| Neutral insulated or Peterson coil earthed power network is widely used in China's 6~66kV distribution networks. It can play an important role in improving power supply reliability of distribution networks. But when single-phase-to-earth fault occurs in this kind of network, the faulty line detection is very difficult to be realized because of the low fault current. In order to solve this problem, many researches are carried out mainly in two fields. One is to improve the precision and sensibility of detecting elements, and the other is to adopt mathematical methods or tools to analyze original fault information in different ways ,so as to find new faulty line detection theory. Great achievements are made after researches of many decades, a lot of kinds of detecting elements and faulty line detection theories are brought forward, but no perfect solution has been achieved till now.Under this background, this paper comprehensively expatiated the basic theory of small current neutral grounding power system and described the characteristics of steady and transient state of single-phase-to-earth fault in detail which occurs in it. Also, this paper introduced every aspects of faulty line detection technology and commented on the strengths and weakness of each theory. And then, a simulation model based on PSB toolbox of MATLAB Simulink is established, which is referring to the actual parameters of a distribution networks.Focusing on the faulty line detection theory which is based on wavelet packets——the most popular topic of faulty line detection research nowadays, an improved faulty line detection method is proposed in this paper by applying both the wavelet packets technology and the correlation analysis technology of digital signal processing. Firstly, each line's transient zero sequence current, which appears in the small current neutral grounding power system when a single-phase-to-earth fault occurs, is decomposed to obtain the suitable outputs of specific frequency bands. And the characteristic frequency band of each line, in which the transient capacity current is most concentrated, is chosen out adaptively according to the maximum energy. Secondly, the correlation analysis between each line's waveform of wavelet packets decomposition coefficient corresponding to its characteristic frequency band is carried out to acquire the correlation matrix and then get each line's integrated correlation coefficient. Finally, faulty line detection, sequenced by the possibilities of grounding fault occurrence, is obtained. Large numbers of MATLAB simulation result showed that this method based on correlation analysis of wavelet packet transformation with muti-frequency bands had ameliorated the existent faulty line detection technology based on the first semiwave method or on the traditional wavelet packets method. It had made great progress in enhancing the accuracy of faulty line detection.But many experiences proved that no single faulty line detection method can be applied in practice universally. So this paper introduced information fusion technology to make full use of all the steady-state and transient-state information of sing-phase-to-earth fault current and proposed a multi-criterion fusion faulty line detection method. Large numbers of MATLAB simulation result showed that this method had integrated many useful faulty line detection methods and can exert the excellences of each one, enlarge the range of right estimation for faulty line. It has more strong adaptability and fault tolerant capability than the single method, and made more progress in enhancing the accuracy of faulty line detection. It ensured the reliability of faulty line detection and has great perspective in the future.
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