Research On Control Strategy Of Medium Voltage Direct Current Traction Power Supply System Based On MMC

Under the background of China’s electrified railway mileage rising and breaking new highs,the negative sequence,harmonic and other problems inherent in the traditional 25 k V AC traction power supply system,as well as the slow speed and transient overvoltage when the locomotive is out of phase,are gradually becoming the "stumbling block" restricting the development of China’s high-speed railway in the future.Due to the influence of various factors mentioned above,it is extremely urgent to improve the power quality of the electrified railway traction system or to explore a better topology and control strategy of the new traction power supply system.Modular multilevel converter(MMC)has the advantages of strong scalability,high operating efficiency,strong fault tolerance,and "black start".Therefore,medium voltage direct current(MVDC)traction power supply system based on MMC is gradually becoming a research hotspot.Compared with the AC traction power supply system of the same voltage level,the MMC-MVDC traction power supply system has greatly improved the power quality problems existing in the traditional AC system.This paper mainly studies the topology and control strategy of MMC and the coordinated control strategy of MMC-MVDC traction power supply system.The main works are as follows:1)This paper mainly analyzes the traction substation topology,MMC topology,the station-level control strategy and valve-level control strategy of MMC in the MMC-MVDC traction power supply system,including double closed-loop control,capacitor voltage sequencing algorithm and the nearest level approximation modulation strategy.Finally,a single-ended MMC traction substation model is built in Matlab/Simulink,and the effectiveness of the control strategy and the feasibility of the MMC traction substation supplying power to the locomotive are verified under three different working conditions.2)Aiming at problems such as increased system losses and shortened component life caused by internal circulation in MMC traction substations,this paper also mainly studies circulation suppression strategies for MMC,with emphasis on analyzing quasi PR suppression methods.First,the second harmonic component in the circulation is accurately obtained by using the notch filter,and then the quasi-PR controller is designed to suppress the circulation of MMC.The structure of this control method is mainly introduced,The Bode diagram of the notch filter and the quasi-PR controller is drawn.Finally,the simulation results show that the performance of the notch filter and the quasi-PR controller is better than that of the PI controller in restraining the circulation.3)The coordinated control strategy for parallel operation of multiple traction substations is studied.This paper mainly focuses on the study of the parallel operation of two traction substations,analyzes the reasons for the poor output voltage sag and current-sharing characteristics in the traditional droop control,and proposes a variable droop control strategy based on the consistency algorithm to dynamically adjust the droop coefficient and achieve good current sharing characteristics.By building a parallel operation model of two traction substations in Matlab/Simulink,the proposed droop control strategy is compared with the traditional droop control strategy.The simulation results show that each traction substation can achieve good current distribution in case of load movement,sudden change,reactive power shortage and fault exit of the substation,and the output voltage of each traction substation can maintain near the reference voltage value,which has certain advantages.