With the aim of exploring an alternative method of nano-doping in cellulose paper and improving flashover strength of oil-impregnated paper, radio frequency (RF) magnetron sputtering method is introduced into the structural modification of insulating paper for converter transformers. In this experiment, insulating paper was treated with ZnO sputtering for 0, 7.5, 15 and 30 min, respectively. The surface morphology of dry paper was observed with a scanning electron microscope. The bonding mode of sputtered ZnO particles with cellulose matrix was investigated via attenuated total reflection infrared (ATR-IR) spectroscopy and mechanical property of the sputtered samples were studied. Surface and volume conductivities of the oilimpregnated sputtered paper were measured. Moreover, the charge dissipation characteristics of sputtered insulating paper was investigated by means of surface potential decay. In addition, trap distribution and carrier mobility of specimen were further obtained. Finally, the DC flashover strength were tested. The results showed that ZnO magnetron sputtering had a distinct influence on the structure of the insulating paper, resulting in the formation of hydrogen bond and chemical bond and an increase of the surface and volume conductivities. ZnO sputtering was found to decrease the initial potential and accelerate charge decay. Moreover, appropriate sputtering enhanced the surface flashover strength.

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.

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