Understanding polymer degradation under voltage stress in PV applications

AC Partial Discharge (PD) specification has been a key design parameter for PV backsheets, as called for in IEC 61730-1. In previous work, we have studied how the structure of polymeric materials can impact their behavior in this test. In this paper, we shift the focus to consideration of potential root causes which can lead to electrical degradation in actual usage (e.g. a PV module producing DC power), and the efficacy of the AC partial discharge test in air and oil to identify these. The suitability of utilizing AC voltage stresses in PV is also challenged in this paper. New experimental evidence is presented as to why testing partial discharge with AC with specimens immersed in the medium is not suitable for testing of solar backsheet materials. PD measurements can be very different for tests performed in air versus oil, and can lead to different pass/fail conclusions for the same material. When tested in oil, materials with voids had a much lower Ve than materials with no voids; in air, there was no difference in Ve for a material prepared with and without voids. With regard to AC versus DC testing the time to breakdown versus voltage was measured for both; these are not linearly related: the AC and DC breakdown voltages for one material study were 25 kV(AC) and 85 kV (DC), more than 3× difference; the voltages which led to breakdown after 10000 seconds were 5 kV(AC) and 50 kV (DC), a 10× difference. The issues raised in this study are active areas of discussion for the TC 82 WG2 Standards Committee.