Effects of inclusions of oxidized particles in XLPE on treeing phenomena

One issue with the use of crosslinked polyethylene (XLPE) insulation in high voltage power cables is the presence of oxidized particles. Oxidation may occur during production of polyethylene, during the extrusion and curing process used to produce XLPE cables, or in subsequent processes/operation of the cables. Oxidized particles are here referred to as organic contaminants. In the present study artificial organic contaminants were introduced in XLPE samples made for electrical treeing measurements. Instead of using the common needle-needle or needle-plane configuration, a wire-plane configuration was used. Organic contaminants have an increasing conductivity and permittivity as function of increasing degree of oxidation. These properties most probably give rise to local electric field enhancements in the material. The morphology of organic contaminants also differs from virgin XLPE, which probably cause bad adhesion between the XLPE matrix and the contaminants. A combination of these factors was found to affect the electrical performance of the insulation. Several different parameters, such as dry and wet ageing, as well as AC voltage and DC voltage stress, were included in the study.