Research On Protection Method Of Three-terminal Hybrid DC Transmission Line

The rapid growth of electricity demand in eastern China and the abundant energy reserves in western China have made China's electricity production and consumption present an inverse distribution.The long-distance and large-capacity transmission system has become an extremely important part of China's power system structure.Compared with AC transmission system,HVDC transmission system has the advantages of less energy consumption and high reliability.With the continuous progress of power electronics technology,new ideas have emerged in the development of HVDC.Many new systems have been proposed,such as flexible HVDC system and hybrid HVDC system.Typical HVDC systems,such as Wudongde HVDC transmission system,use traditional HVDC at the supply end and flexible HVDC at the receiving end,which can not only ensure that the transmission system has a strong ability to resist overvoltage,but also avoid the occurrence of commutation failure at the receiving end.Compared with a single type of HVDC transmission system,it has better advantages.As an important part of ensuring the stable operation of power system,the protection performance affects the stability of power system.Because of the particularity of HVDC topology,the existing HVDC line protection methods cannot be well applied to HVDC system,so it is of significance to study the protection of HVDC line.The hybrid HVDC transmission model built in this thesis is based on the Wudongde HVDC project as a reference background.Aiming at the actual typical scenario,suitable protection methods for HVDC transmission lines are studied.Firstly,the simulation model of hybrid HVDC true bipolar system is constructed,and the equivalent circuit of line fault and the calculation method of fault current under different working conditions are analyzed.The topology structure and operation mechanism of HVDC system are analyzed.A simplified simulation model of three-terminal HVDC system is built based on the system parameters of a practical project.Aiming at the typical short-circuit faults at the outlet of converter station of supply-end LCC(Line Commutated Converter)and two-receiving-end mixed MMC(Modular Multilevel Converter),the fault characteristics are analyzed,the equivalent models of different fault stages are established,the calculation methods of fault current at different fault stages are calculated,and the characteristics of fault current are verified by simulation.Secondly,a single-end hybrid HVDC transmission line protection method based on time-frequency spectrum is proposed.Based on the analysis of high frequency transient attenuation characteristics of internal and external faults under different working conditions,the protection criterion is constructed by using the magnitude of frequency domain in the time-frequency spectrum of transient current after fault,and the complete protection method is determined.The effects of transition resistance,fault distance and noise on the protection performance are considered,and the effectiveness of the protection principle is verified by simulation.Finally,a lorigitudinal protection method based on the power increment direction is proposed.By collecting the information of current and voltage at both ends of the line,ignoring the influence of high frequency transient,the protection criterion and corresponding protection logic are constructed by using the incremental direction of power in the post-fault line.The anti-interference performance of the protection method against fault location,transition resistance and noise is verified by simulation.