Discrimination between magnetizing inrush and fault currents based on energy dissipation criterion

In the transformer differential protection, the second harmonic restraint and the dead angle restraint principles are the two major criterions to discriminate inrush current from fault currents. But in the super-high voltage system, there is also plenty of second harmonic in the fault current. The second harmonic here is of great probability to result in the tripping delay. Meanwhile, the dead angle restraint principle would result in misjudge due to the disappearance of dead angle during the CT saturation. To overcome these shortages, this paper presents a new method to discern inrush current from fault currents using energy dissipation criterion. The least square curve-fitting is adopted to fit the inrush current so as to overcome the shortage of the Fourier algorithm which is sensitive to frequency change. The results of dynamic simulation verify the feasibility of the proposed method.