Research On Low-frequency Stability Of High-speed Railway System With Multiple Vehicles Paralleled

With the large intergration of power electronic converters,some low-frequency oscillations(LFOs)happen in the vehicle-grid system of high-speed railway,which result in traction blockades and influence the operation of railway systems.To tackle this issue,the CRH5 electric multiple units(EMUs)-accessed vehicle-grid system is taken as the instance.Through modeling the output impedance of the traction power grid and the input impedance of the electric vehicles,the whole system's stability is researched.First,the mathematical impedance model of the vehicle-grid system is deduced.The dq-frame impedance of the traction power grid is derived.For the rectifiers in CRH5 EMUs,considering the second-order generalized integrator-based phase locked loop,a more accurate dq-frame impedance model of the single-phase rectifier is put forward.The input impedance of the dual interleaved converters is also derived.Furthermore,through designing the perturbation injection method,the dq-frame impedance measurement approach based on Hilbert transform for single-phase systems is proposed,which solves the failure problem caused by the traditional approach at frequencies higher than the fundamental frequency.The impedance simulations on the grid and rectifiers verify the validity and accuracy of the proposed measurement and modeling methods.Also,the impdance characterastics of the single-phase rectifier is revealed.Second,based on the concept of forbidden region,the forbidden region-based criterion for multi-input multi-output system's stability issues is pur forward,which has less conservertism than other small-gain stability criteria.Using the proposed stability criterion,the condition of the LFO for vehicle-grid systems can be well analyzed.In addition,the impacts of the electrical parameters and control paramters on stability are discussed in detail by the analysis of the proposed criterion and simulations,which provides references for the mitigation of LFO.Last but not least,the stability of the vehicle-grid system is studied systematically and comprehensively by different stability analysis methods.By comparing their results,the characteristics and applicability of different methods are summarized.It is proved that the generalized Nyquist stability criterion works better for accurate analysis of the vehicle-grid system.It can accurately predict the stability boundary and oscillation frequency of LFO.Experiments are implemented to validate the analysis.Moreover,the influence of the control paremters of rectifiers on the low-frequency negative impedance is discussed,and the mechanism of LFO is elaborated from the perspective of negative impedance.Some design-oriented suggestions are also proposed according to the conclusions.