This paper deals with the thermal characteristics of traditional mineral oil and alternative dielectric fluids. Candidates of alternative dielectric fluids used in the transformer were silicon oil, HMWH, synthetic ester and natural ester. In this study, the transformer which uses synthetic ester and natural ester was focused. 3 ph, 66 kV, 16.5 MVA power transformer and 3 ph, 22 kV, 2.3 MVA dist…