Researching On Fault Line Selection Setting Based On Two Current Transforms For Grounding Fault In Small Current Neutral Grounding System

The operation mode that uses the non-neutral ground and the ground connection by extinction coil is called low current grounding system. It is widely used in the power distribution network of our country. The quantity of the distribution network is greatness, and the area it covers is extensive. The network takes on the role of power supply to the large numbers of customers directly. Along with the development of the social economy, people's requirements of the electricity are increasing and the quality of it is of higher demanding at the same time. Therefore, if we can select out the faulty line quickly and efficiently when the fault happens, it will do lots of good to the system security, personal safety, operation stabilization and the reliability of the power supply. But as a result of the operation characteristic of the low current grounding system, the selection of faulty line when the single-phase fault happens in the system is a difficult problem that have not been solved during these years. This paper summarizes the specialty of all kinds of theories and schemes. The existed methods are mostly fit for the power distribution network with three-phase or single-phase current transformer. For the low current grounding system with only A-phase and C-phase current transformer in the distribute system, when the single-phase grounding fault happens, because there is no B-phase current transformer, the zero-sequence and the negative sequence current can not be used for the faulty line selection, the exited methods can not be applied well too. Based on these situations, the paper puts forward some questions that need to be solved.After analyzing the transient characteristics of the low current grounding system with only A-phase and C-phase current transformer when the single-phase fault happens, this paper brings forward that, as a result of the common reactance between the three phases, when the B-phase grounding fault happens, the fault line can be selected out by the use of A-phase or C-phase current, thereby it solves the question that the exited methods can not be used for the fault line selection when the B-phase grounding fault happens. This method uses the wavelet transform that has multi-resolution analysis and good time-frequency local character to pick up fault transient information, so the faulty line selection is achieved. This method is not only fit for the low current grounding system with two phase current transformers, but also adapts to the low current grounding system with other current transformers. Accordingly, it overcomes the question that the exited faulty line selection method fails when only two-phase current transformers are installed in the system. The theory analysis and a great deal of simulation experiments improves that the line selection method advanced in this paper is exact and reliable, it is fit for the low current grounding system with all kinds of grounding modes and it is of great robustness and feasibility.The paper finished the software and hardware design of the faulty line selection equipment of the low current grounding system with two phase current transformer, and we have accomplished the development of the device.