| Underground power cables have become the mainstream of urban power distribution network,because they have the advantages of ground space saving for city modernization.However,the cables are prone to become defective due to the complex and harsh operating environment of cable attachment joints and the varying levels of installation and construction techniques.The defects in the cable joints lead to frequent cable accidents.In order to effectively predict the operation condition of cable joints and avoid accidents caused by defects,this paper adopts a multi-physical field coupling analysis of cable joints and evaluates the hazard level of cable joints based on the analysis results.In this paper,the finite element method is used to model and calculate the multi-physical field coupling of the cable joint system,and the simulation results are verified by multi-physical field measurement experiments on 10kV XLPE distribution cables.Based on the simulation and experimental results,a method is proposed to calculate the internal defect status of the cable insulation to assess the hazard level of the cable joint system.In order to establish a more efficient physical model of cable joints with experiments,this paper provides an in-depth understanding of the stress and insulation structure of 10kV XLPE distribution cables and their intermediate joints,and then establishes a nondestructive model of cable joints.The accident mechanism and defects of cable joints are also analyzed,and three typical defects are selected to establish a physical model of defects,while the corresponding size and type of defects are made on the actual cables to prepare the work for the experiments.To investigate the mechanism of the electric-thermal-force triple field coupling in cable joints,the mathematical models and boundary conditions of the electric,temperature and stress fields of the cable joints are studied separately,including the settings of the finite element parameters of the physical model in the finite element analysis software.The multi-physics field coupling simulation and calculation analysis of the cable joints is carried out.Firstly,the distribution of the electric field strength of each model is calculated,the heat loss in the electric-thermal coupling calculation is loaded as a load into the temperature field analysis module to calculate the temperature distribution of the overall system,and then the temperature distribution in the thermal-force coupling calculation is loaded as a load into the stress field analysis module to calculate the stress distribution between the insulation interfaces.An experimental study on the multi-physics coupling of 10kV XLPE distribution cable joint system was conducted to verify the simulation results.First,we built a distribution cable experimental platform and made a self-made insulating layer interface pressure measurement device for force field measurement,and completed the installation and calibration of the multi-physics field mapping module.The results of the interface pressure measurements are compared with the values of the interface pressure in the stress field analysis to verify the correctness of the simulation results.A calculation method is proposed to characterize the status of defects inside the cable joint.Firstly,a hazard factor is proposed to characterize the ratio of the resistance at the defect to the resistance of the XLPE insulation of the same cable length,and the hazard factor values corresponding to each model are calculated based on the thermal load.Secondly,the hazard factor is characterized by the surface temperature difference method based on the temperature characteristic analysis;finally,the state of the defect inside the cable insulation is calculated by the hazard factor and the real-time cable current. |
