A new detection scheme for realisation of magnetising inrush current in transformers

The computational model developed has been used to predict the pattern of magnetising inrush current for any combination of switching parameters in both single-phase and three-phase transformers. The pattern of second harmonic current during the inrush condition has been found to be inconsistent with the conventional second harmonic-restraint algorithm used in transformer differential relays. The reason is that the largest percentage second harmonic current may not necessarily appear in the first cycle but during any other of the initial few cycles. The new detection algorithm uses the real-time integration value of the magnetising inrush with a time span of one cycle. This method has been found to be consistent under any switching condition. Such a scheme can be readily tailored to replace the second harmonic tuned filter circuit used in the conventional second harmonic-restraint differential relay