An Ultrasonic Detection Method for Interface Defects of Three Post Insulators in Gas-insulated Transmission Lines

Interface defects induced during the manufacturing process, transportation and assembly in three post insulators would affect the safe and stable operation of gas insulated transmission lines (GIL). In this paper, an ultrasonic detection method for interface defects in three post insulators was presented. A narrow-pulse longitudinal wave straight beam contact transducer with a diameter of 6 mm and a frequency of 5 MHz was used in an ultrasonic system for detecting interface defects of a 550 kV three post insulator. The propagation models of ultrasonic waves in defective and pristine post bases of the insulator were proposed. Ultrasonic velocities and acoustic impedances of the epoxy composite insulation and grounding insert of the insulator were tested. A $6mm^{*}$4 mm air gap at the interface between the epoxy composite insulation and grounding insert of post base II was identified by noticing the normalized amplitudes and widths of reflected waves as defect identification features. Ultrasonic scanning tests with variable step sizes were carried out and a three-dimensional image of the defect was obtained by the normalized amplitude of the reflected wave at each position. Upon comparing with an X-ray image of the insulator, it was observed that interface defects in three post insulators can be detected by the ultrasonic scanning method accurately.