Modelling And Simulation Research On Traction Power Supply System Based On MMC-MTDC

In recent years,in order to meet the need of China’s economic development,electrified railways have developed rapidly.However,electric locomotive is a single-phase,non-linear large dynamic power load,which brings great challenges to the construction and operation of traction power system.Electrified railways have two major problems: power quality and phase separation,which restrict the development of high-speed and heavy-load electrical railway.Modular multi-level converter multi-terminal DC transmission system has the advantages of independent control of active power and reactive power,and does not need reactive power and commutation voltage provided by the grid.It can realize multi drop point power receiving and multi power supply.MMC-MTDC has been widely used in new energy grid connection,island power supply and other aspects.Aiming at the shortcomings of the current traction power supply system,this paper combines the actual situation of China’s traction power supply system and the advantages of multi-terminal DC transmission,and proposes a traction power supply system structure based on MMC-MTDC.This structure converts the grid-side three-phase AC power to DC power through three-phase MMC,and then inverts the DC power to single-phase AC power through single-phase MMC to provide power to the traction network.Completely eliminate electrical phase separation and improve power quality issues.It can be seen from the multi-terminal DC transmission that the key of the traction power supply system based on MMC-MTDC studied in this paper is to improve the power flow and make the rated voltage close to the rated value.Therefore,this article main work is as follows:1)This paper studies the basic topology and working principle of MMC,discusses the three working states of MMC sub-modules,and studies the most common modulation strategy of MMC is nearest level modulation.The inner loop controller and outer loop controller of the converter is designed according to the MMC mathematical model,and the outer loop controller that supplies power to the passive network is explained,and the control method is verified by simulation.Finally,the structure of the single-phase MMC inverter and its double closed-loop control strategy are designed.2)Two types of single-phase MMC circulation suppression strategies are proposed.Based on the circulation characteristics of single-phase MMC,a single-phase MMC circulation control method based on differential flat theory is proposed.The control strategy is based on a nonlinear mathematical model,which can effectively suppress the double-frequency circulating current component in the internal current.Compared with the PI circulation controller,this method does not require an accurate mathematical model of circulation.When considering the multi-frequency even-harmonic circulating current in the converter,a low-pass filter is used to extract the DC component of the instantaneous power and the effect is poor,and the phase shift lag is designed.The component extractor can obtain the DC quantity more quickly and effectively,which greatly reduces the complexity of harmonic separation.3)Coordinated control strategy for traction power supply system.Aiming at the problem of large fluctuation of traction load,an improved droop control strategy is proposed based on the proportional relationship between the instantaneous power of the bridge arm and the square of the DC voltage.This strategy introduces a PI controller to avoid rectifier station power changes when traction load changes large overshoot phenomenon,the strategy takes into account both the stabilization of the DC voltage and the improvement of the power flow,while improving the power flow,the DC side voltage is close to the rated value.