Research On Ultra-low Frequency Dielectric Loss Measurement Technology For Power Cables

With the acceleration of urbanization and the rapid development of the power industry,cross-linked polyethylene(XLPE)cables are widely used in urban distribution networks for their excellent electrical and mechanical properties such as high breakdown strength,high insulation resistance,wear resistance,and impact resistance.The design life of cross-linked polyethylene cables is generally 30 years,but during operation,due to various factors such as mechanical stress,thermal stress,electrical stress,environmental stress,etc.,it is easy to cause a decrease in the insulation level of the cable,leading to insulation breakdown and failure of the operating cable,thereby affecting power supply reliability.Therefore,conducting research on the measurement of cable insulation status is of great significance for timely detection of potential faults and ensuring the safe and stable operation of the power system.In this paper,an insulation condition diagnosis method based on ultra-low frequency dielectric loss measurement is proposed for the purpose of insulation condition diagnosis of aging cross-linked polyethylene cables.The main tasks include:(1)This paper analyzes the mechanism of dielectric loss of cross-linked polyethylene cables,analyzes the characteristics of cross-linked polyethylene cables under ultra-low frequency voltage using mathematical modeling and finite element simulation methods,and demonstrates the theoretical basis for using 0.1Hz sine wave voltage to measure the dielectric loss of cables to reflect the aging degree of cross-linked polyethylene cables.(2)Aiming at the problems of harmonic interference in ultra-low frequency dielectric loss measurement and spectral leakage and barrier effect in asynchronous sampling,a method for ultra-low frequency dielectric loss measurement based on Prony algorithm-quasi synchronous sequence is proposed.Using the Prony algorithm and data identification method,the fundamental frequency of the sampled voltage signal is calculated.Through the Newton interpolation algorithm,the quasi-synchronous interpolation reconstruction of the voltage and current signals is realized,and the quasi-synchronous sequence of the sampled signal is obtained.By using Fast Fourier Transform(FFT)for spectrum analysis and combining it with the equivalent circuit model of cable insulation,accurate calculation of ultra-low frequency dielectric loss can be achieved.The effectiveness of this method for ultra-low frequency dielectric loss measurement has been verified through simulation testing.(3)Aiming at the problem that ultra-low frequency dielectric loss measurement signals are susceptible to interference from noise factors such as surrounding power equipment and environment,a noise reduction method based on EMD-SVD is proposed.The measured signal is subjected to Empirical Mode Decomposition(EMD)to obtain a finite number of Intrinsic Mode Function(IMF)components.The energy gradient method is used to identify the IMF components,and then Singular Value Decomposition(SVD)based on singular value curvature is used to achieve noise reduction of the reconstructed IMF components.Simulation tests show that the proposed method has significant noise reduction performance compared to existing methods and can better achieve noise reduction for ultra-low frequency dielectric loss measurement signals.