Research Of Small Signal Interaction Between The DFIG Based Wind Farm And The LCC-HVDC

Due to the inverse distribution of wind power resources and load centers mostly, the abundant wind power in remote areas requires to be largely transmitted along longdistance.The traditional High Voltage Direct Current(LCC-HVDC) links have obvious advantages than the AC lines when transmission distance reaches a certain length. The problem of shaft torsional vibration may be caused when Steam turbines are in close connection with the LCC-HVDC. Whether the same problem exists and whether dynamic interaction may be caused between the control systems are worthy of further studying when the double-fed induction wind turbines(DFIG) communicate with the LCC-HVDC. In order to solve the problem, the dynamic interaction of small signal between the DFIG based wind farm and the LCC-HVDC has been studied by the modal analysis method with the interacted system of the DFIG based wind farm, the LCC-HVDC and the grid as the research object.Firstly, mathematical models of the DFIG based wind farm connecting into the grid and the LCC-HVDC connecting into the grid are built, which are set up in MATLAB / SIMULINK. The validity of the model has been verified through the contrast of simulation results with the demos. Secondly, a mathematical model and small signal model of the system is set up by connecting the DFIG based wind farm, the LCC-HVDC and the equivalent grid. Finally, the various modes and the main factors affecting the existence of various modes of the system are studied by the eigenvalue analytical method and participation factor analytical method in modal analysis method. And, how the controller parameters, the strength of the grid and other factors affect each mode of the system has also been studied by the root locus method, while the interaction between the DFIG based wind farm and the LCC-HVDC has been focused on the analysis. The results show that the current control loop of HVDC has great influence on the DC voltage, the terminal voltage and phase-locked loop(PLL) of DFIG. The PLLof HVDC also affects the PLL and DC voltage loop of DFIG. The PLL of DFIG has certain influence on the PLL and the current control loop of HVDC. The weaker the grid strength is, the more intense the interactions are, and so on.