New pilot protection for VSC-HVDC transmission lines using natural frequency characteristic of current

The natural frequency signal of DC lines is more obvious than that of AC lines. Combining the system structure, it is found that the natural frequency of VSC-HVDC is related with line length. Theoretically, the natural frequency differential is zero for external fault and it is a huge value for the internal fault. Accordingly, a new pilot protection principle and its primary criterion for HVDC transmission line are proposed based on current spectrum analysis at both ends. For the dead zone at the midpoint of the line, a secondary criterion is proposed. At the same time, the system of VSC-HVDC is built in PSCAD and various faults are simulated. In the protection principle, only current information of both converters are used and synchronization is not needed for the transmission of natural frequency. The simulation results show that the protection principle can identify the faults reliably and rapidly in different conditions, and is not affected by the fault resistance.