oil type transformer, reason for the temperature rise

In the oil type transformer are generally used in a class of insulation, insulation heat resistance of 105 ℃.
As the ambient temperature is generally lower than 40 ℃, so the transformer winding temperature generally reach the limit operating temperature.
oil immersed distribution transformer ,Even in a short time to reach 105 ℃, because the time is very short, the insulation of the winding is not directly dangerous.