Research On Topology Reconstruction And Coordinated Control Of DC Offshore Wind Farm

With the development of offshore wind farm to the larger capacity and the farther distance from the shore, the DC offshore wind farm has been paid more and more attention. It has become an important direction for offshore wind power research to deeply analyze the DC wind farm and propose innovative solutions.In this paper, the serial parallel type DC wind farm is analyzed. The operation control technology, topology reconstruction, fault protection and recovery strategy after removing the fault of serial parallel type wind farm are researched. The main work completed as follows:(1) Based on the working principle of the PMSG wind turbine and the characteristics of the series parallel type wind farm, DC wind turbine and its control strategy are established. It enables DC wind turbines to achieve maximum power tracking control and output voltage limiting control to meet the needs of the series parallel type offshore DC wind farm. The simulation software PSCAD/EMTDC is used to model a DC wind farm, the simulation results verify the validity of the model and its control strategy.(2) The topological structures of the series parallel type offshore DC wind farm presented in the literatures are analyzed. Due to the lack of redundancy in the existing transmission system, the transmission line will have an impact on the whole operation of the wind farm after short circuit fault, which is not conducive to the development of the offshore DC wind farm. Based on the advantages of the existing topological structure, this paper presents a new topology for the series parallel type wind farm. In the new topology, asymmetric bipolar operation mode is used in the grid side inverter by two independent converters unit which are connected in series. The connection point of two independent converters forms the midpoint of the DC transmission system, and the other end of the independent converters respectively forms the positive pole and the negative pole of the DC lines. For the wind farm side, the DC wind turbine is connected in series to form a cluster. Each cluster is parallel connected with the positive, neutral point and the negative pole to deliver power to the grid side inverter. At the same time, the auxiliary circuit with DC circuit breaker is added in the wind farm to enhance the redundancy of the system, which is helpful to the fault crossing and recovery of the wind farm.(3) Based on the improved topology, the fault protection of wind farm control strategy when the transmission line short-circuit is designed. That is, through the coordination and cooperation of AC and DC circuit breakers and converters, the wind farm in fault can use the redundant of system for power transmission, and reduce the influence of transmission line fault of the system. At the same time, the recovery strategy of wind farm after removing the fault is designed, namely through the switch devices and converters action, the function of the transmission mode of the wind farm before the fault is realized. The design of the wind farm topology and the control strategy of fault protection and restoration are simulated by PSCAD/EMTDC.