Objective This study aims to reveal the influence law of temperature gradient on the insulation electric field distribution characteristics of the ±800 kV converter transformer valve-side bushing under high-current operation, thereby improving the scientificity and reliability of bushing insulation design in ultra-high-voltage direct current (UHVDC) transmission projects.

Method Most existing studies on converter transformer bushings focus on their thermal or electric field characteristics, typically using 2D or 3D models to analyze steady-state temperature rise and electric field distribution under constant temperature. However, the nonlinear characteristics of dielectric parameters of insulating materials with temperature and electrothermal coupling effects are often overlooked. To address this gap, this study proposed a 2D coupled simulation method considering the temperature dependence of materials, and explored the evolution mechanism of insulation performance from the perspective of electrothermal interaction. A 2D finite element model was established to simulate the structure of a ±800 kV valve-side bushing, with measured temperature-dependent conductivity data of resin-impregnated paper (RIP) from existing literature incorporated. Based on COMSOL Multiphysics, the temperature field under current-carrying conditions was calculated, and electrothermal coupled electrostatic field analysis was further conducted to quantitatively reveal the influence law of temperature gradients on electric field distribution.

Result Simulation results show that under a 6.736 kA DC current, the maximum temperature of the bushing reaches 130.7 ℃, mainly concentrated in the middle-lower part of the conductor rod and the flange area, with a radial temperature gradient of up to 37.7 ℃ at the capacitor core near the flange. The temperature gradient causes significant changes in the spatial distribution of material conductivity, resulting in the radial electric field strength in the capacitor core shifting from the original 6.2~7.6 kV/mm to 2.8~12.4 kV/mm. The maximum electric field strength increases by 63.2%, and the electric field distribution pattern reverses from "weak in the middle and strong at both ends" to "weak inside and strong outside".

Conclusion The temperature gradient significantly affects the spatial distribution of conductivity in the capacitor core material, resulting in electric field distortion. The two-dimensional electrothermal coupled simulation quantitatively reveals the interaction mechanism between the temperature field and the electric field. The research findings provide a theoretical basis and technical support for the insulation design optimization and performance verification of ±800 kV converter transformer valve-side bushings.