Study Of Advanced Co-phase Traction Power Supply System Based On Cascaded Converter

Recently,the research of advanced co-phased traction power supply system is increasingly emphasized at home and abroad.Being an alternative for all phase separation device,it has turned out to be the perfect solution for power quality problems,such as negative sequence,harmonics and reactive power.With the ever increasing application of wide-band gap power electronic devices,it provides good opportunity for the development of the advanced co-phased power supply system based on multilevel converter.Firstly,the current situation and problems of the traction power supply system is analyzed,showing the necessity and urgency of this research.Secondly,considering the development of wide band gap power electronic devices,a new type of co-phased traction power supply system based on cascaded converter is designed.Finally,the feasibility of the system is verified from three aspects:theoretical analysis,simulations and hardware experiments.In the feasibility study,based on the principle of the single-phase three level NPC inverter and cascaded inverter,the control strategies are analyzed respectively in this paper.The causes of single-phase inverter DC side neutral point unbalance problem is analyzed,and the balance strategy is studyed.On the basis of current multilevel modulation strategy,modulation strategies are further studied,including carrier modulation strategy,carrier phase shift modulation strategy and space vector modulation.Moreover,structure characteristics of the single-phase inverter and cascaded inverter are used to design modulation strategy proposed in this paper,which adopts the carrier cascade modulation strategy in the single module and the carrier phase shift control strategy between the modules.Simulations and experiments are carried out to verify its feasibility.To verify the stability,traction impedance model of the system is studied and a three traction substations-seven electric locomotives model is established and analyzed by the improved PQ decomposition method.The power flow result shows that the minimum voltage of traction network can still meet the minimum conditions of locomotive running.At the same time,the relationship between the output power of each traction substation with different positions is analyzed,namely,the closer the locomotive load distance substation is,the larger the output power of the substation.Illustrated by the case of two traction substations in parallel,the grid circulation is analyzed to demonstrate its droop characteristic.Also,the paralleling principle of a single traction substation is studied and the grid circulation is restrained by applying the improved droop control strategy,thus achieving the stable operation.Then,simulations and experiments are carried out to verify the stability.In this paper,four models were built,including single-phase three level NPC inverter,cascaded inverter,the power flow calculation model,traction substation parallel operation of advanced co-phased power supply system.Further simulations were taken to verify the validity of the former theoretical analysis and the above control strategies.Finally,low power experimental platforms based on FPGA are built and programmed using Verilog HDL.The experimental results show the stability and practicability of the system,which provides the basis for the application of a new breakthrough in advanced co-phased power supply system.