Transformer inrush current mitigation using controlled switching and magnetic flux shunts

The inrush current is a transient current that results from a sudden change in the exciting voltage across a transformer’s windings. It may cause inadvertent operation of the protective relay system and necessitate strengthening of the transformer’s mechanical structure. Many methods were reported in the literatures for reduction and mitigation of transformer inrush currents. This paper represents a study of techniques that have been proposed for transformer inrush current mitigation. A new, simple and low cost technique to reduce inrush currents caused by transformer energization is presented here. In this method, a controlled switching approach with a grounding resistor connected to transformer neutral point and a magnetic flux shunt is used. By energizing each phase of the transformer in sequence, the neutral resistor behaves as a series-inserted resistor and thereby significantly reduces the inrush currents. The dimensions of the magnetic flux shunts are chosen such that the inrush current amplitude is further reduced. The proposed method has been tested by computer simulation using 2-D FEM (two-dimensional finite element method) by Maxwell software. The obtained results show that the proposed method is efficient in reduction of transformer inrush current and is much less expensive since there is only one resistor involved and the resistor carries only a small neutral current in steady-state.