The Modified Thermal Circuit Model for Three-phase Unbalanced Operation of Distribution Transformers

Distribution transformers (DT) are the core equipment of distribution power network. During the operation of DT, the internal temperature distribution is not uniform. Hotspot temperature is the most representative parameter. It can reflect the temperature change and insulation aging rate of the DT to guide the actual operation. The hot-spot temperature can be calculated by building the thermal circuit model of transformers. Both the IEEE and IEC standards provide methods for building the thermal circuit model based on the top oil temperature rise and the bottom oil temperature rise. These thermal circuit models are all single-phase model. They assumed that there is no heat exchange for each phase of transformers when the three-phase load are same. However, these models are not applicable to the three-phase unbalanced transformers. The three-phase unbalanced operation of DT is unavoidable in practice. In order to solve this problem, a modified three-phase thermal circuit model for DT was proposed in this paper. It considered the influence of the three-phase unbalanced operation. It modified the traditional thermal circuit model which is based on the top oil temperature rise. The modified model was compared with the standard models through the actual examples. The results showed that the modified model proposed in this paper can reflect the hot-spot temperature of each phase for DT. Compared with the traditional thermal circuit models, the modified model considered the effect of three-phase unbalanced operation. It can accurately calculate the hot-spot temperature of the DT in theory.