High-sensitivity Ground Fault Detection Based On Crosswise Cooperation In Low-resistance Grounding System

The low-resistance grounding system adopted by many major cities distribution networks has the advantages of quickly cutting off grounding fault,low overvoltage level,elimination of resonant overvoltage,using cables and electrical equipment with lower insulation,and convenient operation and maintenance,etc.Compared to non-solid earthed networks,the low-resistance grounding system has a higher fault current when single-phase grounding,and needs to be cut off or isolated faults immediately.Therefore,additional grounding protection should be separately configured.The current value setting of zero-sequence overcurrent protection of China’s 10 k V low-resistance grounding distribution network is generally 40A～60A.The maximum grounding resistance of about 85Ω～130Ω can be detected,therefore,the existed protection cannot protect high-impedance grounding faults.High resistance grounding fault protection problem has always been one of the difficulties in grounding protection of low-resistance grounding systems.Single-phase ground fault characteristics of low-resistance grounding system are focused in this paper.By establishing the zero-sequence equivalent network of grounding faults at different places,the distribution law of the zero-sequence currents in each protection installation is analyzed.It is clear that when a single-phase ground fault occurs,the zero-sequence current amplitude of the fault line is slightly larger than the zero-sequence current amplitude of the neutral point,both of which are higher than 10 times the amplitude of the zero-sequence current of any sound line.And it is independent of the ground transition resistance.In addition,the zero-sequence current amplitude of a healthy line at a metallic ground fault may be higher than the fault line and the neutral zero-sequence current amplitude at a high-impedance grounding fault.Based on the distribution law of zero-sequence current in single-phase grounding faults,the idea of crosswise cooperation between the same-level protections is proposed.The limitation that the grounding protection setting value must be higher than the maximum capacitance current of the line is broken out,which significantly reduces the threshold value of the protection starting current.Combined with the characteristics of traditional inverse-time protection,the zero-sequence inverse-time overcurrent protection for low-resistance grounding system is proposed.The high-sensitive zero-sequence stage current protection for low-resistance grounding system is proposed,which segments the traditional time-fixed zero-sequence overcurrent protection further.Both algorithms give detailed tuning principles,and considering the grounding transformer protection as the backup protection of the outgoing line protection.Taking the zero-sequence inverse-time overcurrent protection algorithm as an example,a multi-level grounding protection scheme suitable for low-resistance grounding systems is proposed,which enables each protection to cut off faults,reduce the power-off range,and improve the reliability of power supply.The protection methods proposed in this paper can improve the protective resistance to1.5kΩ,with effective reliability,selectivity and sensitivity.Among them,the high-sensitive zero-sequence stage current protection algorithm does not need to develop a new protection device,and it can be realized by modifying the fixed value on the original device,of which the engineering application prospect is broad.Finally,the correctness of various ground fault characteristics analyzed in this paper is verified by MATLAB/SIMULINK simulation and experiment data,and the effectiveness and reliability of the proposed protection method are also verified.