| With the continuous development of power electronics technology,the DC distribution network based on Modular Multilevel Converter(MMC)is widely concerned by scholars with the advantages of large power supply capacity,small voltage loss and high system transmission efficiency.Although the future development of DC distribution network is promising,the fault routing and fault location technologies for DC lines are still not perfect,especially for single pole grounding faults,which account for more than 90% of the probability of occurrence in DC distribution network lines,and reliable fault routing and fault location solutions are urgently needed.In this thesis,based on the MMC-based single-ended radial DC distribution network model,the characteristics of single-pole grounding faults in DC distribution networks are theoretically analyzed and simulated,and fault routing and fault location schemes for single-pole grounding faults in DC distribution networks are studied based on their fault characteristics.The details of the study are as follows:Firstly,this thesis analyzes the topology,mathematical model,voltage and current inner-outer loop decoupling control,and modulation methods of MMC converters.The topology,wiring methods,and grounding methods of DC distribution networks are studied,and their advantages and disadvantages are discussed.A simulation model of a single-end radial DC distribution network is built in MATLAB/SIMULINK.The theoretical analysis of the fault mechanism and characteristics of single-pole grounding faults in DC distribution networks was conducted,and the findings were validated using the constructed simulation model.Secondly,a DC distribution network single-pole ground fault selection scheme based on the improved injected sinusoidal signal is studied for the zero-mode current and zero-mode voltage electrical components that are unique to the DC distribution network single-pole ground fault.By injecting the improved sinusoidal signal into the MMC converter,the zero-mode impedance at the first end of the DC line is characterized as inductive and apparently capacitive for fault line identification.The initiation and pole selection criteria of single-pole ground fault are studied,the whole single-pole ground fault selection process is investigated,and the selection scheme is verified by building a simulation model of single-ended radial dc distribution network based on the improved injection method,and the simulation analysis shows that the selection scheme has better transition resistance and noise immunity.The comparison and analysis with the directional polarity routing method based on the zero-mode current mutation studied in this thesis further proves the superiority of the active injection method.Finally,on the basis of the selected faulty lines,fault location is performed on the faulty lines of the DC distribution network.In this thesis,a double-end ranging fault location method based on the improved injection method is studied,in which the ranging devices at the first and last ends of the line are put into the faulty line separately to form an RLC second-order underdamped circuit.Least squares fitting is used to extract the characteristic parameters of the discharge voltage and current to derive the equivalent impedance of the ranging circuit at the first and last ends of the line,and the fault distance is solved by using the equation of the relationship between the fault distance and its.A single-pole ground fault ranging model of the DC distribution network is built in MATLAB/SIMULINK,and simulation experiments are conducted considering the line distribution capacitance,transition resistance,fault location,sampling frequency,and noise respectively.Compared with the previous DC distribution network line fault location methods,this method eliminates the influence of line resistance and inductance on fault location and improves the accuracy of singlepole ground fault location in DC distribution networks. |
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