Investigation on the nonlinear thermal-electrical properties coupling performance of converter transformer

Thermal state and insulation ability of converter transformer are significant to evaluate condition, while the impacts of nonlinear thermal-electric coupling impact are often ignored in the design analysis, and might result in misestimating its actual performance. In this paper, a bilateral thermal-electric coupling method is proposed to investigate the thermal and insulation performance of converter transformer on basis of an actual size. In addition, the influence of non-uniform temperature on the overall winding losses and the nonlinear thermal electric coupling of insulation system are considered. Firstly, the thermal-electrical parameters coupling characteristics of insulation system in converter transformer are investigated based on the built experimental platform. Results indicate that temperature distribution would change the electrical performance all the time, while electrical-dependent characteristic of oil increases with electric field in U type curve. Then, the impact of non-uniform temperature is discussed based on the built bilateral thermal-electric coupling model, and it is proved that temperature would decrease obviously in considering the losses calculated by bilateral coupling. Finally, the nonlinear thermal-electric coupling performance of converter transformer is studied. It indicates that comprehensive factors of temperature dependence and electric filed dependence have great influences on the insulation properties. Significantly, the electric field of pressboard would decrease by 12.8% under hybrid voltage condition.