Investigation On Control Strategies Of DFIG Connected To DC Grid Based On Uncontrolled Rectifier On Stator Side

With the rapid development of direct current(DC)transmission and DC microgrid,the research on DC grid-connected operation and control strategy of renewable energy generation equipment has received more and more attention.Wind power systems based on doubly-fed induction generator(DFIG)account for a large proportion of renewable energy generation.Therefore,it is of great significance to study the DC grid-connected topology and control strategy of DFIG In the multiple DC grid-connected structures of DFIG,the DC-connected system of DFIG based on stator-side uncontrolled rectifier has gained extensive attention and research due to its small structure and small converter capacity.However,the traditional control strategy based on the alternating current(AC)grid voltage phase-locked loop cannot be applicable because the stator end of the DFIG is not directly connected to the AC grid.Furthermore,the stator voltage is seriously distorted due to the stator side uncontrolled rectifier and harmonic currents and torque ripple are generated,how to achieve power decoupling control,stator frequency independent control,reduce torque ripple and harmonic current are the research problems.Therefore,it is very urgent and important to study the high-performance control strategy of the DC gird connected operation of DFIG under the uncontrolled rectifier on the stator side.In this thesis,the DFIG connected to DC grid with uncontrolled rectifier on the stator side is taken as the research object,and the problems of power decoupling and stator frequency control,torque ripple suppression and stator-rotor harmonic current suppression of DFIG are studied.The following innovations are obtained:1?Aiming at the problem of power decoupling and stator frequency control of DFIG connected to uncontrolled rectifier,a closed-loop control method of the stator flux angle based on the second-order generalized integrator(SOGI)is proposed to realize the stator flux linkage orientation and stator frequency control at the same time.Firstly,the indirect stator frequency control method based on controlling the stator flux linkage amplitude is applied,the stator and rotor current model is used to observe the stator flux linkage in order to avoid the pure integral link in the voltage model.Based on this,a stator flux orientation control and stator frequency control strategy based on phase locked loop of stator flux is proposed to reduce the influence of power variation on stator frequency control.In order to avoid the dependence on the mutual inductance and stator inductance parameters of DFIG,a closed-loop control method of the stator flux linkage angle based on SOGI is proposed,which can achieve the stator frequency control and stator flux linkage orientation control at the same time.Finally,the robustness and steady-state performance of various stator frequency control strategies are systematically compared,and the optimal stator frequency control method for DFIG connected to dc grid through uncontrolled rectifiers on the stator side is obtained.2?In order to reduce the torque ripple caused by the stator voltage harmonics of DFIG connected to the uncontrolled rectifier,a direct resonant control method based on torque error for torque ripple suppression is proposed.When the stator voltage distortion of DFIG is low,the 5th and 7th harmonics are mainly considered,and the resonant controller with single resonance frequency is used to achieve the torque ripple suppression.When the stator voltage distortion is high,multiple harmonics need to be considered,and an improved repetitive controller capable of processing fractional delay and DC component suppression is proposed to achieve torque ripple suppression.Finally,considering the variation characteristics of the stator frequency when improving the operating efficiency of the system,a frequency-locked loop is proposed to quickly obtain the stator frequency of the DFIG Based on the acquired stator frequency,an adaptive repetitive controller is designed to suppress torque ripple to ensure torque ripple rejection with the wide range of stator frequency variations.3?In order to reduce the harmonic current loss caused by the stator harmonic voltage,a method of reducing the stator and rotor harmonic current and suppressing the torque ripple based on the two-axis direct resonant control is proposed,which can simultaneously suppress the torque ripple and reduce the stator and rotor harmonic current.Firstly,the fundamental and harmonic equivalent circuit of the DFIG are established.Based on the analysis of the relationship between the harmonic currents of DFIG and the harmonic voltages of the stator and rotor,the sinusoidal operation of the stator current is realized by injecting the appropriate harmonic voltage on the rotor side,and the stator and rotor harmonic current loss are both reduced.On this basis,the operating mechanism of direct resonant control based on torque error for suppression torque ripple is analyzed in depth.An improved dual-axis direct resonant control strategy is proposed to reduce the loss of the stator and rotor harmonic currents while suppressing the torque ripple,in order to improve the system operation efficiency.