Modified thermal circuit model for distribution transformers in three-phase unbalanced operation

The three-phase unbalanced operation of distribution transformer is unavoidable in practice. Under this situation, accidents arising from internal overheating occur easily. Therefore, the hot spot temperature analysis and calculation is of great concern. Thermal circuit model method is widely used to calculate the hot spot temperature of power transformer for the advantages of simple calculation and good accuracy. However, the existing thermal circuit model is a kind of single-phase model, ignoring the interphase thermal coupling effect. To make the model suitable for distribution transformer, we modify the single-phase thermal circuit model by analyzing the internal heat source, reconsidering the influence of environment factors, and implementing the temperature-rising test to calculate the interphase thermal coupling coefficient. The thermal circuit model presented in this paper can accurately calculate the hot spot temperature of distribution transformers both in balanced operating and unbalanced operating. This model can provide a reference for the safe operation of distribution transformers.