Research On Control Strategy Of Three-phase VIENNA Rectifier In Stationary αβ Frame

Recently,the electric vehicles have been widely used for the advantages of high utilization and pollution-free process.The core of the electric vehicles is the on-board or offboard charging power supply.As for the high-power off-board chargers,AC/DC+DC/DC two-stage topology is commonly used,among which the three-phase three-level VIENNA rectifier has gained much attention due to its reduced switching devices,zero dead time and high reliability,and has been widely used in the field of high voltage and high power.During the operation of the three-phase VIENNA rectifier,the input current and output voltage will decline in quality under the circumstances of unbalanced supply voltage,high current harmonics and load variation,which threatens the stable and dynamic performance of the system.Considering the influence on VIENNA rectifier,an improved control strategy is established in stationary frame,including a double-loop and a balance loop,to adapt to different working conditions.The proposed output control strategy can regulate the DC voltage well even with changing load or no load.In order to adapt the grid unbalance and suppress the current harmonics,the research on input current control is carried out by paralleling extra resonant controllers or repetitive controller.The specific research contents are as follows.In Section Ⅱ,the principle and the average model in three frames of the VIENNA rectifier is given.The control strategy in stationary frame is preferred for simple and uncoupled structure,less transformations and it can suit unbalanced grid so that the VIENNA rectifier is controlled in stationary frame.The voltage control strategy is designed to satisfy various conditions,the PI controller is selected to regulate the voltage at the steady state.In consideration of changing load,piecewise PI parameters method is employed to meet the requirement of dynamic response and in consideration of no-load characteristic,BurstMode is used to deal with the overvoltage.In the balance loop,the midpoint voltage is balanced by changing the injected zero sequence component in CB-PWM.Finally,the proposed VIENNA rectifier voltage control strategy is verified in the experiments.In Section Ⅲ,in order to adapt to unbalanced grid and reduce the current harmonics,the input current of the VIENNA rectifier is controlled by proportional controller paralleling multiple resonant controllers in stationary frame.Firstly,the equivalence between the PR controller in stationary frame and the PI controller in rotation dq frame is derived.Secondly,the design rules of the PR controller are given according to the frequency response characteristics and the Bode diagram.So as to realize the digitization,the expressions of the quasi-resonant controller in discrete domain are studied using different discrete methods.After comprasion,different discrete methods are chosen depending on different application.Finally,the effectiveness of the proposed VIENNA rectifier current control strategy is verified by simulation and experiments.Compared with other methods,the proposed strategy can suit the unbalanced gird and suppress the harmonics well.In Section Ⅳ,to simplify the control structure,save program running time and obtain better harmonic suppression effect,a novel current control strategy in VIENNA rectifier is proposed based on PR controller paralleling repetitive controller.Firstly,the discrete model of VIENNA rectifier is established by the method of zero-order hold(ZOH).Secondly,the design criteria of each parameter in repetitive controller are studied.According to the stability condition,the upper bound of the gain in repetitive controller is derived and the stable interval is given,which is related to the leading number and the attenuation coefficient in repetitive controller.In addition,considering the influence of various sampling frequency and the changing inductance caused by the input current,the controller parameters should be modified and the range of the stable interval varies correspondingly.Finally,the effectiveness of the proposed VIENNA rectifier current control strategy is verified by simulation and experiments.Compared with traditional P+ RC,the proposed strategy shows better steady and dynamic performance while compared with multiple resonant controllers,the proposed strategy can reduce the current THD further.