Effect of dynamic mechanical load on water treeing of XLPE cables

This paper presents results from water tree ageing experiments of XLPE cables exposed to both voltage stress and mechanical tension. The main purpose was to examine possible water tree enhancement due to dynamic mechanical tension forces, likely to occur in power cables connected to floating off-shore constructions. All experiments were performed using samples taken from a 12 kV triple extruded XLPE cable with copper conductor. A test rig was made for both static and dynamic stretching of approximately 1m long cable sections without conductor. Simultaneously strains with a maximum of 6% elongation and an effective 50 Hz AC voltage of 14 kV were applied across the cable insulation. During ageing the cable samples were submerged in tap water kept at 30°C. Both the density and growth rate of bow-tie water trees and vented water trees from the conductor screen were found to increase significantly with increasing mechanical tension. No significant growth enhancement was found in case of dynamic mechanical load compared to that of static mechanical tension. Very few vented trees were found to originate from the insulation screen. - This indicates that also the contribution from compression and tension stresses frozen in during the manufacturing process need to be considered.