This image shows PV modules that have a failed bypass diode. Bypass diodes are installed in modules to protect the PV cells from damage during shading. There is generally 3 of them within the junction box, each protecting a string of 20 cells which is why you can clearly see 1/3rd of the module is a lot hotter than the surroundings.
A failed bypass diode causes reduced voltage in a module, and because modules are wired in strings, this then has a knock on effect reducing the overall efficiency of a much larger part of the system. This is what is known as the domino effect and is why small faults such as these can have a disproportionate effect on a sites performance ratio.
If one module usually has 60 volts then a faulty bypass diode will cause that voltage to drop to 40 volts for example. That 33% reduction would be easy to spot if every module was tested individually, but they are tested in long strings of around 20 modules. So 20modules x 60v = 1200V and now the ‘missing’ 20v caused by the faulty diode represents just under 2% difference. Add to the mix that PV modules by their very nature have varying voltages +/- 3% and change with the weather, it could be quite easy to miss a fault such as this when relying on string testing alone and is akin to finding a needle in a haystack.
A drone based thermal survey removes all the guess work, highlighting problems and flushing out faulty modules very quickly.
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