Research On DC Fault Protection Based On Hybrid Bypass In MMC-HVDC

Modular Multilevel Converter based HVDC technology has become a new kind of flexible transmission,which fits new energy grid connected,isolated island power supply,transmission and distribution of large city power grid,AC power grid interconnection and other applications.Modular multilevel converter based on half-bridge sub-module is widely used in engineering because of less elements used.But the multilevel converter based half-bridge does not have the ability of DC fault isolation,as DC fault occurred it can only rely on the ac breaker of ac circuit to remove the fault current,which will take a long time to recovery system running.So on the practical engineering have to choose the more expensive cable as transmission medium.In this paper,the bypass bridge arm is added to the half-bridge sub-module,so that it can be able to cope with frequent overhead line DC fault.And this paper focuses on the DC fault protection based on hybrid bypass and system recovery strategy in the double side MMC-HVDC system.In this paper,the development of the flexible HVDC technology is introduced firstly,and the present study of MMC-HVDC dc fault isolation in the domestic and foreign is summarized,and elaborating the research significance of this article.Secondly,the principle of the flexible DC,the inverter topology structure,working principle,normal operation and DC fault operation characteristics of the converter based on the half-bridge sub-module converter are analyzed,and the mathematical model is established as the basis for simulation.Then,the simulation model of double-ended 21-level half-bridge MMC-HVDC system is built in PSCAD/EMTDC,and the internal and external loop controller is established.According to the circulation of the generation and control of principle,designing the corresponding controller and circulation inhibition.Designing the start controller that without overcurrent in 2-terminal MMC-HVDC.According to the fluctuation mechanism of capacitor voltage,the capacitor voltage sharing voltage control scheme is established,and combining with the nearest level approximation modulation strategy,the modulation wave generation module,sub module capacitor voltage sharing module and the control system for generating IGBT trigger pulse are designed.Starting control,stable operation,power control step changes and three kinds of DC fault simulation in double-ends MMC-HVDC system is analyzed.Finally,analyzing mechanism of dc fault isolation by hybrid bypass and the double bypass thyristor,and making comparison between the hybrid bypass and the double bypass thyristor on fault isolating effect.It is validated that the dc fault isolated by hybrid bypass is quickly.The paper studies mainly the dc fault protection by hybrid bypass and reclosing control strategy,and designing a kind of reclosing method witch is small current and not harm of overvoltage,which make the overhead line flexible HVDC system can deal with transient faults and permanent faults.When the dc fault is permanent fault,the inverter that based on hybrid bypass unlocks to running as STATCOM.The feasibility of DC fault protection and reclosing control strategy is proved by simulation in PSCAD/EMTDC.