Design Of Offshore Wind Power Gathering System And Control Strategy Of Multi-Terminal VSC-HVDC Transmission

Offshore wind power using VSC-HVDC transmission has become the preferred option due to its advantages of longer transmission distance,control flexibility and easy expansion into multi-terminal systems.In view of this trend,this paper focuses on the design of offshore wind power collection system and the study of multi-terminal flexible control strategy.Firstly,the economy of large-scale offshore wind power collection system is studied,then the grid-connected control strategy of the system is deeply studied,and the frequency of the system caused by the VSC-HVDC transmission of offshore wind power is studied.The main research work is as follows:(1)Based on the characteristics of the internal electrical collection system of offshore wind farm,a mathematical model reflecting the economic influencing factors is established.Aiming at the lowest investment cost of the system,improved FCM algorithm and combined it with the prim algorithm,and considering the constraints of cable loss factors,formulated a topological optimization scheme of aggregate system with good economy and less cable line loss,and carried out an example analysis.(2)The mathematical model of offshore wind power VSC-HVDC transmission is established and control strategy is designed.Firstly,the mathematical models of VSC and wind turbine are established.According to the models,corresponding control strategies are put forward,including the control strategy of wind turbine part,fan grid connection and WFVSC and GSVSC for offshore wind power VSC-HVDC transmission are discussed and the effectiveness of control strategies of fan is verified by simulation.(3)Based on the control strategy of offshore wind power two-terminal flexible transmission,the operation characteristics and advantages of traditional control strategies for multi-terminal are analyzed,including master-slave control,DC voltage deviation control and voltage drop control and simulation verification.On this basis,the limitation of fixed sag factor in voltage drop control is improved,and a flexible control factor is obtained by combining fixed sag factor with logistic function.then,aiming at the frequency instability caused by the low inertia characteristics when the offshore wind power flexible direct current transmission is merged into the weak AC network,a multi-terminal control strategy for offshore wind power with improved frequency stability is proposed,and its effectiveness under different operating conditions is verified by simulation.