Research On Co-phase Power Supply Technology Of Electrified Railway Based On MMC

As an important way of travel and freight transportation,railway transportation is closely related to national strategy and national life.With the planning of the “eight vertical and eight horizontal” railway network in China,the railway traction power supply system is developing in the direction of high speed and heavy load.At present,there are major problems such as negative sequence,harmonics and three-phase unbalance in the traditional traction power supply system.The electric phase separation device between the traction substations also restricts the development of high-speed railway.With the research of power electronics technology and multi-level technology,the emergence of continuous co-phase power supply technology based on three-phase to single-phase converter has brought about an opportunity to solve the above problems.Due to the excellent characteristics of the modular multilevel converter(MMC),in order to make full use of its advantages in large capacity and high power occasions,the MMC can be applied to the continuous co-phase power supply system to eliminate step-up transformer,omit output filter,reduce volume and cut costs.Therefore,this paper takes MMC-based continuous traction power supply system as the research object,and focuses on the circuit characteristics,modulation method and control method of three-phase to single-phase converter.First of all,the paper analyzes the types,advantages and disadvantages of multilevel converters,studies the topology and working principle of MMC,and establishes its mathematical model.Based on this,the four-quadrant operating characteristics are analyzed,and the single-phase MMC circulating component and its suppression strategy are studied.Secondly,the control strategy of modular multilevel converters is mainly studied,including the modulation strategy of MMC and the voltage control strategy of submodule capacitors.The modulation principle,advantages and disadvantages of various modulation strategies are also analyzed.In addition,the paper studies an improved nearest level modulation strategy,analyzes its superiority and applicability,and finally verifies it by simulation.Then,the paper studies the topology and working principle of MMC-based three-phase to single-phase converter.The control strategies of the three-phase side and single-phase side are introduced respectively.The control strategy of active and reactive decoupling is adopted on the three-phase side,and a voltage effective value control strategy is proposed on the single-phase side.In this way,the system can output a relatively stable voltage value when the load fluctuates and ensure the voltage quality.Then the effectiveness of the control strategy is verified by simulation.Finally,the simulation model of MMC-based three-phase to single-phase converter system is built on Matlab/simulink platform.The paper analyzes the steady-state and dynamic simulation results of intermediate DC voltage and single-phase AC output voltage under various conditions,and verifies the feasibility and correctness of the system.