Effect of aluminium inclusions on the dielectric breakdown of polyethylene

A narrow size distribution of irregular aluminium particles was blended into power cable insulation grade polyethylene. Some batches of the resulting material were then melt-filtered to reduce the size of particles present and narrow the distributions further. The failure statistics of the loaded polymers were then determined under AC ramped stress. The data have been fitted to extreme value statistics, and trends with size and density of defect can clearly be identified. In addition, for the filtered material, a minimum breakdown field can be associated with a given filter size: a result of commercial importance. Some indications exist to suggest that different modes of failure operate at high and low fields. Candidates for these modes are analysed and discussed in terms of the distributions of defects present. Local field enhancement due to the included flaws were calculated using finite-element techniques. The results are compared with a percolation model of breakdown. Predictions are found to quantify accurately the reduction in the characteristic strength of the material over the narrow range of defect concentrations examined.