Pressure wave propagation technique to investigate water treeing degradation in polymers

A pressure wave propagation (PWP) technique has been used to evaluate electrical aging of polymeric insulation caused by water treeing. The technique is non-destructive and it seems to give both the site information of trees and the space charge distribution in materials. Comparison was made between a non-aged XLPE sample and an ac voltage aged sample respectively. The aged sample was obtained by applying an ac electrical field of 15 kV/mm for about one month in order to grow water trees at moistened conditions. A dc field (14.3 kV/mm) was applied to the sample for 15.4 hours to measure the charge distribution using the PWP technique. For the non-aged sample however, only a dc voltage was applied to the sample for obtaining the reference results. Experiments show that heterocharges were accumulated in the aged sample; however, only a few charges were observed in the nonaged sample. Two different relaxation processes were observed in the aged sample, a fast decay followed by a slow relaxation. The former may be due to the polarization of the moisture in the sample, the latter may represent the real space charge relaxation. The PWP technique makes it possible to quantitatively investigate the relationship between the space charge and the pre-breakdown processes in solid insulating materials