Research On Low Voltage Ride Through Technology Of Double-fed Wind Turbine

Our country has abundant wind energy resources, especially in the western region. With the construction of million kilowatts of wind power base, wind power industry has developed rapidly in recent years, and has entered an important period of rapid development. Currently, uninterrupted operation of wind turbines under the failing of power grid, particularly low voltage ride through, has become an important part of wind power technology, and has been included in the grid rule. In order to meet the further development of the new energy, the thesis studied the low voltage ride through technology of DFIG. The main contents are as follows:Firstly, the thesis analyzed the structure and variable speed constant frequency basic principle of DFIG, and established the mathematical model which including the mathematical model of three-phase stationary coordinate system and the two-phase synchronous rotating coordinate system. On this basis, the paper analyzed the control strategy, as well as the stator, rotor and DC side transient of DFIG. It laid the theoretical foundation for the follow-up work.Secondly, the thesis analyzed the existing low-voltage ride through technology of DFIG. For grid voltage drop, the thesis improved control strategy of DFIG, and then designed a Low Voltage Ride method of DFIG under the voltage dip situation. When the grid voltage drops slightly, the method can improve low-voltage ride-through capability by using improved control strategies, avoiding to add hardware protection circuit that has an impact on transient process. Then we build a simulation model in MATLAB, the simulation proved that the proposed control scheme could be effectively suppressed during the fault rotor overcurrent and DC link overvoltage under different voltage sag depth. It demonstrates the effectiveness of the proposed control scheme.Finally, the thesis analyzed the working principle and mathematical model of STATCOM. On the basis of this, the thesis studied the control strategy and the capacity of STATCOM selection problem and used STATCOM to improve the stability of the wind farm. When the grid voltage drops, STATCOM can conduct reactive power to grid and help wind turbine to achieve low voltage ride through. Meanwhile, the thesis analyzed the reactive power compensation capacity of STATCOM under different voltage sag depth. Simulation results show that when symmetrical grid failure occurs, the STATCOM can provide dynamic reactive power support to the grid, and inject some reactive current to enhance the PCC voltage, reduce the voltage drop depth of the wind farm to assist wind turbine generator to relize low-voltage ride through. But when voltage drops severely, reactive power compensation capacity of STATCOM will be affected.