Research On Voltage Fluctuation Of Electrified Railway Traction Network And Multi Vehicles Coupling System

With the rapid development of high-speed railway in China,more and more electric multiple units(EMUs)and HXD locomotives(hereafter called Vehicles)have been accessed to the traction power supply system.The electrical coupling problem of vehicle-traction network system is more and more serious,for example,the traction blocking phenomenon appears in electric power vehicles in multi-areas.The structure of traction power supply system network,dynamic characteristics of the electrical quantity,nonlinearity of the electric power vehicle and complex electrical coupling relationship are the key factors that influence the stability of Multi vehicles-Traction network system.Starting with the power vehicle traction blocking problem caused by multi vehicles accessed to traction power supply system,the corresponding research idea is ’Unstable electrical reproduction->Stability analysis of vehicle-traction network system->Analysis of time-varying characteristics and the extraction of unstable modes->Research on load characteristic optimization with nonlinear control strategy and dynamic reactive power compensation’.The main work of this paper is presented as follows.1)Combined with the actual situation of the traction blockades,the simulation model of the ’Vehicle-Traction Network’ system is established.Considering all-around infinite tunnel model and electromagnetic theory,the impedance,the mutual impedance and distributed capacitance of the tunnel wires are calculated.Besides,the coupling coefficient between the loop of traction power supply network and the viaduct piers is obtained.The detailed simulation model of traction power supply system is realized.By analyzing the short circuit impedance,traction network and rail voltage,the model is validated.Based on the transient current control,the model of power Vehicle rectifier is built.Finally,the combined simulation of the vehicle-traction network system is realized,the instability of the electrical quantity of the traction power supply system with multiple vehicles accessed is reproduced,and the main influence factors are analyzed.2)Aiming at the problem of the train traction blockades,the stability of the’Vehicle-Traction Network’ system is studied.The mathematical model and small-signal analysis model of EMUs-traction network cascade system are built.The impedance ratio of the vehicle-traction network system is derived,which is the main influence factor of vehicle-traction network system’s instability.Then,the defects of Nyquist stability criterion are analyzed.Based on Bode graph,this paper proposes a feasible strategy to suppress this phenomenon.By changing the sampling period of the feedback signal of the EMUs rectifier,the load module is changed to match the impedance of the source module.3)In view of the time-varying characteristics of the unstable electrical quantity of the’vehicle-traction network’ system,the modal components of the unstable the electrical quantity are identified.The modulation theory is used to analyze the mechanism of the symmetrical frequency component of the signal transmitted in the single phase two level pulse rectifier.Based on the modal analysis and signal reconstruction,this phenomenon is considered as the voltage fluctuation problem of power quality.According to the time-varing characteristic of traction network voltage under Multi-amplitude and multi-frequency modulation,the envelope extraction of the traction network voltage is studied and a number of voltage signal envelopes are obtained based on ELMD(Ensemble Local Mean Decomposition).Then the symmetric difference Teager energy operator is used to detect the mutation time of the modulated frequency signal,which can realize the segmentation of the ELMD envelope of the signal.Finally,the EEMD(Ensemble Empirical Mode Decomposition)decomposition of the segmented signals is carried out to achieve a detailed analysis of the voltage fluctuations and frequency estimation.Experimental results show that the method can effectively extract the traction power system voltage’s fluctuation characteristic and estimate the frequency and amplitude of the voltage fluctuation.4)In order to suppress this phenomenon,a strategy to optimize the load characteristics of EMUs rectifier by using the auto disturbance rejection control(ADRC)is put forward.First,a first-order mathematical model is established to describe the characteristics of EMUs’ rectifier.Second,the nonlinear state observer and feedback control law are designed for state observation and disturbance estimation.By constructing a new nonlinear state error feedback function,the ADRC is improved.This function’s convergence effect is better than the linear state error feedback function’s,and it can solve the problem of the jitter problem of the control force to some extent.Third,the outer loop controller for PWM rectifier voltage is designed.The improved ADRC can deal with the contradiction between overshoot and rapidity effectively.Its performance is better than traditional proportional integral(PI)controller,which is applied in all of the EMUs at present.Finally,the full model of EMUs-traction network electrical coupling system is constructed.Compared with the multivariable controller,the improved ADRC is verified to be able to suppress the low frequency voltage fluctuation of traction network more efficiently through the simulation results.5)Aiming at the voltage fluctuation caused by the multiple vehicles intensively accessing to the traction power system,a MMC_STATCOM scheme is given.First,the mathematical model of the single leg is modeled,and the current of the main circuit is analyzed.Second,the capacitor voltage control strategy of MMC_STATCOM submodule is analyzed,and the stability analysis is carried out by using the Hurwitz criterion.Third,the mathematical model of this type of MMC_STATCOM is derived,which is suitable for the dq cross decoupling.The MMC STATCOM simulation model is built by using Matlab/simulink,and the model is verified by accessing the Resistance-Inductance Load.Finally,the dynamic performance of the system is analyzed by the analysis of MMC STATCOM accessed to multi vehicle-network system,which is effective to restrain the phenomenon of voltage fluctuation caused by multi vehicle-traction network.