In a recently released joint report, NERC and the Texas Regional Entity (TRE) have issued an analysis and recommendations related to the second Odessa, Texas area power disturbance which occurred on June 4, 2022 (Odessa II Report). The report outlines in detail the failure modes and reliability risks resulting from the use of Inverter-based resource (IBR) control systems. NERC and TRE recommend that regulated utilities take multiple immediate actions to assure the preservation of reliability as the transition to IBR based renewable generating resources continues. Changes may to interconnection study and approval processes may be required going forward.
The Odessa I and Odessa II disturbance events are the most recent to highlight the inability of current IBR technology projects to “ride through” power disruptions to avoid tripping offline or reducing output during and after routine system disturbances. Until recent years, the power system was designed with the generation fleet consisting predominately of synchronous generation that through inertia, would ride through disturbance events such as routine trip and reclose operation of transmission lines.
In both Odessa disturbance analysis reports a sizable number of solar PV resources over a wide area responded abnormally to routine disturbance, resulting in a poor recovery form the event. Many solar PV sites reduced power output. The unexpected reduction in solar PV power output for the Odessa II event totaled over 1,700 MW. Many of the solar PV resources that responded abnormally were large Bulk Electric (utility scale) facilities.
Causes of abnormal solar PV performance
The table below provides a list and comparison of the causes of abnormal solar PV performance for the 2021 and 2022 Odessa disturbances.
|Table 1.1: Causes of Solar PV Active Power Reduction
|Cause of Reduction
|Odessa 2021 Reduction [MV]
|Odessa 2022 Reduction [MV]
|Inverter Instantaneous AC Overcurrent
|Passive Anti-Islanding (Phase Jump)
|Inverter Instantaneous AC Overvoltage
|Inverter DC Bus Voltage Unbalanced
|Inverter DC Bus Voltage Unbalanced
|Incorrect Ride – Through Configuration
|Plant Controller Interactions
|PLL Loss of Synchronism
|Feeder AC Overvoltage
*In addition to inverter-level tripping (not included in total tripping calculation)
**Power supply failure
NERC and TRE note that changes to dynamic models and model management practices, particularly Electromagnetic Transient (EMT) models, will be needed to reflect new inverter control strategies implemented in IBR inverters. Therefore, all Generator Owners (GOs) making changes to their inverters should submit updated EMT models to their respective Transmission Planner (TP) and Panning Coordinator (PC) to ensure accurate models are maintained and that the GO is compliant with all applicable and future NERC Reliability Standards.
The report goes into great detail regarding each of the failure modes and provides information useful for transmission owner planning departments to address power system performance.
Recommended Follow Up Activities and Key Findings
The Odessa II report notes that ERCOT has worked with all GOs with solar PV facilities involved in the 2022 Odessa Disturbance to ensure that mitigating measures are being implemented to address the unreliable performance issues observed. Key findings include:
- ERCOT has followed up extensively with every affected facility to seek corrective actions to mitigate future performance issues.
- Inverter manufacturers have been developing mitigating actions to address the causes of reduction that occurred at multiple facilities. These corrections include the following:
- Disabling passive anti-islanding protection or increasing trip thresholds
- Increasing ac overvoltage protection settings closer to equipment ratings
- Lengthening frequency protection timers to avoid tripping on instantaneous spikes in frequency calculations
- Deploying inverter firmware updates to mitigate possible dc bus voltage imbalance issues
- Enabling appropriate fault ride-through modes of operations
- Modifying fault ride-through settings to ensure stable inverter response
The report observes that significant deficiencies exist for inverter-based resources both in positive sequence and EMT models. This includes the use of standard library models that do not match actual inverter control operation, incorrect parameterization of the IBR control models, insufficient IBR control model fidelity (i.e., missing protections or controls), and lack of IBR control model quality checks.
TRC clients should review the Odessa II Report in detail and consider implementing changes to their interconnection study and approval processes going forward. IBR controls management will continue to be a high priority area for future regulatory compliance changes.
Your Trusted Regulatory Advisor:
The forgoing report documents a significant disturbance event which will impact future regulatory decisions. S. TRC closely follows the national and state regulatory trends in all regions of North America. Our approach to power system engineering, planning, design, construction and commissioning testing, balances solutions that incorporate industry reliability risk trends, mandatory reliability standard requirements, regulatory guidance, compliance obligations, best practices, operational goals, and budgets. With expertise in power system engineering, planning and operations, TRC supports public utilities and private energy providers in their efforts to stay ahead of the curve and to meet or exceed regulatory requirements as they evolve.
This regulatory update is provided as a service to TRC’s utility clients, helping to keep you informed of forward-looking issues that will impact your company’s electric system reliability risks along with related topics regarding regulatory developments to help you achieve your company’s business goals.