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 ti 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 Fm at the needle tip. It was shown that the treeing inception time ti has a maximum for an optimum barrier position &x. This optimum barrier position minimizes the maximum electric field and the electric energy density Des at the pin tip and consequently maximizes the tree inception time ti. Finally, a very useful relation relating the initial tree length with electrical properties of the insulating materials was derived.
Keywords: dielectric barrier