Modeling and analysis of three-phase four-wire distribution transformers with mid-tap on the secondary side

In this paper, three-phase four-wire power distribution transformers with a grounded mid-tap on the secondary side are modeled in phase coordinates. These kinds of distribution transformers are commonly used in distribution systems to supply single-phase lighting loads and three-phase power appliances simultaneously. Using the proposed models, along with appropriate three-phase power flow programs, the inherent characteristics of distribution transformers can be retrieved correctly. The major considerations involve copper loss, core losses, phase angular displacement between primary and secondary voltages and off-nominal tapping of the transformers. Based on the proposed models, three distribution transformers with unbalanced loads are analyzed in detail in this paper. The simulation results show that the proposed models can accurately emulate the performance of a physical distribution transformer in a distribution system even under unbalanced conditions.