Wet Ageing of two High Voltage Cable Designs for Offshore Wind Inter Arrays

New inter array submarine high voltage (Um>36 kV) cables can include so-called wet designs allowing water molecules to enter the cable core during service. Two high voltage $U_{m}=52 \mathrm{kV}$ XLPE cables with the same insulation material but different cable screens were subjected to a wet ageing test based on CIGRE TB 722. The results show a large difference in residual breakdown strengths with one cable having a Weibull scale parameter of about $70 \mathrm{kV} / \mathrm{mm}$ and the other $27 \mathrm{kV} / \mathrm{mm}$ after ageing. The cause for the lower values was vented water treeing from the cable screens. Results from SEM analyses show that all examined interfaces between the cable screen and the XLPE insulation reveal smooth surfaces with no defects or contaminations at the inception sites. The ions causing vented water trees then likely origin from the bulk of the screen materials. However, no difference in sodium content using ICPMS was observed for the two cable screen materials.