Research On Control And Protection Strategy Of Flexible Hvdc Transmission System Based On MMC

The flexible HVDC system based on modular multilevel converter has become the development direction of the future HVDC transmission system because of its better development advantages than the traditional DC transmission technology.This paper starts with the system steady operation control mode and does more in-depth the theoretical analysis and verification for the control strategy of steady-state operation condition,fault operating characteristic and fault protection strategies.Specially,the controlment of steady state operation is basis,fault operating characteristic is medium,and fault protection strategies is studying core.In this paper,based on the topology of the system,this paper introduces the operation mode of the submodule of the system and the basic operation principle of the system.It does in-depth study for the control mode of every layer though dividing the system into the control layer of system level,the control layer of converter station level and the control layer of converter valve.In the control layer of the system level,it establishes the steady state control mode which is combined with the outer loop voltage control and inner loop current control.The mathematical model of the MMC-HVDC system,which is based on the switch function as the core,is established by establishing the system steady state operation strategy.It improved the inner loop control strategy of the system through the analysis of the mechanism of the internal circulation of the converter valve and the fault characteristics of the earth fault of the single phase in the MMC-HVDC system.By studying the PWM trigger pulse of MMC-HVDC system,this paper introduces the commonly used modulation strategy-the nearest level approach.Then on steady operation of the system is based,it does in-depth theoretical study and characteristic analysis for the grounding fault of single phase of AC system,short circuit of phase to phase,the grounding fault of two-phase and the grounding fault of monopolar of DC system.Through the research on the fault characteristics,the corresponding fault protection control strategy is designed.The correctness of fault characteristic and the feasibility and practicability of fault protection control strategy are verified by simulation model.At last,the conclusion of this paper is summarized,and the future work is prospected,which will lay the theoretical foundation for the future application in the practical engineering.