Study On Harmonic Instability Mechanism And Mitigation Methods In High-speed Railway Traction Power Supply System

The harmonic instability issue often causes voltage distortion and capacitive devices explosion in the high-speed railway traction power supply system.Moreover,the instability,block and outage are also happened in the control system of high-speed train.These acci-dents seriously disrupt the order of railway transportation and even endanger its safety.The mechanism of harmonic instability is not yet clear.Distinguished by the frequency components contained in the unstable voltage and cur-rent waveforms,the harmonic instability can be mainly divided into three categories.The first one is those frequency coupled with the grid fundamental frequency,including the fre-quency component of f_xand 2f₀-f_x,in which f_x is the perturbation frequency,and f₀ is the grid fundamental frequency.The second one is those without frequency coupling,including the frequency component of f_x.The third one is those coupled with the sampling frequency,in-cluding the frequency components of f_x and kf_sa±f_x,where f_sa is the sampling frequency.Seen from the frequency range in which the instability issue occurs,the first one mainly occurs in the low-frequency from a few hertz to several tens of hertz,and the second one mainly oc-curs in the intermediate-frequency from 100 Hz to half of the switching frequency.The third one mainly occurs in the high-frequency range above half the switching frequency.Hence,the harmonic instability in the interaction system of“high-speed train—traction power sup-ply network”(hereinafter referred as“train and traction network”)includes a series of small-signal instability phenomena,which may occur in a wide frequency range.Much attention needs to paid to the impedance modeling of the train-network interaction system in a wide frequency range,especially in the high frequency range which is above half the switching frequency.Meanwhile,it is of great importance to evaluate and suppress the harmonic insta-bility issues occurring in different frequency ranges.Therefore,this thesis conducts theoretical researches on the harmonic instability of the high-speed railway traction power supply system,aiming to revealing and eliminating the broadband harmonic instability issue caused by the interaction between the train four-quadrant converter control system and the traction power supply network.On the analysis of the occurrence rule,occurrence mechanism and stability criterion of harmonic instability in the high-speed railway traction power supply system,an effective suppression scheme is proposed.The main work of the thesis can be summarized as follows.(1)The small signal averaging model of train-network interaction system has been established.For the traction power supply network,considering the external power grid,traction substation,contact network and other components,a simplified parameter model and a detailed parameter model for the traction power supply network have been established.For high-speed trains,considering the voltage loop,current loop,phase-locked loop and PWM modules,the small-signal averaging models of the train input impedance in the dq coordinate system,i.e.rotary coordinate system,and the??coordinate system,i.e.,stationary coordi-nate system,have been established.On the basis of frequency scanning method,the small-signal averaging model of train-network interaction system is verified.It provides a theoreti-cal method for quantitative analysis of harmonic instability in the low-frequency and mid-frequency domains.(2)A multi-frequency averaging model of train-network interaction system has been established.With the frequency increasing,the influence of control loop on the train input impedance characteristics is weaken,on the other hand the effect of sideband harmonic components caused by the A/D sampling and PWM modulation processes is strengthen.How to accurately assess the influence of sideband harmonic components is the key to im-proving the accuracy of high-frequency models.Therefore,the thesis has extended the effec-tive frequency upper limit of mathematical model to the sampling frequency.The 2^nd and 4^thorder multi-frequency averaging model have been established to evaluate the effects of the sampling sideband harmonic components and the PWM sideband harmonic components.The frequency scanning method is used to analyze the accuracy of the multi-frequency aver-aging model of train-network interaction system,which provides a theoretical method for quantitative analysis of harmonic instability in the high-frequency domain.(3)The mechanism and influence factors of harmonic instability have been re-vealed.When the impedance interaction points between the traction network's output im-pedance and the train's input impedance fall into the negative damping region of the train's input impedance,the train-network interaction system has the risk of harmonic instability.The accurate evaluation of the negative damping characteristics of the train's input imped-ance is the key to evaluating the harmonic instability.Based on the established small-signal averaging model and multi-frequency averaging model of the train-network interaction sys-tem,the analytical expressions of the train's input impedance negative damping in the low-frequency,intermediate-frequency,and high-frequency domains have been derived,respec-tively.Furthermore,according to the negative damping expression of train's input imped-ance,the influence rule of control parameters on the harmonic instability can be found out intuitively,and the occurrence mechanism of harmonic instability in a wide frequency range can be revealed.Simulation and experimental results have validated the theoretical analysis.(4)A method for suppressing the harmonic instability based on virtual impedance control has been proposed.The harmonic instability is caused by the negative damping characteristics of train's input impedance.The key to suppressing the harmonic instability is to reduce the negative damping area of train's input impedance.Based on the negative damping expressions of the train's input impedance,virtual impedance control strategies based on voltage feedforward variables and current feedback variables have been designed,which can be used to suppress harmonic instability issues in the low frequency,intermediate frequency and high frequency ranges.Additionally,in order to reduce the impact of time de-lay characteristic on the virtual impedance suppression in the high frequency range,a time delay compensation link connected in series with the virtual impedance controller is de-signed to effectively compensate the high frequency negative damping characteristic.Simu-lation and experimental results have validated that the proposed virtual impedance control strategy can effectively suppress the harmonic instability in a wide frequency range,which provides a solution to eliminate the harmonic instability issue of the train-network interac-tion system.