The Influence of Heat Transfer Coefficient α of Insulating Liquids on Power Transformer Cooling Systems

The power transformer plays an important role in electric power systems. One of the conditions for the proper operation of the transformer is to ensure a sufficiently low temperature. This condition can be met if the heat exchange is effective. Heat transfer depends, among other things, on the electrically insulating liquid. The thermal property describing the ability of a liquid to transfer heat is the heat transfer coefficient α. At the design stage of the transformers, it is most often assumed that the value of the α coefficient is constant and equal to 100 W·m −2 ·K −1 . Such simplifications can lead to the improper design of the transformer since this factor depends on many factors. The article presents the results of research on the dependence of the heat transfer coefficient α on the type of electrical insulation liquid, the thermal load of the cooled surface, and the length of the heating element. Four types of electrical insulating liquids were considered: mineral oil, synthetic ester, natural ester, and natural ester with reduced viscosity. The obtained results prove that the type of electrical insulating liquid and the thermal surface load value affect the α coefficient. The length of the heating element did not affect the α factor.