Research On Analysis And Suppression Research Of Low-frequency Oscillation In Traction Supply System With Weak Grid

As Chinese electric railways developing rapidly,a large number of new electric trains have been applied.Due to the high switch frequency of the train converters,low-frequency oscillation frequently occurs in the railway traction power system.Extreme oscillations of voltage and current may cause trains fail to operate normally,which significantly affects the safe and stable operation of the railway system.Low-frequency oscillation is more likely to happen in the electric railway system under the condition of the weak grid.To address this issue,actual railway systems and the CRH5 train are taken as examples to research the key factors influencing the stability on the method of impedance ratio criterion.The original cause why low-frequency oscillation occurs in the situation of weak grid is elaborated,and oscillation suppression approaches enhancing the stability of the system are proposed.Firstly,the equivalent impedance model of the power supply network and the train converters are respectively established to cascade the train-network system.According to the constituents of the power supply network,which is composed of utility grid,main traction transformer in the traction substation and catenary network,the impedances are established to deduce the impedance model of the power supply network.The impedance range of these three parts in the actual power supply network is obtained by investigating the parameters of the traction power system in several places.Meanwhile,based on the structure of CRH5 train and its control strategy,the closed-loop dq impedance model expression of the train converter is derived considering SOGI,PLL,DVC,ACC and other control loops.Secondly,on the basis of the impedance model of train network system,the negative resistor in the dd impedance of the train converter and its cause is further clarified.By deriving the eigenvalue expression of the sequence impedance ratio matrix,the mechanism of the mismatch between the impedance of power supply network and the input impedance of train converters is researched thoroughly.Then influential factors on the both leading to LFO are verified analytically and mathematically.The short-circuit ratio index SCR_t of the train network system is given,and the corresponding values in different cases are list to verify the similarity to the short-circuit ratio in the traditional power system.Thirdly,according to the structure and design parameters of part of the Sichuan-Tibet railway power supply network,the impedance of each site under normal power supply and fault conditions are calculated respectively.The maximum of trains allowed to be connected in various places under corresponding conditions are discussed using theory and simulation models.What’s more,the actual impedances of various power supply network are used to verify the given oscillation mechanism,and corresponding SCR_t are calculated,which provides the reference range of the power supply network to different weak extend.Finally,based on the previous analysis of the oscillation mechanism and influential factors,methods to improve the stability on the grid side and the train side are respectively given.Design principles for reference in the construction planning of the power supply network to eliminate oscillation are given.On the other hand,a feedforward link to realize the virtual capacitor to enhance the stability of the system is proposed,which considers the requirements of stability and dynamic characteristics.The effectiveness of the suppression is verified by simulation.