Water Treeing Behaviour in XLPE Insulation under the Influence of SAP and Development of Evaluation Method

Under identical electric field and environmental conditions, no observable water trees were formed in the reference specimen consisting solely of the insulation material. In contrast, specimens with SAP tape attached to the insulation surface exhibited microscopic water trees on the order of several tens of micrometers within the insulation. This indicates that even in D.I.

water without external salt, Na ions originating from the SAP can create a localized ionic moisture environment that initiates water tree formation.

Furthermore, comparison between D.I. water and a 1 Molar concentration(M) NaCl solution revealed that, in the NaCl solution, numerous long water trees with lengths up to approximately 400 μm were formed, confirming that the ionic concentration plays a decisive role in water tree growth.

Finally, taking into account that SAP tape does not directly contact the insulation layer in actual submarine cable structures, possible indirect transport pathways were considered, through which Na ions and moisture derived from SAP can reach regions near the insulation via interfaces, defects, and electric field effects in the multilayer structure. The results suggest that Na-based SAP contributes to improved moisture sealing performance, while at the same time posing a potential risk in terms of long-term insulation reliability. Accordingly, the need for developing Na-free or low-ionic SAP materials and optimizing structural design is discussed.