NRES 00001 Published On: 04/26/2021
Question: For Dispatchability and Operational Flexibility, inverter-based resources can be dispatched downward incredibly quickly and can ramp upwards just as quickly if you hold headroom. Multiple studies have been conducted as well as real-world operations of solar providing Automatic Generation Control. You should consider a class of inverter-based resources that are procured to provide dispatch flexibility rather than just must-take. Inverter-based resources are required to be capable of providing VAR support and have a broader range of reactive power that can be provided compared to fossil. Are you capturing this in your reliability criteria?
Answer: DESC is aware of operational projects where solar provides Automatic Generation Control that is beneficial for other utilities. Traditionally, DESC models the resources that have been proposed and offered on the DESC system, and those proposed assets did not include solar providing AGC. DESC recognizes that part of the Stakeholder process is gaining feedback on the type of assets modeled and will consider these suggestions.
NRES 00002 Published On: 12/21/2021
Question: In which modeling were the uprates evaluated?
Answer: The associated modeling was performed for DESC’s Power Generation group to determine whether the uprates were a cost-effective upgrade.
NRES 00003 Published On: 12/21/2021
Question: The cost function analyzes carrying time, not revenue requirements. Is that not accurate?
Answer: With the associated financial variables, DESC can calculate the revenue requirement values from our output.
NRES 00004 Published On: 12/21/2021
Question: Do you mind commenting on potential gas transmission expansion associated with potential replacement resources and whether/how those gas transmission capital costs will be factored in?
Answer: Each candidate resource in the Retirement Study will have firm transportation costs associated with it. These will be part of the cost of resources as we build the model inputs.
NRES 00005 Published On: 03/18/2022
Question: In discussion of case 1, 4, and the conclusion new tie lines with SOCO interface are identified but not explored or price estimated in any way. Does DESC have plans to further explore the feasibility of additional tie lines with SOCO to potentially accommodate an incremental power purchase option?
Answer: Presumably replacing the existing capacity at Williams Station would not incur significant electric transmission costs, so this wasn’t seen as a scenario that needed to be studied as part of the TIA. New ties are being studied.
NRES 00006 Published On: 03/18/2022
Question: Were any n-1-1 violations rerun in powerworld for those dispatches?
Answer: DESC takes into account these violations and solves n-1-1 contingencies when possible.
NRES 00007 Published On: 03/18/2022
Question: So, in the summer, solar capacity is being dispatched at 100%?
Answer: Solar capacity was dispatched at approximately 81% of maximum rating.
NRES 00008 Published On: 03/18/2022
Question: What assumptions were used for dispatch of PV and BESS for each SUM Pk, WIN Pk, Spring cases?
Answer: In general, the peak cases assume all new resources will be fully dispatched, including solar and storage resources at Wateree. In the winter case, little to no dispatch of solar was included.
NRES 00009 Published On: 03/18/2022
Question: How were the power flow cases redispatched with the new generation mix?
Answer: Given that these units are new combine cycles, they are fully dispatched. At peak, all of the traditional baseload resources are dispatched. In the summer all contracted solar is also dispatched. Furthermore, all new CTs were dispatched first ahead of older technologies.
NRES 00010 Published On: 04/20/2022
Question: Solar/storage modeling, does this mean that you would assume the storage must be charged by the paired solar for the life of the project?
Answer: Paired solar and storage units are modeled only as a PPA. For these units, the battery is assumed to be charged only by the paired solar resource. DESC also models utility-owned battery storage units that are not under these constraints.
NRES 00011 Published On: 04/20/2022
Question: Does DESC plan to maintain the hard limit on battery charging for the lifetime of the paired solar + storage asset in their generic expansion plans?
Answer: Yes. For the purposes of modeling generic resource in the PLEXOS capacity expansion model, DESC intends to model paired solar plus storage PPAs under the configuration that the energy for the storage component is sourced from the paired solar component.
NRES 00012 Published On: 04/20/2022
Question: Does DESC plan on using the data set from the KCHS station to calibrate its models if it chooses not to use the NSRDB data?
Answer: No. DESC requests further input from Stakeholders on how these differences can be reconciled, but currently intends to use the data collected at actual solar sites on the system over a fewer number of years to inform the reliability study
NRES 00013 Published On: 04/20/2022
Question: Using PASA and LOLE to determine ELCC of resource. Does DESC plan to use this approach to determine a PRM? If so, how does DESC take into account battery storage charge and discharge scheduling, ITC constraints, etc.?
Answer: This process is a work in progress. The PASA model may not accurately reflect the contribution of storage resources as currently modeled and is exploring use the ST model to better characterize these units.
NRES 00014 Published On: 04/20/2022
Question: When is DESC likely to decide which ELCC method it goes with?
Answer: At this time, DESC intends to use a planning reserve margin for the 2023 IRP informed by the ongoing LOLE study. DESC also intends to use the LOLE analysis to inform the ELCC for solar and storage units for the 2023 IRP. However, DESC is committed to continuing to evaluate reliability approaches with Stakeholders through the IRP Advisory Group process.
NRES 00015 Published On: 04/20/2022
Question: Does DESC have plans to compare the NREL data to actual GHI values at other locations?
Answer: No. DESC requests further input from Stakeholders on how these differences can be reconciled, but currently intends to use the data collected at actual solar sites on the system over a fewer number of years to inform the reliability study.
NRES 00016 Published On: 06/17/2022
Question: While the Stakeholder Session VII provided some preliminary results for the portfolio LNPVs, it did not provide any information on selected candidate technologies for replacement. In the next stakeholder session, please provide this information, along with a discussion on why DESC believes each technology was selected (or not selected) by the model.
