An analysis of influence factors on traveling wave protections is depicted and it is found that voltage waveform depends on the smoothing reactor and the DC filter so that the voltage change rate decreases and can cause miss-trip. To avoid this， the directional traveling wave can be adopted whose change rate is mostly influenced of the fault impedance. The absolute voltage step and the earth module wave are seriously affected by the line attenuation and dispersion of the traveling wave. Therefore， the important reason of miss-trip is delay of reach time of both set values while far distant fault happens. A new principle of traveling wave protection based on polarity comparison， independent from fault impedance is also presented which can reliably detect the high impedance fault. Simulation results with EMTDC show the conclusions and the new principle proposed are correct and effective.