The following transformer gets overheated and I was looking for most probable the causes associated with it. 1. Rating : 7.5 MVA, 0.85 power factor, Power Transformer (Manufacturer - Kirloskar) 2. Connected Load : Upto 6 MW 3. Oil Natural Cooling System 4. Normal Atmospheric Temperature : 25 C 5. Clauses : The transformer performs well most of the times and at peak load that occurs mostly at 0600 hrs and 1800-2100 hrs. However, on a hot day the temperature of the transformer exceeds the permissible limits. The reading are as follows: Oil Temperature : 50 C, Winding Temperature : 73 C. The connected load at this time was noted to be upto 4 MW and the atmospheric temperature was noted to be about 30 C. This should, practically, never cause the transformer to overheat. 6. Observation : The transformer's vent-pipe seemed to be missing, or perhaps this transformer never used one. Also, this is a pretty new transformer, installed in 2001. 7. My assessment : I thought that the dielectric strength of the oil has deteriorated and/or sludge formation within the tank has occurred. I am planning to filter the oil in the tank. What are your recommendations/feedback?