Inverter-Based Resources During a Cascading Failure: Present State and Future State

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1 1 Inverter-Based Resources During a Cascading Failure: Present State and Future State IEEE PES GM CFWG Panel 08/08/2018, Portland, OR Ryan Quint, NERC (ryan.quint@nerc.net) Andrew Groom, AEMO (andrew.groom@aemo.com.au) Mohit Singh, ComEd (mohit.singh@comed.com), Presenter

2 2 Disturbances Influenced by Inverter-Based Generation Blue Cut Fire Disturbance, August 16, 2016 Bulk system PV inverters Inverter behavior contributing to event (major causes): Miscalculation of frequency Momentary Cessation Loss of Generation/ Widespread Tripping South Australian Blackout, September 28, 2016 Bulk system wind plant inverters Inverter behavior contributing to event: LVRT settings South Australian System Event, March 3, 2017 DER PV inverters Inverter behavior contributing to event: Likely VRT settings Cascading Failure Event Loss of Generation/ Widespread Tripping Canyon 2 Fire Disturbance, October 9, 2017 Bulk system PV inverters Inverter behavior contributing to event (major causes): Momentary Cessation Instantaneous overvoltage tripping PLL synchronization failure Transient interactions DC reverse current tripping Loss of Generation/ Widespread Tripping

3 Blue Cut Fire Disturbance, August 16, 2016: Key Finding # Hz Largest solar PV loss (~700 MW) due to underfrequency tripping Inverter sensed near instantaneous frequency of < 57 Hz and tripped

4 Blue Cut Fire Disturbance, August 16, 2016: Key Finding #2 Inverters have three modes of operation: Continuous Operation: injecting current into the grid Trip: cease injecting current, disconnect from grid, wait ~5 mins, automatically return to service if voltage and frequency within bounds Momentary Cessation: momentarily cease injecting current during voltages outside continuous operating range 0.9 to 1.0 pu)

5 Blue Cut Fire Disturbance, August 16, 2016: NERC Alert

6 South Australian Blackout, 28 September, Pre-event: Severe weather forecast across state AC interconnector targeted at import limit of 435 MW AC interconnector at 507 MW pre-event DC interconnector at 106 MW Grid assessed as secure for N event Automated stability assessment tools were clear 16:18 hrs; 1,826 MW lost load; 850,000 customers affected

7 South Australian Blackout, 28 September, 2016: Sequence of events 7 Severe weather Loss of three EHV lines Wind speeds greater and more destructive than forecast; Tornadoes damaged three 275kV lines, 170km apart. Six voltage dips in 2mins, each triggering windfarm LVRT Multiple EHV faults Loss of 456 MW from nine wind farms in <7s; Wind farms not physically disconnected; Repeated LVRT protection caused loss of output Wind farm protection Loss of AC tie System Black MW deficit picked up on AC Interconnector Flow exceeds stable transfer levels Loss of Synchronism protection opens interconnector ~890 MW loss of infeed with only ~3,000 MW.s inertia; RoCoF ~6Hz/s; UFLS unable to arrest frequency decline to above 47 Hz 100 sec

8 8 South Australian System Event, March 3, sec Fine, clear day Explosive failure of 275 kv CVT at 15:03, damages adjacent 275 kv busbar Three EHV faults in 2 sec near main South Australian load and generation centre

9 South Australian System Event, March 3, 2017: Loss of generation, rooftop PV and load 9 Sequence of 275 kv faults resulted in: Loss of 410 MW of thermal generation Disconnection of 400 MW of customer load Loss of output from around 40% of installed rooftop PV systems Loss of 150 MW of rooftop PV output around, vs 732 MW installed, due to time of day Estimated from net change in system demand little firm data Timing of generation, load and rooftop PV changes was very important

10 Canyon 2 Fire Disturbance, October 9, 2017 Smoke-induced L-L fault events caused by Canyon 2 Fire Both fault cleared normally Fault Event 1: 220 kv L-L Fault < 3 cycle clearing Fault Event 2: 500 kv L-L Fault < 3 cycle clearing

11 11 Canyon 2 Fire Disturbance, October 9, 2017 ~15 minutes Event 1: = 682 MW Event 2: = 937 MW

12 Canyon 2 Fire Disturbance, October 9, 2017: Causes Continued use of momentary cessation Numerous other inverter-related issues: Ramp rate interactions with return from momentary cessation Interpretation of PRC voltage ride-through curve Instantaneous voltage tripping and measurement filtering Phase lock loop (PLL) synchronization issues DC reverse current tripping Transient interactions and ride-through considerations

13 13 Present State: Assessing Vulnerability Major Vulnerabilities Identified Momentary cessation LVRT settings Inverters react too quickly to waveform anomalies Recommendations Generator Owners should coordinate with their inverter manufacturer(s) to eliminate momentary cessation (MC) to the greatest extent possible. For inverters where MC cannot be eliminated (e.g., use another form of ride-through mode), MC settings should be changed; for example, reducing the MC low voltage threshold to the lowest value possible Review inverter response to waveform distortions and transients Review grid codes and incorporate modern plant capabilities Review operational risks from aggregate response of DER and bulk load

14 14 Future State Near-Term Apply recommendations from NERC Stay alert for undiscovered problems with existing/planned fleet Update generation connection requirements Improve system modelling, especially in light of low system strength Legacy modelling tools may not accurately predict performance under extreme conditions EMT models should be considered Long-term Regard predictable disturbance response of inverter connected plant critical to system resilience, particularly N-X or multiple events Regard performance of DER and bulk load potentially as important as transmission connected plant for system disturbance response Standardize models for control systems (similar to IEEE exciter and governor models) Expect surprises from new technology both in terms of new capabilities and new challenges

15 Thank You! 15