| The urban rail transit DC traction power supply system shows problems such as stray current and rail potential,low utilization rate of train braking energy and low energy conversion frequency.Therefore,high voltage DC traction power supply system(HVDC power supply system)was proposed to solve the above challenges or shortcomings by relevant scholars.The HVDC power supply system has high voltage,large capacity,and bidirectional energy flow requirements for the DC transformer,one of its key equipment.However,most of the existing DC transformers have high switching device stress,large number of devices,large device volume,high hardware cost,and complex control.Improved multilevel voltage-balancing DC-DC converter(IMVBDC)is proposed and studied in this paper.Firstly,this paper analyzes the configuration of IMVBDC and the self-balancing principle of DC support capacitor group,and explains the zero current soft switching characteristics and current voltage stress characteristics of switching devices.The research shows that IMVBDC has the advantages of modular multilevel,zero current soft switching and low switching device stress,which meets the performance requirements of HVDC power supply system.At the same time,the simulation models are built to verify the above analysis.In terms of control strategies,this paper first analyzes the modulation mode of IMVBDC standard module,controls the charging and discharging of resonant unit by changing the on-off state of switching device,and then studies the voltage-balancing control to eliminate the differential pressure of DC support capacitor.On this basis,the control strategies to suppress the input current and output voltage ripple of IMVBDC are proposed,and the integrated control strategy is proposed to reduce the input current and output voltage ripple simultaneously.Finally,the correctness of the above control theory analysis is verified by simulation.Finally,a 7-level IMVBDC prototype is designed and built.Based on the verification of IMVBDC basic performance,voltage-balancing control and ripple control,the train operation experiment of HVDC power supply system is carried out.The experimental results show that IMVBDC has the advantages of high voltage,bidirectional flow of energy and high efficiency,which meets the requirements of HVDC power supply system.It also verifies that HVDC power supply system has the advantages of stray current and track potential suppression,train braking energy utilization and high frequency of energy conversion process. |
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