Let’s face it, accurate and reality-based modeling of the dynamic performance of inverter-based resources is not easy. The quality of the analysis performed, and thus the reliability of the bulk power system (BPS), depends on the accuracy, fidelity, and appropriate parameterization of the models used. The first step to improving inverter-based resource model accuracy is to have a firm understanding and documentation of what settings are installed (or intended to be installed) on site, since this information provides the foundation for a model that accurately represents both the facility’s ride-though and normal operation performance.
Detailed coordination between all stakeholders is necessary to ensure an understanding of the equipment settings and capabilities and ensure that the models are created reflecting the inverter-based resource’s performance. The models must have been sufficiently tested to ensure BPS stability prior to interconnection, and correct and up-to-date models must be used throughout the lifetime of the facility. When sufficient due diligence is not carried out regarding an inverter-based resource, those in charge of BPS reliability are left to carry out their studies with unverified building blocks. This can cast serious doubt on the outcomes of those studies and creates significant risk to BPS reliability.
In an attempt to minimize this reliability risk, the North American Electric Reliability Corporation (NERC) will be issuing a Level 2 Alert that provides recommendations focused on improved performance of inverter-based resources, in particular, solar photovoltaic (PV) facilities. This will allow NERC to obtain data to understand the extent of solar PV performance issues. This includes improving ride-through performance and coordination with equipment manufacturers to obtain and provide detailed information regarding operating modes and ride-through capabilities.
How Bad Can It Really Be?
NERC has analyzed numerous large-scale disturbances. One of the main themes uncovered is that the ride-through performance of the affected inverter-based resources was not sufficiently captured in the resource’s model or reliability studies. Modeling inaccuracies have been observed in numerous NERC disturbance reports, and they were a key finding once again in the most recent 2022 Odessa Disturbance Report. Of the approximately 1700 MW of inverter-based resources involved in the disturbance studied, none of the positive sequence or electromagnetic transient (EMT ) models were able to recreate their facility’s ride-through performance.
This is not a direct reflection on planning coordinators and transmission planners since, as mentioned above, producing an accurate dynamic model is not easy and the planning coordinators and transmission planners are doing the best they can with the inputs they receive. However, the BPS cannot afford to continue to be studied with inaccurate models, just as it is not a good idea to keep adding objects on top of a frozen pond without really knowing the objects’ weight. The reliability of the BPS now comes with the same uncertainty as the metaphor—at some point, the errors between expected (studied) performance and actual performance will cause the ice to break at an unexpected time.
We Need to Start Somewhere
Industry needs to work together to address the systemic lack of predictable ride-through performance of inverter-based resources. While the upcoming Level 2 NERC Alert is focused predominantly on generator owners of solar PV resources, the recommendations can and should be applied to all inverter-based resources.
Recommendations within the alert aim to provide guidance for generator owners to coordinate with their respective equipment manufacturers (of the inverter and plant controller) to:
- Ensure the inverter and facility-level protection settings are set to maximize ride-through capability without damaging equipment and to avoid inadvertent tripping operations.
- Determine whether or not the inverters used at the generator owner’s facilities have been observed to exhibit inadvertent protection operation in any other facilities, and, if so, to complete mitigations promptly.
- Ensure that inverter and plant controller modes and parameters are set to maximize support to the BPS during and after a disturbance and that these settings are coordinated with the installed protections to avoid inadvertent trip operations.
- Provide, when necessary, detailed technical justifications for observed performance deficiencies.
- Coordinate such that the overall reactive power capability of the inverter-based resource is not artificially limited in the inverter or plant controller.
In addition to these recommendations, this alert includes data collection for NERC to assess the “extent of condition” of potential risks to the BPS. The Level 2 Alert requires mandatory information reporting where generator owners will submit protection, ride-through, and other performance information to allow NERC to understand these risks clearly.
While this NERC Alert is not a silver bullet that will mitigate all reliability risks, it will be a good first step towards quantifying the extent of inverter-based resource ride-through performance issues and provide a foundation for future work that will help to improve BPS reliability. This, coupled with the recent Federal Energy Regulatory Commission (FERC) reliability directive in Docket No. RM22-12-000, is a big step in the right direction of mitigating these ride-through issues. Frankly, these issues should have been collectively behind us as an industry twenty years ago.
If industry can work together to adopt the recommendations in the upcoming NERC Alert and make significant improvements to dynamic model accuracy and fidelity, we should see better correlation between what is observed in studies and what happens in the real world.
Alex Shattuck, Senior Engineer
North American Electric Reliability Corporation