Carbonized-channel defects of the insulator core rod is one of the main factors inducing brittle fracture of composite insulators. Existing studies mainly focus on the dielectric loss of the insulation under alternating electric fields and the conductive loss of carbonized channels

while the plasma discharge and its thermal effects caused by electric field distortion are often neglected. This omission leads to an incomplete evaluation of insulator performance under carbonization defects. To address this issue

this paper establishes an electro-thermal-plasma multiphysics coupling model to systematically analyze the electric field strength

current density

electron density

ionization loss

and temperature-rise characteristics under different carbonized channel lengths. In addition

110 kV composite insulator samples with varying carbonized channel lengths were fabricated

and ultraviolet and infrared imaging instruments were employed to measure discharge and temperature-rise distributions

respectively. The results show that carbonization significantly increases the relative permittivity of the defective core rod region

causing electric field distortion at both ends of the carbonized channel and inducing intense corona discharge. The heat generated by discharge

combined with Joule heating in the channel

leads to an overall rise in temperature with pronounced increases at both ends. The ultraviolet imaging results reveal that the observed corona discharge spots coincide with the regions of high electron density predicted by simulation. The temperature-rise distribution along the carbonized channel exhibits a segmented pattern

with the highest temperature appearing at both ends. The relative temperature-rise errors at the head and tail of the channel are 3.74% and 3.15%

respectively

confirming the accuracy of the model. This study provides a theoretical foundation for infrared-ultraviolet dual-light joint diagnosis and condition assessment of composite insulators.