Analysis of Insulation Margin of UHV AC Transformer at Low Temperature

As the developing of UHV project, the environmental factors put forward much higher requirements for the design, operation and maintenance of UHV AC transformer. The lowest temperature in winter of northern area in our country can reach -40° C, in order to study its influence on insulation margin of transformer, the changing rules of dielectric parameters of each material in transformer at low temperature have been tested in this paper. And its effect on the distribution of electric field in main insulation has been analyzed by simulation method. Eventually, the change of insulation margin of main insulation in transformer at different temperature has also been calculated. The research results show that the relatively dielectric constant of oil increases linearly with the decrease of temperature, and that of oil-impregnated pressboard decreases exponentially on the contrary. Nevertheless, their breakdown strength shows the same trend of falling firstly and then rising as the temperature increasing, and reaches the valley value at -10°C, which is more serious for oil. Moreover, the simulation result shows that the electric field distribution transfers to oil in oil-paper insulation system, and insulation margin decreases firstly and then increases as the temperature decreasing, which also reaches the minimum value at -10°C. Considering the dielectric characteristic, it can be concluded the electric field distribution, especially the insulation margin of oil in compound insulation should be confirmed at -10°C during the design process of transformer.