Key technologies of VSC-HVDC and its application on offshore wind farm in China

In recent years, wind power industry has been flourishing and in China the focus has been gradually shifted from land-based to offshore wind farms. There are many advantages for offshore wind farms, such as abundant wind energy reserves, high utilization of the wind turbine capacity, not taking up land resources and so on. As wind turbine technology improves, offshore wind farms have been expanding quickly and are located further away from the onshore grid. The power transmission problem has become one of the key issues for restricting the development of offshore wind farms. For example, the technology of high voltage direct current transmission based on voltage source converter (VSC-HVDC), suitable for long-distance transmission of offshore wind energy, has become one of the current research focuses. This has resulted in higher requirements on the aspects of converter voltage level, system dynamic performance, and network power quality. The paper first discusses converter topology for offshore wind farm grid integration. Two VSC-HVDC projects for connecting offshore wind farms, which are located in Shanghai and Dalian of China, are presented in detail. Based on the two projects, the structure, control methods and application of modular multi-level converter are presented. The control strategies of VSC-HVDC are then discussed, focusing on double closed-loop vector control, direct power control, deadbeat control and the control methods for unbalanced grid voltage. This can provide good theoretical foundation for the grid integration of large offshore wind farms.