X-ray phase imaging of electrical treeing in filled epoxy

Epoxy resins are widely used as insulating material in electrical apparatus. Recently, X-ray phase imaging has a lot of attention as a method to grasp tree shape and propagation. However, observation of trees in filled epoxy is challenging because silica particles absorb X-ray and disturb the acquisition of a clear image of treeing. In this paper, image subtraction is applied to overcome this issue and time evolution of treeing is observed. The visibility of electrical trees in a filled sample are evaluated through the noise analysis in spatial frequency domain. As a result, it is suggested that trees in a sample in which 1.5μm, 5μm or 20μm silica is filled by 10wt% can be detected and the electrical trees in epoxy resins with 1.5μm and 5μm silica fillers are successfully observed.