Ultrasonic Methods To Detect Defects In Epoxy Insulation For GIS

Because of its advantages such as high reliability,small ground space requirements,long maintenance cycle,and low environmental impact,the gas-insulated metal-enclosed switchgear(GIS)is widely used in large-scale hydropower projects and urban high-voltage power grids with a complex terrain and a narrow area.As one of the weak and important links,the continuous development of internal voids,cracks,surface micro-cracks could trigger partial discharges or surface flashover under the action of electrical stress and mechanical stress,threatening safe GIS operation.The existing on-line monitoring technology of basin insulator defects is not sensitive to the discharge of voids,and the off-line detection technology is still immature due to environmental interference and limited of equipment volume.In this paper,the ultrasonic pulse reflected method is proposed to detect voids and cracks in the epoxy composite of basin insulator,and the critical refracted longitudinal waves(LCR)method is used to detect the micro-cracks on the surface of the material,so as to study the effectiveness of the ultrasonic wave detecting basin insulator defects.It is of great significance to reduce the operation of defective insulator in the process of production and installation,improve the early warning of insulator defects and reduce the occurrence of power grid accidents.In order to study the effectiveness of ultrasonic pulse-echo method to detect the voids and cracks in epoxy composite,some epoxy composite specimens were made from similar materials and manufacturing processes to make 252 k V GIS basin insulators.Some voids with different diameters or cracks with different diameters and orientations were artificially made in the specimens with different thicknesses.An ultrasonic pulse-echo system was set up,and ultrasonic tests were carried out on the specimens with voids or cracks.A method to calculate the depth of a defect was proposed by the propagation time of defect reflected waves.The results showed that a depth of 50 mm,a diameter of 2 mm void,and a diameter of 1 mm crack in epoxy composite insulation were detected by the ultrasonic system using a 1 MHz transducer.In this paper,in order to study the effectiveness of ultrasonic critical refracted longitudinal wave method to detect surface micro-cracks on the epoxy composite materials,micro-cracks on the insulator surface were made artificially and an ultrasonic testing system with critically refracted longitudinal waves was built for the ultrasonic test on micro-cracks with different orientations and depths.A method was proposed to evaluate a crack by the defect reflected wave using a single transducer and decreasing the amplitude of receivedwaves using the double transducers.The results show that surface micro-cracks with a depth of 0.5?2mm and different orientations could be detected using 2.5MHz ultrasonic variable angle transducers under the modes of the single transducer and double transducers.The relationship between the heights of cracks and the amplitudes of waveform is linear.Compared with the mode of the single transducer,crack orientations have less influence on the detection using the double transducers.