When under a voltage load, PV installation wiring creates an electric field to which the following applies: the stronger the voltage, the stronger the field; and the larger the interconductor clearance, the weaker the field. Unduly strong electric fields between two conductors ionize the ambient air, resulting in a disruptive discharge and a superheated arc flash that can cause a fire. In AC systems, arcing current and hence the power generated by it exhibit a zero crossing 100 times per second, which means that the arc is likely to be extinguished. But no such zero crossing occurs in DC systems, and thus the arcing power remains constant. Owing to the power source characteristics of a PV system, an arc flash in such a system can pose a far greater hazard than in a standard AC installation. And as a PV installation’s operating current is only marginally weaker than its short-circuit current, fuses are unable to protect such a system against short circuits. Moreover, string fuses cannot reliably avert excessive string reverse current or protect modules and string wiring against such a current.
Short circuits are not the only cause of arcing between conductors (parallel arc flashes). A loose connection at a loose terminal or in a faulty connector can also provoke an arc flash (referred to as a series arc flash), which, with a little bit of luck, will only damage the element affected. However, such arc flashes can also cause a fire in adjacent terminals, or even in the building that the PV system is mounted on.
An arc flash or fire can also occur if a fuse that is not rated for direct current is tripped or if a switch that is not rated for direct current is activated. Devices that are rated solely for 250 V DC are in most cases only compatible with very weak direct current and often fail under relatively low direct currents such as more than 24-48 V. Fuses and circuit breakers should only be removed or opened with the system completely powered down and should never be used to switch direct current under load. The use of devices that are not rated for the relevant system current and voltage is negligent and extremely hazardous.
Worldwide, various fires have been caused by PV system arcing, including in Switzerland (a PV installation fire at a farmhouse and in a Mont Soleil junction box). In principle, hazardous arc flashes can be detected when they first arise and before they can cause a fire. A device that does this was developed at the Bern Technical University PV Lab in collaboration with the Alpha Real Company between 1994 and 1998 and underwent extensive field testing in various PV installations under the auspices of an EU project. The device can detect arcing at a distance of up to 100 m, but owing to a lack of vendor interest the concept was not pursued further at the time. Arc-flash-induced module damage that came to light in the autumn of 2006 [4.17] may well spur renewed interest in this already developed and tested arcing detector, which could potentially be integrated into inverters at low cost with a view to providing additional protection [4.18].
Conditions are far more propitious today than in the past for the use and low-cost realization of arcing detectors. Moreover, interference voltage limit values are now available for the DC side of PV inverters (e. g. EN 61000-6-3), which means that devices where interference is efficiently suppressed now need no additional suppression elements. In addition, virtually all inverters now integrate a controller (microprocessor or microcontroller), which by dint of being under only partial load for the most part can reliably perform specific additional arcing detection tasks via a software add-on, thus under certain circumstances easing the workload of a greater or lesser portion of the detection hardware; sophisticated detection methods are also realizable for these devices. FurtherFurthermore, inverter controllers can precisely detect which devices are activated and deactivated and can thus simply disregard any arcing signal that has been erroneously generated in this phase. Figure 4.75 displays the structure of an arcing detector based on the concept that was developed up until 2008. See [4.18] for a detailed description of this device.
Arcing detectors mainly serve to protect the PV system (element 1 in Figure 4.75) solar generators and DC wiring against hazardous module or wiring arcing and against the fires that arcing can potentially cause in roofs or other adjacent structures. The possible locations where such arcing could occur are displayed in Figure 4.75. An arcing detector can detect arcing not only in a PV system’s main DC wiring, but also in its individual strings. Using resonance cycle technology, the arcing
detector (2) detects the high-frequency oscillations generated by arcing in the installation. The intelligent detection unit (3) analyses the signals detected by the arcing detector and only generates an output signal in the presence of hazardous arcing on the DC side, and can differentiate between series and parallel arcing. A DC power supply (4) and (to extinguish series arcing) a DC-compatible switch (5) are only needed with a stand-alone arcing detector, which is considerably more expensive. Only elements 2 and 3 are necessary for the integration of an arcing detector into an inverter (6), in which case elements 4 and 5 can be dispensed with. An arcing detector can also detect parallel arcing between the positive and negative poles (relatively long arcing). This type of arcing is extinguished using a short-circuit switch (not shown in Figure 4.75).
The intelligent detection unit (3) can be realized in the system hardware or solar modules via frequent output signal scanning (2) by the inverter controller. Hybrid solutions are also an option, in which case the intelligent detection unit is hardware based to one degree or another, thus allowing for a far lower inverter controller scanning rate, which in turn reduces processor load. Integration of an arcing detector into an inverter obviates the cost of the most expensive components (4) and (5) and allows for a far more refined and reliable arcing detection process. This controller configuration for inverter power electronics in many cases would probably also allow for input-circuit short-circuiting in order to extinguish parallel arcing, and in many cases this function can be implemented at no additional cost.