Study on Improvements to the Dielectric Breakdown Strength of Extruded Dielectric Cables

This report relates to the study on insulation materials for obtaining extruded dielectric cables with high dielectric breakdown strength. Firstly, the dielectric breakdown strength (EB) of a variety of plastics was evaluated by the use of recessed sheet samples. As a result, the EB for high density polyethylene (HDPE) proved favorable. It was further ascertained that in the case of polyethylene the difference in crystallization due to molding conditions also exerts an influence on the EB. Consequently, in the following step a number of model cables insulated with HDPE were produced by changing the production conditions, such as heat treatment successive to extrusion, etc. Under optimum production conditions an upgrade of approx. 60% of the HDPE cable with EB value over the conventional XLPE cable should be obtained. As a result of measuring the morphorogical properties for insulators under each production condition the EB showed a trend to increase as the degree of crystallization increased, both with XLPE and HDPE, so long as they were within the scope of identical brands. In the relation to insulator lamella thickness the EB of XLPE improved as lamella thickness increased, whereas HDPE indicated an adverse trend.