Faulty Line Selection Study In Small Current Grounding System Single-phase-to-ground Fault Based On Wavelet Packets Analysis Of A'trous Algorithm

In the low voltage distribution network of our country, neutral un-effectual grounded system(NUGS) can be divided into two ones according to different neutral grounding: neutral un-grounding system(NUS) and neutral resonant grounding system(NES). So far, when a single phase ground fault occurs, selecting fault line accurately has not been solved in the running system still. Single-phase ground fault will go through a complex transient process in NUGS and the transient characteristic features are several to several dozen times larger compared to the amount of steady state.Wavelet packet analysis as a time-frequency localization analysis tool, it can solve the problem that the division of the wavelet frequency not detailed enough, more efficient extraction of frequency components of transient, then realize the Fault line detection criterion. In recent years, many researchers have done some relevant researchs and explorations and achieved some good results. However, the extraction of fault features is incomplete and the faulty line detection is insensitive.This paper, on the base of analyzing the steady-state and transient characteristics of single-phase ground fault in NUGS, proposed a new method of fault line detection. The method is based on the characteristics that the algorithm of wavelet packet decomposition of porous properties and multi-band component can maintain linear phase reconstructed, it also combined the selected frequency band(SFB) of the system can adaptively choose the combination of subspace to rebuild the multi-band components. The method solved the problem that the traditional wavelet packet does not have linear phase because the nature of the multi-band component can not be directly reconstructed single, make full use of the greatly limits the use of the fault transient zero sequence current components of the SFB, the given failure value of each outlet can comprehensively reflect the information of the amplitude and polarity, greatly increased the sensitivity of the chosen of the fault lines and enhanced anti-jamming capability, anti-resistance capacity of the transition. A large number of simulation test results verified the accuracy of the method. Also found that, in the NES, for the impact of the arc suppression coil, SFB of high-frequency transient component is very little when small fault angle high impedance ground fault angle occurs. Although the line can give the correct results of the election, but the anti-jamming ability, alignment sensitivity is getting worse.When occurs small fault angle high impedance ground fault in NES, in order to overcome the insufficient sensitivity problem of the line selection based on high frequency transient components, based on the analyzes of the decaying DC component of the single-phase ground fault in system NES, this paper proposed a new method to select out the fault line, which make full use of the trous algorithm of wavelet packet transform to extract effective information of the decaying DC component of zero sequence current, so as to realize the fault line detection based on decaying DC component. This method can be used as an auxiliary criterion of high-frequency transient line selection, the two complement each other. Simulation shows that this fault line detection method has high sensitivity and reliability for various small angle faults.Lots of theoretical analysis and simulation analysises show that the effects of two proposed method of the fault line detection can complement each other. Combining the two methods, it can detect the fault line correctly and with high sensitivity and reliability.