Technical review of DER interconnection is important to ensure that the DER will not cause reliability or safety issues for the power system. Technical reviews are typically designed and conducted by utilities. Regulators can, however, instruct or encourage utilities to implement changes to their technical review process to more efficiently and equitably assess DER interconnections.
As DER adoption accelerates, having effective review processes in place is critical to ensure that utilities are not overwhelmed and that DER customers do not face excessive delays, especially if their interconnecting systems are unlikely to cause issues. There are several tools and processes that can help more efficiently process growing numbers of applications.
DER interconnection applications undergo a technical review process that involves screening criteria to determine if the DER can be approved for installation quickly or needs more in-depth review.
Interconnection applications are subject to a technical review process by utilities prior to interconnection to ensure that the proposed DER system will not interfere with the safe and reliable operation of the distribution system. This review involves the implementation of various technical screening criteria (often referred to as screens) through a structured process. Regulators will typically either directly promulgate these screening requirements, or review and approve utility proposals to do so, or some combination thereof. These screens may either be developed by regulators and utilities themselves or may be adopted and adapted from model screens developed by other regulatory agencies....
Technical screening criteria, which are a series of technical questions about the proposed interconnected system, are intended to account for both the key operating parameters of the proposed interconnecting system as well as the actual conditions of the circuit to which the system intends to interconnect. Ultimately, these screens are designed to ensure that the level of review to which interconnecting systems are subjected is commensurate with their potential impact on the local distribution system. Technical screening criteria are typically framed in a technology-neutral format and often cover all generating facilities connecting to the distribution system....
Once an application for interconnection is received by the utility, there may be a series of initial application review questions to quickly determine if the system requires a more in-depth review or might immediately require a more detailed study. One important concern for utilities is whether or not the system uses precertified equipment, such as inverters certified under UL 1741... which guarantees a minimum threshold of safety and ensures the system will behave in a reliable and predictable manner. Using precertified equipment is typically required for systems seeking expedited, or fast tracked, approval. Precertified equipment, however, does not guarantee fast track approval.
In practice, systems will still be subject to initial technical review screens, usually in the form of simple, binary yes-no questions and statements, which cover fundamental operational behavior of the system (e.g., “Aggregated DG does not cause protective device to exceed 87.5% of short circuit interrupting capability”).... In many technical review processes, proposed systems that do not satisfy these qualifications may be subjected to a series of more in-depth supplemental review screens. If the proposed system passes these supplemental screens, then it may still qualify for a fast-tracked approval.
Excerpts from pages 44 - 45 of NREL: An Overview of Behind-the-Meter Solar-Plus-Storage Regulatory Design
The application and technical screening process can be put into a block decision-tree format, as the example below shows.
As the diagram shows, if the DER equipment is not certified or fails the screens, a detailed impact study may be required. These are costly and time consuming and should be avoided if possible.
Systems that fail both initial technical review screens and supplemental technical review screens will be subject to a detailed impact study to determine the extent of expected technical impacts and what mitigation measures, if any, can and should be taken before the system can be allowed to interconnect. These studies are typically paid for by the DER applicant and are only conducted if the DER applicant agrees to pay for the study. Impact studies, which can be performed by the utility themselves or be required from third-party groups, can be quite costly, especially relative to the cost smaller scale DER systems. Regulators are therefore often tasked with determining a fair and appropriate fee that utilities can charge or require of customers for their completion, as well as a reasonable time frame for completing the study.
Managing a significant number of interconnection applications can become administratively burdensome for utilities, and subjecting each application to a detailed review may exhaust utility resources or cause application processing times to exceed regulated time limits. Categorizing interconnection requests through a series of
increasingly more involved technical screens... helps reduce the administrative burden for utilities and average application times for customers without sacrificing the reliability of the power system. Furthermore, streamlining applications this way helps utilities focus their resources on reviewing proposed interconnections which are most likely to cause potential issues.
Excerpts from pages 45 - 46 of NREL: An Overview of Behind-the-Meter Solar-Plus-Storage Regulatory Design
Impact studies use power flow analyses to determine 'hosting capacity,' that is, the maximum level of installed capacity of a particular technology that a feeder can support without affecting safe and reliable operation.