Low-voltage-ride-through Of DFIG Wind Turbines Based On The State Feedback Linearization

With increasing penetration level of wind turbines (WT) generation into the grid, theinfluence of the operation of WTs to the grid is growing, the requirement of the Grid Codeabout the wind power in most countries is stricter, and it is more and more important toimprove the low-voltage-ride-through (LVRT) capability of WTs when the grid faultoccurs. Doubly fed induction generator (DFIG) is a popular WT system due to its higheconomical efficiency and favorable control performance. Meanwhile, as the stator in theDFIG WT connects to the grid directly, the capacity of the rotor excitation converter isrelatively small, the DFIG WT is very sensitive to grid faults. Consequently, conductingresearch into the LVRT method of the DFIG WT to improve its LVRT capability is ofimportant significance.In this paper, the mathematical models of the DFIG WT and its Back-to-backexcitation converter in static three-phase ABC coordinates and in rotary two-phase dqcoordinates are built. The operating principle and the conventional vector control strategyare analysed. Then a control strategy based on the state feedback linearization is designed,and a2-MW DFIG WT model is established in MATLAB/Simulink environment, thecomparison between the simulation of the state feedback linearization control and theconventional vector control is made. The simulation results indicate that the state feedbacklinearization control has better dynamic performance and robustness under the slightbalanced dip of the grid voltage.Then the transient characteristics of the DFIG WT under the balanced grid faultcondition are analysed, the LVRT strategies that should be adopted for different degrees offault cases have been put forth: while the grid voltage drops slightly, the DFIG WT canrealize uninterrupted operation by the state feedback linearization control; while the gridvoltage dips sharply, the DFIG WT can achieve LVRT by adding hardware protection ofthe rotor Crowbar and the DC bus Crowbar. The value range of the bypass resistor of therotor Crowbar, as well as the optimum switching time of the rotor Crowbar is mainlydeduced, simulation studies have been validated the effectiveness of the proposed control strategies.Finally, a10-kW DFIG WT based on dSPACE experimental prototype is established.Grid Side Converter experiment by both the conventional vector control and the statefeedback linearization control is completed. The experiment results show that the statefeedback linearization control is superior to the conventional vector control in bothsteady-state performance and dynamic performance, verifying the correctness of thetheory. Then Rotor Side Converter is debugged, and grid-connected operation of the DFIGWT is realized.