| With the invention and continuous progress of voltage source converter technology,the related theories of flexible DC transmission continue to improve,and the related projects that have been put into operation and are under construction are gradually increasing,which gradually becomes a very important research direction in the field of DC transmission in the future.VSC-MTDC(multi terminal HVDC based on voltage source converter)network is a high-voltage DC transmission system composed of three or more converter stations and their DC transmission lines.It has a wide application prospect in new energy grid connection,large capacity long-distance power transmission and new urban DC distribution network.However,compared with the large-scale upgrading of traditional AC transmission system,the relay protection principle for flexible DC transmission system is not mature.Due to the particularity of the primary system structure and fault transient,most of the protection principles in the traditional AC system can not be used in the flexible DC transmission system.In particular,when the flexible DC system constitutes the DC power grid,the fault at any point in the network may cause the power loss of the whole multi terminal flexible DC power grid,which has a large range of influence.Therefore,the line protection principle of the multi terminal flexible DC power grid becomes the bottleneck of the development of the multi terminal flexible DC power transmission.Firstly,the primary system modeling of four terminal flexible direct transmission network in PSCAD / EMTDC is completed in the paper.By configuring bridge arm overcurrent protection and bridge arm differential protection for the converter,the outage time of MMC type converter under different boundary conditions,different fault types,different transition resistance and inductance parameters is simulated,and the upper limit of feasible shutdown time for multi terminal flexible and straight power line protection is determined.This paper analyzes the complex refraction and morphological characteristics of fault traveling wave based on two different current limiting reactor arrangements.According to Peterson's law,the ratio relationship between incident wave,reflected wave and refracted wave under two boundary conditions is obtained,and its correctness is verified by simulation.Based on the analysis of the traveling wave frequency variation characteristics of the multi terminal flexible straight transmission line,the characteristics of the wave speed difference between the line mode and the zero mode are revealed.Based on the above research,this paper proposes a longitudinal protection scheme for multi terminal flexible DC power grid.Based on the variation of amplitude and ratio of incident wave,refracted wave and reflected wave in time domain in case of line fault in multi terminal flexible DC power grid,a permissive pilot protection is proposed,and the applicable length range of line which can guarantee the feasibility of the principle is discussed.The simulation results show the advantages of the proposed scheme compared with the existing protection principles in balance selectivity,sensitivity and reliability,and the universality of the primary structure of the system. |
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