Assessment of Inrush Current for Power Transformers by Three-Dimensional Representation

For power transformers, the magnitude of the first peak of inrush current is ten times more than the rated load current. The main feature of this current is its high amplitude and second harmonic content. An uncontrolled inrush current may lead to the inadvertent operation of the circuit over-current protection systems. Furthermore, the magnetic stress produced by the inrush current may destroy mechanical structure and windings of transformers. In addition, this current has undesirable effects on electricity quality, extra loss, and reduction of useful life of transformer. In this paper, the peak value of inrush current in three-phase power transformer with Y-Y connection has been calculated and shown with a three-dimensional representation and its harmonic spectrum has been evaluated. It should be mentioned that, this phenomenon occurs because in four-wire Y-Y connection three-phase power transformers, the probability of saturation in phases is lower in contrast with other possible connection of three-phase transformers.