Study On Detection Of Mandrel And Debonding Defects Of Composite Insulators Based On Ultrasonic Guided Wave

Insulator is one of the key devices for the safe operation of power system,which plays an important role as suspended wire in the overhead transmission line.Fast and accurate detection of debonding within the composite insulators seems vitally important in guaranteeing the safe operation of power grids.Ultrasonic guided wave testing technology,as a novel non-destructive testing method,has been developed greatly in recent years owing to its rapid,accurate,lowcost and wide-range detection advantages.The technique has been widely used in the nondestructive testing of pipes and anchors at present.This paper studies the propagation characteristics of ultrasonic guided waves in composite insulator;investigates the relationship between the debonding size of insulator and the energy attenuation of guided waves;and proposes a new method for segmented detection of debonding defects of composite insulator that is based on the energy attenuation of ultrasonic guided waves.To study the propagation characteristics of ultrasonic guided waves in composite insulators,a simplified model of bilayer composite insulator was constructed.The dispersion curves for longitudinal(L)and torsional(T)modes in the composite insulator were obtained by theoretical analysis,based on which the propagation law was studied.The experimental results showed that the amplitude attenuations of L and T-modes propagating in composite insulator did not differ greatly under excitation of guided waves with different cycles.Nevertheless,from the perspective of easy identification,10-cycle excitation was appropriate.L and T-mode guided wave attenuations were insensitive to the size of skirt,which made detection of skirt defects impossible.The same guided wave modes propagated at the same speed on the silicone rubber and rod,but differed in energy distribution.However,fittings on both ends of the composite insulator exerted great influences on the experimentally received waveforms.The L and Tmodes decayed exponentially with the increasing number of skirt groups.The attenuation curves of 30 kHz L-mode and 50 kHz T-mode were plotted and fitted,while the maximum propagation velocity curves for guided modes at certain frequencies were obtained.Traditional composite insulators defect detection methods were mainly concentrated in the external structure defects of composite insulators rod,and the defects in rod could not be found effectively,so a new method with ability to estimate defect sizes was presented in this paper: L modal guided wave detection based on matching pursuit.Also,the range of frequency for guided wave detection was determined.Finite element simulation and experiment were carried out with excitation guided wave with 10 cycles at 40 kHz and axial defects in 8mm and 2mm according to theory analysis,which showed that L modal guided wave could detect defect of composite insulators rod accurately.In the meanwhile,matching pursuit algorithm was used on the basis of waveform dictionary constructed by Gabor atoms,which showed that the relative errors of axial defect estimated by matching pursuit algorithm is 5.4% and 16.35% respectively.Based on theory analysis,finite element simulation and experiment,it is advisable and effective to detect defect of composite insulators rod and estimate the specific defect size with L modal guided wave based on matching pursuit.In order to inspect debonding defects of composite insulator accurately and nondestructively,we proposes a new method for segmented detection that is based on the attenuation coefficient of ultrasonic guided waves.According to the theory of ultrasonic guided wave propagation,a simplified model of bilayer composite insulator was constructed,and the dispersion curves of longitudinal(L)and torsional(T)guided wave modes were plotted.On the basis of the theoretical analysis,experiments of detecting debonding of composite insulator were carried out.The experimental results showed that: as the guided waves attenuated excessively during propagation in the silicone rubber,only segmented detection was possible for long composite insulator of high voltage level;the attenuation coefficient of guided waves would decrease,when there was debonding within the detection range;the size of debonding would influence the attenuation coefficient,so we could determine whether there was debonding defects as well as the size of defects by inspecting the attenuation coefficient of guided waves;the attenuation coefficient of longitudinal mode and torsional mode could be used to determine whether there was debonding defects and estimate the size of circumferential debonding.Based on study above,it is feasible to defect detection of composite insulator with ultrasonic guided wave technology.