A Novel Algorithm to Identify Magnetizing Inrush Current

With the increasing capacity, decreasing saturation magnetic flux of transformer and remanence produced by DC resistance testing, the ultra-saturation phenomena will be induced in transformers. In this situation, magnetizing inrush current generated by transformer will tend to a sinusoidal waveform. The differential protection based on second harmonic restrained method and principle of dead angle will be no longer applied due to second harmonic component and dead angle is too small. An intelligent adaptively algorithm to identify magnetizing inrush based on the difference between 2 times fundamental wave phase angle and second harmonic phase angle, and aperiodic component is put forward by analyzing the ultra-saturation phenomena in transformers. The algorithm firstly determines whether the current has the character of magnetizing inrush or not intelligently by calculating the difference between 2 times fundamental wave phase angle and second harmonic phase angle. And then, second harmonic component is increased adaptively by aperiodic component to guarantee protection block if the character exists; otherwise, second harmonic component is decreased by another fixed ratio to guarantee protection operation. This algorithm identifies ultra-saturation magnetizing inrush much more quickly and exactly compared with the algorithms in the past. And it can make the protection have a faster operation speed when the transformer has internal fault. Dynamic simulation experiment proves its validity and superiority. The algorithm can block the differential protection reliably even when second harmonic component is less than 8%.