Evaluation of permissible loading for indoor oil-immersed distribution transformers

Failures of power transformers can cause high financial damage. Due to this fact it is advisable to know the insulation condition, operating life, and loading capacitance of transformer to ensure its economical operation and secured service. The most important parameter in transformers life expectancy is the insulation temperature level which accelerates the rate of aging of the insulation. In this study, permissible steady state loading of indoor transformers is evaluated from the top-oil temperature rise model. To verify the proposed model experiments are applied for both indoor and outdoor transformers and the results of top-oil temperature rise corresponding to these tests are compared. Since the thermal transfer is different for indoor and outdoor transformers considering their operating conditions, their top-oil temperature rise differs from each other. Due to this fact the permissible loading factors of these two types transformers are different.