Fiber-optic composite insulators have been widely adopted in converter stations

but multiple operational anomalies involving these insulators have occurred in recent years. Such failures primarily manifest as fiber ablation and breakage within the metal flange

yet the underlying causes remain unclear. This paper analyzes the failure mechanism of fiber-optic composite insulators at a ±500 kV converter station from the perspective of internal electric fields. Results indicate that fiber ablation and breakage within the metal flange are caused by the combined effects of low-resistivity inner coatings and gas bubbles/voids within the filling compound. The low-resistivity inner coating of the optical fiber provides a low-resistance path for leakage current flowing from the low-voltage end to the high-voltage end. Consequently

a significant current flows radially from the metal flange to the inner coating

resulting in high electric field strengths on the surface of the optical fiber within the flange. X-ray scanning and dissection experiments reveal that there exist cavities and numerous bubbles within the low-voltage end's filling compound. This increases the compound's equivalent resistivity

further amplifying current flow toward the low-resistivity inner coating and exacerbating surface field distortion on the fiber. The electric field within bubbles adhering to the fiber surface is further amplified by enhanced interfacial charges induced at the tight buffer-bubble interface and the bubble-filler interface. This results in the most severe field distortion within these bubbles

triggering gas discharge within them. This discharge erodes the fiber

ultimately causing fiber breakage inside the metal flange.