Research On Low-frequency Oscillation Phenomenon Of Electrified Railway Based On Frequency Domain Analysis

With the rapid development of China's high-speed railways,a large number of AC-DC-AC electric locomotives have been put into operation in the power grid,and the interaction of the traction network and train coupling system has become more complicated.In recent years,low-frequency oscillations of railway electrification systems caused by improper electric matching between high-speed trains and traction power supply systems have occurred frequently.This phenomenon often results in the traction lockup of electric trains and faulty action of traction networks,which seriously affects the normal transportation of the hub.This thesis takes the high-speed railroad car network coupling integrated system as the research object,studies the low-frequency fluctuation phenomenon of the traction network voltage and current,establishes a traction network and train coupling model,reproduces the low-frequency oscillation phenomenon,analyzes the characteristics of the oscillating waveform,and focuses on the impact factors of low-frequency oscillation and its suppression measures,which provides a theoretical support for the actual governance of the low-frequency oscillation of the traction network and train coupling system in the electrified railway.Firstly,this thesis simulates the time-domain simulation of the traction network and train coupling integrated model.The simulation accurately calculates the parameters of the equivalent impedance of the traction network side transformer,the equivalent impedance of the power supply line and the capacitance of the ground.The equivalent circuit of the network side is modeled using the equivalent Circuit of Thevenin Circuit.Taking the HXD2B locomotive as an example,the thesis introduces the topological structure of the AC/DC transmission locomotive,the working principle of the circuit and the transient current control strategy of the converter.,also builds an integrated model of the vehicle network based on the Matlab/Simulink platform and verifies the validity of the model.Secondly,the characteristics of the coupling system are analyzed based on the simulation results.The thesis analyzes the change of positive and negative alternating instantaneous power when the oscillation occurs,and compares it with the instantaneous power when the system is in stable operation,also analyzes the phase characteristics of the network voltage and current.What's more,FFT analysis is carried out to investigate the relationship between simple harmonic and low frequency oscillation when multiple locomotives running simultaneously.Furtherly,in order to analyze the phenomenon of low-frequency oscillation,the factors affecting the stability are analyzed according to the impedance ratio criterion.Based on the analysis of transient direct current control strategy,a single-input single-output transfer function model for locomotives is obtained.Bode graph theory is used to indicate the stability of the current loop control parameters,the voltage loop control parameters,the length of the power supply line and the power supply capacity of the traction network.And the dominant pole theory system is used to solve the critical value of the main influence parameter,which provides theoretical reference for the parameter setting in actual engineering.Finally,focusing on the above stability analysis results,the effectiveness of low-frequency oscillation suppression measures was highlighted.The temporary vibration suppression of the engineering field is realized by adjusting the number of locomotives.Through adjusting the voltage of the converter parameters,joining passive virtual impedance suppression strategies,and joining the low-pass filtering feedback damping compensation strategy,the above suppressions improve coupling system stability from the source;and then improve the system anti-interference ability.The correctness and effectiveness of the above-mentioned suppression method are verified based on time-domain simulation model,and the superiority of the three suppression methods are compared,and the optimized suppression solution is proposed.