Axial water ingress in watertight MV XLPE cable designs

When the outer sheath of a polymeric cable becomes severely damaged, humidity can enter the insulation system potentially increasing the relative humidity above a critical value (70% RH) making initiation and growth of water trees possible. The main purpose of this work is to determine how fast water vapour will diffuse axially in the cable. Sensitive relative humidity and temperature sensors were placed within the outer sheath at different axial positions. After drying (evacuation) a hole was cut at the cable end facilitating water ingress. Numerical calculations of axial water diffusion were performed using Comsol. The results so far show that the axial water vapour diffusion in the cable is slow and dependent of the air gap close to the swelling tape. After 130 days the humidity at 0.5 m had increased by about 40%, and the sensor at 1 m had increased by 20%. Numerical calculations of the water diffusion in the same section show a slower increase. The actual axial liquid water penetration is yet not determined. The numerical calculations show that this is an important factor, as the calculations are in more agreement with measurements when adjusting the position of the water front.