Research On The Line Protection Scheme And The Transient Characteristics Of Multi-Terminal Flexible Dc Grids After Line Faults

With the rapid development of power electronics technology and the continuous improvement of new energy consumption capacity,the high Voltage Direct Current(HVDC)transmission technology with Modular Multilevel Converter(MMC)as the core component,is becoming an important direction for the development of smart power grid in the future.It has obvious application advantages in scenarios such as new energy grid connection,multi-region grid system interconnection,and island power supply.Modular multilevel converter based high voltage direct current(MMC-HVDC)system presents low damping characteristics.When the DC side line fails,the injection current at the fault point is provided by multiple converter stations,and the fault current will increase to 6 ～ 7 times of the normal operating current in a very short time,which will cause the converter station to lock,and even cause the power grid to shut down,seriously affecting the safe and stable operation of the system.Therefore,the research of line protection scheme of MMC-HVDC system becomes an urgent problem to be solved.Based on the topological structure and characteristics of the half-bridge MMC-HVDC system,this thesis analyzes the fault characteristics and voltage travel wave of the DC line of the MMC-HVDC system,and proposes two kinds of line protection principles for the DC transmission lines of different lengths,including fault starting,fault pole discrimination,fault type and fault range recognition,and fault isolation.In order to ensure the fast and reliable operation of the relay protection device when the DC side line fails in the flexible DC power grid system,the DC side line fault protection scheme realizes dual configuration.The main theoretical research and simulation work are as follows.(1)On the basis of understanding the basic principle,grounding method and control strategy of the MMC-HVDC system,by analyzing the MMC topology,working principle and comparing the functional differences under different topologies,the equivalent half-bridge MMC is established circuit model.Finally,a four-terminal MMC-HVDC system is built on the PSCAD/EMTDC simulation platform,and the feasibility of the model is verified by simulation operation modeling.(2)Taking the half-bridge MMC-HVDC system as the research object,by analyzing the fault characteristics of the DC side line under three different fault types,the equivalent circuit model is obtained,and then the mathematical calculation expressions of the fault current and fault voltage are derived.Finally,the correctness of its theoretical analysis is verified by simulation.(3)By analyzing and comparing the line boundary characteristics when internal and external faults occur on the DC side line,and according to the obvious difference of the highfrequency transient energy of the current when internal and external faults occur,wavelet transform is used to extract the high-frequency component of the current for fault identification.For four terminal flexible DC power system,a fault direction identification method is proposed,which includes protection starting and using voltage polarity difference of current-limiting reactor.After finishing the fault simulation in PSCAD/EMTDC and processing the data with MATLAB,the feasibility of the proposed protection scheme is verified.(4)By analyzing the propagation characteristics of the fault traveling wave on the DC transmission line,the Peterson equivalent circuit is constructed,and there is a significant difference between the exponent coefficients of the propagation items of the line-mode voltage component of the fault traveling wave inside and outside the fault area,the standard fitting function is used to extract the exponential coefficients for fault location,and the theoretical analysis shows that the fault identification criterion is basically not affected by the transition resistance.A fault pole selection criterion based on the sum of the squares of data difference is proposed.The line protection is realized by using the single-ended quantity,and the fault isolation is realized by the mutual coordination and cooperation between the circuit breakers.The simulation verifies that the proposed protection scheme can quickly and identify different distances and different types of faults,meet the requirements of the system for protection quickness and reliability,and has strong ability to withstand transition resistance and antiinterference ability.