Study On Comprehensive Fault Line Selection Method Based On Fault Types For Single-Phase Earth Fault

Distribution faults are mostly single-phase earth faults, the accuracy of fault line selection in small current neutral grounding system is affected by many factors, so fault line selection for single-phase earth faults in small current neutral grounding system has been a difficult point in the field of relay protection.Detailed analysis on the variation of zero sequence voltage, transient zero sequence current and steady state zero sequence current of distribution single-phase earth fault in different neutral grounding modes was made in this dissertation. The model of arc grounding fault was built and simulation was carried out, the increases of zero sequence voltage caused by PT breaking fault, line breaking fault and ferroresonance fault were compared with the zero sequence voltage increase caused by single-phase earth fault, the method to correctly judge single-phase earth fault was got.Three new methods for fault line selection are proposed in this dissertation, they are active power method to eliminate unbalanced current, fault line selection method based on the comparison of the square value of wavelet transform modulus maxima of transient fault zero sequence current and fault line selection method based on grey relational analysis of zero sequence current. This dissertation proposed an improved type of Fourier algorithm to calculate the effective value of physical quantities accurately, and it eliminated the effect of interference signals. Transient characteristics of the grounding fault contain abundant fault informations, wavelet transform can extract useful fault information from the transient fault signals accurately. A new criterion of fault line selection based on the comparison of the square value of wavelet transform modulus maxima of transient fault zero sequence current was proposed in this dissertation, and it enlarged the fault differences of fault branch and non-fault branches. Fault line selection method based on grey relational analysis of zero sequence current uses the zero sequence current correlation degree of fault branch and non-fault branches to construct an adaptive fault line selection criterion. Research shows that single fault line selection method has obvious effect in some conditions, but isn't suitable for all conditions. It can suit more fault conditions when several methods were used to make comprehensive analysis on the various information produced by fault and use them comprehensively, the accuracy and effective range of fault line selection method also can be raised. Comprehensive method for fault line selection based on several methods is a good way to raise the accuracy and effective range of fault line selection, and it is a research frontier of the fault line selection field.Necessity of the identification of different types of faults were analyzed in this dissertation, comprehensive method for fault line selection was realized by the combination of fault line selection methods based on steady component and transient component using the weighted coefficient method. Different single-phase earth faults have different characteristics, transient component is relatively rich under the conditions of arc grounding fault and instantaneous grounding fault, and the steady component is relatively rich under the condition of resistance grounding fault. So, it is necessary to select weights of different methods reasonably based on different fault characteristics in order to achieve the best fault line selection results. Multi-fractal theory is used to calculate zero sequence voltage of stable and unstable grounding fault,and the relationship between multi-fractal spectrum range and fault type was got. According to different fault zero sequence voltage multi-fractal characteristics, the degree of the fault tending to stable and unstable grounding fault can be determined, thus it provides a basis for determining the weighted coefficient of fault line selection methods based on steady component and transient component used in information fusion method. Fuzzy theory is a method to deal with complex uncertainty problems, this dissertation made an analysis on multi criteria information fusion fault line selection method based on fuzzy theory, and pointed out insufficient point that the traditional fuzzy theory can not completely express all the fault information. Intuitionistic fuzzy sets is the extension of the traditional fuzzy sets, it can use fault information more completely, and achieve multi criteria information fusion fault line selection better.A comprehensive method for fault line selection based on intuitionistic fuzzy information integration theory was proposed in this dissertation, and the problems of determining membership degree and non-membership degree of fault characteristics for fault line selection was rationally solved. Weights of different fault characteristics were calculated according to the multi-fractal spectrum of zero sequence voltage, it solved the problem that previous weighted coefficient method can't select weights of different methods reasonably based on different fault characteristics. The comprehensive method based on intuitionistic fuzzy theory determines the weighted coefficients of fault line selection methods based on steady component and transient component reasonably according to the degree of the fault tending to stable and unstable grounding fault, information fusion of fault characteristics was made. It uses the fault information completely and raises the accuracy of fault line selection.The comprehensive method for fault line selection based on intuitionistic fuzzy theory proposed in this dissertation make use of the aforesaid three kinds of single fault line selection methods to determine the membership degree of fault steady characteristics, transient characteristics and grey correlation characteristics separately, it uses zero sequence voltage multi-fractal spectrum to calculate weights of the three methods in order to make information fusion, and finally achieve comprehensive fault line selection. A lot of experiment and simulation test results confirmed the reliability and superiority of intuitionistic fuzzy comprehensive method for fault line selection.