| The urban population has greatly increased due to the continuous development of our national economy and the accelerating process of urbanization,which aggravates the traffic burden of urban transportation.Urban rail transit has the characteristics of large volume,high density,high environmental friendliness and punctual comfort,which means developing urban rail transit has become the key to solve the above urban population and traffic inconvenience problems.Recently,On-board energy storage system(ESS),as a novel traction technology,has been installed on the urban rail vehicles to supply the power demand during traction stage and to recover the energy during braking stage to improve the system effciency.Compared with the traditional catenary networks,the advantages of this operation mode are as follow:urban transportation systems can work in zones without catenary infrastructure,the visual pollution caused by the dense urban catenary networks can be eliminated,energy transmission does not need to go through the catenary which will minimize the system losses,catenary malfunctions can be completely avoided and so on.In this paper,several key issues based on on-board ESS in urban rail transit are studied such as intermittent power supply mode,system operation mode,optimal configuration of ESS,energy management strategy(EMS)of ESS and control strategy of DC-DC converter and so on.The aim is to reduce the annual average cost of the system and improve the operating efficiency in the system design stage,reduce the energy loss of the ESS and improve the DC-DC converter's performance in the online operation stage which has theoretical significance and practical value for the related technologies such as optimal sizing and EMS of the ESS in urban rail vehicles.Firstly,to improve the efficiency of new energy urban rail transit and reduce the system cost,the fast swap charging mode is proposed based on the study on the power supply mode of the existing on-board ESS in urban rail vehicles,Meanwhile,a system conduction efficiency model based on different power supply modes and a calculation method for the operating efficiency of ESS are proposed.Additionally,the system operating cost function including the whole life cost of energy storage components is analyzed.Finally,the advantages of the proposed fast swap charging mode in terms of economy and system efficiency are verified by case studies and simulation results.Secondly,in order to properly configure the capacity of on-board ESS in urban rail vehicles,the optimal sizing of on-board ESS is studied.The method to calculate the annual average cost based on particle swarm optimization is analyzed and an the ESS based on single energy storage element is optimized.Meanwhile,the optimizationmethod of battery initial purchase quantity based on departure condition and charging time is put forward.Moreover,the fast swap-fast charging hybrid mode based on reducing the number of charging stations is proposed and the hybrid ESS is optimized.Finally,the advantages of the proposed different charging modes in terms of economy is verified by case studies and simulation results.Thirdly,to make full use of the high-energy and high-power characteristics of the two energy storage elements,lithium-ion batteries and supercapacitors(SCs),the EMS of hybrid energy storage system(HESS)is studied.In the first,the advantages and disadvantages of the existing topology and EMS of HESS are analyzed.Then,a single objective optimization and rule-based controllor based on minimum loss of HESS is proposed.The SC state of charge(SOC)penalty in standard multi-objective cost function is replaced with SC SOC constraints in each traction stage.Moreover,a simplified method based on variable-step prediction and single optimization algorithm is proposed to reduce the computational burden.Finally,the proposed EMS is verified by simulation results and experimental results.Fourthly,the battery system is very sensitive to its charging and discharging peak current.In order to extend its lifetime and improve the dynamic characteristic of the on-boad ESS,a model predictive control(MPC)for DC-DC converter with peak current limiting function is studied.Starting from the shortcomings of standard PI controller,double loop control structure and model predictive control for DC-DC converters in ESS,the improved MPC is studied.First of all,a novel MPC current controllor with constant switching frequency is proposed according to the above shortcomings.Meanwhile,a current reference design is proposed to reduce the peak value of the inductor current during the dynamic process without requiring additional control loops.Additionally,Luenberger observer is used to improve the robustness of the controller.Finally,the validity and advancement of the proposed improved MPC of DC-DC converters are verified by simulation results and experimental results. |
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