The influence of superstructures on the electrical breakdown of partially crystalline polymers

Abstract

The knowledge of the electric strength of solid synthetic polymers is rather incomplete to date. More detailed information on breakdown mechanism might contribute to a better use of these solid insulants in a single-material system. The present contribution attempts to show the relation between the morphological superstructure formed in a partially crystalline polymer and the electric breakdown mechanism in a highly inhomogeneous 50 Hz alternating field. More particularly, the electrical breakdown as a function of time suggested to us an interface defect present in the material. Test specimens of polyethylene insulants newly extruded under different working conditions produced different results in our time-graded breakdown tests. Samples with only partial breakdown show the same discharge channels in a form dependent on the manufacturing conditions. Combining these observations with the fact that partially crystallized polymers form superstructures while crystallizing upon cooling, it is suggested that, besides the well-known influences of mechanical stress and of voids, areas of different resistance to electric stress have formed in these morphological structures.