Analysis of Inception Sites of Vented Water Trees in a Laboratory Aged High Voltage XLPE Subsea Cable

The use of lead sheaths in future subsea high voltage power cable designs will not be possible, or subject to a strict authorization processes because lead is a candidate for future inclusion by the European Chemicals Agency in the REACH Annex. “Wet design” subsea cables with no metallic barrier have been developed and are available for voltages up to 66 kV. Using the wet design also for higher voltages is very attractive to avoid the use of lead or metallic sheaths. However, in such cables, water molecules can permeate into the polymeric cable insulation system during service. As subsea high voltage cable cores have up to now been in dry service conditions, the knowledge on their performance when subjected to water is not known.<br/><br/>In this work, XLPE cable cores have been subjected to wet ageing following the recommendations in CIGRE TB 722. The electrical ageing was performed at 500 Hz with an equivalent electrical stress at the conductor screen of 10 kV/mm. The cable was helically spiralized into 2 mm thick samples. The vented water trees which appeared from the semi-conductive screens were made visible using a water pre-treatment supersaturating the tree channels. The water trees were located, and the inception sites were cryo-microtomed. The sites were assessed using Energy Dispersive X-Ray analysis (EDX) and Electron Energy Loss Spectroscopy (EELS) to determine the chemical composition of the section analyzed by Scanning Transmission Electron Microscopy (STEM). More than 5 vented water tree inception sites from the same cable were analyzed. The overall composition of inorganic elements was analyzed using Inductively Coupled Plasma Mass Spectrometry (ICPMS).<br/><br/>The cable insulation and semi-conductor sections that were analyzed by ICPMS revealed the presence of ionic species such as Na⁺ or K⁺. More localized investigation at the inception sites of the water trees was conducted by EDX and EELS analysis with respect to inorganic elements, and the results were compared to those obtained by ICPMS.