The oil-filled terminal adopts a solid-liquid composite insulation composite structure composed of silicone rubber (SiR) stress cone and silicone oil (SO) inside. Compared to the cable body, when the composite insulation interface is invaded by moisture or contains air gaps or impurities, it can cause electric field distortion. This distortion can trigger creepage and even flashover along the surface of the stress cone, significantly impacting the service life of the oil-filled terminal. Among them, moisture intrusion is recognized as the main factor causing insulation deterioration in cable terminals, and cable termination failure rate caused by it account for about 50%. Therefore, the moisture migration process and equilibrium characteristics between SO and SiR solid-liquid medium in the oil-filling terminal need to be further studied. In this paper, the moisture migration law between SO-SiR composite insulation system in oil-filled terminals is systematically studied, the swelling model and mechanism of SO in the terminals are discussed, and the moisture equilibrium characteristics of SO-SiR composite insulation systems under the effect of temperature and swelling are clarified.
Firstly, this study conducted moisture absorption experiments on SO and SiR under various temperature and humidity conditions. The water content of SO and SiR at different temperature and humidity equilibrium states was measured. Using the indirect equilibrium theory, a moisture equilibrium curve for the SO-SiR composite insulation was plotted. The results show that the water content of SO has a linear relationship with the relative humidity at the same temperature, and the saturated water content of SO changes exponentially with temperature. The water content of SiR has a nonlinear relationship with relative humidity, and the saturated water content of SiR does not change with temperature. As the temperature rises, moisture migrates from the SiR to the SO.
In addition to the moisture migration between the SO and the SiR duplex medium in the oil-filled terminal, the SO will also diffuse into the SiR. This diffusion destroys the physical and chemical cross-linking results of the SiR, and affects the moisture absorption characteristics of the SiR. Therefore, it is necessary to clarify the physical mechanism underlying the swelling of SiR by SO. The results show that the SO swells into the SiR in the form of free state and bound state according to the Langmuir diffusion process. With increasing time, the swelling rate increases as a logarithmic function. With increasing temperature, the equilibrium swelling mass remained unchanged, but the swelling rate increased. Under the SO (solvent)-SiR (solute) system, the elastic free energy of the system increased due to the swelling of SO, which was offset by the Gibbs free energy. Finally, the total free energy is zero, and the swelling reaches equilibrium.
On this basis, the moisture equilibrium curve of SO-SiR composite insulation was further optimized. After the swelling of SO, the free volume of SiR increases, which can dissolve more water. However, SiR with different degrees of swelling still exhibits the same water absorption characteristics as unswollen SiR. Combined with the moisture dissolution characteristics of SO, the moisture equilibrium surface diagram of SO-SiR composite insulation under temperature and swelling was drawn. With increased swelling, water molecules migrate from the SO to the SiR. Through this surface diagram, the water content of SO and SiR under different equilibrium states can be obtained, and the operation and maintenance of oil-filled terminals can be guided.