Large Load Oscillation Event

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Patrick Gravois

Operations Engineer – Operations Analysis

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LFLTF Meeting

March 4, 2025

ERCOT Initial Observations and Actions

• On July 24, 2024, ERCOT observed an oscillation with ~40 MW swings in the telemetry of a large load
• Oscillation was not observed in any line telemetry or closest PMUs
• ERCOT directed interconnecting TO (Oncor) to investigate the cause of the oscillation seen in the load telemetry
• Initially reported that oscillation may be associated with circulating currents due to customer transformer configuration and TO metering arrangement. Supplemental power quality equipment was being installed at the load to verify
• ERCOT continued to see intermittent oscillations in load telemetry when load consumption was greater than 130 MW
• Existing power quality meter at POIB showed much smaller oscillation than seen in load telemetry
• ERCOT requested TO of POIB to configure relay to capture high resolution data (PMU/DFR)
• ERCOT received PMU data (30 sample/sec) from the POIB in early September that showed a ~10-12 MW oscillation on the line to the load
• Interconnecting TO reported to ERCOT there was likely a harmonic distortion issue causing the oscillation and they were still investigating

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ERCOT Received Additional Data – Late September

• ERCOT observed load consumption had increased in early September from ~140 MW to ~225 MW
• ERCOT requested additional PMU data to verify if oscillation magnitude had increased
• PMU data showed ~15-20 MW peak to peak oscillations when consumption was > 200 MW
• Voltage was stable, oscillation seen in current magnitude
• Still believed to be local harmonics issue with minimal reliability risks
• TO confirmed load consumption not expected to increase in near future

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October 25 Event

• ERCOT observed the load dropped ~300 MW within a single telemetry scan on Oct. 25
• Load had increased consumption to ~330 MW over the previous week
• ERCOT sent RFI to interconnecting TO and requested PMU data from TO of POIB
• PMU data showed no fault preceding drop in consumption; load reduced ~300 MW over 24 second period
• Oscillation magnitude preceding reduction was ~25 MW peak to peak and ~7.5 Hz (later determined true oscillation mode was ~23 Hz from higher resolution data)
• Cause of reduction was reported to be an offsite telecommunication failure that triggered a load control issue

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ERCOT Received DFR Data – Late October

• ERCOT requested TO of POIB to manually trigger DFR to capture higher resolution data when load is consuming above 300 MW
• ERCOT received 20 sample/cycle DFR data showing a ~23 Hz oscillation with ~50 MW peak to peak magnitude on Oct. 28
• ERCOT determined oscillation is a reliability risk that needs immediate mitigation
• ERCOT and interconnecting TO confirmed load is the source due to dissipating energy levels of oscillation on lines leaving POI station

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Mitigation Action – November 1, 2024

• ERCOT notified interconnecting TO that control room will give an Operating Instruction to reduce consumption of the load to 120 MW (below level at which oscillation originally appeared in July)
• TO notified load owner and operator that they will need to reduce consumption
• ERCOT met with TO and load owner/operator and agreed that load could be reduced to level that oscillation is no longer present in load telemetry and verified with PMU data
• ERCOT gave Operating Instruction and load reduced consumption to 300 MW; oscillations were no longer present in load telemetry

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Follow Up Investigation

• ERCOT received PMU data (60 samples/sec) from Nov. 1 showing 23 Hz oscillation was present before load reduced and mitigated by reducing load consumption to 300 MW
• Load telemetry continued to show no oscillation in following weeks at reduced consumption
• DFR data (20 samples/cycle) still showed significant harmonic distortion at 3^rd harmonic but not a 23 Hz oscillation
• ERCOT notified TO and load owner/operator that site shall remain at 300 MW until root cause of 23 Hz oscillation is identified and mitigated and harmonic distortion is improved to meet TO requirements

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Initial Test – Mid December

• Load owner and operator developed test plan to help determine root cause of oscillation
• Test plan involved increasing load consumption in 2 MW intervals while collecting data
• ERCOT approved test plan and coordinated with load operator to monitor for oscillations during test
• Oscillation was observed when load consumption increased to ~320 MW
• Load ramped back down to 300 MW after test

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Second Test and Return to Full Consumption – Early January

• Data collected in December helped load owner/operator determine root cause of oscillation
• Root cause determined to be older firmware versions on certain equipment
• Older firmware versions updated over the holidays
• Load operator submitted new test plan which was approved by ERCOT
• No oscillations observed during test as load ramped consumption to ~330 MW
• Load allowed to remain at full consumption; no oscillations observed since January
• TO verified that harmonic distortion had significantly improved while still not in compliance with IEEE 519
• Load owner/operator continues to work with TO to improve harmonic distortion

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Key Takeaways

• ERCOT will take action to mitigate observed oscillations coming from large loads, up to potentially disconnecting the load from the system
• Direct communication between ERCOT and large load owners/operators should be established to identify and mitigate observed oscillations coming from large loads
• High resolution data (PMU/DFR) is essential to identify oscillations and determine oscillation magnitudes and frequency modes
• ERCOT, TO, and load owners/operators need to have good understanding and maybe requirements of actual Load performance (steady state and dynamic) to ensure reliable integration
• Lack of industry standard to define the large load performance needed for electric grid reliability and equipment security
• Lack of the accurate models to properly represent these large loads need to be addressed

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Questions?

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