When a transformer is energized the phenomenon of magnetizing inrush current occurs and causes a pseudo tripping signal to the differential relay which leads to the problem of mal operation or false tripping of the relay. In order to avoid this false tripping of the differential relay and for safe operation of the transformer the distinction of inrush current with internal fault current is very important. Conventionally, second harmonic restraint relay is used but as size of power system network is increasing day by day the electrical network is becoming more and more complicated and some disadvantages of conventional system are slowly understood. Therefore, some other methods which can also be used for proper distinction between inrush current and internal fault current are highlighted in this paper. Different techniques used for discriminating inrush current from internal fault current are discussed and some conclusion has been drawn.

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Second Harmonic Equivalent Instantaneous Inductance (eii) Instantaneous Magnetizing Inductance (imi) Morphological Gradient Algorithm (mga) Sinusoidal Proximity Factor (spf) Waveform Singularity Factor (wsf) Finite Element Method (fem) Inrush Current Wavelet Transform