With the adoption of＂AC-DC-AC＂ type of trains in high speed railways （HSR）, the wide spectrum of locomotive current may induce high frequency resonance in a traction power supply system （see Fig. 1）. The harmonic currents generated by the train will be seriously amplified by resonance at some specific frequencies. The resonance could cause serious distortion of voltage and current, and largely increase the harmonic current. Moreover, the series resonant over-current may damage railway an＇esters and locomotive converters , even interrupt the normal rail transportation.