Investigation on distribution of electro-thermal coupled field and improved design of ±1100 kV converter valve-side bushing

Compared to bushings with lower voltage levels, the +/- 1100 kV converter valve-side bushing has thicker radial insulation and longer axial insulation resulting in a more uneven temperature distribution and more severe electric field distortion, especially under high-ambient-temperature and high-current conditions. Improving the distribution of the electro-thermal coupled field of the bushing is critical for the safe operation of converter transformers. This study discusses the distribution of electro-thermal coupled field of a typical bushing and proposes a new method to evaluate the conductivity of multiple capacitor layers under inhomogeneous axial temperatures. The effects of four bushing designs are investigated as per the adjustment of the capacitor screen structure, the replacement of the current-carrying structure, and the modification of resin-impregnated paper material. The results show that merely adjusting the capacitor screens is not feasible due to size limitations. By decreasing heat accumulation, adopting a single-tube structure and increasing the thermal conductivity of resin-impregnated paper material can reduce electric field distortion to a certain extent. Unlike the other three methods, hierarchically controlling the material resistance effectively balances the internal and external electric fields. Simulation results demonstrate the feasibility of these methods to improve the distribution of the electro-thermal coupled field in the bushing.