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A new type of composite insulator mechanical failure named decay-like fracture happens rather often in UHV AC transmission lines. However, the cause from the point view of electric field is not clear. To investigate the effect of electric field on the occurrence and development mechanism of decay-like fracture, the 3D tower-conductor-insulator models for ZB2-type and CZ34-type transmission tangent towers are established and analysed based on the finite element simulation. The conclusion can be drawn that the tower type has a significant effect on the electric field distribution along the axial direction of the insulator string, and the degradation is more likely to occur in high-voltage side of the B-phase string, which is consistent with the fracture failure investigation. Simulation results also show that the withstand electric field strength is 297kV/m. The distribution of surface potential and electric field under the actual operating voltage provide a new idea for the condition monitoring of the decay-like fracture of UHV AC composite insulators in transmission lines, as well as a reference for further research in the field of decay-like fracture failure.

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