Transformer hot-spot temperature estimation for short-time dynamic loading

On the transformer, the effect of thermal stress is aging of the solid insulation. Excessive thermal stress could damage the solid insulation. To avoid this, transformer condition monitoring systems could use thermal models to forecast the operating temperatures during dynamic loading. There are several thermal models of varying complexity, including the thermal models stipulated by the IEC and IEEE standards. In this paper, the thermal model which is referred to by IEC Std. 60076-7 as the differential method, is modified, so it accounts for the oil viscosity dependence on temperature. The model is validated using hot-spot temperature measurements from a 40-MVA transformer, OFAF-cooled, 21/115 kV which is located in the subarctic climate region.