The grid is changing, and we are starting to feel excited and frightened at the same time. If you aren’t, you should be. Because this stuff is frighteningly exciting. Many of us, including our friends, family, neighbors, and colleagues, may have rooftop solar panels, battery storage in our garages, electric vehicles in our driveways, or converter-connected devices strewn about our houses. Technological advancements are posing new challenges and risks for engineers across the world. Our focus has turned to ensuring a reliable and resilient bulk power system (BPS) in the face of these rapid changes.
In my unique role as one of the many stewards of reliability of the North American BPS, I interact with wide reaches of the electric industry. Maintaining reliability of the grid is a collective responsibility of multiple entities – utility planners, distribution and transmission engineers, generating resources, regulatory agencies, research institutions, academia, consultants, software vendors, and equipment manufacturers. NERC has worked exceptionally hard over the past few years to bring the right people together to address some of the most difficult challenges. Integration of massive amounts of distributed energy resources (DERs) is one of those challenges. Enter stage: SPIDERWG.
The NERC System Planning Impacts of Distributed Energy Resources Working Group (SPIDERWG) was formed with one purpose: provide a venue for the industry to come together to address these challenges because the future is not giving us a choice otherwise. You may be thinking “what does NERC have to do with the distribution system?”, and that is a valid question. The answer is simple: the aggregate behavior of DER is already having a noticeable impact on BPS performance (see Figure 1). It is incumbent upon us to handle these impacts now and into the future.
Figure 1: Individual Load Bank Responses to Angeles Forest Disturbance
Based on discussions with grid planners across the industry, it was immediately clear that the modeling, verification, studies, and coordination of DER needed attention. SPIDERWG created sub-groups to address these issues in depth, with a clear mission: “don’t admire the problem; address it.”
Modeling: Developing DER Modeling Capabilities
A planner needs a set of sufficiently representative models to adequately study the impacts of DER on the BPS. NERC previously published two Reliability Guidelines on DER modeling (here and here), and recently published a guideline on parameterizing the DER_A dynamic model (see Figure 2) . For planners to reasonably populate this model, they need information from distribution entities regarding aggregate DER connected to their grid. In some ways, we need a translation between the BPS planner needs and the distribution entity data available to help facilitate this exchange. SPIDERWG is developing guidance on this topic and reviewing the NERC MOD-032-1 standard to ensure sufficient data is available for the purposes of transmission planning. Lastly, powerflow and transient stability programs need to adapt to the new nature of modeling and studies. SPIDERWG is tracking areas of improvement in DER modeling and will be making recommendations to software vendors for improvements to representing and studying DERs for BPS reliability assessments.
Figure 2: DER_A Block Diagram
Verification: Ensuring DER Models Accurately Reflect Reality
With the ability to accurately model aggregate DER, focus turns to verifying that the data entered into the models is a reasonable representation of actual or expected performance. This requires some degree of verification of the data used for modeling versus actual system data, which can include forecast information and any disturbance data available. For example, ISO-NE in 2014 forecasted their distributed solar PV in 2019 would be 1,500 MW, yet their nearest-term 2019 forecast shows roughly 3,500 MW. Figure 3 shows how their forecasting methods are improving; however, year-over-year they continue to underestimate the penetration of DER. We can collectively work together to minimize these potential issues, and have much to learn from entities like ISO-NE who are leading the way sharing their findings and expertise. Further, how are planners tracking the vintage of IEEE Std. 1547 (or other local requirements) for the existing and future installations of DER? How does that translate into modeling assumptions? Can that be compared with historical disturbances? How are unbalanced faults accounted for? SPIDERWG seeks to provide guidance for these issues.
Figure 3: ISO-NE Annual Forecasts in Distributed Solar PV [Source: ISO-NE]
Studies: Performing Reliability Studies Involving Increasing Penetrations of DER
With suitable DER models that planners can use to represent DER in reliability assessments, focus turns to how increasing penetrations of DER can affect BPS reliability in planning assessments. For BPS faults and contingency events, will DER tripping or dynamic response impact primary frequency response, dynamic voltage response, transient stability, etc.? Can future DER be programmed and utilized to support BPS reliability? How can DER management systems (DERMS) help provide these services? What are the worst case study scenarios under high penetration DER conditions? For example NERC TPL-001-4 requires that “Peak Load” and “Off-Peak Load” be studied; however, is this gross or net load conditions? Which of the two is more impactful to study? SPIDERWG is performing a comprehensive review of the NERC TPL-001-4 standard and identifying areas for clarity regarding DER. Assumptions like these are critical to capture for identifying potential reliability issues. With DER being rapidly deployed across North America, the conventional assumptions and studies used to conduct NERC TPL reliability assessments need to be re-evaluated and adapted for the coming new grid.
Coordination: Crossing the Border of Transmission and Distribution
We collectively keep hearing that the transmission and distribution entities are not communicating or sharing sufficient data to help facilitate effective data exchange. Further, entities who are communicating are challenged with understanding the needs on both sides. Industry guidance and clarity is needed for helping the transmission entities obtain the information they need to maintain a reliable BPS. Similarly, the distribution entities need a clear understanding of why this information is needed, and what type of information is needed. The following situation is a great example: transmission planners do not need to know where every DER exists on the distribution system, but they need information regarding aggregate DER information and other key data to make engineering judgments for their studies. As the penetration of DER increases, the detail and accuracy of the data becomes more important.
Where Do We Go From Here?
For the time being, we are focusing on the planning impacts of aggregate amounts of DER. But we regularly hear that we fundamentally need to “plan it like we operate it, and operate like we plan it”. Our focus will need to turn from the planning horizon to the operations horizon. Many operators face significant challenges regarding DER modeling in energy management systems, data availability, forecasting accuracy, and unknown DER tripping, among other issues. The future is full of exciting new challenges for the North American power grid.
If you are interested in participating in the NERC SPIDERWG and any of its sub-groups, feel free to reach out to me (firstname.lastname@example.org) or JP Skeath (email@example.com). We’re happy to have you involved!
Senior Manager, Advanced Analytics and Modeling