A Pilot Protection Scheme for MMC-HVDC Transmission Lines Based on Transient Energy

DC line protection is the key to the development of MMC-HVDC. The boundary conditions provided by DC current-limiting reactors facilitate the design of fault protection algorithms. The voltage-drop characteristics of the current-limiting reactor and the shunt characteristics of adjacent lines are combined in this paper. Then a pilot protection scheme for DC lines based on boundary transient energy is proposed. The difference of the transient energy ratio at the boundary of the line is used to identify internal faults and external faults. Then the transient energy ratio of the positive and negative poles can be calculated to identify the faulty pole. The characteristics of the fault boundary are obviously different under different faults. The implementation of the scheme is simple and there is no need to synchronize data at both ends of the line. The proposed scheme is tested in a four-terminal MMC-HVDC system and the results show that the scheme has the ability to fast and accurately identify faults. It has sufficient sensitivity to faults with large transition resistance, and has strong anti-noise ability.