Internal discharges in composite insulators often cause abnormal heating, interface failure and erosion of the core rod. However, the mechanism and the exact process of the degradation of HTV shed and FRP core rod caused by internal discharges are still unclear. In this work, abnormal heating insulator samples are taken from an AC 500 kV extra-high voltage (EHV) transmission line. Macro- and micro-scopic morphological observation, hardness tests, industrial CT tests and chemical property tests are applied on the samples. The cause of heating and the deterioration mechanism of each part of insulators is further explored. Cavities and air holes are found in the HTV skirts in this batch of insulators, which are generated during manufacturing. These cavities lead to the distortion of electrical field and become the origin of the discharges. Decomposition of polydimethylsiloxane leads to the hardening and chalking of the HTV shed. It is also found that the adhesive in the rod-housing interface fails during operation. Epoxy resin in core rod decomposes in a multi-factor aging environment, leading to the decay-like aging of the core rod surface. As the aging progress develops inward, the insulator will eventually lose its mechanical properties. It is inferred that the internal discharge is the main reason of heating and deterioration of composite insulators. Moreover, long-term discharge will lead to the decay-like aging of core rods.
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