Control Strategy Research And Stability Analysis Of A Three-phase VIENNA Rectifier

With the rapid development of Internet Data Centers and Electric Vehicles,the construction of safe and reliable DC power supply system provides a strong guarantee for the research work in this field.Currently,the mainstream of DC power supply system for data center generally adopts the two-stage cascade structure of three-phase VIENNA rectifier and DC/DC converter.The safe and reliable operation of DC power supply system mainly depended by the performance of the three-phase VIENNA rectifier.This thesis takes the three-phase VIENNA rectifier as the research object,and focuses on the current inner loop control strategy operated in the mixed mode,the current finite set model predictive control strategy(FCS-MPC)and the AC cascade small signal stability analysis between the three-phase VIENNA rectifier with the weak grid.A current inner loop control strategy based on the duty ratio feedforward and quasi-proportional resonance control(QPR)is proposed,and the optimal parameters design method of the control loop is given,which can effectively improve the quality of grid-current waveform of the VIENNA rectifier when operated in the mixed mode.With the decrease of output power and filter inductance,the grid-current may transfor from the continuous operation mode to the intermittent operation mode,which resulting in the distortion of the grid-current.To solve this problem,firstly,the small signal mathematical of the VIENNA rectifier operated in the continuous and intermittent operation mode is established,respectively.According to the principle of unity power factor,an ideal duty cycle feedforward link is introducedl to form a composite current inner loop control strategy.The feedforward channel generates the basic control amount rapidly,and the QPR controller is used to generate the control increment to correct the control error,which effectively improves the dynamic response performance of the VIENNA rectifier and the quality of the steady-state response.An improved FCS-MPC strategy is proposed for the VIENNA rectifier to realize the multi-objective optimization and improve the quality of grid-current waveform.By analyzing the internal relation between the switching states with the neutral-point-potential fluctuation,the neutral-point-potentia fluctuations is directly calculated by the predicted value of the load current and the grid-current.And the multi-objective tracking characteristic of MPC control variable is adopted to realize the balanced control of the neutral-point-potential.In addition,in view to the problem of the grid-current distortion caused by the control delay,a two-beat delay compensation algorithm is adopted to improve the quality of the grid-currentIn view to the disadvantage of only one voltage vector is used in each sampling interval,and the output voltage and current harmonics spread in a wide range of frequency due to the variable switching frequency of the FCS-MPC.An improved fixed operation frequency with the cost function-based modulation scheme(CFM-MPC)is proposed for a three-phase VIENNA rectifier.This method combines the advantages of model predictive optimal control and SVPWM modulation.According to the value of the cost function,three optimized voltage vectors are selected,and the switching action time of the selected are directly calculated by the cost function value of the selected vectors.The method has the advantages of multi-objective optimization,fast response and good quality of the grid-current waveformAn equivalent criterion for small signal stability analysis of the three-phase AC cascade system is presented,and the stability between the three-phase VIENNA rectifier with weak grid is improved by introducing a parallel virtual impedance at the input of the rectifier.The reduced order small signal model of the VIENNA rectifier for the stability analysis of the three-phase AC cascade system is built,and the stability influence factors of the cascade system between VIENNA rectifier with the weak grid is analyzed.The results obtained show that the impedance of d axis can be used to simplify the stability analysis of three-phase AC system,and the feasibility of improving the stability of AC cascade system by introducing parallel virtual impedance is studiedTheoretical analysis,simulation optimization and experimental verification are adopted in this thesis.The mathematical model for the design of the control system and impedance model for the stability analysis are established,and the corresponding simulation model and physical experiment platform of the three-phase VIENNA rectifier system are constructed.Simulation and experimental results are used to verify the effectiveness of the proposed control strategy and the stability analysis.Meanwhile,the comparison under different working conditions are given to verify the superiority of the control strategy proposed in this thesis.The results obtained in this thesis can provide theoretical reference and technical support for improving the grid-current and the neutral-point-potential balance of the three-phase VIENNA rectifier.