Research On Fault Identification And Restart Strategy Of ±500kV Flexible HVDC Grid With Overhead Line

Focusing on the large-scale renewable energy convergence,efficient use and flexible consumtion of clean energy,the pattern of source-grid-load friendly interaction,the MMC-HVDC technology develops rapidly.At present,the converters connect to flexible HVDC grid through overhead line.While the reliability of to the power supply is enhanced,the fault propagation mechanism and characteristics are more complex,and due to the small inertia constant of flexible HVDC grid,fault current rises fast which is a great challenge for the rapid removal of DC fault line and system recovery.The paper bulid the four-terminal zhangbei HVDC grid on the basis of engineering parameters,the overhead line protection and restart strategy is studied,the main research contents and innovations are as follows:Aiming at the high voltage,large capacity VSC-based HVDC grid modeling problem,thevenin equivalent converter bridge arm model and current-limiting and capacitor commutated hybrid DC circuit breaker is proposed The sub modules consisting power electronic devices such as IGBTs and diodes are equivalent to a series connection of resistance and voltage source,which reduces the number of network nodes and simplifies the network matrix,thereby greatly reducing the time for establishing the entire system network equation in the simulation process which greatly improve the simulation speed.Meanwhile,the DC circuit breaker proposed in this paper can rapidly restrain the rise rate of fault current and reduce the amplitude of fault current,and provide enough time for system to detect faults,which can effectively avoid fault misjudgement and improve system reliability.The DC circuit breaker has small difficulty,large opening capacity and good dynamic performance.In view of the transmission mechanism and characteristics of fault characteristics of flexible DC grid,this paper respectively from different different fault locations,fault types and different transition resistance angle,by combining theory and simulation,analyzes the typical electrical transient characteristics,provides a good foundation for the protection of overhead DC lines in flexible DC power systems.The fault characteristics of the flexible DC grid are different from the traditional point to point DC system,and the fault components propagate in the whole DC power grid.In this paper,traveling wave protection and undervoltage differential protection are used as main protections,while undervoltage and over current as well as longitudinal differential protection are used as backup protection configurations.Aiming at the problem that the characteristic of electric quantity caused by excessive transition resistance at fault point is not obvious,a fault identification scheme based on sample value integration is proposed.An identification method based on transient component spectrum characteristics is proposed through the observation of voltage traveling wave spectrum identification of permanent fault and transient fault,to avoid the DC circuit breaker reclosing secondary over-current hazards.After the characteristics of fault line and non-fault line are clearly distinguished,a fault line selection strategy based on single-end electrical quantity and communication based on flexible network is proposed.In view of the high incidence of instantaneous failure on overhead lines,the traditional scheme of restarting of flexible HVDC transmission system is likely to cause secondary overvoltage and overcurrent damage to the system,and the corresponding restart scheme of flexible DC grid fault is proposed.Considering the characteristics of fast rise and high amplitude of current in the case of overhead line fault,this paper puts forward that the current amplitude should be reduced by limiting the current of DC breaker to avoid the blockage of converter.Coordination and utilization of line protection and DC breaker to realize fault line fast isolation;meanwhile,the wind farm side of the converter step-down demodulation control strategy to avoid over-voltage converter station after the action of over-voltage.The simulation results show that there is no impact on current and voltage during the whole recovery process,and the system is safe and stable.