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The Arc Triumph
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In recent posts we spoke about fire risks associated with PV systems and about the attractiveness of PV systems to thieves. Well, we’d like to mention one more menace… electric arcs.
What is an electric arc? It is the continuous flow of current through the air. What does it look like? Watch this YouTube movie:
Any electric installation is exposed to the risk of arcs, but solar installations are particularly sensitive to this issue, because of the continuous DC current and the high currents (>10 A) and voltages (300-1000 V). Furthermore, typical PV systems have hundreds and even thousands of connectors, each a potential arc generator if there is a disconnection or faulty connector in a current carrying wire. The arc can be a source of electrocution, in addition to generating heat which can lead to a fire. Most installations today are only several years old but as they age their wiring and contacts will deteriorate, making them more prone to arcing.
An arc in a PV installation belongs to one of three categories: Serial arc, caused by wiring discontinuity such as a loose connector or a connector defect; Parallel arc, caused by insulation problems between two cables; Parallel-Earth arc, usually caused by internal leakage in a PV module, or by a cable insulation problem involving the ground.
The electrical scheme in Figure 1 shows several possible arc locations in a PV installation.
The danger of fires caused by electric arcs in PV installations is not just some doomsday scenario; such fires have already occurred. The fire on the roof of a Target store in Bakersfield, California, is one example. In the 2009 fire, a Parallel-Earth arc was the cause of the fire. A second fire along a row of modules was caused by subsequent arcing.
Different arcs have different electric characteristics. These can be used to detect an arc occurrence. But since the majority of arcs would occur at the module and string level – i.e. between two adjacent modules or strings (since that is where most connectors are located) a central inverter would not be able to detect an arc, let alone terminate it.
Figure 3 - An arc detection scope comprises the units capable of detecting the arc. In this illustration an arc occurs between two modules. The arc is visible to the string where it occurred and to the neighboring string, but not to the rest of the system. On top of the limited detection capabilities of a traditional system, in the few cases where a central inverter can detect an arc its only recourse is to shut down the inverter. But this would only discontinue a serial arc. Any parallel arcs that may exist within a string or between strings would be uninterrupted, causing considerable damage. To make matters worse, shutting down the inverter can increase the current of a parallel arc, compounding the risk.
Distributed photovoltaic architectures have electronics at the module level, and can make use of them for arc detection and termination. That is precisely what the SolarEdge system does. A PowerBox, the SolarEdge power optimizer, can detect an arc, and upon detection it shuts the module down, thus extinguishing the arc and preventing its re-occurrence. Multiple detection by several PowerBoxes increases sensitivity and provides system owners with a reliable built-in solution. The SolarEdge PV monitoring system alerts that an arc has occurred and identifies its location, so rapid replacement of the faulty units can take place. The SolarEdge arc solution has been tested on a suite of arc scenarios, including cases to be included in the upcoming UL1699 PV arc standard.
In the following movie you can find a demonstration of an arc in a traditional PV system vs. an arc in a SolarEdge system:
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