As before, we use classic electrical measuring methods according to VDE 0126-23 or I-V curve measurements with sufficient irradiation for troubleshooting and analysis. Similar to the thermography, the dependence on solar radiation makes the practical measurement considerably more difficult (season, time of day, facades- east -/ west plants etc). Dark I-V curves, on the other hand, are performed at night and have been used in solar cell research since the 1960s and are slowly finding their way into the field test. Dark I-V curves are similar in shape to IV curves performed by daylight, but do not give indication of short circuit current, module power, etc. The mere comparison of dark-IV curves in this shown fault case of an east / west roof system (4 equal strings with 10 modules, one inverter) indicates PID due to the very different curve (string 1,3,4), while String 2 almost resembles an ideal characteristic and would be picked up too fast as a reference. However, electroluminescence images of all 4 strings clearly show that modules from min. 2 different batches have been installed. A large part of the installed modules have massive loads due to PID and contact errors (busbar, finger …).

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