Research On Fault Characteristics Analysis And Protection Method Of MMC-HVDC

In recent years,the research on modular multilevel converters(MMC)mainly focuses on system modeling,simulation and control system and so on.The control and protection of HVDC transmission system based on MMC mainly focus on the research of DC fault ride-through based on full bridge sub-modules,self-resistance MMC-HVDC with DC fault isolation capability and DC fault protection methods based on the DC circuit breaker.The fault handling of flexible HVDC transmission line relies on the fast and reliable identification of fault.At present,in the flexible DC transmission project,the protection strategy of DC line is based on the traditional LCC-HVDC system,which takes the traveling wave protection as the main protection and the differential undervoltage protection as the backup protection.However,both traveling wave protection and differential undervoltage protection are based on differential quantities of signals and are prone to misoperation.Flexible HVDC transmission system using overhead lines for large capacity and long-distance transmission will become the focus of future development.Therefore,it is of great practical significance to carry out research on flexible DC line protection to improve its rapidity and reliability.The main work of this paper is as follows.Firstly,this thesis introduces the development,current situation and prospect of the flexible HVDC technology,expounds the current situation of MMC-HVDC DC fault and protection methods,states the existing problems in current flexible HVDC protection,and clarifies the necessity of the following research.Secondly,this thesis analyzes the basic principle of MMC-HVDC,and mainly studies its operation characteristics,mathematical model and control strategy.On the basis of the research,this thesis introduces the Zhangbei four-terminal flexible DC power grid project,which is planned to be constructed in the future,and builds a primary model based on PSCAD/EMTDC with actual engineering data.Thridly,based on the model of Zhangbei four-stage flexible HVDC transmission system,this thesis constructs a bipolar HVDC model for simulation,analyzes the fault mechanism and fault characteristics of the fault on the dc side of the flexible HVDC transmission line,and simulates and analyzes the faults,including line-breaking faults,unipolar grounding faults and bipolar short-circuit faults.Fourthly,based on the existing line protection methods,this thesis proposes two new protection method based on traveling wave protection of hvdc.One is a new traveling wave protection method based on control tracking differentiator(TD),which can improve the anti-interference capability.The other is Directional longitudinal protection method based on traveling wave energy.This method mainly presents the fault direction identification criterion,the fault pole discrimination algorithm and the complete steps of the final protection realization.Finally,this thesis constructs a high-speed protection experimental platform based on the hardware simulation system of FPGA.This platform and MATLAB are used to carry out hardware in the loop simulation experiments,which verify the correctness of the algorithm based on the tracking differentiator and the high-speed characteristics of the platform.