Research On Vector Control Technology Of DFIG Used In Wind Energy Generation System

With the situation of energy source exhaustion and environmental pollution, the wind energy generation is gaining more and more attention around the world. The variable speed constant frequency (VSCF) wind energy generation system with doubly fed induction generator (DFIG) not only realizes flexible connection of mechanical-electric system, enhances wind-electric energy conversion efficiency, but also possesses the characteristics of small cubage, light weight because the transducer as AC excitation of DFIG only deals with slip power in double direction. So the research and application of the DFIG progress at a rapid rate in wind power generating application. VSCF wind energy generation system with DFIG and its control technology is studied in this paper. The primary contents and original contributions of this dissertation are as follows:The steady state mathematical model and its equivalent-circuit model of DFIG are established, and the interrelations of key physical parameters such as power, rotor current and slip are discussed. For this basis,issues about operational characteristics,power floating and static operation performance are qualitative analyzed. It also discusses the influence of winding translation and frequency translation on rotor voltage, rotor current, active power and reactive power, and indicates essence of that the transducer capacity is slip ratio of generator capacity.Mathematical models in the synchronous rotary coordinate of the grid-connection, stand-alone and no load DFIG are established respectively. The excitation vector control strategies based on stator-flux orientation of grid-connection, stand-alone and no load DFIG are also put forward. The simulation models of DFIG on three states on MATLAB/Simulink environments are established. The simulation results verify the DFIG vector control strategy, which realizes good static and dynamic performance and obtains decoupling control of active power and reactive power.The reference value of DFIG vector control system is generally the active power, the electromagnetic torque, or the rotor speed. The former two may be termed 'current-mode control', and the last may be termed 'speed-mode control'. It presents that the system dynamic characteristics under speed-mode control are superior to the dynamic characteristics under current-mode control and brings forward the state space explanation that the speed-mode control can control the wind turbine speed which indicates the wind energy that the system achieves. It designs the gains of two typical PI controllers in the current inner loop and the speed outer loop under speed-mode control. Simulation results in MATLAB/Simulink are brought forward to verify the good static and dynamic performance of the designed closed-loop control system.The effect of unbalanced grid voltage on wind energy generation system is studied, and the essential reason is analyzed that DFIG traditional excitation vector control system which built in the synchronous rotary coordinate can not effectively control the negative sequence component of rotor current.By orienting the positive and negative components of stator flux at the same time, it deduces mathematical model of grid connection DFIG under the positive and negative rotary coordinates for the first time, and puts forward DFIG excitation vector control strategy suitable for the condition of unbalanced grid voltage. It classifies the control targets of negative control system into four kinds, and draws significative conclusions by analyze the simulation results that it can minimize oscillations in either active power, or electromagnetic torque, or stator or rotor currents with the proposed control strategy.At last, it designs and sets up an experimental set of small capacity. Based on the stator flux-oriented vector control, this paper puts forward the excitation vector control strategies for the grid-connected, stand-alone and no-load states of DFIG respectively. It also presents the difference between the no-load cutting-in control and direct cutting-in control. The experiment investigation was made to further verify the effect of the direct cutting-in strategy and excitation vector control strategy of grid-connected DFIG. The results offer a good review of cutting-in process and the dynamic and static performance of the grid-connected DFIG system.It designs and implements a laboratory platform of VSCF wind energy generation system with DFIG which main part is a set of dual PWM voltage-source converters that includes the rotor-side converter and grid-side converter. A number of experiment items are performed on this platform, which including the grid-side converter control, the DFIG output active and reactive power decoupled control at the super synchronous, synchronous and sub synchronous speed, cut in control at DFIG rotor side. The experimental results testify the validity of the theory and the control strategy proposed in this dissertation.