Reaearch On Method Of Single-phase Ground Fault Line Selection In Small Current Grounding System

At present,the growing national economy puts forward higher requirements for the stability of power system operation.Small current grounding system is generally applied in medium and low voltage power systems in China,and most of the faults are single-phase ground faults.When such a fault occurrs,it shows short transient period,weak signal and difficult in fault feature extraction.Therefore,it is difficult to select line exactly,and it is necessary to study new line selection methods.In this paper,a new method of single-phase ground fault line selection based on improvement Empirical Wavelet Transform(EWT)and D-S evidence theory was put forward by analyzing the single-phase ground fault characteristics of small current grounding system.Firstly,the simulation model of small current grounding system was built by Matlab/Simulink,and the fault data of different neutral grounding methods,fault occurrence time,fault transition resistance and fault distance were obtained.Then,the spectrum of zero-sequence transient current in each feeder was analyzed by smoothing pretreatment.Through the EWT decomposition,the intrinsic mode function which can best represent the original signal was extracted,and then the energy peak value,the intrinsic mode function energy and the energy relative entropy were calculated to judge the possibility of each line fault respectively.The feasibility of the three methods was verified by the simulation data of several typical examples.Finally,the basic probability assignment function of each method was constructed after verifying the feasibility,and the D-S evidence theory was used to combine the three line selection results at the decision-making level to obtain the final results.In this way,three line selection methods would be complementary,and made the accuracy of line selection higher.In order to verify the practicability of the line selection scheme studied in this paper,a simulation experiment circuit was established in the laboratory,which follows the Technical Specification for Single-phase Ground Fault Line Selection Device for Small Current Grounding System,in order to obtain more realistic fault data.The fault data was collected using the data acquisition circuit based on stm32f7 hardware platform and was sent to the PC.Then the fault data was analyzed and processed by MATLAB to get the final result of line selection.The simulation experiment and simulation fault experiment proved that the line selection method used in this paper can identify both line faults and bus faults.It comprehensively utilized the characteristics of zero-sequence current amplitude,peak value,energy and correlation between fault line and non-fault line after fault occured,which could be applied to many different fault situations,and the effect of line selection was better.