Harmonic Stability Analysis Of Vehicle-grid System In High-speed Railways Based On Harmonic Linearization Method

The high-speed railway vehicle-grid system is a nonlinear power-electronic-based system.Due to the wide time-scale and frequency coupling characteristics of power electronic converters,harmonic distortion or oscillations will be often observed in wide frequency ranges,which will cause harmonic instability problems in the system.According to its frequency range,the harmonic stability problem can be divided into low-frequency oscillations,harmonic resonance and harmonic instability.At present,the impedance-based method is a main tool to analyze the stability of the vehicle-grid system.Since the power electronic converters in electric locomotives are interrelated and the multi-frequency coupling characteristic is obvious,the traditional small-signal impedance modeling method can not be directly applied to the vehicle-grid system.First,a Thevenin-Thevenin equivalent circuit model of the taction network is proposed,considering the advantages and disadvantages of the multi-transmission line model and the lumped parameter equivalent circuit of π type.Then the traction network considering the distributed parameter characteristics is simplified as a dual port circuit with controlled voltage or current sources and the model is verified by the actual field measurement.Last,the proposed dual-port equivalent circuit model is linearized by using the harmonic linearization method to obtain the impedance model suitable for the impedance-based stability analysis.Second,based on the harmonic linearization method,the mirror frequency response of SOGI-PLL is derived.Considering the influence of the time delay,the closed-loop impedance model of single-phase converters based on mirror frequency is established,and then the mirror frequency response of DC voltage ripples is aslo obtained.Aimming for imaging the harmonic transfer relationship of the coupling frequency through converters,the concept of harmonic signal flow diagram is introduced,and then harmonic signal flow diagrams of the SOGI-PLL and single-phase converters are respectively given based on the proposed impedance model..Additionally,based on the impedance-based method,the stability analysis of the vehicle-grid system can be accessed.The influence of the vehicle-grid system topology parameters,controller parameters of converters,time delay and DC voltage ripples on the system stability is discussed.In addition,this paper analysis and summarizes the parameters under where low frequency oscillations,harmonic resonances and the harmonic instability may occur.The hardware in the loop(HIL)simulation platform is built for the verifivcation.Finally,based on the proposed impedance model and the simulation results,it can be concluded that the obtained impedance model of the vehicle-grid system can be used for the harmonic stability analysis and the system topology parameters such as the inductance of the traction network are more related to low-frequency oscillations;parameters of the LCL filter can alert the mode of harmonic oscillations;time delay has obvious influence on the system stability at high-frequency ranges,which can lead to high-frequency problems such as harmonic instability;DC voltage ripples may cause low-frequency oscillations,so it can not be ignored.