Abstract: To improve the sensitivity and reliability of the traveling wave relay,a novel surge impedance directional relay based on S transform was presented.The back-side bus surge impedance was defined by analyzing the characteristic of initial traveling waves generated from forward faults.The forward impedance circle and the reverse impedance circle,which were set according to the back-side bus surge impedance and the forward line surge impedance of the relay respectively,constitute the double impedance circle operating criteria.S transform was adopted to obtain the initial traveling waves of single frequency to calculate the surge impedance.The fault direction was determined by the location of the calculated impedance in the double impedance circles.The PSCAD/EMTDC simulation results show that the relay can quickly and reliably discriminate between the forward fault and the reverse one,and its performances are slightly influenced by various factors（e.g.different fault inception angles,fault distances,fault resistances,structures of busbar,etc）.The ultra high speed directional protection containing the proposed relay is feasible in the engineering field.