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The fact of a dominant resonance frequency existed in the transient process of short-circuit fault is discovered by analyzing transient characteristics of single-phase earthed fault. At this frequency, the amplitude of zero modal current in faulty feeder is much larger than that in healthy feeder, and their polarity is approximately opposite. Transient dominant frequency zero modal current in faulty feeder is 90deg lagged behind zero modal voltage of bus bar, while for healthy feeder it is 90deg leading zero modal voltage of bus bar. Vast simulations under diverse fault conditions have been done by using ATP program. The transient dominant frequency signal is extracted based on Prony algorithm and the corresponding fit data and wave-form is obtained, which completely proved the above conclusions.