Synchronous Oscillation of Direct Drive Offshore Wind Farm Connected to HVDC System

Since 2010, wind power has been connected to the grid on a large scale, resulting in great changes in China's energy structure. By the end of 2017, China's cumulative wind power installed capacity was 164 million KW, mainly concentrated in the "Three North" areas of China. By 2020, the installed capacity of wind power in China has reached 210 million KW. Wind power generation is know for it’s fluctuation, nonlinear complexity and multiple time scales. Its grid connected operation will have a certain impact on the power system. In addition, due to the characteristics of energy and load reverse distribution in China, when dealing with long-distance transmission, access to VSC-HVDC would be the first thought because of its advantages of large capacity, high voltage level and low loss. At present, China mainly uses wind and fire binding to connect to HVDC transmission system to realize long-distance transmission of wind energy. During the 13th Five Year Plan period, China added about 130 million kilowatts of inter provincial power transmission, built DC and other transmission channels, and maximized the proportion of renewable energy transmission. At present, China is in the stage of large-scale cluster grid connection of renewable energy. Significant changes have taken place in the power supply structure and transmission mode of the power system, and oscillation has become the norm. However, research ragarding the subsynchronous oscillation mechanism of wind farm and grid connected HVDC system is not through, and it is impossible to talk about the complete theoretical system and recognized modeling method. In addition, the decentralized access of renewable energy with high penetration rate in China is developing rapidly, and the access proportion of power electronic equipment on the load side is high, which will aggravate the occurrence of oscillation. The generation and interaction of subsynchronous oscillation are complex due to different modes and load structures, but the related research is based on the subsynchronous oscillation characteristics of high proportion power electronic devices. Therefore, the research on the generation process of subsynchronous oscillation of direct drive wind turbine will provide a theoretical basis for leading the industrial upgrading of wind turbine. The interaction of subsynchronous oscillation of multiple groups of direct driven wind farms is studied to provide a theoretical basis for improving the friendliness of wind power.