Research On Control Strategy Of Doubly-Fed Wind Power Generator Under Unbalanced Grid Voltage

In the wind power generation system,the doubly fed induction generator(DFIG)has become the mainstream model for the development and utilization of wind energy.Because it has the characteristics that the active power and the reactive power can be independently adjusted,and the variable speed constant frequency operation capability for maximum wind energy tracking can be performed,and the capacity of the excitation converter is much smaller than the capacity of the wind power system.Our country's wind power development adopts a highly centralized model,and wind farms are mostly located at the end of the grid.There are various steady-state,transient,symmetrical,and asymmetrical circuit faults and various power harmonics in long-distance transmission lines,making wind turbines in a harsh working environment.The wind power system operation control strategy based on the ideal grid environment design cannot meet the grid-connected and high-performance operation requirements under the actual grid conditions,so it directly affects the operational stability of the grid and the power quality.In order to make full use of wind energy and ensure the safe and stable operation of the power system,the grid codes of various countries in the world put forward strict requirements on the operational reliability of wind turbines,mainly reflecting two aspects: first,the wind turbine can effectively resist all kinds of grid disturbances and keep it from running off-grid;the second is to improve the fault ride-through capability of the wind power system under transient voltage sudden changes and the support capability for faulty power grid recovery.When the grid voltage is unbalanced,even a small unbalanced voltage will cause a high degree of asymmetry of the stator and rotor currents of the motor.The asymmetrical current will cause uneven heating of the stator and rotor windings of the motor,electromagnetic torque ripple,power output to the grid,including harmonic components,and other adverse effects.And with the increasing capacity of the wind turbine assembly machine,when the wind turbine reaches a certain scale,the wind turbine that does not have the self-regulating control capability under the unbalanced voltage will cause the instability of the power grid,and even eventually be disassembled from the power grid.It will seriously threaten the safe and smooth operation of the power system.The development of power conversion technology using power electronic devices provides a hardware foundation for the development of wind power generation systems from constant speed constant frequency operation to variable speed constant frequency operation.In the process of converting wind energy into electric energy,the performance of the generator and its control system not only affects the performance,efficiency and power quality of the whole system,but also affects the conversion efficiency of wind energy to mechanical energy and its structure and operation mode.For the doubly-fed induction motor variable speed wind power generation system,the control is realized by the AC excitation converter,which must have the ability of energy bidirectional flow,good input and output characteristics,and sufficient reactive power.The doubly-fed asynchronous wind turbine uses two back-to-back,two-level voltage-type PWM converters connected by DC link for AC excitation to achieve variable-speed constant-frequency operation and maximum wind energy tracking.PWM converter mainly has power bidirectional flow,input current sinusoidal and less harmonic content,adjustable power factor,adjustable DC link voltage,good stability against load disturbance,and can effectively reduce the capacity of DC link energy storage capacitor.Therefore,the operation control of DFIG is mainly the control of the excitation converter.The operating state of the dual PWM power converter is controllable,and both can be switched between rectified and inverted states,thereby achieving bidirectional flow of power.When the DFIG is in the sub-synchronous state,the grid-side converter operates in a rectified state,the machine-side converter operates in an inverting state,and energy flows from the grid to the motor;When the DFIG is operating in the super-synchronous state,the grid-side converter operates in an inverter state,the machine-side converter operates in a rectified state,and energy flows from the motor to the grid.The switching of the operating states of the two converters is determined by the operating region of the doubly-fed induction machine.And the power converter adopts a high frequency self-shutdown device and a space vector PWM modulation method,which can eliminate low-order harmonics and has good input and output characteristics.It has less influence on the grid and doubly-fed induction motors,and can meet the excitation requirements of DFIG in harmonic characteristics.Among them,the grid-side converter mainly realizes the functions of DC bus voltage stability,input current sine balance and power factor stability.The motor-side converter mainly realizes the functions of controlling the active power and reactive power of the DFIG output.For this purpose,effective control and adjustment of the output current of the grid-side converter and the motor-side converter must be realized.The vector control method selects the appropriate phase angle as the basis of the coordinate transformation,realizes the decoupling of the AC motor torque and flux linkage,and thus the independent decoupling control of the torque component and the excitation component,and obtains excellent dynamic characteristics.The symmetrical component method shows that the unbalanced grid voltage can be decomposed into symmetric positive sequence components and negative sequence components,and the positive sequence components appear as DC quantities in the two-phase synchronous rotating coordinate system.The negative sequence component appears as the AC component of the double grid frequency in the two-phase synchronous rotating coordinate system.The traditional proportional-integral PI vector control can only adjust the DC quantity and cannot control the AC quantity,so it can only be used in the ideal grid environment.The resonant controller has the characteristics of large enough amplitude gain for the target frequency signal and fast attenuation of the amplitude of other frequency signals.It is especially suitable for suppressing the fluctuation component with typical oscillation frequency,and has been widely used in the converter technology.This thesis proposes to add a resonant link structure proportional integral resonance PIR controller based on the proportional integral controller.The proportional integral link is used to control the DC component and the resonant link is used to control the AC component,which can effectively control the doubly-fed wind turbine under unbalanced grid voltage.In this thesis,the mathematical model of the grid side and motor side converters in the ideal grid voltage environment is established firstly,and then the mathematical model of the grid side and motor side converters in the unbalanced grid voltage environment is established.It includes the model in the three-phase stationary coordinate system and the model in the two-phase synchronous rotating coordinate system,and derives the instantaneous power model of the converter.On this basis,the control relationship between voltage,current and power is analyzed,and the control targets of grid-side converter and motor-side converter are determined,and the command current for achieving each control target is designed.Based on this mathematical model,a vector control strategy based on proportional second-order generalized integral resonant controller is designed.The grid-side converter and the motor-side converter are controlled separately to analyze their performance in the three-phase grid voltage imbalance environment.Finally,a doubly-fed wind power generation system with a rated capacity of 2MW was built in the MATLAB/Simulink simulation platform.The traditional proportional integral vector control strategy and the vector control strategy based on the proportional second-order generalized integral resonant controller were used to simulate the experiment.The results of comparative analysis prove the effectiveness of the proposed control strategy.