Panel Session – IBR Oscillations: Real-World Events and Root Cause Analysis 

Sudipta Dutta, Sr. Technical Leader, EPRI

Thursday July 20, 2023

IEEE PESGM, Orlando, FL

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SSO Events & Learnings

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Sampling a handful events

Reported SSO events 2007-2023

Y. Cheng, IEEE PES IBR SSO Task Force, “Real-World Subsynchronous Oscillation Events in Power Grids with High Penetrations of Inverter-Based Resources,” IEEE Transactions on Power Systems, 2021

Reported SSO events 2007-2023 (contd.)

> 20 documented events attributed to either SSCI or weak grid interactions

Y. Cheng, IEEE PES IBR SSO Task Force, “Real-World Subsynchronous Oscillation Events in Power Grids with High Penetrations of Inverter-Based Resources,” IEEE Transactions on Power Systems, 2021

Rob O Keefe, “January 24 & March 10, 2023 SSO Events in AEP,” EPRI SSO Workshop, Charlotte, April20, 2023

https://eandt.theiet.org/content/articles/2022/02/how-renewables-caused-scottish-grid-double-heart-attack/

January 24, 2023 SSCI event in AEP territory

• Cause: Fault caused Type3 WP to go radial to series compensated transmission line

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• Effect:
• High peak magnitude line currents (4000+ Amp peak)
• High peak magnitude line voltages (600+ kV peak)
• Dominant sub-synchronous frequency of 21 Hz obtained from frequency spectrum of wind plant phase currents while radial through series cap
• Wind plant tripped in slightly less than 0.5 seconds due to high main power transformer differential current

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• Concerns: Wind plant had SSO/SSCI study and control tuning to damp SSCI prior to interconnection in 2018

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Recorded ABC phase currents and voltages during event at the Wind Plant

Schematic of wind plant location relative to series compensation and fault location

Rob O Keefe, “January 24 & March 10, 2023 SSO Events in AEP,” EPRI SSO Workshop, Charlotte, April20, 2023

March 10, 2023 SSCI event in AEP territory

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• Cause: 4 WFs totaling 937 MW capacity interconnecting toward one end of a 345 kV line series compensated at two locations; 2 different transformer bushing faults at station at one end of line left WFs radial to both series caps and other wind/solar farms downstream, SSCI immediately apparent

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• Effect:
• SS currents/voltages increased but then WFs kept SSCI from further increasing in magnitude, didn’t damp out completely
• Both series caps automatically bypassed by their own protection about 1.2-1.3 seconds after fault clearing
• 1 WF tripped later

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• Concerns: All WFs had SSO/SSCI study and control tuning to damp SSCI prior to interconnections. Three of these had experienced similar events in 2017-2018 and further SSO damping control tuning and model validation was performed after that

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Phase currents and voltages recorded at one of four wind plants during event

Rob O Keefe, “January 24 & March 10, 2023 SSO Events in AEP,” EPRI SSO Workshop, Charlotte, April20, 2023

June 12, 2023 voltage oscillations in Glasgow

• Cause: Under investigation

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• Effect:
• Nearly all traffic lights in Glasgow Scotland failed due to voltage oscillations on the 400 kV grid leading to “spikes” on the distribution grid.
• Voltage oscillation was ~ +/-10% of nominal.
• The oscillation frequency was around 8Hz.

• Concerns:
• Similar events in the vicinity: “Scotland’s grid suffered two ‘heart attacks’ during the night of 24 August 2021”.
• Severe voltage disturbances (duration ~20 and 25 sec) on two occasions, 30 minutes apart. Oscillations for the 400kV system ranged between approximately 355kV and 435kV with a frequency of around 8Hz.

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https://www.msn.com/en-gb/travel/news/glasgow-traffic-lights-out-across-city-as-engineers-scramble-to-fix-fault/ar-AA1cqQ87

https://news.stv.tv/west-central/power-spike-knocks-out-traffic-lights-across-glasgow

https://eandt.theiet.org/content/articles/2022/02/how-renewables-caused-scottish-grid-double-heart-attack/

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Recorded voltages during the 2021 event

~20s

~30min

August 24, 2021 voltage oscillations in Scotland

• Suspected cause: Newly commissioned Moray East offshore WP interacted with HVDC Spittal DC/AC converter within very weak north Scotland grid. Peterhead generation was offline (that could have provided stabilizing MVA).

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• Effect: Several WPs tripped off. North Scotland came close to complete blackout. Disturbances reached Torness nuclear station, which came close to tripping, narrowly avoiding serious consequences for central Scotland.

