The effect of accelerated water tree ageing on the properties of XLPE cable insulation

The influence of accelerated water treeing test (AWTT) on the properties of 10 kV cross-linked polyethylene (XLPE) cable insulating materials was investigated in this paper. The dielectric and physicochemical properties of both aged samples and unaged samples were tested. Dielectric property investigation found a new peak of the dielectric loss tangent with an activation of 0.14 eV in the low-frequency domain for aged samples. With increased ageing time, the conductivity in low frequency region of less than 50 Hz increases obviously, while it nearly keeps unchanged in the frequency above 50 Hz. Physicochemical investigation of FTIR, DSC, XRD and density measurement results indicated that AWTT ageing of XLPE insulated cables will lead to decreased crystallinity and density, and methyl group at the outer insulation layer cannot be found in aged samples. The melting temperature (T_m) was also decreased with ageing time. It was suggested that water was introduced from the outer layer of insulation and the degradation was developed by a combination of mechanical forces and electric force. During the ageing process, water will choose a path between surface of lamella and amorphous region, which consequently lead to the increase of low frequency conductivity and decrease of lamella thickness. Furthermore, a model was proposed to explain how the microstructure of XLPE cable insulation will change during AWTT ageing process. It is shown that XLPE cable insulation will be changed both in dielectric and physicochemical properties before any water tree was found, and a combination of dielectric and physicochemical methods is effective to detect the degradation of the XLPE cable insulation materials.