# Simulation Research on the Effects of Air Gaps and Ambient Temperature on a 27.5 kV Power Cable
## 4 Cable Electrical Field Research  
In order to study the damage mechanism of partial discharge leading to further increases in the cable air gap, the specific distribution of the interface electric field in the XLPE/EPDM composite interface discharge area under AC voltage was investigated using the cable model. Then, the electric field distribution characteristics between air gap composite interfaces were simulated and calculated by the electrostatic field analysis  
Figure 6: The temperature change diagram of the weak line of a cable with time. (**a**) 385 mm. (**b**) 430 mm. module, and the insulation degradation mechanism of the 27.5 kV cable caused by the air gap was analyzed. The air gap defect mainly occurred between the primary insulation layer and stress control tube in the cable model, as shown in Figure 7a. The air gap length was 20 mm, and the thickness was 2 mm. The electric field simulation calculation of the cable model with air gap defects was carried out, and an air gap electric field cloud map was obtained, as shown in Figure 7b. When there was an air gap defect inside the cable terminal, the electric field inside the air gap was seriously distorted, the maximum air gap electric field strength reached 6.5 \(\times\) 10\({}^{6}\) V/m, and the ionization field strength of super air (3 \(\times\) 10\({}^{6}\) V/m) was doubled, which is prone to partial discharge inside the cable, aggravation of cable insulation deterioration and cable aging, which affect the performance and stability of the cable. By simulating the interior of the cable under different temperature changes, an air gap temperature diagram under different temperature changes was obtained, and the influence of different temperature changes on the air gap was studied, as shown in Figure 8. The results show that a decrease in ambient temperature has a greater influence on the temperature in the air gap than an increase in ambient temperature. Figure 7: Cable electric field diagram. (**a**) Air gap free. (**b**) Air gap. Further research identified the maximum temperature of the cable under different temperatures and different environmental temperature changes, as shown in Figure 9. When the ambient temperature decreased from 20 \({}^{\circ}\)C to 0 \({}^{\circ}\)C, the air gap temperature increased the most, reaching 319.87 K. When the ambient temperature was 20 \({}^{\circ}\)C to 0 \({}^{\circ}\)C, the air gap temperature inside the cable was the highest. Under the alternating action of internal and external cold and heat, the interlayer stress changed, and the interface was not tightly bonded. With deformation inside the cable, the air gap developed further, and the cable deteriorated. Cable air gaps are prone to partial discharge, and the energy generated by discharge caused the air gap temperature to change sharply, which further aggravated the deformation of the air gap and aging of the cable; air gaps may cause potential threats to the stability and security of a traction power supply system. In order to study the influence of air gap thickness on the internal temperature of a cable during operation, a cable model with different air gap thicknesses was built, as shown in Figure 10. The air gaps between  
Figure 8: Cable temperature distribution under different environmental temperature changes. Figure 9: Cable max temperature under different environmental temperature changes. the stress control tube and the primary insulation layer were 1 mm, 2 mm, 3 mm, 4 mm, 5 mm and 6 mm. The electro-thermal coupling simulation of air gaps with different thicknesses was carried out, and electro-thermal field diagrams of air gaps with different thicknesses were calculated, as shown in Figure 11a,c. It was found that when an air gap increased from 1 mm to 6 mm, the temperature and the temperature distribution range of the air gap decreased.