Resaerch On Control Method For Frt Of HVDC Systems Connected Dfig Wind Farms

With the development and maturity of wind power technology,the installed capacity of wind power and its application in power system are increasing year by year.With its excellent technical performance and application value,modular multi-level converter has a certain advantage in the long-distance transmission direction of large-scale wind power system.In order to improve the stability and reliability of MMC-HVDC grid-connected system for DFIG wind farm,it is necessary to study and analyze the dynamic characteristics and corresponding control strategies of the system under different types of grid faults,which has significant engineering application value.Firstly,this paper establishes the DFIG mathematical model and the control strategy of the rotor side network converter,and analyses the MMC-HVDC main circuit model and the control strategy of the two sides converter station.The electromagnetic transient model of MMC-HVDC system of doubly-fed wind farm is built on PSCAD/EMTDC simulation platform.The reliability of the model is verified by simulation analysis of joint start-up,grid connection and wind speed disturbance.The fault traversing process of MMC-HVDC system in doubly-fed wind farm is analyzed in three stages after the fault occurs.Aiming at the problem of MMC DC voltage oscillation caused by DC component of DFIG stator current when the frequency changes,an improved frequency-up control scheme is proposed,and an auxiliary load-reduction control scheme is designed and cooperated with it in the wind turbine controller.The simulation results show that the proposed scheme can significantly suppress the oscillation of MMC DC voltage and AC frequency of wind farm,and significantly improve the FRT capability of the system.Based on the improved control strategy of frequency-up and auxiliary load-down,an asymmetric fault simulation model of MMC-HVDC system in DFIG wind farm is established.According to different control objectives,the active power and negative sequence current fluctuations can be suppressed by setting the current instruction value of the outer loop of the converter station.In addition,a DC voltage fluctuation controller is designed for the system with high demand for DC voltage and active power transmission,which effectively weakens the impact on MMC converter station and wind farm on the other side of DC cable.Two kinds of conventional asymmetric faults are set up to verify the effectiveness of the proposed control scheme.The unified modeling platform is realized based on GUI component layout and GUI component programming function of MATLAB graphical user interface.In the symmetrical fault interface window of the receiving end,the simulation and comparison of four control strategies,namely,traditional frequency-up method,improved frequency-up method,only frequency-up method and frequency-up-assisted load-down method,can be realized under different voltage sags.The control effects of different control targets can be viewed in the receiving end asymmetrical interface window.The platform is based on the graphical user interface to realize human-computer interaction.The designed operation platform is practical and easy to implement,and can be used in engineering applications.