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.
Comparison of breakdown-voltage of contemporary and veteran photovoltaic backsheets
Abstract The ability of designated electrically insulating materials in a module, including the backsheet, to act as insulators is a key safety requirement for photovoltaic technology. The recently developed test standards IEC TS 62788-2 (including a method for direct-current breakdown-voltage) and IEC TS 62788-7-2 (for ultraviolet [UV] weathering) are examined for the first time together in this study. Thirty-six contemporary backsheets were compared using the breakdown-voltage test, before and after UV weathering, according to the recent A3 test condition. Twenty-eight backsheets, extracted from a variety of veteran photovoltaic modules, were also examined using the breakdown test. Additional characterizations were performed to interpret the breakdown-voltage results, including: surface and cross-sectional optical microscopy failure analysis; cross-sectional scanning electron (SEM) microscopy with EDS energy-dispersive X-ray spectroscopy (EDX) failure analysis; Fourier-transform infrared spectroscopy (FTIR) to identify the base resin and its chemical integrity; and differential scanning calorimetry (DSC) to identify phase transitions that might adversely affect degradation during weathering. The electrical insulation for the backsheets is discussed relative to: the industry safety requirement; features observed to facilitate the degradation of insulation; the validity of the artificial UV weathering test; the validity of the sheet geometry used for specimens in this study; and the validity of subsequent mechanical testing.
Partial Discharge Detection in High Voltage Equipment
Electrical discharges that do not completely bridge the electrodes are called partial discharges. Although small, these discharges have been known for more than fifty years to cause progressive deterioration. Since 1945 the use of new high voltage dielectrics has required the use of sophisticated detection techniques. Discharge detection has grown into an indispensible tool for the evaluation of modern insulation. The subject has received little attention in books on high voltage techniques although a good deal is scattered in various papers and reports. The object of this book is to collect this knowledge and present it as a coherent study. Topics covered include the behaviour detection and measurement of discharges, the choice of detection method and procedure, the location of discharges and discharge detection in practice.