Research On Bidirectional Isolated DC-DC Converter For Braking Energy Charging Storage And Discharging

When the train is in braking condition,the traction motor converts mechanical energy into electrical energy,which generates a large amount of braking energy and feeds back to the traction network.This part of energy will increase the pressure of the traction network and affect the stability and reliability of the system.In order to avoid this problem,most of the subway systems in China use brake resistors to consume this part of energy.Although this way can stabilize the traction network voltage in time,it will lead to energy waste and increase the temperature of subway tunnels,and also increase the cost of ventilation and air conditioning.In view of this problem,it is necessary to recycle this part of energy to save energy and protect the environment.Firstly,this thesis analyzes the traction power supply system and determines the parameters of the traction network under the condition of no load.Through simulation and calculation analysis,the braking energy and power generated in the process of train braking are determined.The capacity and power of the energy storage device in the energy storage system,namely the supercapacitor module,are determined by comparing the design experience of the energy storage system in the simulation and the actual circuit.Secondly,in order to make the electric energy flow bidirectional between the traction network and the ultracapacitor module,the isolated bidirectional DC-DC converter is used as the bridge of the electric energy transmission between the traction network and the ultracapacitor module.The working principle,soft switching characteristics and power characteristics of isolated bidirectional DC-DC converter are analyzed and the control parameters are optimized to minimize the effective current value of the converter and realize soft switching in the full load range.Thirdly,the serial-parallel mode of the ultracapacitor module array is determined,and according to the amount of energy the ultracapacitor module needs to store,the energy constraint method and the power constraint method are used for analysis,and finally the parameters of the ultracapacitor module are determined.In order to ensure the rapidity and reliability of the control part of the energy storage system,the double closed-loop control strategy of the traction network voltage outer loop and the energy storage inductance current inner loop was selected.The MATLAB/Simulink simulation model was established to simulate the control strategy of the energy storage system,and the correctness and accuracy of the control process design was verified.Finally,a low-power experimental prototype of bidirectional DC-DC converter based on three-level half-bridge + H-bridge topology was built.The working frequency of the prototype was 15 k Hz,and the energy storage device was supercapacitor module.The core of the control algorithm is designed based on FPGA and Verilog HDL hardware description language,and the working state is changed according to the voltage changes in the charging and discharging process.The energy storage system can normally charge and discharge at constant current,which verifies the feasibility of the control strategy.