Effect of Extrusion Process on Electrical Properties of 35kV Polypropylene Cable Insulation

Polypropylene (PP) cable insulation is with low production energy consumption, excellent insulation properties, and recyclability, which presents a good application prospects in the medium voltage (MV) cables. However, as a multiphase polymer composed of a polypropylene matrix and elastomer, the aggregation structure of PP insulation undergoes significant changes during the extrusion process with thermal and mechanical stress, thereby affecting its electrical performance. This study compares the electrical properties of hot-pressed PP insulation samples with those of cable cross-section samples post-extrusion. The results reveal a 11 % decrease in the AC breakdown strength of the inner insulation layer after extrusion. Further analysis indicates that the size of elastomers in the inner insulation layer increases by 85 % after extrusion, leading to the disruption of the microcrystalline structure of PP insulation and a 2.5 % reduction in crystallinity. Additionally, after heat treatment at 200° C to eliminate thermal history, both crystallinity and breakdown strength recover to the levels of the hot-pressed samples. These findings demonstrate that the extrusion process is the primary cause of the decline in the electrical performance of PP insulation. This research provides theoretical insights and practical guidance for the production of PP-based cables.