Inrush restraint algorithms for transformer differential relays

When a power transformer is energized, the resulting inrush current can cause a differential relay to issue a false trip signal. Traditionally, the second harmonic content of the inrush current has been used as a form of pattern recognition to distinguish between this current and legitimate internal fault current. Other forms of pattern recognition have been suggested, particularly one in which the extensive horizontal or flat section of the inrush waveform is used as the distinguishing feature. In this paper, these two methods are compared, for a wide variety of conditions, including asymmetrical current transformer saturation, for which the waveshape is quite similar to that of inrush current. It is concluded that the flat section method may have an advantage over the second harmonic method of restraint.