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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.

High power selective laser melting (HP-SLM) has been used to manufacture the 300M steel single tracks. The influences of laser power and layer thickness on the formability, microstructure and mechanical properties of single tracks were researched. The results show that the optimized parameter window for HP-SLM of 300M steel is determined as laser power 500–1800 W, scanning velocity 400–600 mm/s, layer thickness 80–160 μm. Laser power has a great influence but layer thickness has little effect on the shape and mode of the molten pool (MP). The microstructure of SLMed 300M steel presents an epitaxial growth of columnar crystals and mainly consists of lath α' martensites with no obvious element micro-segregation. Laser power has little effect on the morphology but a great influence on the size and misorientation of the microstructure. However, layer thickness has little effect on the morphology and size but significant influence on the misorientation of the microstructure. The average microhardness increases firstly (~453 to ~551 HV) and then decreases (~551 to ~533 HV) when laser power varies from 500 to 2000 W. However, the average microhardness nearly remains constant (~530 HV) when layer thickness varies from 80 to 200 μm.