The Research Of The Distant Short-circuit Fault Current In Rapid Transit Systems Based On Wavelet Transform

This dissertation deeply studies the transient models of distant short circuit fault currents for the dc rapid transit railway and presents a detailed analysis of the wavelet transforms to discriminate the remote short circuit current from the train starting current.Firstly, the dc railway systems throughout the world are surveyed, the various short circuit fault conditions and protection equipment requirements are discussed. A dc transit system can never be fully protected because the protection cannot always distinguish between distant faults and train load currents for both of the currents have the similar magnitude and the rate of change.The difficulty of analyzing the distant fault current lies in the running steel rails as the return conductors. The rails are affected by the skin effect for its large permeability and size. The track time constants are not constant due to the skin effect. For the calculation of the rails impedance, the method of equivalent cylindrical conductor is developed. The simulation results give satisfaction.After this, the traditional Fourier transform is applied to the distant current and the train starting current. Then the wavelet transform is introduced for the further analyzing.Current singularities based on wavelet transform are analyzed. Two kinds of wavelet transforms are firstly used to analyze the former two currents, one for continuous wavelet transform and the other for dyadic wavelet packet. Then the discrimination criterions based on wavelet transform are firstly constructed. Simulation results show that the wavelet transform is suitable for the discrimination of the distant fault current from the train starting current.In the last, the main research works are summarized and the problems required to further study are included.