| Development of electric vehicles(EVs)is a key way to solve the environmental problems and energy crisis,especially for China.As an indispensable infrastructure for EVs,fast charging system is the urgent need of national economy and people’s livelihood.Two-stage power convertion of fast charging system is composed of Vienna rectifier and resonant converter.It has been and widely used in the market owing to the advantages of lower cost and higher power density.However,universal dual-port\Vienna rectifier has neutral-point(NP)potential oscillation,which results in poor system reliability.Usually,the outputs of fast charing systems are regulated only by DC-DC converter as the voltage of DC bus is constant and higher than the peak value of grid voltage.Obviously,it is unable to meet the requirement of high efficiency in all charging operations.Therefore,the wide voltage range and flexible power supply voltage levels of multi-port Vienna rectifiers are the key solution for above problems.And it has become the emphasis and difficulty of research,which are considered by academy and engineering fields.However,relevant theories and technologies of multi-port Vienna rectifier is imperfect.The essential reasons are the unclear imbalance mechanism,low power quality on AC and DC sides,poor control ability of voltages with unbalanced loads.Therefore,the reliability of two-port Vienna rectifier is improved in this paper.On this basis,high-quality modulation technique is studied.Then topological structures of multi-port Vienna rectifiers are designed.In the end,high-performance control strategies are researched.The main research contents and innovations of this paper are summarized as follows:To suppresse the oscillation of NP potential of dual-port Vienna rectifier,this paper simplifies SV modulation using the carrier-based PWM(CB-PWM)based on level conversion.Then,the effect of voltage vectors on the NP voltage is analyzed.In the end,the adjustment factor of offset is calculated to minimize the low-frequency NP voltage oscillation.The simulation and experimental results indicate that the amplitude of NP voltage oscillation is reduced 57%approximatively after balance control.This method reduces capacitance and device stress on DC side,and improves the life and operation reliability of charging system.To eliminate input currents distortion on account of power factor angle and DC unbalanced voltages,DC imbalance operation mechanisms of Vienna rectifiers are studied based on the characteristic of the forced commutation topology.An overlapped component injection scheme(OCIS)using carrier modulation is proposed in this paper.In this sheme,overlapped componention components are injected during abnormalintervals,which are identified handily.Simulation and experimental results show that the OCIS attenuates the Total Harmonic Distortion of input current comparing with traditional modulations(THDi=1.1%with balanced dc voltages and THDi=1.4 with imbalance).DC imbalance operation mechanisms of Vienna rectifiers and low harmonic modulation could be used for improve the power power quality for AC side.Then it is the theoretical basis and technical support for charging system as green energy-utilizing facility.To improve the power quality on AC and DC sides of Vienna rectifiers,this paper modifys the low harmonics modulation and proposed a segmented component injection scheme(SCIS).Firstly,the average value of neutral current are calculated in a fundamental period.Secondly,circuit operation during clamping invervals are analyzed,then the unqiue compensations are injected in these intervals to eliminate current distortion.Meanwhile,optimized components are calculated accurately and injected into continuous intervals to suppress the fluctuation of neutral-point current.In the end,the AC and DC performance using OCIS and SCIS is compared and evaluated by input current harmonic and voltage RMS of fluctuation.The SCIS can not only eliminate current distortion and suppress DC voltage fluctuations,but also be effective when DC voltage is unbalanced.In brief,the high-quality modulation technique for AC and DC sides is preferred modulation scheme for multi-port Vienna rectifiers.To overcome the issue of multi-output instability under the unbalanced loads disturbance,the multi-port Vienna rectifier with output-current-distribution structure is configurated combining a new current path of neutral leg.Hence,the output voltages are controlled stability under arbitrary loads.In addition,feedforward control strategy of Vienna rectifier in injected current regulating mode and model predictive control strategy of neutral leg in distributed current regulating mode are studied emphatically.The simulation and experimental results indicate that,supplying for the random unbalanced loads,the multi-port Vienna rectifier realizes the fast speed response and strong robustness of regulated output voltages using proposed high-performance control strategies.In conclusion,this paper researches on the dual-port Vienna rectifiers which have been widely used in the market firstly.The oscillation of NP potential is suppressed to improve charging system reliability.Furthermore,the research and development of multi-port Vienna rectifiers are discussed in theory and practice.DC imbalance operation mechanisms and low harmonics modulation of Vienna rectifiers are researched.On this basis,the high-quality modulation technique of multi-port Vienna rectifiers for AC and DC sides is proposed.Moreover,flexible power supply with multiple voltage levels and wide-range multi-outputs are relized by topological reconfiguration,operating modes and control strategy design.Finally,the adaptabilities are improved between charging systems and electric vehicles. |
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