Research On Flexible DC Transmission Technology For Offshore Wind Farms

Since the 21 st century,under the dual challenges of energy shortage and climate change,it has become an inevitable trend to vigorously develop renewable energy.Wind energy is inexhaustible and is a globally recognized renewable energy source.With the increasing maturity of offshore wind power generation technology and increasing installed capacity,flexible DC transmission at offshore wind farms has drawn great attention from domestic and foreign scholars.However,the traditional DFIG(Double-Fed Induction Generator)wind power converter system can not meet the needs of the development of offshore wind power DC transmission,therefore,this paper proposes a new type of DFIG converter system for offshore wind farm flexible DC transmission.Based on the new DFIG converter system of offshore wind farms,this paper mainly studies and designs the converter of the terminal station in this system,The main contents include:First,the status quo of the converter of the terminal station in wind power generation system is studied.The types of power topology,modulation methods,control strategies and harmonic suppression method commonly used in the converter of the terminal station system are given.This paper briefly introduces the composition and characteristics of a new offshore wind farm flexible DC transmission system.Second,the design of 5kW converter is carried out.The mathematic model,the analysis of control strategy,the construction and simulation analysis of the simulation platform,the design of each hardware module,the development and design of the software code and the build of hardware platform were achieved in sequence,and carried out experimental research on this basis,completed off-grid and grid-connected debugging of the converter of the terminal station,and verified the feasibility and correctness of the control strategy.Among them,the sampling scheme in the converter system is a highlight of this article.The sampling scheme is based on the DB bus communication scheme of the external ADC7606 conversion chip.On the one hand,the scheme improves the sampling accuracy of the system,on the other hand,the scheme greatly simplifies the signal conditioning circuit hardware design complexity.Then,for the above three-phase unbalanced of AC-side current and high THD(Total Harmonic Distortion)of grid-connected current during the commissioning of the converter,the converter system performance is optimized.Through many improvements in hardware and software,Coupled with a large number of comparative analysis of experiments,the two problems of the AC side three-phase imbalance and the high grid-connected current THD weresuccessfully solved.Subsequently,in order to further analyze the power quality of the AC output side of the converter system of the terminal station,the DFT algorithm based on the discrete Fourier transform was used to detect the every harmonic of AC current,the overview of discrete Fourier transform DFT algorithm,the simulation analysis based on the DFT algorithm of AC current harmonic detection and the test verification based on the DFT algorithm of AC current harmonic detection were carried out in sequence,among them,in the 5kW bidirectional converter system,the effect of AC current Harmonic detection based on DFT algorithm is equal to the consequence based on the quality analyzer device! at the meanwhile,this paper improves the 3_Phase_PLL technology provided by TI's official website,and proposes a new 3_Phase_PLL technology based on the DFT algorithm of discrete Fourier transform to improve the electric energy quality of the AC side output of the converter,reduce grid-connected current's THD,and restrain the harmonic pollution of the grid!Finally,to study the complete HVDC DFIG converter system for offshore wind farms,the DFIG stator-side and rotor-side converter experimental platform based on dSPACE was built.Specifically,the hardware design,welding and debugging of the system were finished in sequence,and finally set up a DFIG stator-side and rotor-side converter based on dSPACE experimental platform.