Study of extruded power cable insulation anisotropy by means of electrical treeing growth tests

The effects of anisotropy in samples of EPR insulating compound taken from an HV power cable have been studied analysing suitable electrical tree growth parameters. Certain samples were cut in the radial direction R, others in the longitudinal and tangential direction L and T with respect to the cable extrusion axis. Point plane geometry specimens were obtained inserting 5 μm tip radius needles into such samples by means of a special hot moulding process. As the adhesion between needle tip and EPR compound could influence the final treeing tests results, the needle insertion procedure and the hot moulding process were optimised. The average Tree Inception Voltage (TIV) and the average Breakdown Voltage (BDV) were measured, during AC ramp voltage tests, for each of the three types of specimens. Finally the Time-to-breakdown (Tbd) was recorded during 15 kV AC constant voltage tests, in the three cases. The overall results (BDV and Tbd) clearly indicate that R specimens offer a “resistance” to treeing growth higher than L ones, while the T specimens behaviour is in between. Therefore it is concluded that, in the bulk of the investigated cable insulation, a significant anisotropy exists. The potential of the above outcome (data and testing procedures) for improving the overall design and/or the manufacturing process of extruded cables for energy transmission applications is recalled