Axial Water Ingress MV XLPE Cable Designs with Watertight Barrier

Abstract

Humidity can enter the insulation system when the outer sheath of a cable becomes damaged. The relative humidity can then increase above a critical value (70 % RH), facilitating initiation and growth of water trees. The purpose of this experimental and numerical work is to determine how fast water vapour will diffuse axially in a soaked cable with a damaged outer sheath.Before drying (evacuation) relative humidity and temperature sensors were placed within the outer sheath at different axial positions. After the dryingprocess a hole was cut at the cable end facilitating water ingress. Numerical calculations of axial water diffusion were performed using Comsol.The numerical calculations show that the axial water vapour diffusion in the cable is slow and dependent of the air gap close to the swelling tape. After 430 days the measured humidity at 0.5 m had increased by about 75 % (initial 20%), and the sensor at 1 m had increased by 40%. Numerical calculations show a slower increase in the relative humidity. The actual axial liquid water penetration extensions, is yet not determined. The numerical calculations show that this is an important factor, as the calculations seem to be in more agreement with measurements when adjusting the position of the water front.