Aging characteristic at different depths in a single composite insulators shed

Estimation of the degree of aging and the remaining lifetime of composite insulators is an important issue. In this paper, one shed of a 500 kV AC composite insulator that has operated for three years on a transmission line was studied. The insulator shed was cut longitudinally into four layers from the upper surface to the bottom surface and tested by the thermally stimulated current (TSC) method, and each layer was also tested by Fourier transform infrared (FTIR) spectroscopy. The results show that the first layer has fewer trap charges than the other layers in the TSC test, but it also has the highest trap level, and the trap depth of the first layer is deeper than that of the other layers. The trap levels and the quantities of shallow traps decrease successively from the second to the fourth layer. In the FTIR spectra, the absorption peaks of the first layer are lower than those of the other layers, and the other three layers have similar absorption peaks. The aging state of the first layer is more serious than that of the other layers. The study indicates that long-term contamination and ultraviolet (UV) radiation are the main reason for the aging differences among the different insulator shed layers.