Fault Line Selection For Single-phase Earthed Fault In Power Distribution Network Based On Injected Active Current

The6~35kV middle-low voltage power distribution networks in China are mainlyapplied with the small current earthed sytem，which consists of two kinds of system，theneutral unearthed system and neutral earthed system via arc-suppression coil.Among the faultin the small current earthed system，the occurence of single-phase earthed sytem rates thehighest probability，when a single-phase earthed fault happened in the small current earthedsystem，there is only a little tiny amount of zero-sequence current caused by the line-to-earthdistribution capacitance of the system，although the three phase line-to-line voltage remainsthe same as the time before the fault so that the normal power supply for the users will not beinterrupted for a certain time.According to the regulations，the small current earthed systemcan still run for another one or two hours with the single-phase earthed fault unsolved.But thesingle-phase earthed fault will cause the rise in voltage of line-to-ground in the unfaultedphase，which can cause two phase short circuit quite easily after long time running，besides，the whole system over-voltage can be caused by the intermittent arc grounding fault，whichcan destroy the safety of the whole power system.So it is a must that quick finding out ot thefault line and the earthed place.Aiming at solution of the problem of finding out the fault line in the single-phase earthedsystem，the researchers at home and abroad have made a lot of progress.Based on thereference of papers in this field at home and abroad，this paper indicates the history anddevelopment on the single-phase fault line selection in the small current earthed system andthe current situation of research made at home and abroad.Many recent existed fault lineselection methods have been discussed in this paper and the character and short-coming ofthem as well.The character of single-phase earthed fault in the small current earthed system inwhich neutral unearthed system and neutral earthed system via arc-suppression coil includedis introduced in this paper，which is made up of the steady character and the transientcharacter of the single-phase grouned fault. The functions，element libraries and process of simulation of power system simulationsoftware PSCAD/EMTDC is introduced in this paper.The single-phase earthed fault model inthe power distribution network is established by PSCAD/EMTDC to simulate the reactivecurrent in the fault line and unfault line，which prove the effectiveness of injected reactivecurrent method prsented in this paper.Concerning the current situation of fault line selection of single phase earthed fault insmall-current earthed system，plus with analysis of the advantage and disadvantage ofcurrently used in line selection techniques，a method which makes use of neutral pointresistant to discriminate the fault line and the normal line during the single-phase earthed faultis proposed in this paper.A resistance is connected at the neutral point when single-phaseearthed fault happens in order to provide the post-fault system with active component of zerosequence current，which is considered as the criterion for fault line selection.At the last partof this paper the system is modeled by simulation software PSCAD，and the simulationresult shows this method works effectively and improve the accuracy of the line selection.