V. Defining How to Address Inadvertent Export
Chapter V Key Takeaways
The recommendations provided in Chapters III and IV are based upon the BATRIES project’s research on the potential impacts to the grid of inadvertent export, which are laid out in Chapter V: Defining How to Address Inadvertent Export. Inadvertent export is power that is unintentionally exported from a DER when load drops off suddenly, such as when an electric water heater switches off, before the export control system responds to the signal to limit or stop export. Inadvertent export events generally occur in behind-the-meter systems. As ESS deployment grows and more systems use export control means, utilities need to understand whether these inadvertent export events could impact the grid, and if so, how they should be accounted for when evaluating export-controlled ESS. Chapter V surveys how current standards treat inadvertent export and provides research findings based on modeling and analysis conducted by the BATRIES team to test the potential impacts of these events. To understand the range of worst-case impacts, the team conducted time-series analysis of an urban feeder and a rural feeder with exporting solar photovoltaic (PV) systems and non-exporting storage distributed along the feeders.
| Research, Modeling, and Analysis Findings Related to Defining How to Address Inadvertent Export: |
| 1. Testing indicates that open loop response times in a number of PCS products are significantly faster than the 30 seconds required by the UL Certification Requirement Decision (CRD) for PCS. These response times support the assertion that thermal impacts are unlikely to be a limiting factor for inadvertent export because both their level (110% maximum) and duration (typically 2-10 seconds) are below any known thresholds for concern. 2. Inadvertent export is a Root Mean Square (RMS) voltage event and fits into an Institute of Electrical and Electronics Engineers (IEEE) defined event category. Therefore, it is appropriate to use the short-term RMS event limit of 110% instead of the steady-state limit of 105%. This creates more headroom for inadvertent export in most feeders. 3. Time-series modeling is an effective way to evaluate RMS voltage impacts caused by inadvertent export. 4. Feeders can host more DER capacity if the DER is export-controlled. This can be viewed as increasing the feeder’s available hosting capacity for nameplate DER or as a more efficient use of existing feeder capacity for DERs. While both the urban and rural feeder assessments supported this finding, the extent to which hosting capacity can be increased will depend on feeder characteristics, as well as the location and size of the exporting DER. 5. DER capacity on the urban feeder could be doubled with export limiting (inadvertent export) compared to steady export, without exceeding RMS voltage rise limits. 6. The rural feeder’s capacity for inadvertent export is very location dependent. The capacity to support DER drops off more steeply in the longer rural feeder. The main limiting factors were found to be coordination of voltage regulator equipment operations and maintaining voltage balance between phases (not seen in the urban feeder). 7. The value of faster control response was more apparent on the rural feeder than the urban feeder. This observation is based on the interactions of line voltage regulators with inadvertent export events. Regulators lead to more step changes in voltage and voltage unbalance. This may be a limiting factor for export-controlled energy storage in long feeders (not seen in the urban feeder). 8. The impact of smart inverter functions such as volt-var and volt-watt is unclear as these functions were not activated during simulation. This needs further investigation in the future. |
A. Introduction and Problem Statement
Distributed energy resources that are configured for non- or limited-export operation using certain export control methods may, under certain conditions, inadvertently output small amounts of power to the grid for short durations of time. This phenomenon is the result of non-instantaneous control system response times due to large swings in generation and load. While not widely considered a significant threat to grid reliability today, these unintentional injections of current onto the distribution system potentially pose power quality risks as a greater number of areas approach higher DER penetrations and as larger energy storage (and solar-plus-storage) systems with greater Export Capacity proliferate.
It is currently unclear if, or the degree to which, grid power injections from inadvertent export may cause power quality disturbances that exceed norms and standards, including ANSI C82.1 specifications.[1]The American National Standards Institute (ANSI) is a private non-profit organization that oversees the development of voluntary consensus standards for U.S. products and services. ANSI accredits … Continue reading Meanwhile, no uniform specification or requirement currently exists for manufacturers to follow regarding ESS response time to limit inadvertent export. Simply put, storage systems may generate inadvertent export at different times and magnitudes, with the potential to create voltage or thermal disturbances that are not well-characterized.
Most interconnection rules do not define how utilities specify or evaluate inadvertent export that occurs while ESS controls are responding. In many cases, utilities screen and study projects with inadvertent export in the same way that they assess projects with full export. Moreover, different utilities in different jurisdictions may have varying requirements for inadvertent export, or dissimilar methods for measuring it. This variation can create challenges for equipment manufacturers, who must consequently create tailored solutions for different utilities. The lack of clarity regarding the impacts of inadvertent export and the optimal way to manage or prevent impacts is a noteworthy interconnection barrier for ESS. Projects may, as a result, be assumed to have impacts they possibly never produce. In turn, these concerns may require more in-depth review, customized equipment design, and/or grid mitigation that adds cost and time to the ESS interconnection process.
This chapter provides analytical results from modeling and simulation research that explore the potential for adverse power quality and other impacts caused by inadvertent export. Based on the results, the chapter provides key findings regarding Power Control System response time requirements to limit inadvertent export, as well as on other considerations for both recognizing and addressing the potential for disturbances caused by inadvertent export. Results can be used to modify existing interconnection procedures, applicable standards, and testing procedures.
Download the chapter file below to read more. Or download the full Toolkit and Guidance for the Interconnection of Energy Storage and Solar-Plus-Storage for all chapters and recommendations.
References
| ↑1 | The American National Standards Institute (ANSI) is a private non-profit organization that oversees the development of voluntary consensus standards for U.S. products and services. ANSI accredits standards developed by others that ensure consistency in product performance and conformance with testing protocols. |
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