Predicting liquid filled transformer loading capability

The IEEE Transformer Loading Guide equations use the top oil temperature rise over ambient to determine the winding hottest spot temperature during an overload. Recent investigations by the author and others have shown that during overloads there is a time lag between the top oil temperature rise and the oil temperature rise in the winding cooling ducts. This phenomena results in winding hottest spot temperatures greater than predicted by the current (1981) IEEE Loading Guide equations. Accurate predictions of the winding hottest spot temperature requires the use of the temperature of the oil entering and exiting the winding cooling ducts. Low flammability liquids such as silicone or high temperature hydrocarbons are not covered in the present Loading Guide. Resistance change with temperature and liquid viscosity are not considered in current Loading Guide equations. Loading Guide equations also assume a constant ambient air temperature during a load cycle. Improved loading equations based on analysis and testing are presented in this paper. The improved equations consider type of liquid, cooling mode, winding duct oil temperature rise, resistance and viscosity changes, and ambient temperature and load changes during a load cycle. A PC BASIC computer program to perform the calculations was developed and will be included in the next edition of the IEEE Loading Guide for Mineral Oil Immersed Transformers