The Study Of The Single-phase Fault Line Selection Of The Small Current Grounding System Based On Wavelet Analysis

The non-effectively grounding method including Ungrounded way and Arc suppression coil grounding and Large resistance grounding is widely used in the distribution network in China.It can continue to supply the load for1～2hours according to the criterion because the fault current is small and the three-phase voltage is still symmetric when single-phase fault occurs in the non-effectively grounding system.And this is the outstanding advantages of the non-effectively grounding system.But the fault line-selection device which has a high accuracy has not occurred until now because of the small fault current.It is obvious that this is not commensurate with the high automated distribution network and is a serious impediment to the improvement of the level of automation of the whole distribution network.So it is meaningful to study the line-section technology of high accuracy in order to improve the level of automation of the whole distribution network.Firstly, this paper summarizes the present existing fault line-selection method and discusses the use occasions,advantages and disadvantages.Secondly, steady-state characteristics,especially transient characteristics are analyzed when single-phase fault occurs in the non-effectively grounding system (including Ungrounded way, Arc suppression coil grounding and Large resistance grounding).The results show that transient signals contain much fault information and we can find better line-selection method if we make full use of these transient signals.Wavelet theory is developed on the basis of the Fourier theory, but it is better in the analysis of non-stationary signals than the Fourier theory.The wavelet analysis can make the time domain and frequency domain localized at the same time and it can decompose the signal into different frequency bands,so it has great advantages in the analysis of non-stationary signals.Also it has the advantages in the analysis of singularity of signals.This paper presents a fault line-selection method based on the first modulus maxima of the coefficients of the wavelet packet transform on the basis of the wavelet theory.This paper builds a simulation model of a small current grounding system by MATLAB and then conducts a large number of simulation experiments. And the results show that the line-selection method is effective and feasible.