The largest part of the investments of transmission and distribution systems is due to transformers used in changing the voltage level. The operating life of transformers is important due to their considerable economic impact on the power systems. To ensure the economical operation and secured service it is important to have knowledge about the insulation condition, operating life, and loading capacitance of transformer. The most important parameter in transformers life expectancy is the insulation temperature level, which accelerates the rate of aging of the insulation. The modeling of the energy balance and heat transfer of transformer is a major task in transformers thermal modeling. The aim of this paper is to present improved thermal models for transformers loaded at prefabricated MV/LV transformer substations and outdoor situations. The top-oil temperature is studied both for indoor and outdoor transformers and the thermal models proposed for them are compared. Since the thermal transfer is different for indoor and outdoor transformers considering their operating conditions, their thermal models differ from each other. The proposed thermal models are verified by the results obtained from the experiments carried out on typical indoor 30 kV/0.4 kV transformer stations at different operating conditions.