Water tree retardant XLPE cable using VLDPE

Investigations were carried out on water tree retardant XLPE (XLVLDPE) cable using very low density polyethylene (VLDPE) which is a new grade of polyethylene. To begin with, when the water tree retardant effects of various types of VLDPE being sold on the market were evaluated by using sheet specimens, extremely good water tree retardant effects were exhibited when the type of VLDPE was selected appropriately. Next, the above selected varieties of VLDPE were tested regarding the extruding into cables. The extrusion of VLDPE was found to be more difficult compared to conventional XLPE, and consequently required the modification of extrusion conditions and a crosslinking agent. The result of the water tree acceleration test of XLVLDPE cables indicated that the generation of bow-tie trees was suppressed to a larger extent than conventional XLPE cables. Although the AC initial breakdown voltage of XLVLDPE cables was lower than that of the conventional XLPE cables, there was almost no decrease in the breakdown voltage due to the water tree test and the breakdown voltage became higher than that of the conventional XLPE cables after the water tree tests. The superior water tree retardation characteristics of XLVLDPE described was deduced to be due to the combined effect of the low crystallinity of the base polyethylene and the presence of the additive material (neutralizing agent and antioxidant)