On June 8th, NERC issued a report that stemmed from a joint NERC/WECC investigation into the loss of solar generation that occurred on August 16, 2016 resulting from line faults due to a nearby wildfire. “By the end of the day, the SCE transmission system experienced thirteen 500 kV line faults and the LADWP system experienced two 287 kV faults … four of these fault events resulted in the loss of a significant amount of solar photovoltaic (PV) generation” including a “single loss of nearly 1,200MW.”
There are two major findings uncovered by the investigation:
- “Inverters that trip instantaneously based on near instantaneous frequency measurements are susceptible to erroneous tripping during transients generated by faults on the power system.”
- “The majority of currently installed inverters are configured to momentarily cease current injection for voltages above 1.1 per unit or below 0.9 per unit. During the Blue Cut fire event, some inverters that went into momentary cessation mode returned to pre-disturbance levels at a slow ramp rate.”
In summary, the ride-through characteristics exhibited by the majority of solar PV inverters did not support grid reliability but rather exacerbated the system event.
NERC is working with inverter manufacturers to provide for better frequency ride-through capabilities and may revise PRC-024 to determine if that standard needs to indicate that “momentary cessation of inverter connected resources is not allowed within the no-trip area of the voltage curves.”
NERC will also be issuing a NERC Alert on this matter (we were told on June 20) that will require a response by solar PV Generator Owners (GOs) – that will ask applicable GOs and Generator Operators (GOPs) if they are aware of such recommended changes. NERC also indicates that they will likely conduct more “in-depth analysis of momentary cessation with higher penetrations of inverter connected resources … to determine if that should be allowed for voltages less than 0.9 per unit or greater than 1.1 per unit.”
GridSME recommends a review of current inverter settings, and proactive resetting of the inverters to a more conservative stance that will allow for ride-through for a wider band of frequency and voltage excursions. GridSME is also recommending involvement from solar GOs and GOPs at NERC, WECC, and CAISO and other forums to ensure that solar providers are fairly represented in these forums.
If you have any questions feel free to contact me or GridSME.
Director of Regulatory Services
Grid Subject Matter Experts