A Non-unit transient travelling wave protection scheme for multi-terminal HVDC system

The rapid and accurate dc line protection scheme is of great value in isolating dc line faults and improving the reliability of multi-terminal HVDC systems. Nowadays, the traveling wave (TW) protection is widely used as the primary protection in HVDC transmission systems. However, the traditional TW protection has the problems of poor resistance to high impedance faults and the setting value depends on simulation. For the hybrid three -terminal LCC/MMC HVDC system, this paper provides a fast non-unit protection scheme. Firstly, the time -domain expressions of the backward line-mode voltage TW after different line faults occur are derived respec-tively. Based on the expressions, a non-unit protection scheme depending on the difference value of the line -mode backward TW is proposed. Finally, the scheme is verified by simulation. The method only needs single -ended data to realize the rapid dc line protection without converter station communication which has a clear theoretical calculation method in setting values and a strong robustness to high-resistance fault.