| With the development of power electronics technology,the continuous development of high-voltage direct current(HVDC)transmission technology based on voltage source converters has attracted a high degree of attention not only in academically but also in engineering.Because the modular multilevel converter(MMC)has unique advantages in the HVDC transmission field,more and more attention has been paid to MMC since it was proposed,especially in the construction of MMC-HVDC based projects.The novel traction power supply system based on the MMC-HVDC technology can completely eliminate the neutral section device and can solve the power quality problem better.This paper studies the novel traction power supply system based on MMC structure,and verifies its feasibility from three aspects: theory,simulation and experiment.This paper presents the structure of a novel traction power supply system based on MMC,calculates the parameters of this system and designs the parameters of sub-module capacitance,bridge arm inductance and transformer.This paper studies the working principle and modulation strategy of MMC.Firstly,the three working states of the MMC submodule are discussed.The working principle of the MMC is analyzed according to the working states of the submodule.Secondly,two common modulation strategies for the MMC are studied: the carrier phase shift and the nearest level modulation strategy.Thirdly,because the fluctuation of the capacitance voltage of the MMC submodule will affect the stable operation of the converter,a capacitor voltage equalization control strategy is designed.According to the differential mathematics equations,the mathematical model of the three-phase MMC rectifier in the dq coordinate system is established.By the mathematical model,the direct current control strategy based on the inner current loop and the outer power loop is studied.Secondly,the circulation current is caused by the fluctuation of the capacitor voltage.The circulation suppression strategy based on double frequency decoupling is studied,and the correctness of the circulation suppression strategy is verified by simulation.Then,the single-phase H-bridge MMC structure and the direct voltage control strategy of single-phase MMC inverter is studied in this paper.Based on the above research,the impedance model of the traction network was analyzed and established,and the simulation model of the entire system connected to the three substations was completed using Matlab/simulink simulation platform.Through system is startup and locomotived with no-load,load-carrying and fault rectification under different working conditions,the system can remain stable,and the voltage on the traction network can be stabilized at an effective value of about 27.5kV.It indicates that the system can cancel the neutral section and realize the consecutive power supply.The system verified the correctness of the modulation strategy and control strategy of the main circuit of the system.Finally,this paper builds a low-power experimental platform.Three-phase MMC rectifier experiment,single-phase MMC inverter experiment,and the entire system experiment are completed on the experimental platform.The experimental results prove the feasibility of the main circuit structure,modulation strategy and control strategy of the system and the system can be used for traction power supply system. |
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