Research On The Suppression Method Of Low-frequency Oscillation In Electrified Railways From Traction Network Side

In recent years,with the stunning development of high-speed railway in China,more and more power electronic converters are accessed to traction power supply system.The phenomenon of low-frequency oscillation in electrified railways occurs from time to time.It seriously affects the safety and stability of railway system and traffic scheduling,which causes increasing attention.In this thesis,the low-frequency oscillation caused by multiple electric locomotives connected to the traction power supply system simultaneously is taken as the research object.Through the mathematical model derivation and the simulation model establishment of vehicle-network system,the low-frequency oscillation phenomenon is reproduced,and the stability analysis and online monitoring are carried out.Finally,the mitigation method of low-frequency oscillation problem in vehicle-network coupling system is proposed from traction network side.The main work is presented as follows.Firstly,the small signal impedance mathematical model in frequency domain and simulation model of the vehicle-network system are established to reproduce the low-frequency oscillation phenomenon.On the one hand,the small signal impedance model on the network side is established based on the actual situation of the traction power supply system with Thevenin’s equivalent circuit,including primary power supply system,traction substation,traction network,etc.On the other hand,considering the coupling effect of dq axis,the accurate small signal impedance model of locomotive four quadrant rectifier is derived in the dq rotating coordinate system.Then,based on Matlab/Simulink simulation platform,the low-frequency oscillation phenomenon of vehicle-network coupling system is reproduced.Secondly,the stability analysis and on-line monitoring of low-frequency oscillation are carried out.Based on the above mentioned small signal impedance model of vehicle-network system,according to generalized Nyquist stability criterion and dominant pole diagram,the influence of vehicle-network system parameters on the low-frequency oscillation is analyzed,such as equivalent inductance of traction network,number of locomotives,parameters of locomotive rectifier,etc.In addition,based on the improved Prony algorithm,the monitoring process of low-frequency oscillation is clarified.The oscillation mode of traction network voltage is identified,and the oscillation signal is generated,so as to take corresponding measures to suppress the occurrence of low-frequency oscillation in time.In the end,the thesis puts forward a method to suppress the low-frequency oscillation of electrified railways by installing Thyristor Controlled Series Compensator(hereinafter referred to as TCSC)on traction network side.The impedance of TCSC consists of fixed compensation and adjustable capacitive reactance.The TCSC additional damping controller can reduce the equivalent impedance of traction network,increase the damping of vehicle-network system,improve the system stability,and suppress the low-frequency oscillation.Furthermore,the effectiveness of the proposed approach is verified by mathematical stability analysis and Matlab/ Simulink simulation experiments.43 figures,4 tables,75 references...