Research On Electrified Railway Cophase Power Supply System

Railway transportation plays an important role in passenger and freight transportation,it has a pivotal position in the transportation industry.Electrified railway has the advantages of high speed,strong transport capacity,low operating costs,safety and comfort,which promotes its rapid development around the world.However,the electrified railway also has some problems that shouldn't be ignored: on the one hand,the electrical locomotive load is a high-power and single-phase nonlinear load,which brings a series of power quality problems to the public power system,such as reactive power,harmonic and negative sequence;On the other hand,the electrified railway power supply system has electrical phase separation,causing the speed and traction of electrical locomotive to suffer loss,which is not conducive to the development of high speed and heavy load.Taking the existing problems of electrified railway power supply system into account,it is an ideal comprehensive solution to realize cophase connected power supply system.Firstly,analysis for the principles and problems of existing cophase power supply theory is carried out,then a cophase power supply system scheme based on uncontrolled rectifier and single-phase PWM inverter is proposed in this thesis,combined with the current development of the electrified railway.Structure and operating principle of the new cophase power supply system are introduced.The rectifier side of the scheme adopts three-phase uncontrolled rectifier,which has several advantages like simple structure,good performance,low costs and high reliability.Compared with the PWM rectifier based on high frequency switching,the proposed scheme can greatly reduce the switching loss and control complexity.For the single-phase inverter,several common modulation strategies are analyzed and compared,and the CPS-PWM modulation mode is selected for its simple algorithm and easy implementation.The double closed-loop controller coordinate with the voltage effective value and instantaneous value is used to control the output voltage of the single-phase PWM inverter,simulation results have verified the correctness and applicability of the scheme in electrified railway power supply system.Secondly,considering the disadvantages of the CPS-PWM inverter,such as high switching loss,complex control method and low reliability,a scheme of cophase power supply system based on uncontrolled rectifier and step wave synthetic inverter is proposed.Three kinds of step wave synthetic and output voltage modulation strategies are described.The step wave synthetic modulation can reduce the switching loss of the inverter side,which improves the operating efficiency.The system model is established and simulated on the MATLAB/SIMULINK platform,simulation results show that this scheme can provide high quality stable voltage for the electrical locomotive.In addition,the amplitude,frequency and phase of output voltage can be fully controlled,which is conducive to the realization of cophase connected power supply system.Finally,a comprehensive evaluation about characteristics of the two kinds of three-phase to single-phase power supply system modes based on uncontrolled rectifier and H-bridge cascaded inverter is given,meanwhile the problems and countermeasures are analyzed.In addition,this thesis illustrated the reasons why the scheme is complied with the development trend of electrified railway,and pointed out the applicability in the cophase connected power supply system.