Determination of oil temperature in transformers

TOP OIL temperature and winding temperature, where the heating is caused by resistance, are determined by direct measurements when a heat run is made on a transformer in accordance with the American Standards Association Test Code for Transformers.¹ However, the average oil temperature is not directly measureable. The Test Code states that the average oil temperature may be approximated by subtracting one-half of the tank temperature gradient (as determined by measurement of the temperature at the top and bottom surfaces of the external cooling means) from the top oil temperature.