## 4 Conclusions  
This paper takes the overall structure of the high-voltage cable's terminal sheath on the roof of an electric locomotive as the research object, establishes the simulation model of the cable terminal sheath in COMSOL, and studies the influence of the air gap, external wet dirt, and interfacial wet dirt on the distribution of the electric field at the interface of the cable terminal and sheath using simulation analysis, then investigates the mechanism of the occurrence of inner-surface discharge, and draws the following main conclusions:  
1. When the outer surface of the cable terminal is clean and dry, the insulating sheath has no obvious effect. When the outer surface of the terminal has serious wet pollution issues, the insulating sheath can extend the leakage on the current creepage path, which reduces the leakage current along the surface, thus reducing the probability of the partial discharge of cable terminals. 2. The position and size of the air gap affect the electric field distribution at the cable terminal-sheath interface. Compared with the positioning without an air gap, the electric field in the same position but with an air gap is about 1.7 times that without an air gap; when the position of the air gap is located at the edge of the cable terminal, the distortion of the electric field distribution is more serious, and the length of the air gap is increased; the maximal electric field and the average electric field within the air gap are also increased, but the magnitude of the increase is slowed down gradually. In general, the size of the air gap has little effect on the electric field distortion at the sub-interface. 3. The location and length of the interfacial wet contamination area affect the electric field distribution at the cable terminal-sheath interface. When interfacial wet contamination exists at the interface, the point of maximum electric field strength generally occurs at the interfacial wet contamination point, and the maximum increase in electric  
Figure 27: Interfacial electric field diagram for internal wet dirt and an air gap. field strength is about 167 percent compared with the scenario without interfacial wet contamination. With an increase in the interface wet contamination length, the maximum aberration electric field on the interface is basically increased linearly. When the length of the interfacial wet contamination is 20 mm, the maximum electric field at the interface is about twice that of the interfacial wet contamination area, with a length of 5 mm. 4. Moisture contamination on the outer surface of the cable termination greatly enhances the non-uniformity of the electric field's distribution on the surface. When there is no wet contamination at the interface and there is an air gap at the interface, the air gap is located at position 1, where the electric field near the air gap is the largest, about 5.0 \(\times\) 10\({}^{5}\) V/m, which is nearly an order of magnitude larger than the electric field near the air gap when there is no external wet contamination; When there is a wet pollution area at the interface but there is no air gap at the interface, the interfacial electric field is maximum when the wet pollution area is located at position C and the length of the interfacial wet pollution area is 20 mm, and it can reach 1.1\(\times\)10\({}^{6}\) V/m. When there are both interfacial wet pollution areas and air gaps at the interface, the presence of air gaps increases the maximum distortion electric field of interfacial wet pollution areas by about 26.6% compared to that in the absence of air gaps, and the maximum value of the electric field can be as high as 1.9 \(\times\) 10\({}^{6}\) V/m. This makes the air gaps susceptible to partial discharges.