Faulty line selection based on HHT detection for hybrid distribution network

The transient zero sequence current within 1/4 cycle before and after the fault is pretreated with morphological filtering when single-phase grounding fault occurs in distribution network with hybrid lines.Select arbitrarily the transient zero sequence current of a line and detect the arrival instant of its initial traveling wave front applying HHT(Hilbert-Huang Transform),base on which the fault occurrence time is precisely determined.The filtered zero sequence current of each line is decomposed with EMD to get its IMF(Intrinsic Mode Function) component with the highest frequency.As the first order differential polarity of IMF component of faulty line is opposite to that of normal line at fault occurrence instant,the faulty line is hence selected properly.EMD decomposition is unique and has good localized characteristic,not only in time domain but also in frequency domain,the analytic results of which have very high time-frequency resolution and accuracy.The influence of CT saturation and phase inversion are avoided.The proposed method is suitable for the distribution network with hybrid lines or unitary lines.