Research On Ultrasonic Detecting Technology Of Stress And Defect And Mechanical Failure Mechanism Of Insulators In GIS/GIL

GIS/GIL insulator is an important component of GIS/GIL,which plays the role of mechanical support,electrical insulation,isolating compartment and so on.Due to poor control of manufacturing process,insufficient protection during transportation or non-standard installation,there may be stress concentration or micro defects inside the insulator.And if the insulator is put into operation without being found in time,it may cause abnormal operation state of insulator,and then would induce discharge breakdown,explosion or other phenomena in serious cases,which directly threatens the safe and stable operation of GIS/GIL.At present,stress detection in GIS/GIL insulator lacks relevant measurement means,the reliability of defect detection is not high,and the correlation mechanism between defect generation,expansion and stress evolution is not clear.Aiming at the above problems,in this paper,the ultrasonic detection methods of thermal stress,mechanical stress,density uniformity and interface defects in GIS/GIL insulator are proposed,and the deterioration mechanism of epoxy-insert interface is studied.The main research contents are as follows.1)An ultrasonic nondestructive testing method for density uniformity of GIS/GIL insulator was proposed.An ultrasonic reflection detection system was established,and the measurement uncertainty of the ultrasonic system was analyzed.Seven standard samples with different Al₂O₃ content were tested,and then a mapping relationship between density and ultrasonic velocity was established.The ultrasonic velocities at 192 locations of epoxy composites in a 126 k V three-phase basin-type insulator were measured.The density distribution of the insulator was obtained through the mapping equation of velocity-density,and a contour plot of density was obtained by the cubic spline interpolation.2)An acoustic-distance difference method based on the ultrasonic critical refraction longitudinal wave(ultrasonic L_CR)and a measuring method for propagation depth of ultrasonic L_CR were proposed respectively.The variation of ultrasonic L_CR propagation depth in epoxy composites was measured by ultrasonic probes with different frequencies,and a mapping function between the propagation depth and detection frequency was established.The thermal stress tests and simulation below the surface of a 252 k V basin-type insulator were carried out,and a thermal stress distribution cloud map of the insulator was obtained,which provided a portable nondestructive testing method for the thermal stress detection and evaluation of GIS/GIL insulator.3)A measurement method of acoustic elastic coefficient of parallel stress in epoxy composites was proposed.The acoustic elastic coefficient of parallel stress and acoustic elastic coefficient of vertical stress of the material are tested by standard samples.Based on the 550 k V three-post insulators,an ultrasonic penetration method for stress detection at epoxy composites inside one post under radial loads was proposed,and an ultrasonic reflection method for stress detection at epoxy composites on the periphery of the insert under axial loads were proposed.The simulation and experimental study of the insulator under radial and axial mechanical load were carried out,respectively.It was found that the ultrasonic detection value of mechanical stress was in good agreement with the simulation value,which verified the effectiveness of ultrasonic detection for mechanical stress in GIS/GIL insulator.4)An adhesion simulation model and a decohesion simulation model of the epoxy-insert interface in GIS/GIL insulator were proposed.The radial load simulation and experiments of550 k V GIL three-post insulators with no defect,a debonded defect and an air gap were carried out,respectively.It was found that a crack was likely to occur at the epoxy around the maximum curvature of the epoxy-insert interface,where the stress was the largest and reaches the tensile failure strength first.Then the crack extended bi-directionally along the bottom of the interface and the inner of the peripheral epoxy,and finally developed to the overall fracture of the post.The failure of three-post insulators under radial loads depended on the tensile strength of the epoxy around the maximum curvature of the interface.The axial load simulation and experimentas of 550 k V GIL three-post insulators were carried out.It was found that the stress at the maximum curvature of the epoxy-insert interface of the compression side of the insulator was the largest under the axial load.It first reached the strength of the epoxy-insert interface and a crack was induced,and then the crack developed along the bottom of the epoxy-insert interface,so as to spread the whole interface and eventually lead to failure.The failure of three post insulator under axial load depended on the shear strength of epoxy-insert interface.Regardless of the radial load or axial load,the fracture of three-post insulator occured in the semi-conductive adhesive layer between the epoxy composites and the insert.5)An ultrasonic testing method based on normalization of reflection wave peak value,an ultrasonic testing method based on reflection wave acoustic time and an ultrasonic phased array detection method for interface defects in the GIS/GIL insulator were proposed,respectively.An ultrasonic reflection transmission propagation model and an ultrasonic phased array identification model of ultrasonic at the multi-interface in the three-post insulator were established.An ultrasonic narrow pulse single-chip probe,an ultrasonic phased array probe and its coupling wedge suitable for the insulator were designed.An ultrasonic single-chip probe reflection detection platform and an ultrasonic phased array detection platform were established,respectively.Through diffirent ultrasonic experiments,a three-dimensional image of normalized reflection wave,an image of based on the ultrasonic acoustic time of reflection wave and a scanning image based on the ultrasonic phased array to characterize the epoxy-insert interface defect were obtained,and the unity of all detection results of the above three methods was verified.