# Simulation Research on the Effects of Air Gaps and Ambient Temperature on a 27.5 kV Power Cable
## 3 Cable Thermal Field Research  
Under preset conditions, the thermal field simulation of the simulation model was carried out to obtain the cable temperature distribution. The simulation results are shown in Figure 4. By comparing the temperatures of different structures, it was found that the core is the heat source of the cable terminal, and the highest temperature is 105 \({}^{\circ}\)C, as shown in Figure 4a. At the same time, it was observed that the temperature deceases with increasing distance from the cable core. When in the shield layer and the glue, the temperature near the stress tube approached the air temperature. When the cable was energized and heated, due to the varying thermal conductivity and temperature increases in different structural materials, the effects of temperature increases and thermal expansion had a strong influence on the deformation of the cable, which easily leads to the generation of air gaps and aggravates the aging and failure of a cable. Therefore, in order to further study temperature increases in different model structures when the cable is energized, internal temperature increases in the cable and terminal were calculated, as shown in Figure 4b. It was found that temperature increases between the outer semiconductor, the insulating layer and the junction of the stress control tube (x = 385 mm and x = 430 mm) is the largest. In practical engineering, the junction of the semiconductor, insulating layer and stress control tube of the cable is more prone to abnormal deterioration and failure [8, 33]. The simulation results of the cable thermal field model were consistent with the actual project, which also proves that the huge temperature difference between the semiconductor layer, insulation layer and stress control tube is an important reason for the deterioration and failure of a cable. In the following research, the thermoelectric coupling simulation based on cables further revealed the damage and aging mechanisms of cables caused by temperature differences. Electrified railway cables are in an exposed environment, and the temperature of the exposed environment will vary greatly over time. Different ambient temperatures caused by different times aggravate the aging of cables. In order to further study the influence of ambient temperature changes caused by different times on the distribution of the cable internal temperature, according to the ambient temperature of typical cable laying [34, 35], the formula that defines the ambient temperature of an electrified railway cable over time is as follows:  
\[T=\left\{ \begin{array}{ccc}(\frac{2}{45})\cdot t+293.15&&0<t\leq 450\\ 313.15&&450<t\leq 1800\\ (-\frac{3}{45})\cdot(t-2700)+253.15&&1800<t\leq 2700\\ 253.15&&2700<t\leq 4050\end{array} \right. \tag{5}\]  
Based on the above definition formula for changes in ambient temperature over time in electrified railway cables, a cable working environment temperature change curve with time was developed, as shown in Figure 5. It was found that t = 0 s to t = 4000 s can be divided into four zones, and t = 0 s to 450 s is the temperature increase zone. T = 450 s to t = 2700 s is the higher-temperature stable region, t = 2700 s to t = 4000 s is the temperature-drop zone, and t = 0 s to 450 s is the lower-temperature stable zone. In the four zones, the temperature difference from t = 1800 s to t = 2700 s--the temperature-drop zone--is the largest, which easily leads to cable air gaps and exacerbates cable aging. Therefore, the temperature difference from t = 2000 s to t = 2040 s in the temperature-drop zone was selected to further study the effect of environmental temperature differences on the cable temperature distribution. Figure 4: Cable temperature and temperature difference distribution simulation. (**a**) Temperature. (**b**) Temperature increases. Figure 5: Cable working environment temperature change curve with time.