## 3 Cable Terminal-Sheath Structure Electric Field Simulation Research
#### 3.2.4 The Cable Termination and Sheath Have Interfacial Wetting and Air Gaps  
In general, the precondition for water vapor to enter the inner surface of the sheath is that there is an air gap channel; that is, the internal wet pollution area and air gap often exist at the interface at the same time. Therefore, it is necessary to analyze the electric field distribution of the sheath structure when the internal wet pollution area is accompanied by the air gap. In order not to lose generality, as shown in Figure 19, we fix the air gap position at the white part of the position (position 2), and set the wet pollution area (blue part) between the air gap and the lower flange to analyze the electric field distribution inside and near the air gap during the period with the most marked effect on the interface. The electric field distribution of the time-division interface with only an air gap, only an internal wet pollution area, and the simultaneous existence of the air gap and internal wet pollution area are compared. As shown in Figure 20, it can be seen that when the internal wet pollution area and air gap exist at the same time, the electric field distortion at the interface is the most marked, and the maximum electric field at the junction of the internal wet pollution and air gap can reach \(2.55\times 10^{5}\) V/m, which is nearly an order of magnitude greater than that when there is only air gap, and nearly 33.3% higher than that when there is only internal wet pollution. This is because the conductivity and relative dielectric constant of the air gap are lower than that of the insulating sheath made from  
Figure 19: Distribution map of the internal humidity and the air gap. Figure 18: Diagram of the influence of different internal wet dirt area lengths. silicone rubber, and the conductivity and dielectric constant are inversely proportional to the electric field strength. Therefore, when the internal wet pollution area is accompanied by the air gap, the existence of the air gap replaces the position of the original part of the insulating sheath; that is, the dielectric constant and conductivity decrease, increasing the electric field strength. Thus, the simulation results are consistent with the theory. Electric Field Distribution of the Cable Termination-Sheath Structure in the Presence of External Moisture Contamination  
According to real-world investigations, the burning discharge of the roof cable's terminal sheath structure is often seen in high-tide wet weather, such as fog and drizzle. In this weather, the outer surface of the cable terminal umbrella skirt is often in contact with water droplets or with a layer of water in a film, resulting in a significant increase in the leakage current along the outer surface of the umbrella skirt; finally, the surface discharge phenomenon occurs. The existence of the water film will also increase the voltage distributed on the insulating sheath and will then increase the electric field intensity at the interface between the umbrella skirt column and the sheath. As shown in Figure 21, the outer surface of the cable's terminal structure with an insulating sheath is assumed to be covered with a layer of wet dirt of uniform thickness. The thickness of the wet dirt layer is set to 1 mm. The conductivity and relative dielectric constant of the wet dirt will be relatively high. To analyze a more serious scenario, we select the relative dielectric constant of the external wet dirt as 81 and the conductivity as 0.6 S/m, to simulate and analyze the electric field distribution on the sub-interface under various conditions. terminal and compare it with the scenario without external wet dirt. It can be seen that the existence of external wet pollution increases the overall interface electric field, resulting in a larger internal electric field in the air gap than when there is no external wet pollution.