Research On The Effectiveness Of Cable Local Defect Detection Based On Frequency Domain Impedance Spectrum

Cables play an irreplaceable role in the transmission of electrical energy and communication signals.In actual operation,they will accelerate the degradation of insulation performance due to defects such as partial aging or damage,and even cause insulation failures.Local cable defects mainly include aging caused by factors such as high temperature,strong electric field,moisture intrusion,corrosion,and radiation,as well as deformation and damage caused by external mechanical stretching,scratching,extrusion,and penetration of foreign objects.Cable fault detection and location methods mainly include impedance bridge method,traveling wave distance measurement method,and partial discharge online detection method,but the impedance method is complicated on-site operation,traveling wave method requires a high degree of impedance mismatch in the defect section,and the partial discharge signal is general Relatively weak,the propagation path is complicated,and some defects will not cause partial discharge.Therefore,the current cable defect detection methods have their own limitations.Some scholars at home and abroad have carried out research on local defect diagnosis methods based on cable frequency domain impedance spectroscopy,which opened up a new direction for cable local defect detection.Based on the current research results at home and abroad,this paper has carried out related researches on the detection and location of local aging,damage and dampness of cables based on frequency domain impedance spectroscopy,and discussed in depth the effectiveness of this method for the detection of different types and degrees of local damage defects.The main contents are as follows:According to the transmission line theory,the frequency domain distribution parameter model of the cable is established,and the analytical calculation model of the frequency domain impedance spectrum and the reflection coefficient spectrum of the cable head end is obtained.The simulation analyzes the characteristics of the frequency domain impedance spectrum of the cable under the frequency band of 100kHz～120MHz.The impedance spectrum periodically attenuates the oscillation with the increase of frequency,and the maximum value appears at the resonance point.The reflection coefficient spectrum with defects is normal The reflection coefficient spectrum curve shows periodic fluctuations in the center.These points can sensitively reflect the cable parameter information,indicating that the method is feasible for cable defect detection.Based on the complex permittivity of the cable insulation and the damage coefficient of the shielding layer,the damage mutual inductance and the damage capacitance coefficient,a simulation model of the reflection coefficient spectrum of the cable head end with local defects is constructed.The equivalent time component is extracted from the real part of the reflection coefficient spectrum at the head end of the cable through the inverse fast Fourier transform(),thereby realizing the detection and location of the cable local defect.The defect location of a certain length of cable depends on the frequency band range and the number of effective sampling points.With the increase of the degree of defects,the positioning results have been significantly improved.An experimental platform for local defect detection and location based on the frequency domain reflection coefficient spectrum at the head end of the cable was built.Four polyethylene(PE)coaxial radio frequency cables with a length of 9.9m and one XLPE power cable with a length of 60m were carried out.Experiments on partial damage and damp defects.The cable wave speed is measured by using a steep pulse generator and an oscilloscope.The effectiveness of the method in this paper for detecting cable defects is explored by changing the frequency band range,the number of sampling points,the signal strength and setting multiple defects of varying degrees.Experimental results show that this method can achieve accurate positioning of local cable damage and damp defects.The positioning effect is determined by the frequency band range and the number of effective points.Increasing the measurement frequency band can increase the number of effective points.When the minimum frequency band and the number of effective points are met,increasing the number of sampling points can effectively improve the positioning effect.The more serious the local defect,the closer the distance from the head end,the more obvious the positioning result.