Research And Application Of Single-phase Grounding Fault Line Selection And Instantaneous-permanent Discrimination In Distribution Network

Under the premise of no power failure,the existing fault detection method only uses the one-dimensional information after the fault to detect and analyze the fault,and the amount of information is single.With the application of neutral point equipment in substations,such as the South Network is vigorously promoting the grounding mode of the arc suppression coil parallel small resistance,after the single phase ground fault occurs in the distribution network,the neutral point will actively change to disturb the original network,which will provide multi-dimensional electrical information for fault analysis.With the existing automation equipment,new incentives are generated for the distribution network when the neutral point changes,new electrical information is formed,single-phase ground fault detection of the distribution network is completed,and the reliability and safety of the network are improved.Firstly,several common topologies of the distribution network are briefly introduced.And combined with the active changes of the neutral point,the flow of the fault detection in distribution network is expounded.Using the abrupt energy of zero sequence voltage as the functional start of the the single phase ground fault detection,and then judging whether the distribution network has fault in combination with the change of the state information of the switch and the main frequency of the zero-sequence current,eliminating false start and miss judgment caused by switching action.Then,the intervention timing of the neutral point change is analyzed to reduce the impact on the distribution network,and the high-frequency information of zero sequence current is used to determine the change time of the neutral point.Secondly,because in the small current grounding system,the free-oscillation frequency of the cable-overhead line is typically between 300 Hz and 3000 Hz.In the initial stage of the fault,the transient information of the distribution network is rich,and the sequential overlap derivative transformation can reflect the transient characteristic of the zero-sequence current.Therefore,the single-phase-to-ground fault line selection method based on SOD transform correlation analysis is proposed.And combined with the zero sequence voltage gradient summation as the starting criterion,the sensitivity and rapidity of the fault line selection are improved.The information of the transient zero-sequence current in the 0~2.5kHz band is extracted by wavelet transform and the information is SOD transformed.After the SOD transformation,the similarity between the healthy lines is improved,and the difference between the fault line and healthy lines is increased,so the correlation analysis of the results of the SOD transform is performed.The discriminant vector of the fault line is formed by correlation analysis,and the fault line is determined according to the discriminant vector.Finally,according to the different grounding mode characteristics of the neutral point,the concept of instantaneous permanent fault of the neutral point through the arc suppression coil in parallel with the small resistance system is described.The fault property is predicted by the area change of the zero-sequence component volt-ampere curve of the fault line and the slope of the zero-sequence current envelope one-time fitting function.The input of the arc suppression coil and the small resistance will also generate new electrical information,and use the amplitude and phase change of the zero sequence current before and after the neutral point change to form the criterion of the fault property,and combine the two criteria to determine the property of the fault.According to the judgment result of the fault property,the SOD-SVM or the zero sequence capacitive reactance characteristic is used to determine the fault branch,if the fault does not disappear after the arc suppression coil is put in,the SOD transform is performed on the zero sequence current,and as the input of the SVM,the second classifier constructed by the SVM determines the fault branch;if the fault disappears before the arc suppression coil is put into operation,the capacitance property of the branch is judged according to the bypass direction of the volt-ampere curve,and the fault branch is determined according to the nature of the capacitor.