many different facilities “unexpectedly” reduced power output due to the protection and controls at each site. 

NERC said that the normally-cleared single-line-to-ground fault “resulted in a total loss of 2,555 MW of generation,” and system frequency dropped to 59.7 Hz. The total responsive reserve service available at the time of the disturbance was 2,442 MW. Total responsive reserve service deployed was 2,343 MW with 1,116 MW from load resources and 1,227 MW from generation.

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