Traveling-wave differential protection has a special advantage in EHV/UHV transmission line protection for its immune ability to distributed capacitive current. However， unbalanced differential current can occur at normal conditions when traveling-wave differential protection is applied. In this papers， four main factors affecting unbalanced differential current are investigated theoretically. They are line resistance model error， interpolated truncation error， data synchronization error and unequal traveling speed between earth sequence and spatial sequence. An applied scheme for traveling-wave differential protection is also proposed in the paper. The filtering algorithm and sampling rate in application are also discussed. EMTP simulation results show that the proposed scheme operates sensitively， rapidly and selectively， demonstrating good potential in field applications.