Simulation of water diffusion in polymeric cables using finite element methods

The major aging mechanism in polymeric medium voltage cable insulation is water treeing. Water trees can grow from contaminations and protrusions in the cable insulation when the relative humidity (W in the cable insulation exceeds a value of about 70 %. Water trees can cause service breakdown of the cable when they reach a critical length. The service life of cables without any metallic water barrier can be prolonged by delaying the water ingress into the cable. The water ingress is dependent on the diffusion rate, water sorption, initial water content in the cable materials, and the temperature of the cable. A numerical model for simulating the water ingress into a cable has been made using a finite element method. For some of the cable materials the water solubility coefficient was concentration dependent, and details of modeling these coefficients are included. Simulations for several cable configurations have been performed to investigate the time it takes for the humidity in the cable insulation to reach the critical threshold for water tree growth. The results have been compared with a simplified method for determining the time to reach 70% RH. The results show that it is possible to include concentration-dependent solubility coefficients in the finite element method. The results from the simulations and the simplified method show good agreement By taking account of the water absorbed by the semiconductors, the time to reach 70%RH is increased by about 10%.