Research On Control Strategy Of An Integrated Onboard Charger For New Energy Vehicles

With increasing dependence of modern society on fossil energy,the world's fossil energy crisis has gradually emerged.Since the fuel vehicles consume a large amount of fossil energy and emit a great deal of automobile exhaust gas,causing serious pollution to the environment,the development of new energy vehicles to supplement or even replace the use of fuel vehicles is of great significance for energy conservation and environmental protection.There are two types of power converter in new energy vehicles,namely onboard battery charger and traction system.In addition to the basic battery charging function,the onboard charger can also perform as distributed energy to the power grid,fully utilizing the energy stored in the battery in spare time.As the power of battery charging and motor driving continue to increase,resulting in larger volume and weight of the corresponding power converters,the improvement of power density in the limited vehicle space becomes essential.A two-stage integrated onboard charger is proposed in this paper,which consists of a dual active bridge(DAB)DC-DC converter and a three-phase four-leg(TPFL)DC-AC converter and auxiliary circuits.A squirrel cage three-phase asynchronous machine is used as traction motor and the integrated system is connected to single-phase grid.By multiplexing the converter structure and the motor windings,motor driving,vehicle to grid(V2G)operation,and battery charging can be realized,which reduces the size and cost of the converter.Meanwhile,electrolytic capacitors are eliminated in the integrated system,further increasing the power density of the integrated onboard charger.In this paper,the structural composition of the integrated system is analyzed in detail.The different connection and function of DAB,TPFL,and auxiliary circuits in three operation modes are described.Then,in traction mode,the DAB generates a pulsating DC bus voltage with zero portions by phase shifting battery side H-bridge;the dynamic mathematical model of the asynchronous motor is established,based on which the switch-table direct torque control is proposed to control the asynchronous motor.By switching the IGBT at the zero portions of the DC bus voltage,the zero voltage switch(ZVS)of the DC bus side H-bridge and the TPFL is realized,which improves the efficiency of the system.In V2 G mode,the pulsating voltage is identically generated on the DC bus as in traction mode,whereas the TPFL adopts the finite-set model-predictive-control(FS-MPC)to realize the ZVS during zero portions.The switching mode of the circuit is decomposed and analyzed so that the working principles of the circuit is better understood.Finally,three different battery charging control strategies are studied,including a boost type control strategy and an extended phase shift control strategy,of which the DC bus voltage is grid rectification waveform,and the third control method is based on FSMPC,whose DC bus voltage is high frequency pulsatingA practical experimental platform is built in the laboratory in order to verify the effectiveness of the control strategies in three operation modes.Each control strategies proposed in the paper is realized,and the transmission efficiency of the converter is measured.Comparison is made between the integrated system and traditional onboard charger topologies,which proves that the integrated onboard charger is superior in terms of efficiency.