Analysis of Single-Phase Grounding Fault Current in Urban Distribution Networks

Nowadays, urban distribution networks are mainly equipped with distribution systems with a low-resistance grounding system and these systems transmit electricity via underground cables, which means that the impact of capacitance current cannot be ignored. As long as there is a single-phase grounding fault in a certain bus, the zero-sequence current flowing through the lines without fault which are connected to the same bus is the capacitance current of these cables separately. In this case, if the cable lengths of non-faulty lines are too long, a large zero-sequence current will be generated in the non-faulty lines, and it may even cause the unwanted operation of protection of non-faulty lines in extreme cases. In this paper, the mechanism of single-phase earth faults in urban distribution systems is analyzed. Then, the calculation method for zerosequence current is displayed for the distribution network with a single-phase earth fault. In the case study part, the correctness of the zero-sequence current calculation method is verified through a tripping accident analysis in a substation in the north of China. Finally, measures are proposed to avoid unwanted operation of protection by shortening the length of the distribution line and increasing the protection device's setting value for zero-sequence current appropriately.