Analysis of a fractured 500 kV composite insulator - identification of aging mechanisms and their causes

In this paper, a fractured 500 kV ac composite insulator, whose fracture characteristics differ significantly from those of brittle fracture, is analyzed in order to identify the aging mechanisms involved and their causes. Several sheath punctures occurred along the insulator which apparently originated from inside the insulator. Removal of the sheath revealed weak adhesion as well as tracking along the sheath-core interface. Weak microscopic as well as macroscopic interfaces were indicated by the insulator´s weak performance in the water diffusion and dye penetration test according to IEC 62217. The core material was analyzed by means of FTIR identifying glass corrosion, ion exchange and hydrolysis as major degradation mechanisms. Based on the results of analysis, a water induced aging process is proposed and recommendations for online monitoring of composite insulators with respect to interfacial aging are given.