The transformer is operated under rated conditions. The loss of heat of the core and the winding causes the temperature of each part to increase. When the heat and the heat are balanced, the temperature of each part tends to be stable. During the inspection, attention should be paid to the ambient temperature, the upper oil temperature, the load size and the oil level height, and compared with the previous numerical comparison. If the upper oil temperature is higher than usual by 10°C under the same conditions, or the load does not change, However, if the oil temperature continues to increase, and the cooling device is operating normally and the temperature meter is not malfunctioning, abnormalities and failures within the transformer can be considered.