Three-phase transformer models for load flow calculations in power systems

Transformer models are important in performing load flow procedure, especially in distribution power systems. This paper summarizes the detailed transformer models with various connection types using phase coordinates, such as two-winding transformers, open wye-delta with a mid-tap grounded on the secondary side and other special connection types. But in some conditions, the nodal admittance submatrices of these transformers become singular and it can not be incorporated into distribution load flow algorithm such as Backward-Forward Sweep Method. Some methods are adopted to remove these difficulties by using line-to-line voltage component or separating zero and none-zero sequence voltages. Numerical examples demonstrate the validity of the proposed models and the algorithm eliminating matrix singularity.