Research For Multi-criteria Grounding Faults In Small Current Grounding System

In the small current grounding system,the single-phase ground fault has a high probability of occurrence and the ground fault is complex and diverse,which leads to the problem that the accuracy of the single line selection is low,and the traditional manual line-checking method reduces the line selection efficiency and expands the fault.With the improvement of the level of automation technology,the line selection device needs to be further improved in terms of automation and line selection accuracy.Therefore,the research on single-phase ground fault multi-criteria line selection algorithm for small current grounding system is an urgent problem to be solved at home and abroad.In order to solve the single-phase ground fault line selection problem better,the transient zero-sequence equivalent model of the neutral point ungrounded and neutral point arc-suppression coil grounding system is analyzed,and the mathematical model of differential equation is established.According to the difference of grounding transition resistance,the expression of fault zero-sequence voltage and zero-sequence current in the over-damped and under-damped transition process is obtained.Through the comparison and analysis of MATLAB simulation and formula derivation results,it is verified that the theoretical analysis results can better reflect the transient characteristics of each line when different transition resistance ground faults.According to the different components in the zero-sequence current expression of different transition resistances,the double-tree complex wavelet with better time-invariance is used to extract the fault information of each line,so as to obtain the fault components of different frequency bands accurately.Corresponding line selection measures are constructed for fault components of different frequency bands.For the neutral point ungrounded system,the FFT analysis is used to obtain the power frequency amplitude and phase information after the fault is stabilized.The information is used to construct the steady fault measure.For the arc suppression coil grounding system,the DC fault coefficient is constructed by using the DC wavelet coefficients decomposed by the double-tree complex wavelet;for both systems,select the double-tree complex wavelet decomposition and reconstruction to the second layer of wavelet coefficients to construct a transient fault measure;The first half-wave line selection measure is based on the first half-wave current extreme point,the zero-sequence voltage change rate sign is consistent with the non-fault line symbol,and the fault line current is equal to the fault line current and the sum of all non-fault line currents and both flowing to the opposite feature.Based on the fault measure value,it is divided into three sections: effective domain,feasible domain and infeasible domain.Corresponding line selection rules are established for the measurement in different intervals,and the basic reliability distribution function is constructed for multiple line selection measures.And use DS evidence theory to achieve multi-criteria information fusion,and then make up for the shortcomings of the single line selection algorithm.Finally,a MATLAB simulation model for single-phase ground fault of 10 kV power distribution system and a static simulation integrated experimental system for distribution network are built.Through the large amount of data simulation and experimental analysis of the line selection method proposed in this paper,the correctness of the method of selecting lines in the text is verified.