Study Of Inrush Current Identification Algorithm In Transformers

The correct operation rate of transformer differential protection is relatively low for a long time, the main reason for that is the existence of inrush current. However, two traditional methods of inrush current identifying: the second harmonic restraint principle and dead angle restraint principle are effected by many factors, such as the decrease of transformer core material's magnetic saturation point and the TA transient saturation, so its reliability is decreased widely. Therefore, it's very urgent to research for a new method of high reliability to identify inrush current.Based on the analysis of the existing full-wave Fourier algorithm, an improved full-wave Fourier algorithm is proposed and applied in the second harmonic restraint principle, then formed an improved second harmonic restraint principle; at the same time , aiming at the problem that when the second harmonic component of inrush current is low, it may induce misjudgment for second harmonic restraint principle, combine a waveform characteristic identification method based on the improved half-cycle Fourier analysis with the improved second harmonic restraint principle, then a synthetic identification method of higher reliability is formed .In addition ,aiming at the decrease or disappearance of the dead angle induced by TA transient saturation, use mathematical morphology to extract the characteristics of dead angle, then an identification method is formed which is of higher reliability than second harmonic restraint principle.Using lots of EMTDC simulation test data, the comparison test of the traditional second harmonic restraint principle and two kinds of identification methods proposed in this paper is proceeded, it's shown that the threshold values of discriminant parameter of the two identification methods are reasonable, and the performance of them are both better than that of the traditional second harmonic restraint principle, and also have good identification effect on symmetrical inrush current.Using multi-group dynamic analogous test data and transformer fault recording data of an actual power supply bureau, the comparison and verification of the traditional second harmonic restraint principle and two kinds of identification methods proposed in this paper is proceeded, the result indicate that, the performance of synthetic identification method is obviously better than that of the traditional second harmonic restraint principle, the identification method based on mathematical morphology is also better than that of traditional second harmonic restraint principle, but not obviously; However, the proposed two identification methods both have good identification effect on symmetrical inrush current.