STUDIES ON THE IMPROVEMENT OF BREAKDOWN STRENGTH OF POLYOLEFINS

Breakdown tests were conducted on a variety of polyethylenes and polypropylenes. High density polyethylene (HDPE) and polypropylene random copolymers (PPR) exhibited the highest breakdown strengths among the examined polyolefins. Model cables insulated with HDPE and PPR were manufactured, and the impulse breakdown strength of the HDPE cables made under some specific manufacturing conditions was 1.6x larger than that of conventional crosslinked polyethylene (XLPE). However, improvements were not observed in the impulse breakdown strength of PPR. The ac breakdown strength of HDPE and PPR cables also did not improve. The reason of the improvement in impulse breakdown strength of HDPE is due to the increased crystallinity of the insulating material, brought about by the heat annealing applied to the cable in a manufacturing process.