Rapid Shutdown for PV Systems: Page 7 of 7
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No additional equipment required. NEC 690.12 allows installers to run uncontrolled PV system circuits a maximum of 10 feet beyond the array area. Therefore, if it is possible to install a PV system so that the conductor length between the array and the inverter is less than 10 feet, the system is compliant with NEC 690.12. For example, imagine a roof-mounted array installed on a single-story ranch home with a wall-mounted inverter directly below the array area. If the conduit run between the array and the inverter is less than 10 feet, this residential string inverter system does not require any remote switches or contactors to comply with 690.12.
Most commercial systems use a centralized architecture with large central inverters or a distributed architecture with multiple 3-phase string inverters. System designers can adapt either of these basic design options to comply with NEC 690.12.
Central inverter systems. Designers typically position high-capacity central inverters at ground level because of their large size and weight. This results in long uncontrolled PV output-circuit conductor runs off the roof. In this scenario, designers can comply with NEC 2014 requirements by specifying contactor combiner boxes that provide arc-fault protection per 690.11, rapid-shutdown capabilities per 690.12 and the dc combiner disconnect requirement per 690.15(C). Bentek, SolarBOS and Solectria Renewables are a few of the vendors offering listed source-circuit combiners with these capabilities.
Where you use roof-mounted contactor combiner boxes to comply with 690.12, you also have to install control circuits for these devices. The normally open contactor in the combiner will remain closed as long as you apply a 24 V signal to the device; if you interrupt the 24 V control circuit, the contactor will open. Pay special attention to verify that shutdown disables the large capacitor banks at the ground-level inverters in less than 10 seconds. Firefighters can initiate rapid shutdown manually using a dc disconnect or a rapid-shutdown controller, or it can occur automatically upon loss of ac power. In the latter case, firefighters can use the main service disconnecting means to initiate rapid shutdown for load-side–connected PV systems, or the PV system disconnecting means to initiate rapid shutdown for supply-side–connected systems.
3-phase string inverter systems. Due to the cost and complexity of designing NEC 2014–compliant central inverter systems—a combiner capable of providing dc arc-fault protection and rapid-shutdown functionality can cost twice as much as a standard disconnecting combiner—system integrators are increasingly installing 1,000 Vdc-rated, 3-phase string inverters, which range in capacity from 15 kW to 60 kW, on low-slope commercial rooftops in place of dc combiners. As long as you install these 3-phase string inverters within 10 feet of the array, the system complies with 690.12. Once firefighters interrupt utility-supplied power, which they can accomplish without any remotely activated switches, no uncontrolled conductors remain further than 10 feet from the array. However, all of the dc circuits within the array remain energized. For load-side–connected systems, firefighters can initiate rapid shutdown by opening the main service’s disconnecting means; for supply-side– connected systems, they must open the PV system’s disconnecting means.
Many inverter vendors—including ABB, Advanced Energy (via its REFUsol acquisition), Chint, Fronius, SMA America and Solectria Renewables—offer 3-phase string inverters with integrated arc-fault protection that you can install directly on low-slope commercial roofs. Increasingly, integrators are installing these 3-phase string inverters at a moderate tilt angle (5°–30°) on low-profile inverter racks and distributing them throughout the array in lieu of dc combiners. Many inverter and BOS vendors—including Advanced Energy, AET, Bentek, Shoals Technologies Group and SMA America—now offer string inverter mounting kits for low-slope roofs, attesting to the popularity of this design strategy.
Systems With Battery Storage
Battery storage adds a layer of complexity to the rapid-shutdown process. Most battery storage systems are designed to power selected loads during a utility power outage. Since standby power is the battery’s primary function, actuating rapid shutdown on loss of utility power defeats the battery’s purpose. Therefore, battery-based systems need a rapid-shutdown initiator that does not depend on loss of ac power. This independent shutdown initiator could be a simple on/off or control switch, such as a push button that controls remotely actuated switches. While the location for this initiation device ultimately depends on the project, the building and the PV system design, the best location is typically in close proximity to the main service equipment.
Wherever you locate the rapid-shutdown device, it must control the battery-backup circuits as well as the PV array circuits. The technical solution used to control the battery-backup circuits may vary depending on the distance between the batteries and the inverter.
Batteries more than 5 feet from inverter. Where integrators install batteries more than 5 feet from the connected inverter, a new requirement in NEC 2014 690.71(H) calls for a battery-bank disconnecting means “at the energy storage device end of the circuit.” In this scenario, you can install a remotely actuated switch at the battery bank to meet the requirements of both 690.12 and 690.71(H). During the rapid-shutdown process, both the roof-mounted switch for the array and the battery-bank disconnecting means open to de-energize all of the PV system circuits, including the backup power circuits. Since battery-based inverters do not have input capacitance, the voltage of the inverter–input circuit will drop below 30 volts as soon as you disconnect the battery and turn off the utility ac power. As with utility-interactive PV systems without battery storage, the ac circuit between the utility service and the inverter automatically shuts down upon loss of utility power.
Batteries within 5 feet of inverter. Where integrators install batteries within 5 feet of the inverter, NEC 690.71(H) does not require a disconnecting means at the battery bank. In this scenario, designers have two options for controlling the backup-power circuits to meet 690.12. They can install a remotely activated switch in the battery circuit, or they can install a remotely activated ac switch in the stand-alone inverter-output circuit. Either of these options will power down the PV system circuits that would otherwise remain energized upon loss of utility-supplied power.
The NEC 2014 rapid-shutdown requirements represent a substantial change in PV system design and deployment—and a significant step forward in PV system safety. As with all new NEC requirements, it will take some time for system designers and installers to learn how to meet rapid-shutdown requirements in a range of applications; it will also take time for plan checkers and inspectors to learn how to enforce 690.12. The first step is to determine which conductors you need to control. Once you have established this, you can evaluate the means of control. As I have illustrated, some PV system design strategies inherently comply with 690.12, while others require additional equipment.
Dozens of stakeholders participated in the process that culminated in the NEC 2014 rapid-shutdown requirements, and many stakeholders are similarly engaged in the process of improving 690.12 as part of the 2017 revision cycle. While it is far too early in the Code-making process to predict what the revised section will ultimately look like, two proposals have broad stakeholder support. The International Association of Firefighters has proposed revising 690.12 to require module-level control, which may provide the highest level of safety for its members. SEIA has proposed a more detailed and restrictive version of the existing combiner-level control provisions.
Bill Brooks / Brooks Engineering / Vacaville, CA / brooksolar.com
Fire Operations for Photovoltaic Emergencies, California Department of Forestry and Fire Protection (CAL FIRE), November 2010
Solar Photovoltaic Installation Guideline, California Department of Forestry and Fire Protection (CAL FIRE), April 2008
“Understanding the CAL FIRE Solar Photovoltaic Installation Guideline, ” Solar America Board for Codes and Standards, March 2011