Study On Fault Line Selection In Non-solid Earth Network Based On The Wavelet Packet Transform

Many countries, including China, adopt non-effectively grounding( isolated, Petersen coil or high resistance earthed)neutral in medium voltage distribution network to reduce outages caused by single-phase-to-earth fault (small current earth fault). When single-phase-to-earth fault occurs, it is difficult to detect the line with an earth fault due to weak current, unstable fault arc and stochastic factors. To date, there is still a lack of a more reliable fault line detection method and a more accurate device for detecting earth fault line, which has seriously blocked the development of reliability of power supply and improvement of power quality. The problem of fault line detection for single-phase-to-earth fault becomes more and more prominent and it is urgent to resolve this problem thoroughly in order to meet the requirements of distribution automation. Therefore, the study of the high accuracy and high reliable automatic fault line detection technology and the corresponding device is significant to the improvement of the power supply reliability, the reduction in the outage loss and the improvement of distribution automation system.In the view of the fault line selection in the neutral indirectly grounded system and based on the deep analysis into the fault line selection in distribution system, the thesis analyzes the principles of various available methods of fault line selection and their traits and restrictions. And it also analyzes the transient and steady features presented after there are single phase-to-earth fault in the neutral non-grounding system and resonant grounded system. The author makes thorough study on the features of the fault transient and ways to utilize the transient information.Fault transient signal is a kind of singular non-stationary signal which has high frequencies. The new wavelet theory, which is superior to the traditional method with the Fourier, is the powerful tool to analyze the transient unsteady signals. Wavelet analysis is widely used to detect singular point of this kind of signal, profit from its favorable localization performance in both time and frequency domain. But it is important to choose suitable wavelet functions. In this dissertation, according to the characteristic of fault transient signal, the principles of choosing wavelet function and scale are summarized and two kinds of B-spline wavelet function are analyzed and simulated. And the derivation of equivalent filter coefficients of B-spline wavelet is presented.Based on the deep analysis into the wavelet theory and analysis of the transient features presented after there are single phase-to-earth faults in the neutral non-grounding system and resonant grounded system, the thesis proposed a novel algorithm for fault line selection in non-solid earth network according to wavelet packet polarity of the first modulus maxima. The new method of fault line selection fully utilizes the transient zero-sequence current of each line after the phase-to-ground fault and the high frequency harmonics in the wavelet analysis.Simulation of fault line selection in the neutral non-grounding system and resonant grounded system is performed in MATLAB. In the simulation, parameters such as grounding position, switch angle, grounding resistance, grounding distance, are all taken into consideration. Results proved that the new algorithm is valid, accurate and reliable.