Research On Line Selection Device Of Non-Effectively Grounded System Based On Fuzzy Theory

Non-effectively grounded neutral point operation mode which’s another name is the Little Current Grounding System, with its power supply reliability, is widely used in medium voltage distribution network. Small current grounding system faults are mostly single-phase-to-earth faults, when the system single-phase ground fault occurs, it will not power off immediately and will continue to run two hours, but if you do not take measures during the fault zone, accidents may lead to greater harm. so fault line selection for single-phase earth faults in small current neutral grounding system has become a major problem faced by electricity workers. In recent decades, there is a lot of power workers and researchers engaged in research of this issue, but failed to find a good solution. Therefore,a further study on line detection for single-Phase earth fault is important and significant.This paper aims to find more effective line selection criterion on the basis of previous research. First, I introduced several neutral grounding operation modes.on this basis, the system operation characteristics in both neutral isolated and neutral Petersen-coil-grounded modes are analyzed,and various fault features when the single-Phase earth fault occurs are discussed. After the comparative line selection techniques are introduced and comprehensive evaluation, it can be found that single line selection technique for certain circumstances have a good line selection effect, but does not fully apply to complex grid system, Therefore, the following conclusions can be drawn. Putting more complementary criterion for effective integration can get a new method to select. By contrast, three complementary fault characteristics,containing steady-state component, transient component and injected signal, are chosen as the integrated line detection basis in order to make better use of the fault information. And three existing line detection methods based on each characteristic are listed and explained, which are zero-sequence current amplitude and phase comparison, wavelet analysis method and power injection method.When single-phase ground fault occurs, the fault features information of the small current grounding system is affected by many factors, including the type of ground fault which is the one of greater impact. So, in order to accurately judge fault lines,it must distinguish the type of ground fault firstly. Fractal theory has a good ability to recognize the singular of the signal. When single-phase ground fault occurs, the system will produce zero sequence voltage and zero sequence currents including a zero sequence voltage and multiple zero sequence current.So system ground fault type can be well discerned through analyzing the zero sequence voltage by using of the multi-fractal theory.On the basic of theoretical analysis,fault membership functions and weighting membership functions can be derived.And comprehensive weighting can be derived by the combination of ground fault type weighting and the objective weighting of each line selection technology. In order to effectively integrate the various line selection criterion, this paper selects the fuzzy theory to achieve integration of information. In the case of different types of ground faults and system models. In the case of different types of ground faults and system models, Integrated line selection technique was simulated in MATLAB. And experimental results show that integrated line selection technique that using fuzzy theory to achieve information integration works well.Finally, on the basis of line selection criterion has been established,line selection device hardware and software were designed. And the various hardware circuit connection diagram and software design flow were showed in this paper and described detail.