Research On Fault Protection Strategy Of DC Distribution Network Based On MMC

With the large-scale grid connection of distributed energy and the large number of DC loads connected,the current load source storage DC characteristics of the distribution network are becoming increasingly obvious,and the DC distribution network is more suitable for the development trend of the future power grid.Thanks to the development of power electronics technology,Modular Multilevel Converter(MMC)has become a research hotspot in the field of DC distribution networks due to its advantages of low switching frequency,low losses,and easy expansion.However,the complexity of control strategies and the fragility of power electronics devices have increased the difficulty of fault protection in DC distribution networks.Based on a thorough study of the theoretical points of MMC,this thesis analyzes the fault characteristics of two typical faults on the DC side of a medium voltage DC distribution network with dual power supply using a combination of theory and simulation,and provides corresponding protection strategies.Firstly,the topology and mathematical model of MMC are analyzed to determine the type of submodule used in this thesis.Then,a dual closed-loop controller design,a collaborative control strategy design combining modulation method and submodule capacitor voltage balance are carried out for MMC.On this basis,an MMC simulation model is established in the Matlab/Simulink environment to provide simulation conditions for the study of fault protection strategies.Then,taking the blocking time of the converter as the boundary,the fault current characteristics of the inter pole short circuit are derived mathematically.A fault current suppression strategy is studied by changing the feedback value of the outer loop controller to adjust the system output power in response to the characteristics of fast rise and high amplitude of fault current before the converter is locked;A fault current suppression strategy based on additional resistors is studied to accelerate the decrease of fault current in response to the high overcurrent level of the system after the converter is locked.Based on the implementation of current suppression strategies,the difference in current direction between adjacent protection configurations for intra zone faults and out of zone faults is analyzed.A current directionality based overcurrent protection method is proposed,which can accurately isolate faulty lines under different transition resistances and fault distances.By organically combining with current suppression strategies,the action time and breaking current level of the protection have been improved.Finally,in response to the problem of unclear fault components and rapid transient processes during single pole grounding,which makes it difficult to extract fault features,this thesis analyzes the formation mechanism and flow direction of fault current,points out the differences in the composition of transient capacitive current at the first end of fault and non fault pole lines,and constructs a single pole grounding fault line selection method based on transient capacitive current energy based on this difference.The principle of this protection method is clear,it is almost unaffected by transition resistance and fault distance,and has strong noise resistance.Numerous simulations have proven its reliability and applicability.