Effect of oil viscosity on transformer loading capability at low ambient temperatures

The applicability of the ANSI loading guide for power transformers is limited to ambient temperatures above 0°C because the thermal model does not account for variations in the oil viscosity and winding resistance. A practical calculation method based on the bottom-oil temperature and the surface temperature at the top of the cooling duct has been developed. Results indicate that the proposed thermal model could provide a sound basis for calculating the overload capacity for ambient temperatures below 0°C or for severe overloads of short duration