Potential-induced degradation ( PID ) is still one of the main reasons for unpredictable power losses in PV power plants

www.pv-tech.org | February 2020 | 55 Potential-induced degradation (PID) of PV modules containing silicon solar cells is an issue with high relevance to the long-term reliability of PV systems [1]. Despite knowledge of methods for mitigation of PID for standard module technologies, there are still new cases of PID arising related to new technologies such as bifacial solar cells or cheaper packaging materials. It was observed that even PV modules designed and specified to be “PID-free” can develop PID under particular outdoor conditions. Especially humid and hot climates in combination with soiling can lead to a change of the electric properties of the module encapsulation, which results in PID degradation of initially “PID-free” modules, in particular when they exhibit a conventional metal frame and a polymeric back sheet, which is to some extent water permeable. This is attributed to the change of the electric conductivity of the glass surface and of the encapsulating materials [2] causing increased leakage currents on the path from the frame across the glass surface and through the module encapsulation layers and thus change of the electric field in the anti-reflective coating (ARC) of the solar cells. In a PV power plant, many modules (each delivering a voltage of about 35V) are switched in series, leading to voltages to ground up to several hundred volts. Both the module glass and the polymer back sheet are no absolute insulators, therefore tiny leakage currents may flow between the cells in the modules and ground. The leakage current that flows under high-voltage stress of 1,000 V amounts to typically some 10μA/m2 for solar modules with soda-lime front glass and EVA encapsulation. This can be measured outdoors e.g. using the PIDcheck test device, as shown above. Module degradation | Potential-induced degradation (PID) is still one of the main reasons for unpredictable power losses in PV power plants. Volker Naumann, Otwin Breitenstein, Klemens Ilse, Matthias Pander, Kai Sporleder and Christian Hagendorf of Fraunhofer CSP examine how the PID susceptibility of PV modules is influenced by environmental stress. It is found that PID may develop in originally PID-resistant modules after a period of one to three years of unsuspicious operation, depending on climatic conditions Increase of PID susceptibility of PV modules under enhanced environmental stress