DC characteristics of cable insulating materials

The insulation performance of PE (polyethylene) under DC voltage was investigated using sheet samples and miniature cables. Pressed sheets of various kinds of PE, which differed in material density and polymerization process, were prepared, and their thermally stimulated current and breakdown strength were measured. To confirm that insulating properties obtained in the experiments using sheet samples can also be obtained in thicker samples, experiments were conducted using miniature cables with 3.5-mm insulation thickness and a conductor cross-sectional area of 60 mm² to evaluate breakdown strength and to measure the thermally stimulated current. Both sets of results showed that HDPE (high-density PE) has excellent insulating properties under DC voltage. The breakdown strength is high and the space charge accumulation is small. These properties of HDPE are considered to be due to its molecular structure, with its short lengths of molecular branches