Research On Insulation State Diagnosis Method Of XLPE Cable Based On Frequency Domain Response Analysis

With the widespread application of cross-linked polyethylene(XLPE)cables,it is crucial to ensure the safe and stable operation of their lines.Under the long-term influence of factors such as electricity,heat and stress,the insulation performance of XLPE cables will significantly decrease or even malfunction,which will seriously threaten the safety of the power system.Therefore,the research on insulation status diagnosis of XLPE cable lines has urgent practical significance.Compared with existing methods for diagnosing cable insulation status,frequency domain response analysis(FRA)has the advantages of reliable data,strong anti-interference ability,no damage to cables,and the ability to identify local defects.In recent years,it has been widely studied by scholars at home and abroad.The FRA method includes methods such as frequency domain dielectric spectroscopy and frequency domain impedance spectroscopy.Therefore,this thesis uses the dielectric loss factor in the low-frequency region to diagnose the overall insulation state of the cable,while using the impedance spectrum in the high-frequency region to diagnose local defects in the cable.In order to study the frequency domain dielectric properties of XLPE cable insulation after overall thermal aging,this thesis conducted an overall accelerated thermal aging test on XLPE cable insulation,and used the frequency domain dielectric spectrum testing system established in this thesis to conduct frequency domain dielectric loss factor testing.The variation law of the dielectric loss factor in the low frequency range of the cable under different aging times was analyzed.In order to explore the relationship between the dielectric loss factor and the degree of aging at low frequencies,referring to the elongation at break of XLPE material proposed in the national standard as a criterion for evaluating the degree of insulation aging of cables,the correlation between the dielectric loss factor and elongation at break in the low frequency range was analyzed.Based on the negative correlation between the two,a functional expression was established between the dielectric loss factor and aging time in the low frequency range,Realized the evaluation of the overall thermal aging degree of the cable.Due to local overheating and mechanical stress,cables often produce local defects.The impedance spectrum information at high frequencies can reflect the changes in local defect parameters of cables.Therefore,this article uses the frequency domain impedance spectrum method to diagnose the insulation status.This article calculates the distribution parameters of cables through open circuit and short circuit impedance spectra,and analyzes the influence of different local defects on the distribution parameters.Through theoretical analysis and experiments,it was found that local thermal aging defects can lead to an increase in equivalent capacitance and conductivity,while local shielding layer damage can lead to an increase in equivalent inductance and a decrease in capacitance at the defect location.Based on the above analysis,this article establishes a theoretical model of transmission lines for cables with local defects,and simulates and analyzes the impedance spectra of cables with different local defects by changing the distribution parameter values of fault segments.A high-frequency impedance spectrum testing platform for cable local defects was established in the laboratory,and impedance spectrum testing was conducted on faults with local thermal aging,local shielding layer damage,high resistance,and low resistance.The consistency with simulation results and rules was verified.Based on the test results of multiple defect samples in the impedance spectrum,the amplitude,period,average wave velocity and other characteristic parameters of the impedance spectrum are extracted,and a multidimensional feature fusion diagnostic criterion is constructed.This achieves the diagnosis of local defects in cables and the identification of fault types,which has certain reference significance for the insulation defect diagnosis of XLPE cables.