Electromagnetic-thermal-mechanical Characteristics And Characterization Method Research On Cable Joint Under Internal Defects

With the rise in the number of cable lines which have been running for more than 15 years in the whole country, the accidents of of power cable accessories happened frequently, especially the explosions of cable terminations and cable joints. The accidents of cable joint explosions can not only lead to a huge economic loss but can also involve personal safety and the social image of the power company. The mechanism and reason of the cable joint explosion have long plagued the staff in the operation and management department, and the difficulties are: 1) the cable joint explosions take place in various forms, including the explosion during the runtime and explosion after examination reparation in power cut; 2) there are many fators that can contribute to the cable joint explosion, including high contact resistance and eccentrical insulation caused by unqualified manufacture, partial discharge caused by internal defects and overvoltage impact. Essentially, explosion is the instantaneous release of enormous energy. From the source and nature, the explosion of cable joint is the process of acceleration of thermal and electrical aging caused by different internal defects and discharge breakdown in the cable insulation layer.In order to analyze the mechanism process of cable joint explosion better, prevent the accidents of cable joint explosion and provide support for the reliability and safe operation of the power grid, this paper will focus on the electromagnetic-thermal– mechanical coupling characteristics of cable joint under the internal defect and the reason leading to the explosion. Firstly, the model of electromagnetic-thermalmechanical coupling fields was studied and example calculation was carried out based on this model, and the accuracy of this model was verified by measurement results on site. Secondly, the electromagnetic-thermal-mechanical characteristics of cable joint under internal defects, such as high contact resistance, gag discharge and arc breakdown, were investigated, and the temperature and stress distribution rules of the cable joint with different internal defects were obtained. On this basis, the thermal-mechanical characterization methods for the different internal defects of cable joint were studied, and the explosion threshold of the cable joint was investigated. The content of this paper can provide methods for mechanism analysis of cable joint explosion and the advance diagnosis of the internal defects. The main contents of this paper are as follows:(1) The numerical model of electromagnetic-thermal-mechanical coupling fields for cable joint was investigated, including the governing equations, boundary conditions and the efficient iteration solution method. Then, a calculation example of 8.7/15 kV cable joint cable was given, and the calculation results were also analyzed. The accuracy of this coupling model was verified by measurement results on site.(2) In order to analyze the thermal-mechanical characteristics of cable joint under internal defects, such as high contact resistance, gag discharge and arc breakdown, the heat loss calculation models under the three different defects were investigated, mainly including the heat loss equivalent calculation model of conductor and connecting tube considering the high contact resistance, heat losss calculation model of the gap discharge in the cable joint insulation based on the plasma-chemical model and heat losss calculation model of insulation arc breakdown based on magnetohydrodynamic(MHD) model.(3) The numerical calculations for the thermal-mechanical characteristics of cable joint under the different internal defects were carried out, and the influence rules of different degree of defect and other factors on the thermal-mechanical characteristics were obtained.(4) Based on the thermal-mechanical characteristics and laws of cable joint under the internal defects, the characterization methods based on temperature, temperature gradient and stress for the internal defect of power cable joint were proposed and the defect condition monitoring can be realized based on these method. Moreover, aiming at the breakdown process existing in the instantaneous explosion of cable joint, the explosive thresholds of cable joint were investigated from the perspective of thermal-mechanical characteristics, and the explosive thresholds are time threshold and arc current threshold.(5) The experimental platforms to simulate the thermal-mechanical characteristics of cable joint under the internal defect of high contact resistance and insulation arc breakdown were set up to verify the accuracy of the proposed model in this paper.