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Project: 261 #T508

When a transformer is operated under overload conditions, the limiting factor is usually the temperature of the hottest spot in the winding. Loading guides such as ANSI C57.92 provide the operator with a set of tables indicating the acceptable load factor as a function of a predetermined loss of life. These tables are voluntarily limited to ambient air temperatures of 0C because they do not take account of variations in oil viscosity and winding resistance. The thermal model proposed in this study allows the transient or steady state hot-spot temperature to be calculated for any combination of load factor and ambient temperature as defined in ANSI C57.92 and the calculated hot-spot temperatures compared to those yielded by the ANSI model.