Research And Design On Single Phase To Ground Fault Line Selection Device For Small Current Grounding System

High-quality power energy such as low voltage fluctuation rate,less high-order harmonic components,and good voltage symmetry are key factors to realize the normal operation of electrical equipment.Generally in China medium-voltage power grid,neutral point is not connected with ground or connected with ground via arc suppression coil,which makes the single-phase grounding fault current level far less than the normal line current.Therefore,medium and low-voltage power grids are also named “small current grounding system”.High reliability is the main advantage of the small current grounding system.According to national standard,small current grounding system can operate for 120 minutes with power system faults.But in this case,voltage of non-grounding phase will rise sharply.It is a severe test for electrical insulation.Arc is easily to be generated and cause huge accident.So it is necessary to diagnose the fault and determine the location of the failed line system as soon as possible in case the single-phase grounding fault occurs in small current grounding system.Regarding mentioned issues,based on current-voltage constraint relationship during single phase short to ground malfunction,this thesis calculate the relationship between changes of voltage/current in steady-state and changes of voltage/current in transient-state.Through wavelet analysis and calculation,the high-frequency components in the transient-state are obtained as the theoretical basis for line selection.A smart grid transient current sensor based on STM32 is designed in this thesis,which can realize functions such as self-induction power acquisition,GPRS communication,fault location,and fault identification.The feasibility and effectiveness of wavelet analysis in solving power transient issues arefully demonstrated by this thesis.The key characteristic of transient current is extracted through Sym8 wavelet which is carried out with 5-layer decomposition,and is also important fault criterion.The square difference of maximum modulus is selected to locate the faulty line.Finally,a 10 k V small-current grounding system model is simulated in MATLAB.The effectiveness of this method is verified through the simulation of line selection calculation modules.