Micro-Water Migration Characteristics in Cable Oil-Filled Terminations and Their Effect on Insulation Performance

To investigate moisture ingress in the insulation of cable oil-filled terminations, this study first simulated moisture migration within a termination. Based on identified accumulation locations, the breakdown voltage of the silicone rubber (SiR)/cross-linked polyethylene (XLPE) interface and the dielectric properties of both silicone oil (SO) and SiR insulation were tested under varying moisture contents. The results indicate that after moisture invades a cable oil-filled termination, it aggregates into water droplets under multiple forces, particularly the dielectrophoretic force, and migrates toward the stress cone. Moisture tends to accumulate in regions with severe electric-field distortion, specifically at the “triple junction” formed by SiR, XLPE, and the stress cone. The breakdown voltage of the SiR/XLPE interface decreases with increasing moisture content in the SO. Similarly, the interfacial breakdown voltage declines with rising moisture in the SiR, though the reduction is less pronounced than that observed with increased moisture in the oil. When the moisture content in the SO rises, its dielectric loss tangent increases significantly. In contrast, when the SiR contains moisture, a new characteristic peak emerges in its dielectric loss spectrum. This peak frequency shifts toward lower frequencies as the moisture content in the SiR increases, accompanied by a slight rise in the peak value.