High temperatures may damage power transformers. These problems leading to the high temperature are more revealed under non-rating operating conditions such as unbalanced supply voltage. The aim of the present study is thermal analysis of such supply and obtaining its temperature distribution. Existing thermal analysis methods through thermal equivalent circuit (TEC) have some drawbacks; in these models, thermal parameters of different regions of transformer such as core, tank, metallic parts and winding are not defined. On the other hand, those models are not applicable for thermal analysis of transformer with unbalanced supply voltage. Here, a novel TEC model is proposed which is able to define temperatures of different components of oil-immersed power transformers individually under unbalanced supply voltage. The merit of the introduced model is that losses of different parts of transformer are considered as heat generating sources which are used as the inputs of thermal model. At this end, a three-dimensional finite-element method is suggested which is able to estimate the losses of different parts of power transformer. Finally, the results of applying the TEC to the transformer are compared with the temperature distribution of finite-element modelling and high accuracy of the TEC model in estimation of the temperatures of different regions are emphasised.