Research On Control Strategy Of Stand-alone DFIG-based Wind Power System Supplying Nonideal Loads

Due to the challenges of energy crisis and environmental pollution, wind energy has been very popular in the whole world as a kind of renewable energy which has enormous environmental benefits and commercial potential. The world has seen an increase of installed capacity and market share of wind power year by year. Variable-speed constant-frequency (VSCF) wind power system has occupied the position of great importance in the area, with the development of wind power technology. A doubly fed induction generator (DFIG)-based VSCF system is one of the main commercial wind turbines due to its reduced converter size and consequent economic advantages.In the dissertation, mathematical models of DFIG and its stator-side converter (SSC) are established in the three-phase stator stationary reference frame and two-phase rotating reference frame at a synchronous speed. A classical vector control scheme is composed of dual closed loops, viz., DC-link voltage control and AC current control loops, for DFIG’s SSC, as well as the stand-alone vector control for rotor-side converter (RSC)—ndirect stator flux oriented control and direct voltage control. As for the grid-connected operation of the DFIG system in the micro-grid, the possibility of DFIG connecting to the grid with droop-control has been verified through simulation and experiment.This dissertation presents the control strategy of a stand-alone DFIG system with unbalanced and nonlinear loads. Under these load conditions, the quality of the stator voltage and current waveforms of the DFIG is strongly affected due to the negative and other harmonic components, which will lead to reduction of the generator’s efficiency or even damage it due to over current, torque and power oscillations. In the meantime, the deteriorated voltage of PCC will reduce the performance of other normal loads connected to it. In order to solve this problem, the traditional compensation method based on proportional integral controllers has been investigated. The improved compensation method in this dissertation is based on a proportional integral with a resonant controller applied in both rotor-side converter and stator-side converter. These current controllers are controlled in the positive synchronous reference frame so that the rotor and stator currents are directly regulated without decomposing sequential components. Under nonlinear load condition, a pair of harmonics can be controlled by a single PIR controller. The control system is simplified through this method. To verify the effectiveness of the proposed control strategy, simulation and experiment results have been presented and discussed in the dissertation.Finally, a brief introduction of the11kW DFIG experiment system has been made at the end of the dissertation. In order to achieve the goal of communication with the micro-grid central control system, a communication system based on CAN protocol has been designed. Man-machine Interaction Interface has been realized with a touching screen and the design of communication program based on Modbus protocol.