Temperature‐dependent electric field distribution in ±800 kV valve‐side bushing insulation for a converter transformer

The electric field distribution is determined for the design and long‐term performance of bushing yet gets complicated when coupled with temperature due to the highly temperature‐dependent conductivity of insulation. An electrothermal coupling model is established based on a ±800 kV converter transformer valve‐side bushing. The measured temperature‐dependent conductivity of insulation is adopted and fitted. The electric field distribution inside a condenser core under the influence of temperature was investigated, and the related mechanism was analysed, considering the different voltage forms, loading currents, and changeable ambient conditions. The temperature gradient within the condenser core is verified, and the electric field migration is observed. It is indicated that the electric field strength shows a strong dependence on temperature and increases along the radial direction. The maximum field strength appears at the outmost shield layer under the operating voltage and the load current 4500 A, which is higher than that under the DC long‐term withstand voltage of 1455 kV without thermal stress. The increased load current, decreased oil temperature, and enhanced valve hall temperature will lead to more serious electric field distortion, and unacceptable dielectric stress may occur. More attention should be paid to the design and operation of bushing for DC applications.