Modelling And Classification Of Typical Defects In HV Cable Sheath And Earthing Systems

With the development of social economy and the expansion of urban scale,high voltage(HV)cables gradually replaced overhead lines and became the main channel for the power transmission in the urban areas.Although the failure rate of cable systems is much lower than that of overhead transmission lines,breakdown of cables may lead to a great economic loss due to the long maintenance time.Patrol inspection is a critical way to detect and eliminate cable defects before they turn into failures.As one of the essential condition variables for inspection,sheath current can efficiently reflect the condition of cable systems.However,the existing diagnostic criteria of sheath current are relatively simple.The relationship between the current characteristics and defect categories has not been established yet.In the last decade,defects of HV cables,represented by buffer layer defects and grounding system defects,occurred frequently.It is imperative that relevant criteria must be developed.In this dissertation,a numerical model of the HV cable system is established based on Maxwell equations.The factors of the electric field distribution in the air gap between the corrugated sheath and the buffer layer are analyzed.Possible reasons for the buffer layer defects are assessed according to the simulation results.The ambient impact on the grounding currents is clarified by calculating the induced voltage in relation to the minor section length,the distance between phases and the load currents.A classification method has been proposed to discriminate the various defects with the logistic regression algorithm.The result shows that when the buffer layer is flooded or contains design defects,the contact area between the corrugated sheath and the buffer layer may cause air breakdown due to the excessive field strength,and the heat accumulation generated by the discharge will hurt the buffer layer and the outer semi-conductive layer.The defect of the HV cable buffer layer will change the leakage currents,while the defect of the grounding system will make the induced currents show corresponding characteristics due to the changes of circuit topology.The feature extraction method of grounding currents by using amplitude ratio and the phase difference can offset the influence of operating environment difference on defects diagnosis,and the introduction of logistic regression algorithm improves the efficiency and accuracy of defects diagnosis.The model has been fully evaluated using a number of criteria and the results show that Accuracy,Precision,Recall,F1 value of the model are more than 92%.