Research On Protection Technology Of Multi-terminal Flexible DC Transmission System Based On Half-bridge MM

The multi-terminal flexible DC transmission system has significant advantages in the centralized grid connection of clean energy and long-distance,large-capacity power transmission.However,when a DC line fault occurs,the fault current rises rapidly,seriously threatening the safety and stable operation of the system.Therefore,researching fault protection schemes for DC lines to achieve fast and reliable fault identification is of great significance for the stable operation of the multi-terminal flexible DC transmission system.The article first summarizes the MMC topology,working principle,and control strategy,and refers to the Zhangbei flexible DC transmission project to build a multiterminal flexible DC grid model.A fault transient equivalent model was established and fault characteristics were analyzed.The operational characteristics of the multi terminal flexible DC grid were analyzed,which provides a theoretical basis for the research and design of protection schemes in this paper.Aiming at the problem of insufficient protection reliability when a high-impedance fault occurs in a DC line of a flexible DC grid,a single-ended information protection scheme based on the sequential overlapping derivative transformation is proposed.The theoretical analysis of the traveling wave of the line-model voltage fault component during internal and external faults is carried out,and the time-domain expression is obtained.Based on the theoretical analysis and simulation results,the difference between the line-model voltage fault components during internal and external faults is obtained,and the forward internal and external fault identification criterion is constructed based on this difference.The direction identification criterion is constructed using the polarity characteristic of the line model current fault component.In addition,the fault polarity selection criterion is constructed by analyzing the variation characteristics of the fault current component.The simulation results show that the protection has sufficient sensitivity to high resistance faults,strong anti-noise ability and good quick action.Aiming at the problem of failure of fault identification criteria in a single-ended information protection scheme when a high-impedance grounding fault occurs at the end of a line,a protection scheme based on double-ended fault information is proposed.The fault feature analysis of the line-model current fault component traveling wave during internal and external faults is carried out,and the time-domain expression is obtained.Based on the theoretical analysis and simulation results,the polarity difference of the line-model current fault component in different fault areas is obtained,and the internal and external fault identification criterion is constructed using this difference.By using the characteristic that the fault current of the fault pole is much greater than that of the non-fault pole,the fault pole selection criteria are established by the ratio of the average positive and negative fault currents.Simulation experiments verify the feasibility and applicability of the double-ended quantity protection scheme.