Fault Line Selcetion In Non-solidly Earthed Network Based On Adaptive Transient Characteristics

In China, the non-solidly earthed network is widely used in distribution network to improvethe reliability of power supply. But when single-phase to earth fault occurs, due to weak currentand unstable fault arc, fault line selection is not well solved. To date many substations still usemanual switching to identify the fault line.This paper discusses the fundamental and the advantage of the various fault line selectionalgorithms at home and abroad currently, analyses the steady and transient characteristics ofsingle-phase to earth fault from the time domain, makes the widely used cable-wire hybrid lineas research subjects, in depth discusses the transient characteristics of different structure feedersfrom frequency-domain, determines the calculation method of selected frequency band (SFB) fordifferent feeders design systems, demonstrates that in the SFB the polarity of zero sequencetransient current in fault line is to be opposite to those in healthy lines, and points out the SFBobtained by transient-state process is applicable to steady-state process. On these basis, thispaper presents a new fault line selection algorithm by comprehensive use of time-domain andfrequency-domain characteristics of transient signals with the advantage of frequency zoom ofwavelet packet, and designing higher targeted and fitness criterion according to the different faultcharacteristics under different operating modes and different fault conditions. Several key issuesof the algorithm are analysed, including selection of wavelet packet function, treatment offrequency aliasing and dislocation in the presence of wavelet packet decomposition andreconstruction, determination of sampling frequency and the scale of wavelet packetdecomposition, treatment of data boundary, choice of data window and the starting of lineselection algorithm, so that the reliability of the algorithm can be improved effectively.Simulation model of non-solidly earthed network is built using PSB toolbox in MATLAB,and a large number of simulation experiments under various fault conditions are done, resultsshow that the faul line selection algorithm given by this paper is less affected by the position offault, the time of fault and transition resistance and so on, and has features of high reliability andstrong anti-interference. It is suitable to ungrounded system and the neutral point arc suppressioncoil grounding system. This method has important theoretical significant and practical value.