Evolution Law Of Electromagnetic Transient And Protection Strategy For Short-Circuit Faults In A Four-terminal MMC-HVDC Grid With DC Circuit Breakers

Zhangbei ±500k V MMC-HVDC project holds the world's record of voltage level.It is also the first dc grid project which takes dc circuit breakers for fault isolation.Using dc circuit breakers to isolate faults will be more challenging due to the problem of the overlap of fault characteristics caused by dc circuit breaker application.Simultaneously,the usage of dc circuit breakers will generate variation of electromagnetic transient characteristics,which sets requirement of unveiling the fault voltage change mechanism by dc circuit breakers.According to the novel characteristic of the Zhangbei project,this thesis analyzes the electromagnetic transient characteristics of short-circuit faults in the MMC-HVDC grid and provides fault protection method and insulation coordination for the MMC-HVDC grid,which provides support for the dc grid security.The main work and achievements of this thesis are as follows:?The electromagnetic transient characteristics analysis model of a four-terminal MMC-HVDC grid.The fault breaking electromagnetic transient characteristics of hybrid dc circuit breakers with different topologies are analyzed.Unveiling the intrinsic characters of hybrid dc circuit breakers during the breaking procedure.A general simulation model of hybrid dc circuit breakers suitable for transient overvoltage calculation is proposed.According to the parameters of the Zhangbei ±500k V MMC-HVDC project,the key electrical equipments in the converter stations are accurately modelled.?Analysis of electromagnetic transient characteristic of short-circuit faults.The fault development process of a bipolar MMC-HVDC grid is analyzed.The electric network analysis model is built by revealing the characteristic of short-circuit current rapid rising in a small period with large time constant.Building state-equation of a four-terminal MMC-HVDC grid by loop analysis,the evolution law of transient current after short-circuit faults in an MMC-HVDC grid is uncovered.By analyzing the development process of transient voltage propagation after short-circuit faults in the MMC-HVDC grid,the generating mechanism of transient voltage after short-circuit faults in an MMC-HVDC grid is obtained.?Fault identification of an MMC-HVDC grid.The characteristic of voltage change rate based on the boundary effect of dc reactor and the fault current based on moving average algorithm are extracted.Aiming at the overlap of fault characteristics caused by dc circuit breaker application in MMC-HVDC grids,the fast and accurate fault identification method for an MMC-HVDC grid with overhead transmission line are proposed.This method can meet the fault protection requirement of the MMC-HVDC grid with dc circuit breakers.?Evolution law analysis for dc circuit breaker switching overvoltage and insulation coordination research.The breaking characteristics of dc circuit breakers are studied,and the influence mechanism of dc circuit breakers on overvoltage are analyzed.Unveiling that the impotant factor of switching overvoltage in the MMC-HVDC grid is the reverse voltage generated by energy dissipation branch of dc circuit breakers during breaking procedure.The switching overvoltage level of key nodes in converter stations under typical fault are scanned;the switching overvoltage law of dc circuit breakers under different faults are comprehensively analyzed;and the insulation coordination scheme for a four-terminal MMC-HVDC grid are proposed.