Research On Low-frequency Stability Of Single-phase Vehicle-grid System Based On State-space Average Model

With the rapid development of high-speed railways,a large number of power electronic converters have been put into use,and when multiple cars are connected to the traction network,low-frequency oscillations of grid-side voltage and current will occur,which will cause the traction network to be blocked and the high-speed railway cannot operate normally..At present,most of the related researches on the low-frequency oscillation phenomenon of the vehicle network system adopt the method based on impedance modeling.This method can intuitively obtain the stability of the system through the Nyquist criterion,but cannot reveal its mechanism in depth.In response to this problem,this paper takes the CRH5 electric multiple units(EMUs)-accessed vehicle-grid system as an example,and uses the state-space average modeling method to analyze the stability of the single-phase vehicle-grid system connected to the non-ideal grid.First,a single-phase vehicle-grid system model connected to a non-ideal grid is established.Using the state-space average modeling method,the main circuit structure of the rectifier connected to the non-ideal power grid is deduced;then,based on the limited information about the rectifier controller of the CRH5 EMU,a closed-loop controller structure using dq voltage and current is built.In order to analyze the low-frequency oscillation phenomenon,the system is simplified to the Thevenin circuit,and the state equation of the single-phase vehicle network system connected to the non-ideal power grid is sorted out,which provides the basis for the subsequent system stability analysis.Secondly,based on the stability criterion of the mixed-potential function theory,the stability analysis of the single-phase vehicle-grid system connected to the non-ideal grid is carried out.A mixed-potential function theory suitable for the vehicle-grid system is proposed and a mathematical model is established.The stability boundary of the system with different parameters can be obtained by calculation,and the stability of the system can be judged by the boundary.Finally,based on the state-space average model,the system equilibrium point is obtained through the state equation,and the system root locus diagram is drawn using the eigenvalue method in the vicinity to obtain the stability of the system.Two different linearization methods are compared to analyze the stability of the single-phase vehicle-grid system connected to the non-ideal grid.The research results show that the analysis method based on the Jacobian matrix theory can simply and clearly obtain the frequency of the low-frequency oscillation of the system,and can analyze the system stability boundary when the system changes with different circuit parameters and control parameters;the stability analysis method based on the Floquet theory disputes.For periodic systems,only the influence of a certain parameter on the stability of the system can be analyzed.In addition,both simulation and semi-physical objects verify the correctness of the theoretical analysis.This paper discusses the influence of different parameters on the low-frequency oscillation of the single-phase vehicle-grid system,and reveals the mechanism of the system’s low-frequency oscillation,and provides reference suggestions for the stable operation of the high-speed railway.