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Voltage source converter (VSC)-based high-voltage direct current (HVDC) and multi-terminal (MT)/DC grid technologies are the new HVDC transmission technologies after ultra-high voltage alternative current (UHVAC) and UHVDC transmission technologies which have been widely used in China. The application of the new technologies has resulted in a rapid increase in the number of schemes in construction and planning worldwide. This has been stimulated by the greater level of functionality available from the VSC technology, which makes it suitable for a wide variety of applications. These include the integration of offshore wind farms, embedded links within AC networks and interconnectors, especially where the AC networks are relatively 'weak'. VSC technology has renewed interest in MT DC systems, which may ultimately lead to wide area DC grids. This study outlines the research and application on MT and DC grids in China with respect to VSC-HVDC key technologies and DC grid key technologies based on the presentation given in the International Workshop on Next Generation Power Equipment held on 23 September 2016 in Xian, China. The briefing details of the VSC-HVDC projects constructed and to be constructed in China are summarised in this study.

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Power system interconnections using high-voltage direct-current (HVDC) technologies between different areas can be an effective solution to enhance system efficiency and reliability. Particularly, the multi-terminal dc grids that could balance and ensure resource adequacy increase asset utilization and reduce costs. In this paper, the technical feasibility of building dc grids using the line-commutated converterbased (LCC) and voltage source converter-based (VSC) HVDC technologies is discussed. Apart from presenting the technical challenges of building LCC dc grids and LCC/VSC hybrid dc grids, the reliability modeling and analysis of these DC grids are also presented. First, the detailed reliability model of the modular multi-level converters (MMCs) with series-connected high-voltage and low-voltage bridges is developed. The active mode of redundancy design is considered for the reliability model. To this end, a comprehensive whole system reliability model of the studied systems is developed. The reliability model of each subsystem is modeled in detail. Various reliability indices are calculated using this whole system reliability model. The impacts of the redundancy design of the MMCs on these indices are presented. The studies of this paper provide useful guidance for dc grid design and reliability analysis. INDEX TERMS LCC-HVDC, VSC-HVDC, MMC, multi-terminal dc grid, reliability analysis, k-out-of-n.