Application Note - Rapid Shutdown in SolarEdge™ Systems
The SolarEdge™ system incorporates many safety mechanisms, ensuring safety for installers, maintenance works and firefighters. Rapid shutdown (RSD) is a safety mechanism which refers to the fast discharge of conductors to a safe voltage level.
In North America, the National Electrical Code (NEC), section 690.12, defines RSD requirements for PV systems on buildings. The requirements were first introduced in NEC 2014, and updated in NEC 2017. SolarEdge inverters installed in North America have complied with these requirements since they have come into effect. Specifically, NEC 2017 690.12 requires that in rooftop PV systems, controlled conductors beyond 1ft (30.5cm) of the array will be reduced to 30 volts or less within 30 seconds. The SolarEdge SafeDCTM feature ensures the DC voltage of a system is reduced to a safe level when the system is shut down, within up to 5 minutes. While in Europe and APAC there is currently no RSD standard, reducing the DC voltage within 30 seconds allows installers, maintenance works and firefighters to handle the system very soon after shutdown, which is of particular importance in case of an emergency. Therefore, adopting RSD is recommended, and may be required in the future by local regulation.
SolarEdge™ Rapid Shutdown Advantage
SolarEdge is among very few solar equipment manufacturers who provide integrated rapid shutdown functionality in compliance with NEC regulations. Other manufacturers offer this capability via external components (contactors, shunt trip breakers, or other remotely controlled switches), which may add complexity and increase the cost. The SolarEdge RSD solution advantages are:
- No additional components: 3rd party solutions typically need extra fuses or circuit breakers, adding cost, field work and potential quality issues that can increase time on site for troubleshooting
- No additional wiring: 3rd party solutions require additional wires for RSD, and they must have suitable voltage ratings in order to be placed in a conduit together with PV system conductors, otherwise they must be placed in a different conduit, increasing cost and labor
Safety Risks and Solutions in PV Systems
Risk — Electrocution
Firefighters and other first responders called to a blaze commonly cut off power to the burning building as a safety precaution. If the building has a PV installation however, the PV modules continue to generate DC voltage, even if the system is not actually connected to the AC grid. In electrical systems, extra-low voltage (SELV) indicates a safe voltage below 120V. Under these conditions there is a low risk of electrocution. PV modules typically have an output voltage of 30-60V, and thre-four connected modules are enough to generate more than 150V. When connected in a string, voltage in residential and commercial installations can reach 600-1500V, which can be dangerous to installers during system installation, maintenance personnel during O&M, and first responders during an emergency.
Effective Solution — Module-Level Shutdown
SolarEdge offers a PV power harvesting system that consists of power optimizers connected to each module, a PV inverter, and module-level monitoring. Additionally, SolarEdge systems have a built-in SafeDC™ safety feature that minimizes safety risks. When power optimizers are connected, modules continue to operate only as long as a signal from the inverter is constantly renewed. If there is no signal from the inverter, or if the inverter is not operating, SolarEdge’s SafeDC™ feature is designed to automatically shut down the DC current as well as voltage in module and string wires. In safety mode, the output voltage of each module equals 1V. For example, if firefighters disconnect a PV system from the Ineffective Solutions electrical grid during daylight and the PV system consists of 10 modules per string, the string voltage will decrease to 10VDC. Since the maximum string length in a SolarEdge system is 50, the string voltage is limited to 50VDC, safely below the risk level. Even under single faults, the solution has been certified to have SELV (<120V) voltages.
Module-level shutdown occurs automatically in any of these cases
- A building is disconnected from the electrical grid
- The inverter is turned off
- Power optimizer thermal sensors for each module detect the rising temperature (threshold 85˚C)