Research On Single-phase AC-DC-AC Converter Based On Advanced Co-phase Traction Power Supply System

Traction power supply system is the only power source of train.Its performance is directly related to the safe,stable,reliable and economic operation of the train.However,due to the particularity of traction power supply system structure and locomotive load,the existing traction power supply system has serious negative sequence,harmonic,reactive power and other power quality problems.In addition,there are a large number of neutral sections in the catenary,which cause the power supply dead zone.Neutral sections limit the speed of the train,and threaten the safety of the train operation.Therefore,an advanced co-phase traction power supply system based on single-phase AC-DC-AC converter is studied in this paper.The power quality problems could be completely solved and the neutral sections can be completely cancelled in the studied system.However,the single-phase converter used in the advanced co-phase traction power supply system is facing the challenges of double-line-frequency ripple and nonlinear load.The harmonic distortion mechanism and suppression strategy of single-phase AC-DC-AC converter in the advanced co-phase traction power supply system are studied in detail in this paper.Firstly,the system structure and working principle of the advanced traction power supply system based on single-phase AC-DC-AC converter are analyzed.According to the development of the switching device,the key system parameters such as input voltage,output voltage,module number and so on of the traction substation are designed.The SPWM modulation strategy for single-phase diode clamped three-level converter is analyzed.The mathematical models of single-phase rectifier and single-phase inverter are derived in detail.The DQ decoupling control strategy of the rectifier and inverter are designed based on the mathematical model.Then,according to the energy flow relationship of each part of the system,the cause of double-line-frequency ripple in the single AC-DC-AC converter is deeply analyzed.Based on the control loop and hardware loop of rectifier and inverter,the influence mechanisms of the double-line-frequency ripple on the input current of rectifier and the output voltage of inverter are revealed.The equivalent model of inverter is established,and the reason of output voltage distortion caused by nonlinear load is revealed.The mathematical relationship between the harmonic of rectifier input current and rectifier port voltage is established,and the full-order harmonic suppression strategy is proposed.In order to enhance the suppression effect of low order harmonic current,the selective harmonic current suppression strategy is proposed.The combination of the two strategies can ensure that the rectifier input current is sinusoidal while there is still a large double-line-frequency.According to the equivalent model of the inverter,the mathematical relationship between the modulation wave and the output voltage of the inverter is established.The full-order harmonic suppression strategy of the inverter is proposed.And in order to enhance the effect,the selective harmonic voltage suppression strategy is proposed.The combination of the two strategies can ensure the sinuosity of the inverter output voltage.Finally,a simulation model is built in MATLAB/Simulink to verify the correctness of the structure and the harmonic suppression strategy of the advanced co-phase traction power supply system based on single-phase AC-DC-AC converter proposed in this paper.And a small scale experimental platform with FPGA as the core controller is built in this paper.Based on Verilog HDL,the program of the full-order harmonic suppression strategy and selective harmonic suppression strategy are coded.The experiments of the single-module AC-DC-AC converter and three-module AC-DC-AC traction substation are completed.The correctness and effectiveness of the advanced traction power supply system and its control strategy proposed in this paper are verified,which lays a good foundation for its engineering application.