Abstract: To improve the reliability and sensitivity of traveling wave pilot protection,a new algorithm of pilot protection based on traveling wave power for transmission lines is proposed.The algorithm uses Peterson equivalent model to analyze the distribution characteristics of initial traveling wave at fault lines,and the definition of initial traveling wave reactive power is given.The initial traveling waves of single frequency are obtained by S transform to calculate the initial traveling wave reactive power,and the ratio of initial traveling wave reactive power at both ends of transmission line is used to make the protection judgment.For the external fault,there is little initial traveling wave reactive power at the end of the protected line near the fault point,but reactive power is great at the end far from the fault point.For the internal fault,the initial traveling wave reactive power is great at both ends of the protected line.Then the internal and external fault of transmission lines can be identified obviously based on the relative value of the initial traveling wave reactive power at both ends.Theoretical analysis and simulation on PSCAD/EMTDC show that the proposed method has higher reliability,faster action less calculation and simply set action threshold value,and the internal and external fault can still be distinguished accurately under the condition of smaller fault inception angle by the presented method,while its performance is not affected by fault types,fault location,grounding resistance and bus configuration.