Research On Parallel Control Strategy Of EV DC Charging Modules

With the Chinese government vigorously promote the construction of new energy vehicles,90 percent of the market structure of new energy vehicles in China today are electric vehicles,therefore,the research on EV battery charging module will become the focus of work in the future.Currently,most electric vehicle dc charging modules have two stages.The first stage is composed of three-phase PWM rectifier,whose main function is to change three-phase alternating current into direct current.The second stage consists of DC/DC chopper,which mainly controls the output dc voltage to meet the charging demand of the battery.The first-stage three-phase PWM rectifier is directly connected with the second-stage DC/DC module to charge the EV battery.The rectifier can be divided into a voltage type rectifier and a current type rectifier.In this paper,voltage source rectifier is adopted as the first stage of the charger to meet the demand of high power factor and thus reduce the harmonic content of the network side current.The Buck circuit is adopted in the second-stage DC/DC converter to meet the condition of step-down charging.The parallel EV dc charging modules can not only increase the charging power of the system and shorten the charging time,but also make the output power shared among modules so as to increase the service life of each module.Therefore,this paper studies the parallel EV dc charging modules.Firstly,this paper analyzes the circuit principle and control method of the dc charging modules of the EV in detail,including the three-phase rectifier of the first stage and the Buck circuit of the second stage.The mathematical model of three-phase voltage type PWM rectifier under the change of coordinate of two-phase synchronous rotation is given,and the double-closed-loop control strategy based on space voltage vector is deduced on this basis.The working principle of Buck circuit is analyzed,the calculation formula of the parameters of Buck circuit is given,and the mathematical model of Buck circuit is built by using the state space method.In this paper,the parallel control strategy of three-phase rectifier is analyzed.Firstly,the zero sequence circulation in parallel is analyzed in detail,and the mathematical model of zero sequence circulation is deduced.An improved SVPWM control strategy with superimposed zero sequence voltage components is proposed to suppress the circulation.The method reduces the hardware cost and only needs to collect the zero sequence circulation.The calculation is simple and the method is practical.Secondly,to solve the problem of unbalanced output current in the second-stage Buck circuit,a current-sharing control strategy based on consistency algorithm is proposed,and a sparse communication network of CAN protocol is established to realize module current-sharing through current information interaction.Compared with the traditional droop control strategy,this method can obtain very high current precision and also very high voltage precision.In addition,compared with the master-slave control strategy,this method improves the redundancy of the system.Matlab/Simulink simulation software is used to simulate the three-phase VSR model based on double closed-loop control and the PWM rectifier simulation model without power grid voltage sensor.Then a single EV dc charging module is built and simulated.Finally,the zero sequence circulation and output current inequality are simulated when four EV dc charging modules are connected in parallel.In addition,the improved SVPWM method with superposed zero sequencevoltage components and the uniform flow control strategy of the consistency algorithm are applied to the parallel system of four electric vehicle dc charging modules,and the simulation verifies the effectiveness of the proposed control strategy.Finally,on the basis of simulation,three parallel experimental platforms of EV dc charging modules are set up in this paper to carry out experimental analysis on the proposed zero sequence circulation control and the current sharing control strategy based on the consistency algorithm.The experimental results show that the proposed strategy has good practicability and effectiveness.