A non-unit adaptive traveling wave protection method for flexible DC transmission lines based on supplement of voltage data

Traveling wave (TW) protection is crucial for ensuring the safety and stability of flexible DC transmission lines. Thereinto, shortening the sampling step is the key to improve the sensitivity of the derivative-based TW protection. However, higher sampling frequency is difficult to engineer and the protection performance will be constrained in case of the high resistance faults. To address this issue, a non-unit adaptive TW protection based on the supplement of voltage data has been proposed. Firstly, the voltage data is realized to complement utilizing the Lagrange's median theorem at the line boundary, which improves the sampling frequency. Then, it has been observed that the fault current variation is negatively correlated with transition resistance. Specially, the proposed protection takes this fully into account by incorporating the adaptive threshold that covers differences among various fault locations. Therefore, the proposed protection can effectively distinguish internal faults with high sensitivity. Finally, a two-terminal ±500kV MMC-HVDC (modular multilevel converter-high voltage direct current) system is built using PSCAD/EMTDC. The simulation results demonstrate that the line faults can be fast and correctly identified, even with 300 Ω fault impedance, in various fault scenarios.