## 3 Cable Terminal-Sheath Structure Electric Field Simulation Research
#### 3.2.3 The Interface between the Cable Terminal and the Sheath Is Wet, and There Is No Air Gap  
Since the insulating sheath structure exhibits an open removable structure, the sealing part of the sheath easily develops poor adhesion, and external water vapor will enter the inner surface of the sheath along the gap. The existence of water vapor will greatly change the surface conductivity and dielectric constant at the interface. Therefore, it is necessary to consider the effect of wet contamination at the cable terminal-sheath interface on the electric field distribution that is found at the interface. Figure 14: The influence of the length of the air gap on the interface’s electric field. Figure 13: The influence of different air gap positions on the interface electric field. Effect of interfacial wet contamination at different locations on the electric field of cable termination-sheath structures  
In this paper, the more serious situation is selected for the simulation. The internal wet pollution area is regarded as a layer of water-based film; its relative dielectric constant is set to 81 and the conductivity is set to 0.6 s/m. To ensure the generality of the simulation results, three internal wet pollution areas are set on the inner surface of the sheath, as shown in Figure 15. The internal wet pollution areas are numbered as a, b, and c from top to bottom. The length of the internal wet pollution area is set to 5 mm and the thickness of the internal wet pollution area is set to 1 mm. Position a, position b, and position c are set as the internal wet pollution area, and their influence on the interface's electric field distribution is simulated. The most marked distortion of the wet electric field exists at the top of the wet electric field area. Because there is no voltage drop in the internal wet pollution, the electric field between the upper and lower vertices of the internal wet pollution area is very small, and the existence of the two vertices is equivalent to two tips with a very small radius of curvature, resulting in a relatively dense equipotential line and a large corresponding potential gradient. Therefore, the electric field at the two vertices is very high. As shown in Figure 16, when the internal wet pollution area is in position a, there is a maximum electric field at the lower vertex of the internal wet pollution area. Compared with the scenario without internal wet pollution, the electric field intensity increases by about 167%. Effect of interfacial wet contamination of different lengths on the electric field of cable termination-sheath structures  
Figure 16: Diagram of the influence of different internal wet contamination positions on the interfacial electric field. Figure 15: Distribution diagram of the internal wet dirt. a is the upper internal wet stain on the inner surface of the sheath, b is the middle internal wet stain on the inner surface of the sheath, and c is the lower internal wet stain on the inner surface of the sheath. The dash circle is the setting range of the inner wet dirt. Due to the randomness of the location of the internal wet pollution area, the length of the internal wet pollution area is also uncertain. The length of the wet sheath has different effects on the structure of the wet sheath. Therefore, taking position C as the benchmark, the electric field distribution of the sheath structure and its interface is simulated when the internal wet pollution area length is 5 mm, 11 mm, 16 mm, and 20 mm. The maximum electric field inside the sheath changes when the length of wet dirt in the sheath is different. The maximum electric field appears in the upper right corner of the inner wet pollution area. As shown in Figure 17, when the length of the inner wet pollution area is 5 mm, the maximum electric field near the inner wet pollution area is 1.10 \(\times\) 10\({}^{5}\) V/m. When the length of the internal wet pollution area is 11 mm, the maximum electric field near it is 1.62 \(\times\) 10\({}^{5}\) V/m.