Research On Identification And Fault Line Selection Of Single-Phase Grounding Fault In Distribution Network

Small current grounding system is widely used in China’s medium voltage distribution network,which has high reliability of power supply and can operate with fault for 1～2h in the case of single-phase grounding fault.However,the probability of single-phase grounding fault is high.With the increase of the number of feeder lines in distribution network,the proportion of cable lines increases greatly,which increases the capacitive current to the ground.If not be cut off in time,the fault is likely to further expand the fault scope and reduce the reliability of power supply.The accuracy of fault line selection in small current grounding system is affected by many factors,especially for neutral indirectly earthed system.The reliability of steady-state fault criterion is greatly reduced due to the compensation of arc suppression coil.Therefore,although experts and scholars have done a lot of research on fault line selection in small current grounding system,a relatively perfect solution has not been obtained.Firstly,based on the summary of domestic and foreign related literature,this paper analyzes the development status and the advantages and disadvantages of the existing fault line selection methods at home and abroad.The change rules of zero sequence voltage and zero sequence current after single-phase grounding fault is analyzed.A simulation model is built in MATLAB/Simulink,and the characteristic of fault signal is summarized through simulation,which is compared with the theoretical analysis.Secondly,in this paper,the phase voltage and zero sequence voltage in the case of unreal grounding caused by the ferromagnetic resonance of power frequency and series resonance are analyzed theoretically in order to make the fault line selection reliable start.An optimized starting criterion for fault line selection of single-phase grounding is presented and verified by simulation analysis.Finally,this paper theoretically explains the method of fault line selection based on phase current increment combined with Fréchet distance and k-means cluster analysis algorithm.And this paper analyzes the phase current increment characteristics of the first monitoring point at the bus line and each line after single-phase grounding fault occurs in distribution network.For the first monitoring point at the bus,fault phase discrimination can be realized by using the characteristics of each phase current increment.For the first monitoring points at the lines,the differences of waveforms between each phase current increment can be used to characterize the transverse feature of the fault,and the differences of waveforms of fault phase current increment among all lines can be used to characterize the longitudinal feature of the fault.Based on these and combining with the Fréchet distance algorithm,the transverse and longitudinal characteristics of the phase current increment are taken as the measures to characterize the fault,and the feature points are mapped to the two-dimensional plane.In order to avoid the setting of threshold,k-means cluster analysis algorithm is used to distinguish the two modes of fault line and healthy line.A cluster analysis fault line selection using the phase current increment characteristics of amplitude in each monitoring point is provided.For trunk lines with multiple branches,the transverse characteristics of the phase current increment combined with the Fréchet distance algorithm are taken as the measures to realize the selection of fault branch based on the level traversal of tree.At last,the correctness of the method is proved by MATLAB/Simulink simulation.There are 75 figures,20 tables and 87 references in the paper.