False trips on transformer inrush - avoiding the unavoidable

The nature of the magnitizing circuit of a transformer requires that under some initial closing conditions there will be a significant current from the closing sourse; appearing to a differential relay as a difference current - a fault. Because, as the wave position at the instant of closing will vary, protection is complicated. Considering that a fault is possible at the moment of energization also adds complications. Over the years there have been a number of different schemes to avoid false tripping during transforer inrush. These have included increased pickup´s, time delays, harmonic blocking and harmonic restraint. Harmonic elements have been useful, as one characteristic of inrush has been a harmonic component.