| The neutral of system is generally indirectly grounded in power distribution network in China. In such system(NUGS), the single-phase-to-ground fault occurs most frequently. To select the fault line and locate the fault point quickly is of great importance for isolating the fault line, as well as improving operating reliability and reducing loss of lost load. However, due to smallness of the current after single-phase-to-ground fault, it is difficult to select the fault line. Until now, none of the existing protection schemes is perfect. As developing of automation in power distribution system, the fault line detection draws more and more attention.Aiming at the problem of fault line selection, this paper reviews systematically the existing theories about fault line detection of the neutral indirectly grounded system, and then points out their defect and drawback. At the same time, the steady and transient feature of single-phase-to-ground fault in various type of neutral indirectly grounded system is analyzed and simulated, this provides theoretical basis for the research on new fault line selection algorithm. Comparison shows the injecting current method(s method) is a preferable fault line selection method. It does not directly use the steady or transient signals in the lines, so overcomes defect of those methods based on such signals. Rooting in the s method, this paper use the zero sequence current of every line after fault as a selection criterion, and use wavelet packet theory to perform fault line selection.The wavelet theory can analyze signal in time domain and frequency domain simultaneously. It is superior to traditional Fourier analysis method, and it is promising to be widely used in power system. This paper introduces the wavelet and wavelet packet theory, finds out theoretical basis for using wavelet packet theory in fault line selection.A new algorithm for selecting fault line based on the wavelet packet theory is... |
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