Study On Location Of Middle Voltage Cable Joints Based On Time Domain Reflectometry

With the improvement of urban modernization in China,the replacement of traditional overhead lines by cables has become an inevitable trend of the times.The reliability of cable joints has always been a core element that affects the safe operation of cable.As a weak part of the insulation of cables,it is prone to deterioration and failure of insulation,further forming cable accidents and causing economic losses.Effective positioning of the joints can greatly reduce the difficulty of cable detection.At present,due to the long transmission distance of the traveling wave,the accuracy of the cable joints’ positioning is poor.This article is based on the time-domain reflection method for positioning the cable joints.It finds the most suitable pluse through simulation analysis,and designs the joints positioning device.The specific research content is as follows:1.Analyze the frequency-dependent Semlyen model and JMarti model in the EMTP-ATP,and compare the wave impedance,phase coefficient,and attenuation coefficient of the two models with theoretical calculations.The theoretical error of the wave impedance and phase coefficient are within the tolerable range.However,the attenuation coefficient in the Semlyen model of the high-frequency signal is smaller than the actual value.The JMarti model is in good agreement with the actual results.The JMarti model is selected for subsequent simulation.The wave impedance gradient curve is calculated and drawn.The concentrated impedance model is used to simulate the joints and the power cable model with the joints is built.Finally,the square wave pulse with a rise time of 1ns,an amplitude of 1k V,and a pulse width of200 ns is selected to achieve the best positioning effect.2.The traveling wave speed according to the cable structural parameters is calculated.A simple square wave pulse source is used to test the cable without shielding layer and the cable with shielding layer.The results show that the traveling wave speed calculated based on the cable structure is accurate.In addition,by calculating the influence of the shielding layer on the attenuation of the traveling wave,since the frequency components of the square pluse are concentrated within1 MHz,the influence of the shielding layer can be ignored.The input pulse parameters can meet the requirements of joints positioning in engineering.The characteristics of the reflected waves at the joints is analyzed by combining simulation and measured waveforms,which provides a criterion for the identification at the joints.3.The overall function of the positioning device is analyzed based on the above test waveforms.The PCB board is designed and debug.It has been verified that the device can accurately locate the intermediate joints and meet various technical requirements.Considering the difficulty of positioning the cable joints based on the time-domain reflection method,each component of the device is optimized and integrated.The designed device has high positioning accuracy for the joints,and it is portable and reliable for the field test.