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The flexible DC interconnection system, which cascades and unitedly controls several converter stations, can effectively improve the new energy consumption and significantly increase the power quality, thus has been widely used in the power grid. This interconnection system generally uses master–slave control strategy, where chooses one station as the master station to ensure the direct-side voltage’s stability and maintain the input and output power balance. However, in some emergency cases, the input or output power can increase to a huge value that even exceeds the capacity of the master station, which not only threatens the safety of the master station but also leads to power imbalance and abnormal fluctuation of the direct-side voltage. Therefore, to avoid the capacity exceedance of the master station, a multi-architecture power distribution strategy is proposed in this study, which sufficiently considers the capacity limitation in the system architecture and innovatively prioritizes the capacity factor in the control strategy of the master station. Furthermore, the proposed strategy also fully considers cases where the system contains different types of slave stations, such as the station with redundant power and the station that can support bidirectional power flow. The strategy then optimizes the power distribution and adjusts the power flow based on the system’s different architectures. The reliability and effectiveness of the proposed strategy are finally verified by simulation.

High voltage direct current (HVDC) grids are envisioned for large-scale grid integration of renewable energy sources. Upon realization, components from multiple vendors have to be coordinated and interoperability problems can occur. To address these problems, a multi-vendor HVDC system can benefit from a partially open control and protection system. Unwanted interactions can be investigated and solved more easily in partially open software compared to when applying black-boxed and vendor-specific software. Although a partially open approach offers these advantages, practical aspects, such as the implementation in a real station architecture, have to be addressed carefully. This paper covers this important topic, first by reviewing the required control and protection functions and second by discussing the choice for certain open and closed software parts, their implementation in physical units as well as the required communication and interfaces. The result from this discussion is a first proposal of a station architecture for a multi-vendor HVDC system using partially open control and protection. This architecture will be a helpful starting point to industry and academia working with research and harmonization on this topic as ad-hoc solutions in terms of practical aspects can be avoided.

Based on completely mastering ±800 kV transmission technologies, the first ±1100 kV direct current (DC) transmission demonstration project is being constructed in China. Combining theoretical analysis and a large number of experiments, the margin of switching overvoltage in converter stations, the configuration scheme and performance of lightning arresters, the shielding angles of ground lines under different geographical conditions, and the maximum air gap of lines have been determined. The switching impulse flashover characteristics of equipotential sphere and typical rod-plane air gaps are also provided. The external insulation is designed differently for light, medium, and heavy pollution areas. For the ±1100 kV project, if the 8 × 1250 mm^2 conductors are applied, the pole gap is 26 m and the height of the line is 25 m, all the electromagnetic parameters will meet the requirement of international electromagnetic standards. The key design points of ±1100 kV converter transformers, smoothing reactors, converter valves, and wall bushings are researched and the 75 mH smoothing reactors, ±1100 kV/5000 A converter valves, and ±1100 kV/5523 A wall bushings are successfully made. The optimised hierarchical connection modes and coordination control measure of the ±1100 kV project are studied and the results can provide sufficient technical support for the demonstration project.