Research On High-gain Resonant Sending Station Converters For Offshore Wind Farms

Offshore wind power will be one of the key areas of the wind power development in the future.The traditional offshore wind power generation system has many shortcomings,which restrict the development of offshore wind power.However,the flexible DC transmission system(HVDC-flexible)has attracted extensive attention in recent years.Therefore,this paper proposes a multi-terminal HVDC-flexible system suitable for offshore wind farms,and focuses on the study of the sending end station converter(SESC).The specific contents are as follows:First,the topology of traditional HVDC-flexible system for offshore wind farms is described.In order to solve its problems such as inconvenient installation and maintenance,multiple stage converting equipment and complex AC network control,this paper proposes a novel multi-terminal HVDC-flexible transmission system for offshore wind farms,which composed of wind turbines and their converters,the SESC,DC grid and the receiving end station converter(RESC)onshore.Because this paper focuses on the SESC with isolation type topology,the research status of the traditional isolation type converters is analyzed in detail,including the characteristics and working principle of the topology of the traditional isolation type converters,as well as the advantages,disadvantages and applicable occasions of the common topologies.The full-bridge isolated topology and its extended topology are studied.On this basis,this paper proposes a novel topology of high-gain resonant SESC.Second,the topology of the proposed high-gain resonant SESC is analyzed in detail,and its working mechanism is studied by combing with the working waveform.Then,the system is modeled and analyzed according to the fundamental wave analysis(FHA)method,and the voltage gain characteristic function of the converter is derived from the model.The function curve is drawn by MATLAB software,and the input impedance,working area and soft switching conditions of the converter are studied and analyzed.According to its characteristics,the resonant parameters of the system are calculated,and the high frequency transformer in the converter is designed.The fast recovery diode,output capacitor and power switching devices are selected.Third,the equivalent circuit model method is used to establish the small signal model of the high-gain resonant SESC.Through the model,the transfer function of the system is obtained and its control-output characteristic curve is drawn by MATLAB software.In order to satisfy certain dynamic and static performance,a controller based on PI compensation is designed according to the deficiency of the control output characteristics of the system.The Bode diagram is used to analyze the characteristics of amplitude-frequency and phase-frequency of the system before and after compensation,and the control effects before and after the compensation are compared and analyzed in MATLAB.Finally,simulation and experimental verification are carried out based on the parameters calculated in the above chapters.The system is simulated and verified by MATLAB/Simulink platform,and the main voltage and current working waveforms of the system under different input voltages and loads are obtained by simulation.Finally,a prototype of 500 W high-gain resonant converter is developed,and an experimental platform is built.The entire converter is tested by d SPACE,and its performance is verified.The experimental results show that the proposed high-gain resonant SESC can achieve 8 times voltage gain,and the ripple of the output voltage is about 1%,which meets the design requirements.