Effect of Tracking and Treeing on Impulse Breakdown in Solid insulator

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Using a standard lightning impulse voltage, the propagation aspects of a creepage discharge over the surface of oil-immersed solid insulators, high density polyethylene (HDPE), epoxy resin (EP) and epoxy glass mat laminate (EGML), have been observed by means of a camera equipped with an image intensifier. The creepage discharge over the surface of EGML revealed to a distinctive aspect which had the luminous discharge spots on the streamers. It was clarified that this phenomenon was attributed to the existence of a glass fiber ingredient in EGML and the discharge energy was increased by the occurrence of the discharge spots.On the other hand, the treeing properties of PMMA (polymethylmethacrylate) were studied by the intermittent lightning impulse voltage to either the point-plane or the point-point electrode system. The forms and extension lengths of the electrical tree revealed clearly the polarity effect on the applied voltage. The growth of electrical trees was greatly restrained when the impulse voltage was applied to the point electrode more than 4 times. This test result was attributed to the electric charge accumulated by the initial tree generation due to the first voltage application.Furthermore, the oriented ECCPE (Extended Chain Crystal Polyethylene) of a large size was newly developed. Electrical treeing discharges of ECCPE were investigated using a needle to plate electrode system. The direction of the tree-growth was completely controlled by the orientation of ECCPE.When the orientation was parallel to an electric field, the tree propagated as a single line without branchings. When the orientation was vertical to the electric field, it propagated to the orientation direction without branchings. Treeing discharge current also revealed the remarkable difference on orientation directions.The results obtained in this investigation are promising for a field of electrical insulations.