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.
Partial Discharge Measurements on Glass-Mica-Epoxy Samples for electrical insulation during Accelerated Thermo-Oxidative Aging
The stator windings of large air-cooled rotating machines are mainly insulated with mica-glass fabric impregnated with epoxy resin. In service, these materials are submitted to thermal, mechanical and electrical stresses. A specific aging arrangement has been set-up allowing artificial thermo-oxidative aging of samples, thanks to the application of a dry O2 pressure at a controlled temperature. Three different materials, one reference material and two candidate materials, of different thermal classes or different thermal conductivity, have been aged and compared. Partial Discharges (PD) analysis of the samples has been conducted and correlated with samples' morphology. Because of the small size of the samples, a specific PD measurement arrangement has also been set-up. After only a few hundred hours aging, delaminations of a few tens of μm were observed within the samples. PD measurements revealed a decrease of the PD Inception Voltage (PDIV) on these aged samples. The PDIV on samples with specific cavity thickness were also in good agreement with Paschen law. PD measurements, especially the PDIV, seem to be a good indicator of the progress of the aging.
Biodiversity, conservation, and sustainable development
This important book highlights the conflicts between economic growth and the conservation of nature in the context of sustainable development. It places particular emphasis on biological diversity and examines possible policies for resolving conflicts which arise from the contrasting goals of conserving the natural environment and economic growth.