Research Of The Fault Line Detection In Non-Direct Grounded System

There are two distribution network grounding technologies in Shenzhen now: direct grounding and non-direct grounding. The non-direct grounding technology adopts resonant grounding technology – it is also called neurtral earthing via arc-suppression coil. It’s very hard to detect the fault line in resonant grounded system. This relay protection problem is difficult to solve in a long time, and it’s also the key of the safe operation of power system.This article analyzes the fault current of single phase fault in non-direct grounding system. The analysis includes two part: the analyze of the steady current and the analyze of the transient current. And then this article analyze several fault line detection technologies, and proposes that steady current-based comprehensive fault line detecting technology is the basic researching direction in fault line detection researching.The two active fault line detection technologies –small disturbance method and zero-sequence admittance method, are analyzed detailedly. The reasons of fault line detection failure with these two method are proposed: for small disturbance method, in order to detect the fault line, the adjustment of detuning degrees of compensation grid is affected by the parameter of the fault line, while it’s not concerned in the fault line detection devices now; zero-sequence admittance method possesses the capacity of avoiding earthing resistance in theory, but the phase angle of zero-sequence admittances of all lines are very similar to 90 degree, so the effect of fault line detecting is affected by the accuracy of the zero-sequence current transformer, the measurement deviation of current transformers may refer to fault line detecting failure.According these propblems, the article propose relevant optimization plans: adopt the optimal detuning degree in small disturbance method; comparing the zero-sequence admittances variations in stead of comparing the zero-sequence admittance values, this method solve the problem that the measurement deviation of current transformer makes fault line detecting failure. This article also proposes a fault location method, and apply it in the comprehensive fault line detection. Finally, a complex fault line detection technology based on small disturbance method and zero-sequence admittance method are proposed, and the effectiveness is verified by digital simulation.