Hot spot temperature models based on top-oil temperature for oil immersed transformers

Thermal behaviors of transformers have a close relationship with the ageing processes of transformer insulation. In oil immersed transformers, feature temperatures such as top-oil temperature and hot spot temperature are recommended to be monitored online. To minimize the risk of failure and to extend the serving life time of transformers, both temperature rises should be controlled within certain range. However, it is difficult to monitor the hot spot temperature of the transformer. Commonly used method is to calculate the rise of the hot spot temperature based on top-oil temperature rise, as recommended in GB/T15164-94 (Chinese standard) and IEEE C57.91. Later on, electrical-thermal analogy models were developed and proved to be accurate in practice. These electrical-thermal models present heat transfer in the transformers as thermal circuits, where each component represents different thermal distribution in the transformers. General model for the thermal circuits were improved by combining fluid transfer theory into the exponential factors. This paper provides an experimental result which was measured from a 5 kV, 100 kVA transformer equipped with fiber optic probes and thermocouples. The paper also summarizes the most often used models for top-oil temperature and especially hot spot temperature calculation. Comments on both models are given in the paper.