Single-phase Modular Multilevel Converter For Energy Storage And Reuse Of Traction Power Supply

The traction load starts and stops frequently,and its braking energy will cause problems such as overvoltage in the traction network,serious imbalance of the power system and energy loss.This thesis study the recovery,storage and reuse of traction load braking energy based on single-phase MMC converter.The MMC converter has the advantages of flexible capacity expansion,high voltage and high power,and can absorb energy and reuse it when combined with energy storage components.However,the traditional MMC converter includes circulating current,which will increase MMC loss and reduce device capacity utilization.This thesis proposes a single-phase MMC without DC link energy storage converter.The main research contents are as follows:First,this thesis analyzes the mathematical model of the single-phase MMC converter with DC link.Since the circulating current of the single-phase MMC converter is completely equal in the two phases,there is a circulating current between the phase and the DC side.The double-frequency circulating current not only brings losses and also increases the secondary ripple on the DC side.This thesis proposes and studies a single-phase MMC double-frequency circulation suppression strategy,and verifies its effectiveness through MATLAB/Simulink simulation and a low-power platform.Then,on the basis of the circulating current characteristics of the single-phase MMC converter,a single-phase MMC without DC link energy storage converter is proposed and studied.This converter cancels the connection between the DC side capacitor and the two bridge arms based on the traditional single-phase MMC converter,and designs the energy storage components in the sub-module directly,which has no interphase circulating current.This thesis studied the constant power control strategy of single-phase MMC without DC link energy storage converter,which can directly absorb energy and feed it back to the power supply network for reuse.On the other hand,to solve the problem of difficulty in sorting and equalizing voltage when the number of sub-modules in the traditional MMC sorting algorithm increases,an improved sub-module equalizing and sorting algorithm is proposed.The algorithm divides the sub-modules into several groups,only sorting one group of sub-modules every cycle,and apply the sorting result to all groups.Letting the sorting group and the number of conductions circulate in each component in a certain cycle.This method significantly reduces the amount of sorting calculations,and the simulation verifies the effectiveness of the algorithm.Finally,a single-phase MMC without DC link energy storage including 16-module lowpower experimental platform and program are designed.The FPGA chip control program based on Verilog HDL language is completed.Experiments verify that the converter and control strategy studied in this thesis have no interphase circulating current,module voltage balance,and achieving bidirectional power control under different transmission powers.