The purpose of this paper is to discuss and categorize the safety issues of photovoltaic systems for firefighters as well as outline how the SolarEdge system can mitigate these safety issues.
Firefighting
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 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. Yet, 3 – 4 connected modules are enough to generate more than 150V. Residential and commercial installations include dozens or hundreds of modules with voltage that can reach 600V.
Ineffective Solutions:
1. Shutdown functions in traditional inverters merely interrupt current flow and voltages remain dangerously high.
2. Automatic DC breakers located on the inverter in the cabinet, cannot disconnect the voltage on the modules, adding cost without decreasing the risk.
3. PV module covering:
a. Spraying foam – this approach has proven to be ineffective because the foam evaporates or slides off the modules before extinguishing the fire.
b. Covering the module with an opaque material – this approach requires the firefighters to climb onto the burning roof, risking electrocution.
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 safety feature that eliminates safety risks. When power optimizers are connected, modules continue in “operation mode” only as long as a signal from the inverter is constantly renewed. In absence of this signal, power optimizers automatically go into safety-mode, shutting down 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 photovoltaic system from the electrical grid during daylight and the photovoltaic system consists of 10 modules per string, the string voltage will decrease to 10V.
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 in the EU to have SELV (<120V) voltages. In North America the solution is NEC 690.12 (2014) certified which is in compliace with the following directives:
1. Automatic shut down of building mounted PV arrays during emergency shut down.
2. Loss of utility power, open AC disconnect or circuit breaker, disconnection of PV array from inverter.
3. 80Vdc limit for all conductors within 10 seconds of inverter shut down.
Module-level Shutdown Occurs Automatically:
-Either when a building is disconnected from the electrical grid
-Or when the inverter is shutdown
-Or when thermal sensors for each module detect the rising temperature (threshold 95˚C)
Fire Prevention
When the connecters and/or cables in a photovoltaic system are damaged, the electric current may pass through the air, causing an electric arc. This generates a lot of heat and can start a fire as well as cause electrocution. As photovoltaic systems start to age and as damage to connector and/or cables increase, and though with low probability, electric arcs may occur.
Ineffective Solution – Inverter-Level Arc Detection: Traditional inverters have a limited ability to detect arcs inside a PV array. As the distance from the inverter increases, the likelihood of arc detection decreases. Additionally, traditional inverters can only detect and terminate serial arcs which occur close to the inverter. Parallel arcs, however, cannot be terminated.
Effective Solution: Module-level Arc Detection and Termination SolarEdge optimizers act as multiple module-level arc detection sensors in an array, which significantly increases detection accuracy. Optimizers can detect serial arcs and can automatically terminate them by eliminating any current in the wires by completely shutting off all modules in the array. Module-level shutdown can further terminate parallel arcs.
Conclusion
SolarEdge’s SafeDCTM system, an adherent feature in SolarEdge power optimizers, the SolarEdge Inverter and the SolarEdge monitoring system. It is the only system that ensures complete safety for firefighters working with PV modules, eliminating the risk of electrocution and electric arcs, and which is certified in Europe as a DC disconnect.
SolarEdge
www.solaredge.com
