Research On Suppression Mechanism Of Low-frequency Oscillation In The Vehicle-grid System Of High-speed Railways Based On The Dynamic Power Compensation

Recently,with the rapid development of the high-speed railway in China,the AC-DC-AC drive electric multiple units(EMUs)and HXD high-power locomotives(hereafter called vehicles)have been put into operation at high density.The electrical interaction between the traction power grid(hereafter called grid)and vehicle converters can easily lead to the low-frequency oscillation(LFO)phenomenon,which results in the traction blockade of vehicles and seriously threatens the security and stability of the railway system.At present,the suppression methods for the LFOs in vehicle-grid electrical coupling systems(hereafter called vehicle-grid systems)mainly focus on the optimization of the control parameters and control strategies of vehicles,while there are few studies on the dynamic reactive/interharmonic power compensation for the grid.Moreover,the LFO suppression methods based on dynamic power compensation has not been systematically studied,and the corresponding suppression mechanism is also unclear.Therefore,the LFO appears when multiple CRH5 EMUs are energized is taken for instance.Based on the research idea of“Dynamic compensator designing → Analysis of LFO suppression effect → dq impedance modeling and verification → Investigation into LFO suppression mechanism →Optimization of the dynamic compensator”,the study on the suppression mechanism of dynamic power compensation on the LFO in the vehicle-grid system of high-speed railway is conducted.The main works of this thesis are summarized in the following.(1)The design of dynamic compensation devices and the analysis for the corresponding LFO suppression effect.When the LFO occurs,a large amount of reactive power and interharmonic currents appear in the traction power grid,so the straight hanging cascaded H-bridge multilevel static synchronous compensator(STATCOM)and active power filter(APF)are successively designed to realize the dynamic reactive power/inter harmonic compensation.Specifically,the STATCOM aims to provide the reactive power compensation,while the APF aims to provide the interharmonic compensation or simultaneously provide additional reactive power compensation;Then,based on the Matlab/Simulink software,the simulation models of the CRH5 EMUs,traction power supply system,STATCOM,and APF are established.The LFO suppression effect of the compensators and the electrical performance of the system under different compensation modes are compared.Finally,the hardware-in-the-loop(HIL)simulation platform is established,which can reflect the real operation condition of the vehicle-grid system.The LFO phenomenon caused by the connection of multiple CRH5 EMUs to the grid is reproduced.Further,the effectiveness of the compensator in eliminating this type of oscillation is verified.(2)The dq impedance modeling and verification of dynamic compensation device.The CRH5 EMU adopts dq current control,and most of the control system of the designed STATCOM and APF also acts in the dq coordinate system,so the dq small-signal impedance modeling method is adopted in this thesis.Considering the main circuit,phase-locked loop(PLL),DC voltage control loop and current control loop,the dq impedance model of STATCOM and APF are respectively deduced.In particular,for the APF current controller in the αβ coordinate system,the method for transforming it to the dq coordinate system is proposed.Then,to improve the efficiency of dq impedance measurement in single-phase AC system,a simple dq impedance measuring approach is proposed.It avoids the need to inject the additional voltage disturbing signal to the system when the perturbation frequency is greater than the fundamental frequency.Through the impedance measuring,the accuracy of the established dq impedance model in low frequency is effectively verified.Meanwhile,the factors leading to the deviation in the APF impedance above 35 Hz are analyzed,and the error matrices introduced by the second-order generalized integrator(SOGI)and the duty ratio signal are deduced,which reveals the inherent difference in dq impedance modeling between the single-phase AC system and the three-phase AC system.Accordingly,the corresponding method for verifying the correctness of the error analysis is proposed.(3)The mechanism analysis of LFO suppression based on dynamic power compensation.Firstly,considering the voltage drop on the grid impedance,the method for calculating the steady-state operating point of the load subsystem is proposed,which ensures the accuracy of the stability analysis results under different grid parameters.Then,based on the generalized Nyquist stability criterion and the active power consumption criterion,the influence of the APF and STATCOM along with their control parameters on the stability margin and passivity of the vehicle-grid system is analyzed;Given that the existing stability analysis is based on the multiple input and multiple output(MIMO)terminal impedances of the source and load subsystems and makes it difficult to explicitly reveal the LFO suppression mechanism of APF and STATCOM,a dq admittance decomposition method for the two compensators is proposed according to their respective interaction with the rectifiers in CRH5 EMUs.The frequency response characteristics of different dq admittance elements of each compensator in low frequency and their impact on the admittance of the entire load subsystem are analyzed.Further,the RLC equivalent circuit of the vehicle-grid system is deduced based on its equivalent single input and single output(SISO)admittance model.The negative conductive and capacitive features of different dq admittance elements of the rectifier and their effects on the low-frequency stability of the vehicle-grid system are analyzed,and based on this,the influence mechanism of the admittance reshaping effect of each compensator on the system low-frequency stability is finally revealed,which has certain engineering application value.The proposed mechanism analysis method can also be adopted to explain the small-signal stabilizing property of other compensation devices.(4)The optimization of the APF control system.The APF is taken as the research object.To avoid the impact of APF on the system dynamics in the non-LFO region,an optimization approach for extracting the reference current of APF is proposed.It corrects the admittance characteristics of APF in the non-LFO region and also reduces the required compensation capacity of APF effectively.Furthermore,on account of considering both the admittance reshaping effect of APF and the dynamic response speed of the system,a parameter designing method for its band-pass filter(BPF)in the αβ coordinate system and low-pass filter(LPF)in the dq coordinate system is proposed.This research provides some references for the optimization of the dynamic power compensator for eliminating the LFO.Based on the discovered knowledge,the readers can also design the control system of the compensation device themselves to obtain the output characteristics desired in real applications.