Rule 21 Engineering Review Process

The existing Rule 21 tariff structure clearly defines interconnection study time lines. The initial review is the first technical review, which is part of the fast-track process (15 days). If necessary, from there the project goes into supplemental review (30 days). The overall review process begins and ends within 45 days, an extremely reasonable and well-defined window. Until recently, many projects fell within this window, and developers could clearly state expectations to their clients. Although detailed studies were an option, utilities exercised this option only in specific circumstances, such as a customer wanting to use inverters not listed for UL 1741. This all changed recently, and detailed studies are the new norm. Grid congestion is the driving force behind this change.

Screening process. As illustrated in Flowchart 1, Rule 21 begins with a review process that consists of 13 screens (Screens A through M), which apply broad rules of thumb to see if a DER system will have a substantial impact on the distribution system. Pass these screens, and your project will sail smoothly toward interconnection. Fail any of these screens, and your project gets sent to supplemental review, which adds three more screens (Screens N through P) that check various grid-impact parameters. Screen N, the penetration test, is a turning point for most projects—this is where a project can start to veer off course dramatically.

Screen N checks to see if there is a potential for power to backfeed through the substation into the transmission network. It aggregates all distributed generation on a bank and nets that value against the absolute minimum load with the contingency of the largest load suddenly dropping offline. This is known as an N-1 contingency scenario, which means that in the worst case the substation could backfeed for what is generally understood to be a brief period.

Reverse power flow onto transmission networks is a precarious situation with the potential to cause wide-scale network problems affecting millions of customers. Unlike distribution feeders, which are radial in nature, the transmission network is a complex series of bus ties and high-voltage switches that typically loops nodes together, creating a mesh with an almost infinite number of pathways the power can travel. Analyzing the potential safety and reliability impacts of backfeeding on a power system at this large scale is extremely complex.

Detailed studies. As shown in Flowchart 2, when a project fails Screen N and enters a detailed study, that process starts with the electrical independence test (EIT), Screens Q and R. This is a 20-day review time line that assesses how much of the project’s generated power will actually impact the transmission network. If the project is deemed electrically independent and passes screens Q and R, it enters an independent study, which carries a 60-day time line. If the project fails one of the electrical independence test screens, it can go one of two ways: into a distribution group study or into a transmission cluster study.

A window to enter the distribution group study opens twice a year, once between March 1 and March 31 and then

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