Module-Level Rapid Shutdown for Commercial Applications

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  • Module-Level Rapid Shutdown for Commercial Applications
    Module-Level Rapid Shutdown for Commercial Applications
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    Ten K Solar uses MLPE to control the dc bus in its DUO PV system to 60 V or less. Due to its matrix cell architecture, the internal module voltage is always less than 16 Vdc, which is insufficient to...
  • MW-scale MLPE
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  • Module-Level Rapid Shutdown for Commercial Applications
  • Low-voltage parallel architecture
  • MW-scale MLPE
  • False sense of security?
  • Low-voltage inversion
  • Integrated MLPE
  • Flex MLPE

The fire service wants module-level rapid shutdown. But is this commercially viable in nonresidential applications? And will this reduce hazards within the array?

While it has proven relatively easy for solar companies to comply with the rapid-shutdown requirements in NEC 2014, many in the solar industry are justifiably concerned about the implications of the revised and more restrictive rapid-shutdown requirements adopted as part of the 2017 cycle of revisions. Specifically, the International Association for Fire Fighters introduced language that seeks to mandate module-level rapid shutdown for PV systems on buildings. This would, of course, require module-level disconnecting devices for all building-mounted PV modules, including those on commercial rooftops, which is a daunting paradigm shift in terms of both system reliability and economic viability.

In this article, I explore different perspectives on the prospects of deploying module-level power electronics (MLPE) in commercial rooftop applications in light of these evolving rapid-shutdown requirements. Generally speaking, there are two sides to the debate. On one hand, fire service representatives and some MLPE vendors contend that module-level rapid shutdown will improve safety for firefighters and first responders. On the other, the Solar Energy Industries Association (SEIA) and some of its most prominent constituents—including  SolarCity, SunPower and Sunrun—point out that there is no scientific basis for using module-level rapid shutdown to protect emergency responders and that doing so could have unintended negative consequences. Staking out a middle ground, UL promises to take a science-based approach as it develops an NEC 2017 rapid-shutdown PV array product safety standard.

Evolving Rapid-Shutdown Requirements

Introduced as part of the 2014 cycle of revisions and significantly revised for the 2017 Code, the goal of NEC 690.12 is to reduce shock hazards for emergency responders.

2014 version. Under NEC 2014, rapid shutdown of PV system circuits on buildings is accomplished by limiting the potential of controlled conductors to 30 V or less beyond 5 or 10 feet of the array, depending on whether the conductors enter the building or travel along its exterior. Markets have generally shrugged off this Code change and continued apace. Certainly, states that adopted NEC 2014 early, such as Massachusetts, experienced some growing pains associated with rapid-shutdown compliance and enforcement. However, system designers were quick to identify and implement a standard set of cost-effective and application-specific approaches to rapid shutdown.

Where the 2014 version of rapid shutdown becomes the law of the land, residential markets shift away from string inverter–based designs in favor of MLPE-based designs. Arguably, the design response is even simpler in commercial applications. Many commercial project designers started switching from central inverter–based to 3-phase string inverter–based designs to meet the dc arc-fault protection requirements in NEC 2011. To ensure that these distributed inverter designs comply with the 2014 version of rapid shutdown, designers simply need to locate these 3-phase string inverters on the roof within 10 feet of the array.

This is good news for installation companies in the largest US solar market, as well as for the North American solar market in general. Though it will throw some AHJs and inspectors for a loop when California adopts NEC 2014 on January 1, 2017, experiences in other states have shown that this level of rapid-shutdown compliance is relatively straightforward and not overly disruptive to business as usual. (For more about the intent and design implications of 2014 rapid shutdown, see Bill Brooks’ article “Rapid Shutdown for PV Systems,” SolarPro, January/February 2015.)

2017 version. Under NEC 2017, the rapid-shutdown language in section 690.12 expands from a mere 133 words to more than 1,100. Many of the people who previously bemoaned the lack of specificity in NEC 2014 may find themselves longing for those halcyon days of yore. Without a doubt, the number one complaint about 2017 rapid shutdown is that it is overly prescriptive.

From a design point of view, there are two main differences between the versions of rapid shutdown. First, the 2017 Code shrinks the “not more than 30 V” zone on the roof from a maximum of 10 feet to not more than 1 foot from the array in every direction, as shown in Figure 1. Second, whereas 2014 rapid-shutdown requirements apply to conductors outside the array boundary only, the 2017 version has requirements both for conductors outside the array boundary and for conductors inside the array boundary. It is fair to say that there is general consensus on the first count. Both firefighters and solar industry stakeholders agree that shrinking the array boundary when controlling PV circuits will tangibly improve safety for proximity firefighting. Opinions differ, however, about what should happen inside the array boundary.

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