The Research On Study On Control And Fault Protection Strategy Of DC Side Based On MMC-MTDC

Compared with traditional DC transmission technology,the flexible DC transmission system based on Modular Multilevel Converter(MMC)has better development advantages and meets the development trend of DC transmission in the future.In this thesis,the MMC-MTDC system same as the research object based on MMC converter station,the working principle,topology structure,control methods at each level,fault characteristics,and protection strategies after the fault are studied and analyzed.Based on the three-terminal MMC DC transmission system,this paper analyzes the topological structure and working principle of the submodules in a single MMC converter station,and then analyzes the working principle of the three-terminal MMC DC transmission system,and then performs mathematics under the DQ axis.The establishment of the model prepares for the subsequent controller design.Then,in the control mode,the method of selecting the nearest level is selected,and the station-level control method of inner-loop current control and outer-loop power control is adopted.In terms of system-level control,this paper compares different control methods after considering the demand for DC voltage and power.A multi-point DC voltage control method based on voltage margin was selected.Finally,a corresponding three-terminal MMC DC transmission system simulation model was built in PSCAD to verify the simulation.Finally,this thesis analyzes the various faults that the MMC-MTDC system will encounter.Firstly,the faults on the AC side are analyzed,and various faults that may be caused on the AC side are proposed.Then the corresponding protection measures are proposed.Then analyzes the types of faults that may be encountered inside the converter station,and proposes solutions by analyzing the causes of the faults.and enter the DC-side fault protection that this article focuses on,and analyze in detail the two types of faults on the DC side transmission line based on the MMC-MTDC system—unipolar ground fault and inter-pole short circuit fault.In-depth analysis,and then proposed a corresponding protection strategy,so that the system maintains stable operation.Through simulation results on the simulation software PSCAD,a multi-point DC voltage control strategy based on voltage margin can be obtained,and the corresponding controller designed can make the system convert the control mode of the converter station automatically under the change of power demand.Finally,youcan control the active power of the entire system.In other words,even if the main converter station is out of operation,the remaining system can continue to maintain stable power supply based on adopting the control strategy of this paper.When any fault occurs on the DC side,due to the multipoint DC voltage control strategy based on voltage margin control,the system will quickly return to a stable state,so that the fault is quickly eliminated,and the stability of the system operation is improved.