Epoxy resins are widely used as insulating material in electrical apparatus. Recently, X-ray phase imaging has attracted a lot of attention as a method to observe tree shape and propagation. However, observation of trees in filled epoxy is challenging because silica particles absorb X-rays and disturb the acquisition of a clear image of treeing. In this study, image subtraction was applied to overcome this issue and time evolution of treeing was observed. The visibility of electrical trees in a filled sample was evaluated through the noise analysis in spatial frequency domain. Since the noise generated by silica particles was much reduced, electrical trees in epoxy resins with 1.5 µm and 5 µm silica fillers were successfully observed.
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