Potential-induced degradation (PID) may occur in PV modules given a prolonged exposure to a high negative potential in the field in wet weather. Under these conditions, measurable leakage currents flow from the rack to the glass surface and then to the cell. Whether or not a PV system will suffer performance losses due to PID depends on the sensitivity of the PV cell, the location of the PV system and the weather. Whether or not the PV system is floating or grounded is also decisive. Temperature-dependent performance recovery of c-Si modules can also be observed, which counteracts degradation. A good understanding of recovery is essential for the restoration of a PID-damaged solar farm. Although a general statement about PID susceptibility can be made with PID tests as per the draft standard IEC 62804, this regenerative behaviour is not addressed.
Therefore, the ZSW test laboratory Solab developed a test method for a climate chamber with artificial sunlight. Based on field data from the ZSW Widderstall testing facility, the PV module is charged cyclically: phases with leakage current and thus PID stress are followed by recovery periods with light and high module temperatures. If the phase length is modulated, the module’s PID behaviour in a different climate zone can be simulated. The chart below shows the development of the normalised performance of a PID-susceptible c-Si module during a cyclic PID test. During the first stress test (moderate climate) with a two-hour PID phase (at 25 °C, 95% relative humidity and -1,000 V) followed by a twohour recovery period with sunlight, the module output remains stable (dark blue curve). Under harsher conditions leaning towards tropical climate with a four-hour stress phase, the module significantly degrades after only a few cycles (light blue line).