Improved differential current protection scheme for CSC-HVDC transmission lines

In the modern power system, current source converted high-voltage direct current (CSC-HVDC) system is playing an increasingly important role due to the advantages such as flexible control, asynchronous interconnection, and long distance transmission. However, the conventional differential current protection has exposed the shortages in operation speed and fault identification. This study proposes a novel differential current protection scheme, which is composed of the differential current unit and blocking unit. Both of them are designed based on the differences in the newly defined differential current between the internal fault and external fault. A +/- 800 kV UHVDC simulation system is established to verify the performance of the proposed protection. Simulation test results show that the proposed protection can accurately distinguish the internal fault from external fault in the whole fault period. The identification performance is not influenced by the fault location or fault resistance. Besides, the requirement of sampling rate is low and the calculation amount can be negligible, providing a favourable condition for the proposed protection scheme to be implemented into the real HVDC control and protection system.