DC electrical breakdown dependence on the radial position of specimens within HVDc XLPE cable insulation

Because of temperature gradients and inhomogeneous cooling rates after extrusion, the performances of HVDC XLPE cable insulation at radial position are not uniformly distributed. DC breakdown strength measurements of HVDC XLPE cable insulation specimens in inner insulation (near conductor shield), middle insulation and outer insulation (near insulation shield) have been carried out and show that DC breakdown strength of specimens in middle insulation is relatively lower than that of in inner and outer insulation at 30, 50 and 70 °C. At 90 °C, there is no significant difference of DC breakdown strength of specimens at radial position. Thus the micro performances of XLPE specimens at different radial position have been studied by FTIR, DSC, SEM and XRD. FTIR measurements show that cross-linked by-products (benzyl alcohol and acetophenone) content in middle insulation is higher; DSC measurements shows that crystallinity of specimens in middle insulation is slightly higher; SEM measurements show that the grains of specimens in middle insulation are unevenly distributed and their sizes have obvious difference; the results of XRD measurements show a higher content of by-products, a higher crystallinity and a larger crystalline interplanar spacing (a larger grain size) of specimens in middle insulation, which are consistent with FTIR, DSC and SEM analysis. Therefore, cross-linked by-products, crystallinity and crystal morphology together make DC breakdown strength of specimens in middle insulation lower.