Research On Reactive Power Control And Voltage Stability Of Grid Connected Wind Farm

In face of the increasing depletion of traditional fossil energy and theenvironmental pollution caused by energy utilization, energy and environmentbecome an urgent problem of social development. Wind energy is widely usedbecause its advantages such as large storage amount, clean, renewable, etc. Andwind power generation technology has been got rapid development.Large-scale wind farms appear constantly. Wind farm connected to the powergrid poses a significant challenge, in which reactive power control and voltagestability problems get great concern and become an urgent need to solve, to thesafe and stable operation of the grid.The new wind farms in china mostly use variable speed constantfrequency (VSCF) doubly-fed induction generator (DFIG) wind turbine, soDFIG has become the mainstream wind turbine type. Against the voltagestability problems caused by the DFIG wind farm is connected to power grid,the reactive power control and voltage stability of wind farm is researched inthis paper.Firstly, in this paper, the wind speed models and aerodynamiccharacteristics of the wind turbine is analyzed, the mathematical model ofDFIG is introduced, the power transfer characteristics of DFIG is discussed, theDFIG reactive power limits is calculated according to the mathematical modeland the power transmission characteristics, and the DFIG operation control areaand operation control mode is analyzed.Secondly, in this paper, the impact of wake effect of wind turbines in thewind farm is analyzed, a calculation method of dynamic equivalent of flat areawind farm, which considers that the power loss and voltage drop is same asbefore equivalent, is proposed, and the research methods of voltage stability ofgrid-connected wind farms is discussed. Through the simulation study of awind farm, the weak bus of wind farm is got through the static voltage stabilityanalysis, and the impact of DFIG operation mode of constant voltage andconstant power factor for transient voltage is analyzed and discussed.As the output active power of wind farm has randomness and volatilitywith wind speed variation，the issued or absorbed reactive power will fluctuatewith the issued active power, which leads to the voltage fluctuation of weakpower grid. Again, in this paper, the relationship between reactive power andvoltage and the reactive power consumption of each component of wind farm is analyzed, the characteristics of each reactive and voltage regulation equipmentin wind farm is discussed. Then, a strategy of two-stage voltage and reactivepower coordinated control, which utilizes the fast and continuous reactivepower regulation capability of static synchronous compensator (STATCOM)and the dynamic reactive power generation capability of DFIG, is proposed inorder to achieve dynamic reactive power compensation, thereby stabilizesystem node voltages.Finally, example analysis demonstrates the effectiveness of the reactivepower and voltage control strategy and proves that the strategy can suppressvoltage fluctuations caused by wind and has a certain voltage support capabilitywhen the system voltage drops.