The Study Of Fault Line Selection Of The Small Current Grounding System Based On Wavelet Analysis

Small current grounding system is extensively used in 6-66 kV medium and low voltage distribution network in China,there is a high probability of single-phase grounding fault in this system.When it occurs in the small current grounding system,the voltage drop of the fault phase is 0,and the voltage of the non-fault phase rises radical 3 times as much as the original one,but the system remains symmetrical and can continue to run for 1-2 hours,thus the reliability of power supply is high.But it is necessary to find out the fault and remove it as soon as possible,so as not to expand the accident or damage the equipment.When the grounding fault occurs in the system,the fault current is mainly determined by the line-to-ground capacitance,and the value is very small and mixed in the larger load current,in addition,the diversity of grounding modes of small current grounding system makes the fault features of the system not clear,which makes it more difficult to extract fault information and judge fault lines.In this paper,advanced wavelet analysis is applied,and the following aspects are studied in detail:(1)This paper summarizes the existing theoretical methods of fault line selection in recent years,and briefly analyses their principles.Through the analysis and comparison of the advantages and disadvantages of various methods,it summarizes the existing problems in fault line selection..(2)The transient characteristics of zero-sequence current(mainly size and direction)of grounding point fault in single-phase grounding system are analyzed in depth.In addition,using MATLAB to build the model to simulate and verify the transient characteristics of zero-sequence current of grounding point fault.(3)Wavelet analysis has the function of localization in time-frequency domain.It can accurately extract the fault parts needed in this paper from fault transient signals.So the transient part of fault zero-sequence current can extracted by wavelet analysis.Coif4 wavelet is used for four-scale analysis,then,according to the principle of singularity detection of wavelet,fault line selection is realized by comparing the magnitude and polarity of modulus maxima of zero-sequence current after each outgoing line fault,which overcomes the limitation of traditional steady-state algorithm.(4)In this paper,a simulation model of 35 kV neutral grounding system(neutral grounding system and neutral grounding system via arc suppression coil)with four outgoing lines is built on the platform of MATLAB/Simulink.Four kinds of classical fault types are simulated.In addition,the simulation of high resistance grounding fault when zero-crossing is easy to be misjudged is carried out.The experimental results show that the wavelet analysis can accurately select the fault line.In order to study the application of wavelet analysis in practice,substations were simulated.The simulation experiments were carried out for different fault time(0.015s,0.018s,0.002s,0.022s,0.025s),high resistance grounding and bus fault.Experiments show that the fault line can be selected by wavelet analysis.