Research On Safety Analysis And High Resistance Grounding Fault Protection For Retrofit Of Low Resistance Grounding In Distribution Network

In recent years,due to the increase of the line cable rate and other reasons,some cities in my country have experienced a sudden increase in the capacitor current and insufficient compensation capacity of the arc suppression coil of the distribution network,which makes the ground fault arc difficult to self-extinguish,causing a series of cable burning and substation fire questions.So one of the effective solutions to the problems is to transform the low current grounding system whose capacitance current seriously exceeds the standard into a low resistance grounding.During the transformation of the low resistance grounding method,different regions face different problems,there are the following problems in the renovation of a certain central region of China.when the high-voltage side fault occurs in the distribution transformer outside the station,the ground potential may rise and cause potential personal safety hazards on the low-voltage side and the difficulty of effectively protecting the single-phase high resistance grounding fault of the distribution network.According to the circuit theorem,this paper analyzes the equivalent circuit of the single-phase grounding fault in low resistance grounding system,and systematically summarizes the fault current characteristics in two periods of steady state and transient state;the generation mechanism,characteristics and commonly used arc models of arc discharge are expounded,and combined with the characteristics of high resistance grounding faults,a suitable arc model is established.In view of the problem of in the process of transformation of low resistance grounding system,the occurrence of grounding faults may cause personal safety hazards on the low voltage side of the distribution transformers outside the station.Firstly,Matlab/Simulink is used to build the line load inversion model of the arc suppression coil grounding system(NES)to the low resistance grounding system(NRS),based on the background that the distribution transformer shell protection and the working protection are grounded together,the occurrence of shell collision faults is simulated,and calculating ground fault current,ground potential rise,and current flowing through the human body under the typical grounding resistance by simulation;meanwhile,a finite element analysis model of the grounding device based on electromagnetic field theory is established to calculate ground potential and step voltage distribution at the fault point;consider the safe current,time and safe voltage of the human body,corresponding protection measures are proposed.Aiming at the problem that the current value is small when the high-resistance grounding fault occurs in low resistance grounding system,the zero-sequence overcurrent protection may not effectively protect.Firstly,the NRS ground fault equivalent circuit is analyzed,obtaining the characteristic of large difference in zerosequence current amplitude and phase between fault and sound lines,and the above characteristics are verified by simulating the zero-sequence current waveform comparison between lines under typical transition resistance;secondly,a protection method to analyze the zero-sequence current waveform of the line by using the Pearson correlation coefficient to obtain the average similarity of the line,so as to select the faulty line is used;then the single-phase grounding fault model of low resistance grounding system and the cybernetic arc model are built,in different systems(cable line system,hybrid line system),by setting different fault types,different fault point transition resistances,different fault point positions,and different fault closing phase angles as fault conditions for simulation to verify the effectiveness of the method.