Study on the traveling wave differential protection and the improvement scheme for VSC-HVDC transmission lines

The traveling wave differential protection is an ideal main protection scheme of VSC-HVDC transmission line which is flexible, reliable and immune to the distributed capacitive current. However, it is seriously influenced by the unbalanced current due to the errors of traveling wave attenuation characteristic, time synchronization and calculating truncation. This unbalanced current would cause protection maloperation, especially when the serious external faults occur. In order to overcome this defect of traveling wave differential protection, the transient characteristic of the fault traveling wave is studied and find that the current traveling wave energy will outclass the energy of differential current when external fault occurs, and will not when internal fault occurs. Based on this characteristic, an improved traveling wave differential protection scheme was proposed. The PSCAD simulation is done to verify that the proposed scheme can correctly discriminate between internal and external faults with high sensitivity and reliability.