A novel protection method for VSC-MTDC cable based on the transient DC current using the S transform

Voltage source converter based multi-terminal high voltage DC (VSC-MTDC) transmission technology is attracting more and more research. In this paper, a phase-mode transform method for bipolar DC cable is proposed to decouple the currents in the cables and the characteristic of the transient DC currents after faults are analyzed. Based on the analysis, a novel DC cable protection method is proposed. Firstly, the frequency components of the mode currents are extracted by the S transforms, and the sudden change points of the high frequency components are extracted to detect the fault and determine the fault time. Then the magnitudes of the zero frequency components are employed to ignore the disturbances. Finally, the internal faults are identified from the external faults by the polarities of the fault current components at the two terminals of the DC cable. A three-terminal VSC-MTDC system is modeled in PSCAD/EMTDC, and various faults are simulated to verify the proposed method. The simulation results show that this protection method can recognize the internal faults under different fault resistance and distance conditions rapidly and effectively.