Ageing of paper insulation within oil immersed power transformers is the determining factor for the transformer expected life. A thermal diagram as shown in the IEC 60076-2:2011 standard specified a hot spot factor method to estimate the hot spot temperature based on heat run test data. The hot spot factor is split into a Q factor, which depends on loss distribution within winding, and an S factor, which depends on oil flow distribution within winding cooling ducts. In this paper, an experiment is conducted on a disc type winding model to study the hot spot factor under fixed winding geometries subjected to wide range of inlet flow rates and uniform or non-uniform power loss injections. The hot spot factor is calculated from measured temperatures within the winding model using thermocouples. The hot spot factor was decoupled by applying first uniform losses to calculate the S factor and then nonuniform losses to calculate the Q factor. It was observed that the hot spot factor is lower for more uniform temperature profiles. It was observed that the S and Q factors are interdependent and cannot be decoupled.
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