Critical Investigation of Heat Transfer in Oil Cooled Transformer

Power transformer represent the largest portion of capital investment in transmission and distribution substations. Transformer functional life is influenced by thermal, mechanical and dielectric stresses. The concept of insulation Thermal life is a useful one, in which it can set a standard level of physical condition below which insulation integrity may be prejudiced by mechanical or dielectric stresses, normally experienced in service. In power transformer, the oil serves a dual purpose as insulating and cooling medium. The heat generated in the transformer is removed by its oil surrounding the source and is transmitted either to atmospheric air or water. Thermal modeling of power transformer top-oil considering viscosity and conductivity and other relevant parameters have been taken into account in the present work and accordingly an important correlation has been derived for ONAF (oil natural air forced) type of cooling. Result shows that the transformer top oil temperature varies remarkably with respect to time. However, within the range of the data covered the plotted figures show that rapid increase in the temperature of the transformer top oil could be achieved with increasing values of time after a certain period of time.