Annealing Effect on the Conductivity of XLPE Insulation in Power Cable

Conductivity (sigma) in XLPE insulation of power cables annealed at 90 degrees C at temperatures between 50 and 97 degrees C has been measured. In all cases there is an initial increase in conductivity that develops a maximum and finally decreases for long annealing times. This maximum appears in the sample annealed 20 days when conductivity is measured at 50 degrees C and shifts gradually to higher annealing times up to 40 days when the measurement is performed at 97 degrees C. A linear behavior of ln(sigma) versus T-1/4 is observed, which implies that the transport mechanism is basically via thermally assisted hopping conduction. Infrared spectroscopy indicates that, during annealing, some chemical species diffuse from the semiconducting shields (SC) into the XLPE. Thermally stimulated depolarization currents technique (TSDC) and intensity-current measurements (I-V) point out as well the presence of this diffusion process that becomes less significant after long annealing times. The initial increase in sigma is explained in terms of the increase in traps density due to the diffusion process from the SC shields. Long term decrease in sigma is justified by the observed decrease of diffusion rate for long annealing times.