The winding temperature-rise characteristics of transformers using C4F7N/CO2 mixed gas

as one of the most promising alternatives to SF6

have not yet been reported. We investigated a 110 kV transformer by employing numerical simulation to analyze the effects of operating pressure and mixing ratio on the temperature rise. A comparison with the SF6 insulation system was conducted to discuss the difference in heat transfer coefficient on the winding surface between the two gases. Finally

temperature-rise tests were carried out to validate the accuracy of the simulation model. The results indicate that both the average winding temperature rise and the hot-spot temperature-rise decrease with increasing gas operating pressure. The temperature-rise of windings immersed in C4F7N/CO2 mixed gas is significantly higher than that in SF6

with the average temperature rise approximately 6 K. The average heat transfer coefficient on its winding surface gradually approaches that of SF6 as the pressure increases

being approximately 70.56% to 86.69% of the latter. The mixing ratio of C4F7N (8%~20%) in the mixed gas has no effect on the winding temperature-rise. These findings provide a theoretical basis and technical reference for the thermal design of C4F7N/CO2 mixed gas transformers.