An Investigation into the Mechanism of Water Treeing in Polyethylene High-Voltage Cables

Water treeing is a well known fracture phenomenon in polyethylene (PE) and crosslinked polyethylene (XLPE) high voltage cable insulation. We studied two phenomena: (1) The insulation material is mechanically fatigued by electrostriction as is shown by measuring the relevant electrostriction constant, which moreover is compared with literature values on other polymers. The observed "electrostriction" is found to originate mainly from the Coulomb force. Using a simple model, the deformation mode at the end of a water tree channel can be described and the strain amplitude of the PE at the water interface can be calculated. (2) Under high voltage, the surface tension at the PE-water interface decreases strongly as we found experimentally. The observation that this decrease is proportional to the square of the applied field strength is explained. The joint effect of the Coulomb force and the reduced surface tension may cause a fracture phenomenon, well known in the field of polymers, called environmental fatigue failure (or thermal softening). The same assumption is made as in "common" environmental stress cracking of PE with aqueous detergents, viz. that the low surface tension plays an essential and not a coincidental role.