Research On Coordinated Method Of DC Fault Current Limiting And Control In Flexible HVDC Transmission System

A flexible DC transmission system based on a Modular Multilevel Converter(MMC)has the ability to independently control active and reactive power,can supply power to a passive network,and does not need to configure a large number of reactive power compensation equipment and land Small area and other characteristics,the application prospect is broad.Model predictive control(MPC)can meet the constraints and non-linear characteristics of complex control systems.It is widely used in MMC control,but it still has shortcomings such as large amount of control calculation and poor transient characteristics.A DC-side short-circuit fault occurs in a flexible DC transmission system based on overhead lines,which affects the safe and stable operation of the flexible DC transmission system.In this regard,the paper conducts theoretical analysis and experimental research around the optimization of the transient characteristics of MMC-HVDC model predictive control and the suppression technology of the DC-side fault current in flexible DC transmission systems.The main work is as follows:First,the topology and working principle of MMC-HVDC based on Half-Bridge Sub-Modules(HBSM)are analyzed.A transient characteristic optimization strategy for predictive control of MMC model is proposed,and the voltage prediction is rolled and optimized The trajectory softening method is applied to MMC control,which not only reduces the calculation amount of the cost equation,but also improves the accuracy of the MPC model parameter calculation.Based on Matlab/Simulink platform,simulations verify the correctness and effectiveness of the proposed method.Secondly,the fault characteristics of unipolar grounding and bipolar short-circuit faults on the DC side of MMC are analyzed,and the fault current expressions before MMC lockout during unipolar grounding and bipolar short circuit faults are derived in detail.Based on the PSCAD/EMTDC simulation platform,Simulation verification for MMC-HVDC system at both ends.Full-Bridge Sub-Modules(FBSM)with fault self-clearing capability is analyzed.A hybrid MMC structure with HBSM and FBSM as basic sub-module units is selected,and fault current coordination of hybrid MMC is proposed.Suppression strategy,the critical proportion of FBSM in hybrid MMC is analyzed,and simulation verification is performed based on PSCAD/EMTDC platform.Finally,according to the principle of the capacitor discharge of the sub-module during the DC side fault of MMC-HVDC,an experimental circuit for simulating DC fault is designed in a low voltage environment.The experimental circuit can well simulate the DC fault characteristics.An experimental scheme to verify the fault current suppression by hybrid MMC is designed.The degree of suppression of fault current by different proportions of FBSM is compared,and the suppression effect of hybrid MMC on fault current is verified.