1.Engineering Research Center of Offshore Wind Technology, Ministry of Education (Shanghai University of Electric Power), Shanghai200090, China;2.College of Electrical Engineering, Shanghai University of Electric Power, Shanghai200090, China;3.Electric Power Research Institute of State Grid Shanghai Municipal Electric Power Company, Shanghai200437, China

Flexible low-frequency transmission has attracted more and more attention due to its superior transmission economy in the transmission scenario of medium- and far-offshore wind power. The existing wind power system with low-frequency transmission adopts constant-voltage and constant-frequency AC current to transmit electric power, which is a simple combination of low-frequency transmission and variable-speed constant-frequency wind power technology. This combination is easy to implement in engineering, but the key factor of the freedom degree of voltage conversion has not been fully considered. In this paper, a variable-voltage operation control method for the wind power system with flexible low-frequency transmission is proposed, which dynamically adjusts the transmission voltage to meet the power generation requirements with different wind speeds. In this operation mode, a matching dynamic minimum DC bus voltage control method for wind turbines is proposed, and a wide-range DC bus voltage frequency support technology is formed for units without DC/DC converters. Furthermore, the influence of variable-voltage operation mode on transient overmodulation problems and power loss is analyzed, and the reason why overmodulation will not occur and the reasons for the decreasing total loss are demonstrated. Finally, the feasibility and the superiority of the proposed control method in terms of system loss, fault transient current and grid frequency support are verified by simulation.