DC-link Voltage Fluctuation And Low Voltage Ride Through Control Of Direct-drive Electrically Excitation Wind Generator

World energy security problem has become increasingly prominent, ecological environment deteriorate also seriously. In this case, wind power as one kinds of green renewable energy, which is an effective way to solve the energy problem, has attracted people’s attention daily. Direct-drive electric excitation synchronous wind generator(D-EESWG) is a new trend in all kinds of wind generator. It’s been followed of simply reliable driving system, unnecessarily expensive permanent magnetic materials, high power generating efficiency and so on. Based on D-EESWG, this thesis research control strategy of restraining DC-link voltage fluctuation, and discuss low voltage ride through(LVRT) under grid voltage dip. The main content is as follows:Firstly, According to character of grid-connected D-EESWG, this thesis mainly introduce the working principle and mathematical models of wind turbine, drive system, electric excitation synchronous generator, rectifier and inverter, which are a foundation for follow-up study.Secondly, new control strategy of restraining DC-link voltage fluctuation is proposed. the grid-side converter based on traditional double closed loop control strategy will not timely change output power when the wind speed changes, which resulting in excessively fluctuation of DC-link voltage is not conducive to stable operation of the power electronic devices and the wind generator. So in this thesis, new strategy of restraining DC-link voltage fluctuation considered DC-voltage deviation information, which is proposed by analysis of DC-link voltage fluctuation mechanism. Grid-side converter based on MPPT will timely change the output power under the effect of information of DC-link voltage deviation, and steady DC-link voltage. The simulation results show that the proposed control strategy can effectively restrain DC-link voltage fluctuations when the wind speed changes.Finally, LVRT control strategy using stored energy in wind generator inertia for D-EESWG is designed. In order to improve LVRT ability of D-EESWG which use uncontrolled rectifier- controlled inverter to connect with grid, this thesis detailedly analyze traditional LVRT control strategy based on DC-link unloading circuit, and then design LVRT control strategy based on inertial energy storage. Under grid voltage dip, the control strategy limit the generator electromagnetic power, which makes most of imbalance power stored by the generator rotor; Also according to the depth of grid voltage dip, grid-side converter provide a certain amount of reactive power to support grid restoration. Simulation analysis using the proposed control strategy and traditional LVRT control strategy based on the unloading circuit in two different depth of grid voltage dip show that the proposed control strategy can effectively improve working performance of D-EESWG under grid voltage dip.