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• Concerns: Closed Hunterston resulted in significant drop in system strength in W. Scotland. Wales/Hunterston HVDC link, not capable of supporting black start (not VSC) and in future could encounter fault conditions that lead to instability. In that situation, Scotland could not use it to import and would have to disconnect. Torness scheduled for closure in 2028, resulting in further loss of stabilizing nuclear generation. Severely weakened Scottish network will mainly depend on ageing Peterhead power station and overhead line imports from England.

https://www.nationalgrideso.com/research-and-publications/electricity-ten-year-statement-etys/electricity-transmission-network/scottish-boundaries

Torness nuclear station

Peterhead gas-fired station (offline during event)

Spittal HVDC end (significant oscillations observed here)

Closed Hunterston thermal unit

Moray East offshore WP

HVDC link

50miles

Analysis and Tools

Positive sequence impedance scanning tools

• Quickly scanning system for impedance over a wide range of operating conditions including contingencies is important to screen for potential risks of resonance-driven instabilities

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• Typically, these tools (EPRI’s ZSCAN, GE PSLF’s Ztools) scan the system from a specific bus

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• Output of these tools is the variation of R and X vs. f

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• Zero crossing of Xsystem indicates resonance and corresponding resistance shows damping of the system

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• ZSCAN can incorporate non-linear impedances of active devices as user input files (new feature)

Plot of R and X vs f for N-0 and contingencies

ZSCAN – User Manual: Rev. 2021.b. EPRI, Palo Alto, CA: 2021. 3002021974

Active device impedance consideration in positive sequence scanning

18-bus system based on IEEE 14 bus system, 4 out of 5 generators replaced with IBRs (2PV and 2WPP)

Scanned bus 1

User-input non-linear impedances

Scanned bus 2

Scans with and without including non-linearity of IBRs shows different results

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Scanned Bus #1

Scanned Bus #2

• Significant difference in Z profile for the case when the non-linearity is considered versus when it is ignored

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• Incorporating non-linearity of impedance from active devices is more pronounced when the scanning bus has more of these devices in the vicinity

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Non-linearity ignored

Non-linearity included

Impedance Scans in EMT

Sub-synchronous oscillation screening through impedance-based frequency scans: Impact of converter control designs and system strength. EPRI, Palo Alto, CA: 2020. 3002020209.

W. Ren and E. Larsen, "A Refined Frequency Scan Approach to Sub-Synchronous Control Interaction (SSCI) Study of Wind Farms," in IEEE Transactions on Power Systems, vol. 31, no. 5, pp. 3904-3912, Sept. 2016, doi: 10.1109/TPWRS.2015.2501543.

Impedance-Based Stability Analysis Methods in Electromagnetic Transient (EMT) Domain: A Description of Analytical Framework, Case Studies, and Considerations. EPRI, Palo Alto, CA: 2022. 3002024748.

Considering frequency coupling is important

Impedance Scanning Block

Investigation of weak grid oscillations

EMT simulation with original inverter settings: ~ 10 Hz oscillation in RMS voltage

EMT simulation with new inverter settings: control cycling at ~ 1Hz

Investigation of observed oscillations when Line 1 is out of service.

• Plant owner provided EMT model of plant
• Inverter OEM provided new inverter settings
• Two distinct oscillations found
• Driver found to be plant level PF control mode causing activation of non-linear controls

Sunitha Uppalapati, Wes Baker, Deepak Ramasubramanian, Hoang Tong, "Use of Advanced System Strength Metrics to Identify Critical Regions of a Power Network during Day-to-Day Operations", CIGRE Cairns International Symposium, 2023

Investigation of weak grid oscillations

Driver found to be plant level PF control mode causing activation of non-linear controls. Two distinct oscillations:

• ~ 1 Hz oscillations due to mode switching into blocking (FRT V threshold 0.8 pu)
• ~ 10 Hz oscillations due to non-linear control

Orange: nonlinear controller disabled

Blue: nonlinear controller enabled

Positive sequence stability models can predict the potential of control mode switching with the appropriate parameterization of the REEC_A model’s VDL1 and VDL2 tables

Recreation of weak grid oscillatory event

• Use of improved robust positive sequence to re-create low short circuit events
• ~ 7 Hz oscillation observed in SCADA measurement
• ~ 9 Hz oscillation observed in simulation

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• Simulation time scale is for computation efficiency

Deepak Ramasubramanian, Wenzong Wang, Pouyan Pourbeik, Evangelos Farantatos, Anish Gaikwad, Sachin Soni, and Vladimir Chadliev, "Positive Sequence Voltage Source Converter Mathematical Model for Use in Low Short Circuit Systems," IET Generation, Transmission & Distribution, vol. 14, no. 1, pp. 87-97, Jan 2020

Learnings

• With growing IBR penetration, it is expected that SSO events will be increasingly reported

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• Proper tools and analysis methods for SSO risk screening and mitigation needed, there are limitations involved with tools and their applicability

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• Challenges:
• Models: availability, compatibility, different vendors, different compilers, model quality; How much of the system should be modeled? How to determine the boundary buses? Which models to use? Inverter manufacturer not known till late in the interconnection
• Education needs for SSO studies
• Industry wide agreement on definitions to avoid confusion
• Cumbersome study approaches. Need for screening criteria (based on Voltage level? SCR value? X in N-x for SSCI?)
• Type 3 WFs known to have difficulty controlling SSCI even when detailed PSCAD SSO control tuning studies have been performed; SS frequencies may change as new IBR projects interconnect nearby

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Thank You

sdutta@epri.com