Research On Single-Phase Grounding Protection Of Low Resistance Grounding Distribution Network Based On Ground Wire Current Distribution

Due to the continuous increase of the capacitance current to the ground in the urban distribution network,the neutral grounding via arc suppression coil system is facing greater challenges in the aspects of single-phase grounding arc self-extinguishing,suppression of over-voltage,arc suppression coil capacity modification and fault location.In order to solve the above-mentioned problems,some urban distribution networks have been or are being transformed into low resistance grounding system,and they are gradually being promoted.Compared with the non-effectively grounded system,the low resistance grounding system has a larger fault current when a singlephase grounding fault occurs,and the distribution network has many branch lines and complex structure,which puts forward higher requirements for its protection performance.Therefore,the research on single-phase grounding protection of low resistance grounding system has important theoretical and application value.Combined with the development of the urban distribution network,the basis for the selection of neutral grounding resistance is analyzed,and it is concluded that the value of 5-15 ohms is more appropriate.On the basis of constructing an equivalent model of a low resistance grounding distribution network,the characteristics of the zero-sequence voltage,the characteristics of the zero-sequence current of the faulty line and the non-faulty line,and the ratio of the zero-sequence current to the neutral current in the case of a single-phase ground fault are analyzed.Taking advantage of the characteristics of the zero-sequence voltage and the zero-sequence current of each line in the case of a single-phase ground fault,as well as the obvious difference in the ratio of the faulted line and the non-faulted line to the neutral point current,zero-sequence inverse-time over-current protection and zero-sequence over-current relay with the resistance-capacitance ration restriction have been proposed,which can be used to solve high impedance fault,the principle and performance of these two types of single-phase grounding protection are analyzed and verified by simulation.The principle and performance of these two types of single-phase grounding protection are analyzed and verified by simulation.According to the structure of the three-core power cable,the grounding method of the metal sheath and related construction standards,an equivalent model of the cable distribution network with the two ends of the metal sheath structure grounded is constructed.Using the symmetrical component method,the superposition theorem and the substitution theorem,the distribution characteristics of the ground wire current and the difference between the ground wire current of the faulted line and the non-faulty line are analyzed,and a simple method to analyze and calculate the ground wire current distribution in the low resistance grounding distribution network is proposed,and through MATLAB/Simulink to build a low resistance grounding distribution network simulation model to verify the above theoretical analysis.Through theoretical analysis and simulation,the ratio of parallel line ground current to neutral ground resistance current in single-phase ground fault,and the characteristics of the ground current of each upper and lower line in the fault line and neutral ground resistance current are studied.Based on the above research conclusions,an inverse time protection based on the ratio of the ground current to the neutral ground resistance current is proposed.The thesis makes a detailed study of the protection principle,setting calculation method,vertical selectivity,horizontal selectivity,and influencing factors.The feasibility and performance of protection in various situations were simulated and verified by building simulation models.