Study On Frequency Coupling And Stability Of Vehicle-Grid System In High-speed Rail Based On Harmonic Linearization Modeling

A large number of power electronic devices are used in the EMUs of high-speed railway.The control strategy and parameters of different vehicles are different.The vehicle coupled with the grid is a typical time-varying nonlinear system.The complex interaction of electrical relations in the vehicle-grid system may lead to three kinds of electrical instability problems: low-frequency oscillation,harmonic resonance,and high-frequency instability.Therefore,it is of great theoretical significance and practical engineering value to study the electrical instability of the high-speed vehicle-grid system.Based on the impedance modeling method of the cascade system,this paper conducts in-depth research on the frequency coupling process and electrical stability of the high-speed vehicle-grid system.The specific research contents are as follows:Firstly,the low-frequency modeling of traditional positive and negative mirror frequency impedance is inaccurate.dq impedance has some problems,such as unclear physical meaning,limited modeling accuracy,complex modeling and measurement process.In the improved model,the impedance solution method is corrected,and higher-order disturbance harmonics are considered.To verify the accuracy of the model,the conversion rules between dq impedance and positive and negative mirror frequency impedance are proposed.Based on the impedance measurement method,the proposed model is compared with two traditional impedance models,which proves that the proposed model has the highest accuracy.Secondly,aiming at the complex unclear harmonic transfer and frequency coupling process,this paper draws a single-phase converter’s harmonic transfer flow diagram based on the harmonic linearization method.The frequency coupling mechanism is analyzed based on the harmonic transfer flow diagram.And frequency expression of single order frequency-single disturbance,high order frequency-single disturbance,and high order frequency-multiple disturbance are calculated.The harmonic transfer and frequency coupling study has been applied to PLL modeling,impedance measurement,and stability analysis.It has achieved a good effect in Matlab/SIMULINK simulation platform.Finally,three kinds of electrical instability phenomena including low-frequency oscillation,harmonic resonance and high-frequency instability are analyzed based on the positive and negative mirror frequency impedance model.The results show that low-frequency oscillation is an inherent property of single-phase converter and may occur due to the mismatch of parameters of the cascade system.Harmonic resonance depends on the capacitive inductive parameters of the traction network and the harmonics introduced in the switching modulation process.High-frequency instability is developed from harmonic resonance because of high frequency negative impedance.These results were verified by the Starsim/HIL hardware-in-the-loop platform.