Answer: DESC provided the list of candidate resources in Session VI. A small number of specifications used in the actual PLEXOS retirement study have changed, these exact resource definitions will be provided to Stakeholders. Because of the nature of a resource optimization, it can be positively stated that the model chose each resource to meet the energy, demand, and reliability criteria at the lowest cost.
NRES 00017 Published On: 06/17/2022
Question: We have some concerns about the manner in which solar and storage are being treated. Specifically, the ITC should be applied regardless of ownership and hybrid batteries should not be constrained to be charged from the paired solar resource for the lifetime of the asset, but rather follow the ITC rules that require renewable charging for only the first five years.
Answer: In the Retirement Study both Company owned resources and PPAs include the benefit of the ITC. Expansion candidates are generic units and DESC will continue to evaluate paired resources as being charged by the solar component. DESC will model PPAs in configurations that are presented to the company through the RFP process.
NRES 00018 Published On: 06/17/2022
Question: DESC should analyze generation options for which no (or minimal) amount of transmission reinforcements would be required.
Answer: DESC will consider evaluating the option to replace some of Williams existing generation with some form of on-site resources in a future TIA scenario.
NRES 00019 Published On: 06/17/2022
Question: We are generally supportive of using the NREL ATB Advanced Technology Cost Scenario for solar and battery storage resources, provided it is updated to the latest available version. However, it is unclear from DESC’s stakeholder session material what the source will be for thermal resources.
Answer: As described in Stakeholder Session V and VI, the inputs for thermal units are sourced from the Company’s “green sheets”.
NRES 00020 Published On: 06/17/2022
Question: At higher levels of thermal unit retirements and increased solar + storage, resource adequacy becomes more energy constrained rather than capacity constrained. As a result, solar can provide significant resource adequacy benefits when combined with other resources. In the winter, solar can provide additional energy mid-day to recharge batteries in time for the second peak demand period in the evening.
Answer: DESC’s current inputs and optimization approach for paired PV solar and battery units allows for mid day charging in the manner described by Stakeholders. The model also has standalone PV solar and grid charged batteries available.
NRES 00021 Published On: 06/17/2022
Question: We agree that DESC should model a saturation effect for battery storage. In our experience, 4-hour storage saturation starts to occur between 15 20% of peak load. As a result, we recommend the following temporary ELCC values for storage: 100% Marginal ELCC at 750 MW ICAP, 80% at 1000MW, 60% at 1250MW, and 40% at 1500 MW. In addition, it is also important to provide PLEXOS with an 8-hour storage candidate with 100% ELCC.
Answer: Thank you for this feedback. DESC intends to employ a temporary schedule of declining ELCC as storage units are added to the system and will use these recommendations to influence the specifications of the 4 hour battery candidates for the 2022 IRP Update though some modifications may be made. DESC is also open to evaluating the ELCC value of an 8 hour battery as proposed by Stakeholders. DESC intends to use the results of the ELCC study to inform the 2023 IRP.
NRES 00022 Published On: 06/17/2022
Question: We recommend ex-post resource adequacy analysis on resulting battery storage portfolios to ensure they meet the reliability criteria of 0.1 days per year (or equivalent metric used by DESC). This “round trip” modeling can identify potential shortfalls or surplus capacity and better design a coal replacement portfolio.
Answer: Based on prior feedback provided by Stakeholders and DESC, the Commission has ordered DESC to use an LOLE based reserve margin analysis and ELCC based assessment of resource capacity value. DESC will comply with the Commission’s orders and use LOLE and ELCC methods through the 2023 IRP.
NRES 00023 Published On: 06/17/2022
Question: DESC does not include a proposal for ELCC or equivalent capacity accreditation for thermal resources. This introduces an implicit bias favoring new CT and CC resources. At a minimum, these resources should be discounted by the unforced capacity (UCAP), as is done in many jurisdictions. In addition, these resources should be reduced further due to the probability of correlated outages. We recommend new CT and CC resources have a 90% ELCC for firm capacity until a more detailed ELCC study can be conducted.
Answer: It’s correct that DESC does not include a value for ELCC or equivalent accreditation for thermal resources. Thermal resources are accredited with their summer and winter capacity. DESC disagrees that these units should be discounted and that there is a bias. Solar and battery resources have forced outage rates that are not used to discount their capacity contribution. The ELCC for solar and battery resources is based on their limited dispatchability. Thermal resource don’t share this limited dispatchability.
NRES 00024 Published On: 06/17/2022
Question: While the NSRDB data may not be perfect, we believe that the limitations of using historical observations from a select number of plants, as proposed by DESC, is significantly less robust. Historical observations would provide a short historical sample (as opposed to 23 years provided by the NREL NSRDB), amplify variability by not capturing geographic diversity and new resource configurations with much higher inverter loading rations (DC:AC ratios) that increase capacity factors and project economics.
Answer: DESC continues to be open to using NSRDB data if it can be validated as consistent with observed values. The Company welcomes Stakeholder suggestions on how this can be accomplished. There is limited geographic diversity in the service territory. DESC’s service area is relatively compact and prospective locations for new solar are concentrated between Columbia and Charleston.
NRES 00025 Published On: 06/17/2022
Question: We request that DESC provide stakeholders with either the full set of historical solar radiation observations taken from KCHS (Charleston International Airport) or direct stakeholders to the source data if it is publicly available.
Answer: This data is proprietary to DTN and cannot be shared publicly. Interested Stakeholders are encouraged to procure solar radiation observation data directly from DTN.