Study Of Fault Line Selection And Fault Location For Single-Phase Earth Fault In Small Current Grounded System

Neutral non-effective grounded system is widely adopted in power distribution networks of 6～35kV in China, it also called small current grounded system, and the single-phase-to-earth fault happens most frequently in the system, almost 80% of all faults in the system. During the period of the fault, the voltage of the grounded phase down to zero and the voltage of the ungrounded phase up to the line voltage. Since the line voltage is still symmetrical under fault conditions, the systems can keep operating for two hours. However the fault line must be detected quickly to avoid more seriously fault and destroying devices. The fault line detection is a difficult problem, especially in the resonance-grounded power system, because the small fault current and arcing effect etc. So the research of the fault line detection is very necessary and valuable.This paper presents a method of fault line selection for single-phase earth fault in small current grounded system based on wavelet transform. It analysis the transient course and characteristics of zero sequence current when a single-phase earth fault happened in small current grounded system. A method of fault line selection is brought forward which uses wavelet transform to extract the transient fault information. Transforming the post-fault transient component with wavelet, the fault line is found through comparing the magnitude and polarity of wavelet transform of zero sequence currents. This method overcomes the defects of traditional fault line selection means, which will misjudge the fault line when the steady zero sequence currents are very small. It is free from the fault resistance and the neutral grounding modes and can find the fault line reliability.There is little research to fault location in small current grounded system in modem Power system. The main reasons are the network structure of distribution system is complicated and the fault characteristics are not very obvious,which bring some problems to the research of fault location. The traveling wave fault location has high reliability accuracy and anti-disturbance capability,and it also immunes from the fault resistance and the neutral grounding modes. Therefore, the traveling wave fault location is introduced in this paper, Combined with A type traveling wave fault location theory, singularity in detecting signal of wavelet analyze can help recognize the head wave, Traveling wave is made to reflect and refract between the fault point and the bar when fault occurs. The fault location wi11 be determined by the travel time and speed.In the end, the simulation model is put up using MATLAB and the algorithm of fault line selection and fault location are verified in this paper. The simulation experiments show that the method is reasonable and effective which uses wavelet transform to realize the fault line selection and fault location in small current grounded system.