| With the rapid development of power electronics technology and power semiconductor devices,the technical bottleneck of DC power transmission and distribution system has been overcome gradually,and the 24 k V flexible DC traction power supply system(TPSS)has received more and more attentions of the researchers.Compared with the existing 25 k V power frequency single-phase AC TPSS,the 24 k V flexible DC TPSS has some attractive advantages,such as no negative sequence problem,no neutral section,long power supply distance of traction substation(TSS),and the integration of renewable energy sources along the railway line is more fridendly,etc.This makes 24 k V flexible DC TPSS a new type of electrified railway power supply mode with great technical advantages and development potential,which provides a brand-new technical scheme for future electrified railway TPSS.However,since the short time of its concept and special application scenarios,many researches on such system are still at the initial stage,and there are a series of key theoretical and technical issues that need to be explored and studied deeply.To promote the development of 24 k V flexible DC TPSS and provide theory,method and technical support for its promotion and application,this thesis focuses on the key issues involved in the control of 24 k V flexible DC TPSS.The main contents of the thesis are as follows.(1)In order to realize the stable control of TSS’s output voltage in 24 k V flexible DC TPSS,the thesis takes the three-phase modular multilevel converter(MMC)in the TSS as research object and proposes a fast dynamic response control method for DC TSS based on model predictive control(MPC)and dynamic reference design method.In this control method,a model predictive current control(MPCC)strategy designed based on the principle of continuous control set model predictive control(CCS-MPC)is adopted in the inner current loop to realize the fast regulation of the AC side current and circulating current of MMC,and meanwhile simplify the inner current loop’s control structure and controller’s parameter tunning;In the outer DC voltage loop,in order to overcome the defects of proportional integration(PI)controller and improve the dynamic performance of MMC’s DC side output voltage,a dynamic reference design method based on extended state observer(ESO)is proposed to generate the reference value of MMC’s AC side current at the time instant k+2.The simulation results show that the proposed control method can efficiently improve the dynamic response performance of TSS’s output voltage when facing the situation that reference value and load’s power suddenly change while taking into account the steady-state performance of system,and thus can effectively deal with the complex operating conditions of the TPSS.(2)In order to realize the coordinated operation of multiple TSSs in the 24 k V flexible DC TPSS,the thesis proposes a decentralized coordinated control method for multiple TSSs considering the suppression of trains’ s regenerative braking failure based on the detailed analysis of the reasons that the traditional virtual inertia control(VIC)may aggravate train’s regenerative braking failure.The control method introduces VIC into TSS’s outer loop control system to generate the reference value for TSS’s output voltage,and then adopts the aforementioned proposed fast dynamic response control method to quickly and accurately track the voltage reference value generated by the VIC;At the same time,to suppress the occurrence of train’s regenerative braking failure to a certain extent,based on making full use of the fully controllable characteristics of TSS,the output characteristic curve of TSS’s inverter area under VIC is optimized,that is,when trian’s regenerative braking power makes TSS’s output voltage higher than a certain value,the TSS enters constant voltage control mode and maintains a constant DC voltage output.The simulation results show that the proposed control method can not only make all the TSSs in the system jointly maintain the system’s power balance and traction network voltage stable without communication links,but also can avoid the occurrence of train’s regenerative braking failure to a certain extent.(3)In order to ensure the safety and stability of the grid connection process of TSS in the 24 k V flexible DC TPSS,the thesis proposes a pre-synchronization control method to combine with the aforementioned proposed decentralized coordination control method.This method adds a pre-synchronization controller to VIC-based outer loop control system of TSS;When the TSS receives a instruction for grid connection,the pre-synchronization controller generates a corresponding adjustment amount to control the difference between the TSS’s output voltage and the voltage at the access point within the allowable range;when TSS is connected to the traction network,the adjustment amount generated by pre-synchronization controller will be adjusted to zero slowly so as not to affect the power sharing and voltage control performance of the system during normal operation.The simulation results show that the proposed pre-synchronization control method can effectively suppress the surge current that may occur in the process of TSS’s grid connection,and the smooth switching from off grid operation to grid connection operation of TSS can be achieved,which can thus help to satisfy the plug-and-play requirement of TSS.(4)In order to connect the photovoltaic power generation to 24 k V flexible DC TPSS for effective utilization,the thesis proposes a topology and control method that connects the photovoltaic(PV)and energy storage to the system from TSS’s 24 k V DC bus.For the proposed topology,a DC power electronic transformer(DCPET)with high and low voltage level conversion,high and low voltage DC bus electrical isolation and bidirectional power flow capacities is connected to the 24 k V DC bus of TSS and a stable 3k V DC bus is constructed on its low-voltage side.The PV power generation and energy storage system are connected to the low-voltage side of DCPET through their respective DC-DC converters,and then connected to the 24 k V flexible DCTPSS.For the proposed control method,the concept of hierarchical control is adopted,including energy management layer and converter control layer.The energy management layer calculates the power reference value of the PV power generation system and energy storage system under the corresponding working conditions according to the system operation principle;in the converter control layer,each converter tracks the power reference value from the energy management layer on the basis of realizing their respective control objectives.The simulation results show that the proposed topology and control method can release and store the PV energy reasonably,and the effective consumption of PV energy and reliable power supply to the train can be achieved.(5)In order to better serve the theoretical and experimental research of 24 k V flexible DC TPSS,the thesis designs and builds a set of flexible DC TPSS simulation experiment platform including three DC TSSs,and the overall structure of the platform,the main circuit and control system part of TSS are introduced in detail.Additionaly,based on the established experimental platform,the aforementioned proposed fast dynamic response control method based on MPC and dynamic reference dersign for TSS and the decentralized coordinated control method for multiple TSSs are verified correspondingly.The experimental results show that the proposed control methods are effective and feasible,and the experimental platform can operate normally and reliably,which can provide a physical verification platform support for the other theoretical and experimental research of 24 k V flexible DC TPSS to a certain extent. |
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