## 3 Cable Terminal-Sheath Structure Electric Field Simulation Research
#### 3.2.2 Air Gap between the Cable Terminals and Sheath, with No Interfacial Wetting  
In order to facilitate the simulation calculation, the air gap model is simplified in this study, and the leakage current flowing through the interface is completely cut off by the air gap analyzed; that is, the radian of the air gap is \(360^{\circ}\), as shown in Figure 10. The dark orange part represents the umbrella skirt column of the cable terminal, the light orange part represents the installed insulating sheath, and the white part represents the air gap  
Figure 8: The electric potential along the surface when the outer surface of the sheathed cable terminal is covered by evenly distributed wet dirt. Figure 7: The influence of the sheath on the creeping electric field with a clean outer surface on the terminal. Figure 9: The electric field along the surface when the outer surface of the sheathed cable terminal is covered by evenly distributed wet dirt. defect. In this paper, the influence of different lengths of air gaps on the interface electric field and external surface electric field is analyzed when there is only an air gap at the interface between the insulating sheath and umbrella skirt column. Effect of different air gap positions on electric field distribution in cable termination-sheath structures  
As shown in Figure 11, five air gaps with a thickness of 1 mm are set on the interface between the insulating sheath and the umbrella skirt column, with the numbers 1, 2, 3, 4, and 5 assigned from top to bottom. Among them, No. 1 air gap is set at the top of the insulating sheath, and No. 5 air gap is set at the three junction points of the intersection of the insulating sheath, umbrella skirt column, and lower flange. The length and thickness of the air gap remain unchanged at 1 mm. We only changed the position of the air gap to study its influence on the interface electric field and the electric field along the outer surface. The frequency domain electric field simulation when there is no air gap at the sub-interface, No. 1 air gap at the interface, No. 2 air gap, No. 3 air gap, No. 4 air gap, and No. 5 air gap at the effective value of 25 kV power frequency is then carried out, and the electric field cloud diagram of the insulating sheath and its vicinity is obtained, as shown in Figure 12. As can be seen from the figure, compared with the scenario without an air gap, the existence of an air gap distorts the electric field near the air gap. In addition, when the air gap is in position 1, the maximum electric field inside the air gap appears in the lower right corner of the air gap block; that is, the inner surface of the insulating sheath and its electric field strength is about 8 \(\times\) 10\({}^{4}\) V/m. When the air gap is at position 5, the maximum electric field inside the air gap appears in the lower left corner of the air gap block; that is, the outer surface of the umbrella skirt and its electric field intensity is also 8 \(\times\) 10\({}^{4}\) V/m. When the air gap is at positions 2, 3, and 4, the electric field distribution in the air gap is more uniform. Figure 11: Diagram of air gap position distribution. Figure 10: The position of the air gap at the interface between the umbrella skirt column and the insulating sheath. Figure 13 shows the electric field distribution diagram on the interface between the sheath and the umbrella skirt column. From the figure, we can intuitively see the distortion of the air gap and the electric field at the interface. Compared with the position without an air gap, the electric field seen at the same position with an air gap is about 1.7 times that without an air gap. Among the fields, when the air gap is at position 2 and position 5, the maximum electric field at the interface is about 8 \(\times\) 10\({}^{4}\) V/m, which is far less than the breakdown field strength of the air gap of 3 \(\times\) 10\({}^{6}\) V/m.