Research On Multimode Control Strategy For Power Conversion System In EVs Intelligent Integrated Station

Under the background of global energy shortages and serious environmental pollution,it is significant to reduce the energy consumption of transportation industry.As associated with energy conservation and environmental protection characteristics of new energy vehicles,the electric vehicles(EVs)have attracted extensive attention with broad development prospects in the future.Construction of energy supply facilities is one of the fundamental factors in promotion process of electric vehicles.EV intelligent charge-discharge-storage integrated station(CDSIS)emerges as a new concept to integrate the advantages of EV charging-swapping station and energy storage power station.The new pattern of integrated station helps to improve the resources utilization of land,batteries and equipments effectively.As the bidirectional power interface between EVs and the grid,the power conversion system(PCS)is responsible for secure and stable operation of CDSIS.In this paper,multimode control related technical issues of parallel connected PCS in CDSIS are analyzed and discussed in depth.LCL filter based bidirectional converter is the optimal choice of multimode PCS in high-power and low-switching-frequency applications due to its better performance on grid current harmonics suppression and wider adaptability to the variation of grid impedance.A mathematical model of LCL filter is built on variables of total inductor,inductor ratio,capacitor and evaluation indices of current harmonic attenuation coefficient and resonant-harmonic angular frequency ratio.The quantitative relationship between filter parameters and current harmonics,resonant frequency,grid impedance are deduced.Combined with the characteristics analysis result of multi-band harmonic sources,an optimized design method of LCL filter based on the multi-objective weighted algorithm is proposed,which integrates various optimization objective including filter performance,volume and adaptability against grid impedance variation.In addition,the mathematical models of paralleled LCL filters in the islanded mode and V2 G mode are built respectively.The effects of parameters difference,grid impedance and module number on system resonant characteristics are analyzed in a comparative way.In the V2 G operation mode,the mathematical models of PCS module and paralleled system are built.The resonance peak suppression mechanism of active damping scheme based on feedback control is analyzed.Then an impedance reshaping strategy based on inverter-side inductor current inner loop and grid-side inductor current outer loop is adopted.To eliminate the current distortion caused by grid voltage harmonics,a full feedforward scheme of grid voltage is proposed.Meanwhile,to improve the parallel operation reliability in the V2 G mode,an optimized charge-discharge power allocation algorithm is developed for achieving state of charge(SOC)balancing of battery packs and apparent power balancing of PCS modules.In addition,the Norton equivalent model is established considering voltage feedforward control strategy.On this basis,the resonance mechanism and characteristics are analyzed,and the stability criterion in dq frame is deduced further.In the islanded operation mode,the mathematical models of PCS module and paralleled system are built.A cascaded dual-loop control strategy based on inverter-side inductor current inner loop and capacitor voltage outer loop is adopted.To improve the steady-state and dynamic current-sharing effects,a compound current sharing control strategy is developed for parallel operation of PCS modules,consisting of periodic current sharing scheme with hybrid communication bus and instantaneous current sharing scheme with adaptive circulating current impedance.In addition,the Thevenin equivalent model is established considering current sharing control strategy.On this basis,the circulating current mechanism and characteristics are analyzed,and the stability criterion in dq frame is deduced further.In the transition operation mode,the mathematical models of PCS module and paralleled system are built.The generation mechanism of voltage and current fluctuations during the transition process is analyzed.It is concluded that the key issue is to make sure the PCS output voltage and grid voltage are consistent in amplitude,frequency and phase.Then a novel seamless transfer control strategy based on status tracking inner loop and smooth switching outer loop is proposed,which avoids voltage spikes and rush currents caused by controller switching and integrates control strategies of various operation modes.In addition,the control model of generic current inner loop is established and analyzed.Based on the generalized Nyquist stability criterion,the selection range of the inner loo p controller parameter in the stability region of resonant frequency is obtained.