Protection Technology Of Unstable High Resistance Grounding Fault In Low Resistance Grounding System

The low resistance grounding system has the advantages of cutting off permanent low resistance grounding faults quickly,lower over-voltage levels,and convenient operation and maintenance.It is being adopted by more and more large or medium-sized urban power distribution networks.Due to the complex environment of the feeder,when the single-phase grounding fault occurs in the low resistance grounding system,the fault resistance may be high.At the same time,causing by the unstable contact between the feeder and the earth,the physical and chemical changes of the medium or other reasons,the grounding fault may be unstable.Traditional protection methods are difficult to deal with it effectively.The long-term existence of unstable grounding fault is easy to lead to bad accidents such as protection override mis-operation,neutral point grounding resistor burnout and even power outages throughout the station,which seriously affects the reliability of power supply.This thesis researches on the logic of fault modeling--feature analysis--fault protection.First of all,combined with common arc black-box models,this thesis proposes a Multi-odd frequency voltage source equivalent arc model which is more suitable for fault transient and steady state analysis.Combining with the fault equivalent circuit of low resistance grounding system,analytical expression of current and unstable fault characteristics in arc grounding faults under different fault resistances is quantitatively calculated.Secondly,in order to solve the problem of unstable high resistance grounding fault protection in low resistance grounding system,this thesis proposes a grounding fault longitudinal differential protection method which uses zero sequence voltage information to adaptively correct the current between two measuring terminals.This method can support relay protection to deal with unstable high resistance grounding fault.Then,focus on unprotected intermittent grounding fault,this thesis combined the thermal stability principle and the situation which means neutral grounding resistor generates heat during arcing and dissipates heat after arc extinguished,a backup protection method for unstable grounding fault in low resistance grounding system is proposed.This method uses zero-sequence current to construct the characteristic value of heat during fault arcing,uses the difference of heat to construct the characteristic value of heat dissipation after arc extinguished.Protect will action after the characteristic value of heat exceeds the limit,handling intermittent ground fault.Finally,the correctness of each method is verified by simulation and field record data.The main work of this thesis fills the shortage of the protection methods for the unstable high resistance grounding faults in the low-resistance grounding system,improves the ability of protection to handle transition resistance to 2kΩ,establishes a comprehensive treatment scheme of unstable grounding fault.It is of great significance for arc grounding fault modeling and quantitative analysis,exploring instability and high resistance ground fault handling modes,and improving power supply reliability of distribution networks.