Study On Control Strategy Of LCL-type Four Quadrant Converter Used In Urban Rail Bidirectional Substation

As the core of the Metro energy feedback device,the four quadrant converter can cooperate with the diode uncontrolled rectifier to act as the energy feedback device during the train regenerative braking,and can also completely replace the existing diode rectifier to play its dual functions of traction rectifier and inverter feedback.When LCL filter is used for filtering,the converter system has resonance problem.When it is serious,there will be large harmonics in grid current,and DC side voltage will be unstable,which threatens the safe operation of the system.In this paper,the mathematical model of LCL type four quadrant converter,the resonance mechanism of LCL filter,the resonance suppression of LCL filter and the stability of converter under weak grid are studied.Firstly,the paper studies the mathematical model of LCL type converter in different coordinate systems,and then gives the resonance mechanism of LCL filter.It is pointed out that the key to the resonance suppression of LCL filter is to increase the appropriate damping and reduce the resonance peak at the resonance frequency.For the passive damping method of capacitor branch series resistance,which is commonly used in engineering,the simulation model is built on Matlab/Simulink to verify its effectiveness.Secondly,the active damping strategy based on gird current feedback is studied,and an improved active damping strategy based on double coordinate system is proposed.The strategy realizes closed-loop control of DC voltage and grid connected current in dq synchronous rotating coordinate system,and active damping strategy inαβstationary coordinate system.After considering digital control,the law of equivalent impedance changing with frequency is analyzed,and the effective range of active damping strategy is found.The analysis results show that there is a frequency point f _Rwith the equivalent resistance of 0,so if the suppression strategy works,it is necessary to design a non-zero equivalent resistance at the resonance frequency f_res.The root locus method is used to analyze the stability of the control strategy,and the active damping control parameters and current controller parameters are designed.The simulation model is built on Matlab/Simulink to verify the effectiveness of parameter design and active damping strategy.Furthermore,this paper analyzes the instability of the converter under the weak power grid and proposes an improved voltage feedforward robust control strategy.The principle of the control strategy is to effectively improve the stability margin of the system at the expense of a certain system bandwidth.Its advantage is that the control strategy is simple and easy to apply in engineering.The effectiveness of the robust control strategy is verified by Matlab/Simulink.Finally,the 50 k W and 2MW four quadrant converters are built,and the parameters and software program of the prototype are introduced.In the 50k W prototype,the passive damping strategy of the series resistance of the capacitor branch is verified.And in the 2MW prototype,the dual functional verification experiments of the converter traction rectifier and inverter feedback are preliminarily completed.