Mechanism Analysis And Mitigation Of Low-frequency Oscillations In Traction Power Systme Under Multiple Trains With Light-load Condition

The reliability and stability of power supply is one of the most concerned problems in safe and stable operation of electrified railway.Recently,the diffusion of modern electric trains equipped with AC-DC-AC traction drive system has introduced new abnormal power supply problems.When multiple trains under the preparation condition in the depot,the low-frequency oscillations(LFO)in traction power supply system have been reported frequently,which result in tractive locking of the train and seriously disrupt the normal railway transport actions.At present,the researches on the LFO is still in the initial stage,and the available documents are not sufficient to explain the mechanism and mitigation solutions well.Therefore,the analysis of the mechanism and mitigation of the LFO in traction power supply system under the multiple trains and light load condition is presented in this dissertation,focusing on the problems of the explanation,mechanism and mitigation on LFO in traction power supply system.The main work of the dissertation is as follows:(1)In the aspect of deep understanding on LFO phenomenon in traction power supply system,the system of synchronous data acquisition equipment is set up to record the voltage and current data of different positions under the actual working condition of the LFO.Then,the voltage and current data of the LFO are processed based on the RMS calculation.The common characteristics of the effective value waveform are same with the envelop of the instantaneous value one,so that it can be more complete and clear to show the characteristics of the LFO phenomenon in traction power supply system.Based on the RMS data and matrix pencil method,the key oscillation mode information of LFO can be extracted successfully,including frequency,damping ratio,residue,eigenvalue.According to the actual measurement analysis,the general characteristics and laws of LFO are detailed summarized.Combined with the above processes,the comprehensive and in-depth understanding on LFO phenomenon in traction power supply system is completed(2)In the aspect of modeling for the LFO analysis,firstly,with the consideration of concrete configurations,the impedance-based model of the train-network interaction system is presented containing the regional power grid,traction power supply system and the train,which consists two parts:the output impedance of source system and the input impedance of load system.Then,based on the state space averaging theory and linearization of equilibrium point,the small-signal model used for the eigenvalue analysis is derived detailed.According to the concrete configurations of train-network interaction system,the time-domain simulation model and the experimental setup are constructed in order to verify the theoretical analysis(3)In the aspect of phenomenon explanation and mechanism revelation for LFO,the stability of interaction system under different conditions of system impedances has been studied based on the impedance-based stability analysis method,which can provide theoretical description on the formation mechanism and potential factors of the low-frequency fluctuation by comparing the results of the impedance-based and simulation methods.Based on the eigenvalue analysis method,the stability and dynamic performance analysis by mean of bode and eigenvalue plot are adopted to explore the causes and the dynamic effects of LFO for real working conditions and reveal the mismatch mechanism between the actual controller and control objectives in detail,combined with the results of frequency analysis,time domain analysis end the experimental verification,which can provide a theoretical explanation for the LFO occurrence in the electric railway(4)In the aspect of mitigation of the LFO,firstly,the suppression suggestion of LFO,based on impedance analysis results,are proposed for the railway authorities,which mainly includes impedance regulation of load subsystem.Then the expanding capacity reconstruction is implemented in North Xuzhou traction substation.The practical operation and compared field tests shows that the reconstructed substation with proposed method successfully removed the LFO.Based on the mismatch mechanism and the effects of controller parameters,an adaptive mitigation scheme is developed,which can adaptively select the suitable parameters according to the actual load power in order to suppress the LFO.The proposed method can be implemented easily with minimal changes to the original system and thus improve the stability and dynamic performance of interaction system.Finally,with the overall consideration of these two mitigation method from the power supply side and train side respectively,supplemented by the optimization from the power supply and railway transport management,the theoretical system of comprehensive mitigation of LFO can be constructedThe means and results of actual measurement analysis,frequency-domain analysis,time domain analysis and the experimental analysis in the dissertation show that the theory system of the mechanism and mitigation for the LFO in traction power systems has been formed including the understanding,modelling and small-signal analysis.It improves the reliability and stability of power supply in electrified railway,which can provide the theoretical and technical guidance for effectively decreasing the impacts from the LFO in traction power supply system.