| Vigorously developing electric vehicle is an effective way to solve the global energy crisis and curb environmental deterioration.DC charging system provides important basic support for the development of electric vehicles.It is the key con-dition to realize the industrialization and application of electric vehicles,and has a significant impact on the development of electric vehicle industry.Three-phase three-level PWM rectifier is known to be an important electronic topology for ener-gy conversion between AC power grid and load,whose fine operation characteristics can not only improve the performance of DC charging system but also optimize the power quality of power grid.Due to the advantages of low costs,high efficiency,and high power density,three-phase three-level PWM rectifiers are widely used in charging pile,wind power generation,rail transit and so on.In practice,good steady-state and dynamic performance of input current are usually required for PWM rectifier.Conventional DC charging systems often use Vienna rectifier to realize AC-DC conversion.However,Vienna rectifier not only has inherent problems such as zero crossing distortion of input current and limit-ed output voltage range,but also can only realize one-way transmission of electric energy,which cannot meet the demand of two-way transmission of energy between vehicle and grid.The T-type three-level rectifier with bidirectional transmission abil-ity can effectively solve this problem,but the inherent neutral voltage deviation and neutral potential oscillation of three-level topology have not been completely solved,and the anti-interference ability needs to be further improved.In order to meet the high power output demand of DC charging system,modular parallel PWM rectifier system is often adopted.However,the zero-sequence circulating current(ZSCC)is generated by the parallel topology,and it leads to the output waveform distortion,which increases the loss of the system and reduces the working efficiency of the sys-tem.These problems seriously restrict the industrialization and application of DC charging system.To this end,the finite-time control theory is introduced in this thesis.Based on this theory,the zero-crossing distortion of the Vienna rectifier,the control of neutral potential for three-level PWM rectifier,and the ZSCC between parallel connected rectifiers have been studied.The research contents and innovations of this thesis are summarized as follows:Some finite-time stability criteria are obtained for switching systems with non-linear disturbances with time delays by strict theoretical proof.Firstly,the definition of finite-time stability and the role of multi-Lyapunov function method in solving the stability problem of switched systems are analyzed.Then,the multi-Lyapunov function is constructed,and the finite-time stability criterion is obtained by means of linear matrix inequality(LMI)and average dwell time(ADT)theory.Switching process exists widely in power electronic circuits.The study of finite-time control theory is an important theoretical basis for the following research contents.Aiming at the inherent zero crossing distortion of input current in three-level Vienna rectifier,a hybrid control strategy based on finite-time controller and reactive power compensation is proposed.Firstly,the cause of the zero crossing distortion of the input current of Vienna rectifier is analyzed.Secondly,the lagging reactive pow-er compensation strategy is used to track the power factor angle,and the formula of reactive power compensation is obtained.Finally,in order to improve the dynamic performance of the three-phase input current,a finite-time controller is designed for the D-Q axis dynamic system of the current inner loop.Simulation and experimen-tal results show that this hybrid strategy can effectively solve the problem of input current zero crossing distortion in Vienna rectifier.Compared with the existing zero-sequence component injection method,this method has more obvious advantages at high modulation ratio.Therefore,this method greatly widens the output voltage range of Vienna rectifier.In order to solve the inherent problems of unbalance and oscillation of neutral potential in DC-link of T-type three-level rectifier,a finite-time control method is proposed.Firstly,the causes of unbalance and oscillation of neutral potential in T-type three-level rectifier are deeply analyzed.Secondly,the mid-point voltage dynamic equation is established.Finally,a finite-time controller is designed and proved strictly.Simulation and experimental results show that the proposed method can effectively solve the problems of unbalance and oscillation of neutral potential in DC-link of T-type three-level rectifier,and has good steady-state and transient performance.Aiming at the ZSCC problem caused by the parallel connection of T-type three-level rectifier,a hybrid control strategy based on finite-time control and feed-forward terms is proposed.Firstly,the causes of ZSCC are analyzed deeply,and the general-ized model of ZSCC generated by parallel T-type three-level rectifier is established.Secondly,the stability of the system parameter perturbation is studied,and a finite-time plus feed-forward controller is designed.Compared with the traditional finite-time controller and PI controller,the proposed control strategy can achieve better ZSCC suppression performance and stronger anti-interference performance without increasing the hardware cost.The effectiveness of the proposed strategy is validated by simulations and experiments.To sum up,based on the finite-time control theory,the stability of variable time delay switching system with nonlinear disturbance is analyzed,and the controller is designed based on finite-time control method.The problems of the zero-crossing dis-tortion inherent in the Vienna rectifier,neutral potential imbalance and oscillation of the three-level PWM rectifier,and the ZSCC between the parallel three-level PWM rectifiers have been solved,respectively.The performance of DC charging system is improved,and the robustness and reliability of the system have been enhanced prominently. |
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