Research On Control Strategy Of Novel Traction Power Supply System Based MMC

As a kind of green transportation,electrified railway has been playing an important role in people’s production and life.From the proposal of "four vertical and four horizontal" in the national railway planning to the supplement of "eight vertical and eight horizontal",China’s railway is moving towards the forefront of the world with a vigorous attitude.However,the problems of power quality and phase separation in the current railway traction power supply system have not been completely solved,which has become a major obstacle to the development of high-speed and heavy haul railway.The modular multilevel converter(MMC)provides a new way to solve these two problems,and the research on building a new traction power supply system is in full swing.This thesis focuses on the research of the MMC converter control strategy in a new MMC-based traction power supply system,and mainly completes the following tasks:1)This paper introduces the background and significance of this research,and analyzes the current research status of MMC and the research status of electric split phase technology,power quality control and in-phase power supply technology in traction power supply system,and puts forward the structure and advantages of the new traction power supply system to be studied in this paper.2)The structure and working principle of MMC sub module are analyzed.This paper introduces two modulation strategies commonly used in MMC: carrier phase shift modulation and nearest level approximation modulation,and studies the corresponding capacitor voltage equalization control strategy for these two modulation strategies.Finally,the modulation strategy and capacitor voltage balance control strategy suitable for the new traction power supply system proposed in this paper are selected.3)The mathematical models of three-phase MMC topology at rectifier station level and single-phase MMC topology at inverter station level are established respectively.Based on the study of the traditional double closed-loop vector control of three-phase MMC in rectifier station,a power decoupling control strategy on account of differential flat theory is put forward,which is suitable for three-phase MMC.The simulation results show that the performance of this strategy is better than that of the traditional double closed-loop vector control.Similarly,applying the flattening theory to the inverter station level design,a flattening control strategy suitable for the single-phase MMC of the new traction power supply system is proposed,and its feasibility is verified by simulation.4)The circulating current characteristics of three-phase MMC at rectifier station level and single-phase MMC at inverter station level are studied respectively.By comparison,it is found that the circulating current of three-phase MMC only flows between phases,and will not flow into DC side,which will not affect DC side;while the circulating current of single-phase MMC will flow into DC side,which will cause double frequency fluctuation of voltage,current and power at DC side.For three-phase MMC circulating current and single-phase MMC circulating current,the quasi proportional integral resonance is used to design the circulating current suppressor,and the simulation results show that the circulating current is effectively suppressed.5)This paper summarizes the work done in this paper,and points out the need for improvement in this study,pointing out the direction for the follow-up researchers.