D. Effective Grounding
Power system effective grounding manages temporary overvoltage during ground faults. With DERs, an overvoltage risk can be created by backfeeding a ground fault when a portion of the system is unintentionally islanded. For certain DERs (such as rotating machines) and interconnection transformer configurations, supplemental grounding is often required to prevent damaging ground fault overvoltage when islanded.
Since inverters act quite differently from rotating machines during ground faults, they generally have less of a need for supplemental grounding. Engineers may be designing unneeded supplemental grounding into inverter-based DER systems by applying concepts based on rotating machines. Not only can this result in extra costs to the DER system, but excess grounding can also have a negative impact on distribution system protection, and should be avoided. Utility practices for effective grounding are now evolving to address inverters appropriately. However, those practices are not yet widespread; therefore regulators should ensure that interconnection procedures properly evaluate the risk for ground faults from inverter-based machines.
The IEEE C62.92.6, Guide for Application of Neutral Grounding in Electrical Utility Systems, Part VI – Systems Supplied by Current-Regulated Sources was published in 2018 to address system grounding with inverters. Part VI of the long-standing recommended practices of the IEEE C62.92 series for power system grounding gives guidance that can be used by utility engineers for inverter-based resources. The guide clarifies important differences between rotating machines and inverter-based DERs. Interconnection rules should reference it, as it includes topics that are not widely known by many engineers who are not intimately familiar with power electronics.
Acknowledging the important differences of inverter-based DERs is the first step to avoid misapplication of the typical grounding concepts and practices used for rotating machines. IEEE C62.92 (including parts I through V) is the accepted power system grounding standard for all resources, including central power plants, transmission, and distribution systems. Part VI contrasts the straightforward characterization of rotating machines with the less well-defined inverter responses. Topics covered in IEEE C62.92.6 include essential areas such as symmetrical component characteristics, ground-fault overvoltage calculations, effective grounding, and the effectiveness or adverse impacts of supplemental ground sources.
Implementing the performance requirements of IEEE 1547-2018 is another critical step in managing overvoltage with DERs. The standard provides definitive overvoltage performance limits to expect when interconnecting a certified DER. As one of several power quality requirements, subclause 7.4 limits any overvoltage, including due to ground faults or load rejection.
IEEE 1547.1-2020 subclauses 5.17 and 5.18 provide testing and certification requirements related to the overvoltage limits, which allow inverter manufacturers to provide data that complement the usage of IEEE C62.92.6. IEEE P1547.2 provides guidance on how to ground inverter-based DERs, and should be referenced during related grounding evaluations.
It is important that utilities perform grounding evaluations with a full understanding of inverters’ unique characteristics, which affect the outcomes of those evaluations. To this end, the standards discussed here should be used in interconnection rules’ grounding requirements. Without knowledge of these standards, engineers may continue to over-specify grounding needs.
The line configuration screen, typically found in Fast Track (such as SGIP 126.96.36.199) acts as a proxy grounding evaluation. As written in SGIP and most jurisdictions today, it does not take into account differences in grounding needs between rotating machines and inverter-based DERs. This can cause projects to fail the screen and/or be subject to unnecessary upgrades. EPRI has researched and written about how to update screening and interconnection practices with regard to inverters, including guidelines for determining supplemental grounding needs.((Electric Power Research Institute, Effective Grounding and Inverter-based Generation: A “New” Look at an “Old” Subject (Sept. 19, 2019), https://www.epri.com/research/products/000000003002015945.))
The recommendations below are couched within the constraints of how screening (including Supplemental Review) is done today. They modernize the existing screening process for effective grounding, without attempting to completely change the screening process. However, interconnection practices may need to evolve more dramatically to use modern analytical tools to streamline processing of all types of DERs for all relevant distribution system concerns (not just effective grounding).
Screening for grounding would ideally be incorporated in the Initial Review from a process efficiency standpoint. However, the data and tools needed to evaluate effective grounding may require more extensive resources (time and expertise) than would typically be available within the Initial Review process. Thus, it may be more feasible to incorporate such screening within Supplemental Review, as noted in recommendation 5 below. Whether such screens are incorporated within Initial Review or Supplemental Review should be determined through discussions with utilities and stakeholders. Note that for intentional islands, grounding requirements will vary from those that apply in grid-connected mode.
- To ensure inverter-based resources are appropriately addressed by technical requirements, any effective grounding requirements for inverter-based resources should align with or reference IEEE C62.92.6, IEEE 1547.2 (once published), and IEEE 1547-2018 subclause 7.4.
- If there are references to grounding reviews in the description of the interconnection studies (e.g., system impact and feasibly studies), then interconnection procedures should require the use of IEEE C62.92.6, IEEE 1547.2 (once published), and the test data from IEEE 1547.1-2020 for the review of inverter-based resources. If references to grounding reviews appear in agreements related to the studies (such as Attachments 6 and 7 of SGIP), they should also align with or reference IEEE C62.92.6, IEEE 1547.2 (once published), and IEEE 1547-2018 subclause 7.4.
As an example, in SGIP attachment 6 (section 6.3), the following language can be added:
Review of grounding requirements shall include review per IEEE C62.92.6 and IEEE 1547.2 for inverter-based DER when additional grounding equipment is considered.
- If the utility requires supplemental grounding, relevant guidance should be provided in the technical requirements document or interconnection handbook.
- Revise the line configuration screen (SGIP 188.8.131.52) by updating the table as follows.
- One of the following three recommendations should be utilized to properly account for effective grounding within Fast Track review. The approach used will vary depending on the ability to integrate necessary tools and available resources. The recommendations are organized in order of increasing complexity.
- Include a new Supplemental Review screen for three-phase inverters as follows. If it is feasible to evaluate this screen during Initial Review, it may be used in lieu of the line configuration screen to evaluate three-phase inverters.
The Line-to-Neutral connected load on the feeder or line-section is greater than 33% of peak load on the feeder or line-section.
- Alternatively, use a tool, such as the Inverter-Based Supplemental Grounding Tool created by EPRI, to determine if supplemental grounding is required to maintain effective grounding. If supplemental grounding is not needed, then the system would pass the screen. If supplemental grounding is required, then provide for the option to modify the DER system to include the necessary grounding equipment, without proceeding to full study before the interconnection agreement is provided.
Additionally, a detailed Hosting Capacity Analysis that incorporates evaluation of temporary overvoltage risk for inverters may be used in lieu of the screen mentioned in recommendation 4. If the aggregate DER rating is below the HCA limit, then this screen would be passed.