An Ultrasonic Nondestructive Testing Method for Density Uniformity of Basin-Type Insulators in GIS

Abstract

A large density difference of a basin-type insulator in gas-insulated metal-enclosed switchgear (GIS) weakens its mechanical strength, which will threaten the safe and stable operation of GIS. In this article, an ultrasonic nondestructive testing method for density uniformity of GIS basin-type insulators was proposed. First, seven standard samples with different Al2O3 content were tested by an ultrasonic testing system, and then a mapping relationship between density and ultrasonic velocity was established. Second, the uncertainty of the testing system was obtained, and the ultrasonic velocities of 192 positions in a 126 kV three-phase basin-type insulator were measured. Finally, the density distribution of the insulator was obtained based on the mapping equation of velocity-density, and a contour plot of density was established by the cubic spline interpolation. The results show that there was a positive linear correlation between the density of standard samples and the ultrasonic velocity, and the determination coefficient was 0.992. Notably, the expanded uncertainty of ultrasonic propagation time for detecting 45 mm-thick EP/Al2O3 composites was only $0.0112~\mu \text{s}$ . The ultrasonic propagation time of detection positions in the insulator was from 31.0634 to $31.4708~\mu \text{s}$ and the difference was greater than $0.0112~\mu \text{s}$ , which illustrated that the testing system meets the requirements for detecting the density of the insulator. According to the contour plot of density, the density distribution of the insulator was shown clearly and efficiently. The density was higher at the bottom edge of the insulator and the upper part of the insert, whereas the density was smaller at the lateral upper part of the insulator and the lower part of the insert. Specifically, the maximum density of the insulator was 2.36 g/cm3 and the minimum density was 2.25 g/cm3. The feasibility of nondestructive testing density by the ultrasonic method is verified, which provides a new method for density evaluation of GIS insulators.