Improvement on Hausdorff Distance and its Application to the Backup Protection Scheme for Hybrid HVDC System

The hybrid HVDC transmission system applying LCC converter as the rectifier, and MMC converter as the inverter combines the advantages of both types of converter, which makes the protection scheme more complicated. A new pilot protection scheme for the three-terminal hybrid DC transmission system applying energy functions is proposed. The energy function for the LCC converter is extended to be applied in MMC converter to derive the energy level of the hybrid system. Furthermore, an improved Hausdorff distance calculation method is proposed to detect the difference in energy levels between normal and abnormal states. The Hausdorff distances and the floating thresholds are calculated to determine the fault pole and the fault line. Finally, the proposed protection scheme is verified by using the simulation model of Wudongde ±800kV three-terminal hybrid HVDC transmission project and compared with the results derived by the absolute distance method. The results show that the proposed scheme in this paper can identify and trip the fault lines quickly and effectively. It also shows that the proposed scheme can avoid the misoperation caused by abnormal data and ineffectiveness caused by high-resistance ground fault up to 600Ω, which improves the practical application value to some extent.