Failure investigation of buffer layers in high-voltage XLPE cables

Abstract

In recent years, insulation failures on HV XLPE cables caused by buffer layers between corrugated Al sheaths and insulation screens have been reported and cause many concerns. Lots of white powder and ablated traces appeared in the buffer layers of these cables. In this paper, the failure characteristics and phenomena were carefully examined through incomplete statistics and the field sectioned cables, respectively. Then, investigations were performed by means of visual examination, optical microscopy, chemical examination, and resistance measurement. Treeing analysis reveals that the XLPE insulation layer is caused by the failure mechanism rather than the cause of it. SEM, EDAX, and XPS analysis demonstrated the formation process of the white powder and the electrochemical reaction between the Al sheath and the white powder. Resistance analysis pointed out that the unqualified copper wire braid and the insulated white powder causes the insulation screen to lose its connection to the Al sheath. Taking the manufacturing environment, laying environment, and laying method of cables into account, water is chiefly responsible for the formation of white powder and the electrochemical corrosion of Al sheath, which is validated with the precipitation experiment. Finally, the effect of white powder on the failure and the cause of electrochemical corrosion were discussed. The failure cause was identified in the white powder and the thin copper wires that induced a potential difference between the Al sheath and the insulation screen and its subsequent partial discharges and breakdown.