| With the continuous improvement and de Velopment of urban rail transit in China,the coverage and mileage of urban rail transit in more and more large and medium-sized cities continue to increase.Since the continuous development of chip technology this year,the cost of fully controlled power electronic equipment has also increased.Continuous reduction,so the opportunities brought to the application of flexible DC transmission technology based on fully controlled devices make it a better solution in the rail transit distribution network.However,in order to ensure the reliability and safety of the flexible direct transmission process,it is necessary to conduct in-depth research on the protection technology in the distribution network of the flexible DC transmission technology.This paper conducts research from three aspects,including grid modeling,fault feature analysis and protection methods:(1)In order to carry out subsequent fault assessment and protection design,the simulation equi Valent model of flexible DC transmission is studied.First,the simplified equi Valent model is analyzed,and the mathematical model of the inverter protection con Verter,flexible DC control algorithm,switching function,dqcoordinate transformation and other mathematical models are specifically protected.The PWM technology and its related control principles are also analyzed.The model was simulated on PSCAD,and it was Verified that the model built can lay a technical foundation for the subsequent analysis of fault characteristics and distribution network protection research.(2)In order to clearly protect the operation state of the DC power grid,on the basis of modeling and simulation,the operating characteristics of each stage of the fault are further analyzed,and the output frequency components of different stages of the fault are further analyzed to obtain The fault frequency domain characteristics are given,and the direction of grid protection configuration is further given based on the frequency distribution char ACteristics of the fault current.(3)On the basis of modeling and fault analysis,the main current protection method of DC power grid is improved,and a principle of current differential protection based on frequency distribution is proposed.Firstly,the frequency distribution of the differential currents within and outside the protection zone is analyzed separately;further,the characteristics of low-frequency and high-frequency bands of the differential current are obtained by the scattered wavelet transform method,finally,the operation characteristics and protection judgment process of the proposed differential current protection are analyzed,and the effectiveness of the proposed protection principle is analyzed using Various factors.. |
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