| With the acceleration of urbanization in China,urban rail transit has developed rapidly compared with other modes of transportation due to its unique advantages of large traffic volume,low energy consumption and high safety.Energy storage technology,as an effective measure to improve energy utilization and power quality of DC grid in urban rail transit system,has attracted wide attention.Supercapacitor is a kind of energy storage device with the features of high power density,low pollution and long lifetime.The energy storage system(ESS)composed of supercapacitor has unique advantages and broad application prospects in urban rail transit system.In order to improve the applicability of supercapacitor ESS in the DC grid of 1500 V urban rail transit,this paper takes modular supercapacitor ESS as the research object,and conducts in-depth reaearch on the key technologies such as power conversion topology,power conversion and energy balance integrated control strategies etc.Based on the problems existing in the application of ESS in engineering,this paper makes a thorough principle analysis of the ESS and puts forward the key technologies in order to find solutions.Firstly,the topology of bidirectional power converter for energy storage system is studied.DC grid voltage of 1500 V urban rail transit is tested online,and the disturbance of DC gird voltage caused by on-line running vehicle is analyzed.The application requirements of supercapacitor ESS in DC grid are clarified.In order to solve the over-current issue of traditional MMDDC caused by the sudden change of DC grid voltage and the pre-charging problem when system starts up,a combined MMDDC topology suitable for ESS is proposed.Based on the analysis under different working modes,the basic control strategy of supercapacitor ESS based on MMDDC is proposed.The feasibility of combined MMDDC for supercapacitor ESS is demonstrated in depth.Secondly,the energy balance control strategy of the energy storage module is deeply studied for the ESS applied in 1500 V urban rail transit DC grid.Different from the system control strategy which treats energy balance and power conversion independdently,the modular energy storage system(MESS)has independent control ability of sub-modules.In this paper,energy balance control and power conversion are achieved by system power circuit,so that the ESS can realize energy balance control of supercapacitor module simultaneously in the dynamic process of system power conversion.Three sub-module energy balancing control strategies are proposed for different applications of DC grid voltage level,and the balancing ability,system current control performance and stability are analyzed in detail.The proposed energy balance strategiese are helpful to simplify the control structure of ESS and realize the integrated control of power conversion and energy balance.Supercapacitor ESS based on combined MMDDC can operate redundantly.Research on fault diagnosis method and redundancy control strategy of power converter can improve the reliability of the system to a certain extent.In the thesis,a fast open-circuit fault diagnosis algorithm is proposed to diagnose sub-module open-circuit faults generated by control unit or power conversion unit.By implementing the proposed redundancy control strategy,the fast removal of fault sub-modules and seamless access of cold backup modules can be realized.Open-circuit fault diagnosis algorithm and redundancy control strategy are helpful to ensure the long-term safe operation of ESS.Finally,the dynamic performance and power quality of ESS are studied in order to improve its control performance and expand its application field.A linear extended state observer is proposed to observe the variation of load current in DC grid and feedforward the observation result into the control system to improve the dynamic response speed of the ESS to suppress the load disturbance to DC grid.To suppress second ripple current in the ESS,a virtual impedance control strategy is proposed to make the ESS have high output impedance at the second harmonic frequency,so as to suppress the second ripple current into the system.In this paper,the key technologies of supercapacitor ESS based on MMDDC,such as over-current problem in charge mode,sub-modue energy balance strategy,open-circuit fault diagnosis and redundancy control strategy,and power quality optimization control,are deeply studied and analyzed.Aiming at these problems,the integration of supercapacitor ESS is sought by combining the modular structure of the system,and proposes some control strategies.The proposed topology and control strategies are verified by the physical simulation experiment platform.The research work of this thesis has certain reference value for the industrial application of modular ESS. |
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