The deterioration of a composite insulator’s mandrel will cause a serious power-grid accident, so it is necessary to find the defect as soon as possible. Infrared imaging is an effective means to detect mandrel-deterioration defects, but sheath-aging defects will cause trouble to screen mandrel deterioration. Therefore, it is urgent to distinguish the two defects. In this paper, two composite insulators that are out of service with different heating characteristics are tested and analyzed, and the temperature-rise characteristics are analyzed by building a simulation model. The results show that the temperature rise of composite insulators with oxidative decomposition of epoxy resin is large, and the range usually extends from the hardware of the high-voltage side to several umbrella skirts. The temperature rise caused by the composite insulator with the fiber completely wrapped by epoxy resin is small, which mainly occurs from the hardware to the first umbrella skirt. The simulation model constructed in this paper is consistent with the experimental results, which verifies the accuracy of the model. The model can distinguish the mandrel-deterioration defect and sheath-aging defect, and has guiding significance and important reference value for the detection rate of the mandrel-deterioration composite insulator and ensuring the safe and stable operation of the power grid.

Abnormal heating of composite insulators of high-voltage transmission lines concentrate at its end, especially in a high-humidity environment. In order to study the influence of end-sheath aging and moisture absorption on abnormal heating of composite insulators, in this paper, we first discuss the appearance test, temperature rise test, and dielectric characteristic test conducted on 110 kV decommissioned composite insulators. Test results indicated the temperature rise in composite insulators increased with ambient humidity, but temperature rise was not severely affected by surface contamination of its shed and sheath; in dry environments, the dielectric constant and dielectric loss factor of high-voltage end sheaths are higher than of those of medium- and low-voltage end sheaths, and the loss effect becomes more severe after moisture absorption in a high-humidity environment. After the tests, the authors established a COMSOL simulation model of composite insulators, to analyze changes in the electric field and thermal field of the end sheath of composite insulators due to the coupling of electric and thermal fields. It was concluded that the dielectric constant of a high-voltage end sheath of the composite insulator increased after moisture absorption, distorting the partial electric field on the surface; meanwhile, the dielectric loss factor increased significantly after water molecules intruded into the aging layer of the sheath as polar molecules. Therefore, the dielectric loss (leakage conductance loss and lossy polarization loss) caused by aging and moisture absorption of the sheath surface under partially high field strength in the high-humidity environment was the leading cause of abnormal heating at the high-voltage end of composite insulators. The conclusion of this paper serves as an important reference for revealing the causes of abnormal heating of composite insulators in high-humidity environments and the influence mechanism of external factors on abnormal heating.