A confocal raman microprobe analysis of partial discharge activity in gaseous voids

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Description/Abstract

Electrical ageing in polymeric insulators used in high voltage cables and transformers is often thought to originate in small gas filled voids within the bulk of the material. Understanding this ageing process is vital as it can lead to electrical treeing and, in some cases, complete electrical failure of the material as an insulator. This paper investigates the chemical processes in electrically aged voids via confocal Raman microprobe spectroscopy (CRMS), a technique well suited to this analysis due to its ability to probe sub surface artefacts in the sample and a resolution of approximately 2 μm in the optical plane. Samples of XLPE layered with sheets of LDPE containing voids of dimensions 0.4 mm x 0.1 mm where aged via partial discharge activity and then probed using CRMS. The surfaces of these voids were then analysed using CRMS in conjunction with both air and oil immersion lenses. Analysis revealed among other things, evidence of fluorescence and oxygen related bi-products. The surfaces of the voids were then exposed via microtomy and, following a Raman analysis, imaged using scanning electron microscopy. It was found that a large change in morphology had occurred, such that micro pits approximately 5 μm in diameter were formed on the void surface.