Analysis of yellowing phenomenon caused by electrical and thermal degradation in the interface of cross-linked polyethylene cable joints

Yellowing phenomena have been frequently observed at the interface between the cross-linked polyethylene (XLPE) insulation and the semiconducting rubber in the joint when a failure occurs at the end bushing in gas (EBG) of the XLPE transmission cable joint. It is known that lubricants deteriorate thermally and electrically, which is related to connection material failure. In this study, in order to analyse the cause of the yellowing phenomenon in the XLPE cable termination, modelled experiments including liquid silicone rubber (LSR) semiconducting rubber and ethylene propylene diene monomer (EPDM) semiconducting rubber were carried out with the long-term electrical and thermal deterioration, and the degree of deterioration was investigated through chemical analysis and visual inspection. As a result, it was found out that the yellowing phenomenon of the lubricant at the joint interface was caused by partial discharges deterioration and was accelerated by thermal deterioration. Also, it is shown that LSR absorbs lubricant, whereas EPDM does not absorb lubricant but extracts surfer compound, which directly affects yellowing and solidification of lubricant. In addition, it is revealed that the micro-bubbles produced in the lubricant may provide a fundamental cause of partial discharges, which results in the yellowing phenomena.