New algorithm to identify inrush current based on improved mathematical morphology

A new algorithm based on improved mathematical morphology was presented for fast discrimination between inrush current and fault current of power transformers. The method is based on the principle that the waveform of the inrush distorts seriously, while the fault current keeps nearly sinusoid. By the geometric explanation of open operation and close operation of mathematical morphology, endpoint effect was overcome. On the basis of self-defined MBT (max-bottom-top-operator) function in mathematical morphology, this paper deduced the formula against sine wave which indicated the relationship between the selected structural elements radius and the value of MBT function, and constructed relative concavo-convex coefficient Kcc which was used for showing the concavo-convex characteristic. According to the Kcc value of differential current, both inrush current and fault current of power transformers can be distinguished. Theoretical analysis and dynamic simulation results show that the method is effective under various fault conditions and simple to be applied. Its performance is hardly affected by CT saturation and frequency fluctuation. Moreover, judgment time of fault is 8~13 ms. © 2009 Chin.Soc.for Elec.Eng.