Analysis And Suppression Of Low Frequency Oscillation Mechanism Of Pet-based High Speed Train-traction Network System

High-speed railway-traction power supply network system(hereinafter referred to as train-nerwork system)appears low-frequency oscillation(LFO),which seriously threatens the safety of railway transportation.Therefore,the power electronic transformer(PET)based railway traction system is taken as the research object in this paper.Through establishing the closed-loop transfer function of the PET based train-network system,the stability of the whole system is analyzed with the dominant pole method.Firstly,the mathematical model of the train-network system is established.Based on the interactive characteristics of the train-network system,the equivalent output impedance model of the traction power supply system is established.According to the average modeling method,the average equivalent circuit of the PET is obtained.Based on the transient direct current control algorithm,starting from the main circuit and control system,the closed-loop transfer function of the train-network system is derived.Moreover,the dominant pole method is used to study the stability of multi-trains connected to the traction network.Theoretical analysis and simulation results determine the occurrence of the LFO in the PET based train-network system.Accordingly,the influence of traction network parameters,train electrical parameters,and controller parameters on the LFO are summarized and verified by simulations.Moreover,the mechanism of the LFO is revealed with the damping ration method.The frequency characteristic and trend are also provided,which would be useful for the system parameter design and the stable operation of the PET-based railway traction system.By employing the adanced control methods to effectively suppress the LFO in this paper,the nonlinear PI controller and active disturbance rejection controller(ADRC)–based strategies are provided.Using the adaptive characteristics of nonlinear functions for errors,a voltage control strategy based on a nonlinear PI controller is designed.Using the disturbance observation and compensation capability of the active disturbance rejection theory,a voltage control strategy based on the ADRC is designed.Considering the jitter behavor of the tradictional ADRC strategy may ruin the performance,an improved continuous immune function is used to make the controller provide a smooth output,which are able to enhance the immunity to large errors.Finally,the analysis of the LFO of the train-network system and the effectiveness of the designed suppression strategy are verified through the hardware-in-the-loop(HIL)test platform.The test results indicate the LFO of the PET based railway traction system will occur according to the analysis results.Moreover,it can be concluded from the performance of different controllers that the traditional PI controller behaves a relatively long adjustment process when the operating conditions change(the dc-link reference voltage variation or load variation),and is accompanied by certain overshoots.The nonlinear PI controller has smaller overshoots and faster response time.The ADRC further optimizes the system performance to achieve no overshoot and fast adjustment.Both of the designed suppression methods can effectively suppress the LFO of the train-network system and improve the system stability.