Research On Line Selection Method For Single-Phase Grounding Fault In Small Current Grounding System

Most power supply systems in China adopt neutral grounding system or arc suppression coil grounding system with small current grounding operation mode.In this system,the probability of single-phase grounding fault of transmission line is the highest.At present,there are many methods of line selection for the study of small current grounding system at home and abroad.Because of the small fault current and many influencing factors in single-phase fault,the single method of line selection in practical application can not cover all grounding conditions and is vulnerable to the influence of external environment,resulting in the low correct rate of line selection for single-phase grounding fault.Therefore,it is of great significance to study how to effectively improve the selectivity of single-phase grounding protection and make the method of line selection develop towards intellectualization.The main works of the thesis are listed as follows:(1)This paper introduces the research background and significance of single-phase grounding fault in small current system,and briefly analyses the principle and shortcomings of current line selection methods used at home and abroad.The steady-state and transient characteristics of each line are analyzed.According to the different grounding modes,the steady-state and transient characteristics of fault line and non-fault line are compared,which lays a theoretical foundation for the research of line selection method.(2)By analyzing the principle of line selection method based on steady state characteristics and transient characteristics,the grounding fault model of small current system is constructed by using the software of Matlab/Simulink,and the advantages,disadvantages and problems needing improvement of each line selection method are evaluated.The influence of three-phase unbalance on grounding line selection is deeply analyzed,and the theory of three-phase unbalance is studied.On this basis,it is established that in order to carry out accurate and fast grounding line selection,it is necessary to start with transient characteristic signals and use intelligent algorithm as the method of line selection.(3)A grounding line selection method based on wavelet neural network based on particle swarm optimization is proposed.The theory of wavelet neural network based on particle swarm optimization is expounded.The principles of wavelet analysis,neural network and particle swarm optimization(PSO)are briefly described,whichprovides the algorithm basis for the proposed line selection method.In theory,using wavelet neural network based on particle swarm optimization for fault line selection has high accuracy and rapidity,which can improve the shortcomings of existing methods.The proposed method is validated by simulation experiments.A single-phase fault model of small current grounding system is built by using MATLAB/Simulink.By changing its parameters,a large number of fault data are obtained.Comparing the simulation data with the grounding line selection method based on wavelet neural network based on particle swarm optimization,the results show that the grounding line selection method based on particle swarm optimization can effectively improve the accuracy and rapidity of the grounding line selection,and in the case of three-phase unbalance,it can still complete the correct line selection,compared with the traditional grounding method.Line selection method has better adaptability.The wavelet neural network optimized by particle swarm optimization has two characteristics of selectivity and rapidity,and is more suitable for grounding fault line selection of small current system.