The North American Electric Reliability Corp. (NERC) has issued a report recommending changes in the performance criteria for inverter-based generation resources, following an incident in Texas this March.
The Panhandle Wind Disturbance report, analyzed an event that started on the evening of March 21 with severe conditions, including freezing rain, snowfall, and high winds, occurring early in the morning of March 22.
The weather conditions caused icing of wind turbines and triggered high wind speed cutoffs of turbines, leading to two bulk power system (BPS) faults between 4 a.m. and 5 a.m Central Time.
In the first event, a phase-to-phase fault occurred on a radial 345-kilovolt (kV) tie line that connects a wind plant to the Electric Reliability Council of Texas (ERCOT) system. The fault cleared normally but tripped 273 megawatts (MW) of wind generation and unexpectedly reduced power of multiple additional wind plants in the area, reducing power output by 492 MW. In all, total active power reduction was 765 MW and frequency dropped to 59.90 hertz (Hz), prompting the need for 524 MW of responsive reserve service (RRS) to be deployed. Frequency recovered to nominal in just under three minutes.
In the second event, another normally cleared phase-to-phase fault occurred on a nearby 345-kV transmission circuit. Multiple wind plants again unexpectedly reduced power output, totaling 457 MW. Frequency dropped to 59.942 Hz and no RRS was deployed. Frequency recovered quickly in 29 seconds.
The reduction in wind power resources affected an area of the Texas Panhandle up to around 200 miles from the initiating fault.
At the time of the disturbances, ERCOT‘s net internal demand was 34.4 gigawatts (GW) with wind producing nearly 21 GW, or 61 percent, synchronous generation producing 13.2 GW or 38 percent of generation, and imports contributing just over 200 MW.
During the events the active power-frequency response controls at some of the wind plants incorrectly sent dispatch signals to individual turbines partly caused by abnormal or unexpected response of the individual turbines after fault clearing. Current from the turbines then rapidly decreased post-fault, causing active power output to drop from about 180 MW pre-disturbance to a low of 116 MW.
The plant controller then regained control of the turbines’ output and latched on to a plant power output of about 163 MW that became the new set point dispatch for the active power-frequency controller even though there was sufficient wind speed for 180 MW of output.
Frequency remained low for a couple minutes and the new dispatched set point held until frequency recovered and the erroneous dispatch from the controller was released. Once frequency returned to near nominal, the plant rapidly recovered back to pre-disturbance output.
Many wind plants from one turbine manufacturer also experienced wind turbine pitch converter faults that tripped multiple turbines at each facility because the turbines were designed to save power during low voltage conditions by dropping some signals.
However, if the signals do not recover in about 700 milliseconds, the controller issues a fault that the pitch converter axis is not responding and the turbine shuts down and goes off-line instantly and requires manual reset. The pitch converter fault occurred on many of the turbines at each affected site, but not all of them were impacted, NERC said.
The events did not meet the criteria for reporting under the Electric Reliability Organization (ERO) Event Analysis Process, but the Texas Reliability Entity requested a brief report from ERCOT as an event of interest given the breadth and magnitude of the reductions.
In response to its analysis of the event data, NERC issued two “high-level” recommendations.
First, NERC reiterated the need for enhanced inverter-based resource ride-through requirements. The reliability organization also submitted a Standard Authorization Request (SAR) to overhaul PRC-024 with a performance-based ride-through standard to mitigate performance issues that continue to occur on the bulk power system for both inverter-based and synchronous generating resources.
PRC-024 governs generator frequency and voltage protective relay settings.
“The March 2022 disturbance illustrates how multiple resources across a geographic area failed to ride through the disturbance but still generally met the minimum requirements established in PRC-024 regarding voltage and frequency protection,” NERC analysts wrote in the report, adding, “PRC-024 is not serving its intended purpose nor is it performance-based to ensure reliable operation of the BPS.”
The disturbances that occurred in March 2022 should be addressed in the future through ERO Enterprise compliance assurance activities, NERC said.
In its second recommendation, NERC said the March 2022 disturbance also illustrated that many of the abnormal performance issues observed in inverter-based resources are generally not modeled or identifiable during the interconnection process and will require effective post-commercial operation mitigation measures.
Many of the wind plants that tripped out of service in March were the result of plant controller interactions, pitch converter failures, abnormal sub-synchronous control oscillation tripping, and uninterruptible power supply failure, NERC said.
NERC strongly recommended “a performance-based standard that addresses abnormal performance issue identification, analysis, and mitigation for all inverter-based resources to ensure that systemic risk issues specific to inverter-based resources are addressed in a timely manner before they reach a point that could result in instability, uncontrolled separation, or cascading outages.”
NERC said generation operators should develop and validate those standards with oversight and monitoring by the Reliability Coordinator and/or Balancing Authority.
