Centralized Protection For Grounding Fault In Low-Resistance Grounding System Based On Zero-Sequence Current Comparison

The application of low-resistance grounding method in distribution network can quickly cut off the fault,which can effectively reduce the overvoltage and prevent the further expansion of the fault.However,when a high-resistance grounding fault or multiple-line grounding fault occurs,because the zero-sequence current of the faulty line is small,the existing zero-sequence over-current protection may easily cause misoperation of the outlet protection or grounding transformer protection.It can result in further expand of failure range and failure time.It may also cause electrical shock to people and livestock,causing serious economic losses and social impact.Therefore,it is of great significance to study high-resistance grounding faults and multiple-line grounding faults in low-resistance grounding systems.This paper firstly analyzes the characteristics of the simple grounding fault of the low resistance grounding system.By analyzing the single-line grounding fault and the zero-sequence equivalent network of the bus grounding fault,the neutral point and links of the amplitude and phase of zero-sequence current between each line are calculated.It is clear that when a simple grounding fault occurs,the zero-sequence current amplitude of the fault line and neutral grounding point is ten times more than the amplitude of any healthy line.The zero-sequence current phase of the faulty line leads that of the healthy line by slightly more than 90°.Then,on the basis of analyzing the characteristics of simple grounding faults,the characteristics of complex grounding faults are further analyzed.By establishing a zero-sequence equivalent network,the simultaneous grounding faults of multiple lines and the simultaneous grounding faults of bus and lines are analyzed.It is clear that when the simultaneous grounding fault of multiple lines occurs,there is at least one fault line whose zero-sequence current amplitude is 5 times larger than that of any healthy line,and its phase leads healthy line by slightly higher than 90°.And then,by analyzing a variety of grounding fault features,two grounding fault neutral protection methods based on zero-sequence current comparison are proposed.One is based on comparing the zero-sequence currents of the lines.By comparing the zero-sequence current amplitude and phase of each line,the line,whose zero-sequence current amplitude is lighter than that of any other line by a certain multiple,and whose phase is advanced by any other line,can be selected as faulty line.The other method is based on the zero-sequence current projection component comparison.It compares the zero-sequence current of each line,and also the ratio of the projected component of the zero-sequence current at the neutral point to the neutral point zero-sequence current amplitude.Finally,the line,whose projected component amplitude of the zero-sequence current is certain times larger than zero-sequence current amplitude of neutral point,can be selected as faulty line.Otherwise it is a healthy line.Finally,MATLAB simulation is used to verify the correctness of the analysis of various grounding fault features,and to verify the reliability of the proposed grounding fault protection method,which based on the comparison of the zero-sequence current of the line,and another method,which based on comparison of the zero-sequence current projection component.