Modelling and analysis of delta-connected distribution transformers with symmetrical neutral-grounding structure for microgrid networks

The conventional grounding methods for the delta-connected secondary windings, such as corner grounding and mid-tap grounding, result in unbalanced phase-to-ground voltages. To overcome this issue, the neutral-grounding method can be employed, but when it operates under unbalanced load conditions, the neutral current flowing through the grounding winding may cause overheating and failures. We then propose a symmetrical method for the grounding of three-phase transformer windings, based on neutral grounding of the delta-connected configuration that can help to distribute the neutral currents. In addition, the proposed method provides a multi-voltage solution relevant for microgrid networks, reduces the costs, and makes it more convenient for planning and design. We formulate the mathematical model of a three-phase neutral-grounded transformer bank with three grounding windings based on the nodal admittance matrices and the coupling-free equivalent circuits. A function block of the equivalent circuit of the model is implemented in MATLAB/Simulink and is verified by field testing. The proposed method can provide a reference for engineers for the design and operation of microgrid networks.