Research On Start-up And Reactive Power Control For MMC-HVDC Connected To DFIG Wind Farm

With the development of wind power generation technology,the scale of wind farms is becoming larger and larger,but some problems also appear.Electricity generated by large offshore wind farms and wind farms in remote areas cannot be absorbed on site.How to connect large-scale wind farms with long-distance grid-connected transmission has always been a research hotspot.Modular Multilevel Converter(MMC)has become an ideal grid connected way for large-scale long-distance wind power transmission because of its low manufacturing difficulty,low harmonic content and decoupling of active and reactive power and so on.In addition,Doubly-Fed Induction Generator(DFIG)is the most widely used wind turbine due to its small size and low cost.In this paper,the start-up and reactive power control of DFIG wind farm through MMC-HVDC grid connected system are studied.The stability of DC capacitor voltage is a necessary prerequisite for the normal operation of MMC-HVDC.When MMC-HVDC is shut down due to failure,the voltage of the sub module drops to zero,and the capacitance of the sub module needs to be charged.In the process of charging,there will be impulse current and over-voltage.It is of great engineering value to propose a new method to suppress the impulse current.In addition,DFIG fans and MMC converter stations have the ability to provide reactive power.It is of great significance to study how to coordinate the distribution of reactive power between fans and MMC,so as to improve the stability of grid voltage.Firstly,this paper analyzes the working principle of DFIG fan which can realize variable speed constant frequency.Based on the two-phase rotating coordinate system,the mathematical model of DFIG is established.Also,Using direct current control method,the double closed-loop control strategies of Grid side converter and Rotor side converter of DFIG is designed.Then,starting from the topology of MMC,the working principle of MMC is introduced.And the control strategy of WFMMC and GSMMC for DFIG wind farm connected to grid by MMC-HVDC is studied,which lays the foundation for the following research on start-up and reactive power control.Secondly,this paper focuses on the application of DFIG wind farm connected to the grid through MMC-HVDC,and carries out the simulation research of MMC-HVDC no-load start on the PowerFactory/DIgSILENT simulation platform.This paper designs the whole process of starting.In order to reduce the inrush current during the transition of GSMMC from uncontrolled rectifier charging stage to controllable rectifier charging stage,a new slow charging control strategy is proposed.The parameter calculation method of the slow charging curve is given and verified by simulation.In addition,for a single DFIG fan,the strategy of grid connection based on grid voltage directional vector is designed and verified by simulation.Finally,the reactive power of DFIG fan stator and Grid side converter is analyzed,and the different effects of crowbar device on DFIG operation state are studied.It is analyzed that MMC can play the role of reactive power supply and support the reactive power of DFIG wind power generation system.In order to give full play to the reactive power capacity of GSC,RSC and MMC,and maintain the voltage stability of the parallel network,the reactive power compensation control strategy considering GSC/RSC/WFMMC is proposed.Based on the PowerFactory/DIgSILENT simulation platform,the modeling is studied.The results show that the proposed coordinated control strategy can make more reasonable use of the reactive power of DFIG and WFMMC,and effectively reduce the voltage fluctuation caused by the wind farm side fault.