| The frequent braking of urban rail trains produces a large amount of regenerative braking energy.Projects of energy feedback can feed the regenerative braking energy into the ac grid directly.However,lacking inertia and damp of motors,conventional feedback systems using PWM control,which can only transport energy,are more susceptible to power fluctuation and system failures.For this problem,a regenerative braking energy feedback system of urban rail trains,using the sychronverter,is researched in this paper.The system is designed to not only compensate the energy of the dc traction network in the rectification mode but also compensate the energy of the grid in the inverter mode,thus realize the bidirectional flow of energy.In addition,the system can track the frequency and voltage of the grid in time,and adjust the power injected into the grid according to their variations.On this basis,the energy storage module is added as an auxiliary module to maintain the stability of the dc bus voltage when the system applied to the power grid regulation.In this paper,the structure of regenerative braking energy feedback system of urban rail train is introduced in detail at first,and the compositions,type selections and functions of the main power circuit,the synchronverter control and modulation module and the energy storage module are analyzed in detail,as well as the charging and discharging control mode of energy storage unit.Secondly,the operating mechanism of the synchronverter is analyzed according to the theory of the synchronous machine,and the mathematical model of the synchronverter is constructed according to the rotor motion equation,electromagnetic transient equation and flux equation of the synchronous machine.According to the actual control objectives,the control schemes of the system in rectification and inverter modes are designed respectively,the control strategies of the system in self-synchronous mode,rectification mode,inverter feedback mode and power grid regulation mode are elaborated in detail,and the switching method of each mode is introduced in detail.Then taking a certain operation section of some urban rail train as an example,the train running speed,acceleration,running distance,current change,dc bus voltage change are calculated by programming.And the traction energy and regenerative braking energy of single train between the two stations are measured,the demand for the regenerative braking energy feedback is determined,and the dc bus voltage variation of the train in traction and regenerative braking conditions is quantitatively measured.According to the calculationresults,the total capacity of the system is determined,and the parameters of each module of the system are designed.Finally,the transient and dynamic characteristics of the system under normal and abnormal conditions of the grid are verified by simulation,which proves the feasibility and effectiveness of the system and achieves the control objectives.For further verification,a digital control experimental platform based on DSP is established.The experimental results are in agreement with the theoretical analysis and simulation results.The research results show that the system can realize the bidirectional flow of energy in the condition of train traction mode and regenerative braking mode.It can provide traction energy for dc traction network and feed regenerative braking energy back to ac grid.Besides,the system can be independently and softly connected to the grid,track the power grid frequency in real time,and keep synchronization between the system and the grid.Furthermore,the results show that the grid can be easily regulated and safeguarded by this feedback system. |
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