## 3 Cable Terminal-Sheath Structure Electric Field Simulation Research  
In the process of train operation, good insulation equipment and internal fixtures tend not to cause partial discharge, and partial discharge occurring in the area is often due to the interfacial bonding not being good or some of the media production being poor, resulting in the emergence of an air gap. The existence of an air gap will lead to distortion of the electric field near the air gap. The electric field inside the air gap is concentrated, it is easy to trigger air gap breakdown and a breakdown caused by thermal effects. Corrosive substances will intensify the carbonation of the material, resulting in the further expansion of the air gap, which may result in overall material breakdown; at the same time, due to the insulating jacket using an open movable structure, the external water vapor will travel along the gaps into the inner surface of the jacket. The presence of water vapor will greatly change the surface conductivity and dielectric constant of the interface, resulting in the effect of interfacial wet contamination; finally, as the roof cable terminals are often exposed to rising damp and wet weather such as fog, drizzle, etc., under such weather, the outer surface of the cable termination umbrella skirt often adheres to water droplets or a layer of water film, resulting in a significant increase in leakage currents along the outer surface of the umbrella skirt and the occurrence of surface discharging phenomena [22; 23; 24; 25]. Therefore, to investigate the root cause of the discharge, based on this speculation, this chapter starts with the electric field distribution characteristics of the cable termination-sheath structure, and the simulation analysis of the interfacial electric field distribution in various working conditions under the influence of the air gap, interfacial wet contamination, external wet contamination, and other factors. Distribution of the Electric Field in the Cable Termination-Sheath Structure When No Sheath Is Installed  
Firstly, a situation where the upper and lower fittings of the cable terminal are not equipped with an insulating sheath is simulated. According to the requirements of the allowable fluctuation range of traction network power supply voltage, the small increase in  
Figure 3: Partial meshing diagram of the cable terminal–sheath model. power frequency voltage has little impact on the interface electric field, so the voltage applied in the subsequent simulation is the power frequency's effective value of 25 kV [26; 27]. Figure 4 shows the equipotential line diagram when the outer surface of the cable terminal structure is free of pollution. It can be seen that its high field strength is concentrated in the interior of the insulating layer between the two semi-conductive layers inside the insulating sheath and at the fracture of the semi-conductive shielding layer, while, on the outer surface of the cable terminal, there is a high electric field value near the edges of the upper and lower fittings and at the same height as the fracture of the semi-conductive shielding layer. First, we apply uniformly wet dirt to the outer surface of the cable terminal. The wet pollution area's conductivity is set to 0.6 s/m, the relative dielectric constant is set to 81 [28; 29; 30; 31], and the wet dirt's position is shown in Figure 5 (left). Figure 5 (right) shows the equipotential diagram when the outer surface of the cable terminal structure is uniformly wet and dirty. It can be seen that the potential distribution of the cable terminal changes greatly due to the existence of external wet contamination. For example, in the first umbrella skirt, when the outer surface is clean, the equipotential line of the umbrella skirt is almost parallel to the axial direction, and the potential distribution decreases from the root of the umbrella skirt to the top of the umbrella skirt.