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Through the research on the traveling wave transform characteristics of the current transformers (CTs), a description method is proposed by using transfer function to replace the phase- and amplitude-frequency characteristics of the current transformers. In the presented technique, the free alternating current of various frequencies is injected into the primary side of the current transformer by using a variable capacitor and a DC voltage source. The transfer characteristics under different frequencies were measured. According to the characteristics, the transfer function was constructed, and then the model of the secondary cable was added. The simulation result validates that the traveling wave changed dramatically after traveling through the CT and secondary cable, thus it will have significant impact on the accuracy of traveling wave protection and fault location devices.