# Simulation Research on the Effects of Air Gaps and Ambient Temperature on a 27.5 kV Power Cable
## 1 Introduction  
Due to the restriction of line paths and other reasons, the traction power supply system of an electrified railway is more and more dependent on power cables for transport electric energy [1, 2, 3, 4, 5, 6]. With the development of high-speed railways, the reliable operation of cables directly affects the stability and safety of a traction power supply system [7, 8, 9]. Cables are one of the weakest links in the traction power supply of an electrified railway [10, 11]. During operation of the power system, the cable inevitably faces the influence of electrical stress and thermal stress [12, 13, 14, 15]. The internal heat and cold of the cable alternates, the interlayer stress changes, the interface is not tightly bonded, and structural stratification occurs, which leads to deterioration of the cable and aggravates the development of air gap defects inside the cable terminal, resulting in electric field distortion [16, 17, 18, 19]. As a result, safe operation of the cable terminal is damaged, posing a potential threat to the overall safety and performance of the system. Therefore, it is of great significance to study the distributions of the temperature field and the electric field inside a cable and the mechanism of air gap development under different air gap damage conditions, as well as the influence of environmental temperature differences on the internal temperature of the cable, to improve cable life and ensure the safe and stable operation of high-speed trains. Aiming at the typical defects of power cable air gaps, a large number of experiments and simulation studies have been carried out. References [20, 21, 22] found that under anyfield strength and temperature gradient, the dielectric properties such as the conductivity of different dielectrics have an impact on the space charge and electric field intensity distribution of the insulation layer of a cable, resulting in degradation of the cable. Therefore, the influence of internal temperature differences and environmental temperature gradients on the degradation of a cable cannot be ignored. Scholars [23; 24; 25; 26] discovered the thermal aging effect caused by temperature on a cable and observed the effect of thermal aging on the electrical properties of crosslinked polyethylene, where temperature had a profound effect on the cable. However, the above research focused on the influence of air gap defects on cable aging, ignoring the cause of cable air gaps, that is, the process of air gap defect aggravation. Traditional power cable air gap defect experimental research has considerable flaws. On the one hand, it is a long process to design the sample air gap defect of a power cable and simulate the increase in the air gap defect of the power cable in an experiment, while an increase in the air gap defect of a power cable due to electrothermal coupling damage is a microscopic process. It is difficult to observe the microscale defect aggravation of power cables on a longer time scale. On the other hand, at present, the main means to study defects of power cables is to simulate the normal operating conditions of power cables and to observe the temperature changes in power cables during experiments by means of infrared cameras. However, due to the complex structure of power cables and the combination of various materials, infrared cameras can only observe temperature changes in the outer layer. The internal temperature variation of power cables cannot be well characterized. However, power cable simulation research can overcome the difficulties in experimental research, such as a complex structure that is not easy to observe and micro-changes that are not easy to detect, to realize the study of power cable air gap defects. In past power cable simulation studies, references [27; 28; 29] used finite element analysis to analyze the influence of air gap defects on cable terminals. It was proven that air gap defects can cause distortion of the electric field in the insulation layer.