| The development of electric vehicles is a key way to solve the energy and environmental crisis and achieve "carbon peaking" and "carbon neutrality" goals.Charging system is an electric energy conversion hub connecting power battery and power grid,and it is also the key infrastructure for the wide application of electric vehicles.With the continually increasing of driving range and the reduction of charging time,the capacity and voltage level of the power battery is required to be higher,and the power of the charging system is gradually increasing.The conventional charging system usually adopts the two-stage power conversion topology of"three-phase PWM rectifier+isolated resonant DC/DC converter".However,the energy is transferred in two stages,so that the system not only requires more power electronic switches resulting in high cost,and the dc bus capacitor volume is large resulting in poor reliability,which restricting the further improvement of system efficiency and power density.High frequency link matrix converter(HLFMC)is a three-phase single-stage isolated AC/DC converter,which can eliminate the dc bus capacitor,and has the advantages of high efficiency,high power density and high reliability.It is an ideal alternative to the conventional two-stage charging system,and has a broad application prospect in the field of high power dc fast charging.However,there are still some theories and technologies related to the HFLMC that need to be modified,including large output current ripple and slow response,poor safety and stability control under non-ideal operation conditions,and poor reliability of series-parallel module.Therefore,the main goal of this paper is to improve the rapidity and stability of charging system.Several key control technologies have been overcame,which include output current ripple suppression,output current high disturbance rejection and fast response control,security and stability control under unbalanced power grid,and modular series-parallel power balance control to meet the performance requirements of "high reliability,high power,fast response and high efficiency" of the fast charging system.The main research contents and innovations of this paper are as follows:To solve the problem of large output current ripple caused by conventional modulation method,the coupling mechanism of dc current ripple and ac current distortion is first analyzed in this paper,and then the internal relationship between vector sequence,action time and output current ripple is revealed.On this basis,a modified MC-SVPWM modulation strategy without zero-vector is proposed.The output current ripple is reduced by adjusting the vector sequence and action time,and the output current spike is eliminated by adjusting the vector sequence at the sector switching time.The experimental results show that,compared with the conventional 6S-SVPWM method,the proposed MC-SVPWM scheme reduces the dc-side current ripple by 35.7%,which reduces the damage to the battery by charging and realizes the safe charging of the battery.To solve the problem of low dynamic response and poor disturbance immunity due to the coupling of input and output in the HFLMC,a three-order mathematical model of the HFLMC is first established in this paper,and the logical relationship between each state variable is revealed.On this basis,combining with Lyapunov stability theory,a dual closed-loop backstepping control method(DCL-BSC)is designed for the HFLMC,including output current outer-loop backstepping controller and grid current inner-loop backstepping controller.The experimental results show that,compared with the conventional PI controller,the rise time of output current is only 2 ms under the proposed DCL-BSC strategy,which greatly improve the dynamic response speed and disturbance rejection ability of the HFLMC.With the help of phase compensation control method,the unity power factor of grid side is realized to improve the power quality of the charging system effectively.To solve the problem of current distortion at ac-side and double frequency fluctuation at dc-side caused by the unbalanced grid condition,an instantaneous power model under unbalanced condition is established in this paper to deduce the reference value of grid current in the two-phase static coordinate system.On this basis,a nonlinear backstepping control strategy considering the unbalanced grid is proposed,including α and β backstepping controller,and the stability of the system is proved according to Lyapunov stability theory.The experimental results show that,compared with the conventional PI controller and PR controller,the proposed control method reduces the THDs 1.7%and 0.2%at ac-side and the double frequency fluctuation at dc-side by 87.5%and 75%,which effectively improves the power quality of ac side and the stability of dc side.The series-parallel HFLMC is an ideal scheme to improve the power and voltage of charging system.To solve the problem of power unbalanced of input-parallel-output-series HFLMC(IPOS-HFLMC),a nonlinear mathematical model of the IPOS-HFLMC system is established first,and the unbalanced mechanism of voltage and current in IPOS-HFLMC modules is revealed.On this basis,a synchronous power balancing backstepping control strategy is proposed,which can realize voltage equalization at the output side and current equalization at the input side simultaneously,so as to ensure the power balance between modules and the stable operation of the system in high power,and it is flexible and expandable with high practical value.To sum up,this paper focuses on the key performance index of the output side and the input side of the charging system and breaks through the problems of high frequency link matrix converter,including large ripple and slow response,safety and stability control under non-ideal working conditions,and poor reliability of modular series-parallel.The research results with great innovations are realized,which lay a solid foundation for ensuring the efficient,fast and safe operation of the HFLMC and its application in charging system,and can be widely used in battery energy storage system,photovoltaic grid-connected inverter and other fields. |
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