In recent years, as Chinese wind power companies venture into deep-sea projects, there has been a significant shift in wind power transmission system innovations. The focus has turned to flexible direct current (DC) and low-frequency transmission technologies, which are proving to be cost-effective for long-distance power transmission.
In recent years, as Chinese wind power companies venture into deep-sea projects, there has been a significant shift in wind power transmission system innovations. The focus has turned to flexible direct current (DC) and low-frequency transmission technologies, which are proving to be cost-effective for long-distance power transmission.
In 2021, the flexible DC transmission project of the Jiangsu Rudong Offshore Wind Farm by Three Gorges Energy was commissioned. This project overcame numerous technical hurdles, including converter station construction, DC submarine cable development, the application of domestic converter valves, and equipment design and testing for marine environments. It marked China's first use of flexible DC technology in offshore wind power transmission. The offshore converter station became the largest in the world at that time in terms of capacity and voltage level. The electricity generated by the 1.1 million kW offshore wind farm is converted to DC by the offshore station and transmitted via a ±400kV DC submarine cable to an onshore converter station, where it is inverted to 500kV AC for grid connection.
Compared to direct current systems, low-frequency transmission offers several advantages. Wind turbines can directly output low-frequency energy, which is then transmitted through a collection system to an offshore platform. After being boosted by a low-frequency transformer via submarine cables, it is converted back to standard frequency at an onshore conversion station before entering the grid. This approach reduces costs as there is no need for additional facilities such as converter stations.
In addition to cost advantages, flexible low-frequency transmission uses voltage-source converters with similar control capabilities as flexible DC systems, such as adjustable flow direction and dynamic voltage support. This enables black start capability for offshore wind farms and better integration with renewable energy sources.
The transformation of low-frequency wind turbines is crucial in these systems. Currently, there are no national standards for their design, which means electrical system designs must adapt to existing frequency standards while ensuring reliability through simulations and physical testing involving collaboration among universities, research institutes, power academies, design firms, and testing centers.
Cao Guangqi noted that "low-frequency machines represent not just breakthroughs in electrical solutions but also refined designs based on operational scenarios." For instance, current low-frequency transformers weigh about 1.5 times more than their standard counterparts, making structural considerations complex when integrating them into overall machine designs.
As deep-sea developments progress, many projects beyond Dachen Island and Zhejiang Yuhuan No.2 Project could benefit from adopting low-frequency technology, expanding its application scope beyond just remote areas facing renewable energy absorption issues due to network limitations.