The effect of residual crosslinking by-products on the local dielectric strength of HV extruded cables

Crosslinked polyethylene (XLPE) is generally obtained by the activation of a chemical agent like dicumyl peroxide (DCP). Many crosslinking by-products are produced by the decomposition of DCP. These by-products are always present in the XLPE matrix after crosslinking and manufacturing. Typical crosslinking by-products from DCP have been measured for five HV extruded cables at five radial positions in the insulation bulk. Measured by-products consisted in acetophenone and cumyl alcohol which are the most significant in HV cables. Measurement method consisted in Fourier Transform Infrared Spectroscopy (FTIR) carried out on peeled ribbons. In order to avoid by-product loss by diffusion and evaporation, ac breakdown strength measurements have been carried out immediately after FTIR. FTIR measurements show that crosslinking by-products content is higher in the bulk of the insulation than near the conductor and insulation shields. In addition, residual amount of cumyl alcohol is generally higher than for acetophenone. No important effect of both by-products on the ac dielectric breakdown strength was observed.