Research On Low-frequency Oscillation Suppression Of Electrified Railway Based On Active Power Filter

With the large-scale operation of AC-DC-AC high-power electric vehicles and electric multiple units(EMU)trains,a large number of power electronic converter devices have been connected to the traction power supply system.It causes many problems such as lowfrequency oscillations in the traction power supply system.This phenomenon result in traction blockades and seriously affects railway safety and orderly Operation.Aiming at the problem of low-frequency oscillations in traction power supply systems,this paper proposes research on the suppression of low-frequency oscillations in electrified railways based on Active Power Filters.Firstly,the vehicle-network coupling system model of the traction power supply system is built.It could be the basis for low-frequency oscillation analysis.Taking the CRH5 EMU trains as the research object,and referring to the control strategy of its 4-quandrant converter(4QC),the impedance model of the vehicle’s 4QC in the dq coordinate system was derived using the impedance-based small-signal modeling method.The vehicles input impedance model could be derived based on the 4QC impedance.The Thevenin equivalent circuit model of the traction power supply system was established,and the closed-loop transfer function of the vehicle-network coupling system was derived in combination with multiple vehicle loads.The stability of the closed-loop system was analyzed using the generalized Nyquist stability criterion.The Matlab / Simulink toolbox is used to build a simulation model of the vehicle network system.It is verified that increasing the number of vehicles will induce low frequency oscillations in the vehicle network system,which is basically consistent with the results of impedance modeling and analysis.It provides a reference for the following mathematical modeling and simulation analysis..Secondly,the paper introduces the basic principle,topology and control strategy of APF,which could be applied to traction power supply system.The small signal input impedance model of APF is derived and established in detail.Combining with the load system,a closedloop transfer function model of the vehicle-network system including APF is established.The generalized Nyquist criterion is used in stability analysis of the vehicle-network system including APF.The result shows that APF could effectively improve the stability of the vehicle-network system,and suppress the low frequency oscillation in the system.The simulation model was used to verify the suppression effect of APF on the low-frequency oscillation of the vehicle-network system,and the influence of APF control parameters on the stability of the system was analyzed.Thirdly,Based on the Bode diagram,it could deduce that APF could reduce the negative impedance region in low frequency band of the load system impedance.Thus it could suppress the instability of the low frequency band of the vehicle-network system.In order to further enhance the stability of the system,a virtual impedance control method based on APF is proposed.By improving the APF control,the equivalent parallel impedance at the APF access point is used to further correct the ‘APF-vehicles’ system impedance to enhance the suppression ability of the APF.A small-signal impedance model of APF was established.The suppression effect of APF-based virtual impedance on the low-frequency oscillation of the traction power supply system was verified by stability analysis,and simulation models.Finally,by setting control blocks in the APF,using the low-frequency oscillation components could realize the adaptive adjustment of the virtual impedance.It improves the suppression effect and adaptability of the low-frequency oscillation in the vehicle-network system.The adaptive control makes the APF could work well in complex load conditions.