Dq Impedance Modeling Of Train Under Transient Direct Current Control And Mechanism Research On Low-frequency Oscillation Of Train-netwokr System With Mixed Trains

In recent years,low-frequency oscillations have occurred frequently in the electrified railway in China,which bring severe challenges to the safe and stable operation of the trainnetwork system.At the same time,in the passenger and freight mixed transportation railways such as the Sichuan-Tibet Railway,there are different types of trains with significant differences in operating parameters and control strategies,so the modeling and low-frequency oscillation analysis of train-network system become more complicated.To tackle these issues,the CRH3 electric multiple units(EMUs)using transient direct current control(TDCC)and the CRH5 EMUs using dq-decoupling control(DQDC)were taken as the main research objects in this paper,and a unified impedance model of the cascade train-network system was established.By combining the model with the stability criterion based on the frequency response of the dominant eigenvalue,the low-frequency-oscillation of the train-network system was studied analytically under the conditions of non-mixed and mixed trains with different control strategies.Firstly,a unified impedance model of the train-network system was established by the small-signal impedance method.For the electric train,the structure and working principle of the traction drive unit of the train were clarified,indicating that the focus of train modeling is the grid-side rectifier,then the different control strategies adopted by the rectifier and the corresponding modeling research status were compared.Based on that,a TDCC train rectifier impedance model considering the complete control structure was established in the dq-frame,which is unified with the existing dq impedance model of DQDC train rectifier.And,the modeling accuracy was verified by the impedance measurement method based on time domain simulation.For the traction network,its lumped parameter equivalent impedance model in the dq-frame was established.Then,the grid-side and train-side impedances unified in the dqframe were cascaded to establish a complete impedance model of train-network system.Secondly,a stability criterion based on the frequency response of the dominant eigenvalue was proposed,and it was verified that the criterion is equivalent to the existing generalized Nyquist criterion.However,the proposed criterion can directly read the oscillation frequency,clearly give the characterization of the system stability margin,and decompose the influence of parameters on the system stability into its influence on the imaginary and real parts of the dominant eigenvalues.Based on the proposed criterion,the stability of the train network system was analyzed under the conditions of non-mixed operation and mixed operation of different control types of trains,and verified by time domain simulation based on Matlab/Simulink platform,which proved the validity of the proposed criterion.Finally,based on the original model,the simplified analytical impedance models of different train rectifiers were obtained by derivation,which reveals the existence of negative resistance introduced by the direct voltage controller in the model.Then,the difference of the generating path of negative resistance in different control types of train impedance models are analyzed.By combining the analytical model with the proposed criterion,the analytical expressions of the stability margins of the systems were further deduced under the conditions of non-mixed operation,thereby clarifying the contribution of negative resistance,and revealing the mechanism of low-frequency oscillation and the influence of key parameters.In addition,the analytical expressions of the system eigenvalues under the mixed operation condition are deduced,and the complexity of the influence of the number of trains and control parameters on the stability of the system is pointed out and verified by analysis.