Research On Method Of Improving Single-phase Grounding Fault Handling Ability Of Low Resistance Grounding System

According to statistics,among the faults occurring in the 10kV distribution system,single-phase grounding faults can account for more than 70%of the total faults.And after a single-phase ground fault occurs in the small resistance grounding system,the fault current is larger,so the fault must be quickly removed to ensure the safe and stable operation of the system.If the system occurs in the transition resistance is relatively small single-phase ground fault,the system will appear larger zero sequence voltage and zero sequence current,the fault is relatively easy to remove.But when the transition resistance is relatively large,the system zero sequence current amplitude decreases,and even will be less than the starting current threshold,resulting in the protection device can not detect the single-phase grounding,so that it may develop into a two-phase or three-phase fault.At present,when a single-phase ground fault occurs in the small resistance grounding system,the substation is generally configured with fixed-time zero-sequence overcurrent protection for processing,and the fixed-time zero-sequence overcurrent protection has an anti-transition resistance of less than 200Ω,which has shortcomings such as insufficient fault handling capability and low sensitivity.If the centralized wire selection device is added in the substation,it will increase a large amount of wiring,which is not conducive to the expansion of the substation.To address the above issues,the following aspects of the paper were studied:Based on the characteristics of single-phase grounding fault,a single-phase grounding selection method for small resistance grounding system based on substation steady-state monitoring information is proposed to determine the line where single-phase grounding occurs by using the monitoring information such as zero sequence current already available in the substation monitoring system without increasing the original hardware configuration.The line selection criteria based on steady-state zero-sequence current amplitude comparison,steady-state zero-sequence power comparison and transient first half-wave method are constructed respectively,and the rectification method of zero-sequence current start threshold is given.The results of theoretical analysis show that the sensitivity of the proposed single-phase ground fault handling method for small resistance grounding system mainly depends on the measurement accuracy of zero sequence current,and the anti-transition resistance can be increased to about 800Ω within the accuracy range of current zero sequence current transformer.In order to further enhance the sensitivity of small resistance grounding system to single-phase ground fault handling,the method of using dual range zero sequence current transformer and zero sequence current transformer with instrument security limit to reduce the zero sequence current start-up threshold is proposed,and the theoretical analysis results show that the above proposed improvement method can effectively reduce the start-up threshold and further increase the transition resistance to 2800Ω.A simulation model of 10 kV distribution network was built based on PSCAD software,and the effectiveness of the proposed method was verified by simulation under various scenarios,and the results showed that the performance of the proposed method was consistent with the theoretical analysis.The effectiveness of the proposed method is further verified by using field experimental data.