In ungrounded distribution networks

the three-phase power flow may diverge because the admittance matrix of transformer is singular. With a given reference for zero-sequence voltage

this paper proposes a novel method for modeling three-phase transformers able to overcome the singularity. The transformer admittance matrix is augmented by adding a constraint of the voltage on the secondary side

certain that the value of the constant zero-sequence voltage reference will not affect the calculated current in the medium voltage (MV) network or the power flow in other segments. The proposed model can be used in the popular three-phase power flows such as backward/forward sweep (BFS) and implicit ZBus algorithms. Numerical tests show the correctness of the proposed method.