Study On New Protection Principle Of VSC-MTDC Transmission Lines

With the introduction of voltage source converter technology,the field of DC transmission has entered the era of Voltage Source Converter based High Voltage Direct Current(VSC-HVDC).In the development of several decades,the relevant theories of flexible HVDC transmission technology have been continuously improved,and related projects that have been put into operation and under construction are gradually increasing,which has become a very important research direction in the field of direct current transmission.Compared with the traditional DC transmission technology,VSC-HVDC technology has the advantages of independent-controllable reactive power and active power,voltage polarity unchanged during power flow,and has a very broad application prospect in many new fields.With the continuous development of VSC-HVDC technology,the voltage level and capacity of the VSC-HVDC project are increasing,and the requirements for the DC transmission line protection system are also increasing.Therefore,studying the transmission line protection principle of flexible HVDC transmission system is of great significance for realizing safe and reliable operation of the system.In the currently-operated VSC-HVDC project,the transmission line protection system still uses the protection principle of the traditional DC transmission line,but the traditional protection scheme does not consider the characteristics of the VSC-HVDC system,and it is difficult to meet the requirements of the system for line protection.In order to find a more suitable transmission line protection scheme for VSC-HVDC system and promote the development of VSC-HVDC protection technology,this paper firstly studies the traveling wave generation process and the bounce phenomenon in the DC system transmission line,and analyzes the traveling wave.The dispersion phenomenon in the propagation process and the modulus transformation theory of traveling wave in DC line;and the structural characteristics and control principle of VSC-HVDC system are studied.After that,the applicability of the boundary protection of multi-terminal flexible DC transmission system is discussed.Secondly,based on the traveling wave protection,the propagation characteristics of the fault traveling wave in the VSC-MTDC system are studied.The waveform correlation coefficient is used as the index to analyze the traveling wave fault characteristics under the fault conditions in the area and outside the area,and then use This feature proposes a corresponding protection action criterion and a corresponding longitudinal protection scheme.Furthermore,this paper also uses PSCAD software to realize the performance analysis of the modeling and completed protection scheme of the Zhangbei four-terminal VSC-MTDC system.Finally,this paper analyzes the folding and reflection of traveling waves under special boundaries and introduces the equivalent mean value theorem(Peterson's law)for simplifying the calculation of folding and reflection coefficients,and realizes the boundary equivalent of HVDC-MTDC system;Based on the research,the fault-rolling and reflection characteristics of the faulty multi-end system are found.Based on this feature,a station protection scheme based on the sudden energy ratio is proposed,and the difficulty of threshold setting is avoided in the design process.Thereafter,the protection performance analysis in multiple scenarios is completed.