Repowering PV Systems for Rapid Shutdown with UL 3741
More PV plants are operational today than at any time in history. Although asset owners view PV systems as “30-year assets,” the system’s components will age at different rates. PV modules, solid-state devices free of moving parts, should last decades in the field. Inverters, on the other hand, are likely to fail much faster. When an inverter or other major balance of system component(s) fails, it’s time to repower the system.
Repowering is a big, broad topic. This article will focus on how repowered projects can meet rapid shutdown (RSD) requirements by installing UL 3741-listed systems or module-level power electronics (MLPE).
What is a repower?
Many terms – such as repowering, revamping, and remediating – describe retrofitting an older system with newer components. For simplicity’s sake, we will use the term “repower” in this article, which covers a wide range of potential projects. A repowered project could be fairly straightforward (for example, replacing an old central inverter with a new one rated for the same capacity) but is more likely to be quite complex (for example, restringing a section of the array, replacing a central inverter with multiple string inverters, adding energy storage and rapid-shutdown-compliant MLPE, etc.).
Project owners, developers, and engineers must align on a repower project’s goals and compliance requirements. One crucial question to ask early in the process is, “Does the project need updating with more recent codes and standards? If so, which edition of the National Electrical Code (NEC) must the project adhere to?”
If the answer is NEC 2014 or later, and you’re working on a rooftop array, you’ll need to meet the rapid shutdown requirements from Section 690.12. If the answer is NEC 2017 or later, you’ll have two options to choose from:
- Install MLPE that is capable of limiting voltage in conductors within and beyond the array boundary. This option is the most common method of compliance, achieved by adding microinverters, optimizers, or other RSD devices to the existing (or updated) array.
- Install a UL 3741-compliant rooftop system. This option is possible but more challenging to achieve for reasons we’ll explain in the next section.
What is UL 3741?
UL 3741 is a system-level listing standard that provides one pathway to meet the rapid shutdown requirements for rooftop arrays in the NEC. Instead of installing MLPE, the system can be designed and installed under the language of UL 3741.
We’ve explained UL 3741 in earlier articles and webinars – and even published an hour-long online course on the topic. The image below shows the most common UL 3741-compliant array layout.
In the array above, assume that the inverter and the racking system are listed together under UL 3741 and installed per manufacturer instructions. The system will be rapid-shutdown compliant as long as the inverter input circuit is within the array boundary (1 ft) and the wire management follows the listing requirements. Sounds simple in theory, right? However, the main goal of any repower project is to reuse as much original equipment as possible. Installing a UL 3741-compliant system can be challenging when the original racking is in good condition, but the inverter needs replacement.
Why can inverter or racking procurement be a headache? Most often, the racking equipment dictates the UL 3741 system-level listings – the listing will be under the racking manufacturer and include a list of compliant inverters and other balance of system components. For example, racking manufacturer Unirac wrote in a May 2023 article that “Unirac now offers the largest number of racking (7 systems) and inverter (38 models) combinations that are UL 3741 compliant in the industry.” Competitor IronRidge announced in a 2024 tech bulletin, “The IronRidge XR Flush Mount System is now listed to UL 3741 with Tesla, Solis, and MidNite equipment.”
Note that the dates of those announcements are somewhat recent. In fact, given the relative newness of the UL 3741 standard, any racking or inverter equipment manufactured before 2022 won’t be listed to UL 3741. Thus, the easiest way to meet the UL 3741 standard for a repower project is to install a new system, which could include replacing the failed inverter(s) and all of the racking. The only way around this is to demonstrate that the original racking and wiring methods meet today’s UL 3741 listing requirements.
If installing a UL 3741-compliant repower system isn’t possible, only one option remains: install MLPE designed for module-level rapid shutdown. Could this entail restringing the entire array? Absolutely. But you’ll be able to reuse the existing racking as long as it’s in working condition.
Each repower project will look different. Asking the right questions early in the process will save many headaches down the line. Start by determining whether the new system will need to be rapid-shutdown compliant and develop an action plan that meets the budget and performance goals for the project.
Mayfield Renewables is a technical consultancy specializing in commercial and industrial PV and microgrid engineering. Contact us today for a consultation.
