| Energy storage technology has been highly valued with the development of distributed generation and grid connection.Battery energy storage technology has been widely used due to its low cost,long life,high security,easy recycling and miniaturization.At the same time,the performance index of the bidirectional isolated AC/DC converter used in the battery energy storage system is also higher and higher with the development of the power system.The bidirectional isolation converter with high power density and high working efficiency becomes an indispensable part of the study of battery energy storage systems.In view of the requirements of the battery energy storage system for its charge and discharge converters in recent years,a two-stage new soft-switching bidirectional isolated AC/DC topology was proposed in this thesis.The preamplifier is a classic three-phase bridge PWM rectifier with outstanding advantages of unity power factor operation and less harmonic pollution to the power grid;the latter soft-switching bidirectional isolated DC/DC converter is compared with the traditional DAB(Dual-Active-Bridge)topology,the number of fully-controlled switching devices is reduced,and the soft switching characteristics can be realized under a large power range and different input-output voltage ratios,and the control is also simpler.The pre-stage three-phase bridge PWM rectifier adopts double closed-loop control strategy and space vector modulation(SVPWM)based on d-q rotation coordinates,and realizes sinusoidal and phase controllable by directly controlling the current tracking current of the grid side so that rectification and inverter under unit power factor is realized;mathematical modeling,working principle analysis and formula derivation of the soft-switching bidirectional isolated DC/DC converter of the latter stage was carried out.Combined with transformer flux,fixed transmission power,working efficiency and soft switching constraints in the circuit the relationship between the duty ratio of each fully-controlled device and the transmission power in the open-loop state was obtained.And then the corresponding duty cycle characteristic was obtained when the transmission efficiency of the converter is the highest.At the same time,based on the charge and discharge control requirements of the battery energy storage system,a two-stage charging method for the Lead-acid batteries and a discharge method based on the limitation of current of the constant voltage mode were proposed.And Combined with the charging and discharging method,the closed-loop voltage/current PI control of the soft-switching bidirectional isolated DC/DC converter was implemented,and finally the overall control of the system was realized.Finally,the overall simulation was carried on based on MATLAB/SIMULINK platform,and the experimental platform through the hardware and software design was bulilt to verify the performance of the proposed topology and control s trategy.The simulation results and experimental re sults shown that the proposed soft-switched bidirectional isolated AC/DC topol ogy and its control strategy were correct and effective.At the same time,the topology has the advantages of high voltage utilization,high power factor,low control difficulty and wide range of soft switching characteristics,and the charge and discharge control of the battery is implemented easily. |
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