Study And Application To Fault Line Detection In Distribution Network Based On Wavelet Analysis

Neutral un-effectual grounded system are widely used in power distribution network in China. The single phase-to-ground fault happens most frequently in distribution network. Due to the small fault current and unobvious fault features after single phase-to-ground fault, it easily leads to the wrong detection of the fault line. Especially for the arc suppression coil grounded system, the remnants current of the grounded point on fault line is very thin, and the phase is somehow changed, thus, it's more difficult to detect the fault line in Peterson-coil grounded system. The problem of fault line detection has been studied for years in our country and abroad, however, it still has not been solved perfectly, which has seriously blocked the development of reliability of power supply and automatization level of distribution network.Firstly, the stable and transient features of single phase-to-ground fault in neutral un-effectual grounded system are deeply researched and analyzed in theory. The experiment results show that there are many high frequency component features in transient fault currents. If these fault features can be analyzed and extracted, so that a novel fault line detection criterion can be formed. Wavelet analysis is localized both in time domain and frequency domain, and can cut any signal into different frequency components, which makes it a suited tool for analyzing unsteady signal and transient signal. Theories and concepts about wavelet analysis are described in detail, which provides theoretical basis for the extraction of fault features. Then, a new method based on wavelet analysis is presented after combining with the advantages and disadvantages of the existing fault line detection methods. The fault line can be detected in terms of the features of the maximal magnitude and the corresponding polarity oftransient zero-sequence current in feature scale. Simultaneously, some problems such as how to select wavelet basic function and determine the feature scales etc, are discussed in this thesis.In order to solve the problem of overvoltege and the difficulty in fault detection when the permanent fault happens, a scheme that the arc suppression coil parallelled with a medium resistance instantly is put forward. Then, a improved method of fault line detection for permanent fault in mains: when permanent fault happends in mains, to choose the fault line firstly by the criterion above, then to choose it primarily by the method of detecting the second break point when the medium resistance is launched in. By using this improved method, both the accuracy and reliability are enhanced, the voltage of neutral point is suppressed effectively when the grounded fault happens, so the probability of overvoltage in the system is strongly depressed, which can overcome the limitation of steady and other transient approaches. This is the innovation of the thesis.A model of arc suppression coil grounded system is built combining the actual datas in the software of MATLAB6.5/Simulink, and various of circumstances of single phase-to-ground fault are simulated and experimented. Lots of experiment results show that, the proposed method in this thesis can detect the fault line quickly and accurately both in isolated neutral system and arc suppression coil grounded system,which doesn' t influenced by transient resistance and the initial phase of fault.