Abnormal Heating and String Breakage in <scp>500 kV</scp> Composite Insulators: A Multi‐Scale Mechanistic Study of Material Aging

ABSTRACT Abnormal heating and string breakage of composite insulators threaten 500 kV transmission reliability. Four in‐service 500 kV insulators were examined for multi‐scale approach combining macroscopic inspection and dissection, water‐diffusion tests, infrared and ultraviolet diagnostics, and microstructural and physicochemical analyses of silicone rubber sheds and epoxy core rods. Insulators show severe chalking, cracking and pore formation of sheds at the high‐voltage end, accompanied by leakage currents of about 400–1100 μA, exceeding those in the middle and low‐voltage ends and indicating progressive interfacial debonding and decay‐like damage of the core rod. Abnormal heating is primarily concentrated at the junction between the high‐voltage end fitting and sheds, and installing grading rings narrows the temperature difference from roughly 4°C–7°C to 2°C–3°C and reduces ultraviolet photon counts by about 60%–80%, demonstrating that improved electric‐field grading effectively suppresses coupled electrical, thermal and discharge stresses. Microstructural and chemical evidence links polydimethylsiloxane chain scission, aluminium hydroxide to aluminium oxide transformation in silicone rubber, and nitric‐acid‐induced degradation and hydrolysis of the epoxy matrix to increased dielectric loss. These results establish a field‐distortion‐driven evolution pathway from shed aging, interfacial moisture ingress, core‐rod degradation, abnormal heating and eventual string breakage, providing a mechanistic basis for optimizing grading‐ring design and condition‐based maintenance of composite insulators.