Research On Control Strategy Of Doubly-fed Wind Power Generation Converter System Under Unbalanced Grid Voltage

With the increased presence of doubly-fed induction generator (DFIG) based wind turbine in the electric power system, modern grid code now have criterion specific to grid-connected wind turbines. Researches on the control and operation of DFIG wind generation under ideal power grid condition are now well developed. More and more researchers focus on the control and operation of DFIG under unbalanced network condition. This dissertation investigates the control and operation of DFIG under unbalanced grid voltage and mainly carried out the mathematical model and the control strategy of machine side converter (MSC) and grid side converter (GSC). Simulation results were given to validate the feasibility of the control strategies.Firstly, the operation of DFIG wind generation has been analyzed. The dynamic mathematical models of DFIG and GSC under balanced grid voltage were built. On the basis of the mathematical models, the dissertation investigates the stator flux oriented traditional vector control of MSC and the principle of SVPWM. In the meantime, the grid voltage oriented traditional vector control of GSC is also analyzed.The influence of negative sequence grid voltage on the DFIG wind power generation system was investigated. The symmetrical component method is employed to analyze the vectors under unbalanced grid voltage. A dynamic model of DFIG under unbalanced grid voltage was built to conclude the positive and negative dq sequence (dual-dq) rotor current control strategy. According to the control strategy and operation requirements of DFIG, four control targets are shown in the thesis as well as the current director calculation.The effects of voltage unbalance on GSC are investigated, the mathematical model of grid side converter under unbalanced grid voltage was proposed to concluded the dual-dq current control strategy, the current director calculation were also shown respectively. To provide enhanced control, the GSC is controlled to eliminate the active power pulsation output from the overall DFIG wind generation system while the RSC is controlled to ensure constant torque. As for a special unbalanced network condition which means single phase power failure in power system, the dissertation also developed proportional resonant control strategy to ensure the converter ride through the failure.This dissertation adopts MATLAB to simulate the operation of DFIG wind generation using different control strategies. The response of DFIG using traditional vector under balanced and unbalanced grid voltage was investigated. Simulations results of DFIG using dual-dq current control under unbalanced grid confirmed the effectiveness of dual-dq current control and coordinated control. The operation of grid side converter under single phase power failure also was investigated through simulation, simulation results proved the effectiveness of the control strategy.