Water-tree property of cross-linked polyethylene-insulated cable under γ-ray irradiation

We studied the influence of γ-ray irradiation on the water-tree properties of cross-linked polyethylene (XLPE), using a special cable to which a high voltage and γ-ray irradiation were applied. We evaluated the water-tree properties of two types of XLPE using different cross-linking agents. One was peroxide crosslinked PE (PO-XLPE) and the other was silane cross-linked PE (Si-XLPE). In an accelerated aging test, we applied a high voltage to the sample sheets in water under three conditions: without γ-ray irradiation, after γ-ray irradiation in water, and simultaneously with γ-ray irradiation in water. In the tests, the water-tree properties of the Si-XLPE appear to be superior to those of PO-XLPE. Perhaps the siloxane unit of Si-XLPE captures the infiltrating water and prevents it from concentrating at defects such as contaminations and voids. On the other hand, the incidence of water trees is retarded under γ-ray irradiation conditions. This phenomenon is related to the increase in the gel fraction, which corresponds to an increase in the degree of cross-linking in XLPE by γ-ray irradiation. Because the mechanical strength of XLPE is also increased by γ-ray irradiation, the water-tree incidence, which begins with the destruction of the material, is retarded.