New Rapid Shutdown Solutions for Grid-Tied PV Systems

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Fronius string inverters will be available this year with SunSpec-compliant RS communications This Symo Advanced is a commercial product, but expect the residential line of Fronius inverters to follow.
An external initiation switch, such as this Bentek product, can be used for systems with string inverters that are not yet compliant with SunSpec protocols or are otherwise lacking RS functionality.
Tigo Energy recently introduced its newest product for RS compliance. A single TS4-F is mounted at each module and receives a signal via a PLC from any external or inverter-integrated SunSpec-compliant initiation device.
The NEP initiation switch is for use with string inverters that lack integrated RS initiation capability. When the button is pushed, PLCs signal the rooftop MLPE to reduce array voltage.
The NEP junction box with RS receiver capability is designed for factory integration by module manufacturers. The junction box receives a power line signal from an initiation device.
The NEP receiver unit is made for field-installation on each module and for use with any SunSpec-compliant initiation device.
MidNite Solar’s SOB system with Birdhouse initiator (center), receiver (left), and sending unit (right).
SolarEdge string inverter (shown) and optimizer systems are listed for RS. The RS initiation device can be the AC PV system disconnect or the main service disconnect.
SolarEdge string inverter and optimizer (shown) systems are listed for RS. The RS initiation device can be the AC PV system disconnect or the main service disconnect.
AC modules, such as this Jinko/Enphase combination, are a simple solution to comply with RS requirements. Any loss of AC power will cause a reduction in PV circuit voltage to below Code limits.
Magnum Energy, a manufacturer of battery-based inverters, supports an AC-coupled RS-compliant system using the Magnum-branded microinverter shown above.
2017 NEC Figure 690.1(b) Identification of PV System Components in Common Configurations

New standards in the 2017 NEC require that grid-tied PV systems have the capability of being quickly and remotely shut down, ensuring firefighter and first-responder safety. So what does this mean for new PV systems?

“Rapid Shutdown of PV Systems on Buildings”—the requirement for quickly deployable voltage control of PV system circuit conductors in or on buildings—was first introduced in the 2014 National Electrical Code (NEC) as Section 690.12. Substantial changes to rapid shutdown (RS) requirements came in 2017 (see “Code Corner” in this issue).

RS systems allow first responders to expect consistency in the method for de-energizing PV system circuits on or in buildings, to help avoid shock hazards. RS requirements have caused a lot of hand-wringing in the solar industry about increased system cost due to the need for additional equipment, and reduced system reliability, since rooftop-mounted electronic equipment has to endure year-round temperature changes and all kinds of weather.

More than a decade of experience with rooftop-mounted module-level power electronics (MLPEs, such as microinverters and DC-to-DC converters) has shown the viability of well-designed, properly installed products. Different system types, including those with string inverters or MLPEs, all now have fairly straightforward RS solutions.

RS on the Horizon

Nearly half of the U.S. has already adopted the 2017 NEC, while the remaining states are enforcing older editions of the Code with different (or no) requirements for rapid shutdown. Subsection 690.12(B)(2) of the 2017 NEC requirements for module-level shutdown goes into effect on January 1, 2019. Until that date, there is no NEC requirement to control PV circuit voltage within the PV array boundary. But there are requirements for outside the 1-foot perimeter around the array boundary—conductors must be limited to 30 volts or less within 30 seconds of RS initiation. For PV systems installed after January 1, 2019 (in areas that have adopted the 2017 NEC), there are three options for controlling voltage within the array boundary. (Options 1 and 3 allowed in 690.12(B)(2) are described in this issue’s “Code Corner,” but are not viable installation choices at this time.) Option 2—referred to as module-level shutdown—requires voltage within the array boundary to drop below 80 V within 30 seconds of RS initiation. A crystalline PV module’s typical open-circuit voltage (Voc) is between 35 and 60 V, although that voltage may be higher as the temperature drops below 77°F. While a single module’s voltage is usually within the acceptable range, two or more modules wired in series could exceed the 80 V limit inside the array boundary.

Comments (1)

Greg Smith_0_0's picture

Great article, Rebekah. Rapid shutdown has, is and probably always will be the most contentious article ever forced onto the industry. I still don't understand why the PV inverter manufacturers are held responsible for preventing rooftop fires when it isn't their equipment that causes them. It would have been much better for the module manufacturers to take some responsibility and design a module level shut off capability. Instead, the onus was shifted to the inverter and a sloppy, vague and ambiguous RS article was shoved into the Code. This has made the industry less safe in the long run, especially for people using micros/optimizers. As storage takes it bite out of the renewable energy cake, it will be interesting to see if people are as scared of batteries as they are of PV.

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