THE ROLE OF A DIELECTRIC BARRIER IN ELECTRICAL TREES INITIATION IN SOLID INSULATION

Konstantinos Theodosiou - Ioannis Gialas

   This paper examines the role of a strong (high permittivity) barrier in a three layer dielectric arrangement. Specimens consisting from two layers of PMMA (Polymethyl methacrylate) and a third layer of ER (Epoxy Resin), were constructed and stressed under an constant AC 50 Hz voltage with a needle-plane arrangement, for a period of 6 hours. Partial discharge (PD) measurements were continuously taken and visual and microscopic observation was taking place at the tip of the needle area. The treeing inception time t_i and the breakdown voltage of the sample were examined as a function of the barrier position &xi, the ratio of the needle-barrier distance to the total distance between the electrodes. Finite element-2 dimensional simulation was run in order to examine the electric field in the specimen, especially the maximum electric field strength F_m at the needle tip. It was shown that the treeing inception time t_i has a maximum for an optimum barrier position &x. This optimum barrier position minimizes the maximum electric field and the electric energy density D_es at the pin tip and consequently maximizes the tree inception time t_i. Finally, a very useful relation relating the initial tree length with electrical properties of the insulating materials was derived.

Keywords: dielectric barrier

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