Research On Fault Characteristic Analysis And Protection Method Of Flexible DC Grid

With the de velopme nt of po wer electronics technology,flexible DC transmission technology based o n modular multile vel converter(MMC)has obvious advantages in po wer control,po wer quality,and po wer supply reliability compared wi th traditional DC transmission.Ho we ver,the de velopme nt s peed of the fault of the fle xible direct transmission system is extremely fast,and the c urrent after the fault can reach the upper limit of the converter’s tolerance within a fe w milliseconds,which poses a huge challenge to the relay protection.This paper takes MMC-HVDC transmission s ystem as the research object to establish a simulatio n model,and uses a combination of theoretical analysis and simulatio n verificatio n to study the MMC-HVDC transmission line fault protection scheme from the following aspects.(1)In order to solve the problem of ins ufficient sensitivity and refusal to operate when the traditional fle xible DC transmission line traveling wave protection has a high resistance fault,a single-ended protection for flexible DC transmission line base d on curvature of initial traveling wave is proposed.Based o n quantitative analysis on dispersion effect of the initial traveling wave at bipolar flexible DC transmission lines and its propagatio n characteristics at the line boundary,the expression of initial traveling wave is derive d.And the relations hip between the bending degree of the initial traveling wave and the fault distance,as well as the fault resistance,is o btained.The notable difference between the curvature of internal and external f aults is used to construct the fault ide ntification criterion.Combined with the fault-pole selection criterion,a complete single-ende d protection scheme for flexible DC transmission lines is implemented.The simulation res ults s ho w that the pro posed sc heme c an quickly and reliably distinguish internal and e xternal faults,and has strong tolerance to fault resistance.(2)Aiming at the pro blems of the current pilot protection in the transmission of a large number of non-logic signals,strict synchronization of data,and de pendence on line boundary elements,a pilot protection for flexible DC grids based on amplitude integral of transient wavelet is proposed.Base d o n the analysis of the propagation characteristics of the fault traveling wave in ring DC grids and the difference between the amplitudes of forward and backward traveling waves of the faulty line and the non-faulty line.Combining with the superimposed energy based start-up criterion and the fault-pole selection criterion,a no vel protection scheme inde pendent of bo undary components and s ync hronization was proposed using the ratio of forward and backward-wave amplitude integrals within a period after the fault.The simulatio n res ults sho w that the pro posed sc heme c an quic kly and reliably ide ntify i nternal and e xternal faults,and has stro ng anti-fault res istance and anti-interference abilities.(3)In vie w of the de ad zo ne pro ble m of the pro pose d c urvature protectio n principle,the corres po nding criterion is adde d to form the main protectio n sche me,and the pilot protectio n base d o n amplitude inte gral of transie nt wa velet is use d as the bac kup protectio n.Combine d with the criterio n of pole se lection,a complete coordinatio n sc he me of main protectio n and bac kup protection of the fle xible and straight s yste m is forme d,and the s pecific protectio n process is give n.The simulatio n res ults s ho w that the ne w criterion can signific antly impro ve the dead zo ne pro blem of the c urvature protectio n principle,and the protection sc heme can reflect vario us types of faults in the fle xible s traight s ystem,and has good reliability and resistance to transition resistance.Finally,the main research contents and results were summarized.