Effect of Heat Dissipation on Mechanical and Electrical Performance of High Voltage Cable Terminations
Power cables are of great importance in power transmission and distribution systems. Terminations and joints are the basic accessories of the power cables and they are required to make connections between cables or to an electrical apparatus. High voltage cables and cable accessories are widely used on rolling stock to achieve the space and weight requirements and can be subject to different types of stresses simultaneously (environmental, mechanical and electrical stresses). Therefore, it becomes critical to study their behavior under these conditions, identify causes of failure and introduce preventative methods, which will eventually lead to the improvement of the design and installation and the increase of life of the cables and their accessories. In the described model, the effect of the cable power losses on the performance of a cable termination has been studied, by combining Joule Heating and Thermal Stress physics, while the electric field distribution is determined before and after the Joule heating effect.
The high voltage cable and its termination consists of multi-layers, such as cable conductor, cable conductor screen, cable insulation, cable insulation screen, cable screen wires, sheath, stress control element, mastic and termination insulation. One of the most important aspects of modelling is to collect the required material properties before development, as this task is the most time consuming in most of the cases. Once the material properties are defined, the electrostatics physics can be used to calculate the electric field stresses on the termination. Once that is established, a current can be applied through the conductor, heating losses are produced, and the temperature distribution of the termination changes. Due to the different material properties (coefficient of thermal expansion most importantly), the layers are expanding at different rates, the termination is being deformed and it is quite probable of gaps to be created between the layers, causing degradation (leading to early-life failure) of the termination.
Finally, the deformed geometry can be stored and imported to a new model. The electric field distribution is then analysed and it is observed that the maximum electric field value is higher than before, while the mechanical deformation is apparent.
This analysis contributes into understanding the effect of the heat dissipation on the cable termination performance and it indicates that the temperature is of great importance. There are a lot of factors to consider when designing and installing a cable termination, especially, in areas where the temperature changes rapidly and the environmental conditions are more severe than usual. However, at this point, only the Joule heating contribution has been studied. Other sources of heating are the harmonics and transient over voltages, which will be included in the newest version of the model. The biggest challenge that needs to be overcome is the measurement of the material properties as they are influenced by frequency and temperature.
