Island Operation Control Strategy Of Offshore Wind Farm Under Typical Faults

2019 was a year of rapid growth of offshore wind power,and the new installed capacity of the global offshore wind power reached a record high of 5.2GW,of which more than 50% of projects under construction are in China.Offshore wind power will be one of the most important plays in China’s energy and power industry in the next few years.Large scale offshore wind farms rely on grid connected operation.Once a fault causes the interruption of external power transmission channels,wind farm will face the risk of becoming an isolated island of electricity.The offshore wind farm is at the end of the main power grid,and the load is far less than the power supply capacity when it becomes an isolated island of electricity,so it cannot operate normally.Dealing with islanding conditions by shutdown only,will waste energy,endanger equipment life and even lead to power oscillation and collapse of the main power grid.More reasonable island operation mode of offshore wind farms should be further studied.In this paper,the island condition of offshore wind farm caused by typical faults is analyzed,and the control strategy is designed to explore the island operation potential of wind turbines to solve the key technical matters of island operation of offshore wind farms.The main contents are as follows:(1)Firstly,Topology and simulation models of two widely used offshore wind turbines and offshore wind power collection systems,and two typical AC/DC offshore wind power grid connected systems.Based on PSCAD / EMTDC,the simulation model of offshore wind farm and its two grid connected systems is built,which lays the foundation for the follow-up research.(2)In the case of permanent faults of grid connected system,offshore wind farms will lose contact with the main power grid for a long time.However,in order to maintain the reliability of wind power generation equipment,it is still necessary to supply power to the offshore booster platform and auxiliary load of wind turbines.Therefore,the big island operation mode of offshore wind farms based on the cooperative operation of diesel generator and doubly fed wind turbines is proposed.On this basis,a black start scheme based on the offshore wind farm is proposed: the grid side converter of the wind turbine is used to compensate the reactive power required by the long-distance high-voltage submarine cable,and the power is sent to the auxiliary machine of the onshore fire motor group from offshore to help the coastal local power grid recover quickly after power failure.(3)Under the temporary blocking condition of FB-MMC-HVDC(Full Bridge Modular Multilevel Converter High Voltage Direct Current)system,the offshore wind farm will appear a temporary island state with a time of no more than 1 s.In order to ensure the safety and stability of the main network system,large-scale offshore wind farms should be avoided to shut down in a temporary island state.By analyzing the status of offshore wind farm and its grid connected system,a temporary Island fault crossing strategy for offshore wind farm is proposed: Changing the fault type to low-voltage by the grounding energy dissipation resistance and combining with the island low voltage control of the wind turbine,which allows the offshore wind farm quickly resume power supply with the unlocking of the converter station.