Study on Multi-terminal flexible DC Pilot Line Protection Based on the Transient Voltages Amplitude Ratio and the Transient Current Characteristics

DC line protection technology is one of the key technical issues to ensure the reliability of flexible DC grids supply. In the practice of flexible DC engineering, DC reactors are installed at both ends of the DC line to reduce the fault hazard and improve the survivability from the fault crossing of the converter station and the grid fault. The line boundary feature provides a breakthrough for the study of the line protection of the flexible DC grid. In this paper, the DC reactors installed at both ends of the line in the DC grid are used as the line boundaries. The transient characteristics of the internal faults and external faults are analyzed. A pilot line protection principle based on the amplitude ratio of the voltage wavelet transform detail coefficient on both sides of the DC reactor and the transient current fault characteristics which could quickly distinguish the internal faults and external faults was proposed. A four-terminal MMC type flexible DC grid simulation model was built on the PSCAD/EMTDC simulation platform, and the simulation was verified. A large number of simulation examples show that the protection scheme is fast, adapt to transition resistance, and is not affected by the inverter blocking. It can be used as the main protection for flexible DC transmission lines.