Pilot Protection Principle Based on Traveling Wave Amplitude Comparison Considering the Characteristics of Transfer Function

In order to accurately describe the propagation process of traveling wave and its attenuation law in the transmission line, and explore fault information that it carries for the transmission line protection, the equivalent circuit of Thevenin type for the lossy uniform transmission line was analyzed and the lumped equivalent circuit of the faulted network was built by transforming the transmission line wave equations in the time domain into the Laplace domain. On the basis of the lumped equivalent circuit, the mathematical relationship of the initial backward traveling waves on both ends of the faulted line was deduced, and the algorithm of the travelling wave based pilot protection was proposed, while the transfer function of the transmission line was taken into consideration. S transform was used to extract the initial traveling waves, and the attenuation ratio of the initial backward traveling waves on both ends of the line was calculated. The proposed algorithm determines whether a fault is internal or external to the protected zone according to the ratio value. The simulation data from PSCAD/EMTDC was applied to verify the proposed algorithm. The simulation results show that the ratio is greater than 0.81 when the internal fault occurs, and it almost equals to zero when the external fault occurs, indicating that the algorithm can reliably identify internal and external faults, and is not affected by the adverse factors of the error of line model, circuit parameter uncertainty, etc.