| With the continuous development of high-power electronic technology,HVDC transmission technology continues to make breakthroughs,and the VSC-HVDC system based on MMC has become the development direction of the DC power grid.With the launch of CIGRE International Power Grid Committee SC6.31 “DC Power Distribution Feasibility Study” in July 2015,it marked that the flexible DC technology researched by medium voltage DC distribution network has been promoted vigorously.A large number of domestic and foreign scholars have carried out relevant research work on the planning and design,operation and control,key equipment,protection and fault diagnosis of DC distribution networks.This paper first introduces the topology structure of the flexible DC distribution network,the working principle of the MMC basic unit of the modular multi-level converter and the selection of the main equipment parameters of the MMC converter station.Second,in order to better solve the needs of unbalanced load power supply under islanding operation mode,an improved islanding control strategy is proposed in Chapter 4.Based on MATLAB's Simulink simulation platform,a simulation model was built to verify that this control strategy can achieve steady-state operation under unbalanced load and ensure power quality on the power supply side..Finally,combing and analyzing typical fault types,researching the demand of fault operation under single-pole faults of DC distribution network,operating characteristics of DC distribution network during fault operation of AC single-phase grounding belts,operating mode and fault of grounding resistance.The relationship between characteristics,relationship between DC reactor and fault characteristics.In the PSCAD simulation platform,an electromagnetic transient model of a 4-terminal flexible DC distribution network was built to simulate the AC-side and DC-side faults,providing a basis for the configuration of protection and the design of protection settings. |
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