Evaluation Method For Dendritic Aging State Of XLPE Cable Based On Non-sinusoidal Excitation

During the manufacturing,transportation,and laying process of cross-linked polyethylene(XLPE)cables,small defects in the insulation layer often occur,which in turn develop into dendritic aging defects.Therefore,it is necessary to study the evaluation and diagnosis methods of dendritic aging in cable insulation.The dielectric response method is the most widely used non-destructive diagnostic method in recent years.The traditional frequency domain dielectric spectroscopy(FDS)testing method uses a standard sine signal as the excitation source,and determines the dielectric loss factor by detecting the phase of the sample current under sinusoidal excitation.However,when dendritic aging defects appear in XLPE cable insulation and exhibit nonlinear characteristics,using only a single fixed amplitude sinusoidal excitation cannot obtain its nonlinear insulation diagnosis information.Moreover,when using traditional frequency domain dielectric spectroscopy testing methods to test the insulation of XLPE cables with voltage levels of 10k V or higher,due to the long testing time,space charges can accumulate in the insulation,which affects the subsequent operation of the cable.Moreover,traditional testing methods also have problems such as poor anti-interference power and difficulty in measuring response current.Therefore,this article proposes a method for evaluating the insulation aging status of XLPE cables based on non-sinusoidal measurement.Using triangular waves as excitation,a precise micro current acquisition unit based on the I/V conversion principle is constructed,and an intelligent frequency domain dielectric spectrum testing system is designed to achieve insulation status evaluation of XLPE cables.Compared with traditional testing methods,this testing system has a higher testing voltage level and incorporates digital filters based on dyadic wavelet transform and adaptive filtering algorithm,resulting in a higher signal-to-noise ratio,better sampling accuracy,and stronger anti-interference ability.This article cultured electrical and water tree aged cable samples with different degrees of aging in the laboratory,and conducted dielectric response tests using a testing system.The results showed that the u-i response hysteresis circle under non-sinusoidal excitation undergoes varying degrees of rotation and distortion with varying degrees of dendritic aging.The greater the aging degree,the more severe the degree of rotation and distortion.Proposed the use of rotation angleαAnd the hysteresis impedance Z_r is used to characterize the rotation and distortion of the hysteresis circle,thereby determining whether there are aging defects in the cable insulation.For traditional dielectric response models,the cable insulation medium is equivalent to a series or parallel circuit of linear capacitance and resistance.For nonlinear regions in the medium after cable aging,this type of model is no longer applicable.Therefore,this article proposes a nonlinear dielectric response model for cables-the H-W model.The nonlinear components in this model can effectively characterize the nonlinear regions in cable insulation,achieving optimal equivalence for aged cables.Moreover,the frequency domain dielectric spectrum obtained based on this model under non-sinusoidal excitation has a fitting degree higher than 95%with the actual measurement results.Therefore,the H-W model can effectively achieve time-frequency domain conversion.By analyzing the frequency domain dielectric spectra of each cable sample,it was found that for samples with the same aging degree,the nonlinear characteristics of the sample in the frequency range of 0.01Hz～1Hz become more obvious with the increase of excitation voltage.In order to achieve quantitative characterization of nonlinear features,nonlinear feature parameters are definedλ,And a relationship between nonlinear feature parameters and branch length was established.The results show that the nonlinear characteristic parameters increase exponential growth with the increase of the tree length,and the exponential function can be used to predict the length of the tree without damage,thus effectively evaluating the insulation condition of the cable.In order to analyze in detail the growth characteristics of tree branches at different growth stages and the distribution of electric field in insulation at different stages,this thesis uses the COMSOL simulation analysis method to simulate and analyze the tree branches at different growth stages under the action of an external electric field.It was found that during the hysteresis stage,the tree branches have dense morphology,and the tips of the branches that are close to each other shield the electric field from each other,thereby weakening the intensity of the electric field,reducing the degree of distortion of the electric field,and minimizing the variation of nonlinear characteristic parameters.The nonlinear dielectric response model of cables proposed in this article and the detection method based on non-sinusoidal excitation can improve the efficiency and accuracy of on-site testing of cable insulation aging status,which has certain theoretical and engineering significance.