Predicting liquid filled transformer loading capability

The IEEE transformer loading guide (ANSI/IEEE C57-92-1981) equations use the top oil temperature rise over ambient to determine the winding hottest spot temperature during an overload. There is a time lag between the top oil temperature rise and the oil temperature rise in the winding cooling ducts. This phenomenon results in winding hottest spot temperatures greater than predicted by the current IEEE loading guide equations. Loading guide equations assume a constant ambient air temperature during a load cycle. Improved loading equations based on analysis and testing are presented. 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 is presented to perform the calculations in a simple step-by-step procedure