Study On Line Selection And Location Of Single-phase Grounding Fault In Distribution Network Based On Grounding Wire Current

With the rapid growth of urban power demand,the proportion of cable lines in the distribution network is increasing day by day.Urban cable distribution network faults occur frequently,among which the most common fault type is single-phase grounding fault.When the single-phase grounding fault occurs in the three-core cable line of the distribution network,the fault current flows to the metal sheath through the fault point.At this time,the grounding wire current flowing through the sheath changes significantly and contains abundant fault information.Based on the fault characteristics of grounding wire current when single-phase grounding fault occurs in the three-core cable line of distribution network,this thesis studies the method of grounding line selection and fault section location.The main research contents of this thesis are as follows:(1)Based on the analysis of transient and steady-state characteristics when singlephase grounding fault occurs in the distribution network,the single-phase grounding fault characteristics of the grounding wire current on the metal sheath upstream and downstream of the fault point of the normal three-core cable line and the fault line are theoretically deduced,respectively.The conclusion that the grounding wire current at the head end of the sheath is strongly correlated with the zero sequence current of the cable core is verified by simulation in PSCAD.Compared with the steady-state grounding wire current,the amplitude of transient grounding wire current is larger and the fault information contained is rich and easy to be extracted.Therefore,it is selected as the fault characteristic quantity.(2)According to the difference of high frequency oscillation component of transient grounding wire current between fault line and normal line,a method of grounding line selection based on characteristic mode component of VMD is proposed.The high frequency oscillation component of the transient grounding wire current of each outlet line is extracted as the characteristic mode component by using the underdecomposition characteristic of Variational Mode Decomposition(VMD).In order to prevent misjudgment caused by the inability of a single criterion to adapt to the diversity of faults in the distribution network,a dual measure of amplitude and polarity of the characteristic mode component is constructed to realize fault line selection.The effectiveness and adaptability of the proposed grounding line selection method are verified by simulation in PSCAD.Compared with the two grounding line selection methods based on a single criterion,the results show that the proposed grounding line selection method can adapt to various fault situations and has high reliability.(3)A method of fault section location based on VDDTW distance is proposed by using the difference of transient grounding wire current upstream and downstream of fault points of fault line after single-phase grounding fault and combining with the Value-Derivative Dynamic Time Warping(VDDTW)distance algorithm which is very advantageous in measuring the similarity of two time sequences.The VDDTW distance of the fault section is larger than the threshold and much larger than that of the healthy section.Based on this principle,the fault location criterion is proposed according to whether the fault line has branches or not,and then the next search path is selected or the fault section is determined.The effectiveness and adaptability of the proposed location method is verified by simulation in PSCAD with or without branch lines,and compared with fault section location method based on DTW distance,the results show that the proposed fault section location method has higher accuracy.The results show that the method of single-phase grounding line selection and fault section location proposed in this thesis based on grounding wire current has high accuracy and applicability,which provides a new feasible and easy to implement technical route for the study on single-phase grounding line selection and location in distribution network.This thesis has 46 figures,19 tables and 99 references.