Research On Single-phase Grounding Fault Line Selection And Distance Measurement In Small Current Grounding System

In China,small current grounding system is widely applied in medium and low voltage distribution network.When the single-phase grounding fault occurs under this system,the fault is allowed to continue to run for 1-2 hours.But long periods of malfunction may widen the scope of the accident.Therefore,fast and accurate detection of the specific fault line and location,and timely removal of the fault,is conducive to the safe and stable operation of the power system.In view of this,this thesis studies the line selection and location of single-phase grounding fault in small current grounding system of distribution network.In this paper,the steady-state and transient characteristics of single-phase grounding fault in small current grounding system are analyzed,and the voltage and current characteristics and differences between fault lines and non-fault lines are analyzed theoretically and verified by simulation.Aiming at the problem that it is difficult to extract fault features from nonlinear and non-stationary transient signals,Complete Ensemble Empirical Mode Decomposition with Adaptive Noise(CEEMDAN)was introduced to extract fault features.As an improved method of Empirical Mode Decomposition(EMD)and Ensemble Empirical Mode Decomposition(EEMD),CEEMDAN effectively overcomes the shortcomings of EMD and EEMD,and can accurately and completely extract the fault feature information of the original signal.The simulation results show that CEEMDAN has better signal decomposition advantages than EMD and EEMD,and provides a signal decomposition tool for fault line selection and location.Aiming at the problem that fault features of transient components are difficult to extract,which leads to low adaptability of fault line selection method,a fault line selection method based on CEEMDNA-energy ratio was proposed.By using CEEMDAN to decompose the transient zero-sequence current of each circuit,a finite number of inherent mode function(IMF)components are obtained,and the corresponding frequency band energy of each IMF component is obtained.At the same time,the correlation number between each IMF component and the original zero-sequence current is calculated,and the correlation number is assigned to the frequency band energy as the weight.Then,the frequency band energy of each line is accumulated to get the energy of each line respectively.Finally,the energy proportion is obtained through normalization processing and compared with the preset threshold value for line selection as the criterion for line selection.Aiming at the problem that fault traveling wave signals are difficult to be extracted,a fault traveling wave extraction method based on CEEMDAN and Teager energy operator(TEO)is proposed.The voltage travelling wave signals at both ends of the line are decomposed by CEEEMAND to obtain a finite number of IMF components,and the zero line mode components at both ends are decomposed by using phase mode transformation to obtain the zero line mode components at both ends.Finally,the time and energy spectrum of the zero line mode components at both ends is obtained by using TEO to obtain the time when the zero line mode components arrive at both ends.An improved double-ended traveling wave location method is proposed based on the propagation process of the zero line mode component to both ends of the line and the double-ended traveling wave method.In this method,the fault distance can be obtained only by obtaining the moment when the zero line mode component reaches both ends of the line without calculating the traveling wave velocity.Wave velocity in every section of cable for the hybrid boc wave velocity on the inconsistent problem,using the zero line mold component to threshold value of the time difference as the fault section head end judgement,and connecting with the zero line mold component in different sections of different transmission characteristics developed a apply to different extents of the hybrid improvement double-end traveling wave ranging method.The fault distance can be obtained when the zero line module component extracted by CEEMDAN-TEO reaches both ends of the line and is substituted into the fault location method.For the fault line selection and fault location method studied in this paper,the simulation analysis is carried out by changing the different factors that affect the transient signal.The simulation results verify the effectiveness and adaptability of the fault line selection and fault location method proposed in this paper.