A Study on Electrical Characteristics of Outdoor Termination with Viscosity of Insulating Oil

Outdoor terminations connecting overhead transmission lines and power cables comprise a stress relief cone, which is made of silicone rubber, and insulating oil in a composite or porcelain insulator. The lower the insulating oil viscosity, the greater is its ease of injection. However, the swelling and the degree of change in the electrical properties of the silicone rubber increase with a decrease in the insulating oil viscosity. Therefore, the effect of the expected change in the electrical properties of the silicone rubber on the outdoor termination quality should be examined before choosing the insulating oil. In this study, specimens made of insulating and semiconducting silicone rubber were made to swell by impregnating them with three types of insulating oils with different viscosities. The AC breakdown strength of the swollen insulating specimens and the volume resistivity and relative permittivity of the semiconducting specimens were measured. The Weibull statistical analysis showed that the lower the insulating oil viscosity, the lower was the AC breakdown strength in overlapping confidence intervals. The volume resistivity and relative permittivity of the semiconducting specimens changed considerably with a change in the viscosity of the insulating oil. In particular, the volume resistivity of the specimen impregnated with the lowest viscosity insulating oil was 1.57 × 10⁶ Ω m, indicating loss of semiconducting properties. Furthermore, the effect of changes in the electrical characteristics of the specimens on the outdoor termination quality was examined by performing an electric field analysis. Compared with the other oils, impregnation with the lowest viscosity insulating oil resulted in the electric field strength at the edge of the outer semiconducting layer increasing by 15.7%.