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NERC urges inverter changes after Calif. fire downed solar generation

A new report recommends that changes be made to inverter settings and standards to avoid a solar power outage similar to one that occurred August 2016 as a result of a wildfire in southern California.

The North American Electric Reliability Corp. and Western Electricity Coordinating Council on June 8 released the results of a study of the Blue Cut fire in the Cajon Pass, an event that led to the fault-induced loss of nearly 1,200 MW of solar photovoltaic, or PV, generation. The fire caused 13 500-kV line faults for Southern California Edison Co. and two 287-kV faults for the Los Angeles Department of Water and Power. No solar PV facilities de-energized as a result of the faults, but they ceased output due to the event.

"With the proliferation of solar development in all interconnections, the results of this disturbance analysis needs to be widely communicated to the industry to alert them of the potential for widespread solar resource loss during transmission faults on the power system," the report said.

The study made two key findings, both pertaining to inverters, which change direct current from a power source to alternating current. The first finding was that inverters that trip instantaneously based on immediate frequency measurements can trip erroneously during power system faults. The second was that most installed inverters are configured to momentarily cease injection of currents if voltages rise above 1.1 per unit or fall below 0.9 per unit, with some inverters affected by the Blue Cut fire slowing ramping back up after the fault-related disturbances.

NERC and WECC made recommendations for each of those issues. To remedy the first finding, inverter manufacturers should change their settings to avoid erroneous tripping, the report advised. That change will add a time delay to frequency tripping that will allow the inverter to "'ride through' the transient/distorted waveform period without tripping," the study said. Solar power operators already are working with inverter manufacturers, California ISO and Southern California Edison Co. to make a plan for implementing those changes.

To address the second finding, the report said inverters that cease output for voltages outside their operating range should be configured to restore output within five seconds. The study also recommended that NERC review whether one of its reliability standards (PRC-024-2) should be revised to indicate that inverters cannot momentarily cease output within the no-trip area of the voltage curves.

The report went on to say that NERC should alert generation owners and operators of the recommended change to inverter settings and the risk of unintended loss of resources. As part of the alert, NERC should urge balancing authorities and reliability coordinators to assess the reliability risk of solar PV momentary cessation and "take appropriate measures."

The recommendations come as U.S. solar energy expands across the country, although it remains a relatively small part of total generation. The U.S. Energy Information Administration estimates that utility-scale solar plants make up about 2% of total U.S. electric generating capacity and around 0.9% of utility-scale generation. Smaller-scale solar systems, such as rooftop and other customer-sited PV systems, produce about two-thirds the total generation of utility-scale solar facilities.