Research On Ac Side Series Quasi Single-stage High-frequency Isolated AC-DC Power Conversion System

With the continuous expansion of energy storage system application,higher requirements are raised for the power and voltage ratings of the corresponding AC-DC converter.Quasi-single-stage high-frequency isolated AC-DC converter has been widely paid attention to in grid-connected energy storage conversion technology due to its advantages of high power density,long working efficiency and long service life.It has a broad application prospect in AC energy storage system.Due to the operating characteristics of the existing single-stage high-frequency isolated AC-DC converter,it is difficult to meet the practical requirements of energy storage system.In order to extend the voltage and power level of this kind of converter to meet the actual demand of energy storage system,a novel quasi-single-stage high-frequency isolated AC-DC power conversion system with AC side series is proposed.The topological structure,steadystate characteristics,harmonic influence mechanism of power grid and closed-loop control strategy are discussed in detail.Firstly,the operating principle of single-module quasi-single-stage high-frequency isolated AC-DC converter is analyzed.Aiming at the coupling problem of internal circulation and adjacent switching cycle current in Dual Active Bridge DC-DC converter module,a novel three-phase shift modulation strategy is proposed.While eliminating internal circulation and realizing current decoupling,The transformer current value is minimized.The working principle is introduced in detail,and the detailed simulation is carried out.Secondly,the overall structure of the quasi-single-stage high-frequency isolated ACDC power converter in series on the AC side is given,and the mathematical modeling is carried out to obtain the relations among the various electrical variables.Because the proposed topology belongs to the input series and output parallel structure,impossible to ensure that the ac filter capacitance value and the loss of power devices of each module completely consistent,cause capacitor voltage imbalance between modules,so the causes of capacitor voltage deviation mechanism are analyzed,and the correctness of theoretical is verified through simulation.Thirdly,the grid-side current characteristics of the proposed converter are analyzed when the AC grid contains low order harmonics,and the influence mechanism of the harmonics on the grid-side current is revealed.In order to eliminate the low-order harmonic interference and realize the zero steady-state error control of the grid side current,a closed-loop control scheme of the grid side current with quasi-PR compound repeated control is proposed.In order to guarantee the global stability,the structure and performance of alignment PR control and repetition control are analyzed,and the parameter design principle of compound controller is discussed.In addition,to solve the problem of capacitive voltage imbalance,an overall closed-loop control scheme including capacitive voltage balance control strategy and grid-side current closed-loop control is presented,and a corresponding simulation is performed.Finally,the experiment platform of two modules in series is built,the hardware scheme based on DSP+FPGA is given,and the software program is designed.The proposed converter and its modulation method are realized through experiments in detail.