Research Of Control Strategy For Three-phase Three-level VIENNA Rectifier

As an important interface for power grid and power electronic device,three-phase rectifiers require good operation characteristics,which can improve the performance of power electronic device and the quality of power grid.Therefore,three-phase rectifiers are used for various applications,such as new energy generation,electric vehicle,telecommunication power system,etc.Three phase three-level Vienna rectifier is a very attractive boost-type power factor converter(PFC)because of good quality of input current,high power density and high efficiency with good applying foreground.The good steady and transient performance of the Vienna rectifier is required for the actual application.In order to achieve the purpose of application,this thesis focuses on the three-phase three-level Vienna rectifier.The three key aspects,including current resonant control,modulation strategies and the model predictive control,are studied.The main research contents of the paper are as follows:The harmonic relationship between input current and the terminal voltage of the Vienna rectifier is firstly analyzed,providing the basis for the current control methods and modulation strategies.The nonlinear factors of Vienna rectifier's parasitic capacitance and the snubber circuit,have an effect on the switching transient of the terminal voltage.The conclusion is that the nonlinear factors will lead to terminal voltage errors mainly at small input current magnitude and low-order odd harmonic components will be generated in the terminal voltage.Two current resonant control methods are proposed to depress the introduced odd harmonics in the input current,namely the proportional-integral-resonant(PIR)controller in rotating frame and the proportional-resonant(PR)controller in state frame.The current controller structure is improved and the controller's parameters are designed.Simulations and the experiments are carried out in a Vienna rectifier platform,which demonstrates the validity of the proposed methods.The results show that the proposed method can reduce lower-order current harmonics and improve input current performance.The resonant control method can suppress the input current's low-order harmonics caused by the Vienna rectifier's nonlinear factors.However,the parameters tuning and the realization are relatively complex.From the perspective of modulation strategies,the terminal harmonic characteristics are analyzed with the sine-wave pulse-width modulation(SPWM)and space vector modulation(SVM)methods.Two modulation strategies are proposed to suppress the input current's low harmonics.Firstly,based on the mathematical expression of the terminal voltage errors and the terminal voltage total harmonic distortions(THD)for SPWM method,the characteristics of zero-crossing clamping terminal voltage errors and terminal voltage THD are analyzed.It is found that the method of clamping terminal voltage to zero level around voltage zero-crossing can reduce terminal voltage errors,which will decrease the low-order harmonics of the input current.With the discontinuous PWM(DPWM)method,a varying area zero-crossing clamping SPWM(ZCC-SPWM)method is proposed.The clamping area around voltage zero-crossing can be changed.Simulations and experiments verified the validity of the proposed method.Compared with the SPWM method,the proposed ZCC-SPWM method can reduce low-order harmonics under wide load range effectively with simple realization.Then,the characteristics of terminal voltage errors and terminal voltage harmonics for SVM method are analyzed.The mathematical expression of the terminal voltage errors and the terminal voltage THD for SVM method are derived.The method of clamping terminal voltage to zero level around zero-crossing can reduce terminal voltage errors and decrease the low-order harmonics of the terminal voltage.Based on the carrier-based SVM and discontinues SVM method,a varying area zero-crossing clamping SVM(ZCC-SVM)method is proposed.With the proposed method,the terminal voltage errors are reduced and the neutral-point voltage fluctuation did not change much by changing the clamping area with a clamped factor.Simulation and experimental results demonstrate the validity of the proposed method with simple carrier-based implementation.It shows that the proposed ZCC-SVM method can reduce low-order harmonics of the input current and has comparative ability to balance neutral-point voltage,comparing with normal SVM method.The model predictive control(MPC)of three-phase three-level Vienna rectifier is studied to promote the input current quality and transient response performance.The finite control set MPC(FCS-MPC)method has variable switching frequency and needs high sampling frequency of good current performance.Moreover,the optimal switching sequence MPC(OSS-MPC)method with fixed switching frequency applies two or three vectors,which is complicated to implement.With the virtual space vector,a MPC method with discrete space vector modulation(DSVM)is proposed.Using the virtual vector generated with the linear combination of real voltage vectors,the number of feasible voltage vectors rises,which can reduce voltage ripples and improve input current performance with constant switching frequency.To reduce the calculation burden,a DSVM-MPC method with region optimization is proposed,which selects the nearby feasible vectors of the reference voltage vector's trajectory.Meanwhile,a trajectory optimal DSVM-MPC method with less calculation burden and data storage is proposed,which calculates nine virtual vectors on-line in sector I at the trajectory of the objective voltage vector with the modulation index.The detailed simulations and experiments verify the proposed optimal DSVM-MPC methods.Compared with normal FCS-MPC method,the proposed DSVM-MPC method can get preferable input current performance with fixed switching frequency and has comparable neutral-point voltage balancing without additional balancing algorithm added to the cost function.