More Details About Safe Solar Interconnections

Safely interconnecting a new distributed generation (DG) solar power system requires TEP to review the unique circumstances of each proposed project, including its location on our local energy grid. The following questions and answers provide more details about this review.

Why does it matter where a proposed DG system is located?

Just like the electric system in your home, our grid is divided into circuits – each of which serves hundreds or thousands of homes and businesses. To provide reliable service, TEP must maintain a constant balance of supply and demand on each circuit without exceeding the circuit’s maximum capacity. This requires energy as well as voltage support, which TEP’s system is designed to provide.

Solar DG systems affect this balance by intermittently exporting energy that requires additional voltage support. If enough DG systems are located on the same circuit, their excess output can sometimes exceed energy demand on that circuit. This pushes energy back through the substation transformers TEP typically uses to deliver power, limiting our ability to regulate voltage on the circuit.

What problems can result from excessive DG energy on a distribution circuit?

Excess DG output can make it difficult for TEP to regulate voltage on distribution circuits, causing power quality issues that can potentially damage sensitive customer electrical equipment. These problems can be exacerbated during power outages, when DG exports can create what electrical engineers call “islanding” – an isolated, energized area with unregulated, fluctuating voltage. Islanding also creates a life-threatening risk for utility crews who may be working in the area on equipment that should be de-energized.

Has TEP tried to limit the number of DG systems installed on the same circuit?

No. TEP has worked with local solar installers to support interconnection of new DG solar power systems for any customers who have wanted them. This allowed solar generating capacity on some circuits to reach or exceed levels that now trigger additional review and requirements under Arizona’s Distributed Generation Interconnection Rules (DGIRs). Those rules were approved by the Arizona Corporation Commission (ACC) and took effect on Feb. 25, 2020.

Which areas are served by circuits that already contain enough DG capacity to trigger additional review and requirements?

TEP has created DG Saturation Maps showing areas served by circuits where DG capacity already exceeds levels referenced in the DGIR screening tests.

Are new DG solar power systems prohibited in areas shown in these maps?

No. But to comply with DGIR requirements, customers in these areas should expect to make modifications to standard DG system designs to avoid any negative impacts to safety, power quality and service reliability. These modifications could include direct transfer trip equipment to prevent “islanding” or the installation of enough energy storage to qualify the system as a Non-Export or Inadvertent Export Generating Facility under the state’s rules. The cost of these upgrades could make DG systems unaffordable for some customers.

Can TEP switch my home onto a different circuit so I can install DG without additional upgrades?

No. Each of our circuits has been developed to serve energy needs in a specific area, and our grid isn’t designed to serve individual customers in one area from a circuit located elsewhere.

How does TEP calculate the DG “hosting capacity” of a circuit as part of the “Screen A” evaluation required by Arizona’s interconnection rules?

TEP sets that capacity based on the circuit’s minimum daytime load over the most recent 12-month period, as measured at the substation. Because that same metric is included as part of the supplemental review, integrating it as part of Screen A provides a preview of how that additional study is likely to turn out. In other words, a proposed system that fails Screen A will also fail the minimum daytime load screen included in the supplemental review.

Why does Screen A also include the option of measuring a circuit’s combined DG capacity against 15 percent of peak system load?

That standard is integrated in electric reliability and safety rules employed in other states, where it usually serves as a rough proxy for minimum daytime load. The minimum daytime load on most TEP circuits usually exceeds that level. Our Screen A evaluation will use whichever figure was higher over the previous 12 months, providing applicants with the best opportunity for approval of a new DG